Page 62 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 49

How Airplanes Fly 49

Fig. 2.21. Wing flap vortices. (Photographer, Jan-Olov Newborg.)

takenly employed by some as a driving mechanism to accelerate the air
over the top of the wing and thus account for the reduction of pressure
causing lift. Let us go back to the rest frame of an observer on a moun-
taintop who is able to take a picture of the directions of air movement
around a wing as it passes. What would the picture look like? It would
look something like Figure 2.22. Remember when studying this figure
that it is a snapshot of a moving wing and that the arrows represent the
velocities in the air at one moment in time. The air is not moving
around the wing but is shifting in a circular pattern. The arrow marked
“1” will become the arrow marked “2” in a moment, and so on. If one