Dr Hankin, The Probkfn of Soaring Flight 221 



the soaring bird to draw on some at present unknown source of 

 energy — kinetic or other — in the air? 



Another observation is of interest. Birds of the smaller size are 

 more lightly loaded than the larger birds. Thus the cheel, with a 

 span of 51 inches, is loaded "5 lb. per sq. ft. as compared with the 

 vulture, with a span of 85 inches and loading of 1-5 lbs. per sq. ft. 

 Each, however, in similar conditions of soarable air ghdes at 

 approximately the same speed. According to aerodynamical theory, 

 the speed of the vulture should be approximately Vs times as great, 

 the loading being three times as great — if we neglect bodily 

 resistance. Even if this is taken into account, their speeds should 

 not be anything Hke the same. The explanation lies probably in 

 "scale effect." The larger bird is, owing to its larger dimensions, 

 able to lift more in proportion to its area, just as the full size 

 aeroplane can compared with the model. Here nature, faced with 

 the problem of making large birds, has avoided the increased 

 percentage of wing weight due to increased span by concurrently 

 increasing the lifting power of the wing with increase in area. 



Apart from the study of natural soaring flight its mechanical 

 equivalent has aroused interest and a good deal of research has been 

 carried out on the Continent. In Germany, Gustav Lilhenthal, 

 brother of the famous Otto Lilhenthal, has, according to a paper 

 pubhshed by him in 1917*, designed a wing section of alleged 

 similarity to that of a soaring bird, and with a glider of this wing 

 cross section a German engineer, Friedrich Harth, claims to have 

 flown 500 metres against a wind of 12 metres per second at a height 

 of 40 metres. 



Such attempts are, however, premature, and the results un- 

 authenticated. Firstly, we want to know whether soaring birds 

 can soar — as apparently they can — for indefinite distances, pro- 

 vided weather conditions are suitable. Secondly, if such flight is 

 as effortless as it appears to be, from what source is the energy 

 obtained and how the bird is able to take advantage of it. When 

 this is discovered, then will be the time for practical apphcation. 

 Its importance for aviation work in tropical climates has already 

 been referred to. 



Dr Hankin's discovery of the soaring flight of dragon-flies and 

 flying-fishes and its similarity in speed and other respects to that 

 of the soaring bird affords a means of more closely investigating the 

 phenomenon of soaring. Observation of birds soaring at 2000 or 

 3000 feet is difficult compared with the observation of insect flight 

 at a few yards' distance. The whole subject demands most careful 

 investigation and merits the attention of all those interested in the 

 scientific development of the world's latest form of locomotion. 



* " Der Einfluss der Flugelf orm auf die Flugart der Vogel" (SitzimgsbericJite der 

 Gesellschaft naturforschender Freunde, Berlin, 1917, No. 4). 



15—2 



