“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 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,[6] 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!”