My students had this happen. At low power one of the 4 would not turn, but at high power it would, but only after the other three were already spinning, so it couldn't lift off without crashing to one side. The problem motor did not spin as freely with power off. We thought the motor was damaged. But after removing the motor and then the propeller, and making sure it was clean (no hair wrapped around the spindle, etc...), and then putting it back together, it worked fine again. I don't know if some dirt got in there that wiggled back out, or they removed a hair, or what, but after doing this, the motor was fine and has been fine ever since.

My students want to write Python code to make the CoDrone flip. We have noticed that if the drone is hovering and you hold it (carefully) and shake it back and forth it will stop any running code. This is a nice feature, but I now wonder if that gyro (or some other) sensor will abort the code if we try to make it flip. The plan is to get it ascending, make it pitch hard and then cut power (mostly), let it coast under the angular momentum, and then re-engage the motors to catch itself before it crashes. They will be trying it today with a blanket under it as a safety net, but I was wondering if this can even work and/or is there a better way. The Python library does not seem to have a flip sequence (but I think the Arduino does). Any advice?