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Aviation History
1911
1911 - 0925.PDF
OcroBEk 28 191 r [/0GHf] £y DrEH.Hankin. MA. DSc. (Copyright Reserved) CHAPTER XXVII.—Movements Round the Transverse Axis in Flapping Flight. IT is a matter of common observation that just before perching a bird usually makes a few flaps of its wings. These flaps may seem a trivial matter to investigate, but it will l>e at once apparent that they are of considerable theoretical interest. My first observation concerning this matter was as follows :— June 26th, 1910.—At Ballia Ravine, 1.34.—A vulture seen gliding up the valley to settle. When near the tree on which it was about to perch, it flapped in order to gain height or speed. The direction of the flaps could be clearly seen to be up and down. Then for a few metres it glided without flapping. Just is to say, stop-flapping occurred with the wings in the advanced position. The explanation of this rotation round the transverse axis is both obvious and simple. In Fig. 46, at A, a bird is shown with its wings in the dihedrally up position. This, as already explained, produces a couple tending to rotate the bird upwards round its transverse axis. Obviously, this result depends on the resistance that the wings or wing tips experience to forward motion through the air. Therefore it is an adjustment that must be more efficient the faster the bird is moving. If the bird is gliding slowly, or it the bird 'wishes to check its speed, a different adjustment is employed. The wings are placed in the advanced position, as shown Fig. 44.—Stages in settling of a vulture. The bird is shown travelling from right to left—at A gliding, B flapping for a short distance. The arrows show the direction of the strokes. C again gliding, but with feet hanging down ; D and E, body beginning to hang down below level of wings; F, after rotating through nearly a right angle, the wings have commenced "stop flapping." As shown by the arrows, the direction of beats in stop flapping is nearly horizontally to and fro. before perching it hung down its body and again flapped. The direction of these flaps was quite clearly seen to be fore and aft. That is to say, these flaps were meant to act as a brake. For the sake of clearness, I show these different stages in the process of settling in Fig. 44. At A the bird is shown gliding. Then at B it is shown flapping with strokes apparently vertically up and down, as shown by the direction of the arrows. At c the bird is again gliding, and its feet are hanging down. At D and E the bird continues to glide, but the legs and also the body are hanging down. At F the bird is again flapping, but with the beat of the wings in a fore and aft direction. That is to say, before this flapping commenced, or as it commenced, the wings rotated through a right-angle. To this form of flapping I propose to give the name of "stop flapping." In a later paragraph I shall explain in Fig. 46 at is. When in this position the wings present a resis tance to dropping downwards through the air that may be regarded as concentrated at a point, which point must be in advance of the centre of gravity. Hence there must be a couple that rotates the bird round its transverse axis. This, in fact, is the method used by cheels, crows, scavenger vultures, parrots, and other birds in settling either with or without stop flapping. It is noteworthy that advancing the wings causes rotation round the transverse axis, but no direct change of course. A scavenger, gliding downwards at a small ancle with the horizon, may be seen suddenly to slightly advance its wings. The result is a slight rotation of the whole bird, including the wings round the transverse axis. That is to say, the angle of incidence is increased. Hence the wings act as a brake, and speed decreases. As the bird gets nearer its perch Fig. 45.—Stages in settling of a cbeel—at A gliding; at B the wings are advanced, in consequence the bird rotates round its transverse axis as shown at C. At D stop flapping has commenced. Fig. 46.—Diagram showing two methods of rotating round transverse axis. At A the wings are placed dihedrally upwards ; at B the wings are shown advanced. Either disposition results in a couple tending to rotate the bird upwards (beak up, tail down) round the transverse axis. the difference between stop-flapping and the flapping used by the poising kingfisher. The latter form of flapping lifts the bird. Stop-flapping, on the other hand, has no appreciable lifting effect, but tends to check the forward motion of the bird through the air. Shortly after making the above observation, I noticed a cheel settling. In this case there was no hanging down of the body before the commencement of stop-flapping. At the moment that the stop flapping commenced, not only did the wings change their plane of action, but also there was a simultaneous rotation upwards of the whole bird round its transverse axis. How this rotation occurred was shown to me by observation of yet another cheel that did not perch by the usual method, but by the procedure shown in Fig. 45. At A this cheel is shown gliding towards its perch with the wings " straight," that is to say, with their centre of effort nearly or quite on a level with their centre of gravity. The first preparation for perching made by this cheel was to put its wings in the " advanced " position as shown at B. The bird immediately began to rotate round its transverse axis, as shown at C. After the rotation had occurred, stop-flapping began, as shown at D. That there may be a further advancing of the wings. The consequent further rotation round the transverse axis acts as a stronger brake, so that the bird may drop vertically on to its perch. In a slight wind, an eagle, for instance, may stop without any stop flapping. It is striking to see the bird gliding along at a height of two or three feet from the ground, suddenly drop its legs, and perch on a shrub or other projection without any apparent effort to check its speed beyond the advancing of the wings and expansion of the tail. While thus checking speed the tail is expanded and depressed, so that its surfaces may, in some cases, be placed almost at a right angle to the line of flight. In the case of the g»een parrot, stop flapping always occurs, and the wings may clearly be seen to be in an advanced position. Rotation round the transverse axis may also occur when the bird is swooping downwards at high speed. If in such a case the rotation is caused by advancing the wings, there is no change of course, but speed is checked. If, on the other hand, in the case of cheels, rotation is caused by placing the wings in the dihedrally - up position there is a change of course and less loss of speed. This
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