Nonlinear Controller Design for UAVs with Time-Varying Aerodynamic Uncertainties

Unmanned Aerial Vehicles (UAVs) are here and they are here to stay. Unmanned Aviation has expanded significantly in recent years and research and development in the field of navigation and control have advanced beyond expectations. UAVs are currently being used for defense programs around the world but the range of applications is expected to grow in the near future, with civilian applications such as environmental and aerial monitoring, aerial surveillance and homeland security being some representative examples. Conventional and commercially available small-scale UAVs have limited utilization and applicability to executing specific short-duration missions because of limitations in size, payload, power supply and endurance. This fact has already marked the dawn of a new era of more powerful and versatile UAVs (e.g. morphing aircraft), able to perform a variety of missions. This dissertation presents a novel, comprehensive, step-by-step, nonlinear controller design framework for new generation, non-conventional UAVs with time-varying aerodynamic characteristics during flight. Controller design for such UAVs is a challenging task mainly due to uncertain aerodynamic parameters in the UAV mathematical model. This challenge is tackled by using and implementing μ-analysis and additive uncertainty weighting functions. The technique described herein can be generalized and applied to the class of non-conventional UAVs, seeking to address uncertainty challenges regarding the aircraft\u27s aerodynamic coefficients

Morphing Unmanned Aerial Vehicles

The performance and dynamic efficiency of an aircraft are significantly influenced by the aircraft shape and configuration. Therefore, the wing which is an important element in the aircraft load response in terms of drag and lift has been given increasing attention through morphing technology. Several governmental programs and academic research projects on morphing aircraft have investigated methods of efficiently changing the wing geometric characteristics in-flight. The present thesis reviews the current knowledge on wing morphing concepts and investigates the type of methods that can be used to model morphing structures. This review includes the principles of the morphing concept, realization of a morphing structure, aspects of morphing structure design, current methods to model morphing structures, challenges, and the perspectives of the morphing UAVs. It concludes that the wing cover skins must possess a high degree of deformability; but they must be able to maintain their shape and structural integrity under the compression, tension, shear and bending characteristics of aerodynamic and flight loads including the effects of added masses. In order to meet these requirements, thermoplastic elastomers and shape memory polymers are suggested as good candidate materials for smart skins. Nevertheless, an excessively flexible skin is exposed to the hazard of sagging under pressure loads. It is suggested that bio-inspired micro air vehicles based on bat wing structure will gain intensive attention since such a structure prossesses a high flexibility with anisotropy and non-linear elasticity

Multi-agent Collision Avoidance Using Interval Analysis and Symbolic Modelling with its Application to the Novel Polycopter

Coordination is fundamental component of autonomy when a system is defined by multiple mobile agents. For unmanned aerial systems (UAS), challenges originate from their low-level systems, such as their flight dynamics, which are often complex. The thesis begins by examining these low-level dynamics in an analysis of several well known UAS using a novel symbolic component-based framework. It is shown how this approach is used effectively to define key model and performance properties necessary of UAS trajectory control. This is demonstrated initially under the context of linear quadratic regulation (LQR) and model predictive control (MPC) of a quadcopter. The symbolic framework is later extended in the proposal of a novel UAS platform, referred to as the ``Polycopter" for its morphing nature. This dual-tilt axis system has unique authority over is thrust vector, in addition to an ability to actively augment its stability and aerodynamic characteristics. This presents several opportunities in exploitative control design. With an approach to low-level UAS modelling and control proposed, the focus of the thesis shifts to investigate the challenges associated with local trajectory generation for the purpose of multi-agent collision avoidance. This begins with a novel survey of the state-of-the-art geometric approaches with respect to performance, scalability and tolerance to uncertainty. From this survey, the interval avoidance (IA) method is proposed, to incorporate trajectory uncertainty in the geometric derivation of escape trajectories. The method is shown to be more effective in ensuring safe separation in several of the presented conditions, however performance is shown to deteriorate in denser conflicts. Finally, it is shown how by re-framing the IA problem, three dimensional (3D) collision avoidance is achieved. The novel 3D IA method is shown to out perform the original method in three conflict cases by maintaining separation under the effects of uncertainty and in scenarios with multiple obstacles. The performance, scalability and uncertainty tolerance of each presented method is then examined in a set of scenarios resembling typical coordinated UAS operations in an exhaustive Monte-Carlo analysis

Validation of morphine wing methodologies on an unmanned aerial system and a wind tunnel technology demonstrator

