Airfoil
how it works

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here are two basic factors holding up a wing in flight. The action of bulk air and molecular air.

By bulk air I mean the simple deflection of air downward toward the ground and relsulting push up on the wing by reaction.
At the molecular level there is an adiabatic temperature rise as the air molecules at the leading edge are compressed. When most of these compressed molecules are released over the top of the airfoil they travel preferentially in a direction parallel to the wing surface [you need an angle of attack to create a high pressure area on the bottom of the wing which blocks the compressed air on the leading edge causing most of it to go over the top]. This is called lift (just pushing the air down could also be called lift).

The fact that the air travelling over the top of the wing is moving faster than the air on the bottom of the wing is not meaningful. This is the chief source of my discontent with conventional explanations of lift.
To see why ...

In the first figure there is a treadmill type upper surface. The air on top is moving relative to the upper surface yet there can be no lift because the air on top has the same quality as the air on the bottom. (In practice you couldn't possibly seal this gizmo but it shows what would be a clear violation of linear momentum conservation if the thing were to rise in the air.)

In the second figure lift is generated because the air coming out from the balloon is sorted and does not hit the top of the wing as often as the bottom air. It's not just moving bulk air, it's molecular sorted air.

After some distance the molecules randomize again and become part of moving bulk air and can no longer generate lift. The Bernoulli Principle if stated as "faster moving air has less pressure than slow moving air" is false by way of incompleteness. "Faster moving air when exiting an area of compression has less pressure than slow moving air" is a more accurate statement.

This tube experiment shows exactly this ...

Clearly, the air must be moving at the same velocity in the upper tube but it draws the water up the other tubes only where it comes from the compression source not further down the line.
I'm sure this is all well understood by others. So why is it so badly explained?

Addendum 4/18/05

I sent this to editorial@discover.com as a reader correction ...

"Bogglers" myths #5 May 2005 issue

Scott Kim maintains that it is solely the deflection of 'bulk' air downward by the airfoil that causes aerodynamic lift. This is false by way of incompleteness.

While a good portion of lift is obtained this way, much is gotten when the high pressure air under the wing (caused by the angle of attack) forces most of the leading edge compressed air over the top of the wing. This air is then adiabatically 'sorted' like the air coming out of the neck of a balloon, i.e. it has less molecular collisions with the top wing surface until such time as the air is thermally re-randomized.

As the wing hits air molecules, they compress like a spring and are driven forward in the direction of the wing. Then they go over the top of the wing as the spring uncoils and wind up exactly where they started from (disregarding turbulence).

This aspect of lift is easily seen in the row-of-tubes-stuck-in-water experiment where air is blown over the entrance of each tube from another tube that is parallel to the surface of the water and connected to the vertical tubes sequentially. As air is forced over the first vertical tube entrance, water rises in it due to lower pressure. Water also rises in the second tube for the same reason but to a lesser extent and so on down the line of vertical tubes till the pressure difference between the moving air in the horizontal tube and the still air in the vertical tube is equalized ... by the aforementioned "re-randomization".

The velocity in the horizontal tube is logically constrained to be the same throughout its travel, else it would 'bunch up'. [The foregoing sentence may be wrong. The velocity could be a tad faster then slower further down but compensated with more atoms per unit cube.] This fact rules out simple velocity alone as the factor determining pressure gradients in favor of "directional adiabatic cooling" by which I mean that the kinetic energy of individual air molecules is converted from three dimensional randomness to a favored direction ... away from their high pressure source and parallel to the wingtop.

A last note:

There is a downward force on the wing that arises from pushing the bulk of the leading edge air over the top of the airfoil. I suspect that the final situation is this.
  • A) - One upward force component from deflecting air downward.
  • B) - One upward component from sorted air going over the top
  • C) - One downward component from pushing leading edge air over the top
So, one could say with equal validity that C cancels A and therefore only B is left to lift the wing ... or ... C cancels out B leaving only A as the cause of lift.

Your pick. ;o)



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