Whirl flutter stability assessment using rotor transfer matrices

Abstract

The aeroelastic instability phenomenon called whirl flutter is of increased interest for the flutter assessment of aircraft, as a new generation of propeller aircraft is being developed. For the modelling of whirl flutter, a description of the motion induced forces on the propeller is necessary. Classical theory uses analytically derived coefficients for this purpose, but with some approximations. This paper presents a new method, called the transfer matrix method, to describe these transfer functions from hub motion to hub forces in the frequency domain and increase fidelity of whirl flutter assessment. Instead of deriving the transfer functions, they are identified from a time domain model of the isolated propeller, for which several fidelity levels already exist. For propellers in axial flow it comes down to two time domain pulseperturbations of the hub DOF to identify the spectrum of transfer matrices. The usage of this method is demonstrated in the paper using simple propeller models and results are compared to the classical method as well as coupled time domain simulations. Results show a very good agreement with both references, but also the potential benefits of including blade flexibility and more complex aerodynamics into the whirl flutter assessment in the frequency domain.