The Aeroplane Speaks. Fifth Edition

"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!" "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 set in effect at a much less Angle of Incidence than the Main Surface. To explain we must trouble the Blackboard again," and this was his effort: