Abstract: The aero-engine Low Pressure (LP) rotor system, usually of large mass, thin and long shaft and long-span supports, has a feature of low stiffness. When rub-impact occurs, the constraint effect must be considered, as it will significantly change the modal characteristics and lead to deviation of dynamic targets from the design value. In this paper, a dynamic modeling method for typical LP rotor systems in aero-engines is proposed based on the beam element method. By combining the harmonic balance and condensation method of degree of freedom in the frequency domain, an effective method is built to solve the nonlinear mode of the complex rotor system. The above methods are achieved using ANSYS and MATLAB software, and successfully applied to the analysis of the modal characteristics of real LP rotors in aero-engines. Results show that the rub-impact significantly increases the modal frequencies of the rotor system. Particularly for the first order modes, the change in the forward whirl and backward whirl modal frequencies can respectively reach 16% and 29%. However, the changes in modal frequencies are always within a certain range and have interval characteristics. While closely related to gyroscopic effect, modal shapes and casing stiffness, the influence of rub-impact on modal frequency is not sensitive to the friction coefficient. Moreover, due to the friction force at the rubbing point, the modal damping of the backward whirl mode may be less than zero when the rub-impact is severe, leading to the unstable backward whirl model.

Key words: flexible rotor, rub-impact, harmonic balance, nonlinear mode, stability