Vfrpilotpb , I recall an incident about eighteen years ago, when a pilot of a B206 reported, after some VFR night flying, that he got into a situation where he temporarily lost sight of the horizon and in those few moments the nose of the helicopter started to go up. He soon realised what was happening and pushed the cyclic forward, fairly quickly to bring the nose down.
A loud banging noise on the cabin roof with severe vibration occurred.
It soon settled down leaving him and his passengers somewhat unnerved. I don’t recall what he did to get out of that situation, but I think he was lucky to get back alive.
My first impression after an external inspection of the helicopter was that there was no obvious damage. The mast had indications of dynamic stop contact just under the head, but not dented and had not exceeded any serviceable damage limits.
A look under the transmission showed that there had definitely been contact, with the spigot plate on the deck being sheared off its mounting rivets. This was the only physical indication of damage that I could find.
This was my first, and to date, last experience of negative G incidents.