An Evaluation of Fault-Tolerant Control Allocation Strategies for eVTOL Aircraft

Electric vertical takeoff and landing (eVTOL) aircraft typically have more control effectors than controlled axes, offering an opportunity to improve flight safety through recovery from control effector failures. Realizing this potential depends on control allocation strategies that intelligently redistribute virtual force and moment commands among redundant effectors under saturation and fault constraints. However, eVTOL aircraft pose unique challenges for control allocation because of varying control effectiveness across the transition flight envelope. This paper presents a geometric evaluation approach to understand the force and moment generation capability of an aircraft using the attainable force and moment set (AFMS)--a convex set in the six-dimensional space of generalized force and moment control inputs that is defined by saturation and fault constraints. Several classical pseudoinverse-based and optimization-based linear allocation methods are evaluated using the NASA Lift Plus Cruise eVTOL aircraft model in static and dynamic simulations. Their performance is benchmarked by their ability to reproduce the AFMS volume in nominal and faulted conditions, while also evaluating control effort and computational burden. Results show that the direct allocation and mixed L1 optimization algorithms consistently realize the full AFMS volume across hover, transition, and cruise regimes, demonstrating robust reconfiguration capability, while pseudoinverse formulations exhibit significant shortfalls. The dynamic simulation demonstrates zero allocation error, improved trajectory tracking, and successful fault hiding while using the direct allocation algorithm. Beyond specific results for the NASA Lift Plus Cruise aircraft, this study establishes a scalable, aircraft-agnostic method for comparing fault-tolerant control allocation algorithms for real-time implementation in overactuated eVTOL flight control systems.