Linearized Inflow and Interference Models from High Fidelity Free Wake Analysis for Modern Rotorcraft Configurations

Abstract

Linearized inflow models have been used to represent dynamic wake effects for control law development and flight dynamics simulation of conventional main rotor / tail rotor helicopters. With the current focus in the industry toward high-speed concepts and electric vertical take-off and landing (eVTOL) aircraft, modern rotorcraft designs are trending away from the conventional main rotor / tail rotor configuration for which low-order dynamic inflow models have been applied and validated. Configurations currently in design and flight testing stages include coaxial, coaxialcompound, tiltrotor, multi-rotor, and augmented lift concepts that involve strong rotor-rotor and rotor-airframe aerodynamic interactions. A procedure for extracting linearized inflow and interference models from higher fidelity comprehensive analyses suitable for advanced rotorcraft configurations has been developed and applied to a modern lift-offset coaxial rotorcraft. A second-order inflow model structure including wake distortion effects due to pitch and roll motion is shown to capture the critical dynamic response characteristics when compared to a nonlinear flight dynamics simulation coupled with a free wake model. This second-order model structure is more accurate and provides better prediction of flight controller performance metrics than a more conventional Pitt-Peters model structure.