418 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



of its feathers, exercise some type of boundary layer control — that 

 is, that there must be some automatic fluid mechanical process in the 

 bird's makeup by which a good portion of the flow over the bird's 

 surface is kept laminar. The difference in porosity measured by Vic- 

 tor Loughheed may be the key to this process. 



In fact, on the basis of this speculation, I was inspired to attempt 

 to duplicate the boundary layer control which I suspected the birds 

 were achieving. By making many small holes in a section of a sail- 

 plane wing and sucking the boundary layer into the wing with a fan, 

 I was able to measure drag reductions of the order of 50 percent when 

 even the power required for the suction fan was considered to be a 

 loss [7]. Later on, it was also discovered on tliis sailplane that this 

 same suction could increase the lifting power of the wing. We may 

 thus further speculate that the bird may be utilizing boundary layer 

 . control, both for high lift and for low drag. 



Recently, a very fascinating discovery was reported by Kramer 

 [8] — that there exists an automatic bomidary layer control in the skin 

 of the porpoise. Examination of the skin of the porpoise disclosed 

 that the porpoise is completely covered with a hydraulic skin one- 

 sixteenth inch thick that is elastic and ducted. Kramer was able to 

 duplicate this natural boundary layer control device by selecting a 

 rubber skin of suitable stiffness and by mtroducing a damping fluid 

 behind the skin. The stiffness was controlled by small rubber stubs. 

 Between the stubs was the damping fluid. 



But it is conceivable that Nature has solved this problem for birds 

 in a manner that is not analogous to the solution for the porpoise. 



The problem of trimming an aircraft for various speeds is par- 

 ticularly vexing on flying-wing aircraft. Since all birds are essen- 

 tially flying-wing aircraft, it is possible that we can learn a trick or 

 two from the w^ay birds apply trimming moments for various flight 

 conditions. We know that the bird's wing is in general fairly highly 

 cambered. Therefore, we can expect large pitching moments. In 

 order to achieve stable flight, these pitching moments must be bal- 

 anced by aerodynamic moments developed by the tail of a conventional 

 airplane or by twisting and deflected elevators at the wing tips on a 

 swept flying wing. 



Let us look at a comparison of a flying- wing sailplane and the black 

 buzzard (fig. 8). Instead of plotting Ci^ versus C'd, as we did before 

 for the linearized polar, Ave have plotted C-J/AR versus 6'd, which is 

 in actuality a plot of the theoretical induced-drag coefficient versus 

 total-drag coefficient Od. The purpose in doing this was to be able 

 to derive some information on the induced drag from aircraft of 

 widely different aspect ratios — namely, 5.7 for the bird and 21.8 for the 

 sailplane. 



