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Old May 7th 09, 02:03 AM posted to rec.aviation.piloting
Dave Doe
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Posts: 378
Default Feathering an engine

In article c8d0834f-8e51-42ad-add6-
, says...
On May 6, 6:44*pm, Dave Doe wrote:
In article o_r_fairbairn-B758D1.12591306052009@70-3-168-
216.pools.spcsdns.net,
says...





In article ,
*"Tim" wrote:


"Orval Fairbairn" wrote in message
news In article
,
wrote:


On May 5, 1:06 pm, Scott Skylane wrote:
As an aside, the drag created by a windmilling propeller, i.e. one not
feathered and attached to a dead engine, creates as much drag as a flat
plate the same size as the area of the prop arc.


Nope. The blades cannot be everywhere at once, and so the area
affected is no larger than the blade area.


Dan


YES! In aeronautical engineering analysis, a windmilling prop is
considered to be a flat disk, with drag numbers to match. Feathering the
prop greatly reduces drag.


So you are saying if I loose power at high altitude in a fixed pitch prop
aircraft, like a Skyhawk, I will have less drag if I stop the prop, as
opposed to letting it windmill?


That is correct!


Please provide some evidence.

--
Duncan- Hide quoted text -

- Show quoted text -


Here is a URL to a thesis that addresses the question. The answer,
based on his evidence, is, it depends.

http://www.goshen.edu/physics/PropellerDrag/thesis.htm


Thanks, so it really depends on the pitch of the propellor.

qv... you have a prop on the end of a shaft that has no engine, just a
braking mechanism (this is where I find it hard to get my head around
the maths! ...

You're say gliding through the air, the prop is freely spinning. Now,
we apply some braking to the shaft and slow down the prop. Basically
(and according to the maths you've shown), the drag will (dependent on
pitch, but for most fixed pitch props), increase. And "at the other
end" the brake will produce heat. The prop will slow and I would expect
the drag to *increase* and the aircraft attitude will need to be lowered
to maintain the same airspeed. But... according to the maths you've
shown, this is all dependent on the pitch of the prop. And, I *assume*
that fixed pitch props are too fine in pitch to be good windmills.

According to the maths, I assume that wind turbines are more efficient
if built really large, and spin slowly, rather than fast (which kinda
makes sense - certainly in known (expected) wind strengths.

--
Duncan