To increase the aerodynamic efficiency of aircraft, in order to reduce the fuel consumption, a novel morphing wing concept has been developed. It consists in replacing a part of the wing upper and lower surfaces with a flexible skin whose shape can be modified using an actuation system placed inside the wing structure. Numerical studies in two and three dimensions were performed in order to determine the gains the morphing system achieves for the case of an Unmanned Aerial System and for a morphing technology demonstrator based on the wing tip of a transport aircraft. To obtain the optimal wing skin shapes in function of the flight condition, different global optimization algorithms were implemented, such as the Genetic Algorithm and the Artificial Bee Colony Algorithm. To reduce calculation times, a hybrid method was created by coupling the population-based algorithm with a fast, gradient-based local search method. Validations were performed with commercial state-of-the-art optimization tools and demonstrated the efficiency of the proposed methods. For accurately determining the aerodynamic characteristics of the morphing wing, two new methods were developed, a nonlinear lifting line method and a nonlinear vortex lattice method. Both use strip analysis of the span-wise wing section to account for the airfoil shape modifications induced by the flexible skin, and can provide accurate results for the wing drag coefficient. The methods do not require the generation of a complex mesh around the wing and are suitable for coupling with optimization algorithms due to the computational time several orders of magnitude smaller than traditional three-dimensional Computational Fluid Dynamics methods. Two-dimensional and three-dimensional optimizations of the Unmanned Aerial System wing equipped with the morphing skin were performed, with the objective of improving its performances for an extended range of flight conditions. The chordwise positions of the internal actuators, the spanwise number of actuation stations as well as the displacement limits were established. The performance improvements obtained and the limitations of the morphing wing concept were studied. To verify the optimization results, high-fidelity Computational Fluid Dynamics simulations were also performed, giving very accurate indications of the obtained gains. For the morphing model based on an aircraft wing tip, the skin shapes were optimized in order to control laminar flow on the upper surface. An automated structured mesh generation procedure was developed and implemented. To accurately capture the shape of the skin, a precision scanning procedure was done and its results were included in the numerical model. High-fidelity simulations were performed to determine the upper surface transition region and the numerical results were validated using experimental wind tunnel data

Mathematical modelling, flight control system design and air flow control investigation for low speed UAVs

The demand for unmanned aerial vehicles (UAVs) has increased dramatically in the last decade from reconnaissance missions to attack roles. As their missions become more complex, advances in endurance and manoeuvrability become crucial. Due to the advances in material fabrication, wing morphing can be seen as an ideal solution for UAVs to provide improvements by overcoming the weight drawback. This thesis investigates the area of aircraft design and simulation for low speed UAVs looking at performance enhancements techniques for low speed UAVs, and their effects on the aerodynamic capabilities of the wing. The focus is on both suitable control design and wing morphing techniques based on current research findings. The low speed UAV X-RAE1 is used as the test bed for this investigation and is initially analytically presented as three dimensional body where the equations relate to the forces and moments acting on the UAV. A linearised model for straight flight at different velocities is implemented and validated against a non-linear model. Simulations showed the X-RAE1 to have acceptable stability properties over the design operating range. Control design techniques, linear quadratic regulators (LQR) and H-infinity optimisation with Loop Shaping Design Procedure (LSDP), are used to design simple control schemes for linearised longitudinal model of the X-RAE1 UAV at different velocities. The effectiveness and limitations of the two design methods show that both designs are very fast, with settling times 2-3 seconds in the height response and remarkably low variation of the results at different velocities. Computational fluid dynamics is then used to investigate and simulate the impact of introducing smart effector arrays on a UAV. The smart effector array produces a form of active flow control by providing localised flow field changes. These induced changes have direct impact on the aerodynamic forces and showed a substantial increase of lift at low angles of attack. There was also a significant increase to the lift to drag ratio at high angles of attack which resulted to a delay in stall

