Mixed norm control of a helicopter

This thesis presents the results of a study into the design and analysis of robust full authority flight control laws to improve the handling qualities for a fly-by-wire actuated Bell 205 helicopter using mixed H2/Hinfinity -optimization. The emphasis in design is to meet stringent US Army handling qualities specifications against the constraint of robust stability. Use of mixed H2/Hinfinity -optimization promises to address an outstanding problem in helicopter flight control which has been the unrealistic mapping of multiple design objectives into a single norm.; A full authority flight control system is expected to improve helicopter handling qualities and remove the pilot from the feedback loop. Control laws are designed to provide fast, reliable, repeatable responses to pilot command inputs for vertical velocity, pitch and roll attitudes, and yaw rate in an optimal fashion. The Bell 205 does not naturally exhibit good handling qualities, as the dynamic characteristics of the helicopter do not comply with military specifications.; A possible solution to robust flight controller design to improve the handling qualities of the aircraft is to incorporate optimal nominal H2 model following performance and Hinfinity robust stability. This is formulated using a mixed H2/Hinfinity controller synthesis where optimal nominal performance is represented as a model following design optimization of an ideal target model based on military specifications for the various aircraft modes. Hinfinity -optimization is addressed via the incorporation of a structured uncertainty description of the aircraft model. Extensive effort has been expended to describe the error dynamics between the response of the model and the actual helicopter from both experimental data and pilot experience to reduce the trade-off between performance and robustness.; The mixed H2/Hinfinity control problem addresses genuine system requirements and results show effective robust controllers designed for both a benchmark problem and for the Bell 205 helicopter. Structured singular value analysis of system robustness is used to identify acceptable variation in aircraft dynamics for which stability is assured. These variations agree well with simulation testing of robust stability limits. The H2/Hinfinity controller on the other hand was found to predict very conservative limits.