visualisation of the lift distribution over a wing

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

In article ,
brian whatcott wrote:

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

I'm perfectly aware of that, but that hardly matters for the scope of
our discussion of the effect of pressure on a wing. The point I'm making
is that all else being equal, more collisions means higher pressure and
fewer means lower pressure, but that pressure is therefore always a
positive value that acts toward the surface to which it is applied.

It is *never* acting away from that surface; i.e. "pulling".

That is the only reason I mentioned a vacuum, because it is a situation
in which there is *by definition* zero absolute pressure.

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

Alan Baker wrote:
/snip/
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

I'm perfectly aware of that...

It took me too long to realise the problem: you have a problem with
saying: "Oh yes, I got it worng."

People who WON'T do that in technical discussions qualify as people who
are just happy to stir up heated debate.

I am going to leave this thread now: wrasslin' with pigs gets the
hands jest too soiled...

Brian W

Alan Baker wrote:
Jim Logajan wrote:
Alan Baker 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.

Brian Whatcott already addressed one of the errors I had in mind. The other
I had in mind was your incorrect assertion "*by definition*". You should
have asserted "*by derivation*". College level texts on statistical and
thermal physics *derive* the gas laws from statistical mechanics; they do
not present them as true "by definition." (Though that would make for short
textbooks!)