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]
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| Fig. 35.—Elastic Aerial Screw with twisted blades resemblingwings (a b c d, e f g h). | |
x, End of driving shaft. v,w, Sockets in which the rootsof the blades of the screwrotate, the degree of rotationbeing limited by steelsprings (z, s). | a b, e f, tapering elastic rods forminganterior or thickmargins of blades of screw. d c, h g, Posterior or thin elasticmargins of blades of screw.The arrows m, n, o, p, q, rindicate the direction oftravel. |
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.
| Fig. 36.—Cayley’s Flying Model. |
The first suggestion on this subject was given by A.J.P. Paucton in 1768. This author, in his treatise on the Théorie de la vis d’Archimède, describes a machine provided with two screws which he calls a “ptérophores.” In 1796 Sir George Cayley gave a practical illustration of the efficacy of the screw as applied to the air by constructing a small machine, consisting of two screws made of quill feathers, a representation of which we annex (fig. 36). Sir George writes as under:—
“As it may be an amusement to some of your readers to see a machine rise in the air by mechanical means, I will conclude my present communication by describing an instrument of this kind, which any one can construct at the expense of ten minutes’ labour.
“a and b, fig. 36, are two corks, into each of which are inserted four wing feathers from any bird, so as to be slightly inclined like the sails of a windmill, but in opposite directions in each set. A round shaft is fixed in the cork a, which ends in a sharp point. At the upper part of the cork b is fixed a whalebone bow, having a small pivot hole in its centre to receive the point of the shaft. The bow is then to be strung equally on each side to the upper portion of the shaft, and the little machine is completed. Wind up the string by turning the flyers different ways, so that the spring of the bow may unwind them with their anterior edges ascending; then place the cork with the bow attached to it upon a table, and with a finger on the upper cork press strong enough to prevent the string from unwinding, and, taking it away suddenly, the instrument will rise to the ceiling.”
Cayley’s screws were peculiar, inasmuch as they were superimposed and rotated in opposite directions. He estimated that if the area of the screws was increased to 200 sq. ft., and moved by a man, they would elevate him. His interesting experiment is described at length, and the apparatus figured in Nicolson’s Journal, 1809, p. 172.