“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.