BIOPHYSICS OF BIRD FLIGHT — RASPET 423 



nor was any demonstration made to indicate suck gains. Perhaps 

 we need to study the dynamic soaring of birds in more detail before 

 we can hope to succeed. 



Tlie last and least understood phase of bird flight is that of flapping. 

 Aerodynamic theories for unsteady lifting of wings have been devel- 

 oped, but still there is much to be learned from the complex flapping 

 motion of flexible wings, having slots which can open or close in 

 various phases of the flapping motion. 



Insofar as the actual motions of flapping flight are concerned, by 

 far the best description is contained in the documentai^y work of 

 Marey [18], who used a time-lapse photographic technique to define 

 the flapping motion of the wings of birds. His three-dimensional 

 models showing the flapping sequence are works of art. Plowever, 

 his studies, while of historic interest, contribute little to an exact un- 

 derstanding of the physical mechanism of bird propulsion by flapping. 



Of the more recent works in the field of flapping flight, there is 

 the work of Kiichemann and Weber [19]. In a chapter of their book 

 entitled "Aerodynamic Proj^ulsion in Nature," the authors make a 

 clear comparison of the oscillating wing and the propeller. 



At the very Ioav speeds of landing and takeoff of birds, the propul- 

 sive efficiency of a propeller would be rather low. However, if the 

 entire wingspan is used to accelerate a large mass of air above it, 

 thereby achievmg a change in momentum with a relatively small ve- 

 locity increment applied to the large mass, the efficiency remains quite 

 high. In fact, if the flapping wing as a propulsor could be designed 

 for airplanes wliich are to take off and land in short distances, it would 

 provide a very important contribution in its high propulsive efficiency 

 at low speeds. 



The actual power required for flapping flight and the propulsive 

 efficiency of the bird have not yet been measured. This is a chal- 

 lenging problem, but one fraught with experimental difficulties. 

 However, with modern miniaturized instrmnents and telemetering, 

 it should be possible to gain some insight into this problem. 



From the zoological side, there has been a very thorough study made 

 of the musculature of buzzards by Fisher [20]. However, the ques- 

 tion of detennining which muscle plays a part in delivering power to 

 the wing has not been satisfactorily answered. If it were, we would 

 be able to determine the power output which these muscles can pro- 

 vide for flapping flight. 



From the standpoint of the biophysics of bird flight, we probably 

 can sum up the state of our present knowledge by saying that we 

 know very little. A few measurements have been made which were 

 quite revealing when the bird was compared to man's creation, the 



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