Modern manned and unmanned aircraft designs have a lightweight and flexible structure to increase the flight performance. This trend is continuing with more flexible aircraft structures that possess a nonlinear behaviour. Typically those very flexible are equiped with a flight control system. This paper addresses trajectory control in longitudinal and lateral motion of a highly flexible aircraft with nonlinear dynamics. A new flight control design approach is introduced that is based on the classical multi-loop control law structure with specific considerations for the nonlinear structural behaviour. The inner loops of the flight control laws shall ensure stability and an optimal shape of the aircraft. The nonlinearities are summed up as uncertainties. Control design strategies in the frequency domain are used to achieve the design objectives. The outer loops are based on the classical multi-loop concept for autopilots. The design method and results in a linear and nonlinear simulation of the very flexible unmanned aerial vehicle X-HALE are presented.

Unmanned aircraft used as high-altitude platform system has been studied in research and industry as alternative technologies to satellites. Regarding actual operation and flight performance of such systems, multibody aircraft seems to be a promising aircraft configuration. In terms of flight dynamics, this aircraft strongly differs from classical rigid-body and flexible aircraft, because a strong interference between flight mechanic and formation modes occurs. For unmanned operation in the stratosphere, flight control laws are required. While control theory generally provides a number of approaches, the specific flight physics characteristics can be only partially considered. This paper addresses a flight control law approach based on a physically exact target model of the multibody aircraft dynamics rather than conventionally considering the system dynamics only. In the target model, hypothetical spring and damping elements at the joints are included into the equations of motion to transfer the configuration of a highly flexible multibody aircraft into one similar to a classical rigid-body aircraft. The differences between both types of aircraft are reflected in the eigenvalues and eigenvectors. Using the eigenstructure assignment, the desired damping and stiffness are established by the inner-loop flight control law. In contrast to other methods, this procedure allows a straightforward control law design for a multibody aircraft based on a physical reference model.

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The outcome of highly flexible aircraft requires new approaches in control design. In this research, we apply the loop separation concept, which consists in two control loops. The inner loop is capable of stabilizing the plant of the flexible aircraft, while is holding shape of the trimmed structure. Once the highly flexible aircraft is artificially transformed in a slightly flexible aircraft, the second loop or outer-loop is designed according to conventional, rigid-body-based control. Three control approaches were evaluated in the inner loop: LQG/LTR, LQR with output feedback and a direct integration approach. The direct integration approach with uncoupled gains presented better performance. The outer loops for speed, heading, sideslip angle and altitude were estimated using non-smooth optimization techniques and they are capable of attaining the commanded reference with low control energy and inside the maneuver requirements, while the inner loop is capable of reducing the elastic strains of the wing.

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