BIOPHYSICS OF BIRD FLIGHT — RASPET 



413 



2 ^ 6 8 /O /£t /4 /S 18 20 22 24 

 AIRSPEED ' METERS /SECONDS 



Figure S. — Velocity polar and glide ratio of a black buzzard. 



Plate 2 shows a sailplane rigged for bird-flight research. A small 

 radio transmitter and receiver are carried, for transmitting data to a 

 data recorder on the ground. The telephoto camera on the nose of the 

 sailplane is used to record the geometi-y of the bird. However, the re- 

 sults obtained with this camera were not helpful, because it was not 

 possible to determine the orientation of the tip feathers from the 

 nonstereoscopic photographs. 



In making these measurements, the sailplane was launched either 

 by a ground tow behind an automobile on a long runway or by an air- 

 plane tow. When the sailplane reached an altitude where upcurrents 

 were strong enough to support it, the pilot would release and soar in a 

 good upcurrent. Ground observers would scan the skies for buzzards, 

 and when one was found, would direct the pilot to the buzzard by 

 radio. When the pilot located the bird he would descend to the alti- 

 tude of the bird and then follow it, staying no more than 5 to 10 meters 

 behind it. At 30-second intervals, the pilot would report the airspeed 

 at which he and the bird were flying and the altitude of the bird above 

 the horizon, measured in wing spans. 



Subsequently plots of the altitude of the bird against time yielded, 

 from the slope of this plot, the difference in sinking speed between the 

 bird and the sailplane. Then, by measuring carefully the sinking 

 speed of the sailplane in the still air of the morning at various air- 

 speeds, one can obtain the speed polar of the sailplane. Adding to this 

 polar the differences in sinking speed between the bird and the sail- 

 plane, we arrive at the speed polar of the bird (fig. 5) . 



