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From: C. Marin Faure (no email)
Date: Sat Aug 16 2003 - 02:56:12 EDT
From: "" <>
Subject: Re: TWL: Bernoulli effect is not why wings fly
>The actual fact is that lift is created by suction above the wing, NOT by
pressure below as you are proposing. The bottom of a typical wing is flat
and HORIZONTAL while there is a convex curve to the top. This means the air
passing over the top has to travel further to arrive at the trailing edge so
it travels faster and the reduced pressure creates a SUCTION on top of the
wing.
This suction thing is a very inaccurate explanation of lift, and it's
unfortunate that so many people believe it. It's easy to understand and
visualize, which is why it's so popular.
The real causes of lift, outside of the basic explantion that lift is the
reaction to the action of moving air down, are incredibly complicated and
very hard to understand and visualize. There are things like the Coanda
effect and spanwise flow, and pressure shifts... enough stuff to make your
head ache. Try making sense of the pressure and airflow patterns around a
wing on a Computational Fluid Dynamics display, and you soon find yourself
thinking that going to your boat and changing the oil with a McDonalds
straw would be a more relaxing experience.
So everyone preaches Bernouli, which even a five-year old can understand,
and eveyone goes away happy. Unfortunately, it's not correct.
If it were true, then planes with symetrical airfoils should not be able to
fly. But they do. Rubber band powered balsa wood planes with wings made
of a simple slab of balsa wood with no curvature whatsoever, should not be
able to fly, but they do.
Like I said, a wing develops lift because it moves air down. If I said
pushes in other posts, I mis-spoke. There is (to me, anyway) a difference
between pushing air down and moving air down. The air that moves down and
creates lift is not necessarily being pushed down under the wing. In fact,
much of the downward movement of air that produces the opposite reaction-
lift- is behind the wing.
Years ago Boeing did a series of tests to study the pattern of wingtip
vortices. The tests consisted of flying a 747 and a 737 through smoke
trails emitted from a bunch of high towers. The altitude of the tests,
while low, was well out of ground effect. If you watch the film of the
planes passing through the smoke streamers, the tremendous downward
movement of the air caused by the wing is instantly apparent. If the wing
were being sucked up, as the popular theory goes, the air under the wing
would be relatively stable. But believe me, it isn't. The smoke makes it
instantly obvious that there is a massive downward (and outward) movement
of air under the wings.
The Bernouli effect- on wings that are designed to create this effect, and
not all wings are- does play a role in why the air is moved down by the
wing. But the Bernouli effect does not create the lift that holds up the
plane, it contributes to the wing's ability to move the air down.
I described what the cross-section of a 747 wing would have to look like to
generate enough lift by the Bernouli effect to "suck up" the airplane. If
you could see this diagram, you would instantly realize why the Bernouli
effect is not why wings fly.
I know it's asking a lot to expect most people to give up this popular
notion of planes being "sucked" up into the air, but if you want to know
the real source of lift, it ain't that.
_______________
C. Marin Faure
GB36-403 "La Perouse"
Bellingham, WA
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