BIOPHYSICS OF BIRD FLIGHT — RASPET 



419 



0.01 



O.OZ a03 0.04- 



TOTAL DRM COEFFtClENT 



O.OS 



Figure 8. — Comparison of span efficiency factors of the black buzzard and tlie Horten IV 



tailless sailplane. 



It is immediately apparent that the slope of the curve for the buz- 

 zard is much steeper than that for the sailplane. This means that if 

 the two had the same aspect ratio, the bird would outperform the sail- 

 plane, especially at the high-lift coefficients used in soaring. In study- 

 ing the reason for the high induced drag of the Horten IV flying- wing 

 sailplane, we found that the elevators at the trailing edge of the wing 

 caused a severe induced drag, owing to the change in the spanwise lift 

 distribution necessary for trimming the sailplane at high angles of 

 attack. 



The question is, then. How does the bird accomplish this trimming 

 without suffering the resultant induced drag rise? Figure 9, taken 

 from Plankin [9], shows the plan form of a buzzard {Otogyps calvus) 

 in various flight modes. At low speeds, the wings are swept forward. 

 In other words, the center of pressure of the wing is moved forward 

 of the center of gravity of the bird. As a result, an upward pitching 

 moment is developed which counterbalances the nose down-pitching 

 moment of the highly cambered wing. 



Whether the trimming by means of forward and backward sweep 

 results in a stable configuration in pitch cannot be determined with- 

 out a knowledge of the camber and the angle of attack distribution of 

 the bird's wing. However, the bird is capable of correcting for in- 

 stability by means of intuitive sensing and associated reflexes. 



The process of trimming to different speeds is clearly seen from 

 figure 9. At very high speeds, the tips are swept back by bending 



