THE AEROPLANE SPEAKS

By H. Barber

(Captain, Royal Flying Corps)

DEDICATED TO THE SUBALTERN FLYING OFFICER

MOTIVE

The reasons impelling me to write this book, the maiden effort ofmy pen, are, firstly, a strong desire to help the ordinary man tounderstand the Aeroplane and the joys and troubles of its Pilot; and, secondly, to produce something of PRACTICAL assistance to the Pilotand his invaluable assistant the Rigger. Having had some eight years'experience in designing, building, and flying aeroplanes, I have hopesthat the practical knowledge I have gained may offset the disadvantageof a hand more used to managing the "joy-stick" than the dreadfulhaltings, the many side-slips, the irregular speed, and, in short, thealtogether disconcerting ways of a pen. 

The matter contained in the Prologue appeared in the Field of May 6th, 13th, 20th, and 27th, 1916, and is now reprinted by the kind permissionof the editor, Sir Theodore Cook. 

I have much pleasure in also acknowledging the kindness of Mr. C. G. Grey, editor of the Aeroplane, to whom I am indebted for the valuableillustrations reproduced at the end of this book. 

CONTENTS

 PROLOGUE

 PART I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB III. THE GREAT TEST IV. CROSS COUNTRY

 CHAPTER I. FLIGHT II. STABILITY AND CONTROL III. RIGGING IV. PROPELLERS V. MAINTENANCE

 TYPES OF AEROPLANES

 GLOSSARY

THE AEROPLANE SPEAKS

PROLOGUE

PART I. THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES

The Lecture Hall at the Royal Flying Corps School for Officers wasdeserted. The pupils had dispersed, and the Officer Instructor, morefagged than any pupil, was out on the aerodrome watching the test of anew machine. 

Deserted, did I say? But not so. The lecture that day had been uponthe Elementary Principles of Flight, and they lingered yet. Upon theBlackboard was the illustration you see in the frontispiece. 

"I am the side view of a Surface, " it said, mimicking the tones of thelecturer. "Flight is secured by driving me through the air at an angleinclined to the direction of motion. "

"Quite right, " said the Angle. "That's me, and I'm the famous Angle ofIncidence. "

"And, " continued the Surface, "my action is to deflect the airdownwards, and also, by fleeing from the air behind, to create asemi-vacuum or rarefied area over most of the top of my surface. "

"This is where I come in, " a thick, gruff voice was heard, and wenton: "I'm the Reaction. You can't have action without me. I'm a veryconsiderable force, and my direction is at right-angles to you, " andhe looked heavily at the Surface. "Like this, " said he, picking up thechalk with his Lift, and drifting to the Blackboard. 

"I act in the direction of the arrow R, that is, more or less, for thedirection varies somewhat with the Angle of Incidence and the curvatureof the Surface; and, strange but true, I'm stronger on the top of theSurface than at the bottom of it. The Wind Tunnel has proved that byexhaustive research--and don't forget how quickly I can grow! As thespeed through the air increases my strength increases more rapidly thanyou might think--approximately, as the Square of the Speed; so yousee that if the Speed of the Surface through the air is, for instance, doubled, then I am a good deal more than doubled. That's because I amthe result of not only the mass of air displaced, but also the resultof the Speed with which the Surface engages the Air. I am a product ofthose two factors, and at the speeds at which Aeroplanes fly to-day, and at the altitudes and consequent density of air they at presentexperience, I increase at about the Square of the Speed. 

"Oh, I'm a most complex and interesting personality, I assure you--infact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr. Hyde. There's Lift, my vertical part or COMPONENT, as those who preferlong words would say; he always acts vertically upwards, and hatesGravity like poison. He's the useful and admirable part of me. Thenthere's Drift, my horizontal component, sometimes, though rathererroneously, called Head Resistance; he's a villain of the deepest dye, and must be overcome before flight can be secured. "

"And I, " said the Propeller, "I screw through the air and produce theThrust. I thrust the Aeroplane through the air and overcome the Drift;and the Lift increases with the Speed and when it equals the Gravity ofWeight, then--there you are--Flight! And nothing mysterious about it atall. "

"I hope you'll excuse me interrupting, " said a very beautiful younglady, "my name is Efficiency, and, while no doubt, all you have said isquite true, and that, as my young man the Designer says, `You can make atea-tray fly if you slap on Power enough, ' I can assure you that I'm notto be won quite so easily. "

"Well, " eagerly replied the Lift and the Thrust, "let's be friends. Dotell us what we can do to help you to overcome Gravity and Drift withthe least possible Power. That obviously seems the game to play, formore Power means heavier engines, and that in a way plays into the handsof our enemy, Gravity, besides necessitating a larger Surface or Angleto lift the Weight, and that increases the Drift. "

"Very well, " from Efficiency, "I'll do my best, though I'm so shy, andI've just had such a bad time at the Factory, and I'm terribly afraidyou'll find it awfully dry. "

"Buck up, old dear!" This from several new-comers, who had justappeared. "We'll help you, " and one of them, so lean and long that hetook up the whole height of the lecture room, introduced himself. 

"I'm the High Aspect Ratio, " he said, "and what we have got to do tohelp this young lady is to improve the proportion of Lift to Drift. The more Lift we can get for a certain area of Surface, the greaterthe Weight the latter can carry; and the less the Drift, then the lessThrust and Power required to overcome it. Now it is a fact that, if theSurface is shaped to have the greatest possible span, i. E. , distancefrom wing-tip to wing-tip, it then engages more air and produces both amaximum Reaction and a better proportion of Lift to Drift. 

"That being so, we can then well afford to lose a little Reactionby reducing the Angle of Incidence to a degree giving a still betterproportion of Lift to Drift than would otherwise be the case; for youmust understand that the Lift-Drift Ratio depends very much upon thesize of the Angle of Incidence, which should be as small as possiblewithin certain limits. So what I say is, make the surface of InfiniteSpan with no width or chord, as they call it. That's all I require, Iassure you, to make me quite perfect and of infinite service to MissEfficiency. "

"That's not practical politics, " said the Surface. "The way you talk onewould think you were drawing L400 a year at Westminster, and working upa reputation as an Aeronautical Expert. I must have some depth and chordto take my Spars and Ribs, and again, I must have a certain chord tomake it possible for my Camber (that's curvature) to be just right forthe Angle of Incidence. If that's not right the air won't get a niceuniform compression and downward acceleration from my underside, and therarefied `suction' area over the top of me will not be as even and cleanin effect as it might be. That would spoil the Lift-Drift Ratio morethan you can help it. Just thrust that chalk along, will you? and theBlackboard will show you what I mean. "

"Well, " said the Aspect Ratio, "have it your own way, though I'm sorryto see a pretty young lady like Efficiency compromised so early in thegame. "

"Look here, " exclaimed a number of Struts, "we have got a brilliant ideafor improving the Aspect Ratio, " and with that they hopped up on to theSpars. "Now, " excitedly, "place another Surface on top of us. Now do yousee? There is double the Surface, and that being so, the proportion ofWeight to Surface area is halved. That's less burden of work for theSurface, and so the Spars need not be so strong and so deep, whichresults in not so thick a Surface. That means the Chord can beproportionately decreased without adversely affecting the Camber. With the Chord decreased, the Span becomes relatively greater, and soproduces a splendid Aspect Ratio, and an excellent proportion of Lift toDrift. "

"I don't deny that they have rather got me there, " said the Drift, "butall the same, don't forget my increase due to the drift of the Strutsand their bracing wires. "

"Yes, I dare say, " replied the Surface, "but remember that my Spars areless deep than before, and consequently I am not so thick now, andshall for that reason also be able to go through the air with a lessproportion of Drift to Lift. "

"Remember me also, please, " croaked the Angle of Incidence. "Since theSurface has now less weight to carry for its area, I may be set ata still lesser and finer Angle. That means less Drift again. We arecertainly getting on splendidly! Show us how it looks now, Blackboard. "And the Blackboard obligingly showed them as follows:

"Well, what do you think of that?" they all cried to the Drift. 

"You think you are very clever, " sneered the Drift. "But you are nothelping Efficiency as much as you think. The suction effect on the topof the lower Surface will give a downward motion to the air above it andthe result will be that the bottom of the top Surface will not secureas good a Reaction from the air as would otherwise be the case, and thatmeans loss of Lift; and you can't help matters by increasing the gapbetween the surfaces because that means longer Struts and Wires, andthat in itself would help me, not to speak of increasing the Weight. Yousee it's not quite so easy as you thought. "

At this moment a hiccough was heard, and a rather fast andrakish-looking chap, named Stagger, spoke up. "How d'ye do, miss, " hesaid politely to Efficiency, with a side glance out of his wicked oldeye. "I'm a bit of a knut, and without the slightest trouble I caneasily minimize the disadvantage that old reprobate Drift has beenfrightening you with. I just stagger the top Surface a bit forward, andno longer is that suction effect dead under it. At the same time I'msure the top Surface will kindly extend its Span for such distance asits Spars will support it without the aid of Struts. Such extension willbe quite useful, as there will be no Surface at all underneath it tointerfere with the Reaction above. " And the Stagger leaned forward andpicked up the Chalk, and this is the picture he drew:

Said the Blackboard, "That's not half bad! It really begins to looksomething like the real thing, eh?"

"The real thing, is it?" grumbled Drift. "Just consider that contraptionin the light of any one Principle, and I warrant you will not findone of them applied to perfection. The whole thing is nothing but aCompromise. " And he glared fixedly at poor Efficiency. 

"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. WhatWILL the Designer say?"

"Never mind, my dear, " said the Lift-Drift Ratio, consolingly. "You areimproving rapidly, and quite useful enough now to think of doing a jobof work. "

"Well, that's good news, " and Efficiency wiped her eyes with her Fabricand became almost cheerful. "Suppose we think about finishing it now?There will have to be an Engine and Propeller, won't there? And a bodyto fix them in, and tanks for oil and petrol, and a tail, and, " archly, "one of those dashing young Pilots, what?"

"Well, we are getting within sight of those interesting Factors, " saidthe Lift-Drift Ratio, "but first of all we had better decide upon theArea of the Surfaces, their Angle of Incidence and Camber. If we areto ascend as quickly as possible the Aeroplane must be SLOW in order tosecure the best possible Lift-Drift Ratio, for the drift of the strutswires, body, etc. , increases approximately as the square of the speed, but it carries with it no lift as it does in the case of the Surface. The less speed then, the less such drift, and the better the Aeroplane'sproportion of lift to drift; and, being slow, we shall require a LARGESURFACE in order to secure a large lift relative to the weight to becarried. We shall also require a LARGE ANGLE OF INCIDENCE relative tothe horizontal, in order to secure a proper inclination of the Surfaceto the direction of motion, for you must remember that, while we shallfly upon an even keel and with the propeller thrust horizontal (which isits most efficient attitude), our flight path, which is our direction ofmotion, will be sloping upwards, and it will therefore be necessary tofix the Surface to the Aeroplane at a very considerable angle relativeto the horizontal Propeller Thrust in order to secure a proper angleto the upwards direction of motion. Apart from that, we shall require alarger Angle of Incidence than in the case of a machine designed purelyfor speed, and that means a correspondingly LARGE CAMBER. 

"On the other hand, if we are thinking merely of Speed, then a SMALLSURFACE, just enough to lift the weight off the ground, will be best, also a SMALL ANGLE to cut the Drift down and that, of course, means arelatively SMALL CAMBER. 

"So you see the essentials for CLIMB or quick ascent and for SPEED arediametrically opposed. Now which is it to be?"

"Nothing but perfection for me, " said Efficiency. "What I want isMaximum Climb and Maximum Speed for the Power the Engine produces. "

And each Principle fully agreed with her beautiful sentiments, but worktogether they would not. 

The Aspect Ratio wanted infinite Span, and hang the Chord. 

The Angle of Incidence would have two Angles and two Cambers in one, which was manifestly absurd; the Surface insisted upon no thicknesswhatever, and would not hear of such things as Spars and Ribs; and theThrust objected to anything at all likely to produce Drift, and verynearly wiped the whole thing off the Blackboard. 

There was, indeed, the makings of a very pretty quarrel when the Letterarrived. It was about a mile long, and began to talk at once. 

"I'm from the Inventor, " he said, and hope rose in the heart of eachheated Principle. "It's really absurdly simple. All the Pilot has to dois to touch a button, and at his will, VARY the area of the Surface, theAngle of Incidence, and the Camber! And there you are--Maximum Climb orMaximum Speed as required! How does that suit you?"

"That suits us very well, " said the Surface, "but, excuse me asking, howis it done without apparatus increasing the Drift and the Weight outof all reason? You won't mind showing us your Calculations, WorkingDrawings, Stress Diagrams, etc. , will you?"

Said the Letter with dignity, "I come from an Inventor so brilliantlyclever as to be far above the unimportant matters you mention. He is nocommon working man, sir! He leaves such things to Mechanics. The pointis, you press a button and----"

"Look here, " said a Strut, rather pointedly, "where do you think you aregoing, anyway?"

"Well, " from the Letter, "as a matter of fact, I'm not addressed yet, but, of course, there's no doubt I shall reach the very highest quartersand absolutely revolutionize Flight when I get there. "

Said the Chalk, "I'll address you, if that's all you want; now driftalong quickly!" And off went the Letter to The Technical Editor, "DailyMauler, " London. 

And a League was formed, and there were Directors with Fees, and severalout-of-service Tin Hats, and the Man-who-takes-the-credit, and a finefat Guinea-pig, and all the rest of them. And the Inventor paid hisTailor and had a Hair-Cut, and is now a recognized Press Expert--but heis still waiting for those Mechanics!

"I'm afraid, " said the Slide-rule, who had been busy making thoselightning-like automatic calculations for which he is so famous, "it'squite impossible to fully satisfy all of you, and it is perfectly plainto me that we shall have to effect a Compromise and sacrifice some ofthe Lift for Speed. "

Thud! What was that?

Efficiency had fainted dead away! The last blow had been too much forher. And the Principles gathered mournfully round, but with the aid ofthe Propeller Slip[1] and a friendly lift from the Surface she was atlength revived and regained a more normal aspect. 

Said the Stagger with a raffish air, "My dear young lady, I assureyou that from the experiences of a varied career, I have learned thatperfection is impossible, and I am sure the Designer will be quitesatisfied if you become the Most Efficient Compromise. "

"Well, that sounds so common sense, " sighed Efficiency, "I suppose itmust be true, and if the Designer is satisfied, that's all I really careabout. Now do let's get on with the job. "

So the Chalk drew a nice long slim body to hold the Engine and thetanks, etc. , with room for the Pilot's and Passenger's seats, and placedit exactly in the middle of the Biplane. And he was careful to make itsposition such that the Centre of Gravity was a little in advance of theCentre of Lift, so that when the Engine was not running and there wasconsequently no Thrust, the Aeroplane should be "nose-heavy" just to theright degree, and so take up a natural glide to Earth--and this was tohelp the Pilot and relieve him of work and worry, should he find himselfin a fog or a cloud. And so that this tendency to glide downwards shouldnot be in evidence when the Engine was running and descent not desired, the Thrust was placed a little below the Centre of Drift or Resistance. In this way it would in a measure pull the nose of the Aeroplane up andcounterbalance the "nose-heavy" tendency. 

And the Engine was so mounted that when the Propeller-Thrust washorizontal, which is its most efficient position, the Angle of Incidenceand the Area of the surfaces were just sufficient to give a Lift alittle in excess of the Weight. And the Camber was such that, as far asit was concerned, the Lift-Drift Ratio should be the best possible forthat Angle of Incidence. And a beautifully simple under-carriage wasadded, the outstanding features of which were simplicity, strength, light-weight, and minimum drift. And, last of all, there was theElevator, of which you will hear more by-and-by. And this is what itlooked like then:

And Efficiency, smiling, thought that it was not such a bad compromiseafter all and that the Designer might well be satisfied. 

"Now, " said she, "there's just one or two points I'm a bit hazy about. It appears that when the Propeller shaft is horizontal and so workingin its most efficient attitude, I shall have a Lift from the Surfacesslightly in excess of the Weight. That means I shall ascend slightly, at the same time making nearly maximum speed for the power and thrust. Can't I do better than that?"

"Yes, indeed, " spoke up the Propeller, "though it means that I mustassume a most undignified attitude, for helicopters[2] I neverapproved of. In order to ascend more quickly the Pilot will deflect theElevator, which, by the way, you see hinged to the Tail. By thatmeans he will force the whole Aeroplane to assume a greater Angle ofIncidence. And with greater Angle, the Lift will increase, though I'msorry to say the Drift will increase also. Owing to the greater Drift, the Speed through the air will lessen, and I'm afraid that won't behelpful to the Lift; but I shall now be pointing upwards, and besidesovercoming the Drift in a forward direction I shall be doing my bestto haul the Aeroplane skywards. At a certain angle known as the BestClimbing Angle, we shall have our Maximum Margin of Lift, and I'm hopingthat may be as much as almost a thousand feet altitude a minute. "

"Then, if the Pilot is green, my chance will come, " said the MaximumAngle of Incidence. "For if the Angle is increased over the BestClimbing Angle, the Drift will rush up; and the Speed, and with it theLift, will, when my Angle is reached, drop to a point when the latterwill be no more than the Weight. The Margin of Lift will have entirelydisappeared, and there we shall be, staggering along at my tremendousangle, and only just maintaining horizontal flight. "

"And then with luck I'll get my chance, " said the Drift. "If he is a bitworse than green, he'll perhaps still further increase the Angle. Thenthe Drift, largely increasing, the Speed, and consequently the Lift, will become still less, i. E. , less than the Weight, and then--what pricepancakes, [3] eh?"

"Thank you, " from Efficiency, "that was all most informing. And now willyou tell me, please, how the greatest Speed may be secured?"

"Certainly, now it's my turn, " piped the Minimum Angle of Incidence. "Bymeans of the Elevator, the Pilot places the Aeroplane at my small Angle, at which the Lift only just equals the Weight, and, also, at which weshall make greater speed with no more Drift than before. Then we get ourgreatest Speed, just maintaining horizontal flight. "

"Yes; though I'm out of the horizontal and thrusting downwards, "grumbled the Propeller, "and that's not efficient, though I suppose it'sthe best we can do until that Inventor fellow finds his Mechanics. "

"Thank you so much, " said Efficiency. "I think I have now at any ratean idea of the Elementary Principles of Flight, and I don't know that Icare to delve much deeper, for sums always give me a headache; but isn'tthere something about Stability and Control? Don't you think I ought tohave a glimmering of them too?"

"Well, I should smile, " said a spruce Spar, who had come all the wayfrom America. "And that, as the Lecturer says, `will be the subject ofour next lecture, ' so be here again to-morrow, and you will be glad tohear that it will be distinctly more lively than the subject we havecovered to-day. "

PART II. THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THEJOB

Another day had passed, and the Flight Folk had again gathered togetherand were awaiting the arrival of Efficiency who, as usual, was ratherlate in making an appearance. 

The crowd was larger than ever, and among the newcomers some of the mostimportant were the three Stabilities, named Directional, Longitudinal, and Lateral, with their assistants, the Rudder, Elevator, and Ailerons. There was Centrifugal Force, too, who would not sit still and created amost unfavourable impression, and Keel-Surface, the Dihedral Angle, andseveral other lesser fry. 

"Well, " said Centrifugal Force, "I wish this Efficiency I've heard somuch about would get a move on. Sitting still doesn't agree with meat all. Motion I believe in. There's nothing like motion--the more thebetter. "

"We are entirely opposed to that, " objected the three Stabilities, allin a breath. "Unless it's in a perfectly straight line or a perfectcircle. Nothing but perfectly straight lines or, upon occasion, perfectcircles satisfy us, and we are strongly suspicious of your tendencies. "

"Well, we shall see what we shall see, " said the Force darkly. "But whoin the name of blue sky is this?"

And in tripped Efficiency, in a beautifully "doped" dress of the latestfashionable shade of khaki-coloured fabric, a perfectly stream-linedbonnet, and a bewitching little Morane parasol, [4] smiling asusual, and airily exclaiming, "I'm so sorry I'm late, but you see theDesigner's such a funny man. He objects to skin friction, [5] andinsisted upon me changing my fabric for one of a smoother surface, andthat delayed me. Dear me, there are a lot more of us to-day, aren'tthere? I think I had better meet one at a time. " And turning toDirectional Stability, she politely asked him what he preferred to do. 

"My purpose in life, miss, " said he, "is to keep the Aeroplane on itscourse, and to achieve that there must be, in effect, more Keel-Surfacebehind the Vertical Turning Axis than there is in front of it. "

Efficiency looking a little puzzled, he added: "Just like a weathercock, and by Keel-Surface I mean everything you can see when you view theAeroplane from the side of it--the sides of the body, struts, wires, etc. "

"Oh, now I begin to see light, " said she: "but just exactly how does itwork?"

"I'll answer that, " said Momentum. "When perhaps by a gust of air theAeroplane is blown out of its course and points in another direction, itdoesn't immediately fly off on that new course. I'm so strong I pull itoff the new course to a certain extent, and towards the direction of theold course. And so it travels, as long as my strength lasts, in a moreor less sideways position. "

"Then, " said the Keel-Surface, "I get a pressure of air all on one side, and as there is, in effect, most of me towards the tail, the lattergets pressed sideways, and the Aeroplane thus tends to assume its firstposition and course. "

"I see, " said Efficiency, and, daintily holding the Chalk, sheapproached the Blackboard. "Is this what you mean?"

"Yes, that's right enough, " said the Keel-Surface, "and you mightremember, too, that I always make the Aeroplane nose into the gustsrather than away from them. "

"If that was not the case, " broke in Lateral Stability, and affectingthe fashionable Flying Corps stammer, "it would be a h-h-h-o-r-ribleaffair! If there were too much Keel-Surface in front, then that gustwould blow the Aeroplane round the other way a very considerabledistance. And the right-hand Surface being on the outside of the turnwould have more speed, and consequently more Lift, than the Surfaceon the other side. That means a greater proportion of the Lift on thatside, and before you could say Warp to the Ailerons over the Aeroplanewould go--probable result a bad side-slip"

"And what can the Pilot do to save such a situation as that?" saidEfficiency. 

"Well, " replied Lateral Stability, "he will try to turn the Aeroplanesideways and back to an even keel by means of warping the Ailerons orlittle wings which are hinged on to the Wing-tips, and about which youwill hear more later on; but if the side-slip is very bad he may not beable to right the Aeroplane by means of the Ailerons, and then the onlything for him to do is to use the Rudder and to turn the nose of theAeroplane down and head-on to the direction of motion. The Aeroplanewill then be meeting the air in the direction it is designed to doso, and the Surfaces and also the controls (the Rudder, Ailerons, andElevator) will be working efficiently; but its attitude relative tothe earth will probably be more or less upside-down, for the actionof turning the Aeroplane's nose down results, as you will see by theillustration B, in the right wing, which is on the outside of thecircle. Travelling through the air with greater speed than the left-handwing. More Speed means more Lift, so that results in overturning theAeroplane still more; but now it is, at any rate, meeting the air as itis designed to meet it, and everything is working properly. It is thenonly necessary to warp the Elevator, as shown in illustration C, inorder to bring the Aeroplane into a proper attitude relative to theearth. "

"Ah!" said the Rudder, looking wise, "it's in a case like that when Ibecome the Elevator and the Elevator becomes me. "

"That's absurd nonsense, " said the Blackboard, "due to looseness ofthought and expression. "

"Well, " replied the Rudder, "when 'the Aeroplane is in position A and Iam used, then I depress or ELEVATE the nose of the machine; and, if theElevator is used, then it turns the Aeroplane to right or left, which isnormally my function. Surely our roles have changed one with the other, and I'm then the Elevator and the Elevator is me!"

Said Lateral Stability to the Rudder, "That's altogether the wrong wayof looking at it, though I admit"--and this rather sarcastically--"thatthe way you put it sounds rather fine when you are talking of yourexperiences in the air to those 'interested in aviation' but knowinglittle about it; but it won't go down here! You are a ControllingSurface designed to turn the Aeroplane about its vertical axis, and theElevator is a Controlling Surface designed to turn the Aeroplane aboutits lateral axis. Those are your respective jobs, and you can't possiblychange them about. Such talk only leads to confusion, and I hope weshall hear no more of it. "

"Thanks, " said Efficiency to Lateral Stability. "And now, please, willyou explain your duties?"

