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The Impossibility of Flying Heavy Aircraft Without Training



 
 
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  #38  
Old March 9th 06, 05:56 PM posted to rec.aviation.piloting
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Default lift, wings, and Bernuolli

[snip]

I think we are pretty much in full agreement, and were merely using
different words, or stressing concepts that don't matter for the image
being discussed by the other.

When an infinite (or 2D) wing starts its takeoff roll, is there not a
vortex generated? Is not some air thrown down at the outset? (Granted
the vortex isn't at the wingtips and has a different axis, but air goes
down behind the wing and up ahead of it, which wasn't the case before
the takeoff roll).

Steady state, the wing keeps throwing air down but it keeps coming back
up (in front of it). I suppose I am using the term "throwing air down"
a bit more sloppily than you are taking it - I do mean it to be the same
as "imparting a downward acceleration". However, in all the pictures
I've seen, air does leave the wing with a downward velocity with a wing
in level flight. I just find the image to be more useful to me than the
"high pressure low pressure" because it better explains where the
pressure comes from.

It still sounds like you are just saying that since some air
is somehow held down so it can later "come back up" you are
simply saying that there is some density increase for higher
pressure air. While true to a tiny extent, this is
irrelevant to the whole issue.


I'm saying it just to answer the (valid) question of "where the air
goes". Just enough of it gets scrunched up to hold the airplane up.
PV=nRT. In this case the V is big (and not really well defined), and
nRT is effectively constant.

Even if the gravity does not
affect the fluid, an underwater wing moved through the water
at an AOA will apply a force to the fluid and cause it to
move down. The longer you do this, the longer that force
will accelerate the ball of fluid down.


Exactly. And this is the reason I hold to "the wing throws the air
down, the earth gets in the way".

You don't need any compression to have pressure. Pressure
is sufficient for lift. A perfectly incompressible fluid
still has pressure.


Pressure is sufficient for lift. but a perfectly incompressible fluid
is an ideal that does not exist. The electrons get squished a little
harder; that supplies the force. Granted this is not important in the
calculation of what pressure does, and the distances involved are less
than miniscule, but it is important conceptually in seeing "just how
things work" on a basic level. "It goes =somewhere=."

I think we are basically in agreement with the physics, just disagree as
to what parts of the physics are important, because we are evaluating
importance differently.

Jose
--
Money: what you need when you run out of brains.
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