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ÖgeNonlinear modeling and sensitivity analysis of}{oleo-pneumatic strut landing gear systems for aircraft landing performance(Graduate School, 2025-06-26)Landing gear systems support the aircraft throughout ground operations and play a crucial role in absorbing impact forces at touchdown. In today`s modern landing gears, mostly oleo-pneumatic struts are utilized as the shock-absorber component in the landing gear system since it has the maximum efficiency to weight ratio and the maximum shock absorber efficiency. The aim of this study is to generate a Simulink-model for the oleo-pneumatic strut landing gear non-linear dynamics and conduct a sensitivity analysis to evaluate the effects of oleo-pneumatic strut landing gear design parameters on the key performance criteria. The vertical motion of a landing gear system is analytically modeled as a double mass-spring-damper system, capturing the dynamics of both the aircraft and the landing gear system as separate masses: upper or sprung mass and lower or unsprung mass, respectively. The forces acting on the upper mass are accounts for the forces transmitted through the shock strut, the normal reaction force component along the strut axis -applicable when the landing gear has an inclination angle (rake angle) - the aerodynamic lift acting upward on the aircraft, and its weight. Similarly, the lower mass experiences the downward restoring forces from the strut and the strut's normal axis, as well as the upward tire-ground reaction force and its own weight. The aircraft touches the ground with its tires that refers the lower mass in the dynamic model and the tire-ground interaction is modeled as one spring. The stiffness characteristics of a tire is usually obtained by experimental data and it can be defined with an exponential curve using tire stiffness related coefficients and the tire diameter. The upper and lower masses are connected to each other with the oleo-pneumatic strut, generally referring to a piston-cylinder assembly and the cylinder. During aircraft landing operation, the oleo strut compresses under load or extends as the load is relieved. It is usually divided into two chambers and these two chambers are connected with an orifice. The upper chamber is typically filled with gas like nitrogen or air and the lower chamber is filled with highly viscous fluids like oil. As the oleo-strut compresses or extends, the damping force in the oleo strut is generated by the flow of viscous fluid into the orifice and the spring force is produced by the compressed air in the upper chamber. The friction forces are also generated by the moving parts in the oleo-pneumatic strut assembly. The proper characterization and constant tracking of the stroke and stroke velocity of the oleo pneumatic shock strut is an essential component of dynamic modeling. This is because these variables influence the strut's response to impact and dictate the force transmission profile that is produced as a result of the impact. The vertical displacement difference between the upper mass and the lower mass is defined as the strut stroke. Similarly, the stroke velocity is the relative vertical velocity of the upper and lower masses. The lift also acts on the aircraft wings because of the horizontal speed during the aircraft landing operation. The lift is usually taken equal to the total weight of the aircraft at touchdown. After touchdown, the lift decreases as the horizontal speed decreases during the landing operation. The reduction of horizontal velocity is represented by a formula that incorporates wheel braking, thrust (or thrust reverser if relevant), and aerodynamic forces: drag and lift. The aerodynamic forces are determined using the aircraft's horizontal velocity, which is derived from the deceleration of that velocity. This introduces additional complexity and non-linearity to the system. By defining constant parameters (e.g., dimensional properties, friction coefficients, tire stiffness) and variable factors (e.g., stroke, stroke velocity), the spring and damping characteristics of the oleo-pneumatic shock strut and tire are established as well as the deceleration in horizontal velocity and consequently decrease in lift force. Considering all the variants on the spring effects, damping characteristics and forces caused by the friction, the vertical motion of a landing gear system demonstrates a complex and non-linear behavior. This study develops a nonlinear Simulink model to simulate the vertical dynamics of an aircraft landing gear system. The model integrates the equations of motion for both the upper and lower masses and is validated by experimental data and reference model from NACA-TN-2755. Two validation cases are taken into account with different touchdown horizontal velocities (a.k.a sink rate) and the higher sink rate results in a tire-bottoming condition, which occurs once tire's vertical deflection goes beyond its allowed deformation limit and this changes the stiffness regime of the tire. The