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[Phys-L] Re: the plane truth



Hi,

Airplane wings are actually fairly complex. Air
moving faster over the top is fairly simple.
Consider a wing that has the following
cross-section. A one meter long straight flat
bottom and a half circle on top. The air passing
along the bottom travels a 1 meter path length.
The air that starts at the leading edge and goes
over the top travels a distance of pi/2. This is
not a very good wing, but it illustrates the
idea. Air coming over the curved top of a wing is
accelerated downward while it it passing over
downward-curving, rear portion ot the wing (
Bernolli effect pressure). The wing in reaction
experiences an upward force.

( My favorite quick demo of this is a spoon in the
flow of a facet the is only partially open. Hold
the spoon vertically by its handle. Have the water
flow over the curved bask of the spoon. The spoon
will be pulled into the stream, and you can see
the deflection of the stream of water.)

Air that is split by the passing of a wing, does
not have to rejoin at exactly the same place. The
upper air might lag a bit, but the amount of lag
cannot continually increase, otherwise there would
eventually be a huge pile of air someplace.

In addition to the Bernolli effect there is the
angle of attack. The bottom of the wing is higher
in the front than at the rear. Air hitting the
bottom of the wing is "reflected" downward. By
equal and opposite reaction the wing experiences
an upward force. ( This is enough to allow one to
fly upside down.

Thanks
Roger Haar
U of AZ




Anthony Lapinski wrote:

In teaching about fluids, I was discussing that airplanes fly because of
Bernoulli's equation (principle). The shape of the wing allows air going
over the wing to travel faster than that going under the wing. Then the
pressure under the wing is higher than that above, providing "lift." Here
are my two issues with this:

1) Exactly how does the air travel faster over the wing? If the plane is
going at a constant speed, then shouldn't the speed of air relative to the
wings be the same? I've heard that the air at the front of the wing must
meet again at the back of the wing. But what would happen if the plane
were to somehow move through a more dense fluid, say, water? Would the
same thing ("lift") happen?

2) I know that some planes can fly upside down. How can this be understood
in terms of lift and Bernoulli's equation?

Can anybody offer insights into these issues?
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