Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/159



§ 107. Wing Form,—Arched Section.—The most salient characteristics of wing form are common to birds of widely different species and habit of life. In spite of variations in detail and in general proportions, there is a certain uniformity of design and construction that cannot fail to impress even the most superficial observer.

The features in common may be taken, on the doctrine of natural selection, as consequent on the form of the fluid motion essential to flight, although, physically speaking, it is the fluid disturbance that depends upon the form of the wing.

To be definite, we may say that the general nature of the fluid motion can be shown to depend upon the major function of the wing, i.e., the support of the weight; the wing form must then conform to the motion so derived, and the detail of the fluid motion in turn will depend upon the more minute character of the wing form. Such indefinite process of adaptation and readaptation as the above implies is one to which the methods of evolution appear to be eminently adapted, but to which the methods of calculation are ill suited; hence much of the difficulty of the subject.

One of the most remarkable, and it may almost be said unexpected, peculiarities of wing form is the dipping front edge or arched section. This is a characteristic in the wing form of all birds capable of sustained flight, but it is only within comparatively the last few years that this feature has been the subject of observation. It is scarcely credible that so marked a peculiarity should have escaped observation for centuries, but it would seem that such is the case.