406 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



flection of the air, simply as a reflection phenomena, and in disregard- 

 ing entirely the suction on the upper surface. For his failure to un- 

 derstand that birds possess automatic stability due to instinctive re- 

 flexes, in addition to that inherent in their geometry, Lilienthal paid 

 with his life. 



The realm of bird flight can be clearly divided into two aspects: 

 that on motionless wings, which is soaring, and that on flapping wings, 

 which is really the working part of flight. The latter is used in take- 

 off and in climbing to altitude, even by soaring birds. It is used as 

 a principal mode of flight by the nonsoaring birds. The soaring 

 phase of flight, or the flight on motionless wings, was divided by 

 Lord Rayleigh in 1883 [2] into three separate categories : (i) Flight in 

 which the path is not horizontal — in other words, gliding; (ii) flight 

 in an air mass which has a vertical component — that is, static soaring ; 

 and (iii) flight in an air mass which is not uniform in velocity. The 

 latter is, in the strict sense, dynamic soaring. Evidently, a good im- 

 derstanding of the first phase, the motionless wing phase, would con- 

 tribute much to an understanding of the biophysics of bird flight. 

 The second kind of flight, much more complicated (flapping flight) , 

 has been theoretically studied, but very little experimental work has 

 been done to support the various theories. It is the purpose of this 

 article to take up in detail the aerodynamics of a bird's wing — in 

 particular, that of motionless wing flight. 



WIND-TUNNEL EXPERIMENTS 



"\\Tien we consider the various tools available to us for studying 

 flight in general, we are apt to resort to the one wliich has been so 

 useful in helping man to fly — namely, the wind tunnel. It was a 

 wind tmmel which helped the Wright brothers to arrive at proper 

 airfoil sections, and the wind tunnel is still used today for subsonic, 

 transonic, supersonic, and hypersonic flow studies. It will be inter- 

 esting, therefore, to look at some results from wind-tunnel work on 

 the measurements of bird aerodynamics and compare these results 

 with some data obtained in flight. From this, we can determine the 

 validity of the wind tunnel in bird-flight work. In figure 1 is shown 

 a velocity polar of a laughing gull, computed from data measured 

 in the wind tunnel and data measured in flight. The velocity polar 

 is clearly seen to consist merely of a plot of sinking speed, which is 

 really a measure of the energy loss in flight, versus the forward 

 velocity of flight. Actually, this is not a polar, but the terminology 

 is that which is used in aviation. It should be mentioned that the 

 laughing gull measured in the wind tunnel [3] was not actually a 

 feathered bird, but rather a clay model sculptured by an artist. The 

 tunnel, however, possessed a rather low turbulence and provided an 



