H.E., Horizontal equivalent.H.E., Horizontal equivalent.
"Please imagine," said the Blackboard, "that the topVis the front view of a Surface flying towards you. Now if a gust blows it into the position of the lowerVyou 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 theVis 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 theVlower 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 pendulumand 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,6i.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 orfin, 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 setin effectat 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 havein effecta 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 ofIncidence, 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 orwashed-outtowards 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."
'Wash out' on both sides."Wash out" on both sides.
"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 Incidencemeans 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 thesuctioneffect (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,7and 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."
4(return)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.
5(return)Skin friction is that part of the drift due to the friction of the air with roughness upon the surface of the aeroplane.
6(return)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.
7(return)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.
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.'s8have 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 springsto 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 whichare 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,"9wail 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! ItMUSTbe 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 ofhim, 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 carefulinspection of the Deviation Curve10of 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.
"Ignition off?"
"Ignition off."
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, thatis. 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'mbumpgetting it, andbump,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 certainangle, 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, hidinginnumerable 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 Driftdecreases 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 pitch11for 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 altitudegravity decreases. There may yet be literally other worlds to conquer.12
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 relievedto 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,13remonstrates with the severity of a Senior Officer. "See the Medical Officer, you young Hun. Go and see a doctor. Vocal diarrhœa, 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, proceedsto 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.
8(return)A.M.'s: Air Mechanics.
9(return)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.
10(return)Deviation Curve: A curved line indicating any errors in the compass.
11(return)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.
12(return)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.
13(return)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 themap 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.
A--B, 150 miles, A--C, 50 miles; direction and miles per hour of wind. C--D, 100 miles; airspeed of aeroplane. A--D, Distance covered by aeroplane in one hour. A--E, Compass course.A–B, 150 miles,A–C, 50 miles; direction and miles per hour of wind.C–D, 100 miles; airspeed of aeroplane.A–D, Distance covered by aeroplane in one hour.A–E, Compass course.
A–B, 150 miles,A–C, 50 miles; direction and miles per hour of wind.C–D, 100 miles; airspeed of aeroplane.A–D, Distance covered by aeroplane in one hour.A–E, Compass course.
"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 mindand 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.