Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/267



§ 141. Introductory.—The phenomenon of "soaring" is not witnessed with sufficient regularity or frequency in the British Isles for it to impress itself on the ordinary observer as a distinct mode of flight. It is not at all unusual in fact to find the term entirely misapplied, and in view of the dictionary definition it is, perhaps, necessary to admit two distinct usages of the word, the one meaning being simply to fly to a height, quite regardless of the mode of flight employed, and the other being that commonly accepted and used in scientific works, the equivalent of the French term vol à voile; it is in the latter signification that the word is now employed.

In the active flight of birds, that is in flight such as that of the pigeon, where the wings are energetically employed (the vol ramé of the French), we are rightly accustomed to regard the work performed in the flapping of the wings as the source of the power expended in flight, and it is a matter of ordinary observation that under normal conditions, when the wing flapping ceases, the flight path takes on the whole a downward trend; under these passive conditions we know that the energy that is still being expended in the continued flight is derived from the descent of the body under the force of gravity, the angle of free descent being indicated by the symbol γ throughout the present work. The conditions under these circumstances are analogous to those of a cycle or motor vehicle when "freewheeling" or "coasting"; the flight path must on the whole be at a sufficient downward inclination to supply the energy necessary for propulsion; the analogy also comprehends the