On Wed, 28 Jul 2004 17:25:31 -0000
 "Jim Wilson" <[EMAIL PROTECTED]> wrote:

Richard Bytheway said:

Well as a physicist (but with no formal aeronautical education), I always
think of it as the wing is pushing air down, which causes an "equal and
opposite force" (to quote Newton) of the air pushing the wing up. Hence
acrobatic aircraft with symmettrical wings can still fly. The key to wing
shape design is to keep the air flow attached to both the upper and lower
surface so that you can change the direction of airflow. Once the flow
detaches (a stall), you are not pushing the air down any more, so it isn't
pushing you up.

This suggests that both bernoulli and the pushing (gravity) are at play,
depending on the airfoil. My (uneducated) guess is the pushing is almost all
of it and that the bernoulli effect only augments:

The "pushing" comes into play in Newtonian flow, such as at hypersonic speeds. In that case, the momentum transfer of many impacts with air molecules and the resulting exhange of momentum might be seen as "pushing" the wing upward.

Also, past stall a wing will see a decrease in lift, but then an increase again - perhaps to an even higher degree than it had prior to stall - at about 45 degrees, like a flat plate. In that case, the airflow on the back side of the "wing" is obviously going to be detached, but there is lift, nonetheless. It seems to me that this could be seens as the airflow pushing the wing up, and the airflow being deflected downward. Not sure how that fits in with Bernoulli, if at all.

However, at normal angles of attack below stall, the Bernoulli principle is, I believe, the explanation for lift. The _resulting_effect_ of the lower pressure on the top surface of the wing than on the bottom, gives a net lift - which *results*in* airflow being deflected (i.e. "pushed" downward). It could be a matter of point-of-view: the direct cause of the lift is the pressure differential, the effect is that airflow is deflected downwards. It's not the other way around - that is, the air that is pushed downward does not cause the pressure differential over the wing surfaces.

That's my further explanation, in any case.


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