Flutter Assessment of a Rotor Blade in Steady Axial Flight based on Indicial Aerodynamics

Abstract

The 7AD rotor blade is assessed for flutter stability in steady axial flight. For the aeroelastic analyses, the multibody model is tightly-coupled with an unsteady aerodynamic model based on the extended Wagner function and related enhancements for the general motion of an airfoil section considering heave and pitch. The mathematical setup of the extended Wagner function in state space considers unsteady aerodynamic contributions related to the blade airfoil as well as rotor inflow and wake periodicity for axial flow in hover flight. A correction for the effective rotor inflow in climb and descent extends the applicability to steady axial flight. As known for articulated rotor blades, also the straight 7AD blade with parabolic blade tip shows the classical bending-torsion coupling which remains for all studied axial flight velocities. According results for climb and descent are evaluated in terms of a relative flutter onset taking hover as reference into account. Within the studied range of climb velocities, trends for axial climb and descent being related to the change in effective rotor inflow can be observed for the flutter onset. Since the effective inflow velocity only decreases slightly within climb flight, the rotor speed for flutter onset remains almost the same as found in hover flight. For the operation of the rotor at moderate descent velocities, the effective inflow velocity is increased and the relative flutter onset is found at a 3 % larger rotor speed than in hover flight. Here, the change of effective rotor inflow obtained is dominant, since it modifies torsion damping within the critical flutter coupling and influences the flutter onset depending on climb or descent flight, respectively.