He also recommends an elastic aerial screw consisting of two blades, which taper and become thinner towards the tips and posterior margins. When the screw is made to rotate, the blades, because of their elasticity, assume a great variety of angles, the angles being least where the speed of the blades is greatest and vice versa. The pitch of the blades is thus regulated by the speed attained (fig. 35).
The peculiarity of Pettigrew's wings and screws consists in their elasticity, their twisting action, and their great comparative length and narrowness. They offer little resistance to the air when they are at rest, and when in motion the speed with which they are driven is such as to ensure that the comparatively large spaces through which they travel shall practically be converted into solid bases of support.
After Pettigrew enunciated his views (1867) as to the screw configuration and elastic properties of natural wings, and more especially after his introduction of spiral, elastic artificial wings, and elastic screws, a great revolution took place in the construction of flying models. Elastic aeroplanes were advocated by D.S. Brown,[14] elastic aerial screws by J. Armour,[15] and elastic aeroplanes, wings and screws by Alphonse Pénaud.[16]
[Illustration: FIG. 35.--Elastic Aerial Screw with twisted blades resembling wings (a b c d, e f g h).
v,w, Sockets in which the roots of the blades of the screw rotate, the degree of rotation being limited by steel springs (z, s).
a b, e f, tapering elastic rods forming anterior or thick margins of blades of screw.
d c, h g, Posterior or thin elastic margins of blades of screw. The arrows m, n, o, p, q, r indicate the direction of travel.]
Pénaud's experiments are alike interesting and instructive. He constructed models to fly by three different methods:--(a) by means of screws acting vertically upwards; (b) by aeroplanes propelled horizontally by screws; and (c) by wings which flapped in an upward and downward direction. An account of his helicoptère or screw model appeared in the _Aeronaut_ for January 1872, but before giving a description of it, it may be well to state very briefly what is known regarding the history of the screw as applied to the air.