Efficiency.
Unless the plane is really scooting, big props are more efficient than little props. We don't put gearboxes on glow engines because they're heavy and with the pounding they get from an engine they're a maintenance nightmare. You used to see them occasionally in Pattern (Hanno Pretner flew twin .60s geared to a common prop in the early 80's), but for sport flying you just increase the engine size and have fun.
With electric motors, the speed at which the motor is efficient is really way too fast for a sensible prop on anything but a speed demon race plane. Put a gearbox on and you lose efficiency because of the gearbox -- but you gain enough efficiency from the bigger propeller and the motor running in it's "sweet spot" that your overall system efficiency goes up.
With an outrunner the tradeoff is similar: outrunner motors are generally less efficient than a comparable inrunner -- but the outrunner motor is more efficient and probably lighter than that inrunner with a gearbox, and it's certainly less of a pain.
You see similar tradeoffs in full-size aviation: piston engines can't come up to their full power potential when they're driving a propeller directly. So you see many, many planes that have a piston engine and some sort of a speed reduction gear to allow the motor to run at its best RPM while driving a big prop efficiently. The extreme example of this is a turboprop, where the turbine is going upward of ten thousand RPM, but it's geared down to a prop that may be turning less than 2000 RPM.
Topic: Gearbox question.. (Read 1084 times)