Twin engine helicopters - allowable engine ground surging
 

I thought about the Rotorhead forum but decided you folks were my best bet.
For you guys / gals that look after twin engine helicopters, what do the individual manufacturers allow in their manuals for ground surging, as a percentage of N1, NG or whatever the nomenclature.
I have been asked a question at my place of employment and am interested in the values stipulated by Sikorsky, Bell etc
I think that the engines naturally will try to fight each other which can lead to "hunting" or small amounts of surging, just trying to see how much is allowed since we have no stipulated limits that I can find EXCEPT none allowed in flight.
Thanks in advance.
PT6ER

Don't know how much detail you are looking for, but, normally the MRGB is geared in such a way that one engine is driving a fraction more than the other, so as to eliminate any hunting.
This can also be achieved by the ECU throttle controls:ok:

Surges are normally on one engine only, when both engines are fighting each other we call it oscilaton. 212/412 have a long history of this. The engines air systems are linked between engines at the TCU. Lots of things can cause an oscilation and can be a major pain to work out. I can't recall having engine oscilations on the ground, they usually happen at higher torque/N1 settings.
Also, I've never seen a helicopter that is geared for one engine to drive more than the other.

surge
 

:confused: YOU HAVE YOUR TERMINOLIGIES MIXED UP.SURGE IN GAS TURBINES NORMALY HAPPENS AT FAIRLY LOW N1 SPEEDS<80% IT IS CAUSED BY THE AIRFLOW THROUGH THE ENGINE STAGNATING .IT IS NORMALY PREVENTED BYTHE ENGINE BLEED VALVE WHICH WILL CLOSE AT A PRESET N1 DEPENDING ON AMBIANT AIR PRESSURE AND TEMPERATURE.HUNTING IS SOMETHING ELSE,GAS TURBINES IN PAIRS DUE TO THE GOVERNING SYSTEM MECHANICS WILL POWER SHARE WITHOUT ANY EXTERNAL CONTROL,HOWEVER IF ONE ENGINE N1 SPEED STARTS TO HUNTTHE OTHER ENGINE WILL COMPENSATE FOR THE INCREASE OR DECREASE OF TORQUE AS ITS GOVERNING SYSTEM WILL DETECT THE FLUCTUATION IN ROTOR RPM.IN MY EXPERIENCE ENGINE GOVERNING SYSTEMS THAT USE AIR IE BENDIX ECT ARE PRONE TO THIS NORMALY DUE TO AIR LEAKS.

Helinon,
you are correct, this is not a true compressor surge but it is the nomenclature Pratt and Whitney use when we speak with them. I guess we understand each other enough to know that this is not a true "surge".
We have no link between the two engines
Our philosophy is to trouble shoot the control sytem (bias etc) and if it continues, change out the fuel control. This usually fixes the problem and the FCU is found to be out of adjustment when we get it into our shop. I guess my question still stands, what is deemed acceptable for hunting/surging by other twin manufacturers?

I appreciate all your responses, thanks!

Surge
 

a short answer is none!