Bernie,
Your calculations are correct,but remember that the given figure for the SOS is in 0% humidity and standard air temp. With normal humidity levels and higher temps I've seen figures as low as 680 mph for the speed of sound. It would be wise to give yourself a bit more head room than 18 mph. Your prop efficiency just goes to s%&t if the tips go supersonic so giving it a wide berth is a good idea.
Your belt question: Yes a belt thats twice as wide will generally carry twice the HP. The alignment of the sprockets ant the quality of the bearings to carry that load are MUCH more critical. By the way if you are trying to use Gates toothed belts for a PSRU make sure you NEVER even SAY the word airplane. They will refuse sale to you. It is a company policy that they will not support sales to any aircraft system.
Bill
-----Original Message-----
From: Berniehb7448@wmconnect.com
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Tue, Jul 7, 2009 6:45 pm
Subject: [FlyRotary] Re: INJECTION OIL IN FUEL
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)
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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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