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Everybody knows that an electric motor KV is a number of RPM per 1V of the supply voltage. Right? Yeah... too bad it's a lie. Not a very big lie, but a lie after all. KV is the number of RPMs when Back EMF generates 1V on the motor coils. Almost the same, but "almost" makes all the difference.
Also, KV relates to the torque motor generates. The more KV, the less efficient the motor gets because it requires more amps to deliver the same amount of torques (in Newtonmeters, Nm). On top of that, the fact that you got a higher KV motor does not mean that propeller will spin faster. Why? Because you need torque to spin the propeller faster. And more KV means, in general, less torque.

More about motors    • RC motors: size, torque, speed, KV, K...