Backwash Causes Lift?

On Oct 8, 2:34 pm, Le Chaud Lapin wrote:
On Oct 8, 11:38 am, Phil wrote:

First, I would like to point out that your post is interesting because
it implies at first something which I disagree with, but then at the
very end of the post, what you say is exactly true.

Let me try to explain:

If the airflow on top of the wing doesn't contribute to lift, then how
can we explain the phenomenon of the wing stalling? When the wing
stalls, it is the airflow over the top of the wing that detaches from
the curve of the wing and becomes turbulent. This causes a radical
loss of lift. To me, this indicates that the airflow over the top of
the wing plays an essential role in providing lift.

What I am saying is that Newton's law is not at play with downwash,
not in the "uppper surface of wing pull down on molecules" sense. Yes,
there is downwash. Yes, the camber of the wing will influence the net
force exerted on the wing. Yes, there will be stalling, turbulence,
etc. all these things will happen.

The key here is that the air molecules that are above the wing cannot
be pulled down by the wing more can they pull up on the wing. Those
air molecules can only causes the lateral forces of friction (laminar
drag), and a perpendicular downward force on the wing which aircraft
designers obviously want to keep from happening.

I know the Bernoulli effect has been invoked historically to (at least
partially) explain the lift produced by the top surface of a wing. I
think another way to look at it is the Coanda effect (http://en.wikipedia.org/wiki/Coand%C4%83_effect). The airflow tends
to follow the curve of the top of the wing, and is displaced
downward. As long as the air flow follows the curve faithfully, you
have good lift. When the airflow detaches in a stall, you lose most
of your lift. This top surface lift is combined with the downward
displacement of air by the bottom of the wing. The wing is
essentially throwing air downward using both the top and bottom
surfaces. This is why a curved wing is a better lift producer than a
simple flat wing. The top surface curve helps contribute to the lift.

I agree that air is being thrown downward by the bottom surface. I do
not think a top surfaces throws air downward. Even this Coanda effect
says that contact, at least initially, is caused by a pressure
differential. From your link above:

"As a gas flows over an airfoil, the gas is drawn down to adhere to
the airfoil by a combination of the greater pressure above the gas
flow and the lower pressure below the flow caused by an evacuating
effect of the flow itself, which as a result of shear, entrains the
slow-moving fluid trapped between the flow and the down-stream end of
the upper surface of the airfoil. The effect of a spoon apparently
attracting a flow of water is caused by this effect as well, since the
flow of water entrains gases to flow down along the stream, and these
gases are then pulled, along with the flow of water, in towards the
spoon, as a result of the pressure differential. Supersonic flows have
a different response."

"greater pressure above the gas flow and the lower pressure below the
flow caused by an evacuating effect..."

This is what I keep saying. I have been using the words "rarefication
and rarefaction" and instead of "evacuating effect", but this is
essentially what I mean.

Now, how does the wing feel the lift? It feels high pressure on its
bottom surface, and it feels low pressure on its upper surface. It is
pushed up from below, and sucked up from above. That is how the
airplane experiences the effects of the downward displacement of air.

I agree with the downward force. I do not agree that there is a
sucking force above, any more than I agree that there is a sucking
force when a purpose sucks on a straw.

Given that the bottom surfaces of the wing is already 14.7lbs/in^2,
one simply needs to reduce the pressure above the wing to cause lift.
This is what I tried to illustrate with my two-pieces-of-paper-
superposed demonstration.

But in many cases the bottom surface has even more than 14.7lbs/^2.

-Le Chaud Lapin-

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?

Phil

Le Chaud Lapin wrote in
oups.com:

On Oct 8, 12:32 pm, Phil wrote:
There isn't any debate about what a wing stall is, and what causes
it. It has been well-explored in wind tunnel testing. The
phenomenon
of wing stall is real-world evidence that the top surface of the wing
is a large contributor to lift. The Bernoulli effect and the
associated Coanda effect are well-understood scientific phenomena.
They explain how the curved top surface of the wing displaces air
downward. Unless someone can come up with a better explanation for
the fact that wing stall destroys lift, I think the only debate is by
people who are determined to ignore the scientific evidence.
On Oct 8, 12:32 pm, Phil wrote:
On Oct 8, 11:46 am, Mxsmanic wrote:
There isn't any debate about what a wing stall is, and what causes
it. It has been well-explored in wind tunnel testing. The
phenomenon
of wing stall is real-world evidence that the top surface of the wing
is a large contributor to lift. The Bernoulli effect and the
associated Coanda effect are well-understood scientific phenomena.
They explain how the curved top surface of the wing displaces air
downward. Unless someone can come up with a better explanation for
the fact that wing stall destroys lift, I think the only debate is by
people who are determined to ignore the scientific evidence.

What's wrong with the supposition that, all other things being equal,
the configuration of the fluid in a smooth stream results in less
pressure on the upper surface than the configuration of the fluid in
turbulence?

In other words, one could argue that the fluid above the wing, during
streaming (sorry for terminology), no longer exerts its full 14.7lbs/
in^2, but during a stall, even though there is still is some reduction
from the full 14.7lbs/in^2, the reduction is not as complete as it
would have been had there been a nice stream...


God you're even dummer as a sockpuppet


bertie

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-

Le Chaud Lapin wrote in news:1191876409.965861.63860
@r29g2000hsg.googlegroups.com:

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.



Wow, easy to see your=re conversant with physics.


Bertie

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?

Le Chaud Lapin wrote:

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.


You really need to look at some video of Tuft testing.

Here's one to start with.

http://www.youtube.com/watch?v=zrwlpHE7P8Q

Bertie the Bunyip writes:

Fair enough, but the wing is behaving in exactly the same way.

The wings of fixed-wing aircraft behave the same way whether they are gliders
or powered.

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-

Bertie the Bunyip wrote:
Mxsmanic wrote in
:

writes:

There is nothing after leaving the rotor disk to change the direction
of air flow.

The airplane, helicopter, gyrocopter, and gyroplane all fly straight
and level for the same reason and it isn't air being deflected
downward.
No heavier-than-air aircraft flies without deflecting air downward,


Yes, they can, and do.

Dynamic lift is usually at play but is by no means required.

Examples where it isn't required???

Mxsmanic wrote in
:

Bertie the Bunyip writes:

Fair enough, but the wing is behaving in exactly the same way.

The wings of fixed-wing aircraft behave the same way whether they are
gliders or powered.




And you can't even read,

Fjukkwit.


bertie