408 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



where D is the drag in force units, p is the air density, F is the 

 velocity of flight, and S is the wing area, including that intercepted 

 by the body. 

 In a similar matter we define the lift coefficient 6'l, 



C- ^ 



where L is the lift in force units. If, now, the velocity polar of figure 

 1 is transformed into a curve of CiJ^ versus Cn^ we obtain figure 2. The 

 reason for plotting against the square of the lift coefficient is quite 

 evident when one sees that the induced drag coefficient — that is, the 

 drag due to lift — is a function of the square of the lift coefficient: 



where AR^ is the effective aspect ratio : 



ARt=^e, 



h being the span and e the span efficiency factor. 

 What one sees from the linearized drag polar of figure 2 is that the 



i 



Co 



/.6 



12 



I.Q 



O OB 



I 



o 



o.e 



.02 .0^ .06 



.08 .10 

 DRRG 



.12 .1^ .17 

 COEFFICIENT 



.20 



Figure 2. — Linearized drag polar of a laughing gull. 



flight-measured point lies on an extension of the linear portion of the 

 wind-tunnel measurements. This indicates that the wind-tunnel 

 results must be in error below a lift coefficient equal to 0.8. 



