Last weekend I've experienced quite serious problem with Reptile X4R 220 racing quadcopter: on full throttle quad was loosing stability. At first, it was starting to drift yaw to the right. Then, if throttle was not lowered, roll and pitch was also becoming unstable and quad was doing crazy things in the air.

I do not have a video footage of those issues, but Blackbox footage looks like this:

yaw unstable at full throttle 1

Roll, Pitch and Yaw sticks are at zero, throttle is raised from aroung 45% up to 90%. When throttle reaches around 80%, gyro starts to record movement on all axises. At one point, yaw gets a kick and reaches more that 200dps. Finally, I lowered throttle and quadcopter stabilized (moment of stabilization is not visible, but trust me, it is there).

The reason for such a behavior became pretty visible as soon as I displayed motor output:

yaw unstable at full throttle motor output

Flight controller requests much more power from Motor #1 than from other motors. Not only in this short scenario. Look at this graph:

yaw unstable at full throttle motors

FC requests more thrust from Motor #1 almost all the time. Why? Motor #1 provides less thrust than other motors. There are few possible reasons:

  1. Damaged propeller
  2. Damaged motor
  3. Damaged ESC
  4. ESC not calibrated

In my case, it was ESC calibration issue. While this might sound strange, ESCs loses calibration from time to time. Usually after a crash that resulted in detached battery.

Proper ESC calibration solved all the problems I was experiencing.