"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip. First of all, I sometimes arrange with the Rigger to wash-out, that isdecrease, the Angle of Incidence on one side of the Aeroplane, and toeffect the reverse condition, if it is not too much trouble, on theother side. "

"But, " objected Efficiency, "the Lift varies with the Angle ofIncidence, and surely such a condition will result in one side of theAeroplane lifting more than the other side?'

"That's all right, " said the Propeller, "it's meant to off-set thetendency of the Aeroplane to turn over sideways in the oppositedirection to which I revolve. "

"That's quite clear, though rather unexpected; but how do you counteractthe effect of the gusts when they try to overturn the Aeroplanesideways?" said she, turning to Lateral Stability again. 

"Well, " he replied, rather miserably, "I'm not nearly so perfect as theLongitudinal and Directional Stabilities. The Dihedral Angle--that is, the upward inclination of the Surfaces towards their wing-tips--doeswhat it can for me, but, in my opinion, it's a more or less futileeffort. The Blackboard will show you the argument. " And he at onceshowed them two Surfaces, each set at a Dihedral Angle like this:

"Please imagine, " said the Blackboard, "that the top V is the frontview of a Surface flying towards you. Now if a gust blows it into theposition of the lower V you see that the horizontal equivalent of theSurface on one side becomes larger, and on the other side it becomessmaller. That results in more Lift on the lower side and less on thehigher side, and if the V is large enough it should produce such adifference in the Lift of one side to the other as to quickly turn theAeroplane back to its former and normal position. "

"Yes, " said the Dihedral Angle, "that's what would happen if they wouldonly make me large enough; but they won't do it because it would toogreatly decrease the horizontal equivalent, and therefore the Lift, andincidentally it would, as Aeroplanes are built to-day, produce an excessof Keel Surface above the turning axis, and that in itself would spoilthe Lateral Stability. The Keel Surface should be equally divided aboveand below the longitudinal turning axis (upon which the Aeroplane rollssideways), or the side upon which there is an excess will get blown overby the gusts. It strikes me that my future isn't very promising, andabout my only chance is when the Junior Draughtsman makes a mistake, ashe did the other day. And just think of it, they call him a Designer nowthat he's got a job at the Factory! What did he do? Why, he calculatedthe weights wrong and got the Centre of Gravity too high, and theydidn't discover it until the machine was built. Then all they could dowas to give me a larger Angle. That dropped the bottom of the V lowerdown, and as that's the centre of the machine, where all the Weight is, of course that put the Centre of Gravity in its right place. But nowthere is too much Keel Surface above, and the whole thing's a BadCompromise, not at all like Our Efficiency. "

And Efficiency, blushing very prettily at the compliment, then asked, "And how does the Centre of Gravity affect matters?"

"That's easy, " said Grandfather Gravity. "I'm so heavy that if I am toolow down I act like a pendulum and cause the Aeroplane to roll aboutsideways, and if I am too high I'm like a stick balanced on your finger, and then if I'm disturbed, over I go and the Aeroplane with me; and, inaddition to that, there are the tricks I play with the Aeroplane whenit's banked up, [6] i. E. , tilted sideways for a turn, and CentrifugalForce sets me going the way I'm not wanted to go. No; I get on best withLateral Stability when my Centre is right on the centre of Drift, or, at any rate, not much below it. " And with that he settled back into theLecturer's Chair and went sound asleep again, for he was so very, veryold, in fact the father of all the Principles. 

And the Blackboard had been busy, and now showed them a picture ofthe Aeroplane as far as they knew it, and you will see that there isa slight Dihedral Angle, and also, fixed to the tail, a vertical KeelSurface or fin, as is very often the case in order to ensure the greatereffect of such surface being behind the vertical turning axis. 

But Efficiency, growing rather critical with her newly gained knowledge, cried out: "But where's the horizontal Tail Surface? It doesn't lookright like that!"

"This is when I have the pleasure of meeting you, my dear, " saidLongitudinal Stability. "Here's the Tail Surface, " he said, "and inorder to help me it must be set IN EFFECT at a much less Angle ofIncidence than the Main Surface. To explain we must trouble theBlackboard again, " and this was his effort:

"I have tried to make that as clear as possible, " he said. "It mayappear a bit complicated at first, but if you will take the trouble tolook at it for a minute you will find it quite simple. A is the normaland proper direction of motion of the Aeroplane, but, owing to a gust ofair, it takes up the new nose-down position. Owing to Momentum, however, it does not fly straight along in that direction, but moves more or lessin the direction B, which is the resultant of the two forces, Momentumand Thrust. And so you will note that the Angle of Incidence, whichis the inclination of the Surfaces to the Direction of Motion, hasdecreased, and of course the Lift decreases with it. You will alsosee, and this is the point, that the Tail Surface has lost a higherproportion of its Angle, and consequently its Lift, than has the MainSurface. Then, such being the case, the Tail must fall and the Aeroplaneassume its normal position again, though probably at a slightly loweraltitude. "

"I'm afraid I'm very stupid, " said Efficiency, "but please tell me whyyou lay stress upon the words 'IN EFFECT. '"

"Ah! I was wondering if you would spot that, " he replied. "And there isa very good reason for it. You see, in some Aeroplanes the Tail Surfacemay be actually set at the same Angle on the machine as the MainSurface, but owing to the air being deflected downwards by the frontMain Surface it meets the Tail Surface at a lesser angle, and indeed insome cases at no angle at all. The Tail is then for its surface gettingless Lift than the Main Surface, although set at the same angle onthe machine. It may then be said to have IN EFFECT a less Angle ofIncidence. I'll just show you on the Blackboard. "

"And now, " said Efficiency, "I have only to meet the Ailerons and theRudder, haven't I?"

"Here we are, " replied the Ailerons, or little wings. "Please hinge uson to the back of the Main Surfaces, one of us at each Wing-tip, andjoin us up to the Pilot's joystick by means of the control cables. Whenthe Pilot wishes to tilt the Aeroplane sideways, he will move the stickand depress us upon one side, thus giving us a larger Angle of Incidenceand so creating more Lift on that side of the Aeroplane; and, by meansof a cable connecting us with the Ailerons on the other side of theAeroplane, we shall, as we are depressed, pull them up and give them areverse or negative Angle of Incidence, and that side will then geta reverse Lift or downward thrust, and so we are able to tilt theAeroplane sideways. 

"And we work best when the Angle of Incidence of the Surface in frontof us is very small, for which reason it is sometimes decreased orwashed-out towards the Wing-tips. The reason of that is that by thetime the air reaches us it has been deflected downwards--the greater theAngle of Incidence the more it is driven downwards--and in order forus to secure a Reaction from it, we have to take such a large Angle ofIncidence that we produce a poor proportion of Lift to Drift; but thesmaller the Angle of the Surface in front of us the less the air isdeflected downwards, and consequently the less Angle is required of us, and the better our proportion of Lift to Drift, which, of course, makesus much more effective Controls. "

"Yes, " said the Lateral and Directional Stabilities in one voice, "that's so, and the wash-out helps us also, for then the Surfacestowards their Wing-tips have less Drift or 'Head-Resistance, ' andconsequently the gusts will affect them and us less; but such decreasedAngle of Incidence means decreased Lift as well as Drift, and theDesigner does not always care to pay the price. "

"Well, " said the Ailerons, "if it's not done it will mean more work forthe Rudder, and that won't please the Pilot. "

"Whatever do you mean?" asked Efficiency. "What can the Rudder have todo with you?"

"It's like this, " they replied: "when we are deflected downwards we gaina larger Angle of Incidence and also enter an area of compressed air, and so produce more Drift than those of us on the other side of theAeroplane, which are deflected upwards into an area of rarefied air dueto the SUCTION effect (though that term is not academically correct) onthe top of the Surface. If there is more Drift, i. E. , Resistance, onone side of the Aeroplane than on the other side, then of course it willturn off its course, and if that difference in Drift is serious, as itwill very likely be if there is no wash-out, then it will mean a gooddeal of work for the Rudder in keeping the Aeroplane on its course, besides creating extra Drift in doing so. "

"I think, then, " said Efficiency, "I should prefer to have thatwash-out, [7] and my friend the Designer is so clever at producingstrength of construction for light weight, I'm pretty sure he won'tmind paying the price in Lift. And now let me see if I can sketch thecompleted Aeroplane. "

"Well, I hope that's all as it should be, " she concluded, "for to-morrowthe Great Test in the air is due. "

PART III. THE GREAT TEST

It is five o'clock of a fine calm morning, when the Aeroplane is wheeledout of its shed on to the greensward of the Military Aerodrome. Thereis every promise of a good flying day, and, although the sun has notyet risen, it is light enough to discern the motionless layer of fleecyclouds some five thousand feet high, and far, far above that a few filmymottled streaks of vapour. Just the kind of morning beloved of pilots. 

A brand new, rakish, up-to-date machine it is, of highly polished, beautifully finished wood, fabric as tight as a drum, polished metal, and every part so perfectly "streamlined" to minimize Drift, which isthe resistance of the air to the passage of the machine, that to theveriest tyro the remark of the Pilot is obviously justified. 

"Clean looking 'bus, looks almost alive and impatient to be off. Oughtto have a turn for speed with those lines. "

"Yes, " replies the Flight-Commander, "it's the latest of its type andlooks a beauty. Give it a good test. A special report is required onthis machine. "

The A. M. 's[8] have now placed the Aeroplane in position facing thegentle air that is just beginning to make itself evident; the engineFitter, having made sure of a sufficiency of oil and petrol in thetanks, is standing by the Propeller; the Rigger, satisfied with a jobwell done, is critically "vetting" the machine by eye, four A. M. 's areat their posts, ready to hold the Aeroplane from jumping the blockswhich have been placed in front of the wheels; and the Flight-Sergeantis awaiting the Pilot's orders. 

As the Pilot approaches the Aeroplane the Rigger springs to attentionand reports, "All correct, sir, " but the Fitter does not this morningreport the condition of the Engine, for well he knows that this Pilotalways personally looks after the preliminary engine test. The latter, in leathern kit, warm flying boots and goggled, climbs into his seat, and now, even more than before, has the Aeroplane an almost livingappearance, as if straining to be off and away. First he moves theControls to see that everything is clear, for sometimes when theAeroplane is on the ground the control lever or "joy-stick" is lashedfast to prevent the wind from blowing the controlling surfaces about andpossibly damaging them. 

The air of this early dawn is distinctly chilly, and the A. M. 's arebeginning to stamp their cold feet upon the dewy grass, but very carefuland circumspect is the Pilot, as he mutters to himself, "Don't worry andflurry, or you'll die in a hurry. "

At last he fumbles for his safety belt, but with a start remembers thePilot Air Speed Indicator, and, adjusting it to zero, smiles as he hearsthe Pilot-head's gruff voice, "Well, I should think so, twenty miles anhour I was registering. That's likely to cause a green pilot to stallthe Aeroplane. Pancake, they call it. " And the Pilot, who is an old handand has learned a lot of things in the air that mere earth-dwellers knownothing about, distinctly heard the Pilot Tube, whose mouth is open tothe air to receive its pressure, stammer. "Oh Lor! I've got an earwigalready--hope to goodness the Rigger blows me out when I come down--andthis morning air simply fills me with moisture; I'll never keep theLiquid steady in the Gauge. I'm not sure of my rubber connectionseither. "

"Oh, shut up!" cry all the Wires in unison, "haven't we got our troublestoo? We're in the most horrible state of tension. It's simply murderingour Factor of Safety, and how we can possibly stand it when we get theLift only the Designer knows. "

"That's all right, " squeak all the little Wire loops, "we're thataccommodating, we're sure to elongate a bit and so relieve yourtension. " For the whole Aeroplane is braced together with innumerablewires, many of which are at their ends bent over in the form of loopsin order to connect with the metal fittings on the spars andelsewhere--cheap and easy way of making connection. 

"Elongate, you little devils, would you?" fairly shout the Angles ofIncidence, Dihedral and Stagger, amid a chorus of groans from all partsof the Aeroplane. "What's going to happen to us then? How are we goingto keep our adjustments upon which good flying depends?"

"Butt us and screw us, "[9] wail the Wires. "Butt us and screw us, anddeath to the Loops. That's what we sang to the Designer, but he onlylooked sad and scowled at the Directors. "

"And who on earth are they?" asked the Loops, trembling for theirtroublesome little lives. 

"Oh earth indeed, " sniffed Efficiency, who had not spoken before, havingbeen rendered rather shy by being badly compromised in the DrawingOffice. "I'd like to get some of them up between Heaven and Earth, I would. I'd give 'em something to think of besides their Debits andCredits--but all the same the Designer will get his way in the end. I'mhis Best Girl, you know, and if we could only get rid of the Directors, the little Tin god, and the Man-who-takes-the-credit, we should be quitehappy. " Then she abruptly subsides, feeling that perhaps the less saidthe better until she has made a reputation in the Air. The matter ofthat Compromise still rankled, and indeed it does seem hardly fit thata bold bad Tin god should flirt with Efficiency. You see there wasa little Tin god, and he said "Boom, Boom BOOM! Nonsense! It MUST bedone, " and things like that in a very loud voice, and the Designertore his hair and was furious, but the Directors, who were thinking ofnothing but Orders and Dividends, had the whip-hand of HIM, and so thereyou are, and so poor beautiful Miss Efficiency was compromised. 

All this time the Pilot is carefully buckling his belt and makinghimself perfectly easy and comfortable, as all good pilots do. Ashe straightens himself up from a careful inspection of the DeviationCurve[10] of the Compass and takes command of the Controls, theThrottle and the Ignition, the voices grow fainter and fainter untilthere is nothing but a trembling of the Lift and Drift wires to indicateto his understanding eye their state of tension in expectancy of theGreat Test. 

"Petrol on?" shouts the Fitter to the Pilot. 

"Petrol on, " replies the Pilot. 

"Ignition off?"

"Ignition off. "

Round goes the Propeller, the Engine sucking in the Petrol Vapour withsatisfied gulps. And then--

"Contact?" from the Fitter. 

"Contact, " says the Pilot. 

Now one swing of the Propeller by the Fitter, and the Engine is awakeand working. Slowly at first though, and in a weak voice demanding, "Nottoo much Throttle, please. I'm very cold and mustn't run fast until myOil has thinned and is circulating freely. Three minutes slowly, as youlove me, Pilot. "

Faster and faster turn the Engine and Propeller, and the Aeroplane, trembling in all its parts, strains to jump the blocks and be off. Carefully the Pilot listens to what the Engine Revolution Indicatorsays. At last, "Steady at 1, 500 revs. And I'll pick up the rest in theAir. " Then does he throttle down the Engine, carefully putting the leverback to the last notch to make sure that in such position the Throttleis still sufficiently open for the Engine to continue working, asotherwise it might lead to him "losing" his Engine in the air whenthrottling down the power for descent. Then, giving the official signal, he sees the blocks removed from the wheels, and the Flight-Sergeantsaluting he knows that all is clear to ascend. One more signal, and allthe A. M. 's run clear of the Aeroplane. 

Then gently, gently mind you, with none of the "crashing on" badPilots think so fine, he opens the Throttle and, the Propeller Thrustovercoming its enemy the Drift, the Aeroplane moves forward. 

"Ah!" says the Wind-screen, "that's Discipline, that is. Throughmy little window I see most things, and don't I just know that poordiscipline always results in poor work in the air, and don't you forgetit. "

"Discipline is it?" complains the Under-carriage, as its wheels rollswiftly over the rather rough ground. "I'm bump getting it; and bump, bump, all I want, bang, bump, rattle, too!" But, as the Lift increaseswith the Speed, the complaints of the Under-carriage are stilled, andthen, the friendly Lift becoming greater than the Weight, the Aeroplaneswiftly and easily takes to the air. 

Below is left the Earth with all its bumps and troubles. Up into theclean clear Air moves with incredible speed and steadiness this triumphof the Designer, the result of how much mental effort, imagination, trials and errors, failures and successes, and many a life lost in highendeavour. 

Now is the mighty voice of the Engine heard as he turns the Propellernine hundred times a minute. Now does the Thrust fight the Drift for allit's worth, and the Air Speed Indicator gasps with delight, "One hundredmiles an hour!"

And now does the burden of work fall upon the Lift and Drift Wires, and they scream to the Turnbuckles whose business it is to hold them intension, "This is the limit! the Limit! THE LIMIT! Release us, if onlya quarter turn. " But the Turnbuckles are locked too fast to turn theireyes or utter a word. Only the Locking Wires thus: "Ha! ha! the Riggerknew his job. He knew the trick, and there's no release here. " Foran expert rigger will always use the locking wire in such a way as tooppose the slightest tendency of the turnbuckle to unscrew. The otherkind of rigger will often use the wire in such a way as to allow theturnbuckle, to the "eyes" of which the wires are attached, to unscrew aquarter of a turn or more, with the result that the correct adjustmentof the wires may be lost; and upon their fine adjustment much depends. 

And the Struts and the Spars groan in compression and pray to keepstraight, for once "out of truth" there is, in addition to possiblecollapse, the certainty that in bending they will throw many wires outof adjustment. 

And the Fabric's quite mixed in its mind, and ejaculates, "Now, whowould have thought I got more Lift from the top of the Surface than itsbottom?" And then truculently to the Distance Pieces, which run fromrib to rib, "Just keep the Ribs from rolling, will you? or you'll see mestrip. I'm an Irishman, I am, and if my coat comes off---- Yes, Irish, Isaid. I used to come from Egypt, but I've got naturalized since the Warbegan. "

Then the Air Speed Indicator catches the eye of the Pilot. "Goodenough, " he says as he gently deflects the Elevator and points the noseof the Aeroplane upwards in search of the elusive Best Climbing Angle. 

"Ha! ha!" shouts the Drift, growing stronger with the increased Angleof Incidence. "Ha! ha!" he laughs to the Thrust. "Now I've got you. Nowwho's Master?"

And the Propeller shrieks hysterically, "Oh! look at me. I'm ahelicopter. That's not fair. Where's Efficiency?" And she can only sadlyreply, "Yes, indeed, but you see we're a Compromise. "

And the Drift has hopes of reaching the Maximum Angle of Incidenceand vanquishing the Thrust and the Lift. And he grows very bold as hestrangles the Thrust; but the situation is saved by the Propeller, who is now bravely helicopting skywards, somewhat to the chagrin ofEfficiency. 

"Much ado about nothing, " quotes the Aeroplane learnedly. "Compromiseor not, I'm climbing a thousand feet a minute. Ask the Altimeter. He'llconfirm it. "

And so indeed it was. The vacuum box of the Altimeter was steadilyexpanding under the decreased pressure of the rarefied air, and by meansof its little levers and its wonderful chain no larger than a hair itwas moving the needle round the gauge and indicating the ascent at therate of a thousand feet a minute. 

And lo! the Aeroplane has almost reached the clouds! But what's this?A sudden gust, and down sinks one wing and up goes the other. "Oh, myHorizontal Equivalent!" despairingly call the Planes: "it's eloping withthe Lift, and what in the name of Gravity will happen? Surely there wasenough scandal in the Factory without this, too!" For the lift varieswith the horizontal equivalent of the planes, so that if the aeroplanetilts sideways beyond a certain angle, the lift becomes less than theweight of the machine, which must then fall. A fall in such a positionis known as a "side-slip. "

But the ever-watchful Pilot instantly depresses one aileron, elevatingthe other, with just a touch of the rudder to keep on the course, andthe Planes welcome back their precious Lift as the Aeroplane flicks backto its normal position. 

"Bit bumpy here under these clouds, " is all the Pilot says as he headsfor a gap between them, and the next minute the Aeroplane shoots up intoa new world of space. 

"My eye!" ejaculates the Wind-screen, "talk about a view!" And indeedmere words will always fail to express the wonder of it. Six thousandfeet up now, and look! The sun is rising quicker than ever mortal onearth witnessed its ascent. Far below is Mother Earth, wrapt in mistsand deep blue shadows, and far above are those light, filmy, etherealclouds now faintly tinged with pink And all about great mountains ofcloud, lazily floating in space. The sun rises and they take on allcolours, blending one with the other, from dazzling white to crimsonand deep violet-blue. Lakes and rivers here and there in the enormousexpanse of country below refract the level rays of the sun and, like somany immense diamonds, send dazzling shafts of light far upwards. Thetops of the hills now laugh to the light of the sun, but the valleys arestill mysterious dark blue caverns, clowned with white filmy lace-likestreaks of vapour. And withal the increasing sense with altitude ofvast, clean, silent solitudes of space. 

Lives there the man who can adequately describe this Wonder? "Never, "says the Pilot, who has seen it many times, but to whom it is ever newand more wonderful. 

Up, up, up, and still up, unfalteringly speeds the Pilot and his mount. Sweet the drone of the Engine and steady the Thrust as the Propellerexultingly battles with the Drift. 

And look! What is that bright silver streak all along the horizon? Itpuzzled the Pilot when first he saw it, but now he knows it for the Sea, full fifty miles away!

And on his right is the brightness of the Morn and the smiling Earthunveiling itself to the ardent rays of the Sun; and on his left, sohigh is he, there is yet black Night, hiding innumerable Cities, Towns, Villages and all those places where soon teeming multitudes of men shallawake, and by their unceasing toil and the spirit within them producemarvels of which the Aeroplane is but the harbinger. 

And the Pilot's soul is refreshed, and his vision, now exalted, seesthe Earth a very garden, even as it appears at that height, with discordbanished and a happy time come, when the Designer shall have at lastcaptured Efficiency, and the Man-who-takes-the-credit is he who hasearned it, and when kisses are the only things that go by favour. 

Now the Pilot anxiously scans the Barograph, which is an instrument muchthe same as the Altimeter; but in this case the expansion of the vacuumbox causes a pen to trace a line upon a roll of paper. This paper ismade by clockwork to pass over the point of the pen, and so a curvedline is made which accurately registers the speed of the ascent in feetper minute. No longer is the ascent at the rate of a thousand feet aminute, and the Propeller complains to the Engine, "I'm losing my Revs. And the Thrust. Buck up with the Power, for the Lift is decreasing, though the Weight remains much the same. "

Quoth the Engine: "I strangle for Air. A certain proportion, and thatof right density, I must have to one part of Petrol, in order to give mefull power and compression, and here at an altitude of ten thousand feetthe Air is only two-thirds as dense as at sea-level. Oh, where is he whowill invent a contrivance to keep me supplied with Air of right densityand quality? It should not be impossible within certain limits. "

"We fully agree, " said the dying Power and Thrust. "Only maintain Us andyou shall be surprised at the result. For our enemy Drift decreases inrespect of distance with the increase of altitude and rarity of air, and there is no limit to the speed through space if only our strengthremains. And with oxygen for Pilot and Passengers and a steeperpitch[11] for the Propeller we may then circle the Earth in a day!"

Ah, Reader, smile not unbelievingly, as you smiled but a few years past. There may be greater wonders yet. Consider that as the speed increases, so does the momentum or stored-up force in the mass of the aeroplanebecome terrific. And, bearing that in mind, remember that withaltitude gravity decreases. There may yet be literally other worlds toconquer. [12]

Now at fifteen thousand feet the conditions are chilly and rare, and thePilot, with thoughts of breakfast far below, exclaims, "High enough!I had better get on with the Test. " And then, as he depresses theElevator, the Aeroplane with relief assumes its normal horizontalposition. Then, almost closing the Throttle, the Thrust dies away. Now, the nose of the Aeroplane should sink of its own volition, and the craftglide downward at flying speed, which is in this case a hundred milesan hour. That is what should happen if the Designer has carefullycalculated the weight of every part and arranged for the centre ofgravity to be just the right distance in front of the centre of lift. Thus is the Aeroplane "nose-heavy" as a glider, and just so to a degreeensuring a speed of glide equal to its flying speed. And the Air SpeedIndicator is steady at one hundred miles an hour, and "That's allright!" exclaims the Pilot. "And very useful, too, in a fog or a cloud, "he reflects, for then he can safely leave the angle of the glide toitself, and give all his attention, and he will need it all, to keepingthe Aeroplane horizontal from wing-tip to wing-tip, and to keepingit straight on its course. The latter he will manage with the rudder, controlled by his feet, and the Compass will tell him whether a straightcourse is kept. The former he will control by the Ailerons, orlittle wings hinged to the tips of the planes, and the bubble in theInclinometer in front of him must be kept in the middle. 

A Pilot, being only human, may be able to do two things at once, butthree is a tall order, so was this Pilot relieved to find the Design notat fault and his craft a "natural glider. " To correct this nose-heavytendency when the Engine is running, and descent not required, thecentre of Thrust is arranged to be a little below the centre of Drift orResistance, and thus acts as a counter-balance. 

