Representing turboshaft engine in integrated torsion vibration study

[Onemorechance]

For a preliminary study on torsion vibrations of the integrated system engine-transmission-propeller I need information on both propeller and engine. The propeller is not such a problem, I can easily find the information I need in order to model it, but that is not the case for the engine.

The engine is an Allison Model 250 C20B turbo-shaft with a free power turbine. The transmission (not the engine transmission part of the engine) consists of a Shaft connecting the PTO shaft to a splitter box, 2 shafts from splitter box to 2 reduction boxes, some flexible couplings, 2 clutch couplings and 2 propellers.
My question is: would it be good enough for a preliminary study to represent the engine by one polar moment of inertia, or would I need several inertias and stiffnesses to represent the engine?

 

[GregLocock]

One will do initially, but you will quickly wish that you had gone to a more complex model when the failures start!

OK, I'm arguing from automotive driveline experience, where the frequency of the crankshaft system is around 200-300 Hz, whereas the problematical driveline torsionals typically run from 6 to 100 Hz. I have seen engine torsionals right through the driveline.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Onemorechance]

Thank you Greg,

This question is based on the following 2 reasons:
1. I have flexible couplings in between different subsystems (engine -splitter box - shafts - reduction gearboxes - shafts - propellers) , especially the flexible coupling between the engine PTO shaft and the splitter box will serve as an energy buffer for the higher frequency vibrations introduced in the system by the engine. This would work as a barrier for higher frequency modes from the engine to couple with the higher frequency modes of the rest of the subsystems.
2. The engine being a gas turbine engine runs smooth (not like a reciprocating engine) and the power turbine is gas - coupled to the gas turbine, therefore I assume that the polar inertia of the power turbine, (power turbine rotor plus reduction gears in between power turbine rotor and PTOutput shaft), would be really adequate to represent the engine.