I have a rudder-elevator model that has a pretty low CG - about 20 cm below the root chord. I should have a total of over 15 hours of mission time on this plane. This was with APM:Plane 3.0(.2/.3). I never had the time to fine-tune the autopilot. The flight performance was pretty acceptable with the minimally adjusted parameters.
I upgraded to 3.1.0 a few days ago. Wanted to do some additional tuning. I started with roll tuning. After increasing P and I a bit the plane started to oscillate in a motion very similar to Dutch Roll. I tried decreasing the parameters and finally reset to the previous parameters. Next I tried autotune - the plane became uncontrollable very quickly - it overshot the bank angles and exhibited Dutch Roll like behaviour when flying straight. I reset the modified parameters again and landed.
Then two days later I went flying again. I thought the plane should fly fine like it did before with the “good parameters”. Nope, the plane started to oscillate and wanted to go into a death spiral as soon as I switched to FBWA. I tried decreasing the roll P parameter, but it didn’t seem to have any effect. I had to switch to manual many time to save the plane. The plane flew perfectly well in manual. No sight of Dutch Roll or any tendency for corkscrewing itself into the ground.
I looked at the logs for both days, but I’m not at all sure what to start with. Do you have any ideas what to look for when examining the logs?