validation reveals a robust correlation between the model and actual behavior, affirming its capacity to effectively simulate landing gear dynamics within the Simulink environment. Comparing with the reference model presented in NACA-TN-2755, the developed simulink model exhibits maximum 6.76\% error difference for the validation case with no tire bottoming and 8.64\% error difference for the validation case with tire bottoming. The maximum error differences mostly pertains to the overall force acting on the aircraft's mass. The created Simulink model demonstrates comparable or superior performance to the reference model for outputs such as aircraft displacement, tire displacement, stroke, and stroke velocity. The primary performance criteria for landing gear design encompass shock absorber efficiency, the overall post touch down duration necessary for the system to achieve stabilization, and the peak vertical force exerted on the aircraft. A One-at-a-Time (OAT) sensitivity analysis is conducted to evaluate the impact of landing gear design parameters on determined performance criteria by utilizing a baseline model derived from the North American T-6 Texan aircraft. To comprehend the relationship between the parameters and the performance requirements, linearity is initially established for each parameter using the coefficient of determination ($R^2$). Sensitivity analysis is conducted solely for the parameters that have a significant correlation with the performance criteria, utilizing the sensitivity index. The simultaneous application of the coefficient of determination and sensitivity index facilitates the assessment of both the existence of a relationship between the design parameter and performance criteria, as well as the magnitude of this relationship. The sensitivity analysis results indicate that the areas of the hydraulic and air chambers have tangible influence on shock absorber efficiency. Hydraulic damping parameters are the primary factors affecting the required total time for the system to be balanced. The maximum vertical force acting on the aircraft mass is predominantly governed by the sink rate but is also strongly affected by the hydraulic damping characteristics. Additionally, air chamber pressure and area moderately impact the maximum vertical force acting on the aircraft mass. On the other hand, factors such as the initial air volume, gas polytropic index, rake angle, and friction coefficients exhibit do not show concrete impact on the chosen performance criteria. Finally, although engine thrust alone does not markedly impact landing gear performance criteria, the deployment of thrust reverser may influence the maximum force exerted on the landing gear strut and alter the load cycle on it during landing. In conclusion, validation of the model using experimental data shows that the generated Simulink model faithfully depicts real-world behavior by means of simulated reactions and sufficiently captures the non-linear dynamics of the landing gear system. Focusing on specified performance criteria: shock absorber efficiency, total stabilization time post-touchdown, and maximum vertical force exerted on the aircraft, this paper offers a comprehensive sensitivity analysis of the design parameters of oleo-pneumatic strut landing gear systems.
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ÖgeThe effect of twist distribution on rotor performance and its applications on rotor blades(Graduate School, 2025-06-26)The long-established industry of aviation has been evolving inexhaustedly throughout the history and the story has to be continued from now on with the new developments. From the fixed-wing aircraft model developed in earlier times in history of the industry, numerous advancements have been investigated. Each investigation had its own reason to improve. Then vertical flight technology came to mind for enabling hovering, vertical takeoff and manoeuvrability. Helicopter rotorcrafts are essential for both military and civilian applications to operate in confined or remote areas. In order to complete missions such as search and rescue and medical evacuation and aerial firefighting, the biggest advantage for helicopters are vertical takeoff, versatility and ability to reach out areas that neither the fixed-wing aircraft nor ground vehicles can go through. The main motivation for improvements in aviation is about lowering the operational cost and maximizing the capacity in every sense. In order to lower the cost, generaly the power consumption of aircraft must be reduced or in a similar way the efficiency must be enhanced. These improvements can occur in structural meaning, in aerodynamic or in propulsive systems. A major contributor to the overall power consumption of rotorcraft is the rotor system, particularly the aerodynamic characteristics of the blades. It is important to say that the aerodynamical efficiency is a key factor to improve a performance for all aircraft types. The aerodynamic loading, lift distribution, and eventually the thrust and power