Plane 4.1.7 strange mid air AHRS / GPS / compass behaviour

ArduPlane Plane 4.1
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Hi everybody,

Last weekend I made some test flights with a flying wing / delta aircraft equipped with a Matek F405SE and Arduplane V4.1.7 Chibi WingF405 version flashed onto it.
For GPS I use a genuine UBLOX M8 from a Pixhawk 4 made by HEX.
I have disabled both the internal and external compasses (If I did everything right).

The airplane was autotuned and flying very very well, and very predictable.
Really the only concerns at the time where the rather large control outputs in manual / stabilized and the lack of expo but still it was very controllable.

During some later test flights, I was doing some LOS flying in Cruise in a figure 8 pattern to keep it within sight. But at times mostly when leveling out after a corner the plane started to dive by approximately 15 degrees it would just continue and not really react to any control input anymore until I switch out of Cruise and into Stabilized. In stabilized I could regain control and switch back into cruise mode seconds later again without any issues ?!?

Unfortunately the logfiles on the SD where of this where corrupted for some reason. So I only have the logfiles that QGCS recorded, I was flying with telemetry on 4G so there is allot of data still without any holes. Here I can see some strange behaviour where it is constantly switching between AHRS error and normal while in the popup message I get AHRS active > DCM active > AHRS active messages all the time.

Since Iam not really sure what to look at, can somebody please put me in the right direction or maybe a topic from a related issue?

I made a youtube video from the Qground control playback of the flight: Arduplane 4.1.7 AHRS issue - YouTube
Key elements in the video are at:
3:40 Take-off everything looking normal
05:00 This is the straight line that already felt uncontrollable, it leads into the issue which is clearly visible by the erratic movements on the end. I switch to stabilize to save the plane from crashing.
05:40 second time but the exact same behaviour. The video does continue some more minutes but the same behaviour shows up all the time.

The Qground logfile and the Param file that where on the plane during this flight can be found here: Delta AHRS issue - Google Drive

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Working from a tlog is better than nothing, but I’m hesitant to come to say anything too hard and fast because of any potential latency.

I noticed the pitch tuning seems poor. In spite of my comment about latency, I notice that the desired pitch and achieved seem track very poorly together, however the roll seems reasonable. I suggest you work on the tune of the pitch. Perhaps another run of autotune with and increased AUTOTUNE_LEVEL from the default 6, to 7.

Screenshot from 2022-03-21 19:46:48
Screenshot from 2022-03-21 19:46:481280×766 184 KB

Each time things went odd you can see a marked separation between the desired pitch and what was going on, then when you changed to stabilized there’s a significant pitch up. The MIXING_GAIN is still at default of 0.5. I think you might want to slowly bump this value up. I’m wondering if the roll command was washing out the desired pitch and leading to the trouble.

There is expo for the manual control modes. Complete Parameter List — Plane documentation
In the past, I have set up expo on my opentx/EdgeTX radio so it’s only there in manual mode. I haven’t tried this parameter but it should do exactly what I did on my radio. I’ve tried radio expo on FBWA/B and cruise and I didn’t like it. Others may have different opinions.

Edit: You might also want to look at MIXING_OFFSET. Elevon Planes — Plane documentation

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Thanks for the feedback @Allister.

I will build a new plane and test again and post the results here, hopefully within 2 weeks.