But what is this stream of bad language from the Exhaust Pipe, accompanied by gouts of smoke and vapour? The Engine, now revolving atno more than one-tenth its normal speed, has upset the proportion ofpetrol to air, and combustion is taking place intermittently or in theExhaust Pipe, where it has no business to be. 

"Crash, Bang, Rattle----!----!----!" and worse than that, yells theExhaust, and the Aeroplane, who is a gentleman and not a box kite, [13]remonstrates with the severity of a Senior Officer. "See the MedicalOfficer, you young Hun. Go and see a doctor. Vocal diarrhoea, that'syour complaint, and a very nasty one too. Bad form, bad for discipline, and a nuisance in the Mess. What's your Regiment? Special Reserve, yousay? Humph! Sounds like Secondhand Bicycle Trade to me!"

Now the Pilot decides to change the straight gliding descent to a spiralone, and, obedient to the Rudder, the Aeroplane turns to the left. Butthe Momentum (two tons at 100 miles per hour is no small affair) heavilyresents this change of direction, and tries its level best to preventit and to pull the machine sideways and outwards from its spiralcourse--that is, to make it "side-skid" outwards. But the Pilot deflectsthe Ailerons and "banks" up the planes to the correct angle, and, theAeroplane skidding sideways and outwards, the lowest surfaces ofthe planes press up against the air until the pressure equals thecentrifugal force of the Momentum, and the Aeroplane spirals steadilydownwards. 

Down, down, down, and the air grows denser, and the Pilot gulps largely, filling his lungs with the heavier air to counteract the increasingpressure from without. Down through a gap in the clouds, and theAerodrome springs into view, appearing no larger than a saucer, and thePilot, having by now got the "feel" of the Controls, proceeds to putthe Aeroplane through its paces. First at its Maximum Angle, staggeringalong tail-down and just maintaining horizontal flight; then a dive atfar over flying speed, finishing with a perfect loop; then sharp turnswith attendant vertical "banks" and then a wonderful switchbackflight, speeding down at a hundred and fifty miles an hour with short, exhilarating ascents at the rate of two thousand feet a minute!

All the parts are now working well together. Such wires as were beforein undue tension have secured relief by slightly elongating their loops, and each one is now doing its bit, and all are sharing the burden ofwork together. 

The Struts and the Spars, which felt so awkward at first, have beddedthemselves in their sockets, and are taking the compression stressesuncomplainingly. 

The Control Cables of twisted wire, a bit tight before, have slightlylengthened by perhaps the eighth of an inch, and, the Controls instantlyresponding to the delicate touch of the Pilot, the Aeroplane, at thewill of its Master, darts this way and that way, dives, loops, spirals, and at last, in one long, magnificent glide, lands gently in front ofits shed. 

"Well, what result?" calls the Flight-Commander to the Pilot. 

"A hundred miles an hour and a thousand feet a minute, " he brieflyreplies. 

"And a very good result too, " says the Aeroplane, complacently, as he iscarefully wheeled into his shed. 

That is the way Aeroplanes speak to those who love them and understandthem. Lots of Pilots know all about it, and can spin you wonderfulyarns, much better than this one, if you catch them in a confidentialmood--on leave, for instance, and after a good dinner. 

PART IV. 'CROSS COUNTRY

The Aeroplane had been designed and built, and tested in the air, andnow stood on the Aerodrome ready for its first 'cross-country flight. 

It had run the gauntlet of pseudo-designers, crank inventors, press"experts, " and politicians; of manufacturers keen on cheap work andlarge profits; of poor pilots who had funked it, and good pilots who hadexpected too much of it. Thousands of pounds had been wasted on it, many had gone bankrupt over it, and others it had provided with safe fatjobs. 

Somehow, and despite every conceivable obstacle, it had managed tomuddle through, and now it was ready for its work. It was not perfect, for there were fifty different ways in which it might be improved, someof them shamefully obvious. But it was fairly sound mechanically, had alittle inherent stability, was easily controlled, could climb a thousandfeet a minute, and its speed was a hundred miles an hour. In short, quite a creditable machine, though of course the right man had not gotthe credit. 

It is rough, unsettled weather with a thirty mile an hour wind on theground, and that means fifty more or less aloft. Lots of clouds atdifferent altitudes to bother the Pilot, and the air none to clear forthe observation of landmarks. 

As the Pilot and Observer approach the Aeroplane the former is clearlynot in the best of tempers. "It's rotten luck, " he is saying, "a blankshame that I should have to take this blessed 'bus and join X ReserveSquadron, stationed a hundred and fifty miles from anywhere; and justas I have licked my Flight into shape. Now some slack blighter will, Isuppose, command it and get the credit of all my work!"

"Shut up, you grouser, " said the Observer. "Do you think you're the onlyone with troubles? Haven't I been through it too? Oh! I know all aboutit! You're from the Special Reserve and your C. O. Doesn't like yourstyle of beauty, and you won't lick his boots, and you were a bit of atechnical knut in civil life, but now you've jolly well got to know lessthan those senior to you. Well! It's a jolly good experience for most ofus. Perhaps conceit won't be at quite such a premium after this war. Andwhat's the use of grousing? That never helped anyone. So buck up, oldchap. Your day will come yet. Here's our machine, and I must say itlooks a beauty!"

And, as the Pilot approaches the Aeroplane, his face brightens and hesoon forgets his troubles as he critically inspects the craft which isto transport him and the Observer over the hills and far away. Turningto the Flight-Sergeant he inquires, "Tank full of petrol and oil?"

"Yes, sir, " he replies, "and everything else all correct. Propeller, engine, and body covers on board, sir; tool kit checked over and in thelocker; engine and Aeroplane logbooks written up, signed, and under yourseat; engine revs. Up to mark, and all the control cables in perfectcondition and tension. "

"Very good, " said the Pilot; and then turning to the Observer, "Beforewe start you had better have a look at the course I have mapped out. 

"A is where we stand and we have to reach B, a hundred and fifty milesdue North. I judge that, at the altitude we shall fly, there will bean East wind, for although it is not quite East on the ground it isprobably about twenty degrees different aloft, the wind usually movinground clockways to about that extent. I think that it is blowing at therate of about fifty miles an hour, and I therefore take a line on themap to C, fifty miles due West of A. The Aeroplane's speed is a hundredmiles an hour, and so I take a line of one hundred miles from C to D. Our compass course will then be in the direction A--E, which is always aline parallel to C--D. That is, to be exact, it will be fourteen degreesoff the C--D course, as, in this part of the globe, there is that muchdifference between the North and South lines on the map and the magneticNorth to which the compass needle points. If the compass has an error, as it may have of a few degrees, that, too, must be taken into account, and the deviation or error curve on the dashboard will indicate it. 

"The Aeroplane will then always be pointing in a direction parallel toA--E, but, owing to the side wind, it will be actually travelling overthe course A--B, though in a rather sideways attitude to that course. 

"The distance we shall travel over the A--B course in one hour is A--D. That is nearly eighty-seven miles, so we ought to accomplish our journeyof a hundred and fifty miles in about one and three-quarter hours. 

"I hope that's quite clear to you. It's a very simple way of calculatingthe compass course, and I always do it like that. "

"Yes, that's plain enough. You have drafted what engineers call 'aparallelogram of forces'; but suppose you have miscalculated thevelocity of the wind, or that it should change in velocity ordirection?"

"Well, that of course will more or less alter matters, " replies thePilot. "But there are any number of good landmarks such as lakes, rivers, towns, and railway lines. They will help to keep us on the rightcourse, and the compass will, at any rate, prevent us from going farastray when between them. "

"Well, we'd better be off, old chap. Hop aboard. " This from the Observeras he climbs into the front seat from which he will command a good viewover the lower plane; and the Pilot takes his place in the rear seat, and, after making himself perfectly comfortable, fixing his safety belt, and moving the control levers to make sure that they are working freely, he gives the signal to the Engine Fitter to turn the propeller and sostart the engine. 

Round buzzes the Propeller, and the Pilot, giving the official signal, the Aeroplane is released and rolls swiftly over the ground in the teethof the gusty wind. 

In less than fifty yards it takes to the air and begins to climb rapidlyupwards, but how different are the conditions to the calm morning ofyesterday! If the air were visible it would be seen to be acting in themost extraordinary manner; crazily swirling, lifting and dropping, gustsviciously colliding--a mad phantasmagoria of forces!

Wickedly it seizes and shakes the Aeroplane; then tries to turn it oversideways; then instantly changes its mind and in a second drops it intoa hole a hundred feet deep, and if it were not for his safety belt thePilot might find his seat sinking away from beneath him. 

Gusts strike the front of the craft like so many slaps in the face; andothers, with the motion of mountainous waves, sometimes lift it hundredsof feet in a few seconds, hoping to see it plunge over the summit in adeath-dive--and so it goes on, but the Pilot, perfectly at one withhis mount and instantly alert to its slightest motion, is skilfully andnaturally making perhaps fifty movements a minute of hand and feet;the former lightly grasping the "joy-stick" which controls the Elevatorhinged to the tail, and also the Ailerons or little wings hinged to thewing-tips; and the latter moving the Rudder control-bar. 

A strain on the Pilot? Not a bit of it, for this is his Work which heloves and excels in; and given a cool head, alert eye, and a sensitivetouch for the controls, what sport can compare with these ever-changingbattles of the air?

The Aeroplane has all this time been climbing in great wide circles, and is now some three thousand feet above the Aerodrome which from suchheight looks absurdly small. The buildings below now seem quite squat;the hills appear to have sunk away into the ground, and the wholecountry below, cut up into diminutive fields, has the appearance ofhaving been lately tidied and thoroughly spring-cleaned! A doll'scountry it looks, with tiny horses and cows ornamenting the fieldsand little model motor-cars and carts stuck on the roads, the latterstretching away across the country like ribbons accidentally dropped. 

At three thousand feet altitude the Pilot is satisfied that he is nowsufficiently high to secure, in the event of engine failure, along enough glide to earth to enable him to choose and reach a goodlanding-place; and, being furthermore content with the steady running ofthe engine, he decides to climb no more but to follow the course he hasmapped out. Consulting the compass, he places the Aeroplane on the A--Ecourse and, using the Elevator, he gives his craft its minimum angle ofincidence at which it will just maintain horizontal flight and secureits maximum speed. 

Swiftly he speeds away, and few thoughts he has now for the changingpanorama of country, cloud, and colour. Ever present in his mind are thethree great 'cross-country queries. "Am I on my right course? Can I seea good landing-ground within gliding distance?" And "How is the Enginerunning?"

Keenly both he and the Observer compare their maps with the countrybelow. The roads, khaki-coloured ribbons, are easily seen but are notof much use, for there are so many of them and they all look alike fromsuch an altitude. 

Now where can that lake be which the map shows so plainly? He feels thatsurely he should see it by now, and has an uncomfortable feeling thathe is flying too far West. What pilot is there indeed who has not manytimes experienced such unpleasant sensation? Few things in the aircan create greater anxiety. Wisely, however, he sticks to his compasscourse, and the next minute he is rewarded by the sight of the lake, though indeed he now sees that the direction of his travel will not takehim over it, as should be the case if he were flying over the shortestroute to his destination. He must have slightly miscalculated thevelocity or direction of the side-wind. 

"About ten degrees off, " he mutters, and, using the Rudder, corrects hiscourse accordingly. 

Now he feels happier and that he is well on his way. The gusts, too, have ceased to trouble him as, at this altitude, they are not nearly sobad as they were near the ground the broken surface of which does muchto produce them; and sometimes for miles he makes but a movement or twoof the controls. 

The clouds just above race by with dizzy and uniform speed; the countrybelow slowly unrolls, and the steady drone of the Engine is almosthypnotic in effect. "Sleep, sleep, sleep, " it insidiously suggests. "Listen to me and watch the clouds; there's nothing else to do. Dream, dream, dream of speeding through space for ever, and ever, and ever; andrest, rest, rest to the sound of my rhythmical hum. Droning on and on, nothing whatever matters. All things now are merged into speed throughspace and a sleepy monotonous d-d-r-r-o-o-n-n-e - - - - -. " But thePilot pulls himself together with a start and peers far ahead in searchof the next landmark. This time it is a little country town, red-roofedhis map tells him, and roughly of cruciform shape; and, sure enough, there in the right direction are the broken outlines of a few red roofspeeping out from between the trees. 

Another minute and he can see this little town, a fairy town it appears, nestling down between the hills with its red roofs and picturesqueshape, a glowing and lovely contrast with the dark green of thesurrounding moors. 

So extraordinarily clean and tidy it looks from such a height, andlaid out in such orderly fashion with perfectly defined squares, parks, avenues, and public buildings, it indeed appears hardly real, but ratheras if it has this very day materialized from some delightful children'sbook!

Every city and town you must know has its distinct individuality to thePilot's eye. Some are not fairy places at all, but great dark ugly blotsupon the fair countryside, and with tall shafts belching forth murkycolumns of smoke to defile clean space. Others, melancholy-lookingmasses of grey, slate-roofed houses, are always sad and dispirited;never welcoming the glad sunshine, but ever calling for leaden skiesand a weeping Heaven. Others again, little coquettes with village green, white palings everywhere, bright gravel roads, and an irrepressible airof brightness and gaiety. 

Then there are the rivers, silvery streaks peacefully winding far, faraway to the distant horizon; they and the lakes the finest landmarksthe Pilot can have. And the forests. How can I describe them? The treescannot be seen separately, but merge altogether into enormous irregulardark green masses sprawling over the country, and sometimes with greatungainly arms half encircling some town or village; and the wind passingover the foliage at times gives the forest an almost living appearance, as of some great dragon of olden times rousing itself from slumber todevour the peaceful villages which its arms encircle. 

And the Pilot and Observer fly on and on, seeing these things and manyothers which baffle my poor skill to describe--things, dear Reader, thatyou shall see, and poets sing of, and great artists paint in the daysto come when the Designer has captured Efficiency. Then, and the timeis near, shall you see this beautiful world as you have never seen itbefore, the garden it is, the peace it breathes, and the wonder of it. 

The Pilot, flying on, is now anxiously looking for the railway linewhich midway on his journey should point the course. Ah! There it isat last, but suddenly (and the map at fault) it plunges into theearth! Well the writer remembers when that happened to him on a long'cross-country flight in the early days of aviation. Anxiously hewondered "Are tunnels always straight?" and with what relief, keeping ona straight course, he picked up the line again some three miles fartheron!

Now at last the Pilot sees the sea, just a streak on the north-easternhorizon, and he knows that his flight is two-thirds over. Indeed, heshould have seen it before, but the air is none too clear, and he is notyet able to discern the river which soon should cross his path. As heswiftly speeds on the air becomes denser and denser with what he fearsmust be the beginning of a sea-fog, perhaps drifting inland along thecourse of the river. Now does he feel real anxiety, for it is the DUTYof a Pilot to fear fog, his deadliest enemy. Fog not only hides thelandmarks by which he keeps his course, but makes the control of theAeroplane a matter of the greatest difficulty. He may not realizeit, but, in keeping his machine on an even keel, he is unconsciouslybalancing it against the horizon, and with the horizon gone he islost indeed. Not only that, but it also prevents him from choosing hislanding-place, and the chances are that, landing in a fog, he will smashinto a tree, hedge, or building, with disastrous results. The best andboldest pilot 'wares a fog, and so this one, finding the conditionsbecoming worse and yet worse, and being forced to descend lower andlower in order to keep the earth within view, wisely decides to choose alanding-place while there is yet time to do so. 

Throttling down the power of the engine he spirals downwards, keenlyobserving the country below. There are plenty of green fields to lurehim, and his great object is to avoid one in which the grass is long, for that would bring his machine to a stop so suddenly as to turn itover; or one of rough surface likely to break the under-carriage. Now isperfect eyesight and a cool head indispensable. He sees and decides upona field and, knowing his job, he sticks to that field with no changeof mind to confuse him. It is none too large, and gliding just over thetrees and head on to the wind he skilfully "stalls" his machine; thatis, the speed having decreased sufficiently to avoid such a manoeuvreresulting in ascent, he, by means of the Elevator, gives the Aeroplaneas large an angle of incidence as possible, and the undersides of theplanes meeting the air at such a large angle act as an air-brake, andthe Aeroplane, skimming over the ground, lessens its speed and finallystops just at the farther end of the field. 

Then, after driving the Aeroplane up to and under the lee of the hedge, he stops the engine, and quickly lashing the joy-stick fast in orderto prevent the wind from blowing the controlling surfaces about andpossibly damaging them, he hurriedly alights. Now running to the tail helifts it up on to his shoulder, for the wind has become rough indeed andthere is danger of the Aeroplane becoming unmanageable. By this actionhe decreases the angle at which the planes are inclined to the wind andso minimizes the latter's effect upon them. Then to the Observer, "Hurryup, old fellow, and try to find some rope, wire, or anything with whichto picket the machine. The wind is rising and I shan't be able to holdthe 'bus steady for long. Don't forget the wire-cutters. They're in thetool kit. " And the Observer rushes off in frantic haste, before longtriumphantly returning with a long length of wire from a neighbouringfence. Blocking up the tail with some debris at hand, they soon succeed, with the aid of the wire, in stoutly picketing the Aeroplane to theroots of the high hedge in front of it; done with much care, too, so that the wire shall not fray the fabric or set up dangerousbending-stresses in the woodwork. Their work is not done yet, for theObserver remarking, "I don't like the look of this thick weather andrather fear a heavy rain-storm, " the Pilot replies, "Well, it's afearful bore, but the first rule of our game is never to take anunnecessary risk, so out with the engine and body covers. "

Working with a will they soon have the engine and the open part of thebody which contains the seats, controls, and instruments snugly housedwith their waterproof covers, and the Aeroplane is ready to weather thepossible storm. 

Says the Observer, "I'm remarkably peckish, and methinks I spy thetowers of one of England's stately homes showing themselves just beyondthat wood, less than a quarter of a mile away. What ho! for a raid. Whatdo you say?"

"All right, you cut along and I'll stop here, for the Aeroplane must notbe left alone. Get back as quickly as possible. "

And the Observer trots off, leaving the Pilot filling his pipe andanxiously scrutinizing the weather conditions. Very thick it is now, butthe day is yet young, and he has hopes of the fog lifting sufficientlyto enable the flight to be resumed. A little impatiently he awaits thereturn of his comrade, but with never a doubt of the result, for thehospitality of the country house is proverbial among pilots! What oldhand among them is there who cannot instance many a forced landing madepleasant by such hospitality? Never too late or too early to help withfood, petrol, oil, tools, and assistants. Many a grateful thought hasthe writer for such kind help given in the days before the war (how longago they seem!), when aeroplanes were still more imperfect than they arenow, and involuntary descents often a part of 'cross-country flying. 

Ah! those early days! How fresh and inspiring they were! As one startedoff on one's first 'cross-country flight, on a machine the first of itsdesign, and with everything yet to learn, and the wonders of the air yetto explore; then the joy of accomplishment, the dreams of Efficiency, the hard work and long hours better than leisure; and what a field ofendeavour--the realms of space to conquer! And the battle still goes onwith ever-increasing success. Who is bold enough to say what its limitsshall be?

So ruminates this Pilot-Designer, as he puffs at his pipe, until hisreverie is abruptly disturbed by the return of the Observer. 

"Wake up, you AIRMAN, " the latter shouts. "Here's the very thing thedoctor ordered! A basket of first-class grub and something to keep thefog out, too. "

"Well, that's splendid, but don't call me newspaper names or you'llspoil my appetite!"

Then, with hunger such as only flying can produce, they appreciativelydiscuss their lunch, and with many a grateful thought for thedonors--and they talk shop. They can't help it, and even golf is a poorsecond to flight talk. Says the Pilot, who must have his grievance, "Just observe where I managed to stop the machine. Not twenty feet fromthis hedge! A little more and we should have been through it and intoKingdom Come! I stalled as well as one could, but the tail touchedthe ground and so I could not give the Aeroplane any larger angle ofincidence. Could I have given it a larger angle, then the planes wouldhave become a much more effective air-brake, and we should have come torest in a much shorter distance. It's all the fault of the tail. There'shardly a type of Aeroplane in existence in which the tail could not beraised several feet, and that would make all the difference. High tailsmean a large angle of incidence when the machine touches ground and, with enough angle, I'll guarantee to safely land the fastest machine ina five-acre field. You can, I am sure, imagine what a difference thatwould make where forced landings are concerned!" Then rapidly sketchingin his notebook, he shows the Observer the following illustration:

"That's very pretty, " said the Observer, "but how about MechanicalDifficulties, and Efficiency in respect of Flight? And, anyway, whyhasn't such an obvious thing been done already?"

"As regards the first part of your question I assure you that there'snothing in it, and I'll prove it to you as follows----"

"Oh! That's all right, old chap. I'll take your word for it, " hurriedlyreplies the Observer, whose soul isn't tuned to a technical key. 

"As regards the latter part of your inquiry, " went on the Pilot, alittle nettled at having such a poor listener, "it's very simple. Aeroplanes have 'just growed' like Topsy, and they consequently containthis and many another relic of early day design when Aeroplanes weremore or less thrown together and anything was good enough that could getoff the ground. "

"By Jove, " interrupts the Observer, "I do believe the fog is lifting. Hadn't we better get the engine and body covers off, just in case it'sreally so?"

"I believe you're right. I am sure those hills over there could notbe seen a few minutes ago, and look--there's sunshine over there. We'dbetter hurry up. "

Ten minutes' hard work and the covers are off, neatly folded and stowedaboard; the picketing wires are cast adrift, and the Pilot is once morein his seat. The Aeroplane has been turned to face the other end of thefield, and, the Observer swinging round the propeller, the engine isawake again and slowly ticking over. Quickly the Observer climbs intohis seat in front of the Pilot, and, the latter slightly opening thethrottle, the Aeroplane leisurely rolls over the ground towards theother end of the field, from which the ascent will be made. 

Arriving there the Pilot turns the Aeroplane in order to face the windand thus secure a quick "get-off. " Then he opens the throttle fully andthe mighty voice of the Engine roars out "Now see me clear that hedge!"and the Aeroplane races forward at its minimum angle of incidence. Tailup, and with ever-increasing speed, it rushes towards the hedge underthe lee of which it has lately been at rest; and then, just as theObserver involuntarily pulls back an imaginary "joy-stick, " the Pilotmoves the real one and places the machine at its best climbing angle. Like a living thing it responds, and instantly leaves the ground, clearing the hedge like a--well, like an Aeroplane with an excellentmargin of lift. Upwards it climbs with even and powerful lift, and thefamiliar scenes below again gladden the eyes of the Pilot. Smaller andmore and more squat grow the houses and hills; more and more doll-likeappear the fields which are clearly outlined by the hedges; and soon thecountry below is easily identified with the map. Now they can see theriver before them and a bay of the sea which must be crossed or skirted. The fog still lingers along the course of the river and between thehills, but is fast rolling away in grey, ghost-like masses. Out to seait obscures the horizon, making it difficult to be sure where water endsand fog begins, and creating a strange, rather weird effect by whichships at a certain distance appear to be floating in space. 

Now the Aeroplane is almost over the river, and the next instant itsuddenly drops into a "hole in the air. " With great suddenness ithappens, and for some two hundred feet it drops nose-down and tiltedover sideways; but the Pilot is prepared and has put his craft on aneven keel in less time than it takes to tell you about it; for well heknows that he must expect such conditions when passing over a shoreor, indeed, any well-defined change in the composition of the earth'ssurface. Especially is this so on a hot and sunny day, for then the warmsurface of the earth creates columns of ascending air, the speed of theascent depending upon the composition of the surface. Sandy soil, forinstance, such as borders this river produces a quickly ascending columnof air, whereas water and forests have not such a marked effect. Thus, when our Aeroplane passed over the shore of the river, it suddenly lostthe lift due to the ascending air produced by the warm sandy soil, andit consequently dropped just as if it had fallen into a hole. 

Now the Aeroplane is over the bay and, the sea being calm, the Pilotlooks down, down through the water, and clearly sees the bottom, hundreds of feet below the surface. Down through the reflection of theblue sky and clouds, and one might think that is all, but it isn't. Onlythose who fly know the beauties of the sea as viewed from above;its dappled pearly tints; its soft dark blue shadows; the beautifulcontrasts of unusual shades of colour which are always differing andshifting with the changing sunshine and the ever moving position of theaerial observer. Ah! for some better pen than mine to describe thesethings! One with glowing words and a magic rhythm to express the wondersof the air and the beauty of the garden beneath--the immensity of thesea--the sense of space and of one's littleness there--the realizationof the Power moving the multitudes below--the exaltation of spiritaltitude produces--the joy of speed. A new world of sensation!

