Fuselage and Tail-Rotor Interference Effects on Helicopter Wake Development in Descending Flight

Under descending flight conditions, strong interactions between the rotor blades and vortical structures within the wake of the rotors and fuselage can be a significant source of unsteady loading on the system, with important consequences for the handling qualities, acoustic signature or vibrational characteristics of the helicopter. In particular, at low forward speed over a small range of descent speeds, an instability in the wake of the main rotor leads to collapse of the structure of the wake into a condition known as the vortex ring state. Little is known about how other elements of the helicopter configuration, such as the fuselage and tail rotor, influence the onset of the vortex ring state, or whether or not the onset of the vortex ring state is dependent on the trajectory of the helicopter. Calculations using a vorticity preserving CFD technique are compared for an isolated rotor, and a configuration consisting of the same rotor plus a fuselage and tail rotor, for two, accelerated, descending flight trajectories that pass through the onset boundary for the main rotor vortex ring state. Although the fully developed vortex ring state of the main rotor is very similar for both trajectories, the way in which the state develops is different depending on whether the onset boundary is crossed from low descent rate or from high. The development of the vortex ring state on the main rotor appears to be strongly influenced by the flow conditions in a critical region above the rear of the rotor disc. The fuselage modifies the flow in this critical region with subtle but significant effect on the character of the vortex ring state, whereas the effect of the tail rotor appears to be minimal.