Abstract: To study the performance improvement potential of variable speed tail rotors by the passively extendable chord, we establish a helicopter flight performance analysis model, including a comprehensive rotor model, a tail rotor model, a fuselage model, and a propulsive trim method. The flight data of the UH-60A helicopter is utilized to validate this model. The results show that the extendable chord has little effect on the power while the helicopter is hovering. During low to medium speed flight, the extendable chord slightly increases the power, while during high speed flight, the power can be significantly reduced. When the tail rotor speed is reduced, the passively extendable chord is adapted to the aerodynamic environment. During high speed flight, the chord length increases with the reduced speed, and the tail rotor power reduction is increased. When the forward speed is 300 km/h and the tail rotor speed is 90% of the nominal speed, the power increases by 10.1%, which then turns to 1.59% when the extendable chord is used. The extendable chord is suitable to be placed in the range of 70%R~90%R (tail rotor radius). The power saving increases with the extendable chord length increase during high speed flight, or when the helicopter take-off weight decreases.

Key words: extendable chord, variable speed, helicopter, tail rotors, flight performance