Flight Evaluation of Variation in Rotor RPM on Low Speed Handling Characteristics of Helicopter

This paper describes multi fold advantages of increasing the Rotor RPM to improve low speed handling of helicopters in high altitude operations, where-in the availability of engine power, control margin and air density ratio play an important role. The primary aim of this work was to reduce the tail rotor collective requirement in hover, sideward flight, spot turns and in take-off and landing at high altitudes. The increase in rotor RPM was also proven beneficial for low speed handling at low altitude with high all up weight. Simulations were carried out for different all up weights and altitudes to ascertain the effect of variation of rotor RPM on tail rotor collective, main rotor collective and torque requirement in hover and sideward flight. Effect of increased rotor RPM on main rotor blade loads (mainly flap and lag) was also examined through a six DOF helicopter simulation program. Based on the results obtained from simulations, comprehensive flight testing was carried out to confirm simulation results. Flight test data showed that the increase in rotor RPM reduces tail rotor collective requirement. The reduction in main rotor collective and torque requirement (per engine) with increased rotor RPM was also observed. Enhancement in wind handling capability over the critical azimuth and spot turn capability was established. Fatigue loads marginally reduced with higher RPM than the nominal RPM. Cockpit vibration levels were similar and change was not perceptible with higher rotor RPM. Vibration on tail drive shaft and tail rotor actuator with higher rotor RPM, reduced during hover, left sideward flight and spot turns. The trends were independent of all up weight and altitude. The effect of increased rotor RPM on Engine, Transmission, Flight controls and Maintenance was also assessed during flight testing.