Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/23



§ 1. Introductory.—The equilibrium and stability of a bird in flight, or of an aerodrome or flying machine, has in the past been the subject of considerable speculation, and no adequate explanation of the principles involved has hitherto been given. The question of stability is studied to the greatest advantage in the case of gliding flight, for the problem is then presented in its simplest form; it is probable that the underlying principles are identical, whether a bird is in active flight, or whether it be poised on rigid pinions, as when soaring, or when merely gliding from a point of greater to one of less altitude.

There are two methods employed by nature to secure stability where animal life is concerned, i.e., mechanical means and nervous control. As an illustration of the first may be cited the case of a quadruped, whose equilibrium when standing is of the mechanical order; and of the second the biped, whose equilibrium is maintained by the action of the nervous system.

Mechanical stability does not necessarily involve an obvious apparatus like the four legs of a table, as might be supposed from the above illustration; the stability of a spinning top for instance, or of a common hoop, is entirely mechanical, but these