I made the booms fairly short to minimize wind resistance in forward flight. I knew that would reduce roll authority, but didn’t realize it would impact yaw.
I’ve done a fair amount of hover testing previously. The plane has started circling before, but in the absence of significant wind, I would just wait it out and eventually the circling would stop. Pitching forward also seemed to help.
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I’m going to investigate changes in motor mounting to make sure I get them vertical. UPDATE-I just measured and my rear motor tube has a 2 degree angle. This makes the channel 8 motor tilted out 2 degrees (which should be OK), but makes the channel 6 motor tilted inwards 2 degrees (which is probably bad).
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I may then tilt the motors out a degree or two (since that supposedly improves yaw authority). UPDATE: I think I’ll start by printing a new motor mount for the channel 6 motor with a 4 degree angle so that it tilts out 2 degrees rather than tilting in 2 degrees.
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I’m also going to look at the motor calibrations and limits to make sure I’m using the entire range of motor throttle.
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There’s a parameter called “Q_M_YAW_HEADROOM”. It defaults to 200 PPM with a range of 0 to 500 PPM. The definition says “Yaw control is given at least this PWM in microseconds range”. I may try increasing this value a small amount. There’s an interesting thread on this topic at https://discuss.ardupilot.org/t/yaw-on-large-aircraft-steals-lift/27180 where they again focus on getting the mounts vertical and on having sufficient power.