BIOPHYSICS OF BIRD FLIGHT — RASPET 



417 



0.02Z 



aozo 

 o.oia 



0.016 

 0.014 

 0.011 

 0.010 



o.ooa 



0.006 

 0.004 

 0.002 



■ BLMK BUZZARD 

 (COMGYPS ATRflTUS) 



~ /^OARING 

 [ I OUOING 



UNOBERGHi 

 SPIRIT OF ST. LOUIS 



THIN ENGINEO MRPLAN£ 

 30.000 POUNDS GROSS UEIGHT 

 300 mph CRUSING SPEED 



Z S 6 10 20 50 60 100 ZOO 500 600 1000 



MEAN REYNOLDS NUMBER 



Figure 7. — Skin friction curves plotted against mean Reynolds number for the black 

 buzzard, a high-performance sailplane, and a high-performance twin-engined airplane. 



as a means for eliminating the objection that we are comparing 

 vehicles in different domains of the viscous-flow regime. 



In figure 7, the drag polar of the black buzzard in its two modes, 

 gliding and soaring, has been transformed into a plot of average skin- 

 friction drag coefficient versus Reynolds number. On the same plot 

 are shown the Blasius curve for pure laminar flow over a flat plate and 

 the Yon Karman curve for turbulent flow over a flat plate. These 

 two curves provide us a standard over the rather large scale and 

 speed domain covered, from birds to large airplanes. 



It should be mentioned that the data for the airplane shown were 

 also obtained in gliding flight, with propellers feathered after the 

 plane had climbed to altitude on its engines. When we look at figure 

 7 we find that the black buzzard's skin-friction coefficient is only 30 

 percent higher than that of the laminar plate, whereas our best man- 

 made flying machine, a sailplane, possesses a skin-friction coefficient 

 330 percent higher than the laminar flat plate flow. And our best- 

 measured airplane has the poorest showing, having 29 times greater 

 skin friction than the laminar flat plate. 



From this curve we can conclude that the many generations of se- 

 lective breeding have resulted in a flying machine, the bird, which still 

 gives man a goal toward which to strive. 



Furthermore, the fact that the high-speed end of the curve of skin 

 friction for the bird came from data points taken in calm air gives 

 some validity to a speculation that the bird must, through the porosity 



