We are testing a large custom-built quadcopter with a motor-to-motor diagonal of about 1.5 meters (quad X-configuration, see attached diagram).
We are still in the development stage and currently working on stabilization.
A few flights have already been carried out. In previous flights the drone was able to fly, but we observed noticeable oscillations (although it never flipped). Today, during another test flight, we tried again — the only change compared to the previous flights was adding a vibration damper plate under the autopilot.
Unfortunately, this time we encountered a critical issue:
Right after takeoff, the vehicle started to oscillate heavily on all axes (roll, pitch, yaw).
We immediately attempted to land, but the system did not detect landing properly, and PX4 still considered the vehicle in-air, so the disarm command was not accepted.
In the 3D replay you can see the drone tipped over (rolled to the side and flipped).
Frame size: ~1.5 m motor-to-motor diagonal
CG not perfectly at the geometric center due to payload distribution
Autopilot mounted slightly away from CG and isolated with a vibration damper
GPS: UM982 dual-antenna RTK module with heading
Flight controller: [CUAV V6X / Holybro 6X etc.]
PX4 firmware version: [e.g. v1.15.x]
Flight mode: [Position]
Attached log file from QGroundControl
Questions:
What could be the root cause of these severe oscillations? Could it be related to PID tuning, vibration, or sensor bias?
Could you please check our logs and share your interpretation?
Any specific suggestions for tuning or sensor setup for large multirotors with off-center CG?
First flight LOG without crash
Last flight LOG