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Date: Fri, 02 Jun 2006 13:42:27 -0600
From: Heidi Johnson <hjjohnson@sasktel.net>
Subject: Re: [FlyRotary] Re: Prop design speed
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
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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?

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