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Date: Fri, 02 Jun 2006 16:10:33 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Prop design speed
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Heidi Johnson wrote:

>20%  [or rather 80% efficent] isn't really that huge, I think the norm for a run of the mill fixed prop is 
>in the range of about 85%, Paul Lipps posted a note someplace just recently saying his props's
>are just over the 90% [ I think] so.. it's a decent [conservative] ball-park number anyway..
>
>Jarrett Johnson
>  
>
It's a notoriously difficult number to nail down, because it depends on 
how fast the engine is turning and how fast the airplane is moving.  The 
figure is 0% during the run-up.  Go to altitude and pull throttle in a 
dive, the figure can shoot up to over 100% (right after the wings come 
off).  I would think a conscientous manufacturer like Catto would quote 
a typical figure that would include a balance of climb and WOT cruise 
performance, the former being low and the latter much higher.

When it's all rolled up though, I don't think the numbers that Joe is 
quoting are all that far off.  If he was getting 80% efficiency, it 
would indicate the prop is pitched at 91".  If he were reaching 95%, the 
prop would be pitched at 77".  Add in a few other fudged factors, like 
where the pitch was measured as Joe has already mentioned, and 
everything looks fairly reasonable.

-- 
         ,|"|"|,               Ernest Christley     |
----===<{{(oQo)}}>===----     Dyke Delta Builder    |
        o|  d  |o          www.ernest.isa-geek.org  |