The torsional stability of a closed-loop dynamic system is evaluated. The system is a typical transport helicopter speed governor, gas-turbine engine and drive train. This is compared with the stability of a similar system in which a nonlinear coupling is included in the drive train, to determine if the coupling will increase the torsional stability of the system [1]. This nonlinear coupling is designed to isolate torsional vibrations in the rotor. The two systems are first linearized, using a decoupled rotor model for the ZTS coupling, and expressed in the form of Laplace transfer functions. Then a Bode Analysis is conducted and gain and phase margins are compared for the two systems. The nonlinear systems are numerically simulated to determine the time response and frequency response to excitation. The ZTS coupling design considered is found to be effective in stabilizing the previously unstable helicopter drive system.

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