“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 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 of 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.”