Simulating HHC/AFCS Interaction and Optimized Controllers using Piloted Maneuvers

Higher harmonic control (HHC) has been shown to be capable of reducing vibration and noise in rotorcraft. The majority of the previous work has concentrated on the HHC itself, and not the interaction between the automatic flight control system (AFCS) and HHC. Limited studies have been done on HHC/AFCS interaction using a linear time invariant (LTI) model and offline inputs including the use of dynamic crossfeeds to improve HHC performance while maneuvering. Most recently, results on HHC/AFCS interaction using nonlinear piloted simulations have been obtained. It has been observed that the nominal T-matrix controller approach, while effective in steady state, does not reduce vibration during a maneuver. Increasing the feedback gain can slightly improve RMS and settling time but magnifies peak response. The aim of the current work is to optimize HHC controller performance in a realistic piloted maneuver, and validate the design on a nonlinear simulation model. Achieving this objective required the following accomplishments: 1) Extraction, reduction, and validation of a linear time invariant model from a nonlinear model. 2) Baseline higher harmonic controller design and validation based on the linear model applied to the nonlinear model. 3) Optimization of the higher harmonic controller design to improve performance during a maneuver while maintaining stability. 4) Fitting ideal crossfeeds with a low-order approximation. 5) Implementation of crossfeeds to further improve vibration controller performance during a maneuver.