Hexacopter crash with PIXHAWK (hard to understand)

Log Analysis -- deprecated Copter Logs -- deprecated
1

Dear all,
I really have hard time to understand what happened while flying my hexacopter. I followed all the steps of the recommended Wiki page (copter.ardupilot.com/wiki/common … using_Logs) : none of the 6 topics revealed problems.
Here is the description of the flight :

  • I started, switched to AltHold, flew up to 80m high, switched to Loiter and continue flying a bit before switching back to AltHold.
  • then I flew down to 20m and switched back to Loiter once at 20m.
  • I continue flying a bit and switched back to AltHold.
    At that step everything seemed to be OK, when suddenly (around T=26000), the drone began to have difficulties to hold its own altitude (???). Immediately, I switched back to Loiter, but the drone still going down (quite slowly). I tried to push full throttle in order to compensate, but with no result, until the drone crashes.
    I definitely do not manage to analyze what happens, I would appreciate your help as this situation does not inspire me any confidence…
    Here is attached the binary file of the flight.
    Many thanks again for your help.
    Best regards,
    Pierre.
2

I would say that one of two things happened here:

  1. Your hexa has a very bad CG, and in order to fly one of the motors is clipping at maximum throttle. This means you have no throttle margin, which means you are essentially underpowered even though you’re hovering at only 60% throttle.

  2. A powerplant failed and in order to hold an attitude, the hexa had to lower the total throttle output.

So, your hexa is doing extraordinarily badly at holding its demanded attitude. This suggests that #1 is correct - you have a very poor center of gravity. During the descent the loiter controller was demanding a roll angle of nearly -45 degrees. The copter wasn’t even achieving 0 degrees. This means that the loiter controller was actually saving you from your own tuning - you had set RATE_PIT_IMAX and RATE_RLL_IMAX to essentially zero. Otherwise, it’d be zooming off to the right.

  • LOG_BITMASK set to log very few messages. Possibly from loading an APM2 param file onto PIXHAWK.
  • Probably bad CG. Hard to say with no RCOU.
  • RATE_RLL_I and RATE_PIT_I are set very (way too) high, but the IMAX values are essentially zero. I suggest settting the I term equal to the P term, and set IMAX to at least 2000 if not 4500.
  • STB_RLL_P and STB_PIT_P are set very low. If you were to do an autotune, it’d likely bring these up to around 3 times their current values. I would set them to at least the default of 4.5.
  • I am also a little suspicious that your FS_THR_VALUE is set wrong, because the failsafe seems to bounce on and off tens of times quite rapidly after it crashes. This could potentially cause an in-air disarm.
  • I would suggest doing an autotune after addressing some of the above.
3

Dear Jonathan,
Many thanks for your quick and so precise answer: this shows me I still have many things to learn about drone :blush:
I will carefully check all points you mentioned and try to guess what’s goes wrong with my drone.
I will come back to you with news from my newly tunes machine.
Your help is a precious starting point: thanks again.
Best regards,
Pierre.