420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



CIRCLING IN SORRflBLE MR FLEX GLIDING S % 



CIRCLING INfllR NOT FLEX GLIDING 12 % 



FULLY SOfiRRBLE OR 

 HHEN NOT DESIRING 



TO GRIN HEIGHT ^ 



(REF3) FLEX GLIDING Z2% 



Figure 9. — Wing configurations at various flying speeds for Otogyps calvus, 



the elbow of the wing. This tends to move the center of pressure of 

 the wing farther back, a nose down-pitching moment and trimming 

 for higher speeds thus being achieved. 



The foregoing explanation of the control of a bird in pitch is ad- 

 mittedly sketchy. It would, however, be entirely possible to carry 

 out experiments on the control and stability of a bird which had been 

 trained to fly in a tminel that could be inclined with the horizon so 

 as to force the bird to fly at different glide ratios and speeds. By 

 adding weight to the bird ahead of, or behind, its center of gravity, 

 it would be possible to introduce pitching moments for which the bird 

 would have to compensate with sweep of the wings. 



SOARING 



So far we have discussed only the aerodynamics of the bird in 

 gliding flight and the bird's stability. Now we will consider the 

 process of gaining energy from the atmosphere — namely, soaring. 

 Static soaring is accomplished by flying in an upward-moving air- 

 mass having a higher vertical velocity than the bird's minimum sink- 

 ing speed. By staying within the confines of such upcurrents, the 

 bird will gain altitude. 



One common cause of upcurrents is orograpliic lifting as the wind 

 passes over a ridge. Birds are capable of soaring on declivities of 

 very small dimensions. However, they also soar on mountainsides, 

 the best example being the soaring of hawks on Hawk Mountain in 

 Pennsylvania. 



With a sailplane fitted with a sensitive instrument measuring the 

 rate of climb, a pilot is able to duplicate the bird's feat of soaring on a 

 ridge. In fact, often a sailplane pilot merely needs to follow the bird 

 in order to find the best lift. 



