Dear all,
My X8 fixed-wing (running under arduplane 3.7.1) stalled and crashed after entering a spiral of death caused by a too high bank angle. My X8 was successfully auto-tuned, it flew a few times before without any issues. After 30 minutes of squares and circles in AUTO, I triggered a RTL, which requested suddenly the max bank angle (45°). Unfortunately, it went over this angle to ~ 60°, turned too sharply, and stalled. Note that it was flying at ~21 meters/sec when this append. When I plot the desired and effective roll and pitch, both agreed very well (in AUTO) until I switched to RTL (see the graph). Did anybody experience any similar behavior? Any hint to prevent that kind of stall?
Best regards
Guillaume Jouvet
Replies
Thank you very much Guillaume for your reply. I don't have more information about my crash. I didn't know that RTL use a different algorithm than AUTO, that could explain the problem. And thankyou for the idea of switching to a preprogrammed landing sequence, we will try. Is it possible to trigger this sequence from a switch on the transmitter? I know that you could go to a waypoint in the AUTO mission, but only using a button in mission planner. A transmitter switch will be a better option for me in case of panic :)
Thank you and regards,
Adolfo.
hi Adolfo,
Unfortunately, I did not find any clear reasons that can explain such a bad behavior. What you describe looks similar to my crash indeed, and I'm surprised that this kind of stall is not more documented. Still, I noticed that my fixed-wing was slightly over-stabilized (too high PIDs). So, I reduced a bit the PID, and also the maximum bank angle by about 10% to prevent this to appen again. Most importantly, I'm no longer trigger any RTL since I think that this artefact is specific to RTL. According to the documentation, RTL does not use the usual L1 control law but something else. Instead of RTL I use a pre-programmed landing sequence (with AUTOLAND=2). In contrast, my fixed-wing always behaved well in AUTO with a good match between requested and achieved roll.
Sorry not to bring any solution to this problem. If you make any progress on your side, please, let us know!
Best regards,
Guillaume
Hi Guillaume,
Did you find any insight in the cause of this crash? I have came across your post because today we had exactly the same problem with a Bormatec Maja. It was flying in FbwA almost leveled, at the speed of 20 m/s; when we switched to RTL, the pixhawk put the throttle at max and banked to 90º (maximum roll angle was configured to 45º), entered in a spiral of death and crashed violently.
I understand that the plane stalled, but I don't understand why the autopilot allowed the roll angle to go over the maximum angle. And the speed was high enough. In fact, we tested the plane in several AUTO missions without a hitch.
I will appreciate any hint to prevent this situation in the future, too.
Thanks!
Adolfo.