Integrated multi-functional morphing aircraft technologies

In the past years, the development of morphing wing technologies has received a great deal of interest from the scientific community. These technologies potentially enable an increase in aircraft efficiency by changing the wing shape, thus allowing the aircraft to fly near its optimal performance point at different flight conditions. This thesis explores the development, analysis, building and integration of two new functional Variable-Span Wing (VSW) concepts to be applied in Remotely Piloted Aircraft Systems (RPAS). Additional studies are performed to synthesize the mass of such morphing concepts and to develop mass prediction models. The VSW concept is composed of one fixed rectangular inboard part, inboard fixed wing (IFW), and a moving rectangular outboard part: outboard moving wing (OMW). An aerodynamic shape optimization code is used to solve a drag minimization problem to determine the optimal values of wingspan for various speeds of the vehicle’s flight envelope. It was concluded that, at low speeds, the original wing has slightly better performance than the VSW and for speeds higher than 25 m/s the opposite occurs, due to the reduction in wing area and consequently the total wing drag. A structural Finite Element Model (FEM) of the VSW is developed, where the interface between wing parts is modelled. Deflections and stresses resulting from static aerodynamic loading conditions showed that the wing is suitable for flight. Flutter critical speed is studied. FEM is used to compute the VSW mode shapes and frequencies of free vibration, considering a rigid or the real flexible interface, showing that the effect of rigidity loss in the interface between the IFW and the OMW, has a negative impact on the critical flutter speed. A full-scale prototype is built using composite materials and an electro-mechanical actuation system is developed using a rack and pinion driven by two servomotors. Bench tests, performed to evaluate the wing and its actuation mechanism under load, showed that the system can perform the required extension/retraction cycles and is suitable to be installed on a RPAS airframe, which has been modified and instrumented to serve as test bed for evaluating the prototype in-flight. Two sets of flight tests are performed: aerodynamic and energy characterization. The former aims at determining the lift-to-drag ratio for different airspeeds and the latter to measure the propulsive and manoeuvring energy when performing a prescribed mission. In the aerodynamic testing, in-flight evaluation of the RPAS fitted with the VSW demonstrates full flight capability and shows improvements produced by the VSW over a conventional fixed wing for speeds above 19 m/s. At low speeds, the original wing has slightly better lift-to-drag ratio than the VSW. Contrarily, at 30 m/s, the VSW in minimum span configuration is 35% better than the original fixed wing. In the other performed test, it is concluded that the VSW fitted RPAS has less overall energy consumption despite the increased vehicle weight. The energy reduction occurs only in the high speed condition but it is so marked that it offsets the increase in energy during takeoff, climb and loiter phases. Following the work on the first VSW prototype, a new telescopic wing that allows the integration of other morphing strategies is developed, within the CHANGE EU project. The wing adopted span change, leading and trailing edge camber changes. A modular design philosophy, based on a wing-box like structure, is implemented, such that the individual systems can be separately developed and then integrated. The structure is sized for strength and stiffness using FEM, based on flight loads derived from the mission requirements. A partial span, fullsized cross-section prototype is built to validate the structural performance and the actuation mechanism capability and durability. The wing is built using composite materials and an electromechanical actuation system with an oil filled nylon rack and pinion is developed to actuate it. The structural static testing shows similar trends when compared with numerical predictions. The actuation mechanism is characterized in terms of actuation speed and specific energy consumption and it was concluded that it functioned within its designed specifications. A full-scale prototype is later built by the consortium and the leading and trailing edge concepts from the different partners integrated in a single wing. Wind tunnel tests confirmed that the wing can withstand the aerodynamic loading. Flight tests are performed by TEKEVER, showing that the modular concept works reliably. From the previous works, it is inferred that morphing concepts are promising and feasible methodologies but present an undesired mass increase due to their inherent complexity. On the other hand, mass prediction methods to aid the design of morphing wings at the conceptual design phase are rare. Therefore, a mass model of a VSW with a trailing edge device is proposed. The structural mass prediction is based on a parametric study. A minimum mass optimization problem with stiffness and strength constraints is implemented and solved, being the design variables structural thicknesses and widths, using a parametric FEM of the wing. The study is done for a conventional fixed wing and the VSW, which are then combined to ascertain the VSW mass increment, i.e., the mass penalization of the adopted morphing concept. Polynomials are found to produce good approximations of the wing mass. Additionally, the effects of various VSW design parameters in the structural mass are discussed. On one hand, it was found that the span and chord have the highest impact in the wing mass. On the other hand, the VSW to fixed wing ratio proved that the influence of span variation ratio in the wing mass is not trivial. It is found that the mass increase does not grow proportionally with span variation ratio increase and that for each combination of span and chord, exists a span variation ratio that minimizes the mass penalty. Using the VSW to fixed wing ratio function, the mass model is derived. To ascertain its accuracy, a case study is performed, which demonstrated prediction errors below 10%. Although the mass model results are encouraging, more case studies are necessary to prove its applicability over a wide range of VSWs. The work performed successfully demonstrated that VSW concepts can achieve considerable geometry changes which, in turn, translate into considerable aerodynamic gains, despite the increased weight. They influence all aspects of the wing design, from the structural side to the actuation mechanisms. The parametric study summarizes the mass penalties of such concepts, being successful at demonstrating that the mass penalty is not straightforward and that a careful selection of span, chord and variable-span ratio can minimize the mass increase.Nos últimos anos, o desenvolvimento de asas adaptativas tem sido alvo de um grande interesse por parte da comunidade científica. Nesta