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Aviation History
1912
1912 - 0102.PDF
(/ycfif FEBRUARY 3, 1912. SOARING FLIGHT. DISCUSSION AT THE AERONAUTICAL SOCIETY/ Definition. 1. Soaring is that mode of flight in which a. bird remains indefinitely aloft without flapping its wings. 2.'If you take a suitable piece of paper and fix a weight to its edge, it will glide to earth when properly launched into the air. (Fig- I-) . 3. Gliding is also a mode of flight, but differs from soaring in that the path of motion is, on the whole, downwards, whereas in soaring there is no permanent loss of altitude. 4. Flapping flight differs from soaring flight in that the bird exerts visible energy. 5. From these facts may be deduced—what is otherwise a matter of elementary perception—that flight of any description involves the expenditure of power, and that in soaring flight this energy must come from the air. 6. The primary question for discussion is, therefore, what form does this energy take ? 7. In general, the accepted answer to this problem may be summed up in the word " wind." 8. Air in relative motion to the earth is wind, and, having mass, wind possesses momentum capable of doing work through suitable mechanisms. wind, but they are all fallacious. In fact, they are as fallacious a& arguments that attempt to demonstrate the possibility of perpetual motion. Whatever may happen in the first instance, by the inertia of the bird or its initial momentum enabling it to resist the wind for a short while, ultimately the bird and its orbit must both travel at the speed of the wind. Then, the bird is manoeuvring in virtual calm— like the fly in the atmosphere of a moving railway carriage—and soaring becomes impossible. " In the absence of wind, the soaring bird must fall, his soaring flight being no longer possible, he is obliged to flap ; it is for this reason that he is seldom an early riser matmal) because the early morning is ordinarily calm, especially in hot countries."— Mmtillard. "Any attempt to found the theory of soaring on the horizontal orbit of flight is doomed to failure."—Lanchester. Oblique Winds. 15. If the wind is not horizontal but, although uniform, has an adequate upward trend, sufficient propulsive component will be established to permit soaring flight to be continued indefinitely. The bird may move when it pleases within the zone of the suitable wind, it may hover over one spot on earth if the condition^ ©_ .«n/«-S- u-S^Sti. 1' L —— " ® FLIGHT SPEED So 1 CUFF 1 - >__—_— 1 WIND fcrH/'SEc -J 0 I Id • J © 7 / r\ N ,,r V h \ \ A ( © 9. A bird's wing is a suitable appliance for extracting energy from the air, as is evident from the phenomenon of soaring. Propulsion. 10. All forms of flight demand relative motion between the object supported and the air. 11. But the body thus supported offers resistance to such relative motion ; consequently it will be blown away in the same direction as the wind unless an opposing force of propulsion enables it to hold its own. 12. In a paper kite, this force is the pull on the string. In an aeroplane, it is the thrust of the engine-driven propellers. In a glider, it is the component of gravity in the line of motion. In a soaring bird it is the horizontal component of the wind pressure on the bird's wing which has a forward direction (i.e., against the wind) by virtue of the resultant being inclined forward of the vertical (Fig. 2). 13. It is impossible to obtain a forwardly inclined resultant from any kind of wing section in a truly horizontal and uniform wind, hence soaring flight is impossible under such conditions (Fig. 3). 14. Many involved arguments have been put forward to prove that a bird can, by suitable evolutions, soar in a uniform horizontal * Opening argument by Mr. A. E. Berriman, Technical Editor of FLIGHT. facilitate such exact equilibrium, or it may, as more often is the case, circle indefinitely round an orbit that is more or less fixed ir> space. " The condors moved in large curves, sweeping in circles, descending and ascending without once flapping. . . the head and neck were moved frequently, and with force, and it appeared as if the extended wings formed the fulcrum on which the move ments of the neck, body, and tail acted."—Darwin. " The soaring bird moves in circles for the same reason that a man sits in a chair, because he wants to stay where he is." —Lanchester. 16. The least upward velocity component that will satisfy this condition of soaring flight is numerically the same as the rate of vertical descent when gliding in calm. 17. If the wind blows parallel to the side of a hill having the same slope as the gliding path of the bird, the wind must have at least the same velocity as the bird's natural flight speed in order to maintain soaring. (Fig. 4) 18. Obstructions such as cliffs, houses, and ships, cause horizontal winds to trend upwards to an extent that is frequently adequate to maintain soaring. " The region of greatest up current is evidently immediately in advance of the upper edge of the cliff, and the useful region ma I02
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