This is from an article by Bill Gieskieng in the Aug-Sept 1998 Aeromodeller, pgs 16-17, about an engine for the International power freeflight class, F1C. This is a highly developed, technically sophisticated area of competition. I think the kind of engine set up discussed here is now widely used.

Basically there is a 2.5 cc--15 size engine which unloads to as much as 33,000 RPM in the air. It is turning a planetary gear setup with 4:1 reduction to give prop RPM's around 8,000. The prop is 12.8 x 11.8 inch. Prop has calculated efficiency of 80.5%. The idea being to use the engine HP to turn a large high pich prop slowly to get the most thrust out of the power train. The gears are said to be very precise and there is little friction loss in the gear train.

I thought this interesting, and wondered if anyone knew more about it.