This thesis deals with the design of an anti-skid brake controller for a generic fighter aircraft. Antiskid brake controllers prevent wheel locking and maximize the coefficient of friction between the tyre and the ground, resulting in lower stopping distance and time. The frictional force is maximized by regulating the slip. A model for the landing gear is first developed, which consists of the translational and rotational motions of the wheel, the equation for the slip and the elastic landing gear strut dynamics. The elastic behaviour of the landing gear is characterized through its modal frequencies, obtained from a Finite element analysis. As the governing equations are nonlinear, with linear elastic deformations of the strut, feedback linearization is used to design the anti-skid controller. The brake controller is found to work well. Its stability is verified through numerical simulations. Both the plant parameters and the sensor measurements are perturbed up to 10% from their nominal values. It is seen that the feedback linearization tolerates these variations quite well. The system is exceptionally tolerant to sensor noises. The torsional stiffness of the strut is found to be more critical than the longitudinal stiffness. Limits on the torsional stiffness that can be tolerated by the controller are found. This determines the limits on the stiffness of the landing gear beyond which gear walk may appear. The thesis concludes with suggestions for future work in this exciting field.

Doctor of Philosophy / Department of History / Donald J. Mrozek / In the late 20th and early 21st centuries, military aviation technology grew expensive and politically divisive, and this is not without precedent. In the 1960s and 1970s, the F-15 Eagle and F-16 Falcon represented a controversial shift both in the cost of development and in tactical doctrine for the United States Air Force (USAF), yet the motivating factors that influenced their design are not fully understood. Most of the literature either has focused on a teleological exploration of technical evolution or has held to a “genius inventor” paradigm, lionizing individual engineers and planners. Other works have focused on these aircraft as factors that changed the Air Force's tactical approach to warfighting or have simply evaluated their combat performance. Although these approaches are valuable, they do not account for the effect that institutional culture and historical memory had on the F-15 and F-16 programs. This dissertation argues that the culture of the fighter pilot community was based on a constructed memory of World War I fighter combat, idealizing a heroic, romanticized image of “Knights of the Air.” This fighter pilot community attempted to influence the F-15 and F-16 programs to conform to their vision of an idealized past. Furthermore, a smaller group of these pilots, calling themselves the “Fighter Mafia” (and later the “Reformers”) radicalized these ideas, rejecting the Eagle and Falcon as not representative of their ideal vision. Through public and political activism, this group affected the discourse of military technology from the mid-1970s to the present. Drawing on David Nye’s work on the connections between technology and cultural historical narratives and identity, this work will demonstrate that culture and institutional historical memory can be important factors in driving the development of military technology.

Direct lift control for aircraft has been around in the aeronautical industry for decades but is mainly used in commercial aircraft with dedicated direct lift control surfaces. The focus of this thesis is to investigate if direct lift control is feasible for a fighter aircraft, similar to Saab JAS 39 Gripen, without dedicated control surfaces. The modelled system is an aircraft that is inherently unstable and contains nonlinearities both in its aerodynamics and in the form of limited control surface deflection and deflection rates. The dynamics of the aircraft are linearised around a flight case representative of a landing scenario. Direct lift control is then applied to give a more immediate relation from pilot stick input to change in flight path angle while also preserving the pitch attitude. Two different control strategies, linear quadratic control and model predictive control, were chosen for the implementation. Since fighter aircraft are systems with fast dynamics it was important to limit the computational time. This constraint motivated the use of specialised methods to speed up the optimisation of the model predictive controller. Results from simulations in a nonlinear simulation environment supplied by Saab, as well as tests in high-fidelity flight simulation rigs with a pilot, proved that direct lift control is feasible for the investigated fighter aircraft. Sufficient control authority and performance when controlling the flight path angle were observed. Both developed controllers have their own advantages and which strategy is the most suitable depends on what the user prioritises. Pilot workload during landing as well as precision at touch down were deemed similar to conventional control.

The demand for turbofan engine performance development is high in the military industry. However, to develop the engine, it is necessary to predict its performance, and engine testing is both time-consuming and costly. Therefore, simulation is an effective approach to predicting the engine’s performance. During this thesis, a low bypass ratio turbofan engine is created in the simulation tool Simulink to investigate the engine performance throughout different flight conditions and maneuvers. The engine model is constructed for the future fighter aircraft at SAAB Aeronautics. The development of a design point has received particular attention throughout the work. After that, the development of proven methods for estimating engine performance of other parts of the flight envelope, resulting in increased model fidelity and enabling simulations of the same engine type but under different conditions and flight cases. To summarize, the tests of the engine model are successful under various design characteristics, conditions, and flight cases. In addition, simulations of the performance evaluation of fighter aircraft engines have been accomplished.

