Attached is a Prop Design program, which I found some time ago, that has the following explanation on its help page. You can play with the numbers to match your prop and see what speed you will get or you can plug in the variables and see what size prop you should have.  I hope you can receive the attached file.

 

 

 

 

----- Original Message -----

From: george lendich

To: Rotary motors in aircraft

Sent: Thursday, December 22, 2005 4:45 AM

Subject: [FlyRotary] Re: Props

 

Thanks Ed,

I like to reduce things down to the simplest formula, provided one doesn't lose track of what's relevant.

So I guess I should look at this as 100 % theoretically efficient - whereas we may be looking at only 80 % efficiency ( in real terms), Do you agree with 80%?

However your saying that the lift may give more speed ( MPH, in real terms) - So are you suggesting that these two (variables) may balance out?

 

Or should we use a fudge factor, and that could be what?

 

I'm not looking for a finite answer, just close enough to be able to access the experts opinion, and we all know what an expert is - " A drip under pressure".

 

George (down under)

Yes, George.  If we assumed no slip in the prop then theoretically the formula will give you the pitch of the prop you would need to screw through the air and achieve your MPH at your RPM input.  This is based simply on the screw principal.  I am certainly not a prop expert, but I also believe that since the prop is  a spinning wing (airfoil), that there is a lift component  in the direction of flight which may result in more speed than the formula could account for.  

 

Your second equation can be reduced further from   (Inches/Minute)/(RPM) = (Inches/Minute)/(Revs/Minute) = inches/Rev

 

Ed A