Page 42 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 29

How Airplanes Fly 29

The Adjustment of Lift

We have said that the lift of a wing is proportional to the amount of
air diverted per time, times the vertical velocity of that air. And we
have also stated that in the rest frame where the air is initially at rest
and the wing is moving, the air is moving almost straight down after
the wing passes. Let us look at this in more detail. Figure 2.9a shows
how the downwash as seen by the pilot and the observer on the
ground relates to the speed of the airplane and the angle of attack of
the wing. The arrow marked “Speed” represents the speed and direc-
tion of the wing through the air. The arrow marked “Downwash” is
the speed and direction of the downwash as seen by the pilot, and the
arrow marked “Vv” is the speed and direction of the downwash as
seen by an observer on the ground. Vv is the vertical velocity of the
downwash and represents the component that produces lift. In this
figure the letter indicates the angle of the wing’s downwash with

Speed
a
α
Vv
Downwash

Speed (x2)
b
α

Vv(x2)
Downwash

Speed
c
α (x2)

Vv(x2)
Downwash

Fig. 2.9. Effects of speed and angle of attack on downwash.