Accurate State-Space Inflow Modeling for Flight Dynamics and Control of a Coaxial-Pusher Rotorcraft

Recent advances in state space modeling of the aerodynamic interactions between rotors in advanced coaxial configurations have allowed for increased understanding of the stability and controllability of such aircraft. However, modern aerodynamic analysis tools such as free wake models have shown that the behavior of the inflow of a coaxial configuration may not exhibit only first order behavior as some models suggest. Furthermore, it is clear that tip-path plane motion of a coaxial rotor can have an impact on the inflow behavior through wake distortion, and so the inflow response is likely higher order in nature. It is unclear what effects such behavior would have on the prediction of handling qualities and the design of control systems. The objective of this paper is to extend a system identification method to the extraction of a second order inflow representation capable of capturing from free-wake models all of the relevant dynamics of the wake, including wake distortion. This method has successfully been used to extract a first order representation of the inflow for both single main rotor helicopters and coaxial rotorcraft. With state-space models of inflow defined for an advanced coaxial configuration, this paper also presents a comparison of the fully-coupled aircraft flight dynamics, and the design of an explicit modeling-following feedback controller, with both the free-wake identified model and a momentum theory based approach.