Originally Posted by
Thomas coupling
ever closer towards its point of no return (what is it in the 135? 82%?).
From the aerodynamic side this will not be an absolute number. It will depend on weight and RoD. Edit: and altitude but this wouldn't have played a role here.
Stall (drag) depends strictly on AoA.
So, while in horizontal flight, a momentary drop to 75% might be recoverable at a certain mass, the same at a higher mass may not. Same applies, once a descent has started. This will increase AoA and thus increase the likelyhood of a stall at an RPM which was still recoverable in horizontal flight.
In a stabilised descent with collective fully down, somewhere in the mid to high 70s, maybe 80 should be about it for most helicopters with relatively high power/disc loading and thus range of collective movement.