THE AEROPLANE SPEAKS
BY H. BARBER, A.F.Ae.S.
(CAPTAIN, ROYAL FLYING CORPS)
WITH 36 FULL PAGES OF "TYPES OF AEROPLANES" AND 87 SKETCHES AND DIAGRAMS
McBRIDE, NAST & CO., LTD.
THE AEROPLANE SPEAKS.
First edition—December, 1916 Second edition—February, 1917 Third edition—April, 1917 Fourth edition—July, 1917 Fifth edition—December, 1917
C. G. G. in the AEROPLANE: "One hopes that the Subaltern Flying Officer will appreciate the gift which the author has given him out of his own vast store of experience, for the book contains the concentrated knowledge of many expensive years in tabloid form, or perhaps one should say in condensed milk form, seeing that it is easy to swallow and agreeable to the taste, as well as wholesome and nourishing. And, besides the young service aviator, there are thousands of young men, and women also, now employed in the aircraft industry, who will appreciate far better the value of the finicky little jobs they are doing if they will read this book and see how vital is their work to the man who flies."
THE FIELD: "Entirely different from any other text-book on the subject, not merely in its form, but in its capacity to convey a knowledge of the principles and practice of flying. Undoubtedly it is the best book on its subject."
THE UNITED SERVICE GAZETTE: "Should be in the hands of every person interested in aviation."
THE OUTLOOK: "As amusing as it is instructive."
THE MORNING POST: "Should be read and re-read by the would be and even the experienced pilot."
PRINTED IN ENGLAND BY BILLING AND SONS, LIMITED GUILDFORD
DEDICATED TO THE SUBALTERN FLYING OFFICER
The reasons impelling me to write this book, the maiden effort of my pen, are, firstly, a strong desire to help the ordinary man to understand the Aeroplane and the joys and troubles of its Pilot; and, secondly, to produce something of practical assistance to the Pilot and his invaluable assistant the Rigger. Having had some eight years' experience in designing, building, and flying aeroplanes, I have hopes that the practical knowledge I have gained may offset the disadvantage of a hand more used to managing the "joy-stick" than the dreadful haltings, the many side-slips, the irregular speed, and, in short, the altogether 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 permission of 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 valuable illustrations reproduced at the end of this book.
PART I.—THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES 1 II.—THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB 15 III.—THE GREAT TEST 27 IV.—CROSS COUNTRY 38
CHAPTER I.—FLIGHT 55 II.—STABILITY AND CONTROL 70 III.—RIGGING 90 IV.—PROPELLERS 115 V.—MAINTENANCE 126
TYPES OF AEROPLANES 130
THE AEROPLANE SPEAKS
THE ELEMENTARY PRINCIPLES AIR THEIR GRIEVANCES
The Lecture Hall at the Royal Flying Corps School for Officers was deserted. The pupils had dispersed, and the Officer Instructor, more fagged than any pupil, was out on the aerodrome watching the test of a new machine.
Deserted, did I say? But not so. The lecture that day had been upon the Elementary Principles of Flight, and they lingered yet. Upon the Blackboard was an illustration thus:
"I am the side view of a Surface," it said, mimicking the tones of the lecturer. "Flight is secured by driving me through the air at an angle inclined to the direction of motion."
"Quite right," said the Angle. "That's me, and I'm the famous Angle of Incidence."
"And," continued the Surface, "my action is to deflect the air downwards, and also, by fleeing from the air behind, to create a semi-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 went on: "I'm the Reaction. You can't have action without me. I'm a very considerable force, and my direction is at right-angles to you," and he looked heavily at the Surface. "Like this," said he, picking up the chalk with his Lift, and drifting to the Blackboard.
"I act in the direction of the arrow R, that is, more or less, for the direction varies somewhat with the Angle of Incidence and the curvature of the Surface; and, strange but true, I'm stronger on the top of the Surface than at the bottom of it. The Wind Tunnel has proved that by exhaustive research—and don't forget how quickly I can grow! As the speed through the air increases my strength increases more rapidly than you might think—approximately, as the Square of the Speed; so you see 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 am the result of not only the mass of air displaced, but also the result of the Speed and consequent Force with which the Surface engages the Air. I am a product of those two factors, and at the speeds at which Aeroplanes fly to-day, and at the altitudes and consequent density of air they at present experience, I increase at about the Square of the Speed.
"Oh, I'm a most complex and interesting personality, I assure you—in fact, a dual personality, a sort of aeronautical Dr. Jekyll and Mr. Hyde. There's Lift, my vertical part or component, as those who prefer long words would say; he always acts vertically upwards, and hates Gravity like poison. He's the useful and admirable part of me. Then there's Drift, my horizontal component, sometimes, though rather erroneously, 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 the Thrust. I thrust the Aeroplane through the air and overcome the Drift; and the Lift increases with the Speed, and when it equals the Gravity or Weight, then—there you are—Flight! And nothing mysterious about it at all."
"I hope you'll excuse me interrupting," said a very beautiful young lady, "my name is Efficiency, and, while, no doubt, all you have said is quite true, and that, as my young man the Designer says, 'You can make a tea-tray fly if you slap on Power enough,' I can assure you that I'm not to be won quite so easily."
"Well," eagerly replied the Lift and the Thrust, "let's be friends. Do tell us what we can do to help you to overcome Gravity and Drift with the least possible Power. That obviously seems the game to play, for more Power means heavier engines, and that in a way plays into the hands of our enemy, Gravity, besides necessitating a larger Surface or Angle to lift the Weight, and that increases the Drift."
"Very well," from Efficiency, "I'll do my best, though I'm so shy, and I've just had such a bad time at the Factory, and I'm terribly afraid you'll find it awfully dry."
"Buck up, old dear!" This from several new-comers, who had just appeared. "We'll help you," and one of them, so lean and long that he took 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 to help 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 greater the Weight the latter can carry; and the less the Drift, then the less Thrust and Power required to overcome it. Now it is a fact that, if the Surface is shaped to have the greatest possible span, i.e., distance from wing-tip to wing-tip, it then engages more air and produces both a maximum Reaction and a better proportion of Lift to Drift.
"That being so, we can then well afford to lose a little Reaction by reducing the Angle of Incidence to a degree giving a still better proportion of Lift to Drift than would otherwise be the case; for you must understand that the Lift-Drift Ratio depends very much upon the size of the Angle of Incidence, which should be as small as possible within certain limits. So what I say is, make the surface of Infinite Span with no width or chord, as they call it. That's all I require, I assure you, to make me quite perfect and of infinite service to Miss Efficiency."
"That's not practical politics," said the Surface. "The way you talk one would think you were drawing L400 a year at Westminster, and working up a reputation as an Aeronautical Expert. I must have some depth and chord to take my Spars and Ribs, and again, I must have a certain chord to make it possible for my Camber (that's curvature) to be just right for the Angle of Incidence. If that's not right the air won't get a nice uniform compression and downward acceleration from my underside, and the rarefied 'suction' area over the top of me will not be as even and clean in effect as it might be. That would spoil the Lift-Drift Ratio more than you can help it. Just thrust that chalk along, will you? and the Blackboard will show you what I mean."
"Well," said the Aspect Ratio, "have it your own way, though I'm sorry to see a pretty young lady like Efficiency compromised so early in the game."
"Look here," exclaimed a number of Struts, "we have got a brilliant idea for improving the Aspect Ratio," and with that they hopped up on to the Spars. "Now," excitedly, "place another Surface on top of us. Now do you see? There is double the Surface, and that being so, the proportion of Weight to Surface area is halved. That's less burden of work for the Surface, and so the Spars need not be so strong and so deep, which results in not so thick a Surface. That means the Chord can be proportionately decreased without adversely affecting the Camber. With the Chord decreased, the Span becomes relatively greater, and so produces a splendid Aspect Ratio, and an excellent proportion of Lift to Drift."
"I don't deny that they have rather got me there," said the Drift; "but all the same, don't forget my increase due to the drift of the Struts and their bracing wires."
