"That definitely isn't good." I had just landed and unplugged my RC airplane, a Flyzone Rapide. As I carried the model back to the pit area and absentmindedly turned its propeller by hand, I heard and felt metal-on-metal clicking…which is not something you should ever have with a brushless motor. There had been no noticeable symptoms of a problem in flight, and the motor was not seized. So I was hopeful that I would be able to find and repair the problem without too much time or money (spoiler: mission accomplished!).
A Little About Brushless Motors
Brushless motors are very simple devices. In high-level terms, a brushless motor breaks down into two coaxial parts, the rotor and stator. The rotor is the spinning part and usually has permanent magnets attached to it. The stator does not move. It typically has copper windings that create magnetic fields when energized. Brushless motors require a dedicated Electronic Speed Control that rapidly toggles the polarity of the magnetic fields in the windings to make the rotor spin.
There are two basic types of brushless motors, inrunners and outrunners. On inrunners, the stator is integrated into the outer housing of the motor. Other than the exposed drive shaft, the rotor is fully enclosed within the stator. Outrunners are nearly the exact opposite. The rotor comprises the outer casing of the motor, while the stator is mostly hidden inside. Parts of the stator are accessible to permit mounting the motor and connecting power wires. Most electric-powered RC airplanes, including the Rapide, use outrunner brushless motors.
The sheer simplicity of brushless motors makes them practically maintenance-free. You just need to check on the ball bearings from time to time. They also tend to be very reliable. There just isn't much that can go wrong. The most common problems I see stem from people pushing their motors too hard and burning out the windings or overheating the permanent magnets…at which point, they're no longer permanent.