BIOPHYSICS OF BIRD FLIGHT — RASPET 409 



Obviously, the clay model was not representative of a feathered 

 bird in flight. In fact, it is doubtful that even a feathered model 

 could accurately duplicate the aerodynamic properties ascribable to 

 the elasticity and mobility of the feathers on a live bird. 



However, one can admire the finesse with which nature has de- 

 signed her flying machines in observing the neat intersection of wing 

 and body in figure 3, which shows a drawing of the laughing gull, 

 taken from Feldmann's paper. In this drawing, the veiy pointed tips 

 of the soaring birds of the sea are conspicuous. In figure 4 the dis- 

 tinctly difl'erent tips of soaring land birds are shown. The question 

 then arises. What is the function of this pointed tip as contrasted with 

 the slotted wingtip of soaring land birds ? 



It has been suggested that since soaring land birds must land and 

 take off from trees, a large span would be a handicap. Therefore, the 

 slotted wingtip serves to diffuse the vortex flow at the tip, permitting 

 the soaring land bird to attain good performance in spite of limited 

 aspect ratio. The sea bird on the other hand is not limited by its en- 

 vironment with respect to aspect ratio. 



However, an analytic investigation by Newman [4] disputes the 

 premise that the slotted tip can reduce the induced drag over that of 

 a solid tip. We are then left without a logical explanation for the 

 slotted tip of soaring land birds. Wind-tunnel tests with smoke 

 streams and a live bird trained to fly in a tunnel could add to our 

 knowledge of this important distinction between soaring land birds 

 and soaring sea birds. 



In order to duplicate this complicated model, the live bird, one 

 might freeze a bird and then test it in a wind tunnel. This was done 

 at the Washington Naval Shipyard wind tunnel some years ago, but 

 again we have the criticism that a change occurs in the elasticity of the 

 support of the feathers, as well as in the feathers themselves, in the 

 process of freezing the bird. Another criticism of the frozen-bird 

 technique lies in the fact that the bird uses its wing muscles even in 

 gliding flight as a means of control. This is necessary, since the bird 

 possesses little or no inherent aerodynamic stability except possibly 

 along the body axis in roll. In yaw and, to a lesser extent, in pitch, 

 the bird with fixed geometry appears to have neutral or negative 

 stability. In other words, the flight of a bird is stabilized by minute 

 invohmtary control deflections. This is similar to the process of 

 walking in man, in his erect posture. 



Another feature of the bird's aerodynamics is the porosity of the 

 feathers. Whether or not this feature plays an important role in the 

 aerodynamics of the bird has not yet been established. Victor Lough- 

 heed is reported to have measured the porosity of the bird's feathers, 

 finding the porosity 10 times greater in the downward direction than 

 in the direction up through the wing feathers. 



