"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,[7] 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."

[Ill.u.s.tration]

"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.]

PART III

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 pa.s.sage 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[8] 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 gra.s.s, but very careful and circ.u.mspect 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?" "b.u.t.t us and screw us,"[9] wail the Wires. "b.u.t.t 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 G.o.d, 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 G.o.d should flirt with Efficiency. You see there was a little Tin G.o.d, 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[10] of the Compa.s.s 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, 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 _b.u.mp_ getting it, and _b.u.mp_, _b.u.mp_, all I want, _bang_, _b.u.mp_, _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 b.u.mps 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 e.j.a.c.u.l.a.t.es, "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.