Hmm. REALLY not understanding circulation

xerj wrote:
None of this, however, takes away from your nice way of presenting in a
simplified way, why it is that the air speeds up over the top of a wing.

What is eluding me is the reason why the pressure is lower above the wing. I
answer it by saying "the air is faster", but that brings me back to the
question: "why does a wing oriented at an angle of attack make the air go
faster?".

There are three types of air pressure. Total pressure is the total
force exerted by all the air molecules in all directions. It is
measured by a barometer. Dynamic pressure is the force exerted by
molecules that have been accelerated in a particular direction. Blow
air against your hand and you will feel dynamic pressure. Dynamic
pressure is measured by a pitot tube. Static pressure is measured by
the Bernoulli principle. It is the pressure of the remaining molecules
that have not been accelerated in a particular direction. Static
pressure is measured by your static port.

Total pressure at any altitude must remain constant and it must equal
the total of dynamic pressure plus static pressure. If you increase
dynamic pressure in one direction by moving air over a wing, blowing it
through a tube, against your hand, whatever, then static pressure must
be reduced in order to keep total pressure constant. A wing increases
dynamic pressure from front to back over the top, so static pressure is
reduced. At the same time, the positive angle of attack on the bottom
of the wing reduces dynamic pressure from front to back, so static
pressure on the bottom is increased. This increase in pressure
differential is not enough to account for total lift, however.

The reduced static pressure above the wing also causes air to move down
toward the wing, much as the reduced static pressure of water flowing
through the nozzle of a garden hose sprayer forces fluid up and out of
the bottle and into the nozzle. This air is caught up in the flow of
the boundary layer and is also forced off the trailing edge of the wing
and down. This actually generates most of the lift -- as air is forced
down toward the wing and then down off the trailing edge, there must be
an opposite and equal reaction and the wing is forced upward. You can
see the air is forced down by the wing by watching an airplane fly over
a cloud or smoke, or low over water. The downward moving air creates a
canyon in the cloud or ripples on the water almost directly under the
airplane.

Air is accelerated over the top of the wing because the top is curved.
Air molecules are forced upward by the leading edge, but because air is
slightly sticky (viscous), it sticks to the wing rather than just
continuing up. It is like holding a water glass sideways under a
faucet; the water instead of just dropping straight down off the side
of a glass instead follows the glass all the way around to the bottom
and then falls off.

Really, an airplane is nothing more than a fan blade. Instead of
whirling around, it moves straight through the air. Air is drawn from
above the wing and forced down behind it, just like a fan blade forces
air through it. People get a little confused by watching wind tunnel
streams, because in a wind tunnel the air is moving and the wing is
stationary, so instead of moving down off the trailing edge the air in
a wind tunnel is blown straight behind the wing.

Instructors like to demonstrate the Bernoulli principle by blowing over
the top of a sheet of paper. The paper rises, demonstrating lift. What
instructors don't usually do, though, is blow under the paper. The
paper still rises, demonstrating lift.

As the angle of attack increases the boundary layer begins to separate
from the wing, causing turbulence instead of lift, but lift continues
to increase because of the greater lift coming from the area towards
the leading edge. At some point, though, the angle of attack becomes so
great that there is not enough lift to overcome the turbulence from the
separating boundary layer and the wing stalls.