Now the bay is almost crossed and the Aerodrome at B can bedistinguished. 

On the Aerodrome is a little crowd waiting and watching for the arrivalof the Aeroplane, for it is of a new and improved type and its first'cross-country performance is of keen interest to these men; men whoreally know something about flight. 

There is the Squadron Commander who has done some real flying inhis time; several well-seasoned Flight-Commanders; a dozen or moreFlight-Lieutenants; a knowledgeable Flight-Sergeant; a number of AirMechanics, and, a little on one side and almost unnoticed, the Designer. 

"I hope they are all right, " said someone, "and that they haven't haddifficulties with the fog. It rolled up very quickly, you know. "

"Never fear, " remarked a Flight-Commander. "I know the Pilot well andhe's a good 'un; far too good to carry on into a fog. "

"They say the machine is really something out of the ordinary, " saidanother, "and that, for once, the Designer has been allowed full play;that he hasn't been forced to unduly standardize ribs, spars, struts, etc. , and has more or less had his own way. I wonder who he is. It seemsstrange we hear so little of him. "

"Ah! my boy. You do a bit more flying and you'll discover that thingsare not always as they appear from a distance!"

"There she is, sir!" cries the Flight-Sergeant. "Just a speck over thesilvery corner of that cloud. "

A tiny speck it looks, some six miles distant and three thousand feethigh; but, racing along, it rapidly appears larger and soon its outlinescan be traced and the sunlight be seen playing upon the whirlingpropeller. 

Now the distant drone of the engine can be heard, but not for long, forsuddenly it ceases and, the nose of the Aeroplane sinking, the craftcommences gliding downwards. 

"Surely too far away, " says a subaltern. "It will be a wonderful machineif, from that distance and height, it can glide into the Aerodrome. "And more than one express the opinion that it cannot be done; butthe Designer smiles to himself, yet with a little anxiety, for hisreputation is at stake, and Efficiency, the main reward he desires, isperhaps, or perhaps not, at last within his grasp!

Swiftly the machine glides downwards towards them, and it can now beseen how surprisingly little it is affected by the rough weather andgusts; so much so that a little chorus of approval is heard. 

"Jolly good gliding angle, " says someone; and another, "Beautifullyquick controls, what?" and from yet another, "By Jove! The Pilot must besure of the machine. Look, he's stopped the engine entirely. "

Then the Aeroplane with noiseless engine glides over the boundary of theAerodrome, and, with just a soft soughing sound from the air it cleaves, lands gently not fifty yards from the onlookers. 

"Glad to see you, " says the Squadron Commander to the Pilot. "How do youlike the machine?" And the Pilot replies:

"I never want a better one, sir. It almost flies itself!"

And the Designer turns his face homewards and towards his beloveddrawing-office; well satisfied, but still dreaming dreams of the futureand. .. Looking far ahead whom should he see but Efficiency at lastcoming towards him! And to him she is all things. In her hair is themorning sunshine; her eyes hold the blue of the sky, and on her cheeksis the pearly tint of the clouds as seen from above. The passion ofspeed, the lure of space, the sense of power, and the wonder of thefuture. .. All these things she holds for him. 

"Ah!" he cries. "You'll never leave me now, when at last there is no onebetween us?"

And Efficiency, smiling and blushing, but practical as ever, says:

"And you will never throw those Compromises in my face?"

"My dear, I love you for them! Haven't they been my life ever since Ibegan striving for you ten long years ago?"

And so they walked off very happily, arm-in-arm together; and if thishasn't bored you and you'd like some more of the same sort of thing, I'djust love to tell you some day of the wonderful things they accomplishtogether, and of what they dream the future holds in store. 

And that's the end of the Prologue. 

CHAPTER I. FLIGHT

Air has weight (about 13 cubic feet = 1 lb. ), inertia, and momentum. It therefore obeys Newton's laws[14] and resists movement. It is thatresistance or reaction which makes flight possible. 

Flight is secured by driving through the air a surface[15] inclinedupwards and towards the direction of motion. 

S = Side view of surface. 

M = Direction of motion. 

CHORD. --The Chord is, for practical purposes, taken to be a straightline from the leading edge of the surface to its trailing edge. 

N = A line through the surface starting from its trailing edge. Theposition of this line, which I call the Neutral Lift Line, is found bymeans of wind-tunnel research, and it varies with differences inthe camber (curvature) of surfaces. In order to secure flight, theinclination of the surface must be such that the neutral lift line makesan angle with and ABOVE the line of motion. If it is coincident with M, there is no lift. If it makes an angle with M and BELOW it, then thereis a pressure tending to force the surface down. 

I = Angle of Incidence. This angle is generally defined as the angle thechord makes with the direction of motion, but that is a bad definition, as it leads to misconception. The angle of incidence is best describedas the angle the neutral lift line makes with the direction of motionrelative to the air. You will, however, find that in nearly all riggingspecifications the angle of incidence is taken to mean the angle thechord makes with a line parallel to the propeller thrust. This isnecessary from the point of view of the practical mechanic who has torig the aeroplane, for he could not find the neutral lift line, whereashe can easily find the chord. Again, he would certainly be in doubt asto "the direction of motion relative to the air, " whereas he can easilyfind a line parallel to the propeller thrust. It is a pity, however, that these practical considerations have resulted in a bad definitionof the angle of incidence becoming prevalent, a consequence of which hasbeen the widespread fallacy that flight may be secured with a negativeinclination of the surface. Flight may conceivably be secured with anegative angle of chord, but never with a negative inclination of thesurface. All this is only applicable to cambered surfaces. In the caseof flat surfaces the neutral lift line coincides with the chord and thedefinition I have criticised adversely is then applicable. Flat liftingsurfaces are, however, never used. 

The surface acts upon the air in the following manner:

As the bottom of the surface meets the air, it compresses it andaccelerates it DOWNWARDS. As a result of this definite action there is, of course, an equal and opposite reaction UPWARDS. 

The top surface, in moving forward, tends to leave the air behindit, thus creating a semi-vacuum or rarefied area over the top of thesurface. Consequently the pressure of air on the top of the surfaceis decreased, thus assisting the reaction below to lift the surfaceUPWARDS. 

The reaction increases approximately as the square of the velocity. Itis the result of (1) the mass of air engaged, and (2) the velocity andconsequent force with which the surface engages the air. If the reactionwas produced by only one of those factors it would increase in directproportion to the velocity, but, since it is the product of bothfactors, it increases as V(2S). 

Approximately three-fifths of the reaction is due to the decrease ofdensity (and consequent decrease of downward pressure) on the top of thesurface; and only some two-fifths is due to the upward reaction securedby the action of the bottom surface upon the air. A practical point inrespect of this is that, in the event of the fabric covering the surfacegetting into bad condition, it is more likely to strip off the top thanoff the bottom. 

The direction of the reaction is approximately at right-angles to thechord of the surface, as illustrated above; and it is, in consideringflight, convenient to divide it into two component parts or values, thus:

1. The vertical component of the reaction, i. E. , Lift, which is opposedto Gravity, i. E. , the weight of the aeroplane. 

2. The horizontal component, i. E. , Drift (sometimes called Resistance), to which is opposed the thrust of the propeller. 

The direction of the reaction is, of course, the resultant of the forcesLift and Drift. 

The Lift is the useful part of the reaction, for it lifts the weight ofthe aeroplane. 

The Drift is the villain of the piece, and must be overcome by theThrust in order to secure the necessary velocity to produce therequisite Lift for flight. 

DRIFT. --The drift of the whole aeroplane (we have considered only thelifting surface heretofore) may be conveniently divided into threeparts, as follows:

Active Drift, which is the drift produced by the lifting surfaces. 

Passive Drift, which is the drift produced by all the rest of theaeroplane--the struts, wires, fuselage, under-carriage, etc. , all ofwhich is known as "detrimental surface. "

Skin Friction, which is the drift produced by the friction of the airwith roughnesses of surface. The latter is practically negligiblehaving regard to the smooth surface of the modern aeroplane, and itscomparatively slow velocity compared with, for instance, the velocity ofa propeller blade. 

LIFT-DRIFT RATIO. --The proportion of lift to drift is known as thelift-drift ratio, and is of paramount importance, for it expresses theefficiency of the aeroplane (as distinct from engine and propeller). Aknowledge of the factors governing the lift-drift ratio is, as will beseen later, an absolute necessity to anyone responsible for the riggingof an aeroplane, and the maintenance of it in an efficient and safecondition. 

Those factors are as follows:

1. Velocity. --The greater the velocity the greater the proportion ofdrift to lift, and consequently the less the efficiency. Consideringthe lifting surfaces alone, both the lift and the (active) drift, beingcomponent parts of the reaction, increase as the square of the velocity, and the efficiency remains the same at all speeds. But, considering thewhole aeroplane, we must remember the passive drift. It also increasesas the square of the velocity (with no attendant lift), and, addingitself to the active drift, results in increasing the proportion oftotal drift (active + passive) to lift. 

But for the increase in passive drift the efficiency of the aeroplanewould not fall with increasing velocity, and it would be possible, bydoubling the thrust, to approximately double the speed or lift--a happystate of affairs which can never be, but which we may, in a measure, approach by doing everything possible to diminish the passive drift. 

Every effort is then made to decrease it by "stream-lining, " i. E. , bygiving all "detrimental" parts of the aeroplane a form by which theywill pass through the air with the least possible drift. Even the wiresbracing the aeroplane together are, in many cases, stream-lined, andwith a markedly good effect upon the lift-drift ratio. In the case of acertain well-known type of aeroplane the replacing of the ordinary wiresby stream-lined wires added over five miles an hour to the flight speed. 

Head-resistance is a term often applied to passive drift, but it is aptto convey a wrong impression, as the drift is not nearly so much theresult of the head or forward part of struts, wires, etc. , as it is ofthe rarefied area behind. 

Above is illustrated the flow of air round two objects moving in thedirection of the arrow M. 

In the case of A, you will note that the rarefied area DD is of veryconsiderable extent; whereas in the case of B, the air flows round itin such a way as to meet very closely to the rear of the object, thusDECREASING DD. 

The greater the rarefied area DD. Then, the less the density, and, consequently, the less the pressure of air upon the rear of the object. The less such pressure, then, the better is head-resistance D able toget its work in, and the more thrust will be required to overcome it. 

The "fineness" of the stream-line shape, i. E. , the proportion of lengthto width, is determined by the velocity--the greater the velocity, thegreater the fineness. The best degree of fineness for any given velocityis found by means of wind-tunnel research. 

The practical application of all this is, from a rigging point of view, the importance of adjusting all stream-line parts to be dead-on in theline of flight, but more of that later on. 

2. Angle of Incidence. --The most efficient angle of incidence varieswith the thrust at the disposal of the designer, the weight to becarried, and the climb-velocity ratio desired. 

The best angles of incidence for these varying factors are found bymeans of wind-tunnel research and practical trial and error. Generallyspeaking, the greater the velocity the smaller should be the angle ofincidence, in order to preserve a clean, stream-line shape of rarefiedarea and freedom from eddies. Should the angle be too great for thevelocity, then the rarefied area becomes of irregular shape withattendant turbulent eddies. Such eddies possess no lift value, and sinceit has taken power to produce them, they represent drift and adverselyaffect the lift-drift ratio. 

From a rigging point of view, one must presume that every standardaeroplane has its lifting surface set at the most efficient angle, andthe practical application of all this is in taking the greatest possiblecare to rig the surface at the correct angle and to maintain it at suchangle. Any deviation will adversely affect the lift-drift ratio, i. E. , the efficiency. 

3. Camber. --(Refer to the second illustration in this chapter. ) Thelifting surfaces are cambered, i. E. , curved, in order to decrease thehorizontal component of the reaction, i. E. , the drift. 

The bottom camber: If the bottom of the surface was flat, every particleof air meeting it would do so with a shock, and such shock would producea very considerable horizontal reaction or drift. By curving it suchshock is diminished, and the curve should be such as to produce auniform (not necessarily constant) acceleration and compression of theair from the leading edge to the trailing edge. Any unevenness in theacceleration and compression of the air produces drift. 

The top camber: If this was flat it would produce a rarefied area ofirregular shape. I have already explained the bad effect this hasupon the lift-drift ratio. The top surface is then curved to produce ararefied area the shape of which shall be as stream-line and free fromattendant eddies as possible. 

The camber varies with the angle of incidence, the velocity, and thethickness of the surface. Generally speaking, the greater the velocity, the less the camber and angle of incidence. With infinite velocity thesurface would be set at no angle of incidence (the neutral lift linecoincident with the direction of motion relative to the air), and wouldbe, top and bottom, of pure streamline form--i. E. , of infinite fineness. This is, of course, carrying theory to absurdity as the surface wouldthen cease to exist. 

The best cambers for varying velocities, angles of incidence, andthicknesses of surface, are found by means of wind-tunnel research. The practical application of all this is in taking the greatest care toprevent the surface from becoming distorted and thus spoiling the camberand consequently the lift-drift ratio. 

4. Aspect Ratio. --This is the proportion of span to chord. Thus, if thespan is, for instance, 50 feet and the chord 5 feet, the surface wouldbe said to have an aspect ratio of 10 to 1. 

For A GIVEN VELOCITY and A GIVEN AREA of surface, the greater theaspect ratio, the greater the reaction. It is obvious, I think, that thegreater the span, the greater the mass of air engaged, and, as alreadyexplained, the reaction is partly the result of the mass of air engaged. 

Not only that, but, PROVIDED the chord is not decreased to an extentmaking it impossible to secure the best camber owing to the thicknessof the surface, the greater the aspect ratio, the better the lift-driftratio. The reason of this is rather obscure. It is sometimes advancedthat it is owing to the "spill" of air from under the wing-tips. Witha high aspect ratio the chord is less than would otherwise be the case. Less chord results in smaller wing-tips and consequently less "spill. "This, however, appears to be a rather inadequate reason for the highaspect ratio producing the high lift-drift ratio. Other reasons are alsoadvanced, but they are of such a contentious nature I do not think itwell to go into them here. They are of interest to designers, but thisis written for the practical pilot and rigger. 

5. Stagger. --This is the advancement of the top surface relative to thebottom surface, and is not, of course, applicable to a single surface, i. E. , a monoplane. In the case of a biplane having no stagger, therewill be "interference" and consequent loss of Efficiency unless thegap between the top and bottom surfaces is equal to not less than 1 1/2times the chord. If less than that, the air engaged by the bottom of thetop surface will have a tendency to be drawn into the rarefied area overthe top of the bottom surface, with the result that the surfaces willnot secure as good a reaction as would otherwise be the case. 

It is not practicable to have a gap of much more than a distance equalto the chord, owing to the drift produced by the great length of strutsand wires such a large gap would necessitate. By staggering the topsurface forward, however, it is removed from the action of the lowersurface and engages undisturbed air, with the result that the efficiencycan in this way be increased by about 5 per cent. Theoretically the topplane should be staggered forward for a distance equal to about 30 percent. Of the chord, the exact distance depending upon the velocityand angle of incidence; but this is not always possible to arrangein designing an aeroplane, owing to difficulties of balance, desiredposition, and view of pilot, observer, etc. 

6. Horizontal Equivalent. --The vertical component of the reaction, i. E. , lift, varies as the horizontal equivalent (H. E. ) of the surface, butthe drift remains the same. Then it follows that if H. E. Grows less, theratio of lift to drift must do the same. 

A, B, and C are front views of three surfaces. 

A has its full H. E. , and therefore, from the point of view from whichwe are at the moment considering efficiency, it has its best lift-driftratio. 

B and C both possess the same surface as A, but one is inclined upwardsfrom its centre and the other is straight but tilted. For these reasonstheir H. E. 's are, as illustrated, less than in the case of A. That meansless vertical lift, and, the drift remaining the same (for there isthe same amount of surface as in A to produce it), the lift-drift ratiofalls. 

THE MARGIN OF POWER is the power available above that necessary tomaintain horizontal flight. 

THE MARGIN OF LIFT is the height an aeroplane can gain in a given timeand starting from a given altitude. As an example, thus: 1, 000 feet thefirst minute, and starting from an altitude of 500 feet above sea-level. 

The margin of lift decreases with altitude, owing to the decrease inthe density of the air, which adversely affects the engine. Providedthe engine maintained its impulse with altitude, then, if we ignore theproblem of the propeller, which I will go into later on, the margin oflift would not disappear. Moreover, greater velocity for a given powerwould be secured at a greater altitude, owing to the decreased densityof air to be overcome. After reading that, you may like to light yourpipe and indulge in dreams of the wonderful possibilities which maybecome realities if some brilliant genius shows us some day how tosecure a constant power with increasing altitude. I am afraid, however, that will always remain impossible; but it is probable that some veryinteresting steps may be taken in that direction. 

THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, fora given power, surface (including detrimental surface), and weight, horizontal flight can be maintained. 

THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, fora given power, surface (including detrimental surface), and weight, horizontal flight can be maintained. 

THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-driftratio is highest. In modern aeroplanes it is that angle of incidencepossessed by the surface when the axis of the propeller is horizontal. 

THE BEST CLIMBING ANGLE is approximately half-way between the maximumand the optimum angles. 

All present-day aeroplanes are a compromise between Climb and horizontalVelocity. We will compare the essentials for two aeroplanes, onedesigned for maximum climb, and the other for maximum velocity. 

ESSENTIALS FOR MAXIMUM CLIMB:

1. Low velocity, in order to secure the best lift-drift ratio. 

2. Having a low velocity, a large surface will be necessary in order toengage the necessary mass of air to secure the requisite lift. 

3. Since (1) such a climbing machine will move along an upward slopingpath, and (2) will climb with its propeller thrust horizontal, then alarge angle relative to the direction of the thrust will be necessary inorder to secure the requisite angle relative to the direction of motion. 

The propeller thrust should be always horizontal, because the mostefficient flying-machine (having regard to climb OR velocity) has, sofar, been found to be an arrangement of an inclined surface driven bya HORIZONTAL thrust--the surface lifting the weight, and the thrustovercoming the drift. This is, in practice, a far more efficientarrangement than the helicopter, i. E. , the air-screw revolving abouta vertical axis and producing a thrust opposed to gravity. If, whenclimbing, the propeller thrust is at such an angle as to tend to haulthe aeroplane upwards, then it is, in a measure, acting as a helicopter, and that means inefficiency. The reason of a helicopter beinginefficient in practice is due to the fact that, owing to mechanicaldifficulties, it is impossible to construct within a reasonable weightan air-screw of the requisite dimensions. That being so, it would benecessary, in order to absorb the power of the engine, to revolve thecomparatively small-surfaced air screw at an immensely greater velocitythan that of the aeroplane's surface. As already explained, thelift-drift ratio falls with velocity on account of the increase inpassive drift. This applies to a blade of a propeller or air-screw, which is nothing but a revolving surface set at angle of incidence, andwhich it is impossible to construct without a good deal of detrimentalsurface near the central boss. 

4. The velocity being low, then it follows that for that reason also theangle of incidence should be comparatively large. 

5. Camber. --Since such an aeroplane would be of low velocity, andtherefore possess a large angle of incidence, a large camber would benecessary. 

Let us now consider the essentials for an aeroplane of maximum velocityfor its power, and possessing merely enough lift to get off the ground, but no margin of lift. 

1. Comparatively HIGH VELOCITY. 

2. A comparatively SMALL SURFACE, because, being of greater velocitythan the maximum climber, a greater mass of air will be engaged fora given surface and time, and therefore a smaller surface will besufficient to secure the requisit lift. 

3. A small angle relative to the propeller thrust, since the lattercoincides with the direction of motion. 

4. A comparatively small angle of incidence by reason of the highvelocity. 

5. A comparatively small camber follows as a result of the small angleof incidence. 

SUMMARY. 

 Essentials for Maximum Essentials for Maximum Climb. Velocity

 1. Low velocity. High velocity. 2. Large surface. Small surface. 3. Large angle relative to Small angle relative to propeller thrust. Propeller thrust. 4. Large angle relative to Small angle relative to direction direction of motion. Of motion. 5. Large camber. Small camber. 

It is mechanically impossible to construct an aeroplane of reasonableweight of which it would be possible to very the above opposingessentials. Therefore, all aeroplanes are designed as a compromisebetween Climb and Velocity. 

As a rule aeroplanes are designed to have at low altitude a slightmargin of lift when the propeller thrust is horizontal. 

ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN AEROPLANE DESIGNEDAS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIGHTMARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE THRUST IS HORIZONTAL

MINIMUM ANGLE. 

This gives the greatest velocity during horizontal flight at a lowaltitude. Greater velocity would be secured if the surface, angle, andcamber were smaller and designed to just maintain horizontal flightwith a horizontal thrust. Also, in such case, the propeller would notbe thrusting downwards, but along a horizontal line which is obviouslya more efficient arrangement if we regard the aeroplane merely from onepoint of view, i. E. , either with reference to velocity OR climb. 

OPTIMUM ANGLE (Thrust horizontal)

The velocity is less than at the smaller minimum angle, and, asaeroplanes are designed to-day, the area and angle of incidence of thesurface is such as to secure a slight ascent at a low altitude. Thecamber of the surface is designed for this angle of incidence andvelocity. The lift-drift ratio is best at this angle. 

BEST CLIMBING ANGLE

The velocity is now still less by reason of the increased angleproducing increase of drift. Less velocity at A GIVEN ANGLE producesless lift, but the increased angle more or less offsets the loss oflift due to the decreased velocity, and in addition, the thrust is nowhauling the aeroplane upwards. 

MAXIMUM ANGLE

The greater angle has now produced so much drift as to lessen thevelocity to a point where the combined lifts from the surface and fromthe thrust are only just able to maintain horizontal flight. Any greaterangle will result in a still lower lift-drift ratio. The lift will thenbecome less than the weight and the aeroplane will consequently fall. Such a fall is known as "stalling" or "pancaking. "

NOTE. --The golden rule for beginners: Never exceed the Best ClimbingAngle. Always maintain the flying speed of the aeroplane. 

By this means, when the altitude is reached where the margin oflift disappears (on account of loss of engine power), and which is, consequently, the altitude where it is just possible to maintainhorizontal flight, the aeroplane is flying with its thrust horizontaland with maximum efficiency (as distinct from engine and propellerefficiency). 

The margin of lift at low altitude, and when the thrust is horizontal, should then be such that the higher altitude at which the margin of liftis lost is that altitude at which most of the aeroplane's horizontalflight work is done. That ensures maximum velocity when most required. 

Unfortunately, where aeroplanes designed for fighting are concerned, thealtitude where most of the work is done is that at which both maximumvelocity and maximum margin of lift for power are required. 

Perhaps some day a brilliant inventor will design an aeroplane ofreasonable weight and drift of which it will be possible for the pilotto vary at will the above-mentioned opposing essentials. Then we shallget maximum velocity, or maximum margin of lift, for power as required. Until then the design of the aeroplane must remain a compromise betweenVelocity and Climb. 

CHAPTER II. STABILITY AND CONTROL

STABILITY is a condition whereby an object disturbed has a naturaltendency to return to its first and normal position. Example: a weightsuspended by a cord. 

INSTABILITY is a condition whereby an object disturbed has a naturaltendency to move as far as possible away from its first position, withno tendency to return. Example: a stick balanced vertically upon yourfinger. 

NEUTRAL INSTABILITY is a condition whereby an object disturbed has notendency to move farther than displaced by the force of the disturbance, and no tendency to return to its first position. 

In order that an aeroplane may be reasonably controllable, it isnecessary for it to possess some degree of stability longitudinally, laterally, and directionally. 

LONGITUDINAL STABILITY in an aeroplane is its stability about an axistransverse to the direction of normal horizontal flight, and withoutwhich it would pitch and toss. 

LATERAL STABILITY is its stability about its longitudinal axis, andwithout which it would roll sideways. 

DIRECTIONAL STABILITY is its stability about its vertical axis, andwithout which it would have no tendency to keep its course. 

For such directional stability to exist there must be, in effect, [16]more "keel-surface" behind the vertical axis than there is in front ofit. By keel-surface I mean every-thing to be seen when looking at anaeroplane from the side of it--the sides of the body, undercarriage, struts, wires, etc. The same thing applies to a weathercock. You knowwhat would happen if there was insufficient keel-surface behind thevertical axis upon which it is pivoted. It would turn off its propercourse, which is opposite to the direction of the wind. It is very muchthe same in the case of an aeroplane. 

