Discrete control theory and dynamic observers applied to rotorcraft stability and trim

The nonlinear differential equations for rotorcraft simulation (in this case, flap-lag-pitch equations) are sampled once a period (or once a blade passage) and the resultant sampled errors in periodicity and trim are considered a discrete system. The algebraic Ricatti Equation is then used to design a controller to trim this discrete system. The resultant controller is applied to the original nonlinear simulation in that the trim errors at each blade passage are fed back to give discrete control changes. When all states cannot be measured, a discrete observer is used to estimate them. The resultant algorithm is shown to be a robust tool for trimming nonlinear set of rotorcraft equations.