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03-07-2009 | 11:45 AM

#57

DarZeelon

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From: Rosh-HaAyin, ISRAEL

RE: Propeller Worms

Quote:

ORIGINAL: buzzard bait

I am surprised no one bit on Dar's comment that when a prop is turning on a plane in the air at a speed that does not generate thrust, the drag is higher than when the prop is stationary. He said the result is that a dead stick landing has a shallower glide path than one at idle. I'm not sure that's true.

This is also really counter-intuitive, so if true, what is the explanation?

It also contradicts the long-standing practice of scale modelers who use rubber bands for power. These guys are constantly trying every trick to squeeze out a little more endurance. And they use free-wheeling props, even when using balsa props which do not have enough momentum to cause much reverse rotation. All they all wrong?

Jim

Jim,

That's because it is true.

The longitudinal drag produced by stopped, aerodynamically stalled props blades, is smaller in magnitude than the longitudinal, 'negative lift' produces by this prop, as it spins significantly more slowly than needed, for forward flight at the same speed.

If you want the longitudinal parallel; a plane descending in a harrier maneuver, will lose altitude much faster than the same plane gliding...

This is because the stalled wings (in a harrier) make less lift than the lifting wings (in a glide).

Windmilling props make even less drag than a stalled prop; certainly less than a prop forced to spin significantly more slowly than necessary...
This is why the rubber guys let the prop windmill, rather than holding it stopped.

An idling prop will not be spun much faster by the ambient air it is flowing through - it does not windmill!

A windmilling prop, on the other hand, will approach pitch RPM speed.

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