Yaw on large aircraft steals lift

@frizensami wrote:

I’m trying to hover a large quadplane for the first time in QSTABILIZE (wingspan ~ 2.7m, about 18 kg MTOW) and every time the autopilot tries to yaw a significant amount, it loses a lot of lift, many times to the point where it doesn’t produce enough lift to stay in the air / take off again even at 100% throttle.

A couple of videos of the behavior (QSTABILIZE mode):

1. https://photos.app.goo.gl/LEdpwssudmawG9DB2 - aircraft takes off, yaws, loses lift, forced to land
2. https://photos.app.goo.gl/pO83bSPtmdXuAvDA2 - aircraft cannot take off, too much attempted yaw on ground

Notable events in the logs:

1. 'EKF Yaw Resets": every single takeoff is followed by an EKF Yaw reset message. I’m using three compasses (1 on the Here GPS (primary), 2 Pixhawk 2.1 internal). I’ve recalibrated in the field twice without any change to these messages.
2. Q_A_RAT_YAW_I and Q_A_RAT_YAW_D was set to 0 throughout to isolate the P gain.
3. Q_A_RAT_YAW_P has been exercised through the entire range of 0 to 0.55, from 0 - 0.2 there’s insufficient control of the aircraft, at 0.3 there’s decent control of the aircraft but once in
   a while it yaws to the point of losing most lift, and as I go towards 0.5 almost any yaw input causes the aircraft to lose most of its lift.
4. Q_A_ACCEL_Y_MAX was reduced from 27000 to 15000 to 10000, mainly to reduce sharp drops in altitude due to large changes in yaw output.

I just need the aircraft to reach a stable loiter and control its heading in wind before I can begin testing transitions, and I don’t have much experience with control theory and PID tuning. I would really appreciate any help with this For example, is there any way I can reduce how much total yaw output the autopilot is capable of commanding? Or is that even the right way to proceed?

Logs (all similar, just different YAW_P / ACCEL_Y_MAX):

1. https://drive.google.com/open?id=1veMu4OIWYxUfcCsSoQZ6ZjpXkKEVIL9d
2. https://drive.google.com/open?id=1ZZy_PRWy3WW2-V51MFZhbjRL3Ij2YXbd
3. https://drive.google.com/open?id=12h6K8e4oryrVsswRuUhQAf2nu6KS1aBl

Many thanks in advance.

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