422 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 



Figure 11. — Bazin's dynamic soaring analogy, 



careful observation and analysis which should be applied to more of 

 the problems of bird flight. 



The third source of energy for soaring is that which Lord Rayleigh 

 described as fliglit through air which possesses velocity fluctuations. 

 On the basis of this thesis, S. P. Langley [13] made a study of the 

 energy available in the wind. However, the actual mechanism of 

 dynamic soaring was not clearly disclosed until Klemperer [14] pub- 

 lished his paper. Reduced to its simplest form, dynamic soaring is 

 merely correcting for the turbulence in the airmass in such a way 

 that potential energy is gained. Klemperer's contribution points a 

 clear path toward the duplication of this process by man. So far, 

 only certain birds are known to utilize dynamic soaring — in particular, 

 the albatross. 



The strict condition to be fulfilled, as Klemperer points out, is that 

 the sailplane or bird must be immobile against pitching under the in- 

 fluence of gusts. Under this condition, an upwardly directed gust 

 results in increasing the angle of attack, thereby lifting the bird or 

 sailplane. A gust having a horizontal component of velocity will re- 

 sult in an increase in effective airspeed, thereby increasmg the lift. 

 In practice, this process might be accomplished on a sailplane by using 

 modern gyroscopes and servo controls. 



A simple model of an analogy for dynamic soaring is shown in 

 figure 11. By oscillating the model along its axis with a higher ac- 

 celeration in the forward direction than in the reverse, the marble 

 is made to climb to the last stage of the model. Interestingly, Bazin 

 [15] and Lanchester [10] invented this analogy independently. 



Idrac [17], in his carefully documented study of the soaring flight 

 of birds, described a second type of dynamic soaring practiced by the 

 albatross. This bird flies an elliptic path, one vortex of which is in 

 an area of high-velocity flow and the other near the water's surface, 

 in wind of relatively low velocity. In other words, this bird utilizes 

 the energy in the boundary layer of tlie earth. 



The last phase of soaring has yet to be accomplished by man, al- 

 though many have tried it. The Russians have recently (1956) 

 flown a sailplane with elastically supported flapping wings capable of 

 being "tuned" to the turbulence. No significant gains were reported, 



