Application of linear subspace stabilization and linear adaptive techniques to aircraft flight control problems. I. The inner loop

Robust flight control laws are developed for a combat aircraft. The control problem is layered into an outer-loop of control, or automatic pilot, controlling motion of the center of gravity of the aircraft and an inner-loop of control that controls the angular motion around the center of gravity. This paper is concerned with application of subspace stabilization theory to accommodate the nonlinear and uncertain nature of the inner-loop control problem. Thei theory provides the required robustness in the inner-loop that controls pitch, yaw and roll rates and also used to control aircraft velocity. Subspace stabilization control and disturbance accommodation techniques allow using either of several control laws to steer the system error state to a certain subspace S (hyperplane) which represents the prescribed ideal tracking error behavior, while suitably bounding the subsequent motion on the subspace S. Linear adaptive control circumvents the use of nonlinear techniques, through the use of observers to estimate the effects of nonlinear or uncertain terms in the system equations of motion. Simulation results for a worked example demonstrate that the proposed autopilot tracks prescribed trajectories accurately.