AERODYNAMIC CHARACTERISTICS OF DUCTED FAN TAIL ROTOR IN SIDE-WARD FLIGHT CONDITIONS

Typical single rotor helicopter requires a tail rotor to compensate the torque of main rotor and to ensure lateral controllability and stability. The ducted fan tail rotor has the advantage of preventing safety accidents caused by high speed rotating rotor as well as external impact. In addition, extra thrust is generated by the shroud, and there is a low risk of entering the vortex ring state(VRS) due to surround rotor shroud. In this study, the aerodynamic characteristics of ducted fan tail rotor in side-ward condition is analyzed to study the VRS state characteristics. When the side-ward velocity ratio, v_y/v_i , is about 0.5, the thrust of open type tail rotor is reduced by about 92.1% due to VRS effects. As the side-ward velocity ratio increases, it gradually overcome from the VRS. While in the ducted fan tail rotor, the thrust increases constantly, so that the VRS does not occur. This is due to the geometrical characteristics of ducted fan tail rotor that has expanded exit plane of rotor shroud.