consumption properties of the rotor are directly influenced by the phenomena called twist distribution. A key factor in rotor design is blade twist distribution, which shows the pitch angle variation over the blade span. Understanding and optimising blade twist can help to increase hover efficiency, enhance forward flight performance, and lower overall energy requirements. Twist distribution thus becomes a strategic aerodynamic instrument for performance improvement as well as a geometric issue. In rotorcraft engineering, the importance of customised twist designs grows more as mission profiles get more varied and demanding. This thesis examines the impact of various twist distributions on rotor performance during hover and forward flight using theoretical and computational techniques. Afterwards, It also developes a radical opinion about passive and active twist implementations in contemporary rotorcraft design. The agreeable rotor topologies may be determined by having a solid understanding of twist mechanics and how it affects the power coefficient and induced flow. This work is supported by a solid basis in MATLAB computer modelling and helicopter theory. The results of this study seek not just to increase energy efficiency but also to help create more adaptive, intelligent rotor systems. This thesis hopes to provide significant insights for future rotorcraft innovation by combining classical aerodynamic theory with contemporary technical solutions. In order to understand the idea beehind the performance improvements in rotorcrafts, the theory of helicopter has to be examined deeply. There are three main principles for helicopter motions. First and the most primary is the Momentum Theory, and it basically defines a relation around the actuator disk with the airflow going downwards in order to create a thrust. Momentum Theory treats the rotor as an actuator disc and uses the ideas of mass, momentum, and energy conservation to explore thrust and power generation. Next theory is called Blade Element Theory (BET) and this theory considers the rotating blades individually to derivate the aerodynamic forces such as lift and drag forces. So overcoming the problem, Blade Element Theory divides the blade into tiny radial parts to compute local lift and drag forces depending on relative velocity and angle of attack. By integrating these two methodologies, BEMT provides a more comprehensive and pragmatic framework for assessing the influence of blade twist, especially when the pitch angle is represented as a function of the non-dimensional radial location. Since the theories are now understood, the effect of twist distributions can be demonstrated. Types of distributions are, untwisted (uniform pitch), linearly twisted, quadratically twisted and the ideally twisted blades. Mathematical models of these distributions must be obtained for analyzing the performance values. The major concern is the achieve a homogeneous inflow ratio (λ) distribution in order to reduce losses and increse the rotor efficiency. A key non-dimensional value in rotorcraft aerodynamics, the inflow ratio measures the axial flow velocity across the rotor disc in relation to the rotor tip speed. Fundamental in both momentum theory and blade element momentum theory (BEMT), it is also huge for understanding induced velocity and overall rotor efficiency. Using real rotorcraft attributes especially those from the UH-60A Black Hawk main rotor, MATLAB does calculations. Power coefficients are calculated for various negative twist angles (e.g., 0°, -4°, -8°, -10°, -12°), and the results show that negatively twisted blades significantly lower power requirements during hover. The findings confirm the aerodynamic advantage of include twist, particularly when tailored for specific flight circumstances. The thesis looks at twist effects in forward flight, when the inflow model turns unequal owing to advancing and retreating blades. The twist distribution helps to balance lift and reduce vibratory loads. Simulations conducted under forward flight conditions show that twist improves efficiency and stabilises rotor dynamics. One of the numerous design components that regulate rotor behaviour is the twist ratio, which is the variation in pitch angle along the rotor blade. The ability to passively or actively adjust the twist ratio allows for improved flying qualities in a variety of operating regimes. The goal of the thesis is to comprehend how twist affects performance characteristics in order to lower the power required for rotorcrafts to hover and fly forward. Moreover, the subsequent aim is to discover how the active twist ratio concepts are implemented. Apart from traditional (passive) twist, the paper investigates active twist systems for real-time modification of blade twist during flight. Morphing rotor blades that use smart materials to change blade geometry without requiring mechanical hinges or joints are thereby produced. There are 2 main types of active twist technologis that investigated throughout this paper. First one is