I have a large octocopter (~10-15kg) built on the Tarot T18 frame. The first test flights were almost fine but there were some twitches in the roll angle, i.e. the copter suddenly flew a few meters to the side. An autotuning did not clear the problem. After changing parameters according to the tuning process instructions and another autotune, the copter finally crashed into a bank, maybe after another of those twitches. I could not prevent the crash as the copter did not react fast enough in the end. Now I am wondering how to safely tune the parameters of a large copter and which are good start values. I never had tuning issues with smaller copters.
The log files do not show any evidence of vibration. However, the desired and actual roll angles do not match at all. I uploaded the autotune logfile and the logfile where the copter crashed here.
I am grateful for any hint and advice.