"Yes; I dare say," replied the Surface, "but remember that my Spars are less deep than before, and consequently I am not so thick now, and shall for that reason also be able to go through the air with a less proportion of Drift to Lift."
"Remember me also, please," croaked the Angle of Incidence. "Since the Surface has now less weight to carry for its area, I may be set at a still lesser and finer Angle. That means less Drift again. We are certainly 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 not helping Efficiency as much as you think. The suction effect on the top of the lower Surface will give a downward motion to the air above it and the result will be that the bottom of the top Surface will not secure as good a Reaction from the air as would otherwise be the case, and that means loss of Lift; and you can't help matters by increasing the gap between the surfaces because that means longer Struts and Wires, and that in itself would help me, not to speak of increasing the Weight. You see it's not quite so easy as you thought."
At this moment a hiccough was heard, and a rather fast and rakish-looking chap, named Stagger, spoke up. "How d'ye do, miss," he said politely to Efficiency, with a side glance out of his wicked old eye. "I'm a bit of a knut, and without the slightest trouble I can easily minimize the disadvantage that old reprobate Drift has been frightening you with. I just stagger the top Surface a bit forward, and no longer is that suction effect dead under it. At the same time I'm sure the top Surface will kindly extend its Span for such distance as its Spars will support it without the aid of Struts. Such extension will be quite useful, as there will be no Surface at all underneath it to interfere with the Reaction above." And the Stagger leaned forward and picked up the Chalk, and this is the picture he drew:
Said the Blackboard, "That's not half bad! It really begins to look something like the real thing, eh?"
"The real thing, is it?" grumbled Drift. "Just consider that contraption in the light of any one Principle, and I warrant you will not find one of them applied to perfection. The whole thing is nothing but a Compromise." And he glared fixedly at poor Efficiency.
"Oh, dear! Oh, dear!" she cried. "I'm always getting into trouble. What will the Designer say?"
"Never mind, my dear," said the Lift-Drift Ratio, consolingly. "You are improving rapidly, and quite useful enough now to think of doing a job of work."
"Well, that's good news," and Efficiency wiped her eyes with her Fabric and became almost cheerful. "Suppose we think about finishing it now? There will have to be an Engine and Propeller, won't there? And a body to 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," said the Lift-Drift Ratio, "but first of all we had better decide upon the Area of the Surfaces, their Angle of Incidence and Camber. If we are to ascend as quickly as possible the Aeroplane must be slow in order to secure the best possible lift-drift ratio; for the drift of the struts, wires, 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's proportion of lift to drift; and, being slow, we shall require a large Surface in order to secure a large lift relative to the weight to be carried. We shall also require a large Angle of Incidence relative to the horizontal, in order to secure a proper inclination of the Surface to the direction of motion, for you must remember that, while we shall fly upon an even keel and with the propeller thrust horizontal (which is its most efficient attitude), our flight path, which is our direction of motion, will be sloping upwards, and it will therefore be necessary to fix the Surface to the Aeroplane at a very considerable angle relative to the horizontal Propeller Thrust in order to secure a proper angle to the upwards direction of motion. Apart from that, we shall require a larger Angle of Incidence than in the case of a machine designed purely for speed, and that means a correspondingly large Camber.
"On the other hand, if we are thinking merely of Speed, then a small Surface, 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 a relatively small Camber.
"So you see the essentials for Climb or quick ascent and for Speed are diametrically opposed. Now which is it to be?"
"Nothing but perfection for me," said Efficiency. "What I want is Maximum Climb and Maximum Speed for the Power the Engine produces."
And each Principle fully agreed with her beautiful sentiments, but work together 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 thickness whatever, and would not hear of such things as Spars and Ribs; and the Thrust objected to anything at all likely to produce Drift, and very nearly wiped the whole thing off the Blackboard.
There was, indeed, the makings of a very pretty quarrel when the Letter arrived. 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 each heated Principle. "It's really absurdly simple. All the Pilot has to do is to touch a button, and at his will, VARY the area of the Surface, the Angle of Incidence, and the Camber! And there you are—Maximum Climb or Maximum Speed as required! How does that suit you?"
"That suits us very well," said the Surface, "but, excuse me asking, how is it done without apparatus increasing the Drift and the Weight out of all reason? You won't mind showing us your Calculations, Working Drawings, Stress Diagrams, etc., will you?"
Said the Letter with dignity, "I come from an Inventor so brilliantly clever as to be far above the unimportant matters you mention. He is no common working man, sir! He leaves such things to Mechanics. The point is, you press a button and——"
"Look here," said a Strut, rather pointedly, "where do you think you are going, 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 quarters and absolutely revolutionize Flight when I get there."
Said the Chalk, "I'll address you, if that's all you want; now drift along quickly!" And off went the Letter to The Technical Editor, "Daily Mauler," London.
And a League was formed, and there were Directors with Fees, and several out-of-service Tin Hats, and the Man-who-takes-the-credit, and a fine fat Guinea-pig, and all the rest of them. And the Inventor paid his Tailor and had a Hair-Cut, and is now a recognized Press Expert—but he is still waiting for those Mechanics!
"I'm afraid," said the Slide-rule, who had been busy making those lightning-like automatic calculations for which he is so famous, "it's quite impossible to fully satisfy all of you, and it is perfectly plain to me that we shall have to effect a Compromise and sacrifice some of the Lift for Speed."
Thud! What was that?
Efficiency had fainted dead away! The last blow had been too much for her. And the Principles gathered mournfully round, but with the aid of the Propeller Slip and a friendly lift from the Surface she was at length revived and regained a more normal aspect.
Said the Stagger with a raffish air, "My dear young lady, I assure you that from the experiences of a varied career, I have learned that perfection is impossible, and I am sure the Designer will be quite satisfied if you become the Most Efficient Compromise."
"Well, that sounds so common sense," sighed Efficiency, "I suppose it must be true, and if the Designer is satisfied, that's all I really care about. Now do let's get on with the job."
So the Chalk drew a nice long slim body to hold the Engine and the tanks, etc., with room for the Pilot's and Passenger's seats, and placed it exactly in the middle of the Biplane. And he was careful to make its position such that the Centre of Gravity was a little in advance of the Centre of Lift, so that when the Engine was not running and there was consequently no Thrust, the Aeroplane should be "nose-heavy" just to the right degree, and so take up a natural glide to Earth—and this was to help the Pilot and relieve him of work and worry, should he find himself in a fog or a cloud. And so that this tendency to glide downwards should not 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 and counter-balance the "nose-heavy" tendency.
And the Engine was so mounted that when the Propeller-Thrust was horizontal, which is its most efficient position, the Angle of Incidence and the Area of the surfaces were just sufficient to give a Lift a little in excess of the Weight. And the Camber was such that, as far as it was concerned, the Lift-Drift Ratio should be the best possible for that Angle of Incidence. And a beautifully simple under-carriage was added, the outstanding features of which were simplicity, strength, light-weight, and minimum drift. And, last of all, there was the Elevator, of which you will hear more by-and-by. And this is what it looked like then:
And Efficiency, smiling, thought that it was not such a bad compromise after 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 working in its most efficient attitude, I shall have a Lift from the Surfaces slightly 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 must assume a most undignified attitude, for helicopters I never approved of. In order to ascend more quickly the Pilot will deflect the Elevator, which, by the way, you see hinged to the Tail. By that means he will force the whole Aeroplane to assume a greater Angle of Incidence. And with greater Angle, the Lift will increase, though I'm sorry 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 be helpful to the Lift; but I shall now be pointing upwards, and besides overcoming the Drift in a forward direction, I shall be doing my best to haul the Aeroplane skywards. At a certain angle known as the Best Climbing Angle, we shall have our Maximum Margin of Lift, and I'm hoping that may be as much as almost a thousand feet altitude a minute."
"Then, if the Pilot is green, my chance will come," said the Maximum Angle of Incidence. "For if the Angle is increased over the Best Climbing Angle, the Drift will rush up; and the Speed, and with it the Lift, will, when my Angle is reached, drop to a point when the latter will be no more than the Weight. The Margin of Lift will have entirely disappeared, and there we shall be, staggering along at my tremendous angle, and only just maintaining horizontal flight."
