01-- Zero One
January 17th 06, 01:40 PM
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.)
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.)