Ramapriya wrote:

When I asked him
recently about how forward thrust happens, he said that the engine
blades simply turn the other direction.

If by "forward thrust" you mean "reverse thrust" where the air is being
pushed toward the front of the plane instead of the back, then yes, the
propellers of some aircraft are capable of twisting enough to provide
reverse thrust. The C-130 Hercules cargo plane is a good example of this
and there are a number of civilian models that have this option, as well.

With nothing to back up my
hunch, I still feel something amiss and implausible in what he said,
but if true, I must confess it represents fantastic braking within the
engine to first get the blades to a stop spinning, and next spin the
other way!

They don't stop spinning nor do they turn in the opposite direction. They
simply twist about their axis enough so that the air is pushed in the
opposite direction.

To demonstrate, drive down the highway with your arm outstretched and your
hand flat. Now twist your arm so that the front of your hand (the side
facing the wind) is angled up, Your arm will tend to rise. Now twist your
arm so that the front of your hand is angled down. Your arm will fall.
This is a very simplistic demonstration of what happens with
thrust-reversing propellers.

--
John T
http://tknowlogy.com/TknoFlyer
http://www.pocketgear.com/products_s...veloperid=4415
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