"And then with luck I'll get my chance," said the Drift. "If he is a bit worse than green, he'll perhaps still further increase the Angle. Then the Drift, largely increasing, the Speed, and consequently the Lift, will become still less, i.e., less than the Weight, and then—what price pancakes. Eh?"
"Thank you," from Efficiency, "that was all most informing. And now will you tell me, please, how the greatest Speed may be secured?"
"Certainly, now it's my turn," piped the Minimum Angle of Incidence. "By means 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 we shall make greater speed with no more Drift than before. Then we get our greatest 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's the best we can do until that Inventor fellow finds his Mechanics."
"Thank you so much," said Efficiency. "I think I have now at any rate an idea of the Elementary Principles of Flight, and I don't know that I care to delve much deeper, for sums always give me a headache; but isn't there something about Stability and Control? Don't you think I ought to have a glimmering of them too?"
"Well, I should smile," said a spruce Spar, who had come all the way from America. "And that, as the Lecturer says, 'will be the subject of our next lecture,' so be here again to-morrow, and you will be glad to hear that it will be distinctly more lively than the subject we have covered to-day."
[Footnote 1: Propeller Slip: As the propeller screws through the air, the latter to a certain extent gives back to the thrust of the propeller blades, just as the shingle on the beach slips back as you ascend it. Such "give-back" is known as "slip," and anyone behind the propeller will feel the slip as a strong draught of air.]
[Footnote 2: Helicopter: An air-screw revolving upon a vertical axis. If driven with sufficient power, it will lift vertically, but, having regard to the mechanical difficulties of such construction, it is a most inefficient way of securing lift compared with the arrangement of an inclined surface driven by a propeller revolving about a horizontal axis.]
[Footnote 3: Pancakes: Pilot's slang for stalling an aeroplane and dropping like a pancake.]
THE PRINCIPLES, HAVING SETTLED THEIR DIFFERENCES, FINISH THE JOB
Another day had passed, and the Flight Folk had again gathered together and were awaiting the arrival of Efficiency who, as usual, was rather late in making an appearance.
The crowd was larger than ever, and among the newcomers some of the most important 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 a most unfavourable impression, and Keel-Surface, the Dihedral Angle, and several other lesser fry.
"Well," said Centrifugal Force, "I wish this Efficiency I've heard so much about would get a move on. Sitting still doesn't agree with me at all. Motion I believe in. There's nothing like motion—the more the better."
"We are entirely opposed to that," objected the three Stabilities, all in a breath. "Unless it's in a perfectly straight line or a perfect circle. Nothing but perfectly straight lines or, upon occasion, perfect circles satisfy us, and we are strongly suspicious of your tendencies."
"Well, we shall see what we shall see," said the Force darkly. "But who in the name of blue sky is this?"
And in tripped Efficiency, in a beautifully "doped" dress of the latest fashionable shade of khaki-coloured fabric, a perfectly stream-lined bonnet, and a bewitching little Morane parasol, smiling as usual, and airily exclaiming, "I'm so sorry I'm late, but you see the Designer's such a funny man. He objects to skin friction, and insisted upon me changing my fabric for one of a smoother surface, and that delayed me. Dear me, there are a lot more of us to-day, aren't there? I think I had better meet one at a time." And turning to Directional Stability, she politely asked him what he preferred to do.
"My purpose in life, miss," said he, "is to keep the Aeroplane on its course, and to achieve that there must be, in effect, more Keel-Surface behind 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 the Aeroplane 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 it work?"
"I'll answer that," said Momentum. "When perhaps by a gust of air the Aeroplane is blown out of its course and points in another direction, it doesn't immediately fly off on that new course. I'm so strong I pull it off the new course to a certain extent, and towards the direction of the old course. And so it travels, as long as my strength lasts, in a more or 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 latter gets pressed sideways, and the Aeroplane thus tends to assume its first position and course."
"I see," said Efficiency, and, daintily holding the Chalk, she approached the Blackboard. "Is this what you mean?"
"Yes, that's right enough," said the Keel-Surface, "and you might remember, too, that I always make the Aeroplane nose into the gusts rather than away from them."
"If that was not the case," broke in Lateral Stability, and affecting the fashionable Flying Corps stammer, "it would be a h-h-h-o-r-rible affair! If there were too much Keel-Surface in front, then that gust would blow the Aeroplane round the other way a very considerable distance. And the right-hand Surface being on the outside of the turn would have more speed, and consequently more Lift, than the Surface on the other side. That means a greater proportion of the Lift on that side, and before you could say Warp to the Ailerons over the Aeroplane would go—probable result a bad side-slip" (see illustration A, over-leaf).
"And what can the Pilot do to save such a situation as that?" said Efficiency.
"Well," replied Lateral Stability, "he will try to turn the Aeroplane sideways and back to an even keel by means of warping the Ailerons or little wings which are hinged on to the Wing-tips, and about which you will hear more later on; but if the side-slip is very bad he may not be able to right the Aeroplane by means of the Ailerons, and then the only thing for him to do is to use the Rudder and to turn the nose of the Aeroplane down and head-on to the direction of motion. The Aeroplane will then be meeting the air in the direction it is designed to do so, and the Surfaces and also the controls (the Rudder, Ailerons, and Elevator) will be working efficiently; but its attitude relative to the earth will probably be more or less upside-down, for the action of turning the Aeroplane's nose down results, as you will see by the illustration B, in the right wing, which is on the outside of the circle, travelling through the air with greater speed than the left-hand wing. More Speed means more Lift, so that results in overturning the Aeroplane still more; but now it is, at any rate, meeting the air as it is designed to meet it, and everything is working properly. It is then only necessary to warp the Elevator, as shown in illustration C, in order to bring the Aeroplane into a proper attitude relative to the earth."
"Ah!" said the Rudder, looking wise, "it's in a case like that when I become the Elevator and the Elevator becomes me."
"That's absurd nonsense," said the Blackboard, "due to looseness of thought and expression."
"Well," replied the Rudder, "when the Aeroplane is in position A and I am used, then I depress or elevate the nose of the machine; and, if the Elevator is used, then it turns the Aeroplane to right or left, which is normally 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 way of looking at it, though I admit"—and this rather sarcastically—"that the way you put it sounds rather fine when you are talking of your experiences in the air to those 'interested in aviation' but knowing little about it; but it won't go down here! You are a Controlling Surface designed to turn the Aeroplane about a certain axis of the machine, and the Elevator is a Controlling Surface designed to turn the Aeroplane about another axis. Those are your respective jobs, and you can't possibly change them about. Such talk only leads to confusion, and I hope we shall hear no more of it."
"Thanks," said Efficiency to Lateral Stability. "And now, please, will you 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 is decrease, the Angle of Incidence on one side of the Aeroplane, and to effect the reverse condition, if it is not too much trouble, on the other side."
"But," objected Efficiency, "the Lift varies with the Angle of Incidence, and surely such a condition will result in one side of the Aeroplane lifting more than the other side?"
"That's all right," said the Propeller, "it's meant to off-set the tendency of the Aeroplane to turn over sideways in the opposite direction to which I revolve."
"That's quite clear, though rather unexpected; but how do you counteract the effect of the gusts when they try to overturn the Aeroplane sideways?" said she, turning to Lateral Stability again.
"Well," he replied, rather miserably, "I'm not nearly so perfect as the Longitudinal and Directional Stabilities. The Dihedral Angle—that is, the upward inclination of the Surfaces towards their wing-tips—does what it can for me, but, in my opinion, it's a more or less futile effort. The Blackboard will show you the argument." And he at once showed them two Surfaces, each set at a Dihedral Angle like this:
"Please imagine," said the Blackboard, "that the top V is the front view of a Surface flying towards you. Now if a gust blows it into the position of the lower V you see that the horizontal equivalent of the Surface on one side becomes larger, and on the other side it becomes smaller. That results in more lift on the lower side and less on the higher side, and if the V is large enough it should produce such a difference in the lift of one side to the other as to quickly turn the Aeroplane back to its former and normal position."
