I’ve solved the vibration now. I used some 25g lead weights from an online auto store (the types of small weights used for balancing car tyres). They come with stick on tape. I put 4 of them on the bottom of the pixhawk and otherwise didn’t change the mounting at all. In todays flight the average vibration dropped from around 60 m/s/s to 7 m/s/s, which is pretty good for such an easy change!
It flew a lot better with that weight added!
The only issue I had today was that the maximum roll/pitch in ALT_HOLD mode was quite limited. It turns out it is the alt_hold angle limiter trying to leave headroom for collective. It seems there is something wrong in the HeliQuad motors backend that it causing it to limit the angle too early. I’ll look into it.
Cheers, Tidge