visualisation of the lift distribution over a wing

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?

Alan Baker wrote:
In article ,
Ed wrote:

On Sat, 28 Nov 2009 19:07:16 -0600, brian whatcott
wrote:

Stealth Pilot wrote:
/snip/ it is the air below pushing
you up that lifts the wing.
Stealth Pilot
In most circumstances, suction on the upper surface contributes about
2/3 rds of the lift, and pressure on the lower surface contributes about
1/3 rd.
That's one reason which rib stitching for rag wings is a biggy.

Brian W
I have never noticed the fabric lifting on my wings, however I have
seen the fuel siphon out of a wing tank due to an improperly applied
fuel cap.

And greater pressure in the tank than outside of it...

Right, but in a sealed metal tank, is all that other fuel PUSHING the
fuel out of the vent since air can't PULL it out?

In article ,
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?

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.

But there is no set of circumstances that can make the number of
collisions be negative.

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

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! :-)

In article ,
Scott wrote:

Alan Baker wrote:
In article ,
Ed wrote:

On Sat, 28 Nov 2009 19:07:16 -0600, brian whatcott
wrote:

Stealth Pilot wrote:
/snip/ it is the air below pushing
you up that lifts the wing.
Stealth Pilot
In most circumstances, suction on the upper surface contributes about
2/3 rds of the lift, and pressure on the lower surface contributes about
1/3 rd.
That's one reason which rib stitching for rag wings is a biggy.

Brian W
I have never noticed the fabric lifting on my wings, however I have
seen the fuel siphon out of a wing tank due to an improperly applied
fuel cap.

And greater pressure in the tank than outside of it...

Right, but in a sealed metal tank, is all that other fuel PUSHING the
fuel out of the vent since air can't PULL it out?

First of all, the tank is not completely sealed. If it were, the fuel
pumps would soon have difficult pumping the fuel out of the tank.

So, yes, the greater pressure inside the tank is pushing the fuel out.

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

In article ,
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! :-)

Please, don't!

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

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.

But there is no set of circumstances that can make the number of
collisions be negative.

Pedantically speaking, outgassing would occur for a while that would create
a force on your wing surface when it is exposed to a vacuum. Pedantically
speaking, I don't see why those couldn't be called negative collisions.

(Last worked on a fancy high-vaccum system back in college, wherein my lab
mate and I attempted to replicate the Lamb-Retherford experiment.)

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.

But there is no set of circumstances that can make the number of
collisions be negative.

Pedantically speaking, outgassing would occur for a while that would create
a force on your wing surface when it is exposed to a vacuum. Pedantically
speaking, I don't see why those couldn't be called negative collisions.

The force they'd create would be in the same direction as the force of
regular collisions: toward the surface. If the outgassing molecules have
momentum away from the surface then the surface must experience a change
in momentum in the opposite direction.

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

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

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.