Vilja är avgörande för politiska beslut om väpnad insats och i Libyen efterfrågades på kort tid stridsflyg i en konflikt som fick ett starkt folkrättsligt mandat. Viljan till insats med stridsflyg, vilket inte använts internationellt sedan 1963, ställdes mot Sveriges tradition av att ställa upp när FN kallar. I uppsatsen analyseras viljan till insats i beslutsprocessen som föregick insatsen, stridsflygets förmåga och interaktionen däremellan. Uppsatsen har en kvalitativ ansats med förklarande ambition där intervjuer utgör en stor del av empirin. Den nationella beslutsprocessen har använts som teoretisk struktur för analysen. Sammanfattning: Resultatet visar att stridsflygets förmåga vid tillfället för beslutsprocessen var särskilt hög. Försvarsmakten uppvisade en stark vilja till insats genom hela beslutprocessen. Den politiska viljan var inledningsvis svag för en insats men växte sig stark inom delar av stridsflygets förmågebredd. Förmågan möjliggjorde tidiga och klara besked från Försvarsmakten vilka var avgörande för den politiska viljan. Tillgängligheten skapade en politisk press på att agera och bidrog till ett högt tempo som, med en stark gemensam vilja som grund, kunde förkorta tiden för beslutsprocessen. / Will is decisive for political decisions for armed intervention and in Libya fighters were on short notice requested in a conflict with a strong international legal mandate. The willingness to send fighters to an operation was challenged by the tradition to contribute when needed from UN.  The purpose is to study the capability and willingness, and the interaction between them, in the decision making process prior to the operation. The paper has a qualitative approach and interviews are a major part of the empirical data. The national decision-making process has been used as a theoretical framework in the research. Abstract: The results show that the capability at the time was high and that the willingness from the Armed Forces was strong throughout the whole decision process. The broad political will was initially weak but grew stronger during the process, but only for participation in a certain role in the operation. The capability enabled the early and clear signals from the Armed Forces that were crucial to the political will. The high availability also created pressure for political action and the high pace of decision making that, with a common will, shortened the process.

This thesis presents an estimator to assist or replace a fighter aircraft’s air datasystem (ADS). The estimator is based on machine learning and LSTM neuralnetworks and uses the statistical correlation between states to estimate the angleof attack, angle of sideslip and Mach number using only the internal sensorsof the aircraft. The model is trained and extensively tested on a fighter jetsimulation model and shows promising results. The methodology and accuracyof the estimator are discussed, together with how a real-world implementationwould work. The estimators presented should act as a proof of concept of thepower of neural networks in state estimation, whilst the report discusses theirstrengths and weaknesses. The estimators can estimate the three targets wellin a vast envelope of altitudes, speeds, winds and manoeuvres. However, thetechnology is quite far from real-world implementation as it lacks transparencybut shows promising potential for future development. / Det här examensarbetet presenterar en estimator för att hjälpa eller ersätta ettstridsflygplans luftdatasystem (ADS). Estimatorn är baserad på maskininlärningoch LSTM neurala nätverk och använder statistisk korrelation mellan tillstånd föratt uppskatta anfallsvinkeln, sidglidningsvinkel och Mach-tal endast med hjälpav flygplanets interna sensorer. Modellen är tränad och utförligt testad på ensimuleringsmodell för stridsflygplan och visar lovande resultat. Estimatornsmetodik och noggrannhet diskuteras, tillsammans med hur en implementeringi verkligheten skulle fungera. De presenterade estimatorerna bör fungera somett “proof of concept” för kraften hos neurala nätverk för tillståndsuppskattning,medan rapporten diskuterar deras styrkor och svagheter. Estimatorerna kanuppskatta de tre tillstånden väl i ett stort spektra av altituder, hastigheter, vindaroch manövrar. Tekniken är dock ganska långt ifrån en verklig implementeringeftersom den saknar transparens, men visar lovande potential för framtidautveckling.

