414 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



In this illustration, the two modes of gliding flight yield two dif- 

 ferent speed polars for the bird. In the soanng mode the bird flies 

 with open tip slots, while in the gliding mode it flies usually on a long 

 descent at relatively high speeds, with tip slots closed. Also, in the 

 latter mode the bird introduces an M -shaped sweepback, whereas in 

 the soaring mode there is a pronounced forward sweep of the wing. 

 Figure 4 shows the black buzzard {Coragyps atratus) in its soaring 

 mode. 



Returning to figure 5, we see that at a speed of 17 meters per second 

 the speed polars cross. Above this speed the bird chooses the gliding 

 phase, for when the bird is gliding its sinking speed is considerably 

 lower than it is with the tip feathers opened. Below 17 meters per 

 second the bird finds that it can reduce its sinking speed by opening 

 the tip slots, and can thereby increase its glide ratio {L/D). The 

 glide-ratio curves represent the distance the bird can fly for each unit 

 loss of altitude. In other words, the black buzzard is capable of glid- 

 mg 23 miles in still air from an altitude of 1 mile at its best glide ratio. 

 This remarkable feat is possible at a relatively slow forward speed 

 of 15 meters per second with tip slots open. 



An interesting biophysical constant can be derived from the velocity 

 polar of figure 5. If we wish to determine the minimum power re- 

 quired for the bird to maintain level flight, we take the product of the 

 minimmn sinkmg speed of 0.62 meter per second and the weight of the 

 bird. This yields the rate of loss of potential energy which must be 

 compensated by muscle power for the black buzzard in level flapping 

 flight. The minimum power required to maintain level flight is 0.019 

 horsepower. For this bird, which weighs 2.3 kilograms, this results 

 in a power loading of 122 kilograms per horsepower. A rough value 

 for the capability of muscles to put out continuous power is 1 horse- 

 power for 50 kilograms of muscle. 



The value of 122 kilograms per horsepower then implies that flight 

 muscles must constitute 42 percent of the bird's weight. If, then, 

 flapping muscles do not constitute at least 42 percent of the black 

 buzzard's weight, we can conclude that this buzzard could not main- 

 tain continuous level flight without help either from upcurrents or 

 from dynamic soaring, in which energy is extracted from the fluctua- 

 tions in the wind. 



In order to compare the aforementioned free-flight method for 

 determining the aerodynamics of a bird in gliding flight with wind- 

 tunnel measurements, the data of figure 5 have been transformed into 

 a linearized drag polar (fig. 6). In this illustration are shown the 

 drag polars of the black buzzard in the two modes of gliding flight 

 and wind-tunnel data for the laughing gull, the cheel (pariah kite), 

 and the Alsatian swift. The same conclusion that was drawn from 



