Aircraft Structures Research Articles

Research on the Flight Performance of Biomimetic Moth Based on Flapping Function Control

Flapping flight is an important mode of insect flight, and its unique flapping motion pattern enables it to fly efficiently in complex environments. This paper takes a biomimetic moth flapping-wing aircraft as the research object and proposes a periodic function composed of two sine functions with different frequencies as the flapping function. This paper explores the effect of this flapping function on the flight performance of flapping-wing aircraft and verifies whether it can be applied to the flight control of flapping-wing aircraft. Firstly, through the study of biomimetic mechanisms, the basic structure of the flapping-wing aircraft is roughly designed; then, the flapping motion is simplified, a rigid wing flapping motion model is established, and the key parameters affecting the average lift are determined. Next, a virtual wind tunnel simulation platform is built, and the key parameters of the flapping function that affect lift generation are simulated and calculated. Finally, an experimental prototype of a biomimetic moth flapping-wing aircraft is designed and manufactured. Through flight experiments, the effects of flapping amplitude, flapping frequency, and mid-position angle in the flapping function on the flight performance of the biomimetic flapping-wing aircraft are verified. The key control parameters are clarified, the control strategy of the flapping-wing aircraft is optimized, and the maneuverability and controllability of the aircraft are improved, providing a theoretical basis and practical support for the development of control methods for biomimetic flapping-wing aircraft.

Friction Stir Welding in Aircraft Structures: Current and Future Prospects

Friction stir welding (FSW) has significant potential to enhance the efficiency of air transportation systems; however, determining and applying its parameters continues to pose challenges. This article review explores current and future challenges, trends, and practices that facilitate the integration of competitive production into new aircraft structure joints. An overview of friction stir welding techniques used for the design and manufacture of aircraft structures are presented, focusing on product optimization through lightweight and high-strength properties. The implementation of various design solutions and advanced manufacturing technologies can enhance metallic fuselage systems’ performance without compromising risk and cost. The paper presents examples of successfully addressed challenges and those driving the development of new practices, design solutions, methods, and tools. Ultimately, aircraft structure joints become more integrated when systems engineering approaches are linked with various design solutions and advanced manufacturing technologies (e.g., Vertical Compensation Friction Stir Welding) in a reusable manner, balancing product performance and producibility. Nevertheless, the field remains in active development, posing challenges for research, development, and practice.

Improvement of the Impact Resistance of Epoxy Prepregs Through the Incorporation of Polyamide Nonwoven Fabric

The impact of introducing a nonwoven polyamide PA 12-E material on the mechanical properties of polymer composite materials based on epoxy autoclave prepreg T107 has been investigated. This study demonstrates that the incorporation of nonwoven fabric does not lead to a decrease in the mechanical properties of the composites. A significant advantage of composites reinforced with nonwoven fabric is their enhanced impact resistance. During a free impact with an energy of 6.67 J per 1 mm of the sample, complete breakdown with fiber destruction occurs in samples without nonwoven material. In contrast, samples containing nonwoven material exhibit damage characterized by stratification without compromising the fibers. The compressive strength after impact increased from 260 to 320 MPa with the addition of nonwoven material. Consequently, the proposed modification of the commercial prepreg will expand the material’s range of applications and enhance safety, particularly in aircraft structures.

Study the effect of material type on aircraft structures

Study the effect of material type on aircraft structures

Towards the validation of manufacturing simulations by means of digital twins: conception, implementation and data acquisition for a composite aircraft moveable manufacturing process