"Yes," said the Dihedral Angle, "that's what would happen if they would only make me large enough; but they won't do it because it would too greatly decrease the total horizontal equivalent, and therefore the Lift, and incidentally it would, as Aeroplanes are built to-day, produce an excess of Keel Surface above the turning axis, and that in itself would spoil the Lateral Stability. The Keel Surface should be equally divided above and below the longitudinal turning axis (upon which the Aeroplane rolls sideways), or the side upon which there is an excess will get blown over by the gusts. It strikes me that my future isn't very promising, and about my only chance is when the Junior Draughtsman makes a mistake, as he did the other day. And just think of it, they call him a Designer now that he's got a job at the Factory! What did he do? Why, he calculated the weights wrong and got the Centre of Gravity too high, and they didn't discover it until the machine was built. Then all they could do was to give me a larger Angle. That dropped the bottom of the V lower down, 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 now there is too much Keel Surface above, and the whole thing's a Bad Compromise, 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 too low down I act like a pendulum and cause the Aeroplane to roll about sideways, 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, in addition to that, there are the tricks I play with the Aeroplane when it's banked up, i.e., tilted sideways for a turn, and Centrifugal Force sets me going the way I'm not wanted to go. No; I get on best with Lateral 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 the Lecturer's Chair and went sound asleep again, for he was so very, very old, in fact the father of all the Principles.
And the Blackboard had been busy, and now showed them a picture of the Aeroplane as far as they knew it, and you will see that there is a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel Surface or fin, as is very often the case in order to ensure the greater effect 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 look right like that!"
"This is when I have the pleasure of meeting you, my dear," said Longitudinal Stability. "Here's the Tail Surface," he said, "and in order to help me it must be set in effect at a much less Angle of Incidence than the Main Surface. To explain we must trouble the Blackboard again," and this was his effort:
"I have tried to make that as clear as possible," he said. "It may appear a bit complicated at first, but if you will take the trouble to look at it for a minute you will find it quite simple. A is the normal and proper direction of motion of the Aeroplane, but, owing to a gust of air, 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 less in the direction B, which is the resultant of the two forces, Momentum and Thrust. And so you will note that the Angle of Incidence, which is the inclination of the Surfaces to the Direction of Motion, has decreased, and of course the Lift decreases with it. You will also see, and this is the point, that the Tail Surface has lost a higher proportion of its Angle, and consequently its Lift, than has the Main Surface. Then, such being the case, the Tail must fall and the Aeroplane assume its normal position again, though probably at a slightly lower altitude."
"I'm afraid I'm very stupid," said Efficiency, "but please tell me why you lay stress upon the words 'in effect.'"
"Ah! I was wondering if you would spot that," he replied. "And there is a very good reason for it. You see, in some Aeroplanes the Tail Surface may be actually set at the same Angle on the machine as the Main Surface, but owing to the air being deflected downwards by the front Main Surface it meets the Tail Surface at a lesser angle, and indeed in some cases at no angle at all. The Tail is then for its surface getting less Lift than the Main Surface, although set at the same angle on the machine. It may then be said to have in effect a less Angle of Incidence. I'll just show you on the Blackboard."
"And now," said Efficiency, "I have only to meet the Ailerons and the Rudder, haven't I?"
"Here we are," replied the Ailerons, or little wings. "Please hinge us on to the back of the Main Surfaces, one of us at each Wing-tip, and join us up to the Pilot's joystick by means of the control cables. When the Pilot wishes to tilt the Aeroplane sideways, he will move the stick and depress us upon one side, thus giving us a larger Angle of Incidence and so creating more Lift on that side of the Aeroplane; and, by means of a cable connecting us with the Ailerons on the other side of the Aeroplane, we shall, as we are depressed, pull them up and give them a reverse or negative Angle of Incidence, and that side will then get a reverse Lift or downward thrust, and so we are able to tilt the Aeroplane sideways.
"And we work best when the Angle of Incidence of the Surface in front of us is very small, for which reason it is sometimes decreased or washed-out towards the Wing-tips. The reason of that is that by the time the air reaches us it has been deflected downwards—the greater the Angle of Incidence the more it is driven downwards—and in order for us to secure a Reaction from it, we have to take such a large Angle of Incidence that we produce a poor proportion of Lift to Drift; but the smaller the Angle of the Surface in front of us the less the air is deflected downwards, and consequently the less Angle is required of us, and the better our proportion of Lift to Drift, which, of course, makes us 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 Surfaces towards their Wing-tips have less Drift or 'Head-Resistance,' and consequently the gusts will affect them and us less; but such decreased Angle of Incidence means decreased Lift as well as Drift, and the Designer does not always care to pay the price."
"Well," said the Ailerons, "if it's not done it will mean more work for the Rudder, and that won't please the Pilot."
"Whatever do you mean?" asked Efficiency. "What can the Rudder have to do with you?"
"It's like this," they replied: "when we are deflected downwards we gain a 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 the Aeroplane, which are deflected upwards into an area of rarefied air due to the suction effect (though that term is not academically correct) on the top of the Surface. If there is more Drift, i.e., Resistance, on one side of the Aeroplane than on the other side, then of course it will turn off its course, and if that difference in Drift is serious, as it will very likely be if there is no wash-out, then it will mean a good deal 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 that wash-out, and my friend the Designer is so clever at producing strength of construction for light weight, I'm pretty sure he won't mind paying the price in Lift. And now let me see if I can sketch the completed Aeroplane."
"Well, I hope that's all as it should be," she concluded, "for to-morrow the Great Test in the air is due."
[Footnote 4: Morane parasol: A type of Morane monoplane in which the lifting surfaces are raised above the pilot in order to afford him a good view of the earth.]
[Footnote 5: Skin friction is that part of the drift due to the friction of the air with roughness upon the surface of the aeroplane.]
[Footnote 6: Banking: When an aeroplane is turned to the left or the right the centrifugal force of its momentum causes it to skid sideways and outwards away from the centre of the turn. To minimize such action the pilot banks, i.e., tilts, the aeroplane sideways in order to oppose the underside of the planes to the air. The aeroplane will not then skid outwards beyond the slight skid necessary to secure a sufficient pressure of air to balance the centrifugal force.]
[Footnote 7: An explanation of the way in which the wash-out is combined with a wash-in to offset propeller torque will be found on p. 82.]
THE GREAT TEST
It is five o'clock of a fine calm morning, when the Aeroplane is wheeled out of its shed on to the greensward of the Military Aerodrome. There is every promise of a good flying day, and, although the sun has not yet risen, it is light enough to discern the motionless layer of fleecy clouds some five thousand feet high, and far, far above that a few filmy mottled 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 "stream-lined" to minimize drift, which is the resistance of the air to the passage of the machine, that to the veriest tyro the remark of the Pilot is obviously justified.
"Clean looking 'bus, looks almost alive and impatient to be off. Ought to have a turn for speed with those lines."
"Yes," replies the Flight-Commander, "it's the latest of its type and looks a beauty. Give it a good test. A special report is required on this machine."
The A.M.'s have now placed the Aeroplane in position facing the gentle air that is just beginning to make itself evident; the engine Fitter, having made sure of a sufficiency of oil and petrol in the tanks, is standing by the Propeller; the Rigger, satisfied with a job well done, is critically "vetting" the machine by eye; four A.M.'s are at their posts, ready to hold the Aeroplane from jumping the blocks which have been placed in front of the wheels; and the Flight-Sergeant is awaiting the Pilot's orders.
As the Pilot approaches the Aeroplane the Rigger springs to attention and reports, "All correct, sir," but the Fitter does not this morning report the condition of the Engine, for well he knows that this pilot always 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 living appearance, as if straining to be off and away. First he moves the Controls to see that everything is clear, for sometimes when the Aeroplane is on the ground the control lever or "joy-stick" is lashed fast to prevent the wind from blowing the controlling surfaces about and possibly damaging them.
