rmp/v means just that, but don't get confused that more load the motor has on it the rpm drop i.e. a prop.

Higher rpm/v motor usually use gear reduction to achive higher torque. The motor is cheaper but yo have to pay for gears.

Gears however allow you better speed control especially at low rpms.

Ducted fan units however need as many rpm's as possible.

There are more exact sciences envolved here.

I think i know what you are trying to make here. i have seen something similar
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