Gear drives are used a a means to increase torque of a motor to be able to run higher torque at lower RPM. Smaller electric motors do a great job of producing lots of power at high RPMs–but to get that power reduced to a RPM that it will run a prop of a reasonable size a gear drive is needed. We provide a different planetary gear drives that are matched to the motor sizes we sell. As a general rule the Maxon 4.4 gear drive is suitable for motors that put out up to 500 watts. The P 29 and P32 gear drives can handle props up to 20 inches and 3000 watts. The P42 drive is best suited for systems over 3000 watts and turning props up to 30 inches in diameter and 5000 watts. The P62 is suitable for well over 10,000 watts.  With a 14mm shaft you can run props that are over 30 inches in diameter!