The air of this early dawn is distinctly chilly, and the A.M.'s are beginning to stamp their cold feet upon the dewy grass, but very careful and circumspect is the Pilot, as he mutters to himself, "Don't worry and flurry, or you'll die in a hurry."
At last he fumbles for his safety belt, but with a start remembers the Pitot Air Speed Indicator, and, adjusting it to zero, smiles as he hears the Pitot-head's gruff voice, "Well, I should think so, twenty miles an hour I was registering. That's likely to cause a green pilot to stall the Aeroplane. Pancake, they call it." And the Pilot, who is an old hand and has learned a lot of things in the air that mere earth-dwellers know nothing about, distinctly heard the Pitot Tube, whose mouth is open to the air to receive its pressure, stammer, "Oh Lor! I've got an earwig already—hope to goodness the Rigger blows me out when I come down—and this morning air simply fills me with moisture; I'll never keep the Liquid steady in the Gauge. I'm not sure of my rubber connections either."
"Oh, shut up!" cry all the Wires in unison, "haven't we got our troubles too? We're in the most horrible state of tension. It's simply murdering our Factor of Safety, and how we can possibly stand it when we get the Lift only the Designer knows."
"That's all right," squeak all the little Wire loops, "we're that accommodating, we're sure to elongate a bit and so relieve your tension." For the whole Aeroplane is braced together with innumerable wires, many of which are at their ends bent over in the form of loops in order to connect with the metal fittings on the spars and elsewhere—a cheap and easy way of making connection.
"Elongate, you little devils, would you?" fairly shout the Angles of Incidence, Dihedral and Stagger, amid a chorus of groans from all parts of the Aeroplane. "What's going to happen to us then? How are we going to keep our adjustments upon which good flying depends?" "Butt us and screw us," wail the Wires. "Butt us and screw us, and death to the Loops. That's what we sang to the Designer, but he only looked sad and scowled at the Directors."
"And who on earth are they?" asked the Loops, trembling for their troublesome little lives.
"On earth indeed," sniffed Efficiency, who had not spoken before, having been rendered rather shy by being badly compromised in the Drawing Office. "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 and Credits—but all the same the Designer will get his way in the end. I'm his 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 quite happy."
Then she abruptly subsides, feeling that perhaps the less said the better until she has made a reputation in the Air. The matter of that Compromise still rankled, and indeed it does seem hardly fit that a bold bad Tin god should flirt with Efficiency. You see there was a little Tin god, and he said "Boom, Boom, BOOM! Nonsense! It MUST be done," and things like that in a very loud voice, and the Designer tore his hair and was furious, but the Directors, who were thinking of nothing but Orders and Dividends, had the whip-hand of him, and so there you are, and so poor beautiful Miss Efficiency was compromised.
All this time the Pilot is carefully buckling his belt and making himself perfectly easy and comfortable, as all good pilots do. As he straightens himself up from a careful inspection of the Deviation Curve of the Compass and takes command of the Controls, the Throttle and the Ignition, the voices grow fainter and fainter until there is nothing but a trembling of the Lift and Drift wires to indicate to his understanding eye their state of tension in expectancy of the Great Test.
"Petrol on?" shouts the Fitter to the Pilot.
"Petrol on," replies the Pilot.
Round goes the Propeller, the Engine sucking in the Petrol Vapour with satisfied gulps. And then—
"Contact?" from the Fitter.
"Contact," says the Pilot.
Now one swing of the Propeller by the Fitter, and the Engine is awake and working. Slowly at first though, and in a weak voice demanding, "Not too much Throttle, please. I'm very cold and mustn't run fast until my Oil has thinned and is circulating freely. Three minutes slowly, as you love 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 Indicator says. At last, "Steady at 1,500 revs. and I'll pick up the rest in the Air." Then does he throttle down the Engine, carefully putting the lever back to the last notch to make sure that in such position the throttle is still sufficiently open for the Engine to continue working, as otherwise it might lead to him "losing" his Engine in the air when throttling down the power for descent. Then, giving the official signal, he sees the blocks removed from the wheels, and the Flight-Sergeant saluting he knows that all is clear to ascend. One more signal, and all the A.M.'s run clear of the Aeroplane.
Then gently, gently mind you, with none of the "crashing on" bad Pilots think so fine, he opens the Throttle and, the Propeller Thrust overcoming its enemy the Drift, the Aeroplane moves forward.
"Ah!" says the Wind-screen, "that's Discipline, that is. Through my little Triplex window I see most things, and don't I just know that poor discipline always results in poor work in the air, and don't you forget it."
"Discipline is it?" complains the Under-carriage, as its wheels roll swiftly over the rather rough ground. "I'm bump getting it, and bump, bump, all I want, bang, bump, rattle, too!" But, as the Lift increases with the Speed, the complaints of the Under-carriage are stilled, and then, the friendly Lift becoming greater than the Weight, the Aeroplane swiftly and easily takes to the air.
Below is left the Earth with all its bumps and troubles. Up into the clean clear Air moves with incredible speed and steadiness this triumph of the Designer, the result of how much mental effort, imagination, trials and errors, failures and successes, and many a life lost in high endeavour.
Now is the mighty voice of the Engine heard as he turns the Propeller nine hundred times a minute. Now does the Thrust fight the Drift for all it's worth, and the Air Speed Indicator gasps with delight "One hundred miles 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 in tension, "This is the limit! the Limit! THE LIMIT! Release us, if only a quarter turn." But the Turnbuckles are locked too fast to turn their eyes or utter a word. Only the Locking Wires thus: "Ha! ha! the Rigger knew his job. He knew the trick, and there's no release here." For an expert rigger will always use the locking wire in such a way as to oppose the slightest tendency of the turnbuckle to unscrew. The other kind of rigger will often use the wire in such a way as to allow the turnbuckle, to the "eyes" of which the wires are attached, to unscrew a quarter of a turn or more, with the result that the correct adjustment of the wires may be lost; and upon their fine adjustment much depends.
And the Struts and the Spars groan in compression and pray to keep straight, for once "out of truth" there is, in addition to possible collapse, the certainty that in bending they will throw many wires out of adjustment.
And the Fabric's quite mixed in its mind, and ejaculates, "Now, who would have thought I got more Lift from the top of the Surface than its bottom?" And then truculently to the Distance Pieces, which run from rib to rib, "Just keep the Ribs from rolling, will you? or you'll see me strip. I'm an Irishman, I am, and if my coat comes off—— Yes, Irish, I said. I used to come from Egypt, but I've got naturalized since the War began."
Then the Air Speed Indicator catches the eye of the Pilot. "Good enough," he says as he gently deflects the Elevator and points the nose of the Aeroplane upwards in search of the elusive Best Climbing Angle.
"Ha! ha!" shouts the Drift, growing stronger with the increased Angle of Incidence. "Ha! ha!" he laughs to the Thrust. "Now I've got you. Now who's Master?" And the Propeller shrieks hysterically, "Oh! look at me. I'm a helicopter. That's not fair. Where's Efficiency?" And she can only sadly reply, "Yes, indeed, but you see we're a Compromise."
And the Drift has hopes of reaching the Maximum Angle of Incidence and vanquishing the Thrust and the Lift. And he grows very bold as he strangles the Thrust; but the situation is saved by the Propeller, who is now bravely helicopting skywards, somewhat to the chagrin of Efficiency.
"Much ado about nothing," quotes the Aeroplane learnedly. "Compromise or not, I'm climbing a thousand feet a minute. Ask the Altimeter. He'll confirm it." And so indeed it was. The vacuum box of the Altimeter was steadily expanding under the decreased pressure of the rarefied air, and by means of its little levers and its wonderful chain no larger than a hair it was moving the needle round the gauge and indicating the ascent at the rate 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, my Horizontal Equivalent!" despairingly call the Planes; "it's eloping with the Lift, and what in the name of Gravity will happen? Surely there was enough scandal in the Factory without this, too!" For the lift varies with the horizontal equivalent of the planes, so that if the aeroplane tilts sideways beyond a certain angle, the lift becomes less than the weight of the machine, which must then fall. A fall in such a position is known as a "side-slip."
