Originally Posted by
NickLappos
Regarding the TR overspeeding, the engine overspeed should catch a runaway before the TR gets to 120%, and the TR has been tested many times to well above that, I have personally seen 125% Nr/NrTR in flight.
Just to help clarify things, I'm assuming the S-76 drivetrain is similar to most other helicopters where the TR is coupled to the MR at some fixed speed ratio, right? And there is an overrun clutch at each engine input to the main rotor gearbox, which allows the gearbox side of the clutch to overrun the engine side of the clutch, but obviously does not allow the engine side to overrun the gearbox side. This basically means the engines themselves can't slow the rotor system by providing a braking load. All the engines can do is reduce fuel flow to the point where no engine torque is transferred to the gearbox.
This also brings up a relevant issue regarding turbine engine operating conditions. One thing turbine engines don't like is a rapid reduction in speed at very low load. This makes maintaining stable combustion very difficult, and turbine engines typically have control laws limiting the rate of speed change at low load for this reason.