Abstract Simulations are used in the development of structures. These methods and their results are increasingly applied on the way toward a more analysis-based certification process for aircraft structures. The absolute basis for being considered as an acceptable means of compliance (AMC) with regard to a certification paragraph is the validation of these methods and their respective results using physical test data. This contribution describes the approach of generating this validation data and its provision by means of a digital twin (Digital twin in Definition & value. Technical report, AIAA & AIA, 12 (2020). https://www.aiaa.org/docs/default-source/uploadedfiles/issues-and-advocacy/policy-papers/digital-twin-institute-position-paper-(december-2020).pdf, 2020) of a manufacturing process. The work is performed for the virtual production of the initial virtual product house (VPH) use case, a composite multispar moveable of a long-range reference aircraft configuration. It includes the definition of the desired validation data, modeling of the manufacturing process by means of an extended formalized process description (FPD) based on Verein Deutscher Ingenieure e.V. (VDI/VDE-Richtlinie 3682-Part 1: Formalised process descriptions-concept and graphic representation. Standard. https://www.beuth.de/de/technische-regel/vdi-vde-3682-blatt-1/230173798, 2015), the choice of required sensors as well as their integration into an existing tooling as well as the structure itself. Furthermore, the realization and data acquisition during the manufacturing process as well as the creation of a digital twin using a data model based on the formalised process are described. Finally, first results for the evaluation of the use case structure are presented. It is shown that is possible to provide a digital twin based on measured sensor data and provide pre-processing capabilities for this raw data to facilitate the validation of manufacturing process simulations. The creation of this digital twin and the overall realization of the manufacturing is greatly facilitated by the use of FPD and a derived digital life data sheet (LDS).

Proof of concept of a nonlinear structural stability constraint for aeroelastic optimisation

Abstract In the past few decades, substantial work has been directed towards the design of aircraft structures that maximise fuel efficiency, improve performance and curtail emissions. Aeroelastic optimisation offers an effective way to devise lightweight and fuel efficient structures, with structural stability constraints often driving the design. To date, the aeroelastic optimisation community has relied mostly on linear buckling predictions for the evaluation of structural stability constraints, mainly because of their conservativeness, computational efficiency and simplicity of implementation. This approach typically leads to overly conservative buckling margins, and this over-conservativeness places a glass ceiling over the load carrying capacity of wing structures, consequently restricting the exploration of regions within the design space where considerable weight savings could be achieved. By contrast to previous works that predominantly rely on linear buckling constraints, the present paper introduces a method to incorporate nonlinear structural stability analysis into aeroelastic optimisations of wingbox-like structures. The method relies on the evaluation of the positive-definiteness of the tangent stiffness matrix, which is an indicator of structural stability. The sign of the stiffness eigenvalues is monitored while tracing the load-displacement equilibrium paths by means of the arc-length method, thus pinpointing the onset of instability. The proposed constraint is tested in a proof of concept structural optimisation of an idealised version of the CRM wingbox. This optimisation shows a $10.9{\rm{\% }} $ reduction in mass with respect to a baseline design that is optimal with a linear buckling approach, promising great potential for application to more realistic aeroelastic optimisations.

Study of Fluid Flow Characteristics and Mechanical Properties of Aviation Fuel-Welded Pipelines via the Fluid–Solid Coupling Method

The welded pipeline structure of aircraft fuel is a complex and diverse entity, significantly influenced by fluid–solid coupling. The refined aviation fuel-welded pipeline model plays a pivotal role in the investigation of its fluid–solid coupling mechanical properties. However, the mechanical analyses of pipelines with welded structures frequently simplify or ignore the influence of the weld zone (WZ). Consequently, these analyses fail to reveal the complex interactions between different weld zones in detail. In this study, a comprehensive and precise fuel-welded pipeline refinement model is developed through the acquisition of microstructural dimensions and mechanical parameters of the weld zone via metallographic inspection and microtensile testing. Additionally, the influence of clamps and brackets under airborne conditions is fully considered. Furthermore, the numerical simulation results are compared and verified using modal and random vibration tests. This paper addresses the impact of diverse fluid characteristics on the velocity field, pressure field, and stress in disparate areas, and it also conducts an investigation into the random vibration characteristics of the pipeline. The results demonstrate that the fluid pressure and velocity exert a considerable influence on the fluid flow state and structural stress distribution within the pipeline. An increase in flow velocity and alteration to the pipeline geometry will result in a change to the local velocity distribution, which in turn affects the distribution of the fluid pressure field. The highest stresses are observed in the weld zone, particularly at the junction between the weld zone and the heat-affected zone (HAZ). In contrast, the stresses in the bend region exhibit a corrugated distribution in both the axial and circumferential directions. An increase in fluid pressure has a significant impact on the natural frequency of the pipeline. This study enhances our comprehension of the mechanical properties of aircraft fuel lines with fluid–solid coupling and provides a foundation and guidance for the optimal design of fuel-welded lines.