But the ever-watchful Pilot instantly depresses one aileron, elevating the other, with just a touch of the rudder to keep on the course, and the Planes welcome back their precious Lift as the Aeroplane flicks back to its normal position.
"Bit bumpy here under these clouds," is all the Pilot says as he heads for a gap between them, and the next minute the Aeroplane shoots up into a new world of space.
"My eye!" ejaculates the Wind-screen, "talk about a view!" And indeed mere words will always fail to express the wonder of it. Six thousand feet up now, and look! The sun is rising quicker than ever mortal on earth witnessed its ascent. Far below is Mother Earth, wrapt in mists and deep blue shadows, and far above are those light, filmy, ethereal clouds now faintly tinged with pink. And all about great mountains of cloud, lazily floating in space. The sun rises and they take on all colours, blending one with the other, from dazzling white to crimson and deep violet-blue. Lakes and rivers here and there in the enormous expanse of country below refract the level rays of the sun and, like so many immense diamonds, send dazzling shafts of light far upwards. The tops of the hills now laugh to the light of the sun, but the valleys are still mysterious dark blue caverns, crowned with white filmy lace-like streaks of vapour. And withal the increasing sense with altitude of vast, 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 new and 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 Propeller exultingly battles with the Drift.
And look! What is that bright silver streak all along the horizon? It puzzled 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 Earth unveiling itself to the ardent rays of the Sun; and on his left, so high is he, there is yet black night, hiding innumerable Cities, Towns, villages, and all those places where soon teeming multitudes of men shall awake, and by their unceasing toil and the spirit within them produce marvels of which the Aeroplane is but the harbinger.
And the Pilot's soul is refreshed, and his vision, now exalted, sees the Earth a very garden, even as it appears at that height, with discord banished and a happy time come, when the Designer shall have at last captured Efficiency, and the Man-who-takes-the-credit is he who has earned it, and when kisses are the only things that go by favour.
Now the Pilot anxiously scans the Barograph, which is an instrument much the same as the Altimeter; but in this case the expansion of the vacuum box causes a pen to trace a line upon a roll of paper. This paper is made by clockwork to pass over the point of the pen, and so a curved line is made which accurately registers the speed of the ascent in feet per minute. No longer is the ascent at the rate of a thousand feet a minute, 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 that of right density, I must have to one part of Petrol, in order to give me full power and compression, and here at an altitude of ten thousand feet the Air is only two-thirds as dense as at sea-level. Oh, where is he who will invent a contrivance to keep me supplied with air of right density and quality? It should not be impossible within certain limits."
"We fully agree," said the dying Power and Thrust. "Only maintain Us and you shall be surprised at the result. For our enemy Drift decreases in respect of distance with the increase of altitude and rarity of air, and there is no limit to the speed through space if only our strength remains. And with oxygen for pilot and passengers and a steeper pitch 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 aeroplane become terrific. And, bearing that in mind, remember that with altitude gravity decreases. There may yet be literally other worlds to conquer.
Now at fifteen thousand feet the conditions are chilly and rare, and the Pilot, with thoughts of breakfast far below, exclaims, "High enough! I had better get on with the Test." And then, as he depresses the Elevator, the Aeroplane with relief assumes its normal horizontal position. Then, almost closing the Throttle, the Thrust dies away. Now, the nose of the Aeroplane should sink of its own volition, and the craft glide downward at flying speed, which is in this case a hundred miles an hour. That is what should happen if the Designer has carefully calculated the weight of every part and arranged for the centre of gravity 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 degree ensuring a speed of glide equal to its flying speed. And the Air Speed Indicator is steady at one hundred miles an hour, and "That's all right!" 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 to itself, and give all his attention, and he will need it all, to keeping the Aeroplane horizontal from wing-tip to wing-tip, and to keeping it straight on its course. The latter he will manage with the rudder, controlled by his feet, and the Compass will tell him whether a straight course is kept. The former he will control by the ailerons, or little wings hinged to the tips of the planes, and the bubble in the Inclinometer in front of him must be kept in the middle.
A pilot, being only human, may be able to do two things at once, but three is a tall order, so was this pilot relieved to find the Design not at fault and his craft a "natural glider." To correct this nose-heavy tendency when the Engine is running, and descent not required, the centre of Thrust is arranged to be a little below the centre of Drift or Resistance, 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 at no more than one-tenth its normal speed, has upset the proportion of petrol to air, and combustion is taking place intermittently or in the Exhaust Pipe, where it has no business to be. "Crash, Bang, Rattle——!——!——!" and worse than that, yells the Exhaust, and the Aeroplane, who is a gentleman and not a box kite, remonstrates with the severity of a Senior Officer. "See the Medical Officer, you young Hun. Go and see a doctor. Vocal diarrhoea, that's your complaint, and a very nasty one too. Bad form, bad for discipline, and a nuisance in the Mess. What's your Regiment? Special Reserve, you say? Humph! Sounds like Secondhand Bicycle Trade to me!"
Now the pilot decides to change the straight gliding descent to a spiral one, and, obedient to the Rudder, the Aeroplane turns to the left. But the Momentum (two tons at 100 miles per hour is no small affair) heavily resents this change of direction, and tries its level best to prevent it and to pull the machine sideways and outwards from its spiral course—that is, to make it "side-skid" outwards. But the Pilot deflects the Ailerons and "banks" up the planes to the correct angle, and, the Aeroplane skidding sideways and outwards, the lower surfaces of the planes press up against the air until the pressure equals the centrifugal force of the Momentum, and the Aeroplane spirals steadily downwards.
Down, down, down, and the air grows denser, and the Pilot gulps largely, filling his lungs with the heavier air to counteract the increasing pressure from without. Down through a gap in the clouds, and the Aerodrome springs into view, appearing no larger than a saucer, and the Pilot, having by now got the "feel" of the Controls, proceeds to put the Aeroplane through its paces. First at its Maximum Angle, staggering along tail-down and just maintaining horizontal flight; then a dive at far over flying speed, finishing with a perfect loop; then sharp turns with attendant vertical "banks," and then a wonderful switchback flight, 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 before in undue tension have secured relief by slightly elongating their loops, and each one is now doing its bit, and all are sharing the burden of work together.
The Struts and the Spars, which felt so awkward at first, have bedded themselves in their sockets, and are taking the compression stresses uncomplainingly.
The Control Cables of twisted wire, a bit tight before, have slightly lengthened by perhaps the eighth of an inch, and, the Controls instantly responding to the delicate touch of the Pilot, the Aeroplane, at the will of its Master, darts this way and that way, dives, loops, spirals, and at last, in one long, magnificent glide, lands gently in front of its shed.
"Well, what result?" calls the Flight-Commander to the Pilot.
"A hundred miles an hour and a thousand feet a minute," he briefly replies.
"And a very good result too," says the Aeroplane, complacently, as he is carefully wheeled into his shed.
* * * * *
That is the way Aeroplanes speak to those who love them and understand them. Lots of Pilots know all about it, and can spin you wonderful yarns, much better than this one, if you catch them in a confidential mood—on leave, for instance, and after a good dinner.
[Footnote 8: A.M.'s: Air Mechanics.]
[Footnote 9: Butt means to thicken at the end. Screw means to machine a thread on the butt-end of the wire, and in this way the wire can make connection with the desired place by being screwed into a metal fitting, thus eliminating the disadvantage of the unsatisfactory loop.]
[Footnote 10: Deviation Curve: A curved line indicating any errors in the compass.]
[Footnote 11: A propeller screws through the air, and the distance it advances during one revolution, supposing the air to be solid, is known as the pitch. 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, on account of the rarity of the air, the speed of the aeroplane increases, then the angle and pitch should be correspondingly increased. Propellers with a pitch capable of being varied by the pilot are the dream of propeller designers. For explanation of "slip" see Chapter IV. on propellers.]
[Footnote 12: Getting out of my depth? Invading the realms of fancy? Well, perhaps so, but at any rate it is possible that extraordinary speed through space may be secured if means are found to maintain the impulse of the engine and the thrust-drift efficiency of the propeller at great altitude.]
