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Aviation History
1923
1923 - 0703.PDF
NOVEMBER 15, 1923 the underside of its wings of a white colour. When circlingthe white area appears a little larger than it does in the dead bird. When soaring at a higher speed than circling, the whitearea appears magnified to more than twice its real size. When thus gliding across the field of view and seen from the side.The white area of the near wing appears magnified and that of the far wing appears diminished. The third peculiar phenomenon to be described is theappearance of colour on the underside of the wings of soaring birds. If one watches a scavenger vulture in soaring flightthe white portion of the underside of its wing appears of a bright chrome yellow colour. If the bird is looked at througha crystal of Iceland spar two images of the bird are seen. One of these images appears yellow, while, on some occasions,the other image appears snow white. If the bird is on a curved course the inside wing has the deeper colour. This descriptionapplies to the Bengal vulture and to the scavenger vulture. In the adjutant bird the colour appears not on the wings, buton the underside of the body. If an adjutant is on a curved course a patch of deeper yellow appears on the side of the bodynear to the inside wing. If any of these birds ceases to take energy from the air, if, that is, by flexing its wings it beginsto fall feet foremost through the air, the yellow colour instantly vanishes. If the bird then changes its mind anddecides to glide on instead of landing, the yellow colour reappears as it extends its wings and begins to regain itsspeed ahead. The next phenomenon to consider is the occurrence of strepupward glides without apparent loss of speed. These have been observed by me in different species of birds, and also inthe case of dragon-flies. In such glides the long axis of the body is tilted up in the direction in which the soaring animalis travelling. The plane of the wings, so far as observation shows, also lies in the line of flight. This is entirely thereverse of what occurs when the bird is in a known ascending current. In such a current the bird is lifted gradually as on arising tide. Its. long axis is horizontal, and the plane of the ' wings, whenever observable, shows an apparent negativeangle of incidence which, as already explained, indicates the positive angle of incidence with the bird's relative wind. Thechange to this apparent negative angle at the moment of entering the ascending current on the windward side of thesteamer has been observed by me in the case of gulls. In the steep upward glides we are now considering there is noapparent negative angle. There is no gradual lifting as on a rising tide. The bird shoots upwards almost as if fired outof a gun. Gulls show an apparent increase of speed at the moment of commencing the upward glide. That this is a realincrease of air speed is made probable by the fact that it is accompanied by an increase of flexing of the wings, which isthe disposition used in high-speed flight. An instance has been observed by me of two scavengers making a steep upward glidefor a distance of more than 200 metres, their course while doing so making an angle of between 30 and 40° with thehorizon. In the case of gulls the steepness of the glide may be much more. It may amount to as much as 70° or 80°with the horizon. If maintenance in the air was the only thing accomplishedby the soaring bird, it might be justifiable to settle the matter by assumptions about its gliding angle and suppositions aboutascending currents. The occurrence of these steep upward glides furnishes a clear proof that something more isrequired. The most astounding fact about these upward glides is thatthey only occur on occasions when there are grounds for sus- pecting that the bird is in a descending current. On the windward side of a steamer there is an ascendingcurrent in which gulls sometimes glide with gradual gain of height as usual in such currents. s At or near the stern of thesteamer, and especially on its leeward side, there is usually a descending current. Often, but only if wind is present, gullscan soar when enveloped in this descending current. It is in this descending current that steep upward glides occur.Gulls that have stayed behind for food may be seen overtaking the ship by flapping flight just above water level. When theyreach the leeward side of the stern, still near water level, they may be seen to make a sudden turn upwards, to cease flapping,and to glide steeply upwards, usually to about the level of the stern flagstaff. When near the level of the top of the flagstaffthev may be near the limit of the descending current. There may be room for arguing that they are outside of it and aidedby a neighbouring ascending current. Hence this glide upwards is of interest in that it begins near water level, whereit is very difficult to believe that an ' strong upward current can be present. In these steep upward glides in a descending current we seethe mysterious force of soarability in its strongest form, and here is where one. should