Interactional Aerodynamics Modeling and Flight Control Design of Multi-Rotor Aircraft

This paper focuses on the integration of generalized rotor-on-rotor interactional aerodynamics models in state-variable form within flight dynamics simulations of multi-rotor unmanned aircraft systems (UAS), and on their subsequent linearization. The aircraft chosen for this investigation is the Malloy TRV-80 coaxial quadcopter. Upon trimming the flight dynamics at hover, linearized models of said dynamics are compared to nonlinear simulations, and with linear and nonlinear adopting dynamic inflow models that do not account for rotor-on-rotor inflow interference effects. Model-order reduction methods are investigated 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. Model-following flight control laws based on these reduced-order linearized models that account for the rotor-on-rotor interactions are developed and tested against similar control laws based on linearized models that do not account for rotor-on-rotor interactions. It is found that, for the coaxial quadcopter configuration in consideration, rotor-on-rotor interactions result in a lower magnitude roll/pitch rate response to lateral/longitudinal velocity and in a lower pitch​​/roll damping. These changes in stability derivatives yield roll and pitch subsidence modes at a lower frequency and roll and pitch oscillation modes with a lower natural frequency. Additionally, roll and pitch on-axis frequency responses indicate that the interactional inflow dynamics results in higher gains and phases at low to medium frequencies (approximately 0.1 to 4 rad/s). No significant improvements are detected by designing model-following control laws based on reduced-order models that account on rotor-on-rotor interactions.