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Old October 4th 07, 05:32 PM posted to rec.aviation.piloting
BDS[_2_]
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Default Why Airplanes Fly - Voids Above A Planar Sheet


"Le Chaud Lapin" wrote...
On Oct 4, 9:13 am, Tina wrote:
If "Getting on the same page" means learning some of the physics of
flying, I'd enjoy knowing how these 'experiments' are related.

Are you suggesting that a table top under the paper is in any way
representative of what goes on in a dynamic airfoil?


Well I was trying to illustrate what goes on between the two sheets of
paper, but I guess a table will do. Technically, if you place one
sheet of paper on top a table, and yank up hard on the paper, there
will be a tendency for the table to lift off the ground, but since the
mass of table is so great, the net pressure upward on table is not
enough to counteract gravity so the table remains at rest (actually,
at a quantum level it does not completely "not move", but for our
purpose we can say that it doesn't).

So I used two sheets of paper because the bottom paper will rise.

So yes, I believe this experiment illustrates an important phenomenon
in aerodynamics. It is not the only phenomenon that plays a role, but
it has one, nevertheless. The decriptions of lift that I read in
flight books seem to ignore it. This weekend I am going to download
material on aerodynamics and read what it says.

There is another experiment that could demonstrate this principle more
dramatically, using an actual airplane wing:

SMOKING CIGARETTES/AIRPLANE WING EXPERIMENT:

I would take an airplane wing, and mount it rails that can move in the
forward and aft directions along what would be the longitudinal axis
of the airplane if if the wing were so attached. Then I would take a
bunch of cigarettes, light them, and hang them up-side-down from a
high ceiling above the wing of the aircraft. The wing would have an
exaggerated AoA, say 30%, no flaps, displaced slightly so that it is
ahead of the hanging cigarettes, but so that the cigarettles cannot
touch. The cigarettes would be lit so that stream of smoke floats
upward.

Then I would use a tremendous force applied to move the wing forward
along the rails, say, by linear induction motor, or whatever, to move
the wing forward, being careful that the apparutus doing so is already
ahead of the wing and connected by steel wire to minimize interference
effects.

You would see that, if the impulse is great enough, not only would the
smoke be diverted from upward and moved in the direction that the wing
went (forward), but the hanging cigarettes themselves would move.

If flat pressure sensors were mounted above the wing, close to the
trailing edge, they would show a momentary decrease in pressure.

If flat pressure sensors were mounted below the wing, close to the
trailing edge, they would show a momentary increase in pressure.

After the force stops, there would be relaxation where the
rarefication above the wing and compression below the wing are
elminiated by flows due to the pressure gradient.

In a real airplane, this is what is happening, but because the the
wing is constantly moving foward, the rarefication above the wingg and
the compression below the wing are never quite normalized by to normal
atmosphere.

The downwash above wing is due to air rushing in to fill the void.

SMOKING CIGARETTE/HARD-COVER BOOK EXPERIMENT:

There is an similar, not-as-dramatic experiment you can do at home
that is closely related to experiment above. Let a piece of stiff
cardboard be your wing. Hold it from the side at an angle of attack,
as above, but don't rest your arm on top of a table. That would
create a boundary condition beneath the wing. Light a cigarette and
inverted so that it is the hot part is near the top of the wing, so
where in middle between leading and trailing edge. Get your arm out of
the way of the void that is about to be created. Now, in one quick
motion, move the cardboard forward. Notice the tremendous net impulse
force that is generated on the cardboard. The smoke will follow.

These things are happening in flight, along with Bernoulli.

Are you really educated as an engineer?


Yes, electrical/software.

-Le Chaud Lapin-



Are you crazy?! Do you know what cigarettes cost these days??!!

BDS