Quad rotors run at different speeds

ArduCopter Copter 3.1
1

I tried today my new APM for the first time and was rewarded with a rather weird crash. At a closer inspection I noticed that the rotorts turned with different speeds. I removed the propellors and did some experiments.

When I start the motors in Stabilize mode on a flat surface one of them starts to gradually turn slower. This can be seen and heard. This behavior can be repeated but it needn’t always be the same motor. I have the impression this does not happen in Loiter mode. I would expect it to be the other way around, if such a behavior should occur at all with a perfectly unmoving quad. The rotors do vary their speed when I turn it and that is to be expected.

Somebode any idea what is wrong?

Thanks in advance!

2

Is the quad perfectly level when starting? If it’s on a slope the lowest motor will spin faster attempting to lift that side up.
If the ESC’s are calibrated correctly then all the motors should ‘start’ simultaneously.

3

I experimented on my desk. I just checked with a plumb rule since you asked and yes, it is perfectly level. I should’ve checked that earlier, that’s one factor eliminated :slight_smile: The ESC’s are DJI Opto ESC’s which cannot be calibrated. The motors start simultaneously and (audibly) with the about same speed. Then one motor decides to turn more and more slow until almost a stand-still. I hardly can imagine that the Ardupilot is stuck.