Influence of blade elasticity on propeller whirl flutter stability

Abstract

The prediction of propeller whirl flutter for turboprop aircraft is usually done using aerodynamic derivatives based on rigid propeller blades. This work investigates the influence of blade elasticity on whirl flutter stability based on numerical simulations. The new transfer-matrix-method is used to incorporate time domain rotor models from the MBS-code Simpack into frequency domain flutter analyses. The blade modeling in Simpack used in this work utilizes beam models for the blade dynamics and strip theory for aerodynamics. A frequency domain transfer function from hub motion to hub loads is identified using time domain perturbations. The identified transfer matrices are coupled with a simple two-DOF pylon model to explore the effect of elastic blade modeling on whirl flutter stability. Different solution techniques are applied to extract frequency and damping for the pylon whirl modes as well as for the global rotor modes. Results show a significant stabilizing effect for the configurations investigated in this work. Due to a reduction in the aerodynamic cross-coupling moment Mz,theta, which usually drives whirl flutter instability, the predictions with elastic blades yield a more stable system, even for very stiff blades. The transfer-matrix-method results match very well with the time domain reference. This paper thus demonstrates an universal method for incorporating complex propeller models into frequency domain flutter analysis of turboprop aircraft.