Significance of Disk Flexing in Viscous-Damped Jet Engine Dynamics

Abstract

A new method of analyzing multishaft jet engine critical speeds and unbalance response is derived to account for flexible bladed disks, asymmetric mounting, and squeeze film bearing damping. The resulting elliptical shaft whirling drives traveling waves in disks at speeds equal to the sum and the difference of the rotating speed and the whirling speed. To illustrate a possible problem that cannot be predicted by current methods, a sample calculation shows a simplified model representing possible engine parameters tuned so that a critical speed associated with backward whirling is lowered into the operating range by reduction in disk stiffness. In contradiction to the current literature, the rotor is shown highly responsive to unbalance at this critical speed.