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Date: Tue, 01 Nov 2005 16:41:14 -0500
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Catto Prop
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David Leonard wrote:

>
> If I can turn my 2-blade 74" prop at 3000k, will I pick up any speed 
> from a 3-blade 74" prop turning at the same speed (through reduced 
> slip)?  If I make the prop shorter can I turn it faster?  I have heard 
> that 900fps prop tip speed is a good upper limit.  Is this a 
> good number to use?  Should I be using that as my cruise tip speed, 
> not max tip speed? 
>  

Dave, read

http://ernest.isa-geek.org/Delta/Library/HowIMakeProp.pdf

There are several sections in there that you won't care anything about, 
but he spends a lot of time discussing how he sizes props.  It's been a 
while since I read it, but I'm fairly certain that his rule of thumb was 
that an inch of prop will give him 100RPM.  Nothing is linear in this 
game, but jumping from a 68 to a 74 would drop you down to around 2500RPM. 

900fps tip speed in cruise is riding the bleeding edge of supersonic and 
leaves precious little headroom when you want to get there fast. 

Another blade will absorb (nearly) as much energy as each of the first 
two.  If you're making 150HP, then each blade would be converting 75HP 
to thrust.  A third blade of the same size and pitch would absorb 
(nearly) another 75HP at the same RPM.  Do you have that much throttle 
left?  The (nearly) is important because there are some side effects 
like friction and blade interference that have an effect and some are 
nearly impossible to quantify.  Luckily, their effects tend to be small.

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