The above illustration represents an aeroplane (directionally stable)flying along the course B. A gust striking it as indicated acts upon thegreater proportion of keel-surface behind the turning axis and throws itinto the new course. It does not, however, travel along the new course, owing to its momentum in the direction B. It travels, as long as suchmomentum lasts, in a direction which is the resultant of the two forcesThrust and Momentum. But the centre line of the aeroplane is pointing inthe direction of the new course. Therefore its attitude, relative tothe direction of motion, is more or less sideways, and it consequentlyreceives an air pressure in the direction C. Such pressure, acting uponthe keel-surface, presses the tail back towards its first position inwhich the aeroplane is upon its course B. 

What I have described is continually going on during flight, but ina well-designed aeroplane such stabilizing movements are, most of thetime, so slight as to be imperceptible to the pilot. 

If an aeroplane was not stabilized in this way, it would not only becontinually trying to leave its course, but it would also possess adangerous tendency to "nose away" from the direction of the side gusts. In such case the gust shown in the above illustration would turn theaeroplane round the opposite way a very considerable distance; and theright wing, being on the outside of the turn, would travel with greatervelocity than the left wing. Increased velocity means increased lift;and so, the right wing lifting, the aeroplane would turn over sidewaysvery quickly. 

LONGITUDINAL STABILITY. --Flat surfaces are longitudinally stable owingto the fact that with decreasing angles of incidence the centre line ofpressure (C. P. ) moves forward. 

The C. P. Is a line taken across the surface, transverse to the directionof motion, and about which all the air forces may be said to balance, orthrough which they may be said to act. 

Imagine A to be a flat surface, attitude vertical, travelling throughthe air in the direction of motion M. Its C. P. Is then obviously alongthe exact centre line of the surface as illustrated. 

In B, C, and D the surfaces are shown with angles of incidencedecreasing to nothing, and you will note that the C. P. Moves forwardwith the decreasing angle. 

Now, should some gust or eddy tend to make the surface decrease theangle, i. E. , dive, then the C. P. Moves forward and pushes the front ofthe surface up. Should the surface tend to assume too large an angle, then the reverse happens--the C. P. Moves back and pushes the rear of thesurface up. 

Flat surfaces are, then, theoretically stable longitudinally. They arenot, however, used, on account of their poor lift-drift ratio. 

As already explained, cambered surfaces are used, and these arelongitudinally unstable at those angles of incidence producing areasonable lift-drift ratio, i. E. , at angles below: about 12 degrees. 

A is a cambered surface, attitude approximately vertical, moving throughthe air in the direction M. Obviously the C. P. Coincides with thetransverse centre line of the surface. 

With decreasing angles, down to angles of about 30 degrees, the C. P. Moves forward as in the case of flat surfaces (see B), but angles above30 degrees do not interest us, since they produce a very low ratio oflift to drift. 

Below angles of about 30 degrees (see C) the dipping front part of thesurface assumes a negative angle of incidence resulting in the DOWNWARDair pressure D, and the more the angle of incidence is decreased, thegreater such negative angle and its resultant pressure D. Since theC. P. Is the resultant of all the air forces, its position is naturallyaffected by D, which causes it to move backwards. Now, should some gustor eddy tend to make the surface decrease its angle of incidence, i. E. , dive, then the C. P. Moves backwards, and, pushing up the rear of thesurface, causes it to dive the more. Should the surface tend to assumetoo large an angle, then the reverse happens; the pressure D decreases, with the result that C. P. Moves forward and pushes up the front of thesurface, thus increasing the angle still further, the final result beinga "tail-slide. "

It is therefore necessary to find a means of stabilizing the naturallyunstable cambered surface. This is usually secured by means of astabilizing surface fixed some distance in the rear of the main surface, and it is a necessary condition that the neutral lift lines of the twosurfaces, when projected to meet each other, make a dihedral angle. Inother words, the rear stabilizing surface must have a lesser angle ofincidence than the main surface--certainly not more than one-third ofthat of the main surface. This is known as the longitudinal dihedral. 

I may add that the tail-plane is sometimes mounted upon the aeroplane atthe same angle as the main surface, but, in such cases, it attacks airwhich has received a downward deflection from the main surface, thus:

The angle at which the tail surface attacks the air (the angle ofincidence) is therefore less than the angle of incidence of the mainsurface. 

I will now, by means of the following illustration, try to explain howthe longitudinal dihedral secures stability:

First, imagine the aeroplane travelling in the direction of motion, which coincides with the direction of thrust T. The weight is, ofcourse, balanced about a C. P. , the resultant of the C. P. Of the mainsurface and the C. P. Of the stabilizing surface. For the sake ofillustration, the stabilizing surface has been given an angle ofincidence, and therefore has a lift and C. P. In practice the stabilizeris often set at no angle of incidence. In such case the propositionremains the same, but it is, perhaps, a little easier to illustrate itas above. 

Now, we will suppose that a gust or eddy throws the machine into thelower position. It no longer travels in the direction of T, since themomentum in the old direction pulls it off that course. M is now theresultant of the Thrust and the Momentum, and you will note that thisresults in a decrease in the angle our old friend the neutral lift linemakes with M, i. E. , a decrease in the angle of incidence and therefore adecrease in lift. 

We will suppose that this decrease is 2 degrees. Such decrease appliesto both main surface and stabilizer, since both are fixed rigidly to theaeroplane. 

The main surface, which had 12 degrees angle, has now only 10 degrees, i. E. , a loss of ONE-SIXTH. 

The stabilizer, which had 4 degrees angle, has now only 2 degrees, i. E. , a loss of ONE-HALF. 

The latter has therefore lost a greater PROPORTION of its angle ofincidence, and consequently its lift, than has the main surface. It mustthen fall relative to the main surface. The tail falling, the aeroplanethen assumes its first position, though at a slightly less altitude. 

Should a gust throw the nose of the aeroplane up, then the reversehappens. Both main surface and stabilizer increase their angles ofincidence in the same amount, but the angle, and therefore the lift, ofthe stabilizer increases in greater proportion than does the lift of themain surface, with the result that it lifts the tail. The aeroplane thenassumes its first position, though at a slightly greater altitude. 

Do not fall into the widespread error that the angle of incidence variesas the angle of the aeroplane to the horizontal. It varies with suchangle, but not as anything approaching it. Remember that the stabilizingeffect of the longitudinal dihedral lasts only as long as there ismomentum in the direction of the first course. 

These stabilizing movements are taking place all the time, even thoughimperceptible to the pilot. 

Aeroplanes have, in the past, been built with a stabilizing surface infront of the main surface instead of at the rear of it. In suchdesign the main surface (which is then the tail surface as well as theprincipal lifting surface) must be set at a less angle than the forwardstabilizing surface, in order to secure a longitudinal dihedral. Thedefect of such design lies in the fact that the main surface must havea certain angle to lift the weight--say 5 degrees. Then, in order tosecure a sufficiency of longitudinal stability, it is necessary toset the forward stabilizer at about 15 degrees. Such a large angle ofincidence results in a very poor lift-drift ratio (and consequentlygreat loss of efficiency), except at very low velocities compared withthe speed of modern aeroplanes. At the time such aeroplanes were builtvelocities were comparatively low, and this defect was; for that reason, not sufficiently appreciated. In the end it killed the "canard" or"tail-first" design. 

Aeroplanes of the Dunne and similar types possess no stabilizing surfacedistinct from the main surface, but they have a longitudinal dihedralwhich renders them stable. 

The main surface towards the wing-tips is given a decreasing angleof incidence and corresponding camber. The wing-tips then act aslongitudinal stabilizers. 

This design of aeroplane, while very interesting, has not proved verypracticable, owing to the following disadvantages: (1) The plan designis not, from a mechanical point of view, so sound as that of theordinary aeroplane surface, which is, in plan, a parallelogram. It is, then, necessary to make the strength of construction greater than wouldotherwise be the case. That means extra weight. (2) The plan of thesurface area is such that the aspect ratio is not so high as if thesurface was arranged with its leading edges at right angles to thedirection of motion. The lower the aspect ratio, then, the less thelift. This design, then, produces less lift for weight of surface thanwould the same surface if arranged as a parallelogram. (3) In order tosecure the longitudinal dihedral, the angle of incidence has to be verymuch decreased towards the wing-tips. Then, in order that the lift-driftratio may be preserved, there must be a corresponding decrease in thecamber. That calls for surface ribs of varying cambers, and results inan expensive and lengthy job for the builder. (4) In order to securedirectional stability, the surface is, in the centre, arranged to dipdown in the form of a V, pointing towards the direction of motion. Should the aeroplane turn off its course, then its momentum in thedirection of its first course causes it to move in a direction theresultant of the thrust and the momentum. It then moves in a more orless sideways attitude, which results in an air pressure upon oneside of the V, and which tends to turn the aeroplane back to its firstcourse. This arrangement of the surface results in a bad drift. Verticalsurfaces at the wing-tips may also be set at an angle producing the samestabilizing effect, but they also increase the drift. 

The gyroscopic action of a rotary engine will affect the longitudinalstability when an aeroplane is turned to right or left. In the case ofa Gnome engine, such gyroscopic action will tend to depress the nose ofthe aeroplane when it is turned to the left, and to elevate it whenit is turned to the right. In modern aeroplanes this tendency is notsufficiently important to bother about. In the old days of crudelydesigned and under-powered aeroplanes this gyroscopic action was verymarked, and led the majority of pilots to dislike turning an aeroplaneto the right, since, in doing so, there was some danger of "stalling. "

LATERAL STABILITY is far more difficult for the designer to securethan is longitudinal or directional stability. Some degree of lateralstability may be secured by means of the "lateral dihedral, " i. E. , theupward inclination of the surface towards its wing-tips thus:

Imagine the top V, illustrated opposite, to be the front view of asurface flying towards you. The horizontal equivalent (H. E. ) of the leftwing is the same as that of the right wing. Therefore, the lift of onewing is equal to the lift of the other, and the weight, being situatedalways in the centre, is balanced. 

If some movement of the air causes the surface to tilt sideways, as inthe lower illustration, then you will note that the H. E. Of the leftwing increases, and the H. E. Of the right wing decreases. The left wingthen, having the greatest lift, rises; and the surface assumes its firstand normal position. 

Unfortunately however, the righting effect is not proportional to thedifference between the right and left H. E. 's. 

In the case of A, the resultant direction of the reaction of both wingsis opposed to the direction of gravity or weight. The two forces R Rand gravity are then evenly balanced, and the surface is in a state ofequilibrium. 

In the case of B, you will note that the R R is not directly opposedto gravity. This results in the appearance of M, and so the resultantdirection of motion of the aeroplane is no longer directly forward, butis along a line the resultant of the thrust and M. In other words, it is, while flying forward, at the same time moving sideways in thedirection M. 

In moving sideways, the keel-surface receives, of course, a pressurefrom the air equal and opposite to M. Since such surface is greatest ineffect towards the tail, then the latter must be pushed sideways. Thatcauses the aeroplane to turn; and, the highest wing being on theoutside of the turn, it has a greater velocity than the lower wing. Thatproduces greater lift, and tends to tilt the aeroplane over still more. Such tilting tendency is, however, opposed by the difference in theH. E. 's of the two wings. 

It then follows that, for the lateral dihedral angle to be effective, such angle must be large enough to produce, when the aeroplane tilts, a difference in the H. E. 's of the two wings, which difference must besufficient to not only oppose the tilting tendency due to the aeroplaneturning, but sufficient to also force the aeroplane back to its originalposition of equilibrium. 

It is now, I hope, clear to the reader that the lateral dihedral isnot quite so effective as would appear at first sight. Some designers, indeed, prefer not to use it, since its effect is not very great, andsince it must be paid for in loss of H. E. And consequently loss of lift, thus decreasing the lift-drift ratio, i. E. , the efficiency. Also, it issometimes advanced that the lateral dihedral increases the "spill" ofair from the wing-tips and that this adversely affects the lift-driftratio. 

The disposition of the keel-surface affects the lateral stability. Itshould be, in effect, equally divided by the longitudinal turning axisof the aeroplane. If there is an excess of keel-surface above or belowsuch axis, then a side gust striking it will tend to turn the aeroplaneover sideways. 

The position of the centre of gravity affects lateral stability. Iftoo low, it produces a pendulum effect and causes the aeroplane to rollsideways. 

If too high, it acts as a stick balanced vertically would act. Ifdisturbed, it tends to travel to a position as far as possible from itsoriginal position. It would then tend, when moved, to turn the aeroplaneover sideways and into an upside-down position. 

From the point of view of lateral stability, the best position for thecentre of gravity is one a little below the centre of drift. 

Propeller torque affects lateral stability. An aeroplane tends to turnover sideways in the opposite direction to which the propeller revolves. 

This tendency is offset by increasing the angle of incidence (andconsequently the lift) of the side tending to fall; and it is alwaysadvisable, if practical considerations allow it, to also decrease theangle upon the other side. In that way it is not necessary to depart sofar from the normal angle of incidence at which the lift-drift ratio ishighest. 

Wash-in is the term applied to the increased angle. 

Wash-out is the term applied to the decreased angle. 

Both lateral and directional stability may be improved by washing outthe angle of incidence on both sides of the surface, thus:

The decreased angle decreases the drift and therefore the effect ofgusts upon the wing-tips which is just where they have the most effectupon the aeroplane, owing to the distance from the turning axis. 

The wash-out also renders the ailerons (lateral controlling services)more effective, as, in order to operate them, it is not then necessaryto give them such a large angle of incidence as would otherwise berequired. 

The less the angle of incidence of the ailerons, the better theirlift-drift ratio, i. E. , their efficiency. You will note that, while theaileron attached to the surface with washed-out angle is operated to thesame extent as the aileron illustrated above it, its angle of incidenceis considerably less. Its efficiency is therefore greater. 

The advantages of the wash-in must, of course be paid for in some lossof lift, as the lift decreases with the decreased angle. 

In order to secure all the above described advantages, a combination issometimes effected, thus:

BANKING. --An aeroplane turned off its course to right or left does notat once proceed along its new course. Its momentum in the direction ofits first course causes it to travel along a line the resultant of suchmomentum and the thrust. In other words, it more or less skids sidewaysand away from the centre of the turn. Its lifting surfaces do not thenmeet the air in their correct attitude, and the lift may fall to suchan extent as to become less than the weight, in which case the aeroplanemust fall. This bad effect is minimized by "banking, " i. E. , tilting theaeroplane sideways. The bottom of the lifting surface is in that wayopposed to the air through which it is moving in the direction of themomentum and receives an opposite air pressure. The rarefied area overthe top of the surface is rendered still more rare, and this, of course, assists the air pressure in opposing the momentum. 

The velocity of the "skid, " or sideways movement, is then only suchas is necessary to secure an air pressure equal and opposite to thecentrifugal force of the turn. 

The sharper the turn, the greater the effect of the centrifugal force, and therefore the steeper should be the "bank. " Experentia docet. 

The position of the centre of gravity affects banking. A low C. G. Willtend to swing outward from the centre of the turn, and will cause theaeroplane to bank--perhaps too much, in which case the pilot must remedymatters by operating the ailerons. 

A high C. G. Also tends to swing outward from the centre of the turn. Itwill tend to make the aeroplane bank the wrong way, and such effect mustbe remedied by means of the ailerons. 

The pleasantest machine from a banking point of view is one in whichthe C. G. Is a little below the centre of drift. It tends to bank theaeroplane the right way for the turn, and the pilot can, if necessary, perfect the bank by means of the ailerons. 

The disposition of the keel-surface affects banking. It should be, in effect, evenly divided by the longitudinal axis. An excess ofkeel-surface above the longitudinal axis will, when banking, receive anair pressure causing the aeroplane to bank, perhaps too much. An excessof keel-surface below the axis has the reverse effect. 

SIDE-SLIPPING. --This usually occurs as a result of over-banking. It isalways the result of the aeroplane tilting sideways and thus decreasingthe horizontal equivalent, and therefore the lift, of the surface. Anexcessive "bank, " or sideways tilt, results in the H. E. , and thereforethe lift, becoming less than the weight, when, of course, the aeroplanemust fall, i. E. , side-slip. 

When making a very sharp turn it is necessary to bank very steeplyindeed. If, at the same time, the longitudinal axis of the aeroplaneremains approximately horizontal, then there must be a fall, and thedirection of motion will be the resultant of the thrust and the fall asillustrated above in sketch A. The lifting surfaces and the controllingsurfaces are not then meeting the air in the correct attitude, with theresult that, in addition to falling, the aeroplane will probably becomequite unmanageable. 

The Pilot, however, prevents such a state of affairs from happening by"nosing-down, " i. E. , by operating the rudder to turn the nose of theaeroplane downward and towards the direction of motion as illustratedin sketch B. This results in the higher wing, which is on the outsideof the turn, travelling with greater velocity, and therefore securing agreater reaction than the lower wing, thus tending to tilt the aeroplaneover still more. The aeroplane is now almost upside-down, but itsattitude relative to the direction of motion is correct and thecontrolling surfaces are all of them working efficiently. The recoveryof a normal attitude relative to the Earth is then made as illustratedin sketch C. 

The Pilot must then learn to know just the angle of bank at which themargin of lift is lost, and, if a sharp turn necessitates banking beyondthat angle, he must "nose-down. "

In this matter of banking and nosing-down, and, indeed, regardingstability and control generally, the golden rule for all but veryexperienced pilots should be: Keep the aeroplane in such an attitudethat the air pressure is always directly in the pilot's face. Theaeroplane is then always engaging the air as designed to do so, andboth lifting and controlling surfaces are acting efficiently. The onlyexception to this rule is a vertical dive, and I think that is obviouslynot an attitude for any but very experienced pilots to hanker after. 

SPINNING. --This is the worst of all predicaments the pilot can findhimself in. Fortunately it rarely happens. 

It is due to the combination of (1) a very steep spiral descent of smallradius, and (2) insufficiency of keel-surface behind the vertical axis, or the jamming of the rudder end or elevator into a position by whichthe aeroplane is forced into an increasingly steep and small spiral. 

Owing to the small radius of such a spiral, the mass of the aeroplanemay gain a rotary momentum greater, in effect, than the air pressure ofthe keel-surface or controlling surfaces opposed to it; and, when oncesuch a condition occurs, it is difficult to see what can be done by thepilot to remedy it. The sensible pilot will not go beyond reasonablelimits of steepness and radius when executing spiral descents. 

GLIDING DESCENT WITHOUT PROPELLER THRUST. --All aeroplanes are, or shouldbe, designed to assume their gliding angle when the power and thrust iscut off. This relieves the pilot of work, worry, and danger should hefind himself in a fog or cloud. The Pilot, although he may not realizeit, maintains the correct attitude of the aeroplane by observing itsposition relative to the horizon. Flying into a fog or cloud the horizonis lost to view, and he must then rely upon his instruments--(1) thecompass for direction; (2) an inclinometer (arched spirit-level) mountedtransversely to the longitudinal axis, for lateral stability; and (3) aninclinometer mounted parallel to the longitudinal axis, or the airspeedindicator, which will indicate a nose-down position by increase in airspeed, and a tail-down position by decrease in air speed. 

The pilot is then under the necessity of watching three instrumentsand manipulating his three controls to keep the instruments indicatinglongitudinal, lateral, and directional stability. That is a feat beyondthe capacity of the ordinary man. If, however, by the simple movementof throttling down the power and thrust, he can be relieved of lookingafter the longitudinal stability, he then has only two instruments towatch. That is no small job in itself, but it is, at any rate, fairlypracticable. 

Aeroplanes are, then, designed, or should be, so that the centre ofgravity is slightly forward of centre of lift. The aeroplane is then, asa glider, nose-heavy--and the distance the C. G. Is placed in advanceof the C. L. Should be such as to ensure a gliding angle producing avelocity the same as the normal flying speed (for which the strength ofconstruction has been designed). 

In order that this nose-heavy tendency should not exist when the thrustis working and descent not required, the centre of thrust is placed alittle below the centre of drift or resistance, and thus tends to pullup the nose of the aeroplane. 

The distance the centre of thrust is placed below the centre of driftshould be such as to produce a force equal and opposite to that due tothe C. G. Being forward of the C. L. 

LOOPING AND UPSIDE DOWN FLYING. --If a loop is desired, it is best tothrottle the engine down at point A. The C. G. Being forward of the C. P. , then causes the aeroplane to nose-down, and assists the pilot in makinga reasonably small loop along the course C and in securing a quickrecovery. If the engine is not throttled down, then the aeroplane maybe expected to follow the course D, which results in a longer nose divethan in the case of the course C. 

A steady, gentle movement of the elevator is necessary. A jerky movementmay change the direction of motion so suddenly as to produce dangerousair stresses upon the surfaces, in which case there is a possibility ofcollapse. 

If an upside-down flight is desired, the engine may, or may not, bethrottled down at point A. If not throttled down, then the elevator mustbe operated to secure a course approximately in the direction B. If itis throttled down, then the course must be one of a steeper angle thanB, or there will be danger of stalling. 

Diagram p. 88. --This is not set at quite the correct angle. Path Bshould slope slightly downwards from Position A. 

CHAPTER III. RIGGING

In order to rig an aeroplane intelligently, and to maintain it in anefficient and safe condition, it is necessary to possess a knowledgeof the stresses it is called upon to endure, and the strains likely toappear. 

STRESS is the load or burden a body is called upon to bear. It isusually expressed by the result found by dividing the load by the numberof superficial square inches contained in the cross-sectional area ofthe body. 

Thus, if, for instance, the object illustrated above contains 4 squareinches of cross-sectional area, and the total load it is called upon toendure is 10 tons, the stress would be expressed as 2 1/2 tons. 

STRAIN is the deformation produced by stress. 

THE FACTOR OF SAFETY is usually expressed by the result found bydividing the stress at which it is known the body will collapse, bythe maximum stress it will be called upon to endure. For instance, if acontrol wire be called upon to endure a maximum stress of 2 cwts. , andthe known stress at which it will collapse is 10 cwts. , the factor ofsafety is then 5. 

[cwts. = centerweights = 100 pound units as in cent & century. Interestingly enough, this word only exists today in abbreviation form, probably of centreweights, but the dictionary entries, even from ahundred years ago do not list this as a word, but do list c. Or C. Asthe previous popular abbreviation as in Roman Numerals] The word listedis "hundredweight. Michael S. Hart, 1997]

COMPRESSION. --The simple stress of compression tends to produce acrushing strain. Example: the interplane and fuselage struts. 

TENSION. --The simple stress of tension tends to produce the strain ofelongation. Example: all the wires. 

BENDING. --The compound stress of bending is a combination of compressionand tension. 

The above sketch illustrates a straight piece of wood of which the top, centre, and bottom lines are of equal length. We will now imagine itbent to form a circle, thus:

The centre line is still the same length as before being bent; but thetop line, being farther from the centre of the circle, is now longerthan the centre line. That can be due only to the strain of elongationproduced by the stress of tension. The wood between the centre line andthe top line is then in tension; and the farther from the centre, thegreater the strain, and consequently the greater the tension. 

The bottom line, being nearest to the centre of the circle, is nowshorter than the centre line. That can be due only to the strain ofcrushing produced by the stress of compression. The wood between thecentre and bottom lines is then in compression; and the nearer thecentre of the circle, the greater the strain, and consequently thegreater the compression. 

It then follows that there is neither tension nor compression, i. E. , nostress, at the centre line, and that the wood immediately surrounding itis under considerably less stress than the wood farther away. This beingso, the wood in the centre may be hollowed out without unduly weakeningstruts and spars. In this way 25 to 33 per cent. Is saved in the weightof wood in an aeroplane. 

The strength of wood is in its fibres, which should, as far as possible, run without break from one end of a strut or spar to the other end. Apoint to remember is that the outside fibres, being farthest removedfrom the centre line, are doing by far the greatest work. 

SHEAR STRESS IS such that, when material collapses under it, one partslides over the other. Example: all the locking pins. 

Some of the bolts are also in shear or "sideways" stress, owing to lugsunder their heads and from which wires are taken. Such a wire, exertinga sideways pull upon a bolt, tries to break it in such a way as to makeone piece of the bolt slide over the other piece. 

TORSION. --This is a twisting stress compounded of compression, tension, and shear stresses. Example: the propeller shaft. 

