I have a flying wing equipped with a APM Mini running ArduPlane 3.4.0. Trimmed and tuned via autotune following the steps in the wiki. Flies very well in Manual and FBWA but I am experiencing a problem that I cannot find the cause for.

Flew a short flight with LOG_BITMASK set to 65535 demonstrating the problem.

When I fly in FBWA and switch to Loiter the plane very aggressively pitches up for a few seconds. This is totally uncommanded by me from the ground. It then levels out and flies as expected. This is also present in RTL.

I have tried retuning and checking over the tune of the TECS system.

I have poked through the logs but have yet to find something to point me in the right direction.

As you can see in the attached screenshot: elevon channels (RCOU.Ch1&2) are deflected full up and pitch sharply turns up (ATT.Pitch). This is uncommanded, (ATT.DesPitch rather tries to compensate and level out). Maybe someone can find more clues from the attached bin log?

Does anybody have any ideas of what could be at play here? Let me know if more info is needed in any way.

2016-01-19 13-52-12.bin

Screen Shot 2016-01-20 at 13.29.54.png

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  • Friends! I thank you for the help and pointers! I finally had the time and weather conditions to work on my wing. I corrected the neutral position in manual mode to ensure level flight. This was all that was needed. Just wanted to give you all some closure on the problem.

    I am very happy to finally have solved the problem. Just in time for flying season in the northern hemisphere. Big thanks! I wish everyone a happy flying/building season!

  • Developer

    You have THR_PASS_STAB turned on (set to 1) which is unusual in a battery flying wing.  Any chance you can turn this off and try again?  I'm wondering if something weird is going on when transitioning from the throttle direct pass through and then switching into a full auto mode.  This is a guess based on no evidence.

    How does the plane fly in manual mode?  If you take your hands off the sticks does it fly straight and level?  It looks like the trims might be slightly out.

    You know, I reckon your going too fast when you enter loiter.  Look at the graph below - when you go into loiter it drops the throttle very rapidly and deflects the surfaces back to their trim values.  I think this causes your plane to go up.  Your TRIM_THROTTLE is 45 which means when your belting along at max throttle in FBWA and the plane is going as fast as it can you then go into Loiter - its doing exactly what you tell it - drop the throttle down - that's why we see such a fast drop.  And at max speed that small deflection when the control surfaces return to their trim values briefly are enough to cause it to gain altitude.

    Its happening quickly.  Once in loiter after 2 seconds the plane has figured out its gone up in altitude from the loiter start alt and starts coming back down.

    You can see in the graph the plane return to the altitude you entered the loiter just before you go back into FBWA which is correct.

    Try flying straight and level at a reasonable speed, maybe 50%-60% throttle and then going into loiter and see if it happens.

    Note I have an APM2.5 flying wing running APM3.4 and I don't see this.  Although I might test again if we can't get to the bottom of this.

    Thanks, Grant.


    • Thank you Grant. I changed THR_PASS_STAB to 0 and TRIM_THROTTLE to 60. Switching over at lower speed indeed helps the problem!

      The log unfortunately got truncated since I did quite a log flight for the APMs flash memory capacity.

      I will retrim the plane. It does actually seem to have a bit too much up trim. Thinking about just adjusting the servo arms right on the model a few millimeters to lower the elevons down a bit at neutral stick pos. Do you think that could work? Or should I do a manual flight with TRIM_AUTO again instead?

      Greatly appreciate the feedback! Thanks! Will be more careful to bring logs next time!

    • Developer

      Its always best to have the plane fly straight and level in manual mode with no trim and hands off the sticks.  That's true for any plane regardless of whether it has an auto pilot or not.  To this end its best to adjust the servo arms physically to achieve this if possible.

      Whenever you make physical changes to a plane its always a good idea to re-run the autotune and re-do the auto trims.

      Let us know how you go.

      Thanks, Grant.

    • 100KM

      I agree regarding checking manual flight. Nice graph Grant! - what software did you use? I used to use flightplot.jar, but that stopped working on most log files a while ago and doesn't seem to be supported anymore.

    • Developer

      I use MAVProxy GCS and as part of that it comes with a bunch of tools such as MAVExplorer.py which is what I used to create the above graph.  Its an excellent tool for analysing logs.

      Thanks, Grant.

  • 100KM

    Can you post a log where you're flying in manual mode? The attached bin you seem to take off in FBWA most of the time. I'd like to see if there's any anomalies in the airframe behaviour.

  • Here is a video demonstrating the problem 

  • Does anybody else have any pointers? I am starting to lose my hope here :( Would be very sad to scrap this project... I see so much potential in the platform

  • 100KM

    Is your plane properly leveled? Your artificial horizon should show 0 degrees pitch in level flying. If it's not level, the APM wants to level out first (pitch or dive) but then, in auto mode, it will try to maintain altitude and you'll have two conditions fighting each other.

    My 2c

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