G. Stolwijk A Hybrid Optimal Control Approach to Multi-Aircraft Formation Flying Master of Science Thesis II A Hybrid Optimal Control Approach to Multi-Aircraft Formation Flying Master of Science Thesis By G. Stolwijk in partial fulfilment of the requirements for the degree of Master of Science in Aerospace Engineering at the Delft University of Technology, to be defended publicly on Thursday November 30, 2017 at 2:00 PM. Supervisor: Dr. ir. S. Hartjes Thesis committee: Dr. ir. S. Hartjes, TU Delft Dr. ir. H.G. Visser TU Delft Dr. ir. E. van Kampen TU Delft This thesis is confidential and cannot be made public until November 30, 2017. An electronic version of this thesis is available at http://repository.tudelft.nl/. III IV Executive Summary With an expected rise in air traffic of 50% [1], new paradigms such as trajectory-based operations (TBO) are in development for air traffic control to enable more efficient aircraft trajectories. Because of this expected development, a great interest is currently shown in methods which can help determine the best trajectory for aircraft to fly given a certain optimization goal. The research field of aircraft trajectory optimization arose as a result, and in general studies in this field are based on optimal control theory using continuous variables. In this thesis, a Hybrid Optimal Control (HOC) framework which can be used in conjunction with existing optimal control software is presented. Hybrid optimal control theory aims to deal with systems that are both discrete time and continuous time from a mathematical perspective. The hybrid optimal control problem is “to find optimal hybrid – i.e., continuous and discrete – control trajectories such that an integral cost index – typically an integral of a function of the hybrid system state and control input – is minimized subject to the system dynamics, initial, terminal and further equality or inequality constraints” [2]. Much of the complexity of a hybrid optimal control problem depends on how much is known a priori about the switching structure between discrete states. A case study on multi-aircraft formation flying for civil aviation is chosen as an application to demonstrate the capabilities of the designed HOC method. In formation flight, trailing aircraft can experience a significant reduction in induced drag by ‘surfing’ the upwash of wing tip vortices generated by a leading aircraft. This in turn leads to a significant reduction in fuel burn for the trailing aircraft, making formation flight a more economical and environmentally friendly flying strategy. In previous research [3], the optimization of commercial flights for minimum fuel consumption was studied using multiple-phase trajectory optimization. However, only two-aircraft formations and three-aircraft formations with a fixed phase structure (the sequence in which aircraft join and leave the formation) are considered. By approaching this case as a hybrid optimal control problem, the phase structure can be left open as a discrete optimization parameter, enabling larger and more optimal formation trajectories. The existing software GPOPS [4], which is already capable of solving multiple-phase optimal control problems, is used as a basis for the developed HOC method. An efficient algorithm for the evaluation of the discrete solution space of the HOC problem, essentially all possible phase structures, is developed through implementation of a branch-and-bound method. In this approach, the discrete solution space is categorized in a tree structure where each tree level represents an additional discrete variable. The algorithm then only evaluates ‘branches’ which show promising characteristics, while ‘chopping’ the other branches. This way, a significant computation time reduction is achieved since a major part of the discrete solution space can be discarded. An initial guess adaptation method which translates trajectories to a correct initial guess for a given phase structure is also developed, as well as an implementation of multi-threaded computing. V The aircraft dynamics are evaluated as a reduced-order point mass model, with the formation flight phases using a single point, multiple mass model to accommodate the presence of multiple aircraft. The induced drag benefit for trailing aircraft is modelled in the aircraft dynamics by implementing a static reduction factor for each trailing aircraft in the relevant formation phases. All phases are connected appropriately using linkage constraints for the state vectors of the linked phases. The used aircraft model characteristics are based on the Boeing 747-400 aircraft. In several case studies, both a three-aircraft and a four-aircraft transatlantic formation flight scenario are evaluated using the developed HOC tool. The three-aircraft case yields a 4,74% overall reduction in fuel burn, while the four-aircraft case yields an 8.90% reduction. However, when using more pessimistic values for the induced drag reduction (50% less benefit), the fuel burn saving for the four-aircraft case is reduced to 3.48%. The fuel burn reduction also comes at the expense of flight time, since aircraft have to make a detour to join the formation. A three-aircraft case with existing scheduled flights to New York (JFK) is also evaluated, and yields an 11.47% fuel burn saving, a higher value because no significant detours have to be flown. In all experiments, a suboptimality is identified because the aircraft order in the formation is not part of the optimization. An alternative method for evaluating the fuel burn for the cruise formation phase, by using an analytical Bréguet range equation instead of a dynamic model is also tested. This method yields promising results; no significant deviation is observed in the fuel burn values compared to the point-mass dynamic model. However, the method turns out to be less computationally efficient for GPOPS. It is found that the developed tool is efficient and functional at evaluating the HOC problem of multi-aircraft formation flight trajectory optimization. Even for the four-aircraft experiments, computation time remains within practical limits at 5 hours. The convergence behavior of GPOPS is identified to be heavily influenced by the accuracy of the adapted initial guess. However, the robustness of the developed HOC tool leaves room for improvement. To conclude, it is found that the usefulness of a HOC approach is very high for the chosen application of trajectory optimization for multi-aircraft formation flying, since a promising fuel saving potential is identified. Given the number of flights that are performed every day, the impact of saving fuel just by optimizing their trajectories using formation flying can be very high on a global scale, both environmentally and economically. VI Table of Contents Executive Summary .................................................................................................................................... V List of Figures .............................................................................................................................................. IX List of Tables ............................................................................................................................................... XI List of Abbreviations .............................................................................................................................. XIII Nomenclature ........................................................................................................................................... XV 1 Introduction .......................................................................................................................................... 1 1.1 Hybrid Optimal Control ............................................................................................................................ 1 1.2 Formation Flying in Civil Aviation ......................................................................................................... 2 1.3 Optimization software .............................................................................................................................. 3 1.4 Research Proposal ..................................................................................................................................... 4 1.5 Thesis Structure .......................................................................................................................................... 5 2 Optimization using hybrid optimal control ............................................................................... 7 2.1 Conventional optimal control and numerical methods review .................................................. 7 2.2 Hybrid Optimal Control Theory .......................................................................................................... 13 2.3 Overview of recent aviation research involving hybrid optimal control ............................... 16 2.4 Analysis of currently common HOC problem-solving techniques ......................................... 18 3 Formation flight and its application for civil aviation ........................................................... 27 3.1 Aerodynamic concept ............................................................................................................................ 27 3.2 Positioning between aircraft in formation ....................................................................................... 28 4 Optimization problem description