called Active Fiber Composites (AFC) basically consists of embedded electrodes and piezoceramic materials and they are capable to be used in high-force applications due to its significant in-plane deformation. Likewise, the other type is called Macro Fiber Composites (MFC) is based on multiple layers (including piezoelectric fibers, kapton film, and epoxy) and provide better energy conversion efficiency and better flexibility than monolithic ceramics. Active Twist Rotor (ATR) systems include these actuators either attached to or incorporated into the composite blade structure. Electrical activation causes them to produce torsional strain, hence dynamically changing the pitch angle of the blade. Dispersed actuation allows for precise twist control over the span. Benefits include reduced noise, vibration damping, and improved aerodynamic loading performance. There are many important factors that affect how well Active Twist Rotor (ATR) systems work. These include the location and orientation of the actuators, the anisotropic behaviour of composite blade materials, the distribution of structural stiffness along the blade, and the dynamic responsiveness of the control system. Of all the actuation technologies that have been looked into, Macro Fibre Composite (MFC)-based solutions stand out as the most promising. This is mostly because they are light, can withstand a lot of wear and tear, and work well with the fiber-reinforced composite materials that are often used to make current rotor blades. MFC actuators are a good choice for advanced rotorcraft applications because they strike a good compromise between mechanical efficiency and structural adaptability, even if ATR systems can be hard to integrate. This study focusses on the aerodynamic importance of blade twist and starts with a detailed theoretical look at how helicopter rotors work. It helps to create and compare mathematical models that show how different twist distributions work in both hover and forward flight modes. The study uses simulation-based assessments to figure out how the UH-60A Black Hawk rotor settings affect performance. In addition to passive configurations, it also looks into morphing blade technologies and evaluates the practicality of active twist mechanisms from both an engineering and a theoretical point of view. The findings contribute valuable insights for aerospace engineers and designers striving to optimize aerodynamic performance and extend operational range in future helicopter platforms. Ultimately, this work bridges classical rotorcraft theory with modern smart material innovations, paving the way for continued advancements in adaptive blade technologies. Focussing particularly on the relevance of blade twist, the paper starts with a thorough theoretical investigation of helicopter rotor aerodynamics. It promotes the evolution and comparison of mathematical models for several twist distributions in both hover and forward flight conditions. Performance impacts are assessed by means of simulations using the UH-60A Black Hawk rotor settings. The paper also offers morphing blade technologies and evaluates the viability of active twist systems from theoretical and technological perspectives. Examining passive and active twist methods concurrently reveals their unique and relative benefits in enhancing rotorcraft performance. The results offer important new information for engineers and designers trying to improve aerodynamic efficiency and increase the flight range of next-generation helicopters. Advancing the subject of rotorcraft morphing systems and enabling future research in flexible blade design, this work combines traditional aerodynamic theory with modern smart material technologies.
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ÖgeApplication of the HEMLAB algorithm to the 7th and 8th AIAA CFD drag prediction workshop cases(ITU Graduate School, 2025)This thesis presents aerodynamic flow analyses conducted using Computational Fluid Dynamics (CFD) methods. The study evaluates the performance of a proprietary node-based finite volume RANS solver, HEMLAB, through cases presented in the 7th and 8th AIAA CFD Drag Prediction Workshops (DPW-7 and DPW-8). In DPW-7, simulations were performed on the NASA Common Research Model (CRM) wing–body configuration under various angles of attack in transonic flow regimes. HEMLAB was coupled with the PyAMG adaptive mesh refinement library, enabling anisotropic refinement in critical flow regions such as shocks and boundary layers, which improved accuracy while reducing computational cost. DPW-8 involved the analysis of both transonic and subsonic cases using ONERA OAT15A and Joukowski airfoils, respectively. Simulations incorporated both structured baseline meshes and adaptively refined grids. Several inviscid flux schemes (Roe, HLLC, and AUSM-plus-up) were tested to evaluate their influence on the accuracy of aerodynamic coefficients. Drag prediction and pressure coefficient distributions were compared against experimental data, and mesh convergence studies were conducted to ensure numerical consistency. The findings demonstrate that HEMLAB produces results consistent with experimental observations. Adaptive meshing enhanced solution resolution and contributed to more accurate drag predictions. These outcomes highlight the solver's capability to handle complex flows effectively and confirm the value of CFD techniques for high-fidelity aerodynamic simulations in industrial applications.
