Inrunners get their nickname from the fact that their rotational core is contained within the motor's can, much like a standard ferrite motor.
Compared to outrunner motors, inrunners tend to spin exceptionally fast, often as high as 11000 RPM per volt, far too fast for most aircraft propellers. However, inrunners lack torque. As a result, most inrunners are used in conjunction with a gearbox in both surface and aircraft models to reduce speed and increase torque
In many cases the inrunner is "ironless" in that there is no iron stator core to magnetize. The wire is run inside the can and held in place by epoxy or other resin material. Because there is no magnetic iron core, ironless in runners have no "cogging", in that they spin freely with no magnetic interaction when power is disconnected. A well-designed ironless inrunner is extremely efficient. This is because there is virtually no iron magnetization loss and very little windage loss in the motor. However, due to the lack of a magnetic stator core, the ironless motor has very low torque but also higher Kv (a unit that is defined as RPM per volt) when compared to an iron core motor.
Modern brushless inrunners can have Iron core windings to address the lack of torque while maintaining high RPM capability, more commonly used in RC car and truck applications
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