212man,
Vne is actually not imposed on any aircraft, it is usually the speed at which we propose to stop testing because any more is unusable. Few machines are qualified at a physical limit due to structural or handling limiting.
The S-76 has Vne at 155 knots mostly because we decided not to spend lots of test time climbing up after we dove to higher speeds. Many helos since then have also qualified that way, I think.
The Vne for the S-76 once set, also set the maximum speed at which we met the Longitudinal Static Stability (LSS) for level flight. We adjusted the pitch bias actuator (a device that reads airspeed and fools the longitudinal stick to build in LSS) to that speed. We needed the actuator (since removed on most models because it was only needed to meet the letter of the requirement, and had little practical value.)
The LSS maneuver in the FAR says to trim level at .9 Vne, then without resetting collective, push to 1.1 Vne and see where the stick trims to hold that new speed. In the old old days, when Vne was 100, the 10 knot push was no problem, and in fact a good measure of how a pilot would work to trim at the new speed. In a fast helo like the S-76, the initial trim was 142 knots, and the push was to 172 knots, and the 30 knot speed increase lead to a screaming dive at about 2800 feet per minute. Some level flight trim check, huh?
At 2800 feet per minute down, the stick trimmed back right where it started, and the flat gradient was deemed unacceptable, so we adjusted the pitch bias actuator to give 1% forward stick there, and called it OK.
I worked with a group of handling qualities professionals a few years back, and we proposed a change to the FARs that is now in the works that basically looks at a 10 knot push and pull around the level flight trim. This seems to work well now.
For Joker's Wild - Regarding the question of very bad behavior in an S-76 in a dual engine cut, I performed most of those back when, and the aircraft handles quite well, about like most modern helos. If you wait too long with the collective up in your armpits, however, the rpm can get low, and things can get busy. I terms of response to engine failure, I believe the S-76 is typical of most modern helos in its rpm decay, but it has exceptionally good control capability at low rpm, and a very wide and forgiving rpm range. I have autorotated it to touchdown at high weight and at high altitude, and I think it is actually quite good, not a 206, but surprisingly easy. The couple of touchdown autos experienced by customer pilots due to fuel starvation and the like have been quite successful, too.