Dynamic modeling and analysis of the supercritical tail drive system of a helicopter considering the mounting platform deformation

With the high-speed and lightweight development of the helicopter transmission system, the tail drive system is gradually adopting a supercritical design. However, the understanding of the vibration mechanism of the supercritical tail drive system isn't yet clear. Hence, considering the angular bending moment introduced by the axial friction force of the spline coupling and the mounting platform deformation excitation force, this paper established the system's finite dynamic equation considering the mounting platform deformation, which is verified by finite element software. The research results showed that: at supercritical speed, when the spline lubrication is poor, the angular bending moment vector of the spline coupling will introduce a self-excited vibration component that is approximately equal to the first-order critical speed of the horizontal tail short shaft to the horizontal tail shafts, and the intermediate reducer gear pair, thereby stimulating the self-excited vibration phenomenon. The mounting platform deformation at the bearing support position at the spline coupling and the horizontal tail shaft, and the intermediate reducer support position would introduce the speed high-order harmonic components 2x, 3x, and 4x into the frequency-domain response. The effect of the self-excited vibration phenomenon is significantly greater than the mounting platform deformation. In engineering, the spline lubrication status can be determined by observing the appearance of the self-excited vibration components in the vibration response of the horizontal tail short shafts. And the mounting platform deformation can be determined by observing the speed high-order harmonic components in the vibration response at system's different positions. The dynamic model established in this paper can provide a theoretical basis for the dynamic analysis and optimization of the supercritical tail drive system of helicopters.