Analysis of Structural Characteristics of the HALE Joined‐Wing Configuration UAV

This study focuses on a high‐altitude long‐endurance (HALE) joined‐wing configuration UAV and completes a preliminary structural design based on aircraft design indices and the fundamental principles of aircraft structure design. Based on the preliminary structural design scheme and aerodynamic shape, a structural finite element model and a vortex lattice aerodynamic analysis model of the UAV are established, and the aerodynamic loads in typical states are obtained. The aerodynamic loads of the UAV are applied to a finite element model using the multipoint row method, and the structural static characteristics are obtained. The study further explores typical structural dynamic characteristics, including natural mode, static aeroelastic, flutter, and gust response characteristics. A comparison with the structural characteristics of a conventional configuration UAV with the same structural weight reveals that the HALE joined‐wing configuration UAV exhibits significant advantages in structural stiffness, mode characteristics, flutter characteristics, and static aeroelastic characteristics, whereas the gust response characteristics are essentially equivalent to those of the conventional configuration. This superiority can be attributed to the reduced wingspan while providing the same lift and effective support effect of the rear wing on the front (Frt) wings, which enhances the structural stiffness of high‐aspect‐ratio HALE UAVs. This configuration is particularly suitable for deployment in high‐subsonic HALE configurations that require close coordination with conventional tactical aircraft. By comparing with the structural characteristics of the conventional configuration UAV with equal structural weight, the paper comprehensively analyzes the structural characteristics of the HALE joined‐wing configuration and obtains the advantages and disadvantages of applying the joined‐wing configuration in HALE aircraft and defines the design direction, which lays a solid foundation for the engineering application in the next stage.

State of the art in mechanical vibration analysis for UAV´s

This article focuses on the study of the state of the art of theoretical and experimental investigations that carried out vibration analysis in an unmanned aerial vehicle (UAV). According to the review of literature, modal analysis is fundamental to determine the dynamic behaviour of the structure since it studies the nature of the vibrations and the behaviour of the UAV structure under different excitations. One unwanted vibration is flutter, which is a phenomenon that causes structural failure, performance degradation and in the worst case, partial or complete loss of the aircraft structure. The fundamental tool for the analysis of vibrations including flutter is the Fast Fourier Transform. As a result of the analysis of the studies reported in this work, some important points to be considered when implementing modal analysis in UAVs are listed.

Fatigue Experiment and Failure Mechanism Analysis of Aircraft Titanium Alloy Wing-Body Connection Joint.

Taking the titanium alloy wing-body connection joint at the rear beam of a certain type of aircraft as the research object, this study analyzed the failure mechanism and verified the structural safety of the wing-body connection joint under actual flight loads. Firstly, this study verified the validity of the loading system and the measuring system in the test system through the pre-test, and the repeatability of the test was analyzed for error to ensure the accuracy of the experimental data. Then, the test piece was subjected to 400,000 random load tests of flight takeoffs and landings, 100,000 Class A load tests, and ground-air-ground load tests, and the test piece fractured under the ground-air-ground load tests. Lastly, the mechanism analysis and structural safety verification of the fatigue fracture of the joints were carried out by using a stereo microscope and scanning electron microscope. The results show that fretting fatigue is the main driving force for crack initiation, and the crack shows significant fatigue damage characteristics in the stable growth stage and follows Paris' law. Entering the final fracture region, the joint mainly experienced ductile fracture, with typical plastic deformation features such as dimples and tear ridges before fracture. The fatigue crack growth behavior of the joint was quantitatively analyzed using Paris' law, and the calculated crack growth period life was 207,374 loadings. This result proves that the crack initiation life accounts for 95.19% of the full life cycle, which is much higher than the design requirement of 400,000 landings and takeoffs, indicating that the structural design of this test piece is on the conservative side and meets the requirements of aircraft operational safety. This research is of great significance in improving the safety and reliability of aircraft structures.

