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Aviation History
1911
1911 - 1057.PDF
DECEMBER a, 1911. anterior margin of the wing. Therefore the wing has to be retired, in order to keep the " lift " vertically over the " weight." Hence in fast horizontal flapping flight no advancing of the wings is to be observed. This description is only approximately correct. The force of flapping has to neutralise not only the weight but also the resistance to forward movement through the air. If a bird is in movement in the air it may be regarded as being acted on by four chief forces, namely, " lift," " weight," " pull " and " drag." In flapping flight the force exerted by the wings may be regarded as compounded of " lift " and '* pull." Of these two forces the " lift " acts vertically, and the " pull" horizontally. Their resultant may be called the " total pull." It is a force acting upwards and forwards. It balances a force compounded of the " weight " and the "drag." This force, which acts downwards and backwards, may be called the "total drag." If a bird is taking energy from the air in soaring flight it is being subjected to a propelling force. Therefore, the forces acting on it may be regarded as resulting in a " total pull " and a " total drag." By examining the position of the wings in different kinds of soaring flight we may be able to arrive at some conclusion as to the direction from which the unknown force of soarability acts. Figs. 62 to 66 show outlines of a vulture when circling and when flex-gliding at different speeds. It will be seen that as the speed of flex-gliding increases the larger is the proportion of wing area in front of the level of the centre of gravity. If a vulture is circling in fully soarable air with effort to gain height, its wings, besides being advanced, are placed in a dihedrally- up position, as shown in the following diagram (Fig. 67). A _ further reference to this employment of the dihedrally-up position will be made when I come to discuss the functions of the wing tips. A section of a vulture when slow flex-gliding may be represented thus (Fig. 68). As already stated, this position is apparently identical with that assumed for gliding with speed ahead in an ascending current. In this latter case the angle of incidence is about 900. In other words, the " total pull " acts in a direction at right angles to the surface of the wing, or nearly so. Therefore, in slow flex-gliding, the unknown force of soarability must also act in a direction approxi mately at right angles to the surface of the wing. If a vulture, when slow flex-gliding, wishes to increase its speed, it slightly increases the flexure of its wings. The secondary quills are thereby relaxed, and assume the following position (Fig. 69). Thus the wings of a fast flex-gliding vulture are disposed in a way which, if imitated by a power-driven aeroplane, would rapidly bring the machine to the earth. That the wings actually assume the position shown is a matter of comparatively easy observation. It is important to realise that the position of the surface of the wing is due to air-pressure, or more particularly by a pressure exerted by soarable air when under the vulture's wing. Flexing the wing, at the carpal-joint, results in relaxing the ligaments that hold the secondary quills in position. This relaxing of the ligaments, of itself, has no power of putting the secondary quills in their new position. It merely allows the feathers to take the position given to them by the pressure of the air. A little consideration will show that in fast flex-gliding the pressure is exerted at right angles to the surface of the wing, as is the case in slow flex-gliding. In Fig. 70, the disposition of the wing in slow flex-gliding is shown at A. The arrows represent the position and direction of the " total pull" and " total drag." The position of affairs in fast flex-glidirg is shown at C. The weight is as before. The resistance to passage through the air is I/UGHTI Hence in flex-gliding the faster the speed the more the wings are advanced. Therefore there seems to be no probable alternative to the con clusion that soarable air exerts a pressure on the under side of the wing of the flex-gliding vulture. It has already been shown that there are no observational or experimental reasons for assuming the presence of ascending currents in the neighbourhood of the bird that could be invoked as an explanation of this pressure. The view that ascending currents, in the ordinary sense of the word, have to do with soarability fails completely to explain why pressure is still exerted at right angles to the surface of the wings when the latter are fully advanced and relaxed as in fast flex-gliding. That is to say, the facts of the case in slow flex- gliding do not necessarily exclude ascending currents. But the facts of the case in fast flex - gliding furnish evidence that ascending currents are not the cause of soarability. Let us con sider the case from another point of view. Adjutant birds have an extensible pouch that hangs down from the lower part of the neck. Towards the end of the monsoon season before they leave Agra for their breeding haunts, this pouch is often extended and may reach a length of sixteen or more inches. Sometimes the pouch is seen extended while the bird is in soaring flight. It can then be seen swaying slowly to and fro in the air. Owing to its weight, and owing to the air pres sure from the forward movement of the bird, the pouch, in general, hangs downwards and backwards. It shows no indication of being pressed forwards and upwards. Rut the air under the wing of the bird, within a few inches of the sway ing pouch, is pressing the quill feathers upwards and forwards with a force that not only sustains the bird but that also propels it at a speed of thirty or forty miles an hour. How could ascending currents exert this tremendous force on the wings of the bird, and yet have as little apparent action on the pouch as they have on a floating feather in its neighbourhood ? There can be no doubt that the pressure on the under side of the adjutant's wing is exerted by air in motion. The motion is in such a direction as to exert pressure at right angles to the surface. An explosion of a gas is exerted at right angles to a flat surface. Therefore air as it passes under the wing of a soaring bird must undergo some change by which energy is liberated and which in the direction of the resulting force resembles the explosion of a gas. Fig. 70.—Diagram showing posi tion of "pull" and "drag," at A in slow flex-gliding, and at C in fast flex-gliding. "At B is shown an imaginary case in which the wings are placed in the fast flex-gliding position, except that they are not advanced. Hence between the 'pull' and the ' drag' there is a couple tending to rotate the bird round its transverse axis." Fig. 67.—Section of a vulture circling. Fig. 68.—Section of a vulture slow flex-gliding. Fig. 69.—Section of a vulture fast flex-gliding. increased owing to the increased speed. Therefore the " total drag " must act in a more backward direction. Hence I have drawn the "total drag" arrow in C pointing more backwards and less downwards than in A. But the " total drag " mutt act in a line with the " total pull." In fast flex-gliding the wing is further advanced, as I have drawn it at C. Hence, as shown at C, the force is still exerted at right angles to the surface and at the centre of area, or thereabouts. If the bird was to increase speed merely by relaxing the secondaries, as shown at B, a couple would originate tending to cause rotation downwards round the transverse axis. In one of the earlier chapters I pointed out that it was incon ceivable that the bird could get energy out of air if air is hom ogeneous, unless the passage of the vulture's wing causes some change or decomposition. Obviously, as shown, air from the point of view of soarability, is, in general, homogeneous. Therefore the conclusion is inevitable that the passage of the wing causes some change or decomposition in the air. If there is decomposition or explosion there must be something in soarable air that can decompose or explode. I propose to return to the questi in of this unknown some thing in a later chapter. (To be continued.) 1059
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