Finite-State Dynamic Wake Inflow Modelling for Coaxial Rotors

Wake inflow modelling is a crucial issue in the development of efficient and reliable computational tools for flight mechanic and aeroelastic analysis of rotorcraft. The aim of this work is the development of a finite-state, dynamic wake inflow modelling for coaxial rotors in steady flight conditions, based on simulations provided by aerodynamic solvers of arbitrary accuracy. It provides models relating the coefficients of an approximated linear distribution of wake inflow over upper and lower rotor discs either to rotor controls and helicopter kinematic variables or to thrust and in-plane moments generated by the rotors. A three-step identification procedure is proposed. It consists in: (i) evaluation of wake inflow due to harmonic perturbations of rotor kinematics, (ii) determination of the corresponding inflow coefficient (and rotor loads) transfer functions, and (iii) their rational approximation. Wake inflow models are predicted through aerodynamic solutions provided by a boundary element method for potential flows, capable of capturing effects due to wake distortion, multi-body interference (like that in coaxial rotor configurations) and severe blade-vortex interaction. They are validated by correlation with the inflow directly calculated by the aerodynamic solver, for a coaxial rotor system subject to arbitrary perturbations.