tese explora-se o desenvolvimento, análise, construção e integração de dois novos conceitos de Asas de Envergadura Variável (VSWs) funcionais a serem aplicados em Sistemas de Aeronaves Pilotadas Remotamente (RPASs). Estudos adicionais são levados a cabo para sintetizar a massa desses conceitos e desenvolver modelos de previsão de massa. O conceito da VSW é constituído por uma parte interna retangular fixa, Asa Fixa Interna (IFW), e por uma parte externa retangular móvel, Asa Móvel Externa (OMW). Um código de otimização aerodinâmica é utilizado para minimizar a resistência ao avanço, determinando os valores ótimos de envergadura para várias velocidades de voo do veículo. Concluiu-se que, a baixas velocidades, a asa original apresenta um desempenho ligeiramente melhor que a VSW, enquanto que a velocidades superiores a 25 m/s, a VSW apresenta um desempenho melhor devido à redução da área das asas e, consequentemente, à redução da resistência total das asas. Para levar a cabo um estudo estrutural, foi desenvolvido um Modelo de Elementos Finitos (FEM) estrutural da VSW, no qual se modelou a interface entre a IFW/OMW. As deflexões e tensões resultantes dos carregamentos aerodinâmicos estáticos mostraram que a asa é capaz de suportar as cargas em voo. A velocidade de flutter é também investigada, sendo o FEM utilizado para calcular as formas dos modos de vibração da VSW e respetivas frequências de vibração livre. Considerou-se uma interface colada ou flexível, confirmando-se que o efeito da perda de rigidez na interface IFW/OMW, tem um impacto negativo sobre a velocidade de flutter. Um protótipo da VSW é construído, utilizando materiais compósitos, e um sistema de atuação eletromecânico é desenvolvido usando um sistema de pinhão e cremalheira movido por dois servomotores. Os testes de bancada, realizados para avaliar a asa e o mecanismo de atuação, mostraram que o sistema é capaz de realizar a extensão/retração da asa, sendo adequado para ser instalado num RPAS. Este RPAS foi modificado e instrumentado para servir de banco de ensaio para avaliação do protótipo em voo. São realizados dois conjuntos de testes de voo: caracterização aerodinâmica e energética. O primeiro incide na determinação da razão de planeio para diferentes velocidades e o segundo é levado a cabo para determinar a energia propulsiva e de manobra ao executar uma missão típica. Nos testes aerodinâmicos ficou comprovado que o RPAS equipado com a VSW é capaz de uma normal operação e ainda que mostra melhorias sobre uma asa fixa convencional para velocidades acima de 19 m/s. A velocidades mais reduzidas, a asa original tem um desempenho ligeiramente melhor do que a VSW. Por outro lado, a 30 m/s, a VSW na configuração de envergadura mínima é 35% melhor do que a asa fixa original. No outro ensaio realizado, conclui-se que o RPAS de envergadura variável tem menos consumo de energia global, apesar do aumento de peso do veículo. A redução de energia ocorre apenas na fase de cruzeiro de alta velocidade, mas foi tão acentuada que compensou o aumento da energia durante as fases de descolagem, subida e espera. Na sequência do trabalho anterior e no âmbito do projeto europeu CHANGE, é desenvolvida uma nova VSW que permite a integração de outras estratégias adaptativas. A nova asa adotou a mudança de envergadura, e a mudança de curvatura nos bordos de ataque e de fuga. Esta adotou uma filosofia de projeto modular, baseada numa caixa de torção, permitindo o desenvolvimento das diferentes tecnologias adaptativas separadamente. A estrutura é divmensionada para resistência e rigidez usando FEM, com base em cargas de voo derivadas dos requisitos da missão. Um primeiro protótipo é construído para validar o desempenho estrutural e a funcionalidade do mecanismo de atuação. A asa é construída usando materiais compósitos e utiliza um sistema de pinhão e cremalheira e um servomotor, para variar a envergadura. Testes estruturais estáticos mostram que as deflexões corroboram as previsões numéricas. O mecanismo de atuação é caracterizado em termos de velocidade de atuação e consumo de energia específica, concluindo-se que funciona dentro do previsto. O segundo protótipo é construído pelo consórcio e os conceitos de bordo de ataque e de fuga são integrados. Testes em túnel de vento confirmaram que a asa suporta o carregamento aerodinâmico. Os testes de voo, realizados pela TEKEVER, mostram que o conceito modular funciona de forma fiável. Baseado nos trabalhos anteriores, conclui-se que os conceitos adaptativos são promissores e viáveis, mas apresentam um aumento de massa indesejável devido à sua inerente complexidade. Por outro lado, os métodos de previsão de massa para auxiliar o projeto de asas adaptativas na fase de projeto conceitual são raros. Deste modo, um modelo de massa da VSW com um dispositivo de borda de fuga é proposto. A previsão de massa estrutural é baseada num estudo paramétrico. Um problema de minimização de massa com constrangimentos de rigidez e resistência é implementado e resolvido, sendo as variáveis de projeto espessuras e larguras estruturais. Para o levar a cabo, um FEM paramétrico da VSW é desenvolvido. O estudo é feito para uma asa fixa convencional e para a VSW, os quais são combinados para determinar o incremento de massa da VSW. Aproximações polinomiais das massas da asa são produzidas, mostrando serem capazes de produzir uma adequada representação. Adicionalmente, são discutidos os efeitos dos vários parâmetros de design da VSW na massa estrutural. Por um lado, verificou-se que a envergadura e a corda têm o maior impacto na massa da asa. Por outro lado, a razão de massas da VSW e da asa fixa provou que a influência da razão de variação de envergadura na massa das asas não é trivial. Verifica-se que o aumento de massa não cresce proporcionalmente com o aumento da razão de variação de envergadura e que para um dado conjunto de envergadura e corda existe uma razão de variação de envergadura que minimiza o aumento de massa. O modelo de massa é derivado usando a aproximação polinomial da razão da VSW com a asa fixa. Para verificar a precisão do modelo, é realizado um caso de estudo que demonstrou erros de previsão abaixo dos 10%. Embora os resultados do modelo de massa sejam encorajadores, mais casos de estudo são necessários para provar a sua aplicabilidade a uma ampla gama de VSW. O trabalho realizado demonstrou com sucesso que os conceitos de VSW podem alcançar consideráveis mudanças de geometria, que se traduzem em ganhos aerodinâmicos consideráveis, apesar do aumento de peso. Estes influenciam todos os aspetos do projeto da asa, desde a parte estrutural até aos mecanismos de atuação. O estudo paramétrico tentou resumir a penalização de massa de tais conceitos, sendo bem sucedido em demonstrar que esta penalização não é simples e que uma seleção cuidadosa de envergadura, corda e razão de variação de envergadura pode minimizar o aumento de peso.This thesis and the associated research was partially funded by the European Community’s Seventh Framework Programme (FP7) under the Grant Agreement 314139

