I wrote a mail regarding my problem ( Copter unstable on launch ).
Here is the answer from them…
Hope sombody can help my with this…
Thank you for your email.
From the logs it seems that it flew great sometimes.
But will diverge during takeoff sometimes (the desired attitude and the actual attitude do not follow each other well).
It seems to be a firmware issue where sometimes abnormal signal is sent to the ESCs.
Please post your log onto our Facebook group and/or the ardupilot forum for arducopter firmware: https://discuss.ardupilot.org/c/arducopter/copter-3-5 , where developers are able to look into the issue.