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Aviation History
1913
1913 - 0328.PDF
I/O GHT MARCH 22, 1013. -NATURE'S AEROPLANES." By E. F. AT I'aytona Beach, Ha., during January, February, and March, 1911, with the self-proferred and very able assistance of Mr. R. E. Acre, the writer attempted to make some additions to the present meagre store of data upon the flight of soaring birds, and the details of their physical construction. The first part of our experiments consisted of securing specimens of large soaring birds from which to obtain our data and information. This part of the experiments was accomplished by the aid of a gun, and could hardly be regarded as work by even the poorest scientist. The stream-line form of the body of these birds we found to be comparatively bad in front. It is evident that here nature ha« sacrificed perfect stream-line with other and more necessary objects in view. Even so, the air cutting qualities of a bird's head are unquestionably far superior to those of a Gnome motor. The cross- section of the body of every bird examined was nearly a perfect circle at its largest point, which is located under the front edge of the wing. From this point backwards the body tapers very rapidly in a vertical direction, but very little horizontally ; its cross-section at the rear edge of the wing being that of a very flat ellipse with its major axis running from port to starboard. Here the fleshy part of the body ends. But the stream-line form continues on past this point and blends imperceptibly into the tail. The tip of this organ is usually about three-quarters of the chord from the rear edge of the wing, and during ordinary flight it is folded so as to be very little wider at this point than at its juncture with the body. Thus the tail and the body form one unit, of very good stream-line form, which reminds one very strongly of the flat fantail Bleriots. The wings are attached with their tips on a level with the top of the body. At their point of attachment they are blended into the body by a multitude of small downy feathers, in such a way as to leave no air- pockets to set up unnecessary head resistance. The designers of aeroplanes would do well to copy the bird, at least in this respect. Although considerable attention was paid to general details of the biids we examined, by tar the greater part of our time was spent in securing the wing outline, and the cross-section of the wing at regular intervals from centre to tip. The process of securing this data consisted of first laying the bird on its back on a large sheet of paper, and tracing the outline of one wing and a half of the body with a lead pencil. This done, a wing was cut off close to the body, and nailed securely to several small Mock*, which were in turn nailed to the bottom of a large box. We took great pains to always nail down the wing as nearly as possible in its normal soaring position. When all was ready, melted paraffin was poured into the box until the wing was covered. After hardening the block of paraffin containing the wing, it was prized out of the box, and sawn into pieces of equal width. By running a flame over the face of these, and laying a piece of tracing paper over it while it was still hot, a very accurate cross-section could be traced off, reproductions of which are herein enclosed. Three features seem to be characteristic of every wing examined. First, the great thickness of the wing near the front edge; second, its rather deep curve; and finally, the steady decrease of both thickness and curvatuie from the wing root towards its tip. The thickening of the front edge of the wing near the body is undoubtedly a structural necessity, as the wing has no diagonal bracing, the strain increases as the point of attachment to the body is neared. The rapidity with which this thickness decreases from the wing root outward would seem to indicate that it was undesirable aerodynamically. It is the opinion of the writer, however, that the ideal form of wing should be just thick enough at all points to be made of proper stream-line form. For gliding flight 01 in an aero plane I can see no reason why a wing should be thicker at the base than at the tip. The thinning of the bird's wing, however, is due to thj necessity of flexibility in the wing tip during flapping flight, and for obvious structural reason. The wing curves secured showed the maximum curvature to be well towards the front edge, and rather deep in all cases. It was our opinion, however, that this was not a permanent condition. A bird's wing is constructed in such a manner as to make the camber variable throughout a wide range. The bird changes this at will, according to the different conditions under which it flies. The wing sections here shown were, of course, taken when the wing was in a limp condition, and without the pressure on them they receive during flight. Due partly to this and partly to the voluntary flattening which is accomplished by an upward rotation of the front edge of the wing around the large wing-bone, the wing-curve in flight is probably much flattened' and changed in shape. The decrease of curvature from the centre of the wing towards the tip was very noticeable. A slight flattening of the wing at the extreme tip is probably good aerodynamically, on account of the Mc tUp of the air off the wing end, but the bird possesses it to an ANDREWS. extent not warranted by this cause. A logical explanation of this decrease in curve was made by Mr. Wilbur Wright, in the presence of the writer. He attributes it to the fact that the tip of a bird's Wing Plans and Wine Sections of Six Common Birds Hawk wing must act as a propeller during a flapping flight, and for this the deeply curved section is not well adapted. Now a word in regard to the flight of soaring birds. This has a'ways proved as interesting to me as it is mysterious. 334
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