An Analysis of Aeroengine Fan Flutter Using Twin Orthogonal Vibration Modes

Abstract

Flutter prediction methods for aeroengine fans at present typically combine a complex aerodynamic analysis with a simple model of the mechanical behavior of the fan. In this paper a more sophisticated model of the mechanical response is used to investigate flutter and provide additional insight into the physical mechanisms involved. The model incorporates twin orthogonal modes, which are two independent vibration patterns similar in shape and resonant frequency but displaced 1/4 wave circumferentially in space. Flutter can be thought of as a self excited vibration in which the response of each blade in one mode generates aerodynamic forces on the blades which drive the twin mode—and vice versa. The flutter frequency can be determined by considering the phasing between the twin modes; whether flutter does actually occur (at this frequency) depends upon the relationship between the aerodynamic force coefficients and the amplitude response of each mode. The greatest tendency to flutter occurs when the twin modes are identical in frequency. For the more practical case of a frequency split between the modes the tendency to flutter decreases with increased frequency separation, and the vibration pattern becomes non-uniform. The non-uniformities include unequal blade amplitudes, unequal interblade phase angles, variation from blade to blade in the temporal phase between twist and flap within each individual blade, and a deflected shape which is not sinusoidal circumferentially.