Investigation of Lift Offset on Flight Dynamics Characteristics for Coaxial Compound Helicopters

Coaxial compound helicopters adopt the lift-offset (LOS) strategy to improve the rotor performance in high-speed flight, which influences the flight dynamics characteristics of the vehicle. Thus, a flight dynamics model and an inverse simulation method are developed to assess the effect of LOS on this helicopter. The trim results demonstrate that LOS reduces the collective and the longitudinal cyclic pitch across the flight range, and it adds the control input of the propeller collective in hover and lower-speed forward flight. The LOS control strategy reduces the power consumption and increases the maximum flight speed. Also, the LOS control strategy is affected by the gross weight. Furthermore, the stability is dependent on LOS due to its effect on the rotor efficiency and flapping motion. From the controllability results, LOS brings about severe coupling between the rolling moment and the collective differential input. Lastly, the pullup-and-pushover and transient-turn mission task elements with different LOS control strategies are assessed with inverse simulation. The results show that a reasonable LOS control strategy could not only reduce the power consumption but also have a positive influence on the oscillation in control inputs during the maneuver.