Helicopter Rotor Wake Dynamics During Simulated Tactical Maneuvers

The wake dynamics of a helicopter undergoing large amplitude, high rate maneuvering flight conditions, typical of those used in some types of military tactics, were simulated using a time-accurate, viscous corrected free-vortex wake method. Five tactical maneuvers were studied: pop-up, pop-down, a pull-up arrested rate of descent, port and starboard rolls, and roll reversals. The maneuver simulations showed that the wake dynamics were accompanied by significant lags and reorganization of the wake structure, which also significantly influenced the time-history of the lift distribution over the rotor disk. In some maneuvers it was found that the rotor operates well inside its own wake, giving rise to particularly large unsteady airload fluctuations. Roll maneuvers were shown to be characterized by a significant change in the asymmetric nature of the wake trailed from the advancing and retreating sides of the rotor disk. A roll reversal maneuver was noted to introduce large temporal lags in the wake developments, the results also depending on whether a port-starboard-port or a starboard-port-starboard roll maneuver was performed. During some maneuvers, especially those involving high disk angles of attack like a quickstop or pull-up, the tip vortices were shown to bundle into a vortical ring structure below the rotor. This ring then passes up through the rotor disk, creating locally high unsteady airloads. Overall, the predicted results demonstrate the large nonlinear wake dynamics that would be produced during combat-like flight maneuvers, and the necessity of simulating large amplitude or short time period maneuvers using a rotor wake model with sufficiently high spatial and temporal resolution if the important features of the wake dynamics are to be properly captured.