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Aviation History
1911
1911 - 0842.PDF
l/ycH; whole wing depression. This gave the same impression as the movement of one of the wings in a double dip. At 7.24.—A cheel making a whole wing depression showed slight movement upwards of secondaries, while at the same time the wing tip went downwards as in a dip movement. Does a whole wing depression mean a slight arching of the wing, which would involve less efficiency, and, therefore, a steering effect ? August 14th, 1910.—At Futteypur, 8.45 a.m.—An eagle seen gliding up wind. Twice a whole wing depression was seen clearly to be accompanied by a rising of the free or hinder ends of the secondaries. Because the wing depression is not accompanied by any increase of flexing, therefore the wing depression must be due to a twisting of the whole wing. (Facts to be described in a later chapter will make clear the meaning of this argument.) August 16th, 7 a.m.—A cheel while gliding showed an elevation of the free ends of the secondaries, that is to say, rotation of the wing, which was seen to be followed by depression. I was very astonished at being able to see this. This last observation was quite unexpected. Though I have no doubt that it represents accurately what actually happens in a wing depression, I am inclined to think that it was of the nature of an accident that the rotation of the wing was presented to my con sciousness as a phenomenon preceding the depression. Since making this observation I have frequently been able to see that in a wing depression the front edge of the wing is depressed and the hind edge is slightly elevated. There can, therefore, be no doubt that the movement consists in a rotation of the wing, and that the depression ensues when the wing is no longer supported as before by the air pressure from below. The above observations, therefore, point to the conolusion that birds may steer in the horizontal plane, either by rotation of the wing tip or by rotation of the whole wing. It is necessary to consider possible criticisms of this conclusion. 1. When describing the flapping and other movements connected with perching, I shall show that the wing, when in use, can be Fig. 23.—Transverse section of a vulture when circling- in fully soarable air. CG, position of centre of gravity. C, carpal joint. M, metacarpal joint. A1, position of alula, shown by dotted line. I, II, III, and IV, tips of phalangeal quills pressed upwards by pressure of the air. V, position of metacarpal quill tips. rotated through an angle of nearly 90 degrees. There is, therefore, independent evidence of wing rotation. 2. That the movement observed cannot be due to relaxation of the secondaries of the depressed wing, that is to say, to a diminution of camber, will be shown when I come to describe the mechanism for altering camber in a later chapter. 3. That the rotation of the wing, or of the wing tip, for steering in the horizontal plane is not accompanied by a rotation of the other wing, or of the other wing tip in the opposite direction, will shortly be proved. 4. In a later chapter I shall have to describe cases in which a slight relaxation of the secondaries of the outside wing may possibly play some part in steering. 5. I hope on a later occasion to describe my observations on the conditions under which tailless cheels are unstable. It will be seen that the facts observed lead to the conclusion^that movements of the tail do not produce steering effects. 6. Cheels may on rare occasions show sudden rotation round the dorso-vcntral axis through as much as 90°, or even a larger angle. I shall describe these rotations in Chapter XXXIII, and shall show that they have nothing in common with ordinary steering movements. CHAPTER XIX.—Canting. Warping of the wings of an aeroplane to equal amounts in opposite directions may conveniently be referred to as " Wright's method." There is a certain resemblance between the warping of the wing of an aeroplane and the rotation of the wing-tip found in birds. One would therefore expect that birds use Wright's method for pre serving lateral stability, or, as it may otherwise be expressed, for producing or checking rotation round the longitudinal axis. But I am acquainted with no evidence that Wright's method is SEPTEMBER 30, 1911. used by birds. That is to say, during a dip movement of one wing, there is no evidence of any upward rotation of the front edge of the wing-tip of the other wing. At the time that I made the following observation, I thought that I had found an instance of the use of Wright's method :— June 30, 1910.—Ballia Ravine. 2.30.—High level clouds only. A few vultures perched, and one in sight in the air. Slight sun shine. A lammergeyer seen circling near. The first quill feather of the outside wing was turned up while the bird faced the wind, but not when the bird was travelling with the wind. The gradual return of the end of the feather to the horizontal position was clearly seen as the bird turned in each of several successive circles. The wind at the time was nearly imperceptible, but occasionally moving leaves slightly. Further experience has shown that the above observation cannot be regarded as an instance of the use of Wright's method. The return of the first quill feather to the horizontal position, mentioned in the above extract, was not its return to the normal position. On the other hand, as shown in Fig. 23, the tips of the phalangeal quills, when circling, are normally turned upwards. The first quill, in the above instance, assuming the horizontal position was of the nature of a half dip movement, as will be further described and explained in a later chapter. The range of movement observed in this instance of the first quill feather was probably less than two inches. The bird was probably of nine feet span or more. That canting in soarable air is not merely a consequence of travelling on a curved course with the centre of gravity below the centre of effort of the wings is shown, firstly, by the facts of canted flex-gliding. In this form of flight, as elsewhere described, the bird is canted though travelling in a straight line. Secondly, a similar conclusion can be drawn from the phenomena shown in circling where the amount of canting is inversely proportional to- the speed. Parrots and pigeons in fast flapping flight on a curved course are always canted. I have seen an adjutant bird become canted while flapping and then cease flapping and begin circling. This observation makes it improbable that the canting was produced by any movement of rotation either of the wing or of the wing tips, as will be apparent when I come to describe the facts of flapping flight. In a later chapter, when describing "dropping turns," I shall mention cases in which canting is pro duced by momentary increase of flexing of one wing. This acts simply by decreasing the supporting area of the wing, which, therefore, drops a short distance through the air, producing the canted position. In Chapter XXI, I shall describe cases in which canting is produced or removed by "arching " of one wing. Of the different methods used by birds for meeting a puff of wind, the following method, that may be described as "wind-canting," is of interest. Sup posing a cheel is ease-gliding to windward against a strong wind. As it gradually glides into an ascending current of air reflected upwards from a tree or building, it is obvious that at a particular moment when the front part of the wing is more affected by the ascending air than the back part, there must be a tendency for the bird to be rotated upwards round its transverse axis. That is to say, it must tend to rotate round this axis in such a way that the beak tends to go up and the tail tends to go down. As this occurs (or appears to occur), the cheel may be seen to rotate itself on its dorso-ventral axis through about 90°. The consequence is that the bird avoids being tipped up round its transverse axis. That is to say, its angle of incidence by this simple manoeuvre has been kept normal. But the bird has become canted, and is now gliding in a. direction at right angles to the wind. Sometimes a dip movement of the leeward wing may be seen, and the bird then gradually turns off and glides away to leeward. While thus gliding away to leeward it loses its canted position and returns gradually to a level keel. I have seen a similar method of dealing with a puff of wind in " storm soarability " in cases in which there was no evidence of any upward current. In such cases of storm soarability I have occasionally seen the bird make steering movements of sut'h a nature as would tend to check the rotation round the dorso-ventral axis. * It is therefore doubtful whether the rotation round the dorso-ventral' axis is due to any action on the part of the bird. Further light will be thrown on this point by the facts to be described in the next chapter. * Instead of turning off from a puff of wind, a bird may cope with it by adopting the disposition for increased speed. That is to say, it remains facing the wind, but elevates the furled tail, places the wings dihedrally down, increases their flexure, and relaxes the secondaries, that is to say, decreases the camber. On two occasions, in addition, I have seen momentary lowering of the legs, but I am not sure whether this movement bad to do with meeting a squall. (7o bi continued.) 844
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