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Old Sep 12, 2012, 09:43 AM
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The interaction between the air molecules and the surface of the wing, and between the air molecules and all adjacent air molecules. For simplicity we avoid to analyze things at a molecular level, and work with pressure and velocity in volumes of air immediately surrounding the wing section. At subsonic speed the pressure waves can travel back against the airflow, and their behaviour and mutual interaction can be predicted, generally using the Navier-Stokes equations (http://en.wikipedia.org/wiki/Navier%...okes_equations), simplified for incompressible flows. For speeds higher than about 0.3 mach compressibility effects start to have some importance, and the simplified equations no longer give sufficiently accurate data. The predictions generally match pretty well against experimental data, but they are a form of approximation and can't be considered as a perfectly accurate tool. That's why they are no substitution for wind tunnels and prototypes.
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Old Sep 12, 2012, 10:03 AM
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Originally Posted by Montag DP View Post
What causes the pressure difference?
Basic answer :The body moving thru the air causes the pressure difference.
HOW it does this depends on size, shape and speed of that body.
WHERE the body is , in relationship with the earth, also make a difference -
Now we are getting into how various shapes and speeds produce lift.
Basically -pressure difference causes lift.
period
There are lots of explanations as to the various factors involved .
Many are true some are just silly.
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Old Sep 13, 2012, 09:11 AM
Texas Buzzard
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Which is it?

I read a lot of posts that indicate that the air molecules are passing over the wing
as if the wing is stationary and a big fan is causing the air to be moved.

In my experience it seems that it is THE WING THAT IS MOVING THROUGH THE AIR. The air is stationary.

This may seem like a small point to some but face it - the wing moves through the air.
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Old Sep 13, 2012, 09:27 AM
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Quote:
Originally Posted by Texas Buzzard View Post
I read a lot of posts that indicate that the air molecules are passing over the wing
as if the wing is stationary and a big fan is causing the air to be moved.

In my experience it seems that it is THE WING THAT IS MOVING THROUGH THE AIR. The air is stationary.

This may seem like a small point to some but face it - the wing moves through the air.
In Newtonian physics, this doesn't matter. All that matters is the relative velocities between the fluid and the objects it encounters. Wind tunnels utilize this fact.
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Old Sep 13, 2012, 10:12 AM
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HOW it does this depends on size, shape and speed of that body.
Yes, but can you explain to me how a thin airfoil at an angle of attack causes the pressure difference?
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Old Sep 13, 2012, 10:14 AM
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In Newtonian physics, this doesn't matter. All that matters is the relative velocities between the fluid and the objects it encounters. Wind tunnels utilize this fact.
Well-- Newtonian physics explain some of the reasons flying objects do what they do- NOT all the info- which makes sense-- Newton had no aircraft to use in hands on testing.
The interaction of air and a wing in a confined space or where air is sucked across a plate or blown across a plate IS different as compared against a wing in unconfined space
I have not seen helicopers being tested in a tunnel--for example

Wind tunnels can provide info about air in reasonably close contact with the wing but pressures above and below need to include unconfined space.
basically , wind tunnels and Newton etc., explain some of what is involved - NOT all of it.
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Old Sep 13, 2012, 10:19 AM
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Yes, but can you explain to me how a thin airfoil at an angle of attack causes the pressure difference?
You are serious?
you can demonstrate it at your desk by moving a piece of cardstock.
Try it, then tell me what you see.
as a hint - where is the pressure highest?
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Old Sep 13, 2012, 10:20 AM
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You are serious?
you can demonstrate it at your desk with a piece of cardstock.
Try it, then tell me what you see.
I know that it does happen. I want to know physically, what causes the pressure to decrease on the top surface and increase on the bottom surface?

Don't give me an example. Can you tell me why it happens?
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Old Sep 13, 2012, 10:26 AM
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Originally Posted by Montag DP View Post
I know that it does happen. I want to know physically, what causes the pressure to decrease on the top surface and increase on the bottom surface?

Don't give me an example. Can you tell me why it happens?
Basics: air has mass it doesn't want to move - 101
the card, held at an angle and moving, pressurizes air on one side and reduces pressure on the other
If you slowed the movement to a crawl - the air would simply equalize faster and lift would be reduced
you can plot all this if you like
It is simply an obvious occurrance .
The difference in pressure is doing the work
The air is simply trying to re equalize and the card goes along for the ride.
If you let go of it it would fall -cushioned of course by the air resistance
which is another demonstration of pressure differences.
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Old Sep 13, 2012, 10:31 AM
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Quote:
Originally Posted by richard hanson View Post
Basics: air has mass it doesn't want to move - 101
the card, held at an angle and moving, pressurizes air on one side and reduces pressure on the other
If you slowed the movement to a crawl - the air would simply equalize faster and lift would be reduced
you can plot all this if you like
It is simply an obvious occurrance .
The difference in pressure is doing the work
The air is simply trying to re equalize and the card goes along for the ride.
If you let go of it it would fall -cushioned of course by the air resistance
which is another demonstration of pressure differences.
I don't quite follow. It seems like you said:

1) Air has mass and doesn't want to move
2) By moving the air, you change the pressure

It seems there's a logical leap between statements 1 and 2. Can you explain why moving the air causes the pressure to change?
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Old Sep 13, 2012, 10:43 AM
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Originally Posted by Montag DP View Post
I don't quite follow. It seems like you said:

1) Air has mass and doesn't want to move
2) By moving the air, you change the pressure

It seems there's a logical leap between statements 1 and 2. Can you explain why moving the air causes the pressure to change?
re read please
By moving the CARD the pressure around the card changes.
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Old Sep 13, 2012, 10:44 AM
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Originally Posted by richard hanson View Post
re read please
By moving the CARD the pressure around the card changes.
Okay, but by moving the card, surely you are moving the air as well? In any case, why does moving the card cause the pressure to change?
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Old Sep 13, 2012, 10:53 AM
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OK Ill play your game -
Please quote some text book referrence which you like . ( I did address that in my first reply)
Do you want me to parrot the basic laws as written by Newton?
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Old Sep 13, 2012, 11:03 AM
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OK Ill play your game -
Please quote some text book referrence which you like . ( I did address that in my first reply)
Do you want me to parrot the basic laws as written by Newton?
There's no game going on here. I just want to know why, in your understanding, a pressure difference arises on a wing, card, or whatever, when it moves through the air at an angle of attack.
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Old Sep 13, 2012, 11:48 AM
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Did I pass the test?
Am I missing a basic understanding -in your opinion?- If so - please enumerate .
By the way I have worked with fluidic control circuits which rely on Coanda effects etc for switching -- this stuf always seemed straightforward me .
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