Very large Pixhawk Copter leans Left and Crashes

I have been wrestling with a very large quad rotor, 4 of 22 x 8 propellers, 4 of 8000mAh batteries, 3.1m long, 1.3m wide, flight weight of 19.5 kg, running Pixhawk clone and Ublox Round GPS and compass. Mission Planner 1.3.57, and firmware 3.5.6.

I’ve got it all tuned for stable flight, and only recently got the Pos Hold to work, however Pos Hold causes the copter to oscillate in pitch and roll. It was crashing on Pos Hold, but reducing the Pos Hols P value back to minimum (0.5) stopped the crashes. It would appear that reducing it more would actually stop the oscillations, but its at minimum.

For the last few flights, the copter flies for a while in Pos Hold Mode then pitches left and lands heavily sidewards. I can’t prevent it with the radio controls with adding throttle or roll. I imagine if I were higher off the ground the results would be much worse.

I can’t see anything in the logs that would cause this problem, but I’m a complete beginner in trying to decipher the logs.

Any help would be much appreciated.

The Designer

Here are the logs of the last 2 flights.

It is a combination of overweight/underpowered and badly tuned.

It holds it all together for a while but either through a disturbance or voltage dropping off (you have no voltage or current logs) it just can’t stabilise itself and the inevitable happens.

Have you done an autotune?
Can you lighten it?

Don’t agree. Yes its heavy, and it was getting through about half the battery and running out of power on 5 cells, but batteries were increased to 6 cells. Drone now takes off at about 1/3 throttle in Stabilize, requiring a quick stab of the throttle when changing to Alt Hold mode, and flights have been over 12 mins in length.

I have been looking at the logs, and in particular Roll and pitch and Desired Roll and Pitch. They don’t match. In fact they go opposite directions.

Also comparing Throttle In (Ch1 In) to see what altitude requests I gave it, and comparing that to Ch 1 to 4 Out. This is a H pattern Quadrotor, so RC outs are Ch1 Front Right, Ch 2 Left Rear, Ch 3 Front Left and Ch 4 Right Rear. When checking out the Channel logs, at the point of trouble, Ch 2 keeps decreasing, Ch 3 and 4 follow, and Ch 1 goes up near full. This happens in pretty much all the logs in question, regardless of RC Ins.

On a previous day, I got a 10 min flight before running out of power, and landed the drone. Changed to a new set of batteries, and every time I tried to take off, the drone wanted to roll left on take off. When I tried to force it off the ground, it picked up, rolled left, hit the ground and rolled upside down. After taking it home and connecting to Mission Planner, I got and occasional message Bad AHRS, and then while its sitting on the table, the artificial horizon leaned left, then disappeared off the top of the screen, then while the screen was all green, it spun around.

I think I’ll just get another Pixhawk. Might be safer. Only I’ve seen other posts elsewhere that said leaning left was an issue with some Pixhawks.


With the oscillations in Pos Hold Mode. Yes, the drone has had an autotune. Flies very stable in Alt Hold and Stabilize Modes. You can go up to it and try to upset it and it recovers instantly.