Helicopter Rotor Response to Wake Encounters in Ground Effect

Interactions between helicopters and aircraft wakes are most likely to occur near airports, especially during takeoff and landing. Under these conditions, helicopter flight is characterised by low forward speeds and the presence of ground effect. Recent studies conducted out of ground effect have suggested that, particularly at low speeds, the mutually-induced deformation of both the rotorwake and the wake of the aircraft with which it is interacting may be an important factor in limiting the thrust and flapping response of the helicopter during a wake encounter. Numerical prediction of helicopter performance in ground effect remains a challenge, however. In particular, certain experimentally observed features of the rotor flow, such as the formation of the characteristic ‘ground vortex’ and the dynamics of its interaction with the remainder of the rotor flow field, require extremely long-term calculations to capture. In this paper, the development of the rotor flow field in ground effect is studied using a CFD-based approach which, through its vorticity conserving properties, is ideally suited to capturing vortical features in the flow that take a large number of rotor revolutions to develop. Preliminary calculations of the rotor’s response to interactions with an aircraft’s wake in ground effect are then presented. The rotor’s response to a wake interaction in ground effect appears to be more severe than its response in free air, principally because the presence of the ground plane restricts the mutually-induced deformation of the wakes of the helicopter and the interacting aircraft.