As wing designs aim for higher aerodynamic efficiency, the underlying aircraft structure becomes more flexible, requiring additional features to alleviate the loads encountered from gusts and maneuvers. While alleviating loads, it is desirable to minimize the deviations from the original flight trajectory. In this work, a dynamic control allocation method which exploits redundant control effectors for maneuver and gust load alleviation is proposed for flexible aircraft. The control architecture decouples the two objectives of load alleviation and rigid body trajectory tracking by exploiting the null space between the input and the rigid body output. A reduced-dimensional null space input is established, which affects the flexible output (but not the rigid body output) when passed through a null space filter to generate incremental control signals. This null space input is determined to maintain the flexible output of the aircraft within specified values, thereby achieving load alleviation. A receding horizon approach to generate the trajectory of the null space input is developed based on linear aircraft models. This receding horizon approach then informs a model predictive control-based control allocator function which can be used as an add-on scheme to a nominal controller. Numerical simulations are used to illustrate the operation of this load alleviation system based on linear models, linear parameter-varying models, and nonlinear models. It is shown that the proposed load alleviation system can successfully avoid the violation of load bounds in the presence of both gust disturbances and maneuvers and with minimal effect on the trajectory tracking performance. A case study to characterize the proposed load alleviation system identified limits of its applicability to nonlinear aircraft and resulted in recommendations for its design parameters. The load alleviation system developed and demonstrated in this work can be applied to aircraft with wing flexibility high enough that the vertical wingtip deflection is around 28-34% of half-span in cruise and the first out-of-plane bending frequency is around 1.05-1.15 Hz. The case study also showed that a preview horizon of 1-2 seconds provides a good compromise for handling both low-frequency maneuvers and high-frequency gust disturbances.