Comparative Analysis of Advanced Control Strategies for Modern Aircraft Dynamics

Modern aircraft design, driven by advancements in materials and aerodynamic optimisations as well as new applications and unconventional configurations, especially in the UAV sector, but also for medium to large transport aircraft, exhibits complex dynamic behaviour that challenges conventional flight control systems. In consequence, several modern control approaches have emerged, which all have in common to strive for a reduced plant dependency by utilising acceleration or state derivative measurements and thereby extending the control scope to the actuator dynamics. This paper presents a comparative analysis of four advanced control approaches using the well-established Non-linear Dynamic Inversion (NDI) as a reference: Local Acceleration Control (LAC), Incremental Non-linear Dynamic Inversion (INDI), Extended Incremental Non-linear Dynamic Inversion (E-INDI) and Actuator Non-linear Dynamic Inversion (ANDI).We draw attention to the strong analogies between these approaches, which are still inadequately recognised in the field. In the first part of the paper each control law is formulated using a consistent, unique nomenclature and applied to a simple flight dynamic system of an aircraft represented as a point mass to facilitate a comparison of their structures. It is shown that some of these approaches converge to the same control law under ideal nominal conditions. In the second part of the paper, the approaches are evaluated by simulation, extending to various non-ideal conditions, including sensor dynamics, rate dynamics, and model uncertainties. These real-world effects compromise the cancellation of feedback loops inherent to the incremental control concepts, causing results to differ in practice despite all analogies. The advantages and problems of the approaches are presented, with particular emphasis on LAC, which shares the benefits of the INDI-based approaches in terms of robustness, disturbance rejection and compensation of non-linearities, but takes additional advantage from a direct actuator level implementation, avoiding asynchronous, counteracting feedback and enabling lower lag times as well as straightforward gain adjustment.