Prop Speed vs. Thrust vs. diameter vs. vs. vs...

[GyroCFI]

I've read a lot of posts on chosing and setting up a propeller, and i've done it before, with a Warp Drive. However with my new bird coming, i'm investigating a new prop for her and the Prop manufacturers don't ask about how much thrust you want, they typically ask for HP of the engine, the diameter, rotation (CW or CCW) and the speed. Speed... that's the clincher.

So what's the magical speed number? I'm assuming somewhere in the neighborhood of 2500RPM?

And then there's the age old argument of size... right now it has a 68" prop but the neighborhood that surrounds the airport will be filing complaints left and right if this engine/prop combo is too loud. So if I went and put a 70 inch prop on it, would it make enough of a benefit to make it worth my while?

thoughts?

[C. Beaty]

Propeller static thrust varies as the two-thirds power of diameter.

Going from 67” to 70” (70/67) = 1.045. Raising that to the 2/3 power = 1.03 In other words, static thrust potentially increases by 3%.

[GyroCFI]

Thanks Chuck, I appreciate the input. If I remember from my FW days, I think we were at about 2400 or 2450 RPM at cruise, would that be a good number to shoot for when talking to the prop manufacturers?

[C. Beaty]

2400-2450 RPM is as good as any. RPM determines propeller blade area for a given power loading. As the RPM increases, required blade area is reduced.

Also, as RPM (tip speed) is increased, noise increases.

[Mac]

Calculations from Martin Hollman: Static Thrust = 7.38 x ((HP x Prop Dia ft) power of 2/3)
eg 7.38 x ((280 x 6.667) power of 2/3) = 1113 lbs.

I took Martin Hollman's "Static Thrust" formula and entered it into a Excel spreadsheet. I changed the formula from feet to inches for prop diameter since inches is predominately used.

Rounded prop tips and maintaining prop tip speed below .8 mach help reduce sound. Thrust is marginally increased above .8 with an unproportional increase in sound (sound - a conversion of hp).