Eigenstructure Control: A Rotorcraft Handling Qualities Engineering Tool

Abstract

This paper details a design methodology to derive flight control laws for a Bo105 helicopter model to meet ADS-33E-PRF handling qualities requirements. The feedback controller consists of angular rate sensors and flow angle rate estimates as feedback variables. The flow angle rate estimates are derived fromangular rate sensor signals using kinematic relationships. Longitudinal cyclic, lateral cyclic, and tail rotor are used as control variables. This core inner loop design using feedback signals derived only from inertial rate sensors meets the sensor redundancy (triplex) requirements for a digital fly-by-wire feedback system. Based on flightmechanics analysis, a lowcomplexity controller structure is postulated by selectively feeding back output signals to individual control channels. The primary objective of the postulated controller is to reduce the strong pitch to roll cross-coupling, inherent in the Bo105 model. However, instead of a loop at a time design procedure, typically adopted in classical control design, the design optimization is carried out by formulating an equivalent eigenstructure assignment problem that embeds the postulated controller. This approach results in a control methodology acting as an effective bridge between classical and multivariable design methods. Finally, using a recently developed tunable explicit model following problem formulation, a two-axis attitude command system is also designed. This command controller provides decoupled attitude command functions for pitch, and roll channels.