Benefits of Dual Propellers for a Coaxial Helicopter

A model of a coaxial helicopter with a rigid rotor system is modified to have dual side-mounted propellers. The aircraft is simulated in trim at different flight speeds to investigate the potential benefits of the dual propellers with regard to fault tolerance and yaw control authority. At low speed, the dual propellers impact on the main rotor system actuator failure ranges is analyzed, demonstrating an increase in the allowable trim range from 25-30% in the nominal platform to 50-60% with the dual propeller configuration. Relaxation of the stall constraint for the dual propellers expands the upper rotor maximum actuator limits to the maximum geometric limit, minimum actuator limits for the lower rotor are constrained by the upper rotor stall limits. Upper rotor minimum and lower rotor maximum actuator limits are determined by tip clearance restrictions. At mid speed, the dual propellers improve the yaw control power in the 50-100 kt range and allow for a Level 1 aggressive yaw response as defined in ADS-33E. At high speed, the dual propellers provide yaw control redundancy and allow for trim when the rudder is fully deflected for nearly the entire flight envelope.