Flutter Assessment of a Rotor Blade in Hover based on Indicial Aerodynamics considering Blade Profile, Rotor Inflow and Wake Periodicity

Abstract

The 7AD 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. As known for articulated rotor blades, also the 7AD blade exhibits a classical bending-torsion coupling. The lowest flutter onset is found for unsteady aerodynamics limited to blade profile, whilst the cases with added rotor inflow and wake periodicity show both the same flutter onset at a 5% larger rotor speed. Here, the influence of rotor inflow plays the major role, since it increases the torsion damping within the critical flutter coupling. Added wake periodicity neither changes frequency nor damping and hence, does not affect the aeroelastic coupling.