look for some clue to the nature of th<?phenomenon. The first question that arises is whether these apparentlydescending currents are really descending. A very interesting observation by Mr. J. D. North bears on this point. Heinforms me that at the stern of a steamer he has observed pieces of paper that he. had thrown overboard rising continuouslywhen enveloped in dilute descending smoke. That is to say, there was an admixture of ascending air that carried the paperwith descending air that carried the smoke particles. Major Turner, in a very suggestive paper on soaring flight, quotesexperiments by Dr. Betz of Gottingen, in which it was found that if an alternating up-and-down movement was given to theair current in a wind channel, a model wing showed negative resistance ; that is, it tried to move against, the wind stream.These facts suggest the theory that in descending currents in which soaring flight has been observed neighbouring masses ofair may be ascending and descending relatively to each other and that, as in the experiment of Dr. Betz, such opposedmovements can do more for the soaring bird than could the ascending component alone. Should soaring flight indescending currents be due to such a cause one might anti- cipate that a descending mass of air striking another mass ofair relatively at rest would be better fitted for such flight than a " contour current " formed by a light wind flowing over arounded surface, for in the former case there would be much more tendency for a mixture of air currents flowing in differentdirections to he formed than in the latter. Further, if alter- nations of air direction have to do with soaring flight one mightanticipate that a certain minimum speed would be necessary before soaring, as distinct from gliding, could begin. Further,one might anticipate that a particular speed of the bird would be appropriate to a particular degree of alternation. Also, ifa bird on entering the descending current lacked the necessary speed, one might anticipate that, or at least would not besurprised, if. at that moment, it showed some form of in- stability. Let us see how far the facts of the case agree withsuch anticipations. In dust-storms of the kind that have been described by meas " primary dust-storms," the dust is raised by steeply descending currents that strike the earth as if coming from agigantic hosepipe. In such descending currents cheels and scavengers make steep upward glides, recalling those made bygulls at the stern of a steamer. Such glides appear always to commence at the moment when the bird is struck by a gust, asshown by movements of the trees below them. A proof that it is at this moment that the bird is entering the descendingcurrent is given by the fact that at this moment the bird shows instability round the transverse axis. It has been foundby me that both soaring and non-soaring birds show this form of instability when leaving but not when entering an ascendingcurrent. One would expect them therefore to show it also when leaving relatively still air for a descending current.This instability is shown or countered by the bird by a sudden upward jolt of the tail and depression of the wings. At themoment of entering a descending current of a dust-storm the bird shows tail-jolting which may be at the rate of four or fivejolts per second. With gulls, on entering the descending current at the stern of a steamer, this form of instability hasbeen seen by me, but it is very transient and difficult to observe. It may be replaced by a trembling of the wings. In the case oftwo scavengers gliding upwards in a dust-storm gust, already mentioned, my notes relate that they were tail-jolting allthrough their upward glide for at least 200 metres, and that it was so marked as to be visible with the naked eye though thetail of the scavenger is much smaller than that of the cheel. This may be regarded as a proof that all through the glide,which was at high speed, they were passing through portions of air that relatively to each other were ascending anddescending. The immediate neighbourhood of dust-storm, gusts contains rising currents, as shown by the movementsof the dust, but in such air other forms of instability occur and no steep upward glides. On one occasion a dust-storm was observed by me whichreached the steamer while we were going through the Red Sea. When struck by gusts of this storm, gulls in the soarablearea glided ahead with very great and sudden increase of speed. On one occasion, near the Straits of Bonifacio, in agale of wind, which came from nearly ahead, some gulls were following the ship in gliding flight. Occasionally one of themturned round and glided for a long distance, at least 100 metres away from the ship to leeward. It did this with its wingsfully extended and flat as if to check speed. Then it turned round to glide agai n towards the ship. It did this with wingsflexed and arched -—that is, in the high-speed wing disposition During this glide up to the ship its speed through the airmust have approached 30 metres per second. The fact of a 703
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