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Re: Judgement on opposing airfoil views pt. 2



On Fri, 27 Aug 1999, Timothy Folkerts wrote:

I still keep coming back to the bird in the cage, or a helicopter in a
hanger.

The bird steps off the perch and starts to hover (must be a humming bird).
It pushes one puff or air down, followed by another, and the bird finds
itself in equilibrium. But soon the downdraft will set up a circulation
pattern - up along the sides and back down the middle. So now the bird is
flying in a downdraft (which it created itself), which means it needs to
flap harder, which sets up a bigger downdraft ....

AHA! That's the problem! There's a flaw in the above description because
you're imagining that the cage is so small that its walls interact with
the air near the bird and "focus" the downdraft back onto the bird. In a
similar fashion, we could show that if we put a tight box around the
rotating blades of a helicopter, the lifting would vanish. Therefore
instead imagine that a tiny fly is hovering in the center of a huge
plexiglas tank having a size of about 10 feet on a side. Put the whole
tank on scales, and the scales detects the weight of the fly whether it
hovers or not.


unencumbered by any factual information whatsoever, i would speculate that...

all of you are assuming the cage/crate/box is airtight? that there is no
flow of air through the sides of the container.





Dr. Lois Breur Krause
Department of Geological Sciences
442 Brackett Hall
Clemson University
Clemson SC 29634

teaching chemistry, physics, astronomy and geology to elementary education
majors.

How We Learn and Why We Don't: Student Survival Guide,
available from International Thompson Publishing, ISBN 0324-011970

http://home.earthlink.net/~breurkrause

krause@clemson.edu