can someone tell me an accurate formula to figure out MPH based off prop pitch & diameter, I know it will only be a guesstimate as drag increases with speed. here's what I'm figuring,
8x3 vs 8x4
on the bench with an 8x3 I'm pulling 18,000 rpm so if I take 18000x3 =54000(inches per min)/12= 4500(feet per min)x60=270000(feet per hour)/5280=51.136(MPH)
so if that theory is correct using an 8x4 prop if on the bench I was pulling 13700 RPM the plane would be traveling at the same speed. right? but useing the 8x4 would allow the engine more room to unload in flight. potentially giving the plane a faster top speed, right? if that is correct where does the diameter of the prop come into play? if I could pull 18000 rpm out of a 7x6 what would that do for the speed? what about a 9x3?

There's a prop calculator program you will have fun with

mvvs.nl/prop-power-calculator.xls You should be able to cut and paste this in the address bar and pull it up.

It is an Excell spread sheet program and requires Microsoft office

If you do not have that program you can down load Open Office for free and run it.

You can download a program called ThrustHP
http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm
No matter what you use as a calculator you are just getting theoretical numbers that may or may not happen in real life.
The theoretical programs do not care about diameter when they calculate speed.

Using your example calculation:
3 inch pitch
18,000 rpm
51.136 mph

As long as it is turning 18k (not more or less) a perfect 3 inch pitch prop in a perfect world will generate the same mph regardless of diameter.

When you get into the real world everything changes. Engine horsepower and torque, prop design, air density, and the airframe affect the actual performance you get. Unfortunately no one can predict that for you.

You have to buy a stack of props and do your own comparisons in the conditions you have to deal with.

carr...thanks for the LINK to the Thrust program. While it may all be theoretical....I found it helpful to compare the expected (and theoretical) effects of 2 different props on the same plane. On one comparison ( 10 x 6 vs 11 x 5) ..my speed went down but my thrust went up using the 11 inch..... which is what I expected and I was looking to quantify. regards, Rich

thanks for the links, I can see playing with it where the diameter comes in to play and the additional HP it requires to turn it at the same rpm

Sport...presume you meant the Thrust Program. How did you ascertain HP equals RPM, please? I'd love to be able to determine that ( theoretically, of course). regards, Rich