Numerical investigation on energy absorption characteristics and damage modes of biomimetic thin-walled structures in aircraft

The crashworthiness of aircraft is essential for both airlines and passengers. Creating lightweight protective structures that achieve a balance between weight and impact resistance while also providing excellent cushioning and energy absorption presents a significant challenge for aircraft designers. The desert beetle's exoskeleton demonstrates remarkable strength and toughness, effectively shielding its body from external threats. This study examines the desert beetle's exoskeleton by developing bio-inspired thin-walled models with three distinct bottom structures based on its cross-section. Numerical simulations were used for calculations and model optimization. The research systematically investigates how various design parameters, such as impact angle, number of support plates, and wall thickness, affect energy absorption performance. The findings indicate that increasing the inclination angle notably decreases the energy absorption capacity of the protective structure, while adding more partitions can significantly enhance this capacity. Additionally, changes in wall thickness directly influence the structure's compressive performance. Furthermore, a comparative analysis of the optimized structure's energy absorption characteristics under various conditions is provided. Ultimately, this paper highlights the significance of design optimization for protective structures in micro aerial vehicles and outlines future research directions to better balance performance and practicality.

Mechanisms of corrosive freeze-thaw damage in AA7075 using time-resolved x-ray microtomography and correlative electron microscopy

Aluminum aircraft structures experience severe corrosion from exposure to aggressive chloride environments, including cyclic freezing and thawing of residual water during ascent and descent, introducing a cyclic freeze-thaw component to the corrosion process. While corrosion mechanisms in aircraft structures are well studied at constant temperatures, the microstructural and mechanistic behavior under freeze-and-thaw conditions remains unclear. To understand transformations induced by cyclic temperature, we used three-dimensional (3D) x-ray computed tomography (XCT) with scanning electron microscopy (SEM) to study the behavior of AA7075-T651 in a simulated seawater environment undergoing freezing and thawing cycles. Rods immersed in saltwater were thermally cycled above and below freezing, and structural changes were intermittently characterized in 3D. Under freeze-thaw conditions, cracks initiated within corrosion pits through ice expansion, causing progressive crevice growth and spalling along inclusions and grain boundaries with intermediate misorientation angles. Damage mechanisms in freeze-thaw and conventional corrosion environments are compared, with correlations to microstructural evolution.

Fatigue Life of Pin‐Loaded Lugs: Friction and Clearance Effects

ABSTRACTLugs serve as crucial linkages in aircraft structures to connect components and transmit loads and motions. Because of their mechanical functions, their failure would lead to severe consequences. To ensure structural safety, factors that could affect the fatigue performance should be well considered and quantified. In this paper, the effects of the inevitable friction and clearance between the lug and load pin are quantified. The fatigue lives of straight and oblique lugs are predicted by using the classical local stress approach and a modified theory of critical distances. Satisfactory correlations between the predictions and extensive experiments are achieved.

Structurization of information flows for optimal design of aviation structures using artificial intelligence

The task of structuring information flows that provide the process of designing a transport category aircraft for creating a strategy for optimal design of aviation structures using artificial intelligence is considered. The task of creating strategies for the optimal design of aircraft structures is currently partially solved. This task requires the creation of additional optimization and information technology, which allows transferring individual design tasks from humans to artificial intelligence. Artificial intelligence should relieve people in solving complex problems, reduce the time for designing new aircraft and increase their efficiency.

Predicting Curing Distortion in Composite Manufacturing-A Fast and Cost-Efficient Numerical Simulation Method.

