Motors and propellers

Reading a motor number

A motor described as 2207 has a stator 22 mm wide and 7 mm tall. The first two digits are width, the last two are height: it is the size of the iron inside, not the outside of the bell. A taller stator of the same width gives more torque and more weight.

Shafts on 5 inch motors and larger are typically 5 mm in diameter, held by a nyloc nut, and propellers in that range are bored for it. Smaller motors, the ones on 3 inch builds and below, are usually a T-mount or a press fit on a shaft of 1.5 mm or 2 mm instead, so the propeller has to match the motor rather than the frame size.

KV is a ratio, not a rating

KV is the number of revolutions per minute the motor turns for each volt applied, with no load. A higher number is not a better motor; it is a motor intended for fewer cells or a smaller propeller.

The consequence is that KV only means something once you know the battery. The same airframe wants roughly 2300 to 2800 KV on 4S and 1600 to 2100 KV on 6S. Fit a 6S pack to motors chosen for 4S and you are asking them to spin far beyond what the propeller and the ESC were sized for.

Rough KV ranges

Starting points, not limits. Propeller pitch shifts all of these.