Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/331

FLIGHT 319 root of the wing posteriorly. The tip and posterior parts of the wing are more active than the root and anterior parts, from the fact that the tip and posterior parts (the ving is an eccentric) always travel through greater spaces, in a given time, than the root and anterior parts. The wing is so constructed that the posterior margin yields freely in a downward direction during the up stroke, while it yields comparatively little in an upward direction during the down stroke : and this is a distinguishing feature, as the wing is thus made to fold and elude the air more or less completely during the up stroke, whereas it is made to expand and seize the air with avidity during the down stroke. The oblique line referred to as running diagonally across the wing virtually divides the wing into an active and a passive part, the former elevating and propelling, the Litter sustaining. It is not possible to determine with exactitude the precise function discharged by each part of the wing, but experi ment tends to show that the tip of the wing elevates, the posterior margin propels, and the root sustains. The wing and this is important is driven by a direct piston action with an irregular hammer-like movement, the pinion having communicated to it a smart click at the beginning of every down stroke the up stroke being more uniform. The following is the arrangement (fig. 32). If rotate, the blades, because of their elasticity, assume a great variety of angles, the angles being least where the Fio. 34. Double clastic wing driven by direct piston action. During the up stroke of the piston the wing is very decidedly convex on its upper surface (a&crf.AA ); its under surface (efgh,A A ) being deeply concave and in clined obliquely upwards find forwards. It thus evades, to a considerable extent, the air during the up stroke. During the down stroke of the piston the wing is flattened out in every direction, and its extremities twixted in such a manner as to form two screws, as seen at a b c d , e f (f /* , B, IV. Tho active area of the wing is by this arrangement considerably diminished during the up stroke, and considerably augmented during the down stroke; the wing seizing the air with greater avidity during the down than during the up stroke. i,j, k, elastic band to regulate the expansion of the wing; I, piston ; m, piston head; n, cylinder. (Pettigrew, lb"0.) speed of the blades is greatest and vice versa. The pitch of the blades is thus regulated by the speed attained (fig. 35). FIG. 32. Elastic spiral wing, which twists and untwists during its action, to form a mobile helix or screw. This wing is made to vibrate by a direct piston action, and by a slight adjustment can be propelled vertically, horizontally, or at any degree of obliquity. a l>, Anterior margin of wing, to which the neurse or ribs are affixed, c d, Pos terior margin of wing crossing anterior one. x, Ball-and-socket joint at root of wing, the wing being attached to the side of the cylinder by the socket, t, Cylinder, r r, Piston, with cross heads (w, tr) and piston head (,?). o o, Stuffing boxes, e, /, Driving chains, m. Superior elastic band, which assists in elevating the wing, n, Inferior elastic band, which antagonizes m. The alternate stretching of the superior and inferior elastic bands contributes to the continuous play of the wing, by preventing dead points at the end of the down and up strokes. The wing is free to move in a vertical and horizontal direction and at any degree of obliquity. (Pcttigrew, 1870.) the artificial wing here represented (fig. 32) be compared with the natural wing as depicted at fig. 33, it will be seen that there is nothing in the one which is not virtually reproduced in the other. In addition to the foregoing, FIG. 33 shows the spiral elastic wings of the Gull. Each wing forms n mobile elix or screw, a b. Anterior margin of left wing; c d, posterior margin of ditto; dg, primary Oi- rowing feathers of left wing; g a, secondary feathers tto; .r, root of right wing with bull and socket joint; /, elbow joint; m, wrist joint; n, o, hand and finger joints. (Pettigrew, 1870.) Professor Pettigrew recommends a double elastic wing to be applied to the air like a steam-hammer, by being fixed to the head of the piston. This wing, like the single wing described, twists and untwists as it rises and falls, and possesses all the characteristics of the natural wing (fig. 34). He also recommends an elastic aerial screiv consisting of two blades, which taper and become thinner towards the tips and posterior margins. When the screw is made to FJG. 35. Elastic aerial screw with twisted blades resembling wings (a I c d, efgh) x, end of driving shaft; 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) ; ab,ef, 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, g, r indicate the direction of travel. (Pettigrew, 1870.) The peculiarity of Professor Pettigrew s wings and screws consists in their elasticity, their twisting action, and their great comparative length and narroivness. 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. Since Professor Pettigrew enunciated his views (18G7) as to the screw configuration and elastic properties of natural wings, and more especially since his introduction of spiral, elastic artificial wings, and elastic screics, a great revolution has taken place in the construction of flying models. Elastic aero-planes are now advocated by Mr Brown, 1 elastic aerial screws by Mr Armour, 2 and elastic aero-planes, wings, and screws by M. Penaud. 3 M. Penaud s experiments are alike interesting and in structive. He constructed models to fly by three different methods : (a) by means of screws acting vertically up wards ; (b) by aero-planes propelled horizontally by screws ; and (c) by wings which flapped in an upward and down ward direction. An account of his helicopte"re or screw model appears in the Aeronaut for January 1872, but before giving a description of it, it may be well to state very 1 "The Aero-bi-plane, or First Steps to Flight," Ninth Annual Report of the Aeronautical Society of Great Britain, 1874. 2 "Resistance to Falling Planes on a Path of Translation," Xintli Annual Report of the Aeronautical Society of Great Britain, 1874, 3 The Aeronaut for January 1872 and February 1S75.