Tady k tomu neco je (Anderson, Eberhardt):
Circulation
As mentioned before, circulation is a measure of the rotation of the air
around the wing, when seen from the rest frame where the air is ini-
tially standing still and the wing is moving. Circulation has been mistakenly 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 adds the speed of the direction and speed of the relative wind (as seen
by the wing) to each of the arrows in the figure, the familiar stream-
lines with upwash and downwash are produced. (jeste je tam toho vic, ale tohle je proc vznika downwash)
