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Date: Fri, 02 Jun 2006 13:25:53 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Prop design speed
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Bill Bradburry wrote:

>Joe Hull says:
>Snip...
>"Thanx Ed. I have a 64"Dia. x 76" pitch prop. It's supposed to be cruise
>prop
>that is designed for 190HP and 220MPH.  But you're right, it's too early
>to
>be cutting on the prop."
>
>OK, here is where I demonstrate my ignorance...
>My rough calculations, with an estimated 20% slip, says this prop with a
>76" pitch, is good for 121 MPH with a 2.85 drive and 160 MPH with a 2.17
>drive assuming a 6000 RPM cruise???
>Where am I going wrong?
>
>Bill Bradburry
>
>
>--
>Homepage:  http://www.flyrotary.com/
>Archive and UnSub:   http://mail.lancaironline.net/lists/flyrotary/
>
>  
>
For 2.85:

6000 RPM / 2.85 = 2105.263157894737 RPM (at the prop)

2105.263157894737 R/M * 76 I/R * .8 = 128000 inch per minute

128000 I/M / 12 I/FT / 5280 FT/Mile * 60 M/Hour = 121.21 MPH

For 2.17
6000 RPM / 2.17 = 2764.976958525346 RPM (at the prop)

2764.976958525346 R/M * 76 I/R * .8 = 168110.599078341 inch per minute

168110.599078341 I/M / 12 I/FT / 5280 FT/Mile * 60 M/Hour = 159.2 MPH

Looks like your numbers are on target, but isn't 20% slip rather large?

-- 
         ,|"|"|,               Ernest Christley     |
----===<{{(oQo)}}>===----     Dyke Delta Builder    |
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