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ÖgeAeroelastic optimization with genetic algorithm(ITU Graduate School, 2025)Aeroelasticity is the field studying the simultaneous and coupled interaction of aerodynamic, elastic, and inertial forces. It has proven to be a critical consideration in aircraft design, as evidenced by numerous early aviation failures that resulted from neglecting these complex interactions. This discipline encompasses both dynamic and static analyses of wing-like structures, addressing phenomena such as flutter, divergence, buffeting, and aileron reversal that emerge from the intricate interplay of these three fundamental force systems. The challenge lies in the highly relationship between design variables and aeroelastic behavior: small parameter changes can dramatically shift the equilibrium between aerodynamic, elastic, and inertial forces, producing disproportionate effects on structural response. Given this complexity, predicting aeroelastic behavior through isolated examination of design variables is impractical, making sophisticated optimization methods indispensable for navigating this coupled multiphysics design space. Optimization, fundamentally, is the process of enhancing an objective function through algorithmic approaches. While various classification schemes exist, the distinction between gradient-based and gradient-free methods is particularly relevant for aeroelastic problems. Gradient-based methods utilize partial derivatives of the objective function to navigate the design space, but their applicability is limited by requirements for differentiability and continuity constraints that can be problematic in complex aeroelastic systems with discontinuous behavior. In contrast, gradient-free methods employ stochastic search strategies, with genetic algorithms being particularly notable for their biological inspiration. These algorithms mirror evolutionary principles, where superior solutions propagate to subsequent generations while inferior ones are eliminated, even incorporating concepts like genetic mutation to explore the solution space. The effectiveness of such optimization frameworks in aeroelastic applications has been validated using standard benchmark functions, including the Ackley and Eggholder functions, demonstrating their capability to handle the complex, multi-modal design spaces characteristic of aeroelastic optimization problems. The practical application of these optimization principles is demonstrated in this study through the aeroelastic optimization of an unswept, zero taper ratio wing using genetic algorithms. The optimization framework considers a comprehensive set of design variables including chord length, wing length, elastic axis location, static unbalance parameter, bending and torsional stiffness, mass distribution properties, and natural frequency characteristics. These variables are strategically categorized as either independent parameters that directly controlled by the algorithm within specified bounds or dependent variables that emerge from the independent choices. While wing length and chord length remain constant, other independent variables such as elastic axis location, static unbalance, and stiffness properties are optimized within prescribed limits. The optimization targets two critical aeroelastic phenomena: flutter and divergence speeds, both representing catastrophic failure modes for wing structures. Flutter analysis employs Galerkin discretization coupled with Theodorsen aerodynamics, identifying the critical speed as the lowest airspeed where structural damping vanishes. Divergence speed is determined through a differential equation governing the torsional degree of freedom. The genetic algorithm optimization framework, validated using benchmark functions such as Ackley and Eggholder functions, selects the lower of these two speeds as the critical constraint. This approach not only determines optimal design configurations but also provides insights into the parametric sensitivity of aeroelastic behavior, revealing how each design variable influences the critical speeds and overall structural performance. The optimization architecture was implemented using the Python programming language. The open-source nature and data structure capabilities make Python suitable for genetic algorithm optimization. The genetic algorithm components were custom-coded rather than relying on built-in or community-provided modules, to maintain full control over the parameters. Flutter speed and divergence speed calculations were implemented as functions to allow repeated use within the