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From: "Bob Darrah" <RDarrah@austin.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-3050278@logan.com>
Subject: Re: [FlyRotary] Re: FW: [VAF Mailing List] Engine Choice
Date: Sun, 29 Feb 2004 09:57:24 -0600
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Jim Sower wrote:

> <... A wind milling prop DOES have hugh drag compared to a
> stopped one ...>
> I know a wind milling prop (prop turning engine) has more drag
> than a stopped prop (frozen engine or not enough airspeed to
> overcome engine compression).  This guy specified a freewheeling
> prop (sprag clutch that is in the overrunning mode).  There is
> no resistance at all to the prop and the only "work" the
> slipstream has to do is overcome the negligible sprag clutch
> free wheeling friction
Snip

It depends:  a free-wheeling prop in flat (high rpm) has considerable drag.
Add some pitch, and the drag goes down rapidly.  As an example, take a
helicopter in autorotation or a auto-gyro.  the prop (rotor) is in flat
pitch and is freewheeling.
>
> I can't argue with your experience, but I had always understood
> that a frozen prop had less drag than a prop wind milling the
> engine, but that the difference was not significant.

My understanding is that the difference is very significant.

 A> freewheeling prop is waaaay different from a wind milling prop.
> Like if you have a sprag clutch and turn up the engine to high
> rpm and then shut it down, the engine stops immediately but the
> prop will keep spinning for a couple of minutes.

Or as long as the wind turns it.
>
> <... When you think wind milling drag ... think prop circle
> area, not just the blades cross section themselves. Square yards
> of drag, not square ft ...>
> I don't think that's exactly the way it works quantitatively.
> Maybe Marc can help.  Oooops!  Marc is not on this list.

Maybe not exactly, but darn close.
>
> As I read it .... Jim S.


And as I read it....Bob Darrah