Prop size

[earthbnd misfit]

2 questions. We had a raaf 2000 which broke a prop. Warp drive. He used another warpdrive, but was 1 inch shorter. He pitched it to the same rpm . But he wasn't getting enough thrust. So he bought a new prop, and it was fine. Does 1 inch make so much difference?
Second question. The larger the reduction and the larger the prop, makes it produce more thrust. Is there a limit you can calculate, where it's less efficient? You can multiply torque but not horsepower.

[Mike Schallmann]

First question --YES

Second Question --YES

[mceagle]

A basic rule of thumb is the larger the diameter the more efficient - within the constraints of tip speed and airframe clearance.
Small increases in diameter can make large increases in thrust.

[Friendly]

That one inch becomes many inches as it goes around to 360 degrees and the inch is traveling faster because it has to travel a futher distance for each turn of the prop. When it is done at x amount of rpm it adds up fast.

[earthbnd misfit]

But i'm still curious, that if there was no limitation to size. Then if we have, say 100 hp. Is there a thrust curve for different sizes. Does the pitch eventually become so flat, that it's creating more drag then thrust?:confused:

[PW_Plack]

Does the pitch eventually become so flat, that it's creating more drag then thrust?

Misfit, if you wanted to take maximum advantage of a much larger diameter, you'd need to adjust the gear ratio to slow it down, not remove pitch.

If you leave gearing the same, and just reduce pitch to keep the same max prop RPM as the smaller prop, the tips of the larger prop will be travelling much faster. Drag will increase dramatically as the prop tips approach supersonic speeds.

[earthbnd misfit]

Yes, i realize i need to gear it down, so the tip speed doesn't exceed 80% sos. But even so, wouldn't you need to reduce the pitch? But i suppose a heli rotor is a huge geared down prop?

[PW_Plack]

...wouldn't you need to reduce the pitch?

Unless you can adjust prop pitch in flight, you want as much pitch as you can use without putting the prop blades into aerodynamic stall at the start of your takeoff roll. That's because as airspeed increases, effective pitch decreases.

If you start out with very little pitch, you'll significantly compromise thrust at cruise speeds, which negates part of the reason you wanted more thrust.

Minimizing tip speed also helps reduce prop noise.