[Footnote 13: Box-kite. The first crude form of biplane.]
The Aeroplane had been designed and built, and tested in the air, and now it 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 and large profits; of poor pilots who had funked it, and good pilots who had expected 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 fat jobs.
Somehow, and despite every conceivable obstacle, it had managed to muddle 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, some of them shamefully obvious. But it was fairly sound mechanically, had a little inherent stability, was easily controlled, could climb a thousand feet 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 got the credit.
It is rough, unsettled weather with a thirty mile an hour wind on the ground, and that means fifty more or less aloft. Lots of clouds at different altitudes to bother the Pilot, and the air none too clear for the observation of landmarks.
As the Pilot and Observer approach the Aeroplane the former is clearly not in the best of tempers. "It's rotten luck," he is saying, "a blank shame that I should have to take this blessed 'bus and join X Reserve Squadron, stationed a hundred and fifty miles from anywhere; and just as I have licked my Flight into shape. Now some slack blighter will, I suppose, command it and get the credit of all my work!"
"Shut up, you grouser," said the Observer. "Do you think you're the only one with troubles? Haven't I been through it too? Oh! I know all about it! You're from the Special Reserve and your C.O. doesn't like your style of beauty, and you won't lick his boots, and you were a bit of a technical knut in civil life, but now you've jolly well got to know less than those senior to you. Well! It's a very good experience for most of us. Perhaps conceit won't be at quite such a premium after this war. And what's the use of grousing? That never helped anyone. So buck up, old chap. Your day will come yet. Here's our machine, and I must say it looks a beauty!"
And, as the Pilot approaches the Aeroplane, his face brightens and he soon forgets his troubles as he critically inspects the craft which is to transport him and the Observer over the hills and far away. Turning to the Flight-Sergeant he inquires, "Tanks 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 the locker; engine and Aeroplane logbooks written up, signed, and under your seat; engine revs. up to mark, and all the control cables in perfect condition and tension."
"Very good," said the Pilot; and then turning to the Observer, "Before we start you had better have a look at the course I have mapped out (see p. 40).
"A is where we stand and we have to reach B, a hundred and fifty miles due North. I judge that, at the altitude we shall fly, there will be an East wind, for although it is not quite East on the ground it is probably about twenty degrees different aloft, the wind usually moving round clockways to about that extent. I think that it is blowing at the rate of about fifty miles an hour, and I therefore take a line on the map to C, fifty miles due West of A. The Aeroplane's speed is a hundred miles 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 a line parallel to C—D. That is, to be exact, it will be fourteen degrees off the C—D course, as, in this part of the globe, there is that much difference between the North and South lines on the map and the magnetic North 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 to A—E, but, owing to the side wind, it will be actually travelling over the 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 journey of 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 calculating the compass course, and I always do it like that."
"Yes, that's plain enough. You have drafted what engineers call 'a parallelogram of forces'; but suppose you have miscalculated the velocity of the wind, or that it should change in velocity or direction?"
"Well, that of course will more or less alter matters," replies the Pilot. "But there are any number of good landmarks such as lakes, rivers, towns, and railway lines. They will help to keep us on the right course, and the compass will, at any rate, prevent us from going far astray when between them."
"Well, we'd better be off, old chap. Hop aboard." This from the Observer as he climbs into the front seat from which he will command a good view over 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 so start 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 teeth of the gusty wind.
In less than fifty yards it takes to the air and begins to climb rapidly upwards, but how different are the conditions to the calm morning of yesterday! If the air were visible it would be seen to be acting in the most extraordinary manner; crazily swirling, lifting and dropping, gusts viciously colliding—a mad phantasmagoria of forces!
Wickedly it seizes and shakes the Aeroplane; then tries to turn it over sideways; then instantly changes its mind and in a second drops it into a hole a hundred feet deep; and if it were not for his safety belt the Pilot might find his seat sinking away from beneath him.
Gusts strike the front of the craft like so many slaps in the face; and others, with the motion of mountainous waves, sometimes lift it hundreds of feet in a few seconds, hoping to see it plunge over the summit in a death-dive—and so it goes on, but the Pilot, perfectly at one with his mount and instantly alert to its slightest motion, is skilfully and naturally making perhaps fifty movements a minute of hand and feet; the former lightly grasping the "joy-stick" which controls the Elevator hinged to the tail, and also the Ailerons or little wings hinged to the wing-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 he loves and excels in; and given a cool head, alert eye, and a sensitive touch for the controls, what sport can compare with these ever-changing battles 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 such height looks absurdly small. The buildings below now seem quite squat; the hills appear to have sunk away into the ground, and the whole country below, cut up into diminutive fields, has the appearance of having been lately tidied and thoroughly spring-cleaned! A doll's country it looks, with tiny horses and cows ornamenting the fields and little model motor-cars and carts stuck on the roads, the latter stretching away across country like ribbons accidentally dropped.
At three thousand feet altitude the Pilot is satisfied that he is now sufficiently high to secure, in the event of engine failure, a long enough glide to earth to enable him to choose and reach a good landing-place; and, being furthermore content with the steady running of the engine, he decides to climb no more but to follow the course he has mapped out. Consulting the compass, he places the Aeroplane on the A—E course and, using the Elevator, he gives his craft its minimum angle of incidence at which it will just maintain horizontal flight and secure its maximum speed.
Swiftly he speeds away, and few thoughts he has now for the changing panorama of country, cloud, and colour. Ever present in his mind are the three great 'cross-country queries. "Am I on my right course? Can I see a good landing-ground within gliding distance?" And "How is the Engine running?"
Keenly both he and the Observer compare their maps with the country below. The roads, khaki-coloured ribbons, are easily seen but are not of much use, for there are so many of them and they all look alike from such an altitude.
Now where can that lake be which the map shows so plainly? He feels that surely he should see it by now, and has an uncomfortable feeling that he is flying too far West. What pilot is there indeed who has not many times experienced such unpleasant sensation? Few things in the air can create greater anxiety. Wisely, however, he sticks to his compass course, and the next minute he is rewarded by a sight of the lake, though indeed he now sees that the direction of his travel will not take him over it, as should be the case if he were flying over the shortest route to his destination. He must have slightly miscalculated the velocity or direction of the side-wind.
"About ten degrees off," he mutters, and, using the Rudder, corrects his course 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 so bad as they were near the ground, the broken surface of which does much to produce them; and sometimes for miles he makes but a movement or two of the controls.
The clouds just above race by with dizzy and uniform speed; the country below slowly unrolls, and the steady drone of the Engine is almost hypnotic 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; and rest, rest, rest to the sound of my rhythmical hum. Droning on and on, nothing whatever matters. All things now are merged into speed through space and a sleepy monotonous d-d-r-r-o-o-n-n-e———." But the Pilot pulls himself together with a start and peers far ahead in search of the next landmark. This time it is a little country town, red-roofed his map tells him, and roughly of cruciform shape; and, sure enough, there in the right direction are the broken outlines of a few red roofs peeping out from between the trees.
Another minute and he can see this little town, a fairy place it appears, nestling down between the hills and its red roofs and picturesque shape, a glowing and lovely contrast with the dark green of the surrounding moors.
So extraordinarily clean and tidy it looks from such a height, and laid out in such orderly fashion with perfectly defined squares, parks, avenues, and public buildings, it indeed appears hardly real, but rather as if it has this very day materialized from some delightful children's book!
Every city and town you must know has its distinct individuality to the Pilot's eye. Some are not fairy places at all, but great dark ugly blots upon the fair countryside, and with tall shafts belching forth murky columns of smoke to defile clean space. Others, melancholy-looking masses of grey, slate-roofed houses, are always sad and dispirited; never welcoming the glad sunshine, but ever calling for leaden skies and a weeping Heaven. Others again, little coquettes with village green, white palings everywhere, bright gravel roads, and an irrepressible air of brightness and gaiety.
