  This paper proposes a nonlinear control architecture for flexible aircraft
simultaneous trajectory tracking and load alleviation. By exploiting the
control redundancy, the gust and maneuver loads are alleviated without
degrading the rigid-body command tracking performance. The proposed control
architecture contains four cascaded control loops: position control, flight
path control, attitude control, and optimal multi-objective wing control. Since
the position kinematics are not influenced by model uncertainties, the
nonlinear dynamic inversion control is applied. On the contrary, the flight
path dynamics are perturbed by both model uncertainties and atmospheric
disturbances; thus the incremental sliding mode control is adopted.
Lyapunov-based analyses show that this method can simultaneously reduce the
model dependency and the minimum possible gains of conventional sliding mode
control methods. Moreover, the attitude dynamics are in the strict-feedback
form; thus the incremental backstepping sliding mode control is applied.
Furthermore, a novel load reference generator is designed to distinguish the
necessary loads for performing maneuvers from the excessive loads. The load
references are realized by the inner-loop optimal wing controller, while the
excessive loads are naturalized by flaps without influencing the outer-loop
tracking performance. The merits of the proposed control architecture are
verified by trajectory tracking tasks and gust load alleviation tasks in
spatial von Karman turbulence fields.
