Rotor aerodynamics during maneuvering flight using a time-accurate free-vortex wake

A time-accurate free-vortex method was developed to predict the evolution of the rotor wake aerodynamics during transient, maneuvering flight conditions. Because of the highly non-linear nature of the rotor wake problem, the numerical algorithm was rigorously examined for stability, accuracy and convergence. Solution convergence was demonstrated with a verified second-order accuracy for both hovering and forward flight conditions. The wake model was then validated for transient conditions using measured rotor responses following rapid ramp increases in collective pitch, and also for oscillatory blade pitch inputs. The methodology was applied to study idealized maneuvering flight problems consisting of imposed angular pitch and roll rates. The results showed that time-dependent effects associated with maneuver-induced wake distortion significantly influenced the rotor aerodynamics and, also, the blade flapping response. The magnitude of the maneuver-induced wake distortion effects was found to vary with rotor operating state as well as with rotor geometric parameters such as the blade flapping frequency. The results also suggested that linearized inflow models using configuration and/or operating state independent wake distortion factors may not be adequate descriptors of the maneuver problem.