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Date: Tue, 7 Jul 2009 20:19:32 -0600
From: Bob White <bob@bob-white.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Speed of Sound was Re: [FlyRotary] Re: INJECTION OIL IN FUEL
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Hi Bernie,

I found a web site with a speed of sound calculator.  

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/souspe.html

It seems mostly dependent on temperature with no density effect for a
particular gas.  Wikipedia seems to agree.

The calculator gives 768 mph at 66F.  40F is 748 mph.  90F is 786
mph.  It seems you are really on the edge, and apt to go super sonic
at the prop tips under some conditions.  Cooler air will give you more
power so more rpms also.  Dry air is specified, so humidity will
undoubtedly change the numbers somewhat.  Haven't found any
calculations for 'wet' air.

Bob W. 

On Tue, 7 Jul 2009 21:45:29 EDT
Berniehb7448@wmconnect.com wrote:

> This is what I figured out as my "formula":
> 
> Max Prop RPM & Speed of Sound
> 
> 1 Mile = 5,280 Feet
> 
> Speed of Sound =  768 mph (I think this is at sea level barometric 
> pressure, I haven't figured out what happens with altitude.)
> 
> pi = 3.1429
> 
> 6 FT (72") D (Diameter) Prop    
> 
> CIRCUMFERENCE =  pi X D =   3.1429  X  6 ft  = 18.857 ft Circumference
> 
> 1 RPM = 18.857 ft/Minute at prop tip
> 
> With 2:1 PSRU Reduction, calculate by prop RPM and Circumference:
> Prop RPM 3,000 (Engine RPM 6,000) =  56,571 ft/Minute = 10.714 miles/minute 
> = 642.84 MPH
> (768 MPH - 642.84 MPH  = 125.16 MPH under Speed of Sound)
> Prop RPM 3,500 (Engine RPM 7,000) =  66,000 ft/Minute = 12.5 miles/minute = 
> 750 MPH 
> (768 MPH - 750 MPH  = 18 MPH under Speed of Sound)
> ================================================
> I paid enough attention in my math classes to think I've got it right. 
> 
> Here's one for you: Does a cogbelt PSRU's powerlimit increase with a wider 
> belt? Like, for example, if a two incher can handle 150 HP, then you might 
> need a four inch belt to handle 300 HP. Something like that?
> Here's another: One maker put an engine RPM limit on the PSRU, and I'm 
> thinking it has to do with the capability of the bearing at the input end. Maybe 
> because these bearings are sealed - they don't run in an oil bath.
> Thank you, Bernie.
> 


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