Tail-Rotor Thrust On A 5.5-Foot Helicopter Model In Ground Effect

Wind-tunnel experiments were conducted to investigate tail-rotor performance for flight conditions in ground effect where helicopter directional control deficiencies have been experienced. Tail-rotor thrust was measured on a small-scale helicopter model for various wind speeds and azimuths. Measurements were made with the fuselage and fin removed to emphasize the primary interference effects of a main rotor in ground effect. For a main-rotor height/diameter ratio h/D = 0.265, the largest thrust reductions occur when (i) the tail rotor rotates in the direction where the top blade moves forward, (ii) wind speeds are above 15 knots, and (iii) wind- azimuths are between 50° and 180°. Opposite rotation of the tail rotor eliminates most of the adverse interference effects of the main rotor. Flow visualization with neutrally buoyant helium bubbles revealed a small, well-defined ground vortex core at moderate and high wind velocities. Interpretation of the measured data showed that the ground vortex and trailing vortex systems of the main-rotor flow field produced tail-rotor thrust perturbations that are due to rotational and axial interference with the tail rotor.