Attitude Oscillations Cause Crash for Large X8 in Autotune

Hi everyone,

I’ve been looking over our vehicle’s logs for the past two days and was hoping someone here might be able to help me understand some of the issues I’ve been struggling with, which resulted in a crash recently.

Some brief stats of the vehicle:
Frame and Class: Octo Quad X8
Flight Controller: Pixhawk 2.1 Cube
Motors: TMotor U8 Pro 100 Kv
Propellers: TMotor 29x9.5
ESC: TMotor Flame 80
Takeoff Weight: 17 kg
Hover Throttle: ~52%

Testing Recap:
The vehicle flew alright, albeit not great with the default PID gains. From there an autotune for roll only was attempted. The autotune completed after several minutes and the vehicle was landed. As the vehicle is almost completely symmetric, the PID gains for pitch were then set to what the autotune found acceptable for roll. After verifying that these new gains were “basically flyable”, another autotune for pitch was conducted.

Everything went well for the first several minutes, then at a certain point the vehicle began to oscillate out of control along both the roll and pitch axes (45 degrees across the vehicle). The safety pilot was unable to recover and the vehicle spiraled into the ground.

The problem seems to stem from a roll and pitch coupling in the vehicle. Even when flying with the default PID gains in stabilize mode this issue could be seen. For example, a large pitch input would result in an uncommanded roll 180 degrees out of phase from the pitch response. This seems to have compounded during autotune until the vehicle became unstable.

What could cause this coupling of roll and pitch? A few things came to mind for me but I don’t know enough about Ardupilot and tuning large copters with low Kv motors to be confident. Here are a few explanations:

  1. Poor vehicle tuning. Yaw was not tuned at this point. Since a combined roll and pitch input equate to yaw, possibly the vehicle was unable to correct to its yaw setpoint.
  2. Hardware damage. Potentially a damaged motor or ESC affecting the response of the vehicle.

Log Files
Default PID Gains:
Roll Autotune:
Pitch Autotune Crash:

I would appreciate any explanations for what happened, or advice for flying this type of vehicle. Thanks.

According to
I’d probably make these changes as a starting point, although some are rough and will need testing:
INS_ACCEL_FILTER maybe lower towards 10, needs testing
MOT_THST_EXPO 0.72, needs testing or motor/prop data

Your Z vibes are high and clipping too - are the props too close to the frame or overflying the center plate? All the usual suggestions apply to vibes…