Backwash Causes Lift?

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-