The curing distortion is a critical determinant of the quality of integrally manufactured composite structures, playing a pivotal role in the design and fabrication of composite. This paper presents two simplified methods in predicting the curing distortion for large-scale composite aircraft structures manufactured through the autoclave process. Firstly, the refined finite element models of the two simplified methods were developed. Then, it was utilized to predict the curing distortion of Ω-shaped composite laminates. The comparative study between the experimental data and numerical results shows that the proposed second simplified method balanced the prediction accuracy and efficiency, which is urgently needed in practice. Finally, using the second simplified method, predictions were conducted for the curing distortion of practical large-scale composite skin structures. The results were in good agreement with the corresponding experiments. This study provides a new solution for the rapid iterative design of large-scale composite structures, as well as the efficient design of frame molds for their manufacturing.

ФОРМОУТВОРЕННЯ НАМОТУВАННЯМ КОМПОЗИТНОГО ПЕРЕДНЬОГО ГОРИЗОНТАЛЬНОГО ОПЕРЕННЯ ЛЕГКОГО ЛІТАКА

A process has been developed for modeling the trajectory of laying reinforcing material (RM) during the formation of composite aerodynamic surfaces (AS). The work provides a brief description of the types of structures of aircraft of complex shape according to the criterion of the possibility of manufacturing by the winding method, in which certain types of surface shapes are highlighted according to the condition of determining the laying trajectory and developing a winding control program (WCP) for CNC machines. It is shown that the process of developing technology for manufacturing structures such as aerodynamic load-bearing surfaces has significant differences from the process of developing technology for axisymmetric products. The general technique for determining the trajectory of laying reinforcing material is shown and new approaches to specifying the reinforcement pattern are described. The general methodology for calculating the WCP for winding the AS is described. Calculation of the movements of the working parts of a winding machine (WM) includes several subtasks: choosing the order of passage of individual turns of the RM, calculating the movements of the working parts of the WM when laying each turn, and determining the program for transition from turn to turn. It is shown that the most important stage in the development of general-type AS winding technology is the choice of a winding scheme that determines the structural-force diagram (SFD) of the structure. For this purpose, the concept of the limiting point of winding has been introduced. The winding pattern of any type of product can be described by an array of coordinates of boundary points and reinforcement angles. An algorithm for calculating the trajectory of RM placement on the surface of the mandrel is presented. The features of choosing the order of passage of turns are described. The order in which the turns pass affects the number of weaves in the structure of the composite material obtained during the winding process and the strength of the product. The optimal order of passing turns according to the strength criterion is determined experimentally through repeated development tests. The features of choosing programs for transition from turn to turn are described. The algorithm of actions of the WM during the transition from one turn to another - the transition program consists of two programs: the program for entering the turn and the program for exiting the turn. The algorithm of transition programs depends on the chosen order of turns and in each specific case depends on the design features of the product. To test the developed methodology, a test calculation of the front horizontal tail of a light aircraft was carried out. As a result of the work done, it was possible to solve the problem of modeling the RM laying trajectory for bodies of complex shape with a fixed type of section, manufactured by the winding method from a polymer composite material (PCM). This technology can be used to produce aircraft structures such as “wing”, “fuselage” of an airplane, helicopter, and the like. Thus, the possibility of winding a general type AS has been proven using the example of the horizontal front tail of a light aircraft. The range of production of wound products has expanded - bodies with an arbitrary convex section and a straight generatrix. A technology has been developed for manufacturing from PCM by winding non-axisymmetric aerodynamic power structural elements such as aircraft wings, helicopter blades, air and water propellers. A surface reinforcement scheme has been found that makes it possible to obtain a variable wall thickness with its decrease from the root to the end rib. The trajectories of laying reinforcing material on a surface with an arbitrary convex section and the movement of working parts for a three-axis winding machine are determined. The developed calculation method can be used to create various aerodynamic structural elements with positive surface curvature from PCM by winding. The use of advanced high-performance technology for automated winding of reinforcing fiber material provides a significant reduction in labor costs in the manufacture of these units, reducing their prices compared to the prices of existing metal and composite aircraft of comparable size and equipment

Effect of Extreme Environments on Adhesive Joint Performance

The presented research on adhesives was conducted with the aim of supporting the design of composite repairs for composite aircraft structures that can withstand specific environmental conditions. Double-sided strap joint specimens of epoxy-based CFRP adherents and straps were bonded by two types of adