I have a question but i am not real sure as to why and why nots....
My question revolves around getting more rpm and why they always run gearboxes as slowing down the rpm of a propeller.
I'm confused as to why we do not use a gearbox to speed up rmp...
I was thinking of a model like a hawker Tempest or a hawker typhoon where the engine/compartment is off set fom the cowls centerline...
In real life they needed the engine higher so the large prop would not hit the ground and encase the engine completely.
in a model with an electric motor you could mount the motor lower to use the cowl opening . and a gearbox would turn the prop and raise the c/l to the correct spot.
But not sure why we cannot use a larger motor and sping a prop faster than the motors speed ?

I have a question but i am not real sure as to why and why nots....
My question revolves around getting more rpm and why they always run gearboxes as slowing down the rpm of a propeller.
I'm confused as to why we do not use a gearbox to speed up rmp...
I was thinking of a model like a hawker Tempest or a hawker typhoon where the engine/compartment is off set fom the cowls centerline...
In real life they needed the engine higher so the large prop would not hit the ground and encase the engine completely.
in a model with an electric motor you could mount the motor lower to use the cowl opening . and a gearbox would turn the prop and raise the c/l to the correct spot.
But not sure why we cannot use a larger motor and sping a prop faster than the motors speed ?

  You could, but it would be a lot less efficient. In general, accelerating a large amount of air a little bit is more efficient than accelerating a small amount of air a lot.

     For IC engines you get the maximum HP by making it run at as high an RPM as possible. If you have a direct drive prop what you gain in the engine by making it spin faster can easily get lost by reduced efficiency. For full-scale applications maximizing the HP into the airframe is the most important factor so you see a lot of reduction drives.

    For stunt it's not necessarily true that you want maximum efficiency, in fact, you want it to be terribly inefficient in level flight. Plus depending on the engine, running a maximally efficient prop (at low revs) can lose you so much shaft HP that you are net worse off. That's what people missed (and are still missing in some cases) when we got high-performance engines back in the 80's.

   For electric it's a bit of a different story, your "engine" probably has reasonable performance over a wide range of RPM (limited on the low end by current capability) so you will probably never get into the compromise of RPM for prop efficiency for any reasonable prop diameters.

     Brett

I guess that leave me a sexond question is can we get 23-24k rpm on a 6-6.5 inch propellor ?

Easily, by direct drive from a motor designed for ducted fan operation.

Is this for speed?  I don't think it'd be suitable for stunt.

I guess that leave me a sexond question is can we get 23-24k rpm on a 6-6.5 inch propellor ?

yep - I currently have a sytem that will do 40,000 rpm on a 6x6 prop (for speed obviously)