BIOPHYSICS OF BIRD FLIGHT — RASPET 



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FLIGHT MEASUREMENT 

 -H LfiUGH/NG GULL 



H BLACK BUZZRRDCSOmN% ' 



nindtunnel measurement 



o LAUGHING GULL (REF.S) 



^ ALSATIAN SNIFT (AEF.ZI) 

 CH£iL(PARIAH KITEXAEFIZ) 



"0 m .0^ .06 .06 .10 IZ .14 .16 .18 .ZO 12 Z^ 

 DRAG COEFFICIENT 



Figure 6. — Linearized drag polar of the black buzzard, laughing gull, Alsatian swift, and 



cheel. 



the single laughing gull measurement is borne out by the complete 

 polars of the black buzzard — namely, wind-tunnel measurements of 

 models of birds cannot yield valid information concerning the aero- 

 dynamic properties of birds in natural flight. For this reason, prog- 

 ress in imderstanding the more difficult phases of flapping flight 

 will only be possible when theory can be supported by flight measure- 

 ments made under natural conditions. In general, the measurements 

 made in wind tunnels tend to ascribe to the bird much higher energy 

 losses than it actually experiences. For this reason, any biophysical 

 conclusions would lead to absurdities if they are based on wind-tunnel 

 measurements made on model or stuifed birds. 



However, the comparison-flight method is subject to some criti- 

 cism at the present state of the art. Since the measurements with 

 wild birds had to be made in the middle of the day when birds were 

 soaring — that is, in a turbulent environment — one cannot absolutely 

 say that the black buzzard possessed the very low drag coefficient 

 which was measured. We say that either it possesses this low drag 

 coefficient or else it must be utilizing a source of energy which the 

 sailplane was not. The only possible means of extracting such en- 

 ergy from the environment lies in dynamic soaring. However, we do 

 have rather positive evidence that the lowest measured drag values 

 are valid for the high-speed points on the speed polars of figure 5, 

 since they were obtained near sunset when the air was quite smooth, 

 during: a elide to roost of a black buzzard. 



