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X8 Heavylifter (24kg payload) abnormal attitude control and crash

Dear all,

I’ve acquired a multicopter from Foxtech and have been trying to put it to work this last few months without success. The frame I’m using has the following characteristics:

Last week I performed some flights which ended with a crash. On the first flight the multicopter performed takeoff in AltHold mode and, once it reached a safe altitude, I changed it to Position mode. At this point I noticed the first odd thing with the multicopter starting to oscillate severely. As a corrective measure I changed the mode to Stabilize and the copter climbed 10m to 20m above ground. This resulted from the fact that the RC channel for the throttle was in the mid position (to have 0 vertical velocity) and then, when the mode was changed, this value was used as a thrust reference instead of a vertical velocity. Then, I safely landed the copter.

With this type of response I have identified that clearly something was wrong with the position control, so I performed a second flight only in Stabilized mode to assess if the attitude control was well tuned. On this second flight, once the copter was in hover position it started to show some oscillation. I also noticed that the attitude response was sluggish. Hence, I started to lower the copter altitude to land it. Almost instantly as I reduced the throttle the oscillations started again and as I reduced the throttle they worsen until the point the copter performed a flip and crashed into the ground.

After reviewing the flight logs I saw that the rate control wasn’t as sharp as I would think, but at this point I can’t find the origin of this effect. Regarding the sensors vibrations, everything looked well behaved and well within the values recommended (below 20dB)

As someone worked with this platform before? Which gains should I use?
Should I look in depth some component of the airframe to ensure it is performing as should be?

The log/bin files can be downloaded from the following links:
1st Flight: https://www.dropbox.com/s/ltz8njlaj2cdebz/1st_Flight.bin?dl=0
2nd Flight (Crash): https://www.dropbox.com/s/c1rj9d7wnqrj7yt/2nd_Flight_CRASH.bin?dl=0

Thank you for your help in advance and I hope we figure this one out.

It is overpowered.

Put on a dummy weight that will emulate the payload that you are going to use.
Set ATC_RAT_RLL and PIT Filter to 6
Set Autotune aggressivity to 0.05
Do a full set of autotune.

Do not fly without the payload or the dummy.

David

You really should consider adding a good power monitor, with current monitoring.

It will depend a bit of the purpose of the machine and the total weight, but some initial adjustment of default parameters might help

Thank you Eosbandi,

With respect to the issue of overpower, do you think that reducing the number of motors would also be a good solution? I’m afraid that with a higher payload any hard landings would brake the landing gear.

This copter “flies” at around 0.2 ThH

Thats not a problem, if the system is well tuned. I doubt that in this occasion…

Have you followed this:
http://ardupilot.org/copter/docs/ac_rollpitchtuning.html#ac-rollpitchtuning

and that:
http://ardupilot.org/copter/docs/autotune.html
Especially the section “Additional Notes”!

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