I had a couple of crashes when the Freya transitioned to inverted.

The problem I had was the fuel line in the tank would fold over and starve the carb when it went inverted. In this scenario, even a header tank wouldn't help because effectively once it folded, no amount of pressure from the tank would go to the header and it would flameout.

I changed to a stiffer fuel line in the tank and did lots of tests by flipping the tank (removed from the heli) with fuel and all the inlets and outlets sealed off to test if the stiffer line would at least let the clunk travel in the tank.