Proposal of a unified control strategy for vertical take-off and landing transition aircraft configurations

This paper proposes a unified controller based on an Incremental Nonlinear Dynamic Inversion (INDI) for vertical take-off and landing (VTOL) transition aircraft configurations. The proposed strategy attempts to encapsulate the control of different flight phases into one universal controller. The INDI control approach is used for dealing with systems that are non-affine in control inputs, which makes it suitable for controlling various kinds of VTOL aircraft configurations. Compared to a conventional cascaded loop, this concept enables the allocation of all desired forces and moments simultaneously. The controller comprises of a reference model, an Onboard Plant-Model, and error controller and a control allocation. While the reference model provides smooth and reasonable trajectories of the command values, the Onboard Plant-Model computes online estimates of the control effectiveness with respect to pseudo controls. Furthermore, the error controller is designed to achieve the desired error dynamics by a proper choice of feedback gains. The control allocation computes physically feasible control effector commands, providing the possibility of considering secondary constraints. The overall approach is extended by the concept of virtual control inputs – nested control feedback loops of the control allocation into the reference model. The control concept is applied to an experimental transition VTOL configuration including tilt rotors and aerodynamic surfaces. The control architecture as well as the aircraft configuration allows the unified design approach of the controller. Consequently the complete flight envelope utilizes only one baseline controller, eliminating the need for distinct controller variants for different maneuvers. Initial aeropropulsive interaction in the form of closed loop pitch oscillations are addressed and thereby enabled flight tests and corresponding results are presented.