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Old October 8th 07, 11:16 PM posted to rec.aviation.piloting
Bertie the Bunyip[_19_]
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Default Backwash Causes Lift?

Le Chaud Lapin wrote in
ps.com:

On Oct 8, 4:14 pm, Phil wrote:
On Oct 8, 3:46 pm, Le Chaud Lapin wrote:

On Oct 8, 2:45 pm, Phil wrote:


Then how do you explain what happens when a wing stalls? When a
wing reaches a high enough angle of attack to stall, the bottom
surface is still deflecting air downward. Yet when the airflow
over the top of the wing detaches and becomes turbulent, most of
the lift of the wing is destroyed. If the attached airflow over
the top of the wing is not generating lift, then why does the
lift disappear when that airflow detaches?


Because the turbulent air on top of a wing during a stall pushes
down on the wing harder than does when the airflow non-turbulent.


-Le Chaud Lapin-


Do you know of any research that supports that theory?


Well, no, it's only speculation.

You might be asking how one could speculate on something so complex,
and the only answer i can give is that everything that I have
speculated about does not seem complex at all. I have only used high
school physics so far. It could be that I am wrong of course, but I
do understand Newton's theory of reciprocity of force.

There is another way to look at the air-over-the-wing-does-not-pull-up
on the wing point of view:

Take a wing, and one single diatomic molecule, say nitrogen, N2.

Put your N2 on top of the wing. Someone will offer to pay you
$1,000,000US, if you can use that N2 molecule to impart a force on the
wing to get the wing to move upward. You can throw the molecule at
the wing as hard as you want. You can drag it side ways. The only
requirement is that you have to keep the molecule in the region above
the wing. You will probably not get the prize.

But, if someone ask you to cause the net lift on the wing to increase
by manipulating the molecule, you might ask:

You: "Must I still impart a force upon the top surface of the wing?"
Challenger: "No, that stipulation has been removed."
You: "Is there already air pressure beneath the wing?"
Challenger: "Yes"
You: "Well, this is easily. I take my N2 molecule and put it in my
pocket. Done."

The net change in lift would be so small, it would be immeasurable by
any equipment we have, but the net lift would increase.

This is the process that is happening with a wing in flight. The air
molecules above the wing, by virtue virtue of a process that is
heavily influence by both the aerodynamics of the leading eade and the
camber of the wing going backwards, has a reduced rate-of-impartation
of their momentum against the top of the wing.

Naturally, the impartations are completely removed, then, assuming
quasi-static conditions under the wing, standard atomosphere would
generate 14.7lbs/in ^ 2. The net lift on the wing would be found by:

(14.7 lbs/in^2 * area-under-the-wing) - (average-pressure-above-wing *
area-above-wing)

In still air, the average-pressure above the wing is the same as the
average-pressure below the wing. That's why a wing that is in still
air, perhaps supspended off the ground by thin steel wires for
dramatic effect, will be neither inclined to move upward nor downward,
because the force on both top and bottom are equal.

In steady-state, pure-stream flow, the pressure above the wing will be
reduced. Ane experimentalist might have unrealistic expectations that
they are going to eliminate the entire 14.7 lbs/in on top of the wing,
so that they get a glorious net pressure of 14.7lbs from the bottom of
the wing, when this is obviously not the case. You can see this by
doing some simple cacluations with a Cessna Centurion 210.

http://www.airliners.net/info/stats.main?id=148

From the link above, this aircraft has a wing area of 175.5 square

feet. We know that at STP, the pressure is 14.7 lbs/in^2. Since
there are 144 square inches in a square foot, the pressure on 175.5 sq
ft sheet of whatever is...

175.5 sq-ft * 144 sq-inches/sq-ft * 14.7lbs/sq-inch = 371,498 lbs.

But the max takeoff weight of this aircraft is only 3800 lbs.

With so much potential for lift, why such a measly 3800lbs (assuming
structural capacity not breached)?


It's that, during flight, at no time will all the pressure on top of
the wing be removed. One can only hope, in the best of situations, to
remove some of it. The pressure goes from two extremes:

1. One the ground, no pressure is removed, wing has no reason to go up
or down, because it is same on top and bottom.
2. Optimum lift situation, maximum pressure is removed. [You can
almost calculate this pressure, in ounces / in^2, based on weight in
flight]

In between 1 and 2 is the situation of turbulence. This situation is
not "as bad" as no-movement-on-the-ground (no pressure removed), and
it is not as good as optimum-flight-maximum-depletion-above-wing-in-
effect. It is somewhere in between.

-Le Chaud Lapin-




Well, thank god you'll never fly.


Ever


Bertie