You are missing the point. Take a small engine, divide the output in half,
and the prop is going to be small.

Why?

Small props do not mind turning fast.

No, they don't mind at all. But as I understand it they aren't as
efficient as large props turning slowly.

Also, if you use the belts for the RPM reduction, you have to put the big
pulley out on the pylon, where you want a small one.

Or you use a gear at the engine. Point is, once you're not direct
drive, the engine speed is no longer required to be the same as the prop
speed.

Jose
(r.a.homebuilt left in, though I don't follow that group)
--
Math is a game. The object of the game is to figure out the rules.
for Email, make the obvious change in the address.