Seeking tips to Loiter-tune a *really heavy* single-rotor model

Dear community,

I have a very heavy single-rotor heli (MTOW ~ 25 kg / 55 lb) with a PixHawk currently loaded with ArduCopter 3.2.1, but that I tentatively plan on updating to the latest stable 3.3.x release.

It currently flies well in Stabilize, but disastrously in Loiter. It’s a long story, but in a nutshell, over the course of some time, several people that weren’t properly communicating with each other or following proper procedures messed around with the flight parameters, to the point where I have little to no idea where to even begin making adjustments. The other extremely important thing to note is that when this beast of a helicopter crashes, it crashes hard and requires costly repairs, so it is very nerve-racking to test-fly, switching it into Loiter basically amounts to a leap of faith, and engaging in “casual” trial and error is simply not an option.

If anyone who’s knowledgeable and interested would like to take a look at the current parameter file and indicate if anything jumps out that seems wildly unusual and potentially the source of the problem, please let me know and I will PM you to arrange to send you the file. OR alternatively, I can list the values of specific parameters, as requested, in this thread, which might actually be better to encourage teamwork than privately sending the parameter file to separate individuals.

Again, it flies very satisfactorily in Stabilize, so I’m not interested in changing those parameters. I figure most likely something is off with the Alt Hold and/or Loiter-specific parameters, or possibly GPS or compass parameters.

What I can tell you is that the model immediately crashed the last 2 times it was switched into Loiter. The first time, some major oscillation was evident for the first second or two in Loiter mode, which most likely put too much stress on the mechanical components of the rotorhead (e.g. servos, ball links, etc.) of this very heavy machine and caused it to mechanically fail. I’m unfortunately not sure whether or not any parameters were changed in between this crash and the next attempt to switch it into Loiter. The second time though, it immediately seemed to just flip on its side, consequently lose all of its lift, and go down sideways.

Sorry this case is a bit convoluted and probably lacking as much information as would ideally be required, but I would be most appreciative of any feedback that could help get this model flying properly in Loiter, and ultimately in Auto/WPNav mode.