As far as I was aware, itwas/is a very widely known phenomena. It was shown in training, with the obvious details being;
a. what it is,
b. how you get into it,
c. how you get out of it.
Generally speaking it requires a heavy hand to get into it, albeit that DA can obviously have a negative affect.
Main problem was encountering at low level, ie rolling and applying power. The techniques of low flying were based around this.
From memory the Ba and later models with the larger chord blades and extra tab on the tail rotore blades did not make life any better !! This was also as the hyd pump and servos were not upgraded from the B model.
Basically it aerodynamic loading past the ability of the servo's to control.
God knows what fatigue affects there is/was, but after having done it and taught it for many years, I haven't seen any negative fatigue effects.
Now vortex ring is a different topic and the AS350, but I won't start on that one................
