A Note on a Phenomenon Affecting Helicopter Directional Control in Rearward Flight

This paper presents the results of n wind‐tunnel investigation of tail‐rotor performance that was initiated because of directional‐control problems that have occurred with helicopters in a low‐velocity tail wind. The investigation has identified significant adverse effects of the main‐rotor wake that include an increase in the adverse fin force, a decrease in the tail‐rotor thrust obtained, and an increase in the tail‐rotor torque required. The adverse effects are the result of the immersion of the tail rotor and fin in a large‐scale vortex generated by the interactions of the main‐rotor wake and the mind in the presence of the ground. When rearward airspeed is increased to a critical value, the free‐stream flow carries the large‐scale vortex away from the tail rotor and fin. This causes a discontinuity in tail‐rotor power and collective pitch required. Model test data are shown that simulate flight results.