After three unsuccessful attempts at finding a mechanical explanation to this problem I've decided to post here to see if anyone has encountered a similar problem and what they found to be the cause.
- This hexa is rather large and powerful, 900mm between opposite motor centers, 370kv motors, 1555 props, 6S 11000mAH battery. The frame is flying in the + configuration.
- The APM (2.5 running 2.9.1b) is almost on top of the CG but weight distribution isn't perfectly symmetrical as a second 3S 2600mAH battery is installed next to it so there is a slight Left/Right imbalance.
- The frame flew perfectly level and responded well to throttle but would spin continuously as soon as it would lift off the ground.
- Default PID settings were used for the first flights. THR_MIN was lowered to 50 as the default idle speed seemed a bit high.
- At first motor misalignment was thought to be the cause. Very slight corrections were necessary to straighten them out but I doubt this was the cause as sometimes the frame would spin CW and sometimes CCW without any physical readjustment.
- The initial flight was done with manual magnetic declination (evaluated at -16.4140° in my area) the following flights were performed with auto declination with the same result.
- Magnetometer calibration had been performed previously but was repeated between flights 2 and 3 with the same result at flight 3. Calibration was performed outsides and far from any metal or strong magnetic fields.
- Unfortunately CTUN and RAW logging were not taken during those flights so I don't have any much data available for analysis.
My current guess is that the default PID values are probably too high given the frame's highish power/weight ratio. I think I will start by setting RATE_YAW_I = 0 and lowering RATE_YAW_P to see how the frame behaves and go from there.