Weerd: X8 octocopter - rotation around vertical axis under load

[roentgen]

Hi,

I grabbed a good deal on a used xFold Travel X8 octocopter. The specs look promising and I was eager to finally fly my Canon R5. Total payload weight is around 3kg - well within the envelope. Empty copter weight is 5kg. Batteries add another 1.5kg. T-Motor MN4012 340kV motors with 15x5 props. DJI Naza M v.2 flight controller. The batteries are two new 6S 12Ah packs.

When I fly the drone with no payload it behaves as expected but when I add a payload I have the following weird behavior: when battery discharges to around 23V the copter has a tendency to land in GPS Attitude mode and no stick input (throttle @ middle). If I push it by moving the throttle stick up it holds altitude but some time later it starts rotating in yaw/horizontal plane. I couldn't see a failing motor though.

Any ideas?

I have several theories I'm not disclosing to avoid confirmation bias.

[ched]

I assume you have done the 'naza' calibration dance and get a good number of satellites locked?
Have you looked at the Nasa pid type settings?
Is the compass suffering from interference - should be as far away from motors and esc as possible.

[roentgen]

No, I didn't do compass calibration. But the very same drone with no payload behaves as expected. 7 satellites locked.

Since it's a prebuilt copter I didn't try modding PID values but in fact I've no idea if the settings have been tampered with.

[ched]

I would try compass calibration first. It could be due to increased weight and drag, maybe a breeze the yaw is drifting and as compass isn't calibrated it might not 'see' the drift and correct it?

If I remember correctly, using the Naza software you can backup the setting - do that asap, then you have a record and the capability to revert back to where you are now.
I can't really remember how the Naza settings work - must be about 4 years since I touched mine.

One silly thing, triple check all props are on correct direction as if yaw is drifting then maybe be 1 prop is reversed and the correction is causing your drift?

Let us know how you get on.

[Bad Raven]

One silly thing, triple check all props are on correct direction as if yaw is drifting then maybe be 1 prop is reversed and the correction is causing your drift?

By direction Ched means facing the right way, as in not running with the rear concave face leading to the passing air. The upper motor props should be convex upwards, but so should the lower motor props and people do fit them reversed with all sorts of consequences!