On Dec 1, 6:05*pm, Jim Logajan wrote:
Scott wrote:
Alan Baker wrote:
In article ,
*Scott wrote:

Alan Baker wrote:

Observation can lead you astray: and that is clearly the case here
if you actually think that air can *pull* on a surface.

Why can't air PULL on a surface? *Air is made up of molecules.
Molecules have mass. *Anything with mass can attract anything else
with mass, can't it?

Gravity?

You're not serious.

Anti-gravity in this case. *If air can push something, why can't it
pull something?

No need to postulate anti-gravity to find a case where air can "pull" on a
surface - just invoke van der Waals force! :-)

I think I see what Alan is getting at: While there is low pressure
on the top of the wing, there is still pressure. There isn't an
absolute vacuum, so some pressure is there. But its a lot less than
that below the wing, so the wing moves upward. As he says, air can't
suck the wing upward, but its pressure can be reduced enough that the
pressure below displaces the wing upward.

Semantics. We argue about downwash (Newton) and pressure
differential (Bernoulli) but they're just two symbiotic approaches to
the same phenomenon. Shoot, the air flowing off the top of the wing is
accelerated and moving downward with respect to the flight path, so
downwash is to be expected.

But there's no downwash when a balloon rises. Just displacement.

Dan