ORIGINAL: B.L.E.
Gear/belt reduction is also very popular with homebuilt airplanes, especially when converting automobile engines to aircraft use. The Rotax two and four stroke engines all have prop speed reduction units (PSRUs) on them.
Speaking of that, here's a picture of my gasser (Rotax 503) with the gearbox and prop. The gearbox is equipped with a centrifugal clutch and a 3.47:1 reduction. The max HP of the motor (50 hp at sea level) is reached at 6500 rpm, so a direct drive is basically impossible. But the gearing translates to about 1850 prop rpm and as you can see, it can swing a HUGE one. The prop is a 68" 3-blade.
For sure, it makes lots of thrust and needs LOTS of left rudder on takeoff...
I'd love to see a practical gearbox for r/c motors, as a big one turning slow can really crank out a lot thrust and is much more efficient than a small one really spinning up.
But I'm sure it'd cost out the wazoo. My rear is still sore from buying this gearbox/clutch assembly you see here
....
LS