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CH02_Anderson 7/25/01 8:55 AM Page 32

32 CHAPTER TWO

critical angle of attack

Lift 1

5 10 15 20
Effective Angle of Attack (degrees)

Fig. 2.11. Lift as a function of angle of attack.

to adopt a visualization tool of looking at the wing as a scoop (Figure
2.12) that intercepts a certain amount of air and diverts it to the angle
of the downwash. To be more accurate, the air diverted at the bottom
of the scoop has a vertical velocity defined by the speed of the wing
and its angle of attack. As one goes farther from the wing, the vertical
velocity decreases until at the top of the scoop the air has a very small
vertical velocity. For wings of typical airplanes it is a good approxima-
tion to say that the area of the scoop is proportional to the area of the
wing. The shape of the scoop is approximately elliptical for all wings,
as shown in the figure. Since the lift of the wing is proportional to the
amount of air diverted by the wing, which is proportional to the area
of the wing, the lift of a wing is proportional to its area.
If one were to move such a scoop through the air, how much air
would it divert? Certainly, if the scoop were to divert a certain amount
of air at one speed, it would divert twice as much air at twice the