Then there are the rivers, silvery streaks peacefully winding far, far away to the distant horizon; they and the lakes the finest landmarks the Pilot can have. And the forests. How can I describe them? The trees cannot be seen separately, but merge altogether into enormous irregular dark green masses sprawling over the country, and sometimes with great ungainly arms half encircling some town or village; and the wind passing over the foliage at times gives the forest an almost living appearance, as of some great dragon of olden times rousing itself from slumber to devour the peaceful villages its arms encircle.
And the Pilot and Observer fly on and on, seeing these things and many others which baffle my poor skill to describe—things, dear Reader, that you shall see, and poets sing of, and great artists paint in the days to come when the Designer has captured Efficiency. Then, and the time is near, shall you see this beautiful world as you have never seen it before, the garden it is, the peace it breathes, and the wonder of it.
The Pilot, flying on, is now anxiously looking for the railway line which midway on his journey should point the course. Ah! There it is at last, but suddenly (and the map at fault) it plunges into the earth! Well the writer remembers when that happened to him on a long 'cross-country flight in the early days of aviation. Anxiously he wondered "Are tunnels always straight?" and with what relief, keeping on a straight course, he picked up the line again some three miles farther on!
Now at last the Pilot sees the sea, just a streak on the north-eastern horizon, and he knows that his flight is two-thirds over. Indeed, he should have seen it before, but the air is none too clear, and he is not yet able to discern the river which soon should cross his path. As he swiftly speeds on the air becomes denser and denser with what he fears must be the beginning of a sea-fog, perhaps drifting inland along the course of the river. Now does he feel real anxiety, for it is the duty of a Pilot to fear fog, his deadliest enemy. Fog not only hides the landmarks by which he keeps his course, but makes the control of the Aeroplane a matter of the greatest difficulty. He may not realize it, but, in keeping his machine on an even keel, he is unconsciously balancing it against the horizon, and with the horizon gone he is lost indeed. Not only that, but it also prevents him from choosing his landing-place, and the chances are that, landing in a fog, he will smash into a tree, hedge, or building, with disastrous results. The best and boldest pilot 'wares a fog, and so this one, finding the conditions becoming worse and yet worse, and being forced to descend lower and lower in order to keep the earth within view, wisely decides to choose a landing-place while there is yet time to do so.
Throttling down the power of the engine he spirals downwards, keenly observing the country below. There are plenty of green fields to lure him, 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 it over; or one of rough surface likely to break the under-carriage. Now is perfect eyesight and a cool head indispensable. He sees and decides upon a field and, knowing his job, he sticks to that field with no change of mind to confuse him. It is none too large, and gliding just over the trees and head on to the wind he skilfully "stalls" his machine; that is, the speed having decreased sufficiently to avoid such a manoeuvre resulting in ascent, he, by means of the Elevator, gives the Aeroplane as large an angle of incidence as possible, and the undersides of the planes meeting the air at such a large angle act as an air-brake, and the Aeroplane, skimming over the ground, lessens its speed and finally stops 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 order to prevent the wind from blowing the controlling surfaces about and possibly damaging them, he hurriedly alights. Now running to the tail he lifts it up on to his shoulder, for the wind has become rough indeed and there is danger of the Aeroplane becoming unmanageable. By this action he decreases the angle at which the planes are inclined to the wind and so minimizes the latter's effect upon them. Then to the Observer, "Hurry up, old fellow, and try to find some rope, wire, or anything with which to picket the machine. The wind is rising and I shan't be able to hold the 'bus steady for long. Don't forget the wire-cutters. They're in the tool kit." And the Observer rushes off in frantic haste, before long triumphantly returning with a long length of wire from a neighbouring fence. Blocking up the tail with some debris at hand, they soon succeed, with the aid of the wire, in stoutly picketing the Aeroplane to the roots 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 dangerous bending-stresses in the woodwork. Their work is not done yet, for the Observer remarking, "I don't like the look of this thick weather and rather fear a heavy rain-storm," the Pilot replies, "Well, it's a fearful bore, but the first rule of our game is never to take an unnecessary risk, so out with the engine and body covers."
Working with a will they soon have the engine and the open part of the body which contains the seats, controls, and instruments snugly housed with their waterproof covers, and the Aeroplane is ready to weather the possible storm. Says the Observer, "I'm remarkably peckish, and methinks I spy the towers of one of England's stately homes showing themselves just beyond that wood, less than a quarter of a mile away. What ho! for a raid. What do you say?"
"All right, you cut along and I'll stop here, for the Aeroplane must not be left alone. Get back as quickly as possible."
And the Observer trots off, leaving the Pilot filling his pipe and anxiously scrutinizing the weather conditions. Very thick it is now, but the day is yet young, and he has hopes of the fog lifting sufficiently to enable the flight to be resumed. A little impatiently he awaits the return of his comrade, but with never a doubt of the result, for the hospitality of the country house is proverbial among pilots! What old hand among them is there who cannot instance many a forced landing made pleasant by such hospitality? Never too late or too early to help with food, petrol, oil, tools, and assistants. Many a grateful thought has the writer for such kind help given in the days before the war (how long ago they seem!), when aeroplanes were still more imperfect than they are now, and involuntary descents often a part of 'cross-country flying.
Ah! those early days! How fresh and inspiring they were! As one started off on one's first 'cross-country flight, on a machine the first of its design, and with everything yet to learn, and the wonders of the air yet to explore; then the joy of accomplishment, the dreams of Efficiency, the hard work and long hours better than leisure; and what a field of endeavour—the realms of space to conquer! And the battle still goes on with ever-increasing success. Who is bold enough to say what its limits shall be?
So ruminates this Pilot-Designer, as he puffs at his pipe, until his reverie is abruptly disturbed by the return of the Observer.
"Wake up, you airman," the latter shouts. "Here's the very thing the doctor ordered! A basket of first-class grub and something to keep the fog out, too."
"Well, that's splendid, but don't call me newspaper names or you'll spoil my appetite!"
Then, with hunger such as only flying can produce, they appreciatively discuss their lunch, and with many a grateful thought for the donors—and they talk shop. They can't help it, and even golf is a poor second to flight talk. Says the Pilot, who must have his grievance, "Just observe where I managed to stop the machine. Not twenty feet from this hedge! A little more and we should have been through it and into Kingdom Come! I stalled as well as one could, but the tail touched the ground and so I could not give the Aeroplane any larger angle of incidence. Could I have given it a larger angle, then the planes would have become a much more effective air-brake, and we should have come to rest in a much shorter distance. It's all the fault of the tail. There's hardly a type of Aeroplane in existence in which the tail could not be raised several feet, and that would make all the difference. A high tail means a large angle of incidence when the machine touches ground and, with enough angle, I'll guarantee to safely land the fastest machine in a five-acre field. You can, I am sure, imagine what a difference that would make where forced landings are concerned!" Then rapidly sketching in his notebook, he shows the Observer the following illustration:
"That's very pretty," said the Observer, "but how about Mechanical Difficulties, and Efficiency in respect of Flight? And, anyway, why hasn't such an obvious thing been done already?"
"As regards the first part of your question I assure you that there's nothing 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," hurriedly replies the Observer, whose soul isn't tuned to a technical key.
"As regards the latter part of your inquiry," went on the Pilot, a little nettled at having such a poor listener, "it's very simple. Aeroplanes have 'just growed' like Topsy, and they consequently contain this and many another relic of early day design when Aeroplanes were more or less thrown together and anything was good enough that could get off 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's really so?"
"I believe you're right. I am sure those hills over there could not be seen a few minutes ago, and look—there's sunshine over there. We'd better hurry up."
Ten minutes' hard work and the covers are off, neatly folded and stowed aboard; the picketing wires are cast adrift, and the Pilot is once more in his seat. The Aeroplane has been turned to face the other end of the field, and, the Observer swinging round the propeller, the engine is awake again and slowly ticking over. Quickly the Observer climbs into his seat in front of the Pilot, and, the latter slightly opening the throttle, the Aeroplane leisurely rolls over the ground towards the other end of the field, from which the ascent will be made.