Adaptive control designs using neural networks (NNs) based on dynamic inversion are investigated for aerospace vehicles which are operated at highly nonlinear dynamic regimes. NNs play a key role as the principal element of adaptation to approximately cancel the effect of inversion error, which subsequently improves robustness to parametric uncertainty and unmodeled dynamics in nonlinear regimes. An adaptive control scheme previously named composite model reference adaptive control is further developed so that it can be applied to multi-input multi-output output feedback dynamic inversion. It can have adaptive elements in both the dynamic compensator (linear controller) part and/or in the conventional adaptive controller part, also utilizing state estimation information for NN adaptation. This methodology has more flexibility and thus hopefully greater potential than conventional adaptive designs for adaptive flight control in highly nonlinear flight regimes. The stability of the control system is proved through Lyapunov theorems, and validated with simulations. The control designs in this thesis also include the use of pseudo-control hedging techniques which are introduced to prevent the NNs from attempting to adapt to various actuation nonlinearities such as actuator position and rate saturations. Control allocation is introduced for the case of redundant control effectors including thrust vectoring nozzles. A thorough comparison study of conventional and NN-based adaptive designs for a system under a limit cycle, wing-rock, is included in this research, and the NN-based adaptive control designs demonstrate their performances for two highly maneuverable aerial vehicles, NASA F-15 ACTIVE and FQM-117B unmanned aerial vehicle (UAV), operated under various nonlinearities and uncertainties.

Modern aerospace systems rely heavily on legacy platforms and their derivatives. Historical examples show that after a vehicle design is frozen and delivered to a customer, successive upgrades are often made to fulfill changing requirements. Current practices of adapting to emerging needs with derivative designs, retrofits, and upgrades are often reactive and ad-hoc, resulting in performance and cost penalties. Recent DoD acquisition policies have addressed this problem by establishing a general paradigm for design for lifelong evolution. However, there is a need for a unified, practical design approach that considers the lifetime evolution of an aircraft concept by incorporating future requirements and technologies. This research proposes a systematic approach with which the decision makers can evaluate the value and risk of a new aircraft development program, including potential derivative development opportunities. The proposed Evaluation of Lifelong Vehicle Evolution (EvoLVE) method is a two- or multi-stage representation of the aircraft design process that accommodates initial development phases as well as follow-on phases. One of the key elements of this method is the Stochastic Programming with Recourse (SPR) technique, which accounts for uncertainties associated with future requirements. The remedial approach of SPR in its two distinctive problem-solving steps is well suited to aircraft design problems where derivatives, retrofits, and upgrades have been used to fix designs that were once but no longer optimal. The solution approach of SPR is complemented by the Risk-Averse Strategy Selection (RASS) technique to gauge risk associated with vehicle evolution options. In the absence of a full description of the random space, a scenario-based approach captures the randomness with a few probable scenarios and reveals implications of different future events. Last, an interactive framework for decision-making support allows simultaneous navigation of the current and future design space with a greater degree of freedom. A cantilevered beam design problem was set up and solved using the SPR technique to showcase its application to an engineering design setting. The full EvoLVE method was conducted on a notional multi-role fighter based on the F/A-18 Hornet.

