Originally Posted by
Nick Lappos
Steve76 asks:
Hello Nick and fellow 76 drivers,
With regard to a internal gearbox failure of the #1 engine, N2 input driveshaft and the subsequent channelling of 100% of the turbines power into the tail rotor. Is it not unreasonable to expect that the tail rotor will overspeed to the point that it suffers a catastrophic failure and disintegrates?.
Nick sez:
You are postulating an internal main rotor gearbox failure, I think, where the engine is still connected to the tail rotor, but disconnected from the main rotor bull gear.
Such a failure would leave the engine driving a much smaller load than the big main rotor, so an upspeed might occur, depending on how big the power reduction is (the reduction from normal drive to just the tail rotor.)
In all cases, the engine overspeed protection will catch the drive up speed if it should get out of hand, since the overspeed protection system is designed to shutdown the engine if the engine shaft breaks, which is a very big power reduction, the biggest possible. The tail rotor is quite healthy at speeds up to somewhere over 130% (I have been to 128 in flight during tests - this is done by professionals, do not try this at home!)
Most likely, the failure you describe would create a situation where the engine would speed up for a second or two, then settle down to about 109% or so, and it would be at about 2% torque, spinning happily with almost no load (just driving the tail rotor). The 109% is because unloaded, but with the cruise collective pitch setting, the engine would be trimmed up a bit by the collective bias. The rotor and the #2 engine would be driving the helicopter, and you would be in single engined flight.
The only problem would be if you decided to then shut down the #1 engine, because that would cause the loss of tail thrust.
Resurrecting this thread as I am new to the 76, (literally just finished initial training).
What am I missing? Trying to figure out why you would lose TR thrust if you shut down #1 engine.