(I) Three planes are more stable than two. [Elevator—main aerofoil—horizontal rudder behind.]
(J) When an aeroplane is gliding (downwards) stability is greater than in horizontal flight.
(K) A large moment of inertia is inimical (opposed) to stability.
(M) Aeroplanes (naturally) stable up to a certain velocity (speed) may become unstable when moving beyond that speed. [Possible explanation. The motion of the air over the edges of the aerofoil becomes turbulent, and the form of the stream lines suddenly changes. Aeroplane also probably becomes deformed.]
(N) In a balanced glider for stability a separate surface at a negative angle to the line of flight is essential. [Compare F.]
(O) A keel surface should be situated well above and behind the centre of gravity.
(P) An aeroplane is a conservative system, and stability is greatest when the kinetic energy is a maximum. (Illustration, the pendulum.)
§ 3. Referring to A. Models with a plane or flat surface are not unstable, and will fly well without a tail; such a machine is called a simple monoplane.
Fig. 4.—One of Mr. Burge Webb's Simple Monoplanes.
Showing balance weight A (movable), and also his winding-up gear—a very handy device.