Partially Structured LQR design for Lateral-Directional Flight Control Law

Flight control engineers use certain control structure depending upon requirements and resource availability while designing flight control laws. Classical control design is carried out by assessing the stability margins of each loop while closing all other loops for each flight condition for scheduled gain control laws. This is repeated one at a time for all the loops and therefore it is very time-consuming. A Linear Quadratic Regulator (LQR) design has inherent stability feature and is more suitable for multi input multi output (MIMO) system but the feedback gain matrix using LQR design has all non-zero elements which uses complete feedback from all the states to drive all the inputs. In this paper, gain suppression algorithm is used following to LQR design to achieve classical control law structure, i.e. to remove feedback from state to particular input. Generic flight control law design procedure is briefly outlined. The advantages and practical issues related to the implementation of LQR design method at the system level from industry point of view are elaborated.