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Aviation History
1911
1911 - 0908.PDF
By DrEJi.Hankin. MA. DSc. (Copyright Reserved) • oOO Fig. 34.—Outline of a vulture when diving. Span when wings extended, 85 ins. ; span when diving, 40 ins. ; weight of bird, 5,520 grammes. The transverse line is drawn through the centre of gravity. CHAPTER XXIV.—Movements of the Outside Wing Tip in Circling. I HAVK already described steering movements of the inside wing tip in circling vultures. I have now to describe certain movements of the outside wing tip still more difficult to observe, and whose meaning is obscure. An example of my first observations relating to this matter is the following:— March 7th, 1910.-—At 1.0.—A large group of vultures circling in Taj Gunj direction. They were from 100 metres up and upwards. Several showed windward dip strongly marked, and also slight arching of outside wind on the windward side of the track. At first I regarded this movement of the outside wing as identical with the arching that I had already observed in cheels. But I gradually noticed that this arching, unlike that of cheels, only affected the outer part of the wing. Then as I became more familiar with the appearance, I recog nised that this false arching was of the nature of a " half-dip movement." But it was only when I arrived in Naini Tal that I was able to make conclusive ob servations that showed that the false arching in question was of the nature indicated. The following are indications of my Naini Tal observations :— June 19th, 1910.—At Ballia Ravine, 12.30.—A black vulture circling showed windward dip (that is to say, of inside wing) on windward side of track. This was seen to be followed by a slight half dip of outside wind. June 22nd, 1910.—At Ballia Ravine, 3.45.—A vulture circling with wings nearly flat showed windward dip followed by a half dip of the outer wing. This was again seen shortly afterwards. There were a few small cloud shadows. I formed the impression that vultures circling in sunshine had wings dihedrally up, while the one or two vultures seen circling with wings flat were in shade. At Ballia Ravine I noticed that, in circling, besides windward and leeward dips, other small dip movements of the inside wing occasionally occurred. In addition, the windward and leeward dips were often followed by slight dip movements of the outside wing tip. These usually last longer than the previously described movements of the inside wing. A half dip of the outside wing may last for several seconds. These movements of the outside wing tip occur more frequently than otherwise on the windward side of the track, that is to say, at that portion of the circle where there is most canting, and presumably most steering. That is to say, the outside wing tip is occasionally rotated downwards at a time when it should be rotated upwards, if Wright's method played a part in steering movements. But the movement of the outside wing tip does not seem to be, or always to be, simply a half-dip movement. Occasionally, at least, it is combined with slight retirement of the phalangeal quiil feathers. This gives the wing tip a rounded appearance. For instance:— August 17th, 1910.— At Jharna Nullah. At 11.50.—Sunshine increasing, and circling of larger birds beginning. Adjutants circling with occasional flapping. Light wind. Some adjutants circling showed clearly rounding of outside wing tip on windward side of track. This was seen three times. Twice also a very slight rounding, merely a slight retirement of the first phalangeal quill Was observed.* + No doubt the succeeding three phalangeal qjn'Us were ako retired. But it was only in the case of the first quill, thanks to its position, that the movement could with certainty be distinguished. 12.6—Adjutants circling overhead a short way up showed half dip of outside wing along windward side as a depression of first phalangeal quill only. This was often combined with a slight retirement of this feather. This latter movement (extent probably about an inch) was presumably an indication of very slight relaxation of secondaries. It could only be seen when the birds were directly overhead or slightly to windward. As soon as they had drifted past to leeward, the quill feathers seemed to approximate, owing to their being seen foreshortened. 12.50.—Two miles along Tundla Road, beyond Jharna Nullah. An adjutant circling, in nearly complete calm, and well canted over, showed on each of several circles observed, slight alula (perhaps a little more than half an inch) of inside wing on up wind side of circle. While I was watching it the wind in creased. The bird then circled with its wings not quite fully extended. No advancement of the alula then occurred. The bird was not so canted as previously. Presumably it was ease- circling. The advancement of the alula, when it occurred, was quite definitely and clearly seen. The bird showed scarcely any leeward drift, so there could not have been much wind at the height at which it was circling. On one other occasion I have seen advancement of the alula in an adjutant. The above is the only instance in which I have definitely noted in my diary that the wing tip was retired during the half dip of the outside wing. It must be obvious that both these movements are very difficult to see. The retirement is best seen when the bird is directly overhead. The dip, on the other hand, would be more easily observed when the bird is seen from the side. But the following instance suggests that the two movements are combined : November 13th, 1910.—At Jharna Nullah. 9.53.—Vultures began circling. 10.14.—Vultures began slow flex-gliding. 10.24.—No half dips or retirement of outside wing tips have been seen as yet, though I had looked carefully for these move ments. 10.30.—Besides columns of birds circling and drifting to lee ward, vultures were now circling without leeward drift over the slaughter-house. Previously, all circling vultures had shown leeward drift. 10.33.—Vultures seen flex-gliding at medium speed. 10.35.—A vulture seen fast flex-gliding. 10.46.—Many, perhaps a hundred, vultures started to windward of me and drifted overhead at a height of 20 or 30 metres. They were gaining height rapidly. They showed many half dips and retirement of outside wing tips. These occurred both on wind ward and leeward sides of the circle. They were seen in every vulture that could be observed. II.32.—Vultures now were circling for the most part with wing tips of both wings slightly retired. This is ease-circling. It had not been observed previously. But vultures at lower levels were circling with wings slightly advanced or straight (tips not retired) and occasionally flapped. A long time before making these observations, I had noticed in circling vultures that the quill feathers of the outside wing tip were occasionally not as fully extended (that is to say, advanced) as those of the inside wing tip. I was, at first, of opinion that this was a sign that the outside wing did less work in circling, and that, consequently, the bird did not find it necessary to use the muscular exertion necessary for full extension. In view of the facts now described, it appears more probable that this lack of full extension is a sign of slight decrease of camber of the outside wing, which decrease, it may be surmised, is favourable for the relatively greater speed of the outside wing when the bird is travelling on a curved course. When describing my observations in Naini Tal, I mentioned certain cases in which I was able to see a decrease of camber of the outside wing in circling, as evidenced by a slight relaxation of the secondaries. Presumably these cases were of half dips with retire ment of the outside wing tip. The fact that while observing the relaxation of the secondaries I made no note of any movement of the wing tips, proves nothing. The relaxation was so very difficult to see that all my attention must have been concentrated on the hind margins of the two wings. Small movements of the wing tip may well have been occurring at the time. Thus, the exact nature of the movement of the outside wing tip in 910
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