NATURE OF WOOD UNDER STRESS. --Wood, for its weight, takes the stressof compression far better than any other stress. For instance: awalking-stick of less than 1 lb. In weight will, if kept perfectlystraight, probably stand up to a compression stress of a ton or morebefore crushing; whereas, if the same stick is put under a bendingstress, it will probably collapse to a stress of not more than about50 lb. That is a very great difference, and, since weight is of thegreatest importance, the design of an aeroplane is always such as to, as far as possible, keep the various wooden parts of its construction indirect compression. Weight being of such vital importance, and designersall trying to outdo each other in saving weight, it follows that thefactor of safety is rather low in an aeroplane. The parts in directcompression will, however, take the stresses safely provided thefollowing conditions are carefully observed. 

CONDITIONS TO BE OBSERVED:

1. All the spars and struts must be perfectly straight. 

The above sketch illustrates a section through an interplane strut. Ifthe strut is to be kept straight, i. E. , prevented from bending, thenthe stress of compression must be equally disposed about the centre ofstrength. If it is not straight, then there will be more compressionon one side of the centre of strength than on the other side. That isa step towards getting compression on one side and tension on the otherside, in which case it may be forced to take a bending stress for whichit is not designed. Even if it does not collapse it will, in effect, become shorter, and thus throw out of adjustment the gap and all thewires attached to the top and bottom of the strut, with the result thatthe flight efficiency of the aeroplane will be spoiled. 

The only exception to the above condition is what is known as theArch. For instance, in the case of the Maurice Farman, the spars of thecentre-section plane, which have to take the weight of the nacelle, arearched upwards. If this was not done, it is possible that roughlandings might result in the weight causing the spars to become slightlydistorted downwards. That would produce a dangerous bending stress, but, as long as the wood is arched, or, at any rate, kept from bendingdownwards, it will remain in direct compression and no danger canresult. 

2. Struts and spars must be symmetrical. By that I mean that thecross-sectional dimensions must be correct, as otherwise there will bebulging places on the outside, with the result that the stress will notbe evenly disposed about the centre of strength, and a bending stressmay be produced. 

3. Struts, spars, etc. , must be undamaged. Remember that, from what Ihave already explained about bending stresses, the outside fibres ofthe wood are doing by far the most work. If these get bruised or scored, then the strut or spar suffers in strength much more than one mightthink at first sight; and, if it ever gets a tendency to bend, it islikely to collapse at that point. 

4. The wood must have a good, clear grain with no cross-grain, knots, or shakes. Such blemishes produce weak places and, if a tendency to bendappears, then it may collapse at such a point. 

5. The struts, spars, etc. , must be properly bedded into their socketsor fittings. To begin with, they must be of good pushing or gentletapping fit. They must never be driven in with a heavy hammer. Thenagain, a strut must bed well down all over its cross-sectional areaas illustrated above; otherwise the stress of compression will not beevenly disposed about the centre of strength, and that may produce abending stress. The bottom of the strut or spar should be coveredwith some sort of paint, bedded into the socket or fitting, and thenwithdrawn to see if the paint has stuck all over the bed. 

6. The atmosphere is sometimes much damper than at other times, and thiscauses wood to expand and contract appreciably. This would not matterbut for the fact that it does not expand and contract uniformly, butbecomes unsymmetrical, i. E. , distorted. I have already explainedthe danger of that in condition 2. This should be minimized by WELLVARNISHING THE WOOD to keep the moisture out of it. 

FUNCTION OF INTERPLANE STRUTS. --These struts have to keep the liftingsurfaces or "planes" apart, but this is only part of their work. Theymust keep the planes apart, so that the latter are in their correctattitude. That is only so when the spars of the bottom plane areparallel with those of the top plane. Also, the chord of the top planemust be parallel with the chord of the bottom plane. If that is not so, then one plane will not have the same angle of incidence as the otherone. At first sight one might think that all that is necessary is to cutall the struts to be the same length, but that is not the case. 

Sometimes, as illustrated above, the rear spar is not so thick as themain spar, and it is then necessary to make up for that difference bymaking the rear struts correspondingly longer. If that is not done, thenthe top and bottom chords will not be parallel, and the top and bottomplanes will have different angles of incidence. Also, the sockets orfittings, or even the spars upon which they are placed, sometimesvary in thickness owing to faulty manufacture. This must be offset byaltering the length of the struts. The best way to proceed is to measurethe distance between the top and bottom spars by the side of each strut, and if that distance, or "gap" as it is called, is not as stated in theaeroplane's specifications, then make it correct by changing the lengthof the strut. This applies to both front and rear interplane struts. When measuring the gap, always be careful to measure from the centreof the spar, as it may be set at an angle, and the rear of it may beconsiderably lower than its front. 

BORING HOLES IN WOOD. --It should be a strict rule that no spar be usedwhich has an unnecessary hole in it. Before boring a hole, its positionshould be confirmed by whoever is in charge of the workshop. A bolt-holeshould be of a size to enable the bolt to be pushed in, or, at any rate, not more than gently tapped in. Bolts should not be hammered in, as thatmay split the spar. On the other hand, a bolt should not be slack in itshole, as, in such a case, it may work sideways and split the spar, notto speak of throwing out of adjustment the wires leading from the lug orsocket under the bolt-head. 

WASHERS. --Under the bolt-head, and also under the nut, a washer must beplaced--a very large washer compared with the size which would be usedin all-metal construction. This is to disperse the stress over a largearea; otherwise the washer may be pulled into the wood and weaken it, besides possibly throwing out of adjustment the wires attached to thebolt or the fitting it is holding to the spar. 

LOCKING. --Now as regards locking the bolts. If split pins are used, be sure to see that they are used in such a way that the nut cannotpossibly unscrew at all. The split pin should be passed through the boltas near as possible to the nut. It should not be passed through both nutand bolt. 

If it is locked by burring over the edge of the bolt, do not use a heavyhammer and try to spread the whole head of the bolt. That might damagethe woodwork inside the fabric-covered surface. Use a small, lighthammer, and gently tap round the edge of the bolt until it is burredover. 

TURNBUCKLES. --A turnbuckle is composed of a central barrel into each endof which is screwed an eye-bolt. Wires are taken from the eyes of theeye-bolt, and so, by turning the barrel, they can be adjusted to theirproper tension. Eye-bolts must be a good fit in the barrel; that is tosay, not slack and not very tight. Theoretically it is not necessaryto screw the eye-bolt into the barrel for a distance greater than thediameter of the bolt, but, in practice, it is better to screw it infor a considerably greater distance than that if a reasonable degree ofsafety is to be secured. 

Now about turning the barrel to secure the right adjustment. The barrellooks solid, but, as a matter of fact, it is hollow and much more frailthan it appears. For that reason it should not be turned by seizing itwith pliers, as that may distort it and spoil the bore within it. Thebest method is to pass a piece of wire through the hole in its centre, and to use that as a lever. When the correct adjustment has beensecured, the turnbuckle must be locked to prevent it from unscrewing. Itis quite possible to lock it in such a way as to allow it to unscrew aquarter or a half turn, and that would throw the wires out of the veryfine adjustment necessary. The proper way is to use the locking wire sothat its direction is such as to oppose the tendency of the barrel tounscrew, thus:

WIRES. --The following points should be carefully observed where wire isconcerned:

1. Quality. --It must not be too hard or too soft. An easy practical wayof learning to know the approximate quality of wire is as follows:

Take three pieces, all of the same gauge, and each about a foot inlength. One piece should be too soft, another too hard, and the thirdpiece of the right quality. Fix them in a vice, about an inch apart andin a vertical position, and with the light from a window shining uponthem. Burnish them if necessary, and you will see a band of lightreflected from each wire. 

Now bend the wires over as far as possible and away from the light. Where the soft wire is concerned, it will squash out at the bend, andthis will be indicated by the band of light, which will broaden at thatpoint. In the case of the wire which is too hard, the band of light willbroaden very little at the turn, but, if you look carefully, you willsee some little roughnesses of surface. In the case of the wire of theright quality, the band of light may broaden a very little at the turn, but there will be no roughnesses of surface. 

By making this experiment two or three times one can soon learn to knowreally bad wire from good, and also learn to know the strength of handnecessary to bend the right quality. 

2. It must not be damaged. That is to say, it must be unkinked, rustless, and unscored. 

3. Now as regards keeping wire in good condition. Where outside wiresare concerned, they should be kept WELL GREASED OR OILED, especiallywhere bent over at the ends. Internal bracing wires cannot be reachedfor the purpose of regreasing them, as they are inside fabric-coveredsurfaces. They should be prevented from rusting by being painted withan anti-rust mixture. Great care should be taken to see that the wireis perfectly clean and dry before being painted. A greasy finger-mark issufficient to stop the paint from sticking to the wire. In such a casethere will be a little space between the paint and the wire. Air mayenter there and cause the wire to rust. 

4. Tension of Wires. --The tension to which the wires are adjusted isof the greatest importance. All the wires should be of the same tensionwhen the aeroplane is supported in such a way as to throw no stress uponthem. If some wires are in greater tension than others, the aeroplanewill quickly become distorted and lose its efficiency. 

In order to secure the same tension of all wires, the aeroplane, whenbeing rigged, should be supported by packing underneath the lowersurfaces as well as by packing underneath the fuselage or nacelle. Inthis way the anti-lift wires are relieved of the weight, and there is nostress upon any of the wires. 

As a general rule the wires of an aeroplane are tensioned too much. Thetension should be sufficient to keep the framework rigid. Anythingmore than that lowers the factor of safety, throws various parts of theframework into undue compression, pulls the fittings into the wood, andwill, in the end, distort the whole framework of the aeroplane. 

Only experience will teach the rigger what tension to employ. Much maybe done by learning the construction of the various types of aeroplanes, the work the various parts do, and in cultivating a touch for tensioningwires by constantly handling them. 

5. Wires with no Opposition Wires. --In some few cases wires will befound which have no opposition wires pulling in the opposite direction. For instance, an auxiliary lift wire may run from the bottom of a strutto a spar in the top plane at a point between struts. In such a casegreat care should be taken not to tighten the wire beyond barely takingup the slack. 

Such a wire must be a little slack, or, as illustrated above, it willdistort the framework. That, in the example given, will spoil the camber(curvature) of the surface, and result in changing both the lift andthe drift at that part of the surface. Such a condition will cause theaeroplane to lose its directional stability and also to fly one wingdown. 

I cannot impress this matter of tension upon the reader too strongly. It is of the utmost importance. When this, and also accuracy in securingthe various adjustments, has been learned, one is on the way to becominga good rigger. 

6. Wire Loops. --Wire is often bent over at its end in the form of aloop, in order to connect with a turnbuckle or fitting. These loops, even when made as perfectly as possible, have a tendency to elongate, thus spoiling the adjustment of the wires Great care should be takento minimize this as far as possible. The rules to be observed are asfollows:

(a) The size of the loop should be as small as possible within reason. By that I mean it should not be so small as to create the possibility ofthe wire breaking. 

(b) The shape of the loop should be symmetrical. 

(c) It should have well-defined shoulders in order to prevent theferrule from slipping up. At the same time, a shoulder should not havean angular place. 

(d) When the loop is finished it should be undamaged, and it should notbe, as is often the case, badly scored. 

7. Stranded Wire Cable. --No splice should be served with twine until ithas been inspected by whoever is in charge of the workshop. The servingmay cover bad work. 

Should a strand become broken, then the cable should be replaced at onceby another one. 

Control cables have a way of wearing out and fraying wherever they passround pulleys. Every time an aeroplane comes down from flight the riggershould carefully examine the cables, especially where they pass roundpulleys. If he finds a strand broken, he should replace the cable. 

The ailerons' balance cable on the top of the top plane is oftenforgotten, since it is necessary to fetch a high pair of steps inorder to examine it. Don't slack this, or some gusty day the pilot mayunexpectedly find himself minus the aileron control. 

CONTROLLING SURFACES. --The greatest care should be exercised in riggingthe aileron, rudder, and elevator properly, for the pilot entirelydepends upon them in managing the aeroplane. 

The ailerons and elevator should be rigged so that, when the aeroplaneis in flight, they are in a fair true line with the surface in front andto which they are hinged. 

If the surface to which they are hinged is not a lifting surface, thenthey should be rigged to be in a fair true line with it as illustratedabove. 

If the controlling surface is, as illustrated, hinged to the back of alifting surface, then it should be rigged a little below the position itwould occupy if in a fair true line with the surface in front. This isbecause, in such a case, it is set at an angle of incidence. This anglewill, during flight, cause it to lift a little above the position inwhich it has been rigged. It is able to lift owing to a certain amountof slack in the control wire holding it--and one cannot adjust thecontrol wire to have no slack, because that would cause it to bindagainst the pulleys and make the operation of it too hard for the pilot. It is therefore necessary to rig it a little below the position it wouldoccupy if it was rigged in a fair true line with the surface in front. Remember that this only applies when it is hinged to a lifting surface. The greater the angle of incidence (and therefore the lift) of thesurface in front, then the more the controlling surface will have to berigged down. 

As a general rule it is safe to rig it down so that its trailing

edge is 1/2 to 3/4 inch below the position it would occupy if in a fairline with the surface in front; or about 1/2 inch down for every 18inches of chord of the controlling surface. 

When making these adjustments the pilot's control levers should be intheir neutral positions. It is not sufficient to lash them. They shouldbe rigidly blocked into position with wood packing. 

The surfaces must not be distorted in any way. If they are held true bybracing wires, then such wires must be carefully adjusted. If they aredistorted and there are no bracing wires with which to true them, thensome of the internal framework will probably have to be replaced. 

The controlling surfaces should never be adjusted with a view toaltering the stability of the aeroplane. Nothing can be accomplished inthat way. The only result will be to spoil the control of the aeroplane. 

FABRIC-COVERED SURFACES. --First of all make sure that there is nodistortion of spars or ribs, and that they are perfectly sound. Thenadjust the internal bracing wires so that the ribs are parallel to thedirection of flight. The ribs usually cause the fabric to make a ridgewhere they occur, and, if such ridge is not parallel to the direction offlight, it will produce excessive drift. As a rule the ribs are at rightangles to both main and rear spars. 

The tension of the internal bracing wires should be just sufficient togive rigidity to the framework. They should not be tensioned above thatunless the wires are, at their ends, bent to form loops. In that case alittle extra tension may be given to offset the probable elongation ofthe loops. 

The turnbuckles must now be generously greased, and served round withadhesive tape. The wires must be rendered perfectly dry and clean, and then painted with an anti-rust mixture. The woodwork must be wellvarnished. 

If it is necessary to bore holes in the spars for the purpose ofreceiving, for instance, socket bolts, then their places should bemarked before being bored and their positions confirmed by whoever is incharge of the workshop. All is now ready for the sail-maker to cover thesurface with fabric. 

ADJUSTMENT OF CONTROL CABLES. --The adjustment of the control cables isquite an art, and upon it will depend to a large degree the quick andeasy control of the aeroplane by the pilot. 

The method is as follows:

After having rigged the controlling surfaces, and as far as possiblesecured the correct adjustment of the control cables, then remove thepacking which has kept the control levers rigid. Then, sitting in thepilot's seat, move the control levers SMARTLY. Tension the controlcables so that when the levers are smartly moved there is no perceptiblesnatch or lag. Be careful not to tension the cables more than necessaryto take out the snatch. If tensioned too much they will (1) bind roundthe pulleys and result in hard work for the pilot; (2) throw dangerousstresses upon the controlling surfaces, which are of rather flimsyconstruction; and (3) cause the cables to fray round the pulleys quickerthan would otherwise be the case. 

Now, after having tensioned the cables sufficiently to take out thesnatch, place the levers in their neutral positions, and move them toand fro about 1/8 inch either side of such positions. If the adjustmentis correct, it should be possible to see the controlling surfaces move. If they do not move, then the control cables are too slack. 

FLYING POSITION. --Before rigging an aeroplane or making any adjustmentsit is necessary to place it in what is known as its "flying position. " Imay add that it would be better termed its "rigging position. "

In the case of an aeroplane fitted with a stationary engine this issecured by packing up the machine so that the engine foundations areperfectly horizontal both longitudinally and laterally. This positionis found by placing a straight-edge and a spirit-level across the enginefoundations (both longitudinally and laterally), and great care shouldbe taken to see that the bubble is exactly in the centre of the level. The slightest error will assume magnitude towards the extremities of theaeroplane. Great care should be taken to block up the aeroplane rigidly. In case it gets accidentally disturbed while the work is going on, itis well to constantly verify the flying position by running thestraight-edge and spirit-level over the engine foundations. Thestraight-edge should be carefully tested before being used, as, beinggenerally made of wood, it will not remain true long. Place it lightlyin a vice, and in such a position that a spirit-level on top showsthe bubble exactly in the centre. Now slowly move the level along thestraight-edge, and the bubble should remain exactly in the centre. Ifit does not do so, then the straight-edge is not true and must becorrected. THIS SHOULD NEVER BE OMITTED. 

In the case of aeroplanes fitted with engines of the rotary type, the"flying position" is some special attitude laid down in the aeroplane'sspecifications, and great care should be taken to secure accuracy. 

ANGLE OF INCIDENCE. --One method of finding the angle of incidence is asfollows:

First place the aeroplane in its flying position. The corner of thestraight-edge must be placed underneath and against the CENTRE of therear spar, and held in a horizontal position parallel to the ribs. Thisis secured by using a spirit-level. The set measurement will then befrom the top of the straight-edge to the centre of the bottom surfaceof the main spar, or it may be from the top of the straight-edge to thelowest part of the leading edge. Care should be taken to measure fromthe centre of the spar and to see that the bubble is exactly in thecentre of the level. Remember that all this will be useless if theaeroplane has not been placed accurately in its flying position. 

This method of finding the angle of incidence must be used under everypart of the lower surface where struts occur. It should not be usedbetween the struts, because, in such places, the spars may have taken aslight permanent set up or down; not, perhaps, sufficiently bad to makeany material difference to the flying of the machine, but quite badenough to throw out the angle of incidence, which cannot be corrected atsuch a place. 

If the angle is wrong, it should then be corrected as follows:

If it is too great, then the rear spar must be warped up until it isright, and this is done by slackening ALL the wires going to the top ofthe strut, and then tightening ALL the wires going to the bottom of thestrut. 

If the angle is too small, then slacken ALL the wires going to thebottom of the strut, and tighten ALL the wires going to the top of thestrut, until the correct adjustment is secured. 

Never attempt to adjust the angle by warping the main spar. 

The set measurement, which is of course stated in the aeroplane'sspecifications, should be accurate to 1/16 inch. 

LATERAL DIHEDRAL ANGLE. --One method of securing this is as follows, and this method will, at the same time, secure the correct angle ofincidence:

The strings, drawn very tight, must be taken over both the main and rearspars of the top surface. They must run between points on the spars justinside the outer struts. The set measurement (which should be accurateto 1/16 inch or less) is then from the strings down to four points onthe main and rear spars of the centre-section surface. These pointsshould be just inside the four centre-section struts; that is to say, as far as possible away from the centre of the centre-section. Donot attempt to take the set measurement near the centre of thecentre-section. 

The strings should be as tight as possible, and, if it can be arranged, the best way to accomplish that is as shown in the above illustration, i. E. , by weighting the strings down to the spars by means of weightsand tying each end of the strings to a strut. This will give a tight andmotionless string. 

However carefully the above adjustment is made, there is sure to be someslight error. This is of no great importance, provided it is dividedequally between the left- and right-hand wings. In order to make sure ofthis, certain check measurements should be taken as follows:

Each bay must be diagonally measured, and such measurements must be thesame to within 1/16 inch on each side of the aeroplane. As a rule suchdiagonal measurements are taken from the bottom socket of one strut tothe top socket of another strut, but this is bad practice, because ofpossible inaccuracies due to faulty manufacture. 

The points between which the diagonal measurements are taken should beat fixed distances from the butts of the spars, such distances being thesame on each side of the aeroplane, thus:

It would be better to use the centre line of the aeroplane rather thanthe butts of the spars. It is not practicable to do so, however, as thecentre line probably runs through the petrol tanks, etc. 

THE DIHEDRAL BOARD. --Another method of securing the dihedral angle, andalso the angle of incidence, is by means of the dihedral board. It isa light handy thing to use, but leads to many errors, and should not beused unless necessary. The reasons are as follows:

The dihedral board is probably not true. If it must be used, then itshould be very carefully tested for truth before-hand. Another reasonagainst its use is that it has to be placed on the spars in a positionbetween the struts, and that is just where the spars may have a littlepermanent set up or down, or some inaccuracy of surface which will, ofcourse, throw out the accuracy of the adjustment. The method of using itis as follows:

The board is cut to the same angle as that specified for the upwardinclination of the surface towards its wing-tips. It is placed on thespar as indicated above, and it is provided with two short legs to raiseit above the flanges of the ribs (which cross over the spars), as theymay vary in depth. A spirit-level is then placed on the board, and thewires must be adjusted to give the surface such an inclination as toresult in the bubble being in the centre of the level. This operationmust be performed in respect of each bay both front and rear. The baysmust then be diagonally measured as already explained. 

YET ANOTHER METHOD of finding the dihedral angle, and at the same timethe angle of incidence, is as follows:

A horizontal line is taken from underneath the butt of each spar, andthe set measurement is either the angle it makes with the spar, ora fixed measurement from the line to the spar taken at a specifieddistance from the butt. This operation must be performed in respect ofboth main and rear spars, and all the bays must be measured diagonallyafterwards. 

Whichever method is used, be sure that after the job is done the sparsare perfectly straight. 

STAGGER. --The stagger is the distance the top surface is in advance ofthe bottom surface when the aeroplane is in flying position. The setmeasurement is obtained as follows:

Plumb-lines must be dropped over the leading edge of the top surfacewherever struts occur, and also near the fuselage. The set measurementis taken from the front of the lower leading edge to the plumb-lines. Itmakes a difference whether the measurement is taken along a horizontalline (which can be found by using a straight-edge and a spirit-level)or along a projection of the chord. The line along which the measurementshould be taken is laid down in the aeroplane's specifications. 

If a mistake is made and the measurement taken along the wrong line, itmay result in a difference of perhaps 1/4 will, in flight, be nose-heavyor tail-heavy. 

After the adjustments of the angles of incidence, dihedral, and staggerhave been secured, it is as well to confirm all of them, as, in makingthe last adjustment, the first one may have been spoiled. 

OVER-ALL ADJUSTMENTS. --The following over-all check measurements shouldnow be taken. 

The straight lines AC and BC should be equal to within 1/8 inch. Thepoint C is the centre of the propeller, or, in the case of a "pusher"aeroplane, the centre of the nacelle. The points A and B are marked onthe main spar, and must in each case be the same distance from the buttof the spar. The rigger should not attempt to make A and B merely thesockets of the outer struts, as they may not have been placed quiteaccurately by the manufacturer. The lines AC and BC must be takenfrom both top and bottom spars--two measurements on each side of theaeroplane. 

The two measurements FD and FE should be equal to within 1/8 inch. F isthe centre of the fuselage or rudder-post. D and E are points marked onboth top and bottom rear spars, and each must be the same fixeddistance from the butt of the spar. Two measurements on each side of theaeroplane. 

If these over-all measurements are not correct, then it is probably dueto some of the drift or anti-drift wires being too tight or too slack. It may possibly be due to the fuselage being out of truth, but of coursethe rigger should have made quite sure that the fuselage was true beforerigging the rest of the machine. Again, it may be due to the internalbracing wires within the lifting surfaces not being accurately adjusted, but of course this should have been seen to before covering the surfaceswith fabric. 

FUSELAGE. --The method of truing the fuselage is laid down in theaeroplane's specifications. After it has been adjusted according to thespecified directions, it should then be arranged on trestles in sucha way as to make about three-quarters of it towards the tail stick outunsupported. In this way it will assume a condition as near aspossible to flying conditions, and when it is in this position the setmeasurements should be confirmed. If this is not done it may be out oftruth, but perhaps appear all right when supported by trestles at bothends, as, in such case, its weight may keep it true as long as it isresting upon the trestles. 

THE TAIL-PLANE (EMPENNAGE). --The exact angle of incidence of thetail-plane is laid down in the aeroplane's specifications. It isnecessary to make sure that the spars are horizontal when the aeroplaneis in flying position and the tail unsupported as explained above underthe heading of Fuselage. If the spars are tapered, then make sure thattheir centre lines are horizontal. 

UNDERCARRIAGE. --The undercarriage must be very carefully aligned as laiddown in the specifications. 

