The Effect of Bending-Torsion Coupling on Fan and Compressor Blade Flutter

A study of the effects of bending-torsion interaction of the flutter boundaries of turbomachinery blading is presented. The blades are modeled as equivalent sections, and the equations of motion allow for the general case of structural, inertial and aerodynamic coupling, in the presence of structural damping. Two different speed regimes are investigated: incompressible flow, and supersonic flow with a subsonic leading edge locus. Flutter boundaries are presented for cascade design parameters representative of current technology fan rotors. These results illustrate that bending-torsion interaction has a pronounced effect on the flutter boundaries for both speed regimes, although the mode frequencies show no appreciable tendency to coalesce as flutter is approached. Several cases of bending branch instability were observed, without incorporating the effects of finite mean lift or strong shocks in the analysis.