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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.