In article ,
Jim Logajan wrote:

Alan Baker wrote:
Because the push is caused by the impact of countless air molecules
with the surface of wing. If those collisions fall to zero (i.e. in a
perfect vacuum) then there is zero push.

I don't see what a change in air density (such as taking the extreme case
of a vacuum) has to do with lift. Unless you are claiming density change as
a requirement?

I believe lift can be reasonably computed using inviscid _incompressible_
flow theory (e.g. as far back as Kutta's 1902 dissertation,) so I don't see
why any change in _density_ - much less the vacuum edge case - needs to be
invoked.

Any change in pressure is *by definition* a change in the number of
particles in the fluid that are impacting the surface.

I never mentioned density.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg