Flight Dynamics of a Coaxial Compound Helicopter with Rotor-on-Rotor Interactional Aerodynamics

This paper focuses on the integration of a generalized rotor-on-rotor interactional aerodynamics model in state-variable form within a flight dynamics simulation and on its subsequent linearization and linear model analysis. The aircraft chosen for this investigation is a generic lift-offset compound coaxial rotorcraft. Upon trimming the flight dynamics to hovering flight with the presence of rotor-on-rotor interactions, the linearized hover models are validated against a baseline model that do not account for rotor-on-rotor inflow interference effects. A model order reduction method is employed to guide the development of linearized models that are tractable for flight control design while still predicting the effect of rotor-on-rotor interactions on the vehicle flight dynamics. The effect of rotor-on-rotor interactional aerodynamics modeling on predictions of tip clearance between the rotors is also investigated. In hover, for the coaxial configuration considered, the model accounting for rotor-on-rotor interactions leads to a greater predicted tip clearance relative to the nominal separation, due to the thrust differential between the two main rotors. Furthermore, compared to the model without rotor-on-rotor interactions, the model with rotor-on-rotor interactions predicts smaller rolling and pitching moment derivatives with respect to lateral and longitudinal velocity, along with reduced roll and pitch stability damping derivatives. These changes in stability derivatives yield roll and pitch subsidence modes at a lower frequency and roll and pitch oscillation modes with a higher natural frequency. The resulting frequency responses show a reduction in the magnitude of the roll response to on-axis inputs (approximately 0.7 to 10 rad/s). In forward flight, the dominant effect is at low frequencies in the pitch axis. These findings are in line with previous studies focusing on model validation of the Sikorsky X2TD™ based on flight-test data.