Unsteady Lifting-Line Free-Wake Aerodynamic Modeling for Rotors in Hovering and Axial Flight

A time-stepping, lifting-line, computationally efficient tool for preliminary design applications is developed to predict the unsteady aerodynamic loads of rotors operating in hovering and axial flight. The velocity field induced by wake vorticity is computed using a free-wake vortex-lattice model, while sectional aerodynamic loads are evaluated through the application of Küssner and Schwarz’s airfoil theory. The vorticity released by the trailing edge is related to the distribution of bound circulation and is convected downstream to form the vortex-lattice wake. The local bound circulation is determined by applying the Kutta–Joukowski theorem for unsteady flows. The proposed unsteady aerodynamic solver is successfully validated by comparison with both experimental data available in the literature and numerical results obtained by a three-dimensional boundary element method computational tool for potential flow. It does not apply to rotors in edgewise flight conditions and when compressibility effects are not negligible.