Dynamic load alleviation of input-redundant flexible aircraft via nonlinear control allocation over invariant manifold

Flexible aircraft design in modern transport aircraft improves fuel efficiency by increasing lift, reducing drag, and minimizing weight. This design increases structural flexibility, requiring special attention to load at critical locations to prevent failure during intense maneuvers or gusts. Traditional maneuver load alleviation is typically achieved through static allocation of control surface deflections or linear aeroelastic models. It is particularly challenging to separate flexible responses from rigid-body responses due to the nonlinear aerodynamic and geometric characteristics of flexible aircraft. This paper proposes a decoupled dynamic load alleviation strategy for input-redundant flexible aircraft exploiting invariant manifold structures of nonlinear systems. The nonlinear modeling considers static and periodically excited aeroelastic responses of flexible aircraft as a linear parameter-varying system with nonlinear steady-state gain, facilitating a novel nonlinear control allocation framework. This framework manipulates a proxy signal across the invariant manifold structure considering both high-frequency and low-frequency aircraft response components to minimize impact on rigid-body responses, achieving effective decoupling. The proposed nonlinear control allocation further minimizes load violation through output-input-constraint conversion. A flexible aircraft wing prototype is developed and tested through simulations and wind tunnel experiments, demonstrating enhanced load alleviation capabilities without compromising rigid-body responses compared to existing methods.