Flutter Assessment of a Double-Swept Rotor Blade in Hover Based on Indicial Aerodynamics Considering Blade Profile, Rotor Inflow and Wake Periodicity

Abstract

The ERATO rotor blade is assessed for flutter stability in hover. For the aeroelastic analyses, the multibody model is tightly-coupled with an unsteady aerodynamic model based on Wagner's function and related enhancements for the general motion of an airfoil section considering heave and pitch. The mathematical setup of the Wagner function is extended for axial flow to include unsteady contributions related to rotor inflow and wake periodicity. Since the unsteady aerodynamic model is based on indicial functions, a separation of these contributions is possible and allows to study their individual impact on rotor blade flutter. According flutter results are extracted in frequency domain for three test cases which differ in the unsteady aerodynamic model for circulation comprising blade profile, rotor inflow and wake periodicity. In addition to the classical bending-torsion coupling which is well known for articulated straight rotor blades, the double-swept ERATO blade exhibits a critical flutter coupling which is composed of two elastic flap bending modes. Due to the forward and backward sweep of the blade layout, these elastic flap modes introduce further torsional blade motion into the aeroelastic coupling.