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Aviation History
1911
1911 - 0752.PDF
IfUSHTj tion of this '* phalangeal quill gap." Since the date of the last entry I have seen it on many occasions. Once I have seen it in the case of a black vulture. I have observed it both in circling, ease- gliding and in the gliding periods of flap-gliding. This dip movement of limited range, in which the phalangeal mass only is moved, I propose to term the " half-dip." During the Fig. 16.—View from in front of phalangeal quills. half-dip, owing to the roiation, air ceases to press on the under surface of the leathers. Bat rotation is not carried far enough for air to press on the upper surface of the quills. Hence, during the half-dip, the feathers Deing relieved from air pressure, whether from above or below, take on their natural curvature as shown in Fig. 16. A half-dip movement causes a steering effect in the same direction as a full dip but to a less extent. I have been able to see this steering effect on several occasions, but perhaps more often than not the effect is too small to be detected. It may be suggested that during a dip the air acts as a drag, by pressing on the upper surface of the quills. This suggestion is an easy explanation of the steering effect. But the phenomenon of the half dip suggests that it is not sufficient. It is possible that facts to be described in later chapters miy be considered to indicate that some more deep-seated action is involved. The chief muscles concerned in rotating the wing tip are shown in Fig. 15. This may be supposed to be a diagrammatic view of the under side of the wing, in which various muscles not concerned with wing tip rotation have been removed for the sake of clearness. The following are the names that I propose for the muscles, with a short description:— 1. "Pronator phalangis." This muscle arises from near the base of the ulna. Its tendon_is inserted onithe, base of the middle phalanx. 2. "Abductor pinnae." This muscle arises)from a tendon that connects the elbow and carpal joints. It is inserted on the outer side of the phalanx. Pulling the tendon of this muscle has a slight effect in rotating the wing-tip but also tends to advance the first primary quills. 3. "Pronator metacarpi." This muscle arises from the under surface of the distal part of the ulna. Its tendon passes in a curved course over the carpal joint and is inserted on to the base of the metacarpal bone. 4. " Supinator longus." This musclej arises from the dorsal surface of the radius. Its tendon (not shown in the diagram) is inserted on the middle phalanx. A small branch of its tendon is inserted into the alula. The action of this muscle is to rotate the front edge of the wing-tip upwards, that is to say to return it to its original position after a dip movement.* 5. " Supinator brevis " is a short supinator muscle lying in the hollow of the metacarpal bone mass.t A second kind of steering action also occurs. This is visible as a momentary depression of the whole wing. The result is that the bird turns towards the side of the wing that is depressed. I propose to bring forward evidence bearing on the question of the nature of the depression movement in Chapter XVIII. Perhaps more often than not in the smaller soaring birds, and sometimes in larger birds, the dip is combined in one movement with depression of greater or less extent. CHAPTER IX.—Diving. Rotation round Transverse Axis. A tendency to dive head downwards, or else losing speed to glide backwards and descend tail foremost is or has been shown by various gliders and aeroplanes. Soaring birds behave as if free of this tendency. But they can dive voluntarily when they wish to descend from a height at speed. A study of the method by which they check their speed when thus diving will be found to be of interest and to lead to the suggestion that they have a perfect method of preserving their longitudinal stability far superior to the use of elevators or horizontal rudders as seen on aeroplanes. * The branch of the supinator longus tendon to the alula was missing in the only specimen of the sarus Orus antigone) that I have dissected. t Rotation of the wing-tip can be produced by pulling the tendons of these muscles. To see this rotation it is necessary to use a bird of large size. The movement is much more easily seen in a bird of ten foot span than in a bird of seven foot span. The bird should be freshly killed, and the structure of the wing should be disturbed as little as possible. Only a small portion of the basal part of the tendons should be dissected out. SEPTEMBER 2, 1911. The following extracts from my diary illustrate the general phenomena shown by birds when diving ;— February 14th, 1910.—At 3.36.—A light west wind and a few- isolated cumulus clouds. At the time of commencing my obser vations only one vulture was visible. It was flex-gliding. Its height was measured with the telemetre and found to be 700 metres. While watching it, I noticed that its speed was greater than usual, and I at once made a measurement. It was found to be 40 metres per second (that is to say 89 miles an hour). It was then seen to be diving downwards, its track making an angle of perhaps 200 or 300 with the vertical. After I had made the measurement,, its speed increased rapidly and greatly. At a height of about 100 metres above the earth it suddenly checked its dive, swerving somewhat from its course while so doing. The bird was then seen to be descending at moderate speed with its wings extended in the horizontal plane and slightly flexed. Its body and legs were hanging down below the level of the plane of the wings, and as it descended it was swaying to and fro like a parachute till it reached the earth. Within one or two minutes about 30 other vultures dived and landed in the same way. Then a vulture was seen which after its dive, and after it had commenced "para chuting," drew up its legs and flex-glided off, having apparently changed its mind. The vultures that had settled rose at 3.45. circling with flapping. Above 50 metres height they circled without flapping. They drifted to leeward and passed me at about 200 metres height. The "windward dip" was seen in several. Also half dips of the outside wing on the windward side of the circles. Above 200 metres height the vultures gained height rapidly. Though I was able to follow several of the diving birds with the binocular it was quite impossible for me to see the method by which their speed when diving was so suddenly decreased. To be able to see how this is done it is necessary to be standing near the carrion so that the birds are seen approaching. Thus having an end on view of their track more can be observed than when they are diving at a speed of probably more than a hundred miles an hour across the field of view. I had seen the actual adjustment used for checking speed when diving some years ago. In view of my increased acquaintance with the subject it was desirable for me to see these movements again. A fortunate chance gave me this- opportunity. The following description is from my diary :— 20th March, 1910.—At 5.0.—I arrived at Futteypur-Sikri just as the body of a leopard, that someone had shot and skinned. A • *^"; Fig. 17.—A vulture diving and checking speed. A, position assumed in diving; B, wings, still flexed, placed in dihedrally up position. Owing to the inertia acting through the centre of gravity and the resistance of the wing-tips forming a couple, the bird rotates to the position shown at C. In this position, instead of descending head first, it is descending legs first, with greatly increased resistance. had been thrown over the edge of the hill a few yards away from the terrace of the dawk bungalow. Vultures were descending. For the most part they came from a distance, gliding downwards at a small angle of descent. One was watched nearly overhead diving downwards. When about 200 metres up it placed its wings, still flexed, in the dihedrally up position, so that the two wings made with one another a dihedral angle of between 900 and 100°. The bird also began to extend its legs; consequently it rotated in the air round its transverse axis, so that instead of de scending head first it descended legs first. As, in consequence, the speed decreased, the dihedral angle of the wings diminished. 754
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