In this thesis, the problem of intercepting highly manoeuvrable threats using seeker-less interceptors that operate in the command guidance mode, is addressed. These systems are more prone to estimation errors than standard seeker-based systems. Several non-linear and optimal estimation and guidance concepts are presented in this thesis for interception of randomly maneuvering targets by seeker-less interceptors. The key contributions of the thesis can be broadly categorized into six groups, namely (i) an optimal selection of bank of lters in interactive multiple model (IMM) scheme to cater to various maneuvers that are expected during the end-game, (ii) an innovative algorithm to reduce chattering phenomenon and formulate effective guidance algorithm based on 'differential game guidance law' (modi ed DGL), (iii) IMM/DGL and IMM/modified DGL based integrated estimation/guidance (IEG) strategy, (iv) sensitivity and robustness analysis of Kalman lters and ne tuning of lters in filter bank using innovation covariance, (v) Performance of tuned IMM/PN, tuned IMM/DGL and tuned IMM/modi ed DGL against various target maneuvers, (vi) Performance comparison with realistic missile model. An innovative generalized state estimation formulation has been proposed in this the-sis for accurately estimating the states of incoming high speed randomly maneuvering targets. The IMM scheme and an optimal selection of lters, to cater to various maneu-vers that are expected during the end-game, is described in detail. The key advantage of this formulation is that it is generic and can capture evasive target maneuver as well as straight moving targets in a uni ed framework without any change of target model and tuning parameters. In this thesis, a game optimal guidance law is described in detail for 2D and 3D engagements. The performance of the differential game based guidance law (DGL) is compared with conventional Proportional Navigation (PN) guidance law, especially for 3D interception scenarios. An innovative chatter removal algorithm is introduced by modifying the differential game based guidance law (modified DGL). In this algorithm, chattering is reduced to the maximum extent possible by introducing a boundary layer around the switching surface and using a continuous control within the boundary layer. The thesis presents performance of the modified DGL algorithm against PN and DGL, through a comparison of miss distances and achieved accelerations. Simulation results are also presented for varying fiight path angle errors. Apart from the guidance logic, two novel ideas have been presented following the evolving "integrated estimation and guidance" philosophy. In the rst approach, an in-tegrated estimation/guidance (IEG) algorithm that integrates IMM estimator with DGL law (IMM/DGL), is proposed for seeker-less interception. In this interception scenario, the target performs an evasive bang-bang maneuver, while the sensor has noisy measure-ments and the interceptor is subject to an acceleration bound. The guidance parameters (i.e., the lateral acceleration commands) are computed with the help of zero e ort miss distance. The thesis presents the performance of the IEG algorithm against combined IMM with PN (IMM/PN), through a comparison of miss distances. In the second ap-proach, a novel modi ed IEG algorithm composed of IMM estimator and modi ed DGL guidance law is introduced to eliminate the chattering phenomenon. Results from both of these integrated approaches are quite promising. Monte Carlo simulation results re-veal that modi ed IEG algorithm achieves better homing performance, even if the target maneuver model is unknown to the estimator. These results and their analysis o er an insight to the interception process and the proposed algorithms. The selection of lter tuning parameters puts forward a major challenge for scien-tists and engineers. Two recently developed metrics, based on innovation covariance, are incorporated for determining the filter tuning parameters. For predicting the proper combination of the lter tuning parameters, the metrics are evaluated for a 3D interception problem. A detailed sensitivity and robustness analysis is carried out for each type of Kalman lters. Optimal and tuned Kalman lters are selected in the IMM con guration to cater to various maneuvers that are expected during the end-game. In the interception scenario examined in this thesis, the target performs various types of maneuvers, while the sensor has noisy measurements and the interceptor is subject to acceleration bound. The tuned IMM serves as a basis for synthesis of e cient lters for tracking maneuvering targets and reducing estimation errors. A numerical study is provided which demonstrates the performance and viability of tuned IMM/modi ed DGL based modi ed IEG strategy. In this thesis, comparison is also performed between tuned IMM/PN, tuned IMM/DGL and tuned IMM/modi ed DGL in integrated estimation/guidance scheme. The results are illustrated by an extensive Monte Carlo simulation study in the presence of estimation errors. Simulation results are also presented for end game maneuvers and varying light path angle errors . Numerical simulations to study the aerodynamic e ects on integrated estimation/ guidance structure and its e ect on performance of guidance laws are presented. A detailed comparison is also performed between tuned IMM/PN, tuned IMM/DGL and tuned IMM/modi ed DGL in integrated estimation/guidance scheme with realistically modelled missile against various target maneuvers. Though the time taken to intercept is higher when a realistic model is considered, the integrated estimation/guidance law still performs better. The miss distance is observed to be similar to the one obtained by considering simpli ed kinematic models.