3D-in-2D Displays for ATC.

This paper reports on the efforts and accomplishments of the 3D-in-2D Displays for ATC project at the end of Year 1. We describe the invention of 10 novel 3D/2D visualisations that were mostly implemented in the Augmented Reality ARToolkit. These prototype implementations of visualisation and interaction elements can be viewed on the accompanying video. We have identified six candidate design concepts which we will further research and develop. These designs correspond with the early feasibility studies stage of maturity as defined by the NASA Technology Readiness Level framework. We developed the Combination Display Framework from a review of the literature, and used it for analysing display designs in terms of display technique used and how they are combined. The insights we gained from this framework then guided our inventions and the human-centered innovation process we use to iteratively invent. Our designs are based on an understanding of user work practices. We also developed a simple ATC simulator that we used for rapid experimentation and evaluation of design ideas. We expect that if this project continues, the effort in Year 2 and 3 will be focus on maturing the concepts and employment in a operational laboratory settings

Advanced Air Vehicles Program (AAVP): Advanced Air Transport Technology Project (AATT)

No abstract availabl

Air Force Institute of Technology Research Report 2013

This report summarizes the research activities of the Air Force Institute of Technology’s Graduate School of Engineering and Management. It describes research interests and faculty expertise; lists student theses/dissertations; identifies research sponsors and contributions; and outlines the procedures for contacting the school. Included in the report are: faculty publications, conference presentations, consultations, and funded research projects. Research was conducted in the areas of Aeronautical and Astronautical Engineering, Electrical Engineering and Electro-Optics, Computer Engineering and Computer Science, Systems Engineering and Management, Operational Sciences, Mathematics, Statistics and Engineering Physics

Aeronautics and Space Report of the President - Fiscal Year 2008 Activities

The National Aeronautics and Space Act of 1958 directed the annual Aeronautics and Space Report to include a "comprehensive description of the programmed activities and the accomplishments of all agencies of the United States in the field of aeronautics and space activities during the preceding calendar year." In recent years, the reports have been prepared on a fiscal-year basis, consistent with the budgetary period now used in programs of the Federal Government. This year's report covers activities that took place from October 1, 2007, through September 30, 2008