Bernie,

Here’s another formula if you want a crosscheck:

 

PropTipSpeed = Sqrt (Radius² X AngularVelocity² + TAS²)

where:

Angular Velocity in radians per second

TAS in feet per second

Radius in feet

PropTipSpeed will come out in feet per second

 

This was taken from the Avweb site.  One of John Deakins articles that someone mentioned here a while back.

 

Bryan

 

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.