Page 63 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 50

50 CHAPTER TWO

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Fig. 2.22. Airflow around a wing as seen from an observer watching on
the ground.

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.
So, why is this happening? First, we have to bear in mind that air is
considered an incompressible fluid for these discussions. That means
that it cannot change its volume and that there is a resistance to the
formation of voids. Now the air has been accelerated over the top of
the wing by the reduction in pressure. This draws air from in front of
the wing and expels if back and down behind the wing. This air must
be compensated for, so the air shifts around the wing to fill in. This is
similar to the circulation of the water around a canoe paddle.
This circulation around the wing is no more the driving force
Eddie Rickenbacker, the U.S. “ace
for the lift on the wing than is the circulation in the water that
of aces” in WWI, was a famous
drives the paddle. It is true that if one is able to determine the
auto racer before the war and
circulation around a wing the lift of the wing can be
owned the Indianapolis Speedway
calculated. Lift and circulation are proportional to each other.
after the war. He also owned
The most obvious result of the circulation around the wing
and built up Eastern Airlines.
is that the air approaches from below the wing. This is the