It is taking me a little longer that I had expected to tune a large scale Heli (Vario VLC) with a Pixhawk running copter_3.3. Have previously setup up and tuned a 800 size with the same setup and found it quite simple.
The problem I have is that in loiter or Poshold mode I get a slow circular oscillation, 'toilet-bowl' effect. I have tuned Stabilise and Althold to give a response I am reasonably happy with (could be a little sharper) but any mode reliant on GPS produced this oscillation.
I had one idea that the GPS and compass location could be the problem? Because I'm running a large electric motor, I wanted to put the 3DR Compass and GPS as far back as I could. At the moment is is right at the back of the boom, roughly 1200cm from the Flight controller which I have mounted on the CG. I once saw in the setup of a multi-rotor (I forget which one) the instruction to not mount the GPS to far away from the CG. This made me think that this could be the source of the problem I have now?
Has anyone experienced this problem and found GPS to be a problem? Or has anyone with a large heli mounted their GPS way back and not seen any similar issues?