visualisation of the lift distribution over a wing

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

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

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.

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.

I'm going to build a thick wing, real thick! About 10,000 feet thick.
While the bottom surface of the wing sits at 29.92" sea level
atmospheric pressure, the upper surface will be *way* up there, in a
lower-pressure area. This airplane is gonna to lift off the ground at
zero airspeed with no power.

In article ,
Beryl wrote:

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.

I'm going to build a thick wing, real thick! About 10,000 feet thick.
While the bottom surface of the wing sits at 29.92" sea level
atmospheric pressure, the upper surface will be *way* up there, in a
lower-pressure area. This airplane is gonna to lift off the ground at
zero airspeed with no power.

Only if you make it from monatomic Unobtanium. Only thing light enough
for the job.

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

Alan Baker wrote:
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.

That assertion is incorrect. You are no dummy so I'm sure you'll correct it
when you realize the errors.

I never mentioned density.

Sorry, but you used the word "vacuum." The notable characteristic of a
vacuum is that its density is zero.

In article ,
Jim Logajan wrote:

Alan Baker wrote:
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.

That assertion is incorrect. You are no dummy so I'm sure you'll correct it
when you realize the errors.

Sorry, but it's not. Pressure is created by particle collisions.


I never mentioned density.

Sorry, but you used the word "vacuum." The notable characteristic of a
vacuum is that its density is zero.

That is *a* notable characteristic.

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

Alan Baker wrote:
In article ,
Jim Logajan wrote:

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

That assertion is incorrect. You are no dummy so I'm sure you'll correct it
when you realize the errors.

Sorry, but it's not. Pressure is created by particle collisions.

Hmmm...looks like Jim expected too much from you: the kinetic theory of
gases has it that pressure may be computed from the temperature AND the
density of gases... that is to say, by retaining the SAME molar quantity
of gas, and raising its temperature (which translates to a higher
velocity), the pressure is increased P.V = R.t and all that....

Put it another way: each "hotter" molecule reverses direction at a
surface with greater force.

Brian W