Thrusting or Sucking (where's Howard Stern when we need him.)

Todd,



Thanks for the illustration. Now I know my problem..



I have some extra bowling balls stuck inside my wing!



Larry

"01" USA





" wrote in
message :

"Ian Johnston" wrote:

: The point of the
: original post was that you have to account for *both*
: pressure and momentum flux to figure out the force.

Thanks - that was exactly my point. Peraps I was not terribly clear.


I'm glad I didn't step on your toes. I probably should have
left it to you to say what your point was, but I couldn't
resist.

As to clarity, I suspect my own attempts were not much
better. The term "momentum flux" seems rather intimidating
for someone who's just trying to figure out what holds his
airplane up. At first glance, pressure seems so much
simpler to understand, and it's pretty easy to see why one
would think that pressure is all there is to the force on
the airplane.

But momentum flux is fundamentally a simple concept. I like
to think of a black box hanging in space with no way to tell
what's inside, but I see that there's someone throwing
bowling balls into the box through a hole in the side, and I
see bowling balls coming out of another hole in the box.
The balls going in have a velocity (speed + direction) and
mass, and they carry momentum into the box. The balls
coming out also have a velocity and mass, and carry momentum
out.

Even though we have no idea what's going on inside the box
(perhaps the balls are bouncing off a plate, or maybe
there's a juggler inside catching, juggling for a while,
then throwing the balls out), we can still tell the exact
force on the box due to the balls going in and out. It's
the force required to change their momentum. If the box
also had some pressure differential on the walls, then the
total force on the box would be the force due to the
changing momentum of the balls going in and out, plus the
force on the box due to the pressure differential.

In the context of lift, we can do the same thing, except
that there aren't any balls going in and out, but there is
air going in and out, and the air carries momentum in and
out, just as the bowling balls did. "Momentum flux" is just
a fancy term for the total force required to change the
momentum of the air flow going into and out of the box
divided by the surface area of the box so that it's in the
same units as the pressure on the surface of the box.

--
T o d d P a t t i s t - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)