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Aviation History
1911
1911 - 0735.PDF
AUGUST 26, 1911. j4£tufyoL3ir&3?igfi£ m OE3o By DrE.HHankm.MA.DSc (Cofiyrig h t Reser ved) o oOO CHAPTER VI.—Ease-Gliding and Lift-Gliding. ON any fine day, after soarability has been established, a large number of birds of different species may be seen circling and gliding over the Jharna Nullah factory. At intervals a cluster of birds becomes separate from the rest and commences to drift, circling more or less directly to leeward. Sometimes as many as half a dozen clusters each containing one or two hundred birds may be seen at one time. Sometimes I have been able to observe that the starting of a cluster of birds to leeward was coincident with the coming of a puff of wind. After the cluster has drifted one to three miles to leeward it breaks up and the birds that had formed it may be seen flex-gliding either directly up-wind to join the original group of birds over the slaughter-house, or else they flex-glide in different directions to join other groups of circling birds. In a light wind the birds may be seen to be flex-gliding at low speed with wings only slightly flexed. If the wind freshens all the birds in sight may be seen suddenly to increase the flexing of their wings, that is to say, there is no dihedrally-up angle and the wings are not advanced. In rare cases cheels and vultures may be seen gliding up-wind in a more or less straight line with gain of height. For this form of flight I propose the term " lift-gliding." In lift-gliding the wings are held in the same position as in circling. In lift-gliding the bird always shows instability round the dorso-ventral axis. By this term I mean the axis that is vertical if the bird is gliding hori zontally. In this kind of instability the bird shows a tendency to rotate for short distances to and fro round the axis in question. The distance through which a bird may lift-glide varies from a few metres up to one or two hundred metres. At the end of a lift-glide the bird may either begin circling, or, flexing its wings, may flex- glide up-wind at increased speed. In either case its previous instability at once vanishes. I shall have occasion to describe lift- gliding more minutely after bringing forward evidence bearing on the question of the nature of soarability. In the presence of a strong wind, currents of air are deflected (OHHltUlI The above diagram is from " Fauna of British India Birds," by Oates and Blanford, as copied in the Journal of the Bombay Natural History Society, Vol. XVII, p. 850. As an index drawing, showing the positions of the principal parts of a bird, it should be of general interest and of particular assistance to those reading this article. that is to say to make the adjustment necessary for flex-gliding at higher speed. Fig. 10 is a looking-glass record of the track of a cheel first circling and drifting with the wind, and then flex-gliding up-wind at slow speed. Some time after morning soarability for cheels has been established these birds may often be seen gliding in irregular curves without gain or loss of height and at moderate speed. I propose the term "ease-gliding" for this form of flight. Scavengers also indulge frequently in ease-gliding. The heavier vultures show this form of flight less often. Fig. 11 shows the track of a cheel while ease- gliding. Ease-gliding of a vulture is shown in Fig. 12. In the case of vultures when ease-gliding, the wings are held flat and the front margins of the two wings are in one straight line, upwards from the walls of high buildings. If the air is soarable cheels appear to avoid, rather than otherwise, such ascending currents. But, in the morning, before soarability has been established, and late in the afternoon, when soarability near the earth decreases, cheels collect on the windward side of such buildings and by taking advantage of the ascending currents remain ease-gliding in the air. I propose to describe this phenomenon in detail on a later occasion. CHAPTER VII.—Canted Flex-Gliding. I have now to describe a very remarkable phenomenon which at once shows that the problem of the nature of soarability must be solved by serious research and not by idle theorising. 737
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