<p class="MsoNormal" style="text-align: justify; text-indent: 0cm; line-height: 120%">Ground resonance is an instability phenomenon in the helicopter-ground system that can cause serious damage to the helicopter in a very short time. The ground resonance is considered as a kind of self-excited vibration, which is caused by the coupling between the lead-lag motion of the blades and the translational motion of the fuselage which lies on the elastic supports. In searches of the general rotor system, the rotor model with elastic support is often considered. According to the mechanism of the ground resonance, a similar instability phenomenon due to the mode coupling can also occur in the general rotor system. However, the existing literatures generally focus on the parametric instability of the rotor system, and the consistency analysis between the mechanisms of the parametric instability in the rotor system and the ground resonance in the helicopter-ground system is lacking. In this work, we investigate the presence of the instability phenomenon in an elastic-supported Jeffcott rotor system and the consistency between the instability mechanisms in the helicopter-ground system and the Jeffcott rotor system. The motion equations of the helicopter-ground system are first obtained based on the two-dimensional equivalent model. Coleman transform is used to linearize the motion equations and the modal analysis of the helicopter system is then performed. To obtain the criteria of the ground resonance, the dynamic parameters of the helicopter-ground system and the elements in the dynamic matrices are analyzed by modal analysis. &nbsp;The results show that the non-diagonal elements in the mass matrix and the negative of the rotating speed square in the diagonal elements of the stiffness matrix are the core factors leading to the ground resonance. The former are the inertial force terms and the latter are stiffness force terms, respectively. They are all induced by the eccentricity of the mass of the helicopter rotor system. The elastic-supported Jeffcott rotor model is then applied for modal analysis and it is found that the similar instability phenomenon does not appear in this model. &nbsp;In the unbalanced Jeffcott rotor system, when considering the torsional degree of freedom, the instability phenomenon caused by the coupling between the lateral mode and the torsional mode will occur. Then we compare the mechanism of the ground resonance of the helicopter system with the mechanism of the lateral-torsional coupling instability of the unbalanced rotor system in two aspects: 1) the elements of the dynamic matrices extracted from the motion equations who are transformed by Coleman transform; 2) the mode shapes in the coupling modal frequency ranges. Results prove the consistency of the instability mechanisms of the two systems.&nbsp; It can be concluded that both the ground resonance and the lateral-torsional coupling instability are caused by the strong coupling between the precession mode and the translational modes of the systems. The unbalanced rotor model is found to be equivalent to the helicopter system with one blade. The above research can provide a reference for further understanding the consistent instability mechanism between the helicopter ground resonance and lateral-torsional coupling instability of the rotor system.</p>

The Consistency of Helicopter 'Ground Resonance' and the Unstable Lateral-Torsional Vibration in Standard Rotor Systems

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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