Applying variations of the quantitative feedback technique (QFT) to unstable, non-minimum phase aircraft dynamics models

Variations of the quantitative feedback technique (QFT) are applied to a multi-input-multi-output (MIMO) flight control problem with unstable, nonminimum phase plants. The authors present specialized techniques that enable the designer to achieve acceptable results for effective plants which are both unstable and non-minimum phase. The weighting matrix function is discussed, and a method of developing frequency dependent compensation is presented. The longitudinal control system, defined as a single-input-single-output (SISO) system, is designed using the loop transmission function. The limitations imposed by right-half-plane poles and zeros are discussed as the design is presented. A straightforward approach to designing a prefilter is also presented. Singular-G and optimal blending methods are used to improve the achievable stability characteristics of the lateral-directional (MIMO) effective plant. A brief discussion of the design of prefilters for a MIMO system is included