The Effect of Aerodynamic Phase Lag on the Twin Vibration Mode Model of Aeroengine Fan Flutter

An earlier paper analyzed aeroengine fan flutter in terms of twin orthogonal vibration modes in the fan. This paper extends the analysis by incorporating variable aerodynamic phase lags between a blade motion and the resulting aerodynamic forces. The extended analysis predicts two independent kinds of flutter. If the total damping within each mode is negative, each mode develops a self-sustaining vibration at its own natural frequency. This was not predicted by the earlier twin mode analysis. Even when the damping in each mode is positive an interaction between the modes can cause flutter under some conditions. This behavior is basically of the form described in the earlier paper, but there are significant detailed changes when variable aerodynamic phase lags are introduced.