1. The aeroplane must be placed in its flying position and sufficientlyhigh to ensure the wheels being off the ground when rigged. When in thisposition the axle must be horizontal and the bracing wires adjusted tosecure the various set measurements stated in the specifications. 

2. Make sure that the struts bed well down into their sockets. 

3. Make sure that the shock absorbers are of equal tension. In the caseof rubber shock absorbers, both the number of turns and the lengths mustbe equal. 

HOW TO DIAGNOSE FAULTS IN FLIGHT, STABILITY, AND CONTROL. 

DIRECTIONAL STABILITY will be badly affected if there is more drift(i. E. , resistance) on one side of the aeroplane than there is on theother side. The aeroplane will tend to turn towards the side having themost drift. This may be caused as follows:

1. The angle of incidence of the main surface or the tail surface maybe wrong. The greater the angle of incidence, the greater the drift. Theless the angle, the less the drift. 

2. If the alignment of the fuselage, fin in front of the rudder, thestruts or stream-line wires, or, in the case of the Maurice Farman, thefront outriggers, are not absolutely correct--that is to say, if theyare turned a little to the left or to the right instead of being in linewith the direction of flight--then they will act as a rudder and causethe aeroplane to turn off its course. 

3. If any part of the surface is distorted, it will cause the aeroplaneto turn off its course. The surface is cambered, i. E. , curved, to passthrough the air with the least possible drift. If, owing perhaps to theleading edge, spars, or trailing edge becoming bent, the curvature isspoiled, that will result in changing the amount of drift on one side ofthe aeroplane, which will then have a tendency to turn off its course. 

LATERAL INSTABILITY (FLYING ONE WING DOWN). --The only possible reasonfor such a condition is a difference in the lifts of right and leftwings. That may be caused as follows:

1. The angle of incidence may be wrong. If it is too great, it willproduce more lift than on the other side of the aeroplane; and if toosmall, it will produce less lift than on the other side--the resultbeing that, in either case, the aeroplane will try to fly one wing down. 

2. Distorted Surfaces. --If some part of the surface is distorted, thenits camber is spoiled, and the lift will not be the same on both sidesof the aeroplane, and that, of course, will cause it to fly one wingdown. 

LONGITUDINAL INSTABILITY may be due to the following reasons:

1. The stagger may be wrong. The top surface may have drifted back alittle owing to some of the wires, probably the incidence wires, havingelongated their loops or having pulled the fittings into the wood. Ifthe top surface is not staggered forward to the correct degree, thenconsequently the whole of its lift is too far back, and it will thenhave a tendency to lift up the tail of the machine too much. Theaeroplane would then be said to be "nose-heavy. "

A 1/4-inch area in the stagger will make a very considerable differenceto the longitudinal stability. 

2. If the angle of incidence of the main surface is not right, it willhave a bad effect, especially in the case of an aeroplane with a liftingtail-plane. 

If the angle is too great, it will produce an excess of lift, and thatmay lift up the nose of the aeroplane and result in a tendency to fly"tail-down. " If the angle is too small, it will produce a decreasedlift, and the aeroplane may have a tendency to fly "nose-down. "

3. The fuselage may have become warped upward or downward, thus givingthe tail-plane an incorrect angle of incidence. If it has too muchangle, it will lift too much, and the aeroplane will be "nose-heavy. " Ifit has too little angle, then it will not lift enough, and the aeroplanewill be "tail-heavy. "

4. (The least likely reason. ) The tail-plane may be mounted uponthe fuselage at a wrong angle of incidence, in which case it mustbe corrected. If nose-heavy, it should be given a smaller angle ofincidence. If tail-heavy, it should be given a larger angle; butcare should be taken not to give it too great an angle, because thelongitudinal stability entirely depends upon the tail-plane being set ata much smaller angle of incidence than is the main surface, and ifthat difference is decreased too much, the aeroplane will becomeuncontrollable longitudinally. Sometimes the tail-plane is mounted onthe aeroplane at the same angle as the main surface, but it actuallyengages the air at a lesser angle, owing to the air being deflecteddownwards by the main surface. There is then, in effect, a longitudinaldihedral as explained and illustrated in Chapter I. 

CLIMBS BADLY. --Such a condition is, apart from engine or propellertrouble, probably due to (1) distorted surfaces, or (2) too small anangle of incidence. 

FLIGHT SPEED POOR. --Such a condition is, apart from engine or propellertrouble, probably due to (1) distorted surfaces, (2) too great anangle of incidence, or (3) dirt or mud, and consequently excessiveskin-friction. 

INEFFICIENT CONTROL is probably due to (1) wrong setting of controlsurfaces, (2) distortion of control surfaces, or (3) control cablesbeing badly tensioned. 

WILL NOT TAXI STRAIGHT. --If the aeroplane is uncontrollable on theground, it is probably due to (1) alignment of undercarriage beingwrong, or (2) unequal tension of shock absorbers. 

CHAPTER IV. THE PROPELLER, OR "AIR-SCREW"

The sole object of the propeller is to translate the power of the engineinto thrust. 

The propeller screws through the air, and its blades, being set at anangle inclined to the direction of motion, secure a reaction, as in thecase of the aeroplane's lifting surface. 

This reaction may be conveniently divided into two component parts orvalues, namely, Thrust and Drift. 

The Thrust is opposed to the Drift of the aeroplane, and must be equaland opposite to it at flying speed. If it falls off in power, then theflying speed must decrease to a velocity, at which the aeroplane driftequals the decreased thrust. 

The Drift of the propeller may be conveniently divided into thefollowing component values:

Active Drift, produced by the useful thrusting part of the propeller. 

Passive Drift, produced by all the rest of the propeller, i. E. , by itsdetrimental surface. 

Skin Friction, produced by the friction of the air with roughnesses ofsurface. 

Eddies attending the movement of the air caused by the action of thepropeller. 

Cavitation (very marked at excessive speed of revolution). A tendency ofthe propeller to produce a cavity or semi-vacuum in which it revolves, the thrust decreasing with increase of speed and cavitation. 

THRUST-DRIFT RATIO. --The proportion of thrust to drift is of paramountimportance, for it expresses the efficiency of the propeller. It isaffected by the following factors: Speed of Revolution. --The greater thespeed, the greater the proportion of drift to thrust. This is due tothe increase with speed of the passive drift, which carries with it noincrease in thrust. For this reason propellers are often geared down torevolve at a lower speed than that of the engine. 

Angle of Incidence. --The same reasons as in the case of the aeroplanesurface. 

Surface Area. --Ditto. 

Aspect Ratio. --Ditto. 

Camber. --Ditto. 

In addition to the above factors there are, when it comes to actuallydesigning a propeller, mechanical difficulties to consider. Forinstance, the blades must be of a certain strength and consequentthickness. That, in itself, limits the aspect ratio, for it willnecessitate a chord long enough in proportion to the thickness to makea good camber possible. Again, the diameter of the propeller must belimited, having regard to the fact that greater diameters than thoseused to-day would not only result in excessive weight of construction, but would also necessitate a very high undercarriage to keep thepropeller off the ground, and such undercarriage would not only produceexcessive drift, but would also tend to make the aeroplane stand onits nose when alighting. The latter difficulty cannot be overcome bymounting the propeller higher, as the centre of its thrust must beapproximately coincident with the centre of aeroplane drift. 

MAINTENANCE OF EFFICIENCY. 

The following conditions must be observed:

1. PITCH ANGLE. --The angle, at any given point on the propeller, atwhich the blade is set is known as the pitch angle, and it must becorrect to half a degree if reasonable efficiency is to be maintained. 

This angle secures the "pitch, " which is the distance the propelleradvances during one revolution, supposing the air to be solid. The air, as a matter of fact, gives back to the thrust of the blades just as thepebbles slip back as one ascends a shingle beach. Such "give-back" isknown as Slip. If a propeller has a pitch of, say, 10 feet, but actuallyadvances, say, only 8 feet owing to slip, then it will be said topossess 20 per cent. Slip. 

Thus, the pitch must equal the flying speed of the aeroplane plusthe slip of the propeller. For example, let us find the pitch of apropeller, given the following conditions:

 Flying speed. .. .. .. .. .. .. . 70 miles per hour. Propeller revolutions. .. .. 1, 200 per minute. Slip. .. .. .. .. .. .. .. .. .. .. . 15 per cent. 

First find the distance in feet the aeroplane will travel forward in oneminute. That is--

 369, 600 feet (70 miles) ------------------------ = 6, 160 feet per minute. 60 " (minutes)

Now divide the feet per minute by the propeller revolutions per minute, add 15 per cent. For the slip, and the result will be the propellerpitch:

 6, 160 ----- + 15 per cent. = 5 feet 1 3/5 inches. 1, 200

In order to secure a constant pitch from root to tip of blade, the pitchangle decreases towards the tip. This is necessary, since the end of theblade travels faster than its root, and yet must advance forward at thesame speed as the rest of the propeller. For example, two men ascendinga hill. One prefers to walk fast and the other slowly, but they wish toarrive at the top of the hill simultaneously. Then the fast walkermust travel a farther distance than the slow one, and his angle of path(pitch angle) must be smaller than the angle of path taken by the slowwalker. Their pitch angles are different, but their pitch (in this casealtitude reached in a given time) is the same. 

In order to test the pitch angle, the propeller must be mounted upona shaft at right angles to a beam the face of which must be perfectlylevel, thus:

First select a point on the blade at some distance (say about 2 feet)from the centre of the propeller. At that point find, by means of aprotractor, the angle a projection of the chord makes with the face ofthe beam. That angle is the pitch angle of the blade at that point. 

Now lay out the angle on paper, thus:

The line above and parallel to the circumference line must be placedin a position making the distance between the two lines equal to thespecified pitch, which is, or should be, marked upon the boss of thepropeller. 

Now find the circumference of the propeller where the pitch angle isbeing tested. For example, if that place is 2 feet radius from thecentre, then the circumference will be 2 feet X 2 = 4 feet diameter, which, if multiplied by 3. 1416 = 15. 56 feet circumference. 

Now mark off the circumference distance, which is represented above byA-B, and reduce it in scale for convenience. 

The distance a vertical line makes between B and the chord dine isthe pitch at the point where the angle is being tested, and it shouldcoincide with the specified pitch. You will note, from the aboveillustration, that the actual pitch line should meet the junction of thechord line and top line. 

The propeller should be tested at several points, about a foot apart, oneach blade; and the diagram, provided the propeller is not faulty, willthen look like this:

At each point tested the actual pitch coincides with the specifiedpitch: a satisfactory condition. 

A faulty propeller will produce a diagram something like this:

At every point tested the pitch angle is wrong, for nowhere does theactual pitch coincide with the specified pitch. Angles A, C, and D, aretoo large, and B is too small. The angle should be correct to half adegree if reasonable efficiency is to be maintained. 

A fault in the pitch angle may be due to (1) faulty manufacture, (2) distortion, or (3) the shaft hole through the boss being out ofposition. 

2. STRAIGHTNESS. --To test for straightness the propeller must be mountedupon a shaft. Now bring the tip of one blade round to graze some fixedobject. Mark the point it grazes. Now bring the other tip round, and itshould come within 1/8 inch of the mark. If it does not do so, it is dueto (1) faulty manufacture, (2) distortion, or (3) to the hole throughthe boss being out of position. 

3. LENGTH. --The blades should be of equal length to inch. 

4. BALANCE. --The usual method of testing a propeller for balance is asfollows: Mount it upon a shaft, which must be on ball-bearings. Placethe propeller in a horizontal position, and it should remain in thatposition. If a weight of a trifle over an ounce placed in a bolt-hole onone side of the boss fails to disturb the balance, then the propeller isusually regarded as unfit for use. 

The above method is rather futile, as it does not test for the balanceof centrifugal force, which comes into play as soon as the propellerrevolves. It can be tested as follows:

The propeller must be in a horizontal position, and then weighed atfixed points, such as A, B, C, D, E, and F, and the weights noted. Thepoints A, B, and C must, of course, be at the same fixed distances fromthe centre of the propeller as the points D, E, and F. Now reverse thepropeller and weigh at each point again. Note the results. The firstseries of weights should correspond to the second series, thus:

 Weight A should equal weight F. " B " " " E. " C " " " D. 

There is no standard practice as to the degree of error permissible, butif there are any appreciable differences the propeller is unfit for use. 

5. SURFACE AREA. --The surface area of the blades should be equal. Testwith callipers thus:

The points between which the distances are taken must, of course, be atthe same distance from the centre in the case of each blade. 

There is no standard practice as to the degree of error permissible. If, however, there is an error of over 1/8 inch, the propeller is reallyunfit for use. 

6. CAMBER. --The camber (curvature) of the blades should be (1) equal, (2) decrease evenly towards the tips of the blades, and (3) the greatestdepth of the curve should, at any point of the blade, be approximatelyat the same percentage of the chord from the leading edge as at otherpoints. 

It is difficult to test the top camber without a set of templates, buta fairly accurate idea of the concave camber can be secured by slowlypassing a straight-edge along the blade, thus:

The camber can now be easily seen, and as the straight-edge is passedalong the blade, the observer should look for any irregularities of thecurvature, which should gradually and evenly decrease towards the tip ofthe blade. 

7. THE JOINTS. --The usual method for testing the glued joints is byrevolving the propeller at greater speed than it will be called upon tomake during flight, and then carefully examining the joints to see ifthey have opened. It is not likely, however, that the reader will havethe opportunity of making this test. He should, however, examine all thejoints very carefully, trying by hand to see if they are quite sound. Suspect a propeller of which the joints appear to hold any thickness ofglue. Sometimes the joints in the boss open a little, but this is notdangerous unless they extend to the blades, as the bolts will hold thelaminations together. 

8. CONDITION OF SURFACE. --The surface should be very smooth, especiallytowards the tips of the blades. Some propeller tips have a speed ofover 30, 000 feet a minute, and any roughness will produce a bad drift orresistance and lower the efficiency. 

9. MOUNTING. --Great care should be taken to see that the propelleris mounted quite straight on its shaft. Test in the same way as forstraightness. If it is not straight, it is possibly due to some of thepropeller bolts being too slack or to others having been pulled up tootightly. 

FLUTTER. --Propeller "flutter, " or vibration, may be due to faulty pitchangle, balance, camber, or surface area. It causes a condition sometimesmistaken for engine trouble, and one which may easily lead to thecollapse of the propeller. 

CARE OF PROPELLERS. --The care of propellers is of the greatestimportance, as they become distorted very easily. 

1. Do not store them in a very damp or a very dry place. 

2. Do not store them where the sun will shine upon them. 

3. Never leave them long in a horizontal position or leaning up againsta wall. 

4. They should be hung on horizontal pegs, and the position of thepropellers should be vertical. 

If the points I have impressed upon you in these notes are not attendedto, you may be sure of the following results:

1. Lack of efficiency, resulting in less aeroplane speed and climb thanwould otherwise be the case. 

2. Propeller "flutter" and possible collapse. 

3. A bad stress upon the propeller shaft and its bearings. 

TRACTOR. --A propeller mounted in front of the main surface. 

PUSHER. --A propeller mounted behind the main surface. 

FOUR-BLADED PROPELLERS. --Four-bladed propellers are suitable only whenthe pitch is comparatively large. 

For a given pitch, and having regard to "interference, " they are not soefficient as two-bladed propellers. 

The smaller the pitch, the less the "gap, " i. E. , the distance, measuredin the direction of the thrust, between the spiral courses of theblades. 

If the gap is too small, then the following blade will engage airwhich the preceding blade has put into motion, with the result that thefollowing blade will not secure as good a reaction as would otherwise bethe case. It is very much the same as in the case of the aeroplane gap. 

For a given pitch, the gap of a four-bladed propeller is only halfthat of a two-bladed one. Therefore the four-bladed propeller is onlysuitable for large pitch, as such pitch produces spirals with a largegap, thus offsetting the decrease in gap caused by the numerous blades. 

The greater the speed of rotation, the less the pitch for a givenaeroplane speed. Then, in order to secure a large pitch and consequentlya good gap, the four-bladed propeller is usually geared to rotate at alower speed than would be the case if directly attached to the enginecrank-shaft. 

CHAPTER V. MAINTENANCE

CLEANLINESS. --The fabric must be kept clean and free from oil, as thatwill rot it. To take out dirt or oily patches, try acetone. If that willnot remedy matters, then try petrol, but use it sparingly, as otherwiseit will take off an unnecessary amount of dope. If that will not removethe dirt, then hot water and soap will do so, but, in that case, besure to use soap having no alkali in it, as otherwise it may injure thefabric. Use the water sparingly, or it may get inside the planes andrust the internal bracing wires, or cause some of the wooden frameworkto swell. 

The wheels of the undercarriage have a way of throwing up mud on tothe lower surface. This should, if possible, be taken off while wet. Itshould never be scraped off when dry, as that may injure the fabric. Ifdry, then it should be moistened before being removed. 

Measures should be taken to prevent dirt from collecting upon anypart of the aeroplane, as, otherwise, excessive skin-friction will beproduced with resultant loss of flight speed. The wires, being greasy, collect dirt very easily. 

CONTROL CABLES. --After every flight the rigger should pass his hand overthe control cables and carefully examine them near pulleys. Removal ofgrease may be necessary to make a close inspection possible. If only onestrand is broken the wire should be replaced. Do not forget the aileronbalance wire on the top surface. 

Once a day try the tension of the control cables by smartly moving thecontrol levers about as explained elsewhere. 

WIRES. --All the wires should be kept well greased or oiled, and in thecorrect tension. When examining the wires, it is necessary to place theaeroplane on level ground, as otherwise it may be twisted, thus throwingsome wires into undue tension and slackening others. The best way, ifthere is time, is to pack the machine up into its "flying position. "

If you see a slack wire, do not jump to the conclusion that it mustbe tensioned. Perhaps its opposition wire is too tight, in which caseslacken it, and possibly you will find that will tighten the slack wire. 

Carefully examine all wires and their connections near the propeller, and be sure that they are snaked round with safety wire, so that thelatter may keep them out of the way of the propeller if they comeadrift. 

The wires inside the fuselage should be cleaned and regreased about oncea fortnight. 

STRUTS AND SOCKETS. --These should be carefully examined to see if anysplitting has occurred. 

DISTORTION. --Carefully examine all surfaces, including the controllingsurfaces, to see whether any distortion has occurred. If distortion canbe corrected by the adjustment of wires, well and good; but if not, thensome of the internal framework probably requires replacement. 

ADJUSTMENTS. --Verify the angles of incidence; dihedral, and stagger, andthe rigging position of the controlling-surfaces, as often as possible. 

UNDERCARRIAGE. --Constantly examine the alignment and fittings of theundercarriage, and the condition of tyres and shock absorbers. Thelatter, when made of rubber, wear quickest underneath. Inspect axles andskids to see if there are any signs of them becoming bent. The wheelsshould be taken off occasionally and greased. 

LOCKING ARRANGEMENTS. --Constantly inspect the locking arrangements ofturnbuckles, bolts, etc. Pay particular attention to the control cableconnections, and to all moving parts in respect of the controls. 

LUBRICATION. --Keep all moving parts, such as pulleys, control levers, and hinges of controlling surfaces, well greased. 

SPECIAL INSPECTION. --Apart from constantly examining the aeroplane withreference to the above points I have made, I think that, in the case ofan aeroplane in constant use it is an excellent thing to make a specialinspection of every part, say once a week. This will take from two tothree hours, according to the type of aeroplane. In order to carry itout methodically, the rigger should have a list of every part down tothe smallest split-pin. He can then check the parts as he examines them, and nothing will be passed over. This, I know from experience, greatlyincreases the confidence of the pilot, and tends to produce good work inthe air. 

WINDY WEATHER. --The aeroplane, when on the ground, should face thewind; and it is advisable to lash the control lever fast, so that thecontrolling surfaces may not be blown about and possibly damaged. 

"VETTING" BY EYE. --This should be practiced at every opportunity, and, if persevered in, it is possible to become quite expert in diagnosing byeye faults in flight efficiency, stability and control. 

The aeroplane should be standing upon level ground, or, better thanthat, packed up into its "flying position. "

Now stand in front of it and line up the leading edge with the mainspar, rear spar, and trailing edge. Their shadows can usually be seenthrough the fabric. Allowance must, of course, be made for wash-in andwash-out; otherwise, the parts I have specified should be parallel witheach other. 

Now line up the centre part of the main-plane with the tail-plane. Thelatter should be horizontal. 

Next, sight each interplane front strut with its rear strut. They shouldbe parallel. 

Then, standing on one side of the aeroplane, sight all the front struts. The one nearest to you should cover all the others. This applies to therear struts also. 

Look for distortion of leading edges, main and rear spars, trailingedges, tail-plane and controlling surfaces. 

This sort of thing, if practiced constantly, will not only develop anexpert eye for diagnosis of faults, but will also greatly assist inimpressing upon the memory the characteristics and possible troubles ofthe various types of aeroplanes. 

MISHANDLING OF THE GROUND. --This is the cause of a lot of unnecessarydamage. The golden rule to observe is: PRODUCE NO BENDING STRESSES. 

Nearly all the wood in an aeroplane is designed to take merely thestress of direct compression, and it cannot be bent safely. Therefore, in packing an aeroplane up from the ground, or in pulling or pushing itabout, be careful to stress it in such a way as to produce, as faras possible, only direct compression stresses. For instance, if it isnecessary to support the lifting surface, then the packing should bearranged to come directly under the struts so that they may take thestress in the form of compression for which they are designed. Suchsupports should be covered with soft packing in order to prevent thefabric from becoming damaged. 

When pulling an aeroplane along, if possible, pull from the top of theundercarriage struts. If necessary to pull from elsewhere, then do so bygrasping the interplane struts as low down as possible. 

Never lay fabric-covered parts upon a concrete floor. Any slightmovement will cause the fabric to scrape over the floor with resultantdamage. 

Struts, spars, etc. , should never be left about the floor, as in suchposition they are likely to become scored. I have already explained theimportance of protecting the outside fibres of the wood. Rememberalso that wood becomes distorted easily. This particularly applies tointerplane struts. If there are no proper racks to stand them in, thenthe best plan is to lean them up against the wall in as near a verticalposition as possible. 

TIME. --Learn to know the time necessary to complete any of the variousrigging jobs. This is really important. Ignorance of this will leadto bitter disappointments in civil life; and, where Service flyingis concerned, it will, to say the least of it, earn unpopularity withsenior officers, and fail to develop respect and good work where men areconcerned. 

THE AEROPLANE SHED. --This should be kept as clean and orderly aspossible. A clean, smart shed produces briskness, energy, and pride ofwork. A dirty, disorderly shed nearly always produces slackness and poorquality of work, lost tools and mislaid material. 

GLOSSARY

Aeronautics--The science of aerial navigation. 

Aerofoil--A rigid structure, of large superficial area relative to itsthickness, designed to obtain, when driven through the air at anangle inclined to the direction of motion, a reaction from the airapproximately at right angles to its surface. Always cambered whenintended to secure a reaction in one direction only. As the term"aerofoil" is hardly ever used in practical aeronautics, I have, throughout this book, used the term SURFACE, which, while academicallyincorrect, since it does not indicate thickness, is a term usuallyused to describe the cambered lifting surfaces, i. E. , the "planes" or"wings, " and the stabilizers and the controlling aerofoils. 

Aerodrome--The name usually applied to a ground used for the practiceof aviation. It really means "flying machine, " but is never used in thatsense nowadays. 

Aeroplane--A power-driven aerofoil with stabilizing and controllingsurfaces. 

Acceleration--The rate of change of velocity. 

Angle of Incidence--The angle at which the "neutral lift line" of asurface attacks the air. 

Angle of Incidence, Rigger's--The angle the chord of a surface makeswith a line parallel to the axis of the propeller. 

Angle of Incidence, Maximum--The greatest angle of incidence at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained. 

Angle of Incidence, Minimum--The smallest angle of incidence at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained. 

Angle of Incidence, Best Climbing--That angle of incidence at which anaeroplane ascends quickest. An angle approximately halfway between themaximum and optimum angles. 

Angle of Incidence, Optimum--The angle of incidence at which thelift-drift ratio is the highest. 

Angle, Gliding--The angle between the horizontal and the path alongwhich an aeroplane at normal flying speed, but not under engine power, descends in still air. 

Angle, Dihedral--The angle between two planes. 

Angle, Lateral Dihedral--The lifting surface of an aeroplane is said tobe at a lateral dihedral angle when it is inclined upward towards itswing-tips. 