algorithm. These functions serve as the objective functions in the genetic algorithm optimization. Parallel processing was utilized for faster generation loops. Although the critical speed calculations within a generation can be distributed across multiple processors, the algorithm waits for all calculations to complete due to the dependency between generations. The impact of genetic algorithm parameters on the optimization process was analyzed. The final optimization was run with tuned parameters. The population size decreased linearly, while the crossover and mutation rates decreased exponentially across generations to improve optimization efficiency. A large population and high mutation and crossover rates enabled the algorithm to explore the broader design variable space and make aggressive changes to search for optimal solutions in the early generations, whereas the reduced population size in later generations shortened runtime, and the lower mutation and crossover rates allowed the algorithm to stabilize without forcing aggressive changes on design variables. The optimization results, stored systematically and visualized through graphs and spreadsheets, demonstrate the genetic algorithm's effectiveness in determining optimal design variables for maximizing aeroelastic stability limits. The framework successfully identifies design configurations that maximize critical speeds while respecting all imposed constraints. This study provides a valuable tool for the preliminary design phase of subsonic aircraft, enabling engineers to optimize wing structures for aeroelastic performance before committing to detailed design work. The methodology presented here can be readily adapted to other wing configurations and extended to incorporate additional design constraints and objectives, making it a versatile framework for aeroelastic optimization in aerospace applications.
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ÖgeQuadrotor actuator fault detection and isolation. a model-based approach(ITU Graduate School, 2025)Unmanned Aerial Vehicles (UAVs), particularly quad-rotors, have gained increasing importance across a wide range of civilian and industrial applications due to their maneuverability, vertical take-off and landing (VTOL) capabilities, and mechanical simplicity. However, their inherently nonlinear, unstable, and underactuated dynamics make them highly sensitive to actuator faults, especially those related to the brushless DC motors that directly influence flight control. Any degradation in motor performance, such as loss of effectiveness (LOE), can compromise flight safety and mission success. This thesis focuses on developing a robust model-based Fault Detection and Isolation (FDI) system capable of detecting and isolating actuator faults in real-time without requiring direct motor feedback. The proposed methodology integrates two complementary observers: a nonlinear Thau observer for estimating system states and detecting deviations in attitude dynamics and a dynamic inversion-based observer that reconstructs actuator behavior from measured accelerations and angular rates. The combination of these two observers enables dual-residual evaluation, enhancing detection sensitivity and fault isolation performance, even under noisy measurements and model uncertainties. The nonlinear Thau observer is designed using the full Newton-Euler dynamic equations of the quadrotor, avoiding linearization and enhancing estimation accuracy across the entire flight envelope. The dynamic inversion observer provides a secondary set of residuals by comparing estimated motor responses with expected motor behavior. An adaptive thresholding technique is implemented to handle noise and varying flight conditions, while a two-stage magnitude estimation mechanism enables both fast detection and reliable steady-state assessment of fault severity. Finally, a fault-tolerant control allocation strategy is incorporated to compensate for the detected faults using the estimated fault magnitudes. The complete system was tested in a simulation environment with a realistic quadrotor model. Various fault scenarios, including sudden and progressive LOE faults affecting different motors, were introduced. The results show that the proposed system effectively detects faults within a range of 0.1 to 0.3 seconds from the moment they occur. The fault magnitude was accurately estimated, enabling the controller to adjust thrust using a fault-tolerant control allocation strategy. This allowed the quadrotor to maintain stability and successfully complete its mission, even with partial motor faults. The simulation results validate that the system is capable of detecting both sudden and gradual Loss of Effectiveness (LOE) faults, estimating their magnitude, and ensuring consistent control performance. Overall, the approach developed in this thesis presents a real-time, lightweight FDI solution that enhances the reliability and safety of quadrotor UAVs, making it suitable for practical application in mission-critical scenarios.