Mechanism and effect of “critical follower speed” on dual-rotor system of aero-engines

In order to investigate the mechanism of “critical follower speed” which could lead to a sustained strong vibration of aero-engine, a dual-rotor dynamic model with intershaft bearing was established. The relationship of the dual-rotor parameters in the case of “critical follower speed” phenomenon was deduced. And the dynamic characteristics of the rotor system under “critical follower speed” were analyzed. The results of investigation revealed that: the phenomenon “critical follower speed” caused the failure of rotor system to cross the critical speed of disk-swing mode, which made vibration extremely sensitive to unbalanced mass distribution of the rotor system. And there were two ratio influential factors: the ratio of polar inertia moment to the diameter inertia moment by disks (simple named “inertia ratio”), and the increasing speed ratio of dual-rotor system. When the inertia ratio of the equivalent high-pressure-disk was equal to 1, or the increasing speed ratio was equal to the inertia ratio of any low-pressure-disk, the rotor system was excited by high-pressure rotor under the “critical follower speed” state. Besides, when the inertia ratio of the low-pressure-disk was equal to 1, or the product of increasing speed ratio and inertia ratio of equivalent high-pressure-disk was equal to 1, the rotor system was excited by low-pressure rotor under the “critical follower speed” state. Additionally, it is suggested that the equivalent inertia ratio design of the blade-disk on aero-engine and the speed increasing ratio of dual-rotor should be strictly controlled.