§ 2. In connexion with this same question of automatic stability, the question must be considered from the theoretical as well as from the practical side, and the labours and researches of such men as Professors Brian and Chatley, F.W. Lanchester, Captain Ferber, Mouillard and others must receive due weight. Their work cannot yet be fully assessed, but already results have been arrived at far more important than are generally supposed.
The following are a few of the results arrived at from theoretical considerations; they cannot be too widely known.
(A) Surfaces concave on the under side are not stable unless some form of balancing device (such as a tail, etc.) is used.
(B) If an aeroplane is in equilibrium and moving uniformly, it is necessary for stability that it shall tend towards a condition of equilibrium.
(C) In the case of "oscillations" it is absolutely necessary for stability that these oscillations shall decrease in amplitude, in other words, be damped out.
(D) In aeroplanes in which the dihedral angle is excessive or the centre of gravity very low down, a dangerous pitching motion is quite likely to be set up. [Analogy in shipbuilding—an increase in the metacentre height while increasing the stability in a statical sense causes the ship to do the same.]
(E) The propeller shaft should pass through the centre of gravity of the machine.
(F) The front planes should be at a greater angle of inclination than the rear ones.
(G) The longitudinal stability of an aeroplane grows much less when the aeroplane commences to rise, a monoplane becoming unstable when the angle of ascent is greater than the inclination of the main aerofoil to the horizon.
(H) Head resistance increases stability.