My Skywalker X8 quadplane has become quite unstable and I could sure use some help figuring out how to fix it. I was running 3.8b4 and upgraded to the 3.8 release but both had issues today. Logs of the flight are at https://www.dropbox.com/sh/7kjhq49gl16jlbg/AACN2W7RGRxOEKZGcOWIUqrEa?dl=0
On VTOL take off the plane goes to 30 meters, pauses for a second, then the rear motor comes on, then it flips (360 degree roll) after which the rear motor increases and once forward motion starts it transitions to forward flight. It does this in both Q_Loiter takeoff and manually switching to FBWA or in an Auto flight with VTOL as the first command. I think it actually flipped twice on one takeoff.
The plane doesn't track at all during a grid mission generated by Mission Planner. It misses waypoints and turns well past them. When it finally turns it turns the opposite direction expected towards the outside of the grid rather than the inside. I've used the same type of plane with the same type of missions before without any problem.
During RTL, which I set to use hybrid mode, the plane doesn't descend from the last waypoint on its way towards the landing point. Then the vertical motors come on and the plane rolls wildly until it gets to around 30 meters high where it becomes stable and does a perfect landing.
If you are interested in the build or want to see a video of it flying properly I blogged about this plane not long ago https://discuss.ardupilot.org/t/a-tale-of-two-quadplanes