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Aviation History
1911
1911 - 0977.PDF
NOVEMBER II, 191 I. ITDGHII Eg 0€fo».. j4jStudyLe£J£>ira By DrE.H/iankin. MA.DSc (Copyright Reserved) 1 oOO CHAPTER XXXI. -Steering Movements in Flapping Flight Flying Foxes. ONE might expect, d priori, that steering movements in flapping flight would be discovered by a study of the larger birds. But, as I have elsewhere stated, vultures and adjutants never indulge in prolonged flapping flight. Periods of flapping are alternated with periods of gliding. While gliding, steering movements (by wing- tip rotation or wing depressions) can be seen. It appears to me probable that they choose the period of gliding for steering, as a rule, and only in exceptional instances make any steering effort while flapping. From my description of the position of the wing-tips in flapping flight, it follows that it is unlikely that the wing-tips play any important part in producing steering effects or canting in flapping flight. We may expect that both these functions are carried out by adjustments affecting the whole wing. In the case of flying foxes (Pteropus mediiis), I have succeeded in discovering steering movements in flapping flight. It will therefore be of interest to describe the flight of these creatures. I obtained the following measurements from three different specimens of the flying fox :— Weight ... 923 grammes Span ... ... 51J ins. Area of one wing i-2l sq. ft. Width of wing 8£ ins. II. III. 860 grammes 640 grammes 484 ins. 44 ins. '962 sq. ft. '914 sq. ft. 8 ins. 8 ins. Loading ... '84 lbs. per sq.ft. '98 lbs. per sq.ft. 77 lbs. per sq.ft. The membrane of the wings of these animals is so soft and extensible that it is difficult to be certain how far the wing measure ments given above correspond to the size of the wings in actual flight. A colony of between one and two hundred flying foxes lives, during most of the year, in a garden in Agra known as the Company Garden. During the day-time these animals may be seen hanging head downwards from the branches of a large tree. During Sep tember, October and November these bats used to commence their flight in twilight. After flapping and gliding round the tree for a few minutes they used to flv off, flapping, in solitary flight to their feeding grounds. During December and January the whole colony disappeared. In February a few returned, but from then up to the time of writing (April) they do not start until it is almost completely dark, so further details of their mode of flight are no longer to be observed. The structure of the wing of the bit, as compared to the wing of a bird, is extremely complicated so far as the numbers of muscles present are concerned. Despite the large size of the flying foxes that I dissected I found that the majority of the tendons were scarcely thicker than a bristle. Most of the muscles have flexing and extending functions. The flying fox has no power of rotating the wing-tip. There is a muscle that can rotate downwards, or turn downwards the middle third of the anterior margin of the wing. This is the part of the anterior margin that is supported by the first two digits and that extends in front of the main bony framework. By turning downwards of this part of the margin the camber can be increased. There is also a muscle whose action is to bend downwards the •uter part of the wing. The bending occurs at the carpal joint, thus producing the appearance of arching that is seen in the flapping flight of this animal. That there is a general similarity between the flight of flying foxes amd the flight of birds is shown by the following extracts from my diary:— September 24th, 1910. 6.30 p.m.—At Company Garden.— Flying foxes seen clearly to move wings quicker during the up stroke than during the down stroke. This, combined with the arching at the end of the down stroke, causes the illusory appearance of a pause during the beat. On several occasions a flying fox seen to cease flapping for making a turn (in the horizontal plane). In each case it recom menced flapping immediately it had turned. A flying fox seen gliding with wings arched. It was seen to check its speed by advancing the wings which were still arched. Advance of wings on down stroke seen in slow flapping flight. Flying foxes frequently seen gliding downwards, at a small angle with the horizon, with wings arched, and at moderate speed. On one occasion a flying fox seen to glide downwards but with wings even and dihedrally down. It was gliding down at a small angle with the horizon, and its speed was seen to greatly increase. September 26th, 1910.—At Company Garden. 6.30.—A flying fox half flapping showed advancing of wings with conse quent rotation round transverse axis. This was for checking speed. A flying fox gliding with wings arched showed increase of arching of one wing for steering. A flying fox gliding downwards (at a small angle with the horizon) with wings nearly flat showed a whole wing depression for steering. I have on other occasions seen increase of arching for steering in flapping flight. The wing appears to be more arched at the end of the down stroke on the side to which the bat wishes to go. But it is a question how far this appearance of increased arching is illusory. It is possible that the arching is not increased in amount, but that it lasts longer. But in the case of a flying fox gliding downwards with wings arched, I have noticed that increase of arching was followed by increased rate of loss of height. As in birds the dihedrally up position is used for causing rotation upwards, as illustrated by the following observations : — October 2nd, 1910.—At Company Garden. 6.30.—A flying fox when gliding was seen to put wings in slightly dihedrally up position for rotation round transverse axis to check speed before stop flapping. Flexing with arching seen for checking speed in gliding. In flapping flight increased arching of one wing seen for steering. This was again seen. On three occasions I formed the impression that the wing is slightly flexed on the down stroke in flapping flight. Despite a somewhat intimate acquaintance with the flight of vultures, I have only on one or two occasions seen one of these birds make a sudden movement to avoid another. No doubt, owing to their habit of flying together, they are expert in judging the move ments of other birds. Flying foxes show no such gregarious habits when flying. They are only together in the air for a few minutes after leaving the tree on which they roost. Hence, despite my comparatively slight acquaintance with these animals, I have very frequently seen them make sudden movements to avoid one another. On one occasion I have seen them apparently in collision in the air. These sudden movements can be seen to result in the beat of the wings being horizontally to and fro. In a few cases only 1 have seen that this is preceded by rotation round the transverse axis caused by advancing the wings. I will close my account of flying foxes with the following extract from my diary :— November 15th, 1910.—At Company Garden.—A flying fox seen gliding with wings arched. Increase of arching was followed by increased rate of descent. A flying fox gliding with wings that showed a wing depression. This was a very slight movement. (Apparently it was a steering movement.) Two flying foxes noticed in collision. As a result there appeared to be rotation both of wings and body to check speed. Flapping seen without arching. Apparently it was half flapping to check speed. A flying fox seen to advance wings, and rotate round transverse axis. Then it rotated round its dorso-ventral axis. It gave me the impression that the object of this manoeuvre was to turn suddenly horizontally. (Each of these two rotations was through about 900. A more detailed account of a similar proceeding in the case of a cheel will be given in Chapter XXXIV.) In arching, the posterior margin of the wing seems to go up. CHAPTER XXXII. — The Functions of the Tail With regard to the question of the functions of the tail, Lillenthal expresses himself as follows :— " As compared with the action of the wings, the tail surface of birds has only a very small importance, since the bird flies very nearly as well as before after loss of the whole of the tail feathers. This is the case not only as regards turning upwards and downwards but also as regards steering in the horizontal plane. A sparrow deprived of its tail flies just as adroitly through a lattice as its intact brother.'" * " Der Vogelflug als Grundlage der Fliegekunst," page 72. 979
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