Motor Squealing

Flying the Demonstrator today, pulling out of a failing hover and giving it a handful of throttle, I noticed a loud squealing noise coming from the motor. It sounded like a failing bearing but it wasn't that. I'm familiar with the general sound of an electric motor, including the rasping noise of a cavitating prop but this one is new to me. It was brief and only happened once in the four flights I had today. It sounded nasty but does anyone know what causes it and is it harmful?

You closed the throttle on the way down and when you opened the throttle quickly the ESC/motor went into a cycle of confusion. The solution is to set the bottom end so that the motor does not stop, then when you open the throttle the esc and motor know which way to spin.

One possibility is loose magnets inside the outer casing

Go back to the thread last week when I said...

At start-up, a brushless controller sends a pulse to the motor and the motor will either spin clockwise or anticlockwise. The controller senses the back emf and determines if the motor is spinning the correct way. It will then send another pulse to keep it spinning the same way or the opposite pulse as necessary. In flight, a windmilling prop can confuse this sequence and the controller/motor combination squeals and shuts down. This is more evident with some controllers and the cheaper ones seems to suffer more. Keeping the motor ticking over at low rpm avoids this problem.

kevinross wrote: One possibility is loose magnets inside the outer casing

This I don't want to hear.

Hi Alan - It may be worth checking that the grubscrews retaining the shaft are tight.
Did you get any flying in after we left ?
Colin