Backwash Causes Lift?

On Oct 2, 9:06 pm, "Dan Luke" wrote:
"Le Chaud Lapin" wrote:
Obviously, any air above the wing can only result in a force downward
on top of the wing. The only force causing the plane to want to move
upward comes from beneath the wing. The effect of any air above the
wing is to cause rarefication above the wing, resulting in lower
pressure, thereby giving the 14.7lbs/in^2 (plus) to do its work. That
"reaction" coming from downward movement of air seems just plain silly
to me.

Then why does the wing stall and cease lifting when flow separates from the
upper surface?

Because the situation of rarefication no longer exists during a stall,
or is significantly abated.

When plane is on the ramp, pressure above and below the wing are
equivalent.

When plane is flying certain critical speed, there is, IMO, pinching
that occurs at the leading edge of wing. This area of high pressure
results in a tendency for air to flow away from that pressure point in
all directions.

1. Flowing forward is not an option - that would make pressure
situation worse.
2. Flowing backward, toward the empennage, is not an option. The
leading edge of wing is there.
3. Flowing upward is possible, since above-the-pressure-point pressure
is less than that induced at pressure point.
4. Flowing backward is possible, since below-the-pressure-point
pressure is less than that induced at pressure point.

But here is the catch. If you take an umbrella, open it, find a friend
with extremely long arms, and ask him to yank the umbrella toward his
torso in one, quick, abrupt motion, he will feel a force immediately.
The umbrella might even invert if the impulse is strong enough.
[Sidenote: In the 1970's, I convinced small children that they could
fly if they jumped of 7ft brick wall with umbrella. Very amusing to
see their faces when they hit ground going just about as fast as they
would have without umbrella.] The force that is felt is due to
pressure building under the curved part of umbrella. But even if the
pressure did not build from compression, a force would still be felt,
becaue the force that was equalizing the pressure under the curved
part will have been removed.

And now the $1,000,000 point:

The air on the "outside" of the umbrella does *NOT* instantaneously
fill the void that is created by yanking the umbrella. A finite
amount of time is required for such air to rush in. If the unbrella
is pulled at even a low speed, the net effect can be felt. Pull it
fast enough, and it will invert or snap.

This is, IMO, a more illustrative way of looking at aerodynamics above
the wing than the canned Bernouilli speech.

1. The pinchage creates pressure.
2. A void is created over the wing, provided that plane is moving fast
enough that air high above win cannot rush in.
3. Air at back of wing participates in futile effort to fill the void.

But the most important thing is the pinchage. That pinching results
in high net speed of air molecules backward. Any air above wing that
tries to rush in and fill void is bombarded backward before it can
"touch" the upper surface of wing. I speculated that, if this point a
view were correct, gliders should have short chords with very long
spans, which, of course, is true.

About stalling:

When the angle of attack is too great, the pinchage is still present,
and depending on the shape of the leading edge, the backward flow is
still pressent, but not at the right angle relative to wing, and
certainly not flowing backward enough to stop the onrush of air coming
from above at back of wing. In the air comes, rushing in, and
pressure builds on top of wing.

But there is an ace in hole. Some books say that a plane will stall
if AOA is above critical angle. I do not think this is quite true. It
would seem that, worst-case-scenario, the dynamics above the wing
become royal mess at huge AOA. However, that mess will be less than
static pressure, and there will still be compression beneath the wing.
So if thrust is great enough, airplane should be able to do whatever
it wants.

-Le Chaud Lapin-