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Aviation History
1909
1909 - 0755.PDF
NOVEMBER 27, 1909. SOARING FLIGHT. By WILBUR BEFORE bringing his second paper, which he read before the American Western Society of Engineers in 1903, to a conclusion, Wilbur Wright devoted a section to the subject of soaring flight (see FLIGHT, NOV. 13th, 1909), to which, as his opening remarks show, he and his brother Orville paid great attention during such times as they were not actually making their own gliding experiments. " In addition to the work with the machine, we also made many observations on the flight of soaring birds, which were very abundant in the vicinity of our camp. Bald eagles, ospreys, hawks and buzzards gave us daily exhibitions of their powers. The buzzards were the most numerous and were the most persistent soarers. They apparently never flapped except when it was absolutely necessary, while the eagles and hawks usually soared only when they were at leisure. Two methods of soaring were employed. When the weather was cold and damp and the wind strong, the buzzards would be seen soaring back and forth along the hills or at the edge of a clump of trees. They were evidently taking advantage of the current of air flowing upward over these obstructions. On such days they were often utterly unable to soar except in these special places. But on warm clear days when the wind was light they would be seen high in the air, soaring in great circles. Usually, however, it seemed to be necessary to reach a height of several hundred feet by flapping before this style of soaring became possible. Frequently a great number of them would begin circling in one spot, rising together higher and higher, till finally they would disperse, each gliding off in whatever direction it wished to go. At such times other buzzards only a short distance away found it necessary to flap frequently in order to maintain themselves. But when they reached a point beneath the circling flock, they too began to rise on motionless wings. This seemed to indicate that rising columns of air do not exist everywhere, but that the birds must find them. They evidently watch each . other, and when one finds a rising current the others quickly make their way to it. One day when scarce a breath of wind was stirring on the ground, we noticed two bald eagles sailing in circling sweeps at a height of probably 500 feet. After a time our attention was attracted to the flashing of some object con- siderably lower down. Examination with a field glass proved it to be a feather which one of the birds had evidently cast. As it seemed apparent that it would come to earth only a short distance away, some of our party started to get it. But in a little while it was noticed that the feather was no longer falling, but, on the contrary, was rising rapidly. It finally went out of sight upward. It apparently was drawn into the same rising current in which the eagles were soaring, and was carried up like the birds. " The days when the wind blew horizontally gave us the most satisfactory observations, as then the birds were compelled to make use of the currents flowing up the sides of the.hills, and it was possible for us to measure the velocity and trend of the wind in which the soaring was performed. One day four buzzards began soaring on the north-east slope of the Big Hill at a height of only 10 or 12 ft. from the surface. We took a position to windward and about 1,200 ft. distant. The clinometer showed that they were 4^° to 5 £° above our horizon. We could see them very distinctly with a field glass. When facing us the under side of their wings made a broad 757 WRIGHT. band on the sky, but when in circling they faced from us we could no longer see the under side of their wings. Though the wings then made little more than a line on the sky the glass showed clearly that it was not the under side that we saw. It was evident that the buzzards were soaring with their wings constantly inclined about 5° above the horizon. They were attempting to gain sufficient altitude to enable them to glide to the ocean beach three-fourths of a mile distant, but after reaching a height of about 75 ft. above the top of the hill they seemed to be unable to rise higher, though they tried a long time. At last they started to glide toward the ocean, but were compelled to begin flapping almost immediately. We at once measured tha slope and the wind. The former was i2|°; the latter was 6 to 8 metres per second. Since the wings were inclined 50 above the horizon and the wind had a rising trend of fully 120, the angle of incidence was about 170. The wind did not average more than 7 metres, 15 miles an hour. For the most part the birds faced the wind steadily, but in the lulls they were compelled to circle or glide back and forth in order: to obtain speed sufficient to provide support. As the- buzzard weighs about "8 lb. per square foot of wing area* the lifting power of the wind at 17° angle of incidence was apparently as great as it would have been had it been blowing straight upward with equal velocity. The pressure was inclined 5° in front of the normal, and th§ angle of descent was 12\°. " On another day I stood on the top of the West Hill directly behind a buzzard which was soaring on the steep southern slope. It was just on a level with my eye and not more than 75 ft. distant. For some time it remained almost motionless. Although the wings were inclined about 50 above the horizon, it was not driven backward by the wind. This bird is specially adapted to soaring at large angles of incidence in strongly rising currents. Its wings are deeply curved. Unless the upward trend amounts to at least 8° it seems to be unable to maintain itself. One day we watched a flock attempting to soar on the west slope of the Big Hill, which has a descent of nearly 9°. The birds would start near the top and glide down along the slope very much as we did with the machine, but we noticed that whenever they glided parallel with the slope their speed diminished, and when their speed was maintained the angle of descent was greater than that of the hill. In every case they found it necessary to flap before they had gone 200 ft. They tried time and again but always with the same results. Finally they resorted to hard flapping till a height of about 150 ft. above the top of the hill was reached, after which they were able to soar in circles without difficulty. On another day they finally succeeded in rising on almost the same slope, from which it was concluded that the buzzard's best angle of descent could not be far from 8°. There is no question in my mind that men can build wings having as little or less relative resistance than that of the best soaring birds. The bird's wings are un- doubtedly very well designed indeed, but it is not any extraordinary efficiency that strikes with astonishment but rather the marvellous skill with which they are used. It is true that I have seen birds perform soaring feats of almost incredible nature in positions where it was not possible to measure the speed and trend of the wind, but whenever it was possible to determine by actual measure- ment the conditions under which the soaring was per-
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