Angle, Longitudinal Dihedral--The main surface and tail surface are saidto be at a longitudinal dihedral angle when the projections of theirneutral lift lines meet and produce an angle above them. 

Angle, Rigger's Longitudinal Dihedral--Ditto, but substituting "chords"for "neutral life lines. "

Angle, Pitch--The angle at any given point of a propeller, at whichthe blade is inclined to the direction of motion when the propeller isrevolving but the aeroplane stationary. 

Altimeter--An instrument used for measuring height. 

Air-Speed Indicator--An instrument used for measuring air pressures orvelocities. It consequently indicates whether the surface is securingthe requisite reaction for flight. Usually calibrated in miles per hour, in which case it indicates the correct number of miles per hour at onlyone altitude. This is owing to the density of the air decreasing withincrease of altitude and necessitating a greater speed through spaceto secure the same air pressure as would be secured by less speed at alower altitude. It would be more correct to calibrate it in units of airpressure. 

Air Pocket--A local movement or condition of the air causing anaeroplane to drop or lose its correct attitude. 

Aspect-Ratio--The proportion of span to chord of a surface. 

Air-Screw (Propeller)--A surface so shaped that its rotation about anaxis produces a force (thrust) in the direction of its axis. 

Aileron--A controlling surface, usually situated at the wing-tip, theoperation of which turns an aeroplane about its longitudinal axis;causes an aeroplane to tilt sideways. 

Aviation--The art of driving an aeroplane. 

Aviator--The driver of an aeroplane. 

Barograph--A recording barometer, the charts of which can be calibratedfor showing air density or height. 

Barometer--An instrument used for indicating the density of air. 

Bank, to--To turn an aeroplane about its longitudinal axis (to tiltsideways) when turning to left or right. 

Biplane--An aeroplane of which the main lifting surface consists of asurface or pair of wings mounted above another surface or pair of wings. 

Bay--The space enclosed by two struts and whatever they are fixed to. 

Boom--A term usually applied to the long spars joining the tail of a"pusher" aeroplane to its main lifting surface. 

Bracing--A system of struts and tie wires to transfer a force from onepoint to another. 

Canard--Literally "duck. " The name which was given to a type ofaeroplane of which the longitudinal stabilizing surface (empennage)was mounted in front of the main lifting surface. Sometimes termed"tail-first" aeroplanes, but such term is erroneous, as in such a designthe main lifting surface acts as, and is, the empennage. 

Cabre--To fly or glide at an excessive angle of incidence; tail down. 

Camber--Curvature. 

Chord--Usually taken to be a straight line between the trailing andleading edges of a surface. 

Cell--The whole of the lower surface, that part of the upper surfacedirectly over it, together with the struts and wires holding themtogether. 

Centre (Line) of Pressure--A line running from wing-tip to wing-tip, andthrough which all the air forces acting upon the surface may be said toact, or about which they may be said to balance. 

Centre (Line) of Pressure, Resultant--A line transverse to thelongitudinal axis, and the position of which is the resultant of thecentres of pressure of two or more surfaces. 

Centre of Gravity--The centre of weight. 

Cabane--A combination of two pylons, situated over the fuselage, andfrom which anti-lift wires are suspended. 

Cloche--Literally "bell. " Is applied to the bell-shaped constructionwhich forms the lower part of the pilot's control lever in a Bleriotmonoplane, and to which the control cables are attached. 

Centrifugal Force--Every body which moves in a curved path is urgedoutwards from the centre of the curve by a force termed "centrifugal. "

Control Lever--A lever by means of which the controlling surfacesare operated. It usually operates the ailerons and elevator. The"joy-stick". 

Cavitation, Propeller--The tendency to produce a cavity in the air. 

Distance Piece--A long, thin piece of wood (sometimes tape) passingthrough and attached to all the ribs in order to prevent them fromrolling over sideways. 

Displacement--Change of position. 

Drift (of an aeroplane as distinct from the propeller)--The horizontalcomponent of the reaction produced by the action of driving through theair a surface inclined upwards and towards its direction of motion PLUSthe horizontal component of the reaction produced by the "detrimental"surface PLUS resistance due to "skin-friction. " Sometimes termed"head-resistance. "

Drift, Active--Drift produced by the lifting surface. 

Drift, Passive--Drift produced by the detrimental surface. 

Drift (of a propeller)--Analogous to the drift of an aeroplane. It isconvenient to include "cavitation" within this term. 

Drift, to--To be carried by a current of air; to make leeway. 

Dive, to--To descend so steeply as to produce a speed greater than thenormal flying speed. 

Dope, to--To paint a fabric with a special fluid for the purpose oftightening and protecting it. 

Density--Mass of unit volume, for instance, pounds per cubic foot. 

Efficiency--Output Input

Efficiency (of an aeroplane as distinct from engine and propeller)--

 Lift and Velocity Thrust (= aeroplane drift)

Efficiency, Engine--Brake horse-power

 Indicated horse-power

Efficiency, Propeller--

 Thrust horse-power Horse-power received from engine (= propeller drift)

NOTE. --The above terms can, of course, be expressed in foot-pounds. Itis then only necessary to divide the upper term by the lower one to findthe measure of efficiency. 

Elevator--A controlling surface, usually hinged to the rear of thetail-plane, the operation of which turns an aeroplane about an axiswhich is transverse to the direction of normal horizontal flight. 

Empennage--See "Tail-plane. "

Energy--Stored work. For instance, a given weight of coal or petroleumstores a given quantity of energy which may be expressed in foot-pounds. 

Extension--That part of the upper surface extending beyond the span ofthe lower surface. 

Edge, Leading--The front edge of a surface relative to its normaldirection of motion. 

Edge, Trailing--The rear edge of a surface relative to its normaldirection of motion. 

Factor of Safety--Usually taken to mean the result found by dividing thestress at which a body will collapse by the maximum stress it will becalled upon to bear. 

Fineness (of stream-line)--The proportion of length to maximum width. 

Flying Position--A special position in which an aeroplane must be placedwhen rigging it or making adjustments. It varies with different types ofaeroplanes. Would be more correctly described as "rigging position. "

Fuselage--That part of an aeroplane containing the pilot, and to whichis fixed the tail-plane. 

Fin--Additional keel-surface, usually mounted at the rear of anaeroplane. 

Flange (of a rib)--That horizontal part of a rib which prevents it frombending sideways. 

Flight--The sustenance of a body heavier than air by means of its actionupon the air. 

Foot-pound--A measure of work representing the weight of 1 lb. Raised 1foot. 

Fairing--Usually made of thin sheet aluminum, wood, or a lightconstruction of wood and fabric; and bent round detrimental surface inorder to give it a "fair" or "stream-like" shape. 

Gravity--Is the force of the Earth's attraction upon a body. Itdecreases with increase of distance from the Earth. See "Weight. "

Gravity, Specific--Density of substance Density of water. Thus, if the density of water is 10 lb. Per unit volume, the same unitvolume of petrol, if weighing 7 lb. , would be said to have a specificgravity of 7/10, i. E. , 0. 7. 

Gap (of an aeroplane)--The distance between the upper and lower surfacesof a biplane. In a triplane or multiplane, the distance between asurface and the one first above it. 

Gap, Propeller--The distance, measured in the direction of the thrust, between the spiral courses of the blades. 

Girder--A structure designed to resist bending, and to combine lightnessand strength. 

Gyroscope--A heavy circular wheel revolving at high speed, the effect ofwhich is a tendency to maintain its plane of rotation against disturbingforces. 

Hangar--An aeroplane shed. 

Head-Resistance--Drift. The resistance of the air to the passage of abody. 

Helicopter--An air-screw revolving about a vertical axis, the directionof its thrust being opposed to gravity. 

Horizontal Equivalent--The plan view of a body whatever its attitude maybe. 

Impulse--A force causing a body to gain or lose momentum. 

Inclinometer--A curved form of spirit-level used for indicating theattitude of a body relative to the horizontal. 

Instability--An inherent tendency of a body, which, if the body isdisturbed, causes it to move into a position as far as possible awayfrom its first position. 

Instability, Neutral--An inherent tendency of a body to remain in theposition given it by the force of a disturbance, with no tendency tomove farther or to return to its first position. 

Inertia--The inherent resistance to displacement of a body as distinctfrom resistance the result of an external force. 

Joy-Stick--See "Control Lever. "

Keel-Surface--Everything to be seen when viewing an aeroplane from theside of it. 

King-Post--A bracing strut; in an aeroplane, usually passing through asurface and attached to the main spar, and from the end or ends of whichwires are taken to spar, surface, or other part of the construction inorder to prevent distortion. When used in connection with a controllingsurface, it usually performs the additional function of a lever, controlcables connecting its ends with the pilot's control lever. 

Lift--The vertical component of the reaction produced by the actionof driving through the air a surface inclined upwards and towards itsdirection of motion. 

Lift, Margin of--The height an aeroplane can gain in a given time andstarting from a given altitude. 

Lift-Drift Ratio--The proportion of lift to drift. 

Loading--The weight carried by an aerofoil. Usually expressed in poundsper square foot of superficial area. 

Longeron--The term usually applied to any long spar running length-waysof a fuselage. 

Mass--The mass of a body is a measure of the quantity of material in it. 

Momentum--The product of the mass and velocity of a body is known as"momentum. "

Monoplane--An aeroplane of which the main lifting surface consists ofone surface or one pair of wings. 

Multiplane--An aeroplane of which the main lifting surface consists ofnumerous surfaces or pairs of wings mounted one above the other. 

Montant--Fuselage strut. 

Nacelle--That part of an aeroplane containing the engine and pilot andpassenger, and to which the tail plane is not fixed. 

Neutral Lift Line--A line taken through a surface in a forward directionrelative to its direction of motion, and starting from its trailingedge. If the attitude of the surface is such as to make the said linecoincident with the direction of motion, it results in no lift, thereaction then consisting solely of drift. The position of the neutrallift line, i. E. , the angle it makes with the chord, varies withdifferences of camber, and it is found by means of wind-tunnel research. 

Newton's Laws of Motion--1. If a body be at rest, it will remain atrest; or, if in motion, it will move uniformly in a straight line untilacted upon by some force. 

2. The rate of change of the quantity of motion (momentum) isproportional to the force which causes it, and takes place in thedirection of the straight line in which the force acts. If a body beacted upon by several forces, it will obey each as though the others didnot exist, and this whether the body be at rest or in motion. 

3. To every action there is opposed an equal and opposite reaction. 

Ornithopter (or Orthopter)--A flapping wing design of aircraft intendedto imitate the flight of a bird. 

Outrigger--This term is usually applied to the framework connecting themain surface with an elevator placed in advance of it. Sometimes appliedto the "tail-boom" framework connecting the tail-plane with the mainlifting surface. 

Pancake, to--To "stall "

Plane--This term is often applied to a lifting surface. Such applicationis not quite correct, since "plane" indicates a flat surface, and thelifting surfaces are always cambered. 

Propeller--See "Air-Screw. "

Propeller, Tractor--An air-screw mounted in front of the main liftingsurface. 

Propeller, Pusher--An air-screw mounted behind the main lifting surface. 

Pusher--An aeroplane of which the propeller is mounted behind the mainlifting surface. 

Pylon--Any V-shaped construction from the point of which wires aretaken. 

Power--Rate of working. 

Power, Horse--One horse-power represents a force sufficient to raise33, 000 lbs. 1 foot in a minute. 

Power, Indicated Horse--The I. H. P. Of an engine is a measure of the rateat which work is done by the pressure upon the piston or pistons, asdistinct from the rate at which the engine does work. The latter isusually termed "brake horse-power, " since it may be measured by anabsorption brake. 

Power, Margin of--The available quantity of power above that necessaryto maintain horizontal flight at the optimum angle. 

Pitot Tube--A form of air-speed indicator consisting of a tube with openend facing the wind, which, combined with a static pressure or suctiontube, is used in conjunction with a gauge for measuring air pressures orvelocities. (No. 1 in diagram. )

Pitch, Propeller--The distance a propeller advances during onerevolution supposing the air to be solid. 

Pitch, to--To plunge nose-down. 

Reaction--A force, equal and opposite to the force of the actionproducing it. 

Rudder--A controlling surface, usually hinged to the tail, the operationof which turns an aeroplane about an axis which is vertical in normalhorizontal flight; causes an aeroplane to turn to left or right of thepilot. 

Roll, to--To turn about the longitudinal axis. 

Rib, Ordinary--A light curved wooden part mounted in a fore and aftdirection within a surface. The ordinary ribs give the surface itscamber, carry the fabric, and transfer the lift from the fabric to thespars. 

Rib, Compression--Acts as an ordinary rib, besides bearing the stress ofcompression produced by the tension of the internal bracing wires. 

Rib, False--A subsidiary rib, usually used to improve the camber of thefront part of the surface. 

Right and Left Hand--Always used relative to the position of the pilot. When observing an aeroplane from the front of it, the right hand side ofit is then on the left hand of the observer. 

Remou--A local movement or condition of the air which may causedisplacement of an aeroplane. 

Rudder-Bar--A control lever moved by the pilot's feet, and operating therudder. 

Surface--See "Aerofoil. "

Surface, Detrimental--All exterior parts of an aeroplane includingthe propeller, but excluding the (aeroplane) lifting and (propeller)thrusting surfaces. 

Surface, Controlling--A surface the operation of which turns anaeroplane about one of its axes. 

Skin-Friction--The friction of the air with roughness of surface. A formof drift. 

Span---The distance from wing-tip to wing-tip. 

Stagger--The distance the upper surface is forward of the lower surfacewhen the axis of the propeller is horizontal. 

Stability--The inherent tendency of a body, when disturbed, to return toits normal position. 

Stability, Directional--The stability about an axis which is verticalduring normal horizontal flight, and without which an aeroplane has nonatural tendency to remain upon its course. 

Stability, Longitudinal--The stability of an aeroplane about an axistransverse to the direction of normal horizontal flight, and withoutwhich it has no tendency to oppose pitching and tossing. 

Stability, Lateral--The stability of an aeroplane about its longitudinalaxis, and without which it has no tendency to oppose sideways rolling. 

Stabilizer--A surface, such as fin or tail-plane, designed to give anaeroplane inherent stability. 

Stall, to--To give or allow an aeroplane an angle of incidence greaterthan the "maximum" angle, the result being a fall in the lift-driftratio, the lift consequently becoming less than the weight of theaeroplane, which must then fall, i. E. , "stall" or "pancake. "

Stress--Burden or load. 

Strain--Deformation produced by stress. 

Side-Slip, to--To fall as a result of an excessive "bank" or "roll. "

Skid, to--To be carried sideways by centrifugal force when turning toleft or right. 

Skid, Undercarriage--A spar, mounted in a fore and aft direction, and towhich the wheels of the undercarriage are sometimes attached. Shoulda wheel give way the skid is then supposed to act like the runner of asleigh and to support the aeroplane. 

Skid, Tail--A piece of wood or other material, orientable, and fittedwith shock absorbers, situated under the tail of an aeroplane in orderto support it upon the ground and to absorb the shock of alighting. 

Section--Any separate part of the top surface, that part of the bottomsurface immediately underneath it, with their struts and wires. 

Spar--Any long piece of wood or other material. 

Spar, Main--A spar within a surface and to which all the ribs areattached, such spar being the one situated nearest to the centre ofpressure. It transfers more than half the lift from the ribs to thebracing. 

Spar, Rear--A spar within a surface, and to which all the ribs areattached, such spar being situated at the rear of the centre of pressureand at a greater distance from it than is the main spar. It transfersless than half of the lift from the ribs to the bracing. 

Strut--Any wooden member intended to take merely the stress of directcompression. 

Strut, Interplane--A strut holding the top and bottom surfaces apart. 

Strut, Fuselage--A strut holding the fuselage longerons apart. It shouldbe stated whether top, bottom, or side. If side, then it should bestated whether right or left hand. Montant. 

Strut, Extension--A strut supporting an "extension" when not in flight. It may also prevent the extension from collapsing upwards during flight. 

Strut, Undercarriage--

Strut, Dope--A strut within a surface, so placed as to prevent thetension of the doped fabric from distorting the framework. 

Serving--To bind round with wire, cord, or similar material. Usuallyused in connection with wood joints and wire cable splices. 

Slip, Propeller--The pitch less the distance the propeller advancesduring one revolution. 

Stream-Line--A form or shape of detrimental surface designed to produceminimum drift. 

Toss, to--To plunge tail-down. 

Torque, Propeller--The tendency of a propeller to turn an aeroplaneabout its longitudinal axis in a direction opposite to that in which thepropeller revolves. 

Tail-Slide--A fall whereby the tail of an aeroplane leads. 

Tractor--An aeroplane of which the propeller is mounted in front of themain lifting surface. 

Triplane--An aeroplane of which the main lifting surface consists ofthree surfaces or pairs of wings mounted one above the other. 

Tail-Plane--A horizontal stabilizing surface mounted at some distancebehind the main lifting surface. Empennage. 

Turnbuckle--A form of wire-tightener, consisting of a barrel into eachend of which is screwed an eyebolt. Wires are attached to the eyeboltsand the required degree of tension is secured by means of rotating thebarrel. 

Thrust, Propeller--See "Air-Screw. "

Undercarriage--That part of an aeroplane beneath the fuselage ornacelle, and intended to support the aeroplane when at rest, and toabsorb the shock of alighting. 

Velocity--Rate of displacement; speed. 

Volplane--A gliding descent. 

Weight--Is a measure of the force of the Earth's attraction (gravity)upon a body. The standard unit of weight in this country is 1 lb. , andis the force of the Earth's attraction on a piece of platinum calledthe standard pound, deposited with the Board of Trade in London. At thecentre of the Earth a body will be attracted with equal force inevery direction. It will therefore have no weight, though its massis unchanged. Gravity, of which weight is a measure, decreases withincrease of altitude. 

Web (of a rib)--That vertical part of a rib which prevents it frombending upwards. 

Warp, to--To distort a surface in order to vary its angle of incidence. To vary the angle of incidence of a controlling surface. 

Wash--The disturbance of air produced by the flight of an aeroplane. 

Wash-in--An increasing angle of incidence of a surface towards itswing-tip. 

Wash-out--A decreasing angle of incidence of a surface towards itswing-tip. 

Wing-tip--The right- or left-hand extremity of a surface. 

Wire--A wire is, in Aeronautics, always known by the name of itsfunction. 

Wire, Lift or Flying--A wire opposed to the direction of lift, and usedto prevent a surface from collapsing upward during flight. 

Wire, Anti-lift or Landing--A wire opposed to the direction of gravity, and used to sustain a surface when it is at rest. 

Wire, Drift--A wire opposed to the direction of drift, and used toprevent a surface from collapsing backwards during flight. 

Wire, Anti-drift--A wire opposed to the tension of a drift wire, andused to prevent such tension from distorting the framework. 

Wire, Incidence--A wire running from the top of an interplane strutto the bottom of the interplane strut in front of or behind it. It maintains the "stagger" and assists in maintaining the angle ofincidence. Sometimes termed "stagger wire. "

Wire, Bracing--Any wire holding together the framework of any part ofan aeroplane. It is not, however, usually applied to the wires describedabove unless the function performed includes a function additional tothose described above. Thus, a lift wire, while strictly speaking abracing wire, is not usually described as one unless it performs theadditional function of bracing some well-defined part such as theundercarriage. It will then be said to be an "undercarriage bracing liftwire. " It might, perhaps, be acting as a drift wire also, in whichcase it will then be de-scribed as an "undercarriage bracing lift-driftwire. " It should always be stated whether a bracing wire is (1) top, (2)bottom, (3) cross, or (4) side. If a "side bracing wire, " then it shouldbe stated whether right- or left-hand. 

Wire, Internal Bracing--A bracing wire (usually drift or anti-drift)within a surface. 

Wire, Top Bracing--A bracing wire, approximately horizontal and situatedbetween the top longerons of fuselate, between top tail booms, or at thetop of similar construction. 

Wire, Bottom Bracing--Ditto, substituting "bottom" for "top. "

Wire, Side Bracing--A bracing wire crossing diagonally a side bay offuselage, tail boom bay, undercarriage side bay or centre-section sidebay. This term is not usually used with reference to incidence wires, although they cross diagonally the side bays of the cell. It should bestated whether right- or left-hand. 

Wire, Cross Bracing--A bracing wire, the position of which is diagonalfrom right to left when viewing it from the front of an aeroplane. 

Wire, Control Bracing--A wire preventing distortion of a controllingsurface. 

Wire, Control--A wire connecting a controlling surface with the pilot'scontrol lever, wheel, or rudder-bar. 

Wire, Aileron Gap--A wire connecting top and bottom ailerons. 

Wire, Aileron Balance--A wire connecting the right- and left-hand topailerons. Sometimes termed the "aileron compensating wire. "

Wire, Snaking--A wire, usually of soft metal, wound spirally or tiedround another wire, and attached at each end to the framework. Used toprevent the wire round which it is "snaked" from becoming, in the eventof its displacement, entangled with the propeller. 

Wire, Locking--A wire used to prevent a turnbuckle barrel or otherfitting from losing its adjustment. 

Wing--Strictly speaking, a wing is one of the surfaces of anornithopter. The term is, however, often applied to the lifting surfaceof an aeroplane when such surface is divided into two parts, one beingthe left-hand "wing, " and the other the right-hand "wing. "

Wind-Tunnel--A large tube used for experimenting with surfaces andmodels, and through which a current of air is made to flow by artificialmeans. 

Work--Force X displacement. 

Wind-Screen--A small transparent screen mounted in front of the pilot toprotect his face from the air pressure. 

FOOTNOTES:

[1] Propeller Slip: As the propeller screws through the air, thelatter to a certain extent gives back to the thrust of the propellorblades, just as the shingle on the beach slips back as you ascend it. Such "give-back" is known as "slip, " and anyone behind the propellorwill feel the slip as a strong draught of air. 

[2] Helicopter. An air-screw revolving upon a vertical axis. If drivenwith sufficient power, it will lift vertically, but having regard to themechanical difficulties of such construction, it is a most inefficientway of securing lift compared with the arrangement of an inclinedsurface driven by a propeller revolving about a horizontal axis. 

[3] Pancakes: Pilot's slang for stalling an aeroplane and droppinglike a pancake. 

[4] Morane parasol: A type of Morane monoplane in which the liftingsurfaces are raised above the pilot in order to afford him a good viewof the earth. 

[5] Skin friction is that part of the drift due to the friction of theair with roughnesses upon the surface of the aeroplane. 

[6] Banking: When an aeroplane is turned to the left or the rightthe centrifugal force of its momentum causes it to skid sideways andoutwards away from the centre of the turn. To minimize such action thepilot banks, i. E. , tilts, the aeroplane sideways in order to oppose theunderside of the planes to the air. The aeroplane will not then skidoutwards beyond the slight skid necessary to secure a sufficientpressure of air to balance the centrifugal force. 

[7] An explanation of the way in which the wash-out is combined with awash-in to offset propellor torque will be found on p. 82. 

[8] A. M. 's: Air Mechanics. 

[9] Butt means to thicken at the end. Screw means to machine athread on the butt-end of the wire, and in this way the wire can makeconnection with the desired place by being screwed into a metal fitting, thus eliminating the disadvantage of the unsatisfactory loop. 

[10] Deviation curve: A curved line indicating any errors in thecompass. 

[11] A propeller screws through the air, and the distance it advancesduring one revolution, supposing the air to be solid, is known as thepitch. The pitch, which depends upon the angle of the propeller blades, must be equal to the speed of the aeroplane, plus the slip, and if, onaccount of the rarity of the air the speed of the aeroplane increases, then the angle and pitch should be correspondingly increased. Propellerswith a pitch capable of being varied by the pilot are the dream ofpropeller designers. For explanation of "slip" see Chapter IV. Onpropellers. 

[12] Getting out of my depth? Invading the realms of fancy? Well, perhaps so, but at any rate it is possible that extraordinary speedthrough space may be secured if means are found to maintain the impulseof the engine and the thrust-drift efficiency of the propeller at greataltitude. 

[13] Box-kite. The first crude form of biplane. 

[14] See Newton's laws in the Glossary at the end of the book. 

[15] See "Aerofoil" in the Glossary. 

[16] "In effect" because, although there may be actually the greatestproportion of keel-surface In front of the vertical axis, such surfacemay be much nearer to the axis than is the keel-surface towards thetail. The latter may then be actually less than the surface in front, but, being farther from the axis, it has a greater leverage, andconsequently is greater in effect than the surface in front.