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Aviation History
1911
1911 - 1013.PDF
NOVEMBER 25. 1911. p GHT Eg 0£?o», jtf„Sfa&o£3jrd.37i$6f By Dr EH.hankin. MA. DSc. (Copyrig h t Reser ved) .0OQ1 CHAPTER XXXV.—Lateral Stability. AN important part of the control of an aeroplane is the means adopted for ensuring lateral stability. The question arises as to the nature of the method employed by birds for subserving this important adjunct to their pawers of flight. I have seen a cheel gliding through the open doorway of a racket court. The doorway was too narrow for the expanded wings of the bird. As it reached the door, it smoothly and evenly canted itself over, glided through in this canted position, and then, when on the other side, presumably returned with equal smoothness to a level keel. I believe it is the perfection of the method employed by birds for thus canting to one side or the other that has prevented my discovering its nature with certainty by direct observation. Some facts to be described in the present chapter will, however, enable us to draw an inference as to the nature of the adjustment. I have already proved that lateral stability is not due to rotation of the wing tips in opposite directions. It is conceivable that lateral stability might be maintained by rotation of the wings them selves in opposite directions. But it is not likely that birds would employ an adjustment in which both wings would be so placed as to tend to check speed ahead, and in which the lifting efficiency of hoth wings would be diminished. Further, such a suggestion is not .supported by any facts of observation. It is possible to observe wing rotation. For instance, I recently saw a cheel, for a fraction •of a second, rapidly rotate its wings to and fro, to a very small •extent round their long axes. This happened while the bird was gliding and about to perch. Probably the movement was antici patory to stop flapping, in which, as elsewhere proved, rotation of the wings occurs or can occur. Also the movement that I have described as " wing depression " has been shown to be due to wing "rotation lasting slightly longer than in this instance. That cheels can catch food thrown to them while they are gliding in the air, and that they always catch the tfood with their feet, never with their beaks, are well- known facts. The details of the extremely rapid move ments by which they accomplish this feat are very •difficult to observe. The fact that one can hardly help feeling amusement or astonishment at the agility of the bird adds to the difficulty of making the observation. It is my experience that the power of minute observa tion is greatly diminished if the consciousness is occu pied by any feeling, whether of surprise, interest, or pleasure. On one occasion I was able to follow the movements of cheels while catching food in the air. I was throwing pieces of bread to cheels from the terrace outside my house. This terrace has a height of about 15 ft. from the ground. If the cheels were gliding in front of me, they had to make a sudden turn and a dive in order to •catch the bread. This happened at first. Then, as if the cheels knew what I was doing, they kept gliding in the air behind me, so that on swooping they travelled in the same direction as the piece of bread, and could catch it more easily. An example of catching a piece of bread after a difficult turn is the following:— October 13th, 1910.—At 4.15.—A cheel was gliding past in front of me about 5 ft. above my level as I threw a piece of bread. When the cheel had reached a point about 10 ft. to the left of the position where the piece of bread was falling, it rotated round its transverse axis through about 900. At the end of this rotation the longitudinal axis of the bird was vertical instead of being horizontal. That is to say, the beak pointed vertically upwards and the tail downwards. Then the cheel rotated through 180'' round its dorso - ventral axis. That is to say, after making this second rotation, its beak pointed downwards and its tail upwards. This movement was quicker than the transverse-axis rotation. 1 could see that the wings were flexed during this second rotation. While it was making these rotations a small feather dropped off. The cheel then swooped downwards, and caught the fading piece of bread at a time when the latter had reached a point about 2 ft. from the ground. While swooping the wings were flexed and there was no flapping. As usual, the cheel caught the bread in its claws, not in its beak. The rotaiion round the transverse axis was presumably due to advancing of the wings, as observed in other cases. At the moment of catching the bread the cheel began gliding upwards (in a curve of long radius). As observed in other cases, this gradual change of course must have been due to placing the wings in the dihedrally-up position. The bird glided upwards, and about its original height. Then, as usually occurs, the claws were brought forward and the head bent down and backwards, as the bird eat the bread without interruption of its gliding flight. (See Fig. 56.) In the above account I have described two methods of producing rotation round the transverse axis. The first, by advancing the wings, causes a sudden rotation and is associated with loss of speed ahead. That is to say, speed ahead is changed into speed feet foremost. This feet-foremost speed obviously was the source of the energy used for rotation round the dorso-ventral axis, and also was a part source of the energy required for the swoop, whose speed was greater than could be accounted for by gravity alone. The second method of producing rotation round the transverse axis was by placing the wings in a dihedrally-up position. This method causes a more gradual turn, and is used in cases in which speed ahead is maintained. Recently I was with a friend at Jharna Nullah, and within a few minutes we saw two cases in which a cheel dropped a piece of meat and caught it before it reached the ground. In each case the cheel was being chased by other birds. Apparently to drop a piece of food and again catch it in this way is a method used by cheels to baffle pursuit Cheels when swooping steeply downwards sometimes show to-and- fro rotations of large range round the longitudinal axis. For instance:— V v A-.*&"'•* Fig. 56.—Movements of a cheel when catching food thrown to it in the air. A, a piece of bread falling while a cheel is gliding past at B. At C the cheel advances its wings. In consequence the bird rotates through 90° round its transverse axis, as shown at D and E. The cheel then rotated 180" round its dorso-ventral axis, as shown at F. The wings are now flexed and secondaries relaxed. The bird then swoops down, as shown at G and H, gradually extending its wings and increasing their camber. At K and L the wings are shown in the dihedrally-up position. This adjustment causes gradual rotation round the transverse axis. The bird consequently glides upwards, catching the bread, M, in its claws as it passes. The cheel carried out these manceuvres while the bread was falling from A to M, a distance of about 15 feet. October 17th, 1910.—3.30.—When feeding my captive adjutant I threw some pieces of meat into the air1 Some cheels swooping for these showed rapid to-and-fro oscillations round the longitu dinal axis. Two of them after checking speed ahead by advancing wings showed rapid rotation round the dorso-ventral axis. During this rotation the wings were only slightly flexed. According to my recollection of this incident, the adjutant was threatening the cheels by snapping its beak at them. In certain cases, therefore, the cheels had occasion to check speed suddenly IOI5
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