Integrated Design of AFCS and HHC for Rotorcraft Vibration Reduction using Dynamic Crossfeeds

Since the 1950's, Higher Harmonic Control (HHC) has been known to be an effective means for alleviating problematic N/rev vibratory loads in rotorcraft. Previously, control laws for vibration suppression have been designed independently from automatic flight control systems. Recently published work has identified a one-way coupling between the two that degrades the performance of the vibration suppression loop during maneuvering flight. This coupling demands that these previously independent systems be viewed as a single integrated AFCS/HHC design challenge in which the impact of the AFCS on the HHC loop must be carefully considered. Integrated design of AFCS and HHC laws therefore requires a mathematical model that is capable of accurately predicting two classes of phenomena: flight dynamics for the AFCS loop and N/rev vibrations for the HHC loop. This work presents a new type of LTI helicopter model that meets these atypical modeling challenges and validates the model against recent wind tunnel test data. The LTI is used to design control laws that include dynamic crossfeeds which serve to decouple the HHC loop from the influence of the AFCS.