Identification of State-Space Rotor Wake Models with Application to Coaxial Rotorcraft Flight Dynamics and Control

Abstract

Title of dissertation: IDENTIFICATION OF STATE-SPACE ROTOR WAKE MODELS WITH APPLICATION TO COAXIAL ROTORCRAFT FLIGHT DYNAMICS AND CONTROL Sean Hersey Doctor of Philosophy, 2019 Dissertation directed by: Professor Roberto Celi Department of Aerospace Engineering Modern aerodynamic analysis tools, such as free-vortex wake models and CFDbased techniques, include fewer theoretical limitations and approximations than classical simplified schemes, and represent the state-of-the-art in rotorcraft aerodynamic modeling, including for coaxial and other advanced configurations. However, they are impractical or impossible to apply to many flight dynamics problems because they are not formulated in ordinary differential equation (ODE) form, and they are often computationally intensive. Inflow models, for any configuration type, that couple the accuracy of high-fidelity aerodynamic models with the simplicity and ODE form of dynamic inflow-type theories would be an important contribution to the field of flight dynamics and control. This dissertation presents the methodology for the extraction of linearized ODE models from computed inflow data acquired from detailed aerodynamic free-vortex wake models, using frequency domain system identification. These methods are very general and applicable to any aerodynamic model, and are first demonstrated with a free wake model in hover and forward flight, for a single main rotor, and subsequently for the prediction of induced flow off the rotor as well, at locations such as the tail or fuselage. The methods are then applied to the extraction of first order linearized ODE inflow models for a coaxial rotor in hover. Subsequent analysis concluded that freevortex wake models show that the behavior of the inflow of a coaxial configuration may be higher-order. Also, tip-path plane motion of a coaxial rotor causes wake distortion which has an impact on the inflow behavior. Therefore, the methodology is expanded to the identification of a second order inflow representation which is shown to better capture from all of the relevant dynamics from free-vortex wake models, including wake distortion. With ODE models of inflow defined for an advanced coaxial configuration, this dissertation then presents a comparison of the fully-coupled aircraft flight dynamics, and the design of an explicit modeling-following feedback controller, with both a free-vortex wake identified model and a momentum theory based approach, concluding that accurate inflow modeling of coaxial rotor inflow is essential for investigation into the flight dynamics and control design of advanced rotor configurations. IDENTIFICATION OF STATE-SPACE ROTOR WAKE MODELS WITH APPLICATION TO COAXIAL ROTORCRAFT FLIGHT DYNAMICS AND CONTROL