The working situation of fighter pilots is often very challenging. The pilots are requested to perform their tasks and make decisions in situations characterised by time-pressure, huge amounts of data and high workload, knowing that wrong decisions might result in fatal consequences. To aid the pilots, several automatic support systems have been implemented in modern fighter aircraft and will continue to be implemented in pace with technological advancements and new demands posed on the pilots. For example, innovations within the information fusion (IF) domain have made it possible to fuse large amounts of data, stemming from different sensors, databases etc., to create a better foundation for making decisions and act than would have been possible if the information sources had been used separately. However, there are both positive and negative effects of automation, such as decreased workload and improved situation awareness on the one hand, but skill degradation and complacent behaviour on the other. To avoid the possible negative consequences of automation, while at the same time ameliorating the positive ones, a human-centred automation (HCA) approach to system design has been proposed as a way of optimizing the collaboration between the human and the machine. As a design approach, HCA stresses the importance of a cooperative human-machine relationship, where the operator is kept in the automation loop. However, how to introduce HCA within the fighter aircraft domain as well as its implications for the interface and automation design of support systems within the field has not been investigated. This thesis investigates the implications of introducing HCA into the fighter aircraft domain. Through literature surveys and empirical investigations, general and domain specific HCA guidelines have been identified. These advocate, for example, that an indication of the reliability of the information and the recommendations provided by the different aircraft support systems must be given as well as that support for appropriate updates of the pilots’ individual and team awareness of the situation must be provided. A demonstrator, mirroring some of the identified guidelines, has been implemented and used to evaluate the guidelines together with system developers within the domain. The evaluation indicated that system developers of modern fighter aircraft implicitly incorporate many of the identified HCA guidelines when designing. However, the evaluation further revealed that to explicitly incorporate these guidelines into the development approach, preferably through the development of a domain specific style guide, would aid the system developers design automated support systems that provide appropriate support for the pilots. The results presented in this thesis are expected to aid developers of modern fighter aircraft support systems by incorporating HCA into the traditional simulator-based design (SBD) approach. This approach is frequently used within the field and stresses early and frequent user-involvement when designing, in which complementary HCA evaluations could be performed to further improve the support systems implemented from an automation perspective. Furthermore, it is expected that the results presented in this thesis will contribute to the research regarding how to incorporate the human operator in the information fusion processes, which has been recognised as a research gap within the IF field. Thus, a further contribution of this thesis is the suggestion of how the HCA development approach could be of aid when improving the interaction between the operator and the automated fusion system. / Arbetssituationen för stridspiloter är ofta mycket utmanande. Piloterna måste utföra sina uppgifter och fatta beslut i stressiga situationer med stora informationsmängder och hög arbetsbörda, samtidigt som val av fel beslut kan leda till allvarliga konsekvenser. För att hjälpa piloterna har flera automatiska stödsystem implementerats i moderna stridsflygplan. Denna trend kommer att fortsätta i takt med nya tekniska framgångar och nya krav som ställs på piloterna. Forskning inom informationsfusion (IF) har bland annat gjort det möjligt att fusionera stora mängder data som härstammar från olika sensorer, databaser m.m. för att på så sätt skapa en bättre grund för att fatta beslut och agera än vad som hade varit möjligt om informationskällorna hade använts separat. Dock har både positiva och negativa effekter av automatisering rapporterats, såsom minskad arbetsbörda och förbättrad situationsuppfattning men även försämrad pilotprestation till följd av att de automatiska systemens prestanda inte övervakas. För att undvika negativa effekter av automation samtidigt som de positiva effekterna stärks har den så kallade människo centrerade automationen (HCA) lyfts fram som en möjlig väg att designa system där samverkan mellan automationen och den mänskliga operatören optimeras. Som en designapproach fokuserar HCA på viken av en samverkande människamaskin relation, där operatören hålls kvar i automatiseringsloopen. Men hur HCA kan introduceras inom stridsflygdomänen och dess implikationer för gränssnitts- och automationsdesign av stödsystem inom domänen har inte undersökts. Denna licentiatavhandling undersöker möjliga implikationer av att introducera HCA inom stridsflygdomänen. Genom litteraturundersökningar och empiriska studier har generalla och domänspecifika HCA riktlinjer identifierats, såsom att piloterna måste erbjudas en indikation angående tillförlitligheten hos den information och de rekommendationer som de olika implementerade stödsystemen i flygplanet har genererat, samt att stöd för att uppdatera piloternas individuella och gemensamma uppfattning av situationen måste ges. En demonstrator, som återspeglar några av de identifierade HCA riktlinjerna, har implementerats och använts för att utvärdera riktlinjerna tillsammans med systemutvecklare inom domänen. Denna utvärdering påvisade att systemutvecklare inom stridsflygdomänen implicit använder sig av många av de identifierade HCA riktlinjerna under designprocessen, men att explicit inkludera dessa i en domänspecifik design guide skulle kunna hjälpa dem att designa automatiska system som erbjuder lämpligt stöd för piloterna. De resultat som presenteras i denna licentiatavhandling förväntas kunna hjälpa utvecklare av moderna stridsflygsystem genom att inkludera HCA i den traditionella simulator-baserade designapproachen (SBD). Denna approach används flitigt inom området och fokuserar på tidigt och återkommande användardeltagande vid designarbetet, där komplementära HCA utvärderingar skulle kunna genomföras för att förbättra de stödsystem som implementeras från ett automationsperspektiv. Det förväntas även att de resultat som presenteras i denna avhandling kommer att bidra till forskningen kring hur operatörer kan påverka fusionsprocessen, vilket har identifierats som ett område där mer forskning behövs inom IF området. Ytterligare ett bidrag av denna avhandling är därför det förslag som ges på hur HCA utvecklingsprocessen skulle kunna användas för att förbättra interaktionen mellan operatören och det automatiska fusionssystemet.