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Aviation History
1909
1909 - 0382.PDF
JUNE 26, 1909. SOARING FLIGHT—HOW TO PERFORM IT.* By O. CHANUTE. THERE is a wonderful performance daily exhibited in southern climes, and occasionally seen in northerly latitudes in summer, which has never been thoroughly explained. It is the soaring or sailing flight of certain varieties of large birds who transport themselves on rigid unflapping wings in any desired direction ; who, in winds of 6 to 20 miles per hour, circle, rise, advance, return, and remain aloft for hours without a beat of wing, save for getting under way or convenience in various manoeuvres. They appear to obtain from the wind alone all the necessary energy, even to advancing dead against that wind. This feat is so much opposed to our general ideas of physics that those who have not seen it some- times deny its actuality, and those who have only occasionally witnessed it subsequently doubt the evidence of their own eyes. Others, who have seen the excep- tional performances, speculate on various explanations, but the majority give it up as a sort of " negative gravity." Personal observation has shown convincingly by analysis that a gull weighing 2-188 lbs., with a total supporting surface of 2'015 sq. ft., a maximum body cross-section of 0*126 sq. ft., and a maximum cross- section of wing edges of 0-098 sq. ft., patrolling on rigid wings (soaring) on the weather side of a steamer, and maintaining an upward angle or attitude of 50 to 7° above the horizon, in a wind blowing 12-78 miles an hour, which was deflected upward 10° to 200 by the side of the steamer (these all being carefully observed facts) was perfectly sustained at its own " relative speed" of 17-88 m.p.h. and extracted from the upward trend of the wind sufficient energy to overcome all the resistances, this energy amounting to 6-44 foot-pounds per second. It was shown that the same bird in flapping flight in calm air, with an attitude or incidence of 3° to 50 above the horizon and a speed of 20-4 miles an hour, was well sustained and expended 5-88 foot-pounds per second, this being at the rate of 204 pounds sustained per horse- power. It was stated also that a gull in its observed manoeuvres, rising up from a pile-head on unflapping wings, then plunging forward against the wind and sub- sequently rising higher than its starting point, must either time its ascents and descents exactly with the variations in wind velocities, or must meet a wind billow rotating on a horizontal axis and come to a pose on its crest, thus availing of an ascending trend. But the observations failed to demonstrate that the variations of the wind gusts and the movements of the bird were absolutely synchronous, and it was conjectured that the peculiar shape of the soaring-wing of certain birds, as differentiated from the flapping-wing, might, when ex- perimented upon, hereafter account for the performance. These computations, however satisfactory they were for the speed of winds observed, failed to account for the observed spiral soaring of buzzards in very light winds, and the writer was compelled to confess : " Now, this spiral soaring in steady breezes of 5 to 10 m.p.h., which are apparently horizontal, and through which the bird maintains an average speed of about 20 miles an hour, is the mystery to be explained. It is not accounted for, quantitatively, by any of the theories which have been advanced, and it is the one performance which has led some observers to claim that it was done through * A paper read before the International Aeronautical Congress,October, 1907. ' aspiration,' i.e., that a bird acted upon by a current, actually drew forward into that current against its exact direction of motion." A still greater mystery was propounded by the few '•-. observers who asserted that they had seen buzzards soaring in a dead calm, maintaining their elevation and ;" their speed. Among these observers was Mr. E. C. Huffaker, at one time assistant experimenter for Professor Langley. The writer believed and said then that he must in some way have been mistaken, yet, to satisfy himself, he paid several visits to Mr. Huffaker in Eastern Tennessee, and took along his anemometer. He saw quite a number of buzzards sailing at a height of 75 to 100 ft. in breezes measuring 5 or 6. miles an hour at the surface of the ground, and once he saw one buzzard soar- ing apparently in a dead calm. The writer was fairly baffled. The bird was not simply gliding, utilising gravity or acquired momentum, it was actually circling horizontally in defiance of physics and mathematics. It took two years and a whole series of further observations to bring those two sciences into accord with the facts. Curiously enough the key to the performance of circling in a light wind or a dead calm was not found through the usual way of gathering human knowledge, i.e., through observations and experiment. These had failed because I did not know what to look for. The mystery was, in fact, solved by an eclectic process of conjecture and com- putation, but once these computations indicated what observations should be made, the results gave at once the reasons for the circling of the birds, for their then observed attitude and for the necessity of an independent initial sustaining speed before soaring began. Both Mr. Huffaker and myself verified the data many times, and I made the computations. ;:.,:. These observations disclosed several facts :— 1st. That winds blowing 5 to 17 m.p.h. frequently had rising trends of io3 to 15°, and that upon occasions when there seemed to be absolutely no wind, there was often nevertheless a local rising of the air estimated at a rate of four to eight miles or more per hour. This was ascertained by watching thistledown and rising fogs along- side of trees or hills of known height. Everyone will readily realise that when walking at the rate of four to eight miles an hour in a dead calm the "relative wind" is quite inappreciable to the senses, and that such a rising air would not be noticed. 2nd. That the buzzard, sailing in an apparently dead horizontal calm, progressed at speeds of 15 to 18 m.p.h., as measured by its shadow on the ground. It was thought that the air was then possibly rising 8-8 feet per second, or 6 m.p.li. 3rd. That when soaring in very light winds the angle of incidence of the buzzards was negative to the horizon —i.e., that when seen coming toward the eye, the after- noon light shone on the back instead of on the breast, as would have been the case had the angle been inclined above the horizon. 4th. That the sailing performance only occurred after the bird had acquired an initial velocity of at least 15 or 18 m.p.h., either by industrious flapping or by descend- ing from a perch. 5th. That the whole resistance of a stuffed buzzard, at a negative angle of 30 in a current of air of 15*52 m.p.h., 384
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