BIOPHYSICS OF BIRD FLIGHT — RASPET 



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10 20 30 40 



PilRSPEED flT Sffl LEVEL IN MILES PER HOUR 



Figure 1. — Velocity polar of a laughing gull. 



environment quite representative of that which one might find in the 

 atmosphere. On comparing the sinking speed obtained from the 

 wind-tumiel measurements, one sees that the sinking speed of the 

 clay model is a little more than double that of the actual bird meas- 

 ured in flight at the speed of 30 miles per hour. The flight measure- 

 ment consisted of a very simple comparison of the flight of the gull, 

 ■while soaring on a ridge on Long Island, relative to that of a sail- 

 plane. The pilot in the sailplane was able to adjust his speed to 

 follow the bird exactly, and at this particular forward speed, the 

 bird and the sailplane flew back and forth on a ridge for about 2 

 hours, neither outclimbing the other. 



Tills is proof that their sinking speeds at tliis forward speed were 

 identical. It is just this concept of comparison flying which I will 

 discuss in connection with some measurements of the black buzzard. 

 Tlie technique was developed to a higher state and used to get the 

 complete measurement of the drag of a bird over the speed range of 

 its flight in the gliding phase. 



However, in order to determine the nature of the aerodynamics of 

 birds in terms of the known parameters used in aeronautics, we must 

 refer the drag to a nondimensional drag coefficient 0-d : 



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