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-

Le Chaud Lapin wrote in
oups.com:

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.



Nope, it's not the same at all.

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's precisely true since th ecritical angle is defined by the stall.





Bertie

On Oct 2, 10:23 pm, Bertie the Bunyip wrote:
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's precisely true since th ecritical angle is defined by the stall.

What is the definition of a stall anyway?

I'm saying that, if you take a plane with certain critical angle,
throw away engine, put on an engine that can generate 10x the thrust,
the plane should still fly, even if you exceed critical angle.

These books imply that the critical angle is angle at with bad things
happen above the wing, and because of that, the plane will fall. I'm
saying that, you can have all the bad things happen above the wing and
still be able to keep the plane aloft due to compression that occurs
beneath the wing.

Of course, I have only been doing this officially 7 weeks, so I might
be wrong.

-Le Chaud Lapin-

Le Chaud Lapin writes:

What is the definition of a stall anyway?

An abrupt loss of lift.

I'm saying that, if you take a plane with certain critical angle,
throw away engine, put on an engine that can generate 10x the thrust,
the plane should still fly, even if you exceed critical angle.

No, the thrust of the engine doesn't matter, unless the engine itself is
supporting the weight of the aircraft with thrust (possible in a few fighter
aircraft). A wing above the critical angle will stall at any speed.

These books imply that the critical angle is angle at with bad things
happen above the wing, and because of that, the plane will fall.

Yes, true.

I'm saying that, you can have all the bad things happen above the wing and
still be able to keep the plane aloft due to compression that occurs
beneath the wing.

The wing is not supported by compression. It is supported by the displacement
of a mass (of air) downward. If this displacement ceases to take place, lift
disappears. A stalled wing does not divert air downward, so it doesn't
generate lift.

Mxsmanic wrote in
:

Le Chaud Lapin writes:

What is the definition of a stall anyway?

An abrupt loss of lift.

I'm saying that, if you take a plane with certain critical angle,
throw away engine, put on an engine that can generate 10x the thrust,
the plane should still fly, even if you exceed critical angle.

No, the thrust of the engine doesn't matter, unless the engine itself
is supporting the weight of the aircraft with thrust (possible in a
few fighter aircraft). A wing above the critical angle will stall at
any speed.

These books imply that the critical angle is angle at with bad things
happen above the wing, and because of that, the plane will fall.

Yes, true.

I'm saying that, you can have all the bad things happen above the
wing and still be able to keep the plane aloft due to compression
that occurs beneath the wing.

The wing is not supported by compression. It is supported by the
displacement of a mass (of air) downward. If this displacement ceases
to take place, lift

This should be entertaining.


Bertie

"Mxsmanic" wrote
Le Chaud Lapin writes:

What is the definition of a stall anyway?

An abrupt loss of lift.

Son, for someone who continually chastises the pilots here for their lack of
knowledge, you sure can come up with some doozies yourself!

BDS

On 3 Oct, 13:27, "BDS" wrote:
"Mxsmanic" wrote

Le Chaud Lapin writes:

What is the definition of a stall anyway?

An abrupt loss of lift.

Son, for someone who continually chastises the pilots here for their lack of
knowledge, you sure can come up with some doozies yourself!


Actually, it's correct, but only because he read it off wickepedia or
something.

It's not like he'd actually ever DO a stall.


Bertie

"Bertie the Bunyip" ...
On 3 Oct, 13:27, "BDS" wrote:
"Mxsmanic" wrote

Le Chaud Lapin writes:

What is the definition of a stall anyway?

An abrupt loss of lift.

Son, for someone who continually chastises the pilots here for their
lack of
knowledge, you sure can come up with some doozies yourself!


Actually, it's correct, but only because he read it off wickepedia or
something.

Here's my take on it - a stall occurs at the angle of attack where the
coefficient of lift stops increasing with angle of attack and begins to
decrease. It continues to decrease beyond this point as angle of attack is
increased further. It is not necessarily an abrupt change - most lift
versus angle of attack curves that I've seen do not have a drastic (abrupt)
drop beyond the peak.

BDS

"Le Chaud Lapin" wrote in message

I'm saying that, if you take a plane with certain critical angle,
throw away engine, put on an engine that can generate 10x the thrust,
the plane should still fly, even if you exceed critical angle.

These books imply that the critical angle is angle at with bad things
happen above the wing, and because of that, the plane will fall. I'm
saying that, you can have all the bad things happen above the wing and
still be able to keep the plane aloft due to compression that occurs
beneath the wing.

Of course, I have only been doing this officially 7 weeks, so I might
be wrong.

You are.

With a stronger engine, the wing would still be stalled. You would be
flying on the thrust of the engine.

Disruption of the flow on the top side of the wing is what defines a stall.
The wing would create only a fraction of the list that it would, unstalled.
--
Jim in NC

Le Chaud Lapin writes:

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.

Pressure beneath the wing is not what lifts the aircraft. It's the twisting
acceleration of air above the wing downward that results in lift.