Sequential loop closure in design of a robust rotorcraft flight control system

Rotorcraft flight control systems present many significant design challenges. First, large variations in the response characteristics of rotorcraft result from the wide range of airspeeds of typical operation (hover to over 100 kn). Second, the assumption of vehicle rigidity often employed in the design of fixed-wing flight control systems is rarely justified in rotorcraft where rotor degrees of freedom can have a significant impact on the system performance and stability. A methodology is proposed for the design of robust rotorcraft flight control systems utilizing quantitative feedback theory (QFT). QFT is a technique that accounts for variability in the dynamic response of the controlled element in the design of robust control systems. It was developed to address a multiple-input/single-output (MISO) design problem, but the extension of this technique to address multiple-input/multiple-output (MIMO) systems was employed to address the Eight control system (FCS) design of a UH-60 Black Hawk Helicopter. This was accomplished by constructing a set of MISO systems mathematically equivalent to the MIMO system. QFT was applied to each member of the set independently, Inherent conservatism in this design technique leads to limitations in its utility. A second approach utilizing the sequential closure of control loops proved an improved method for designing of robust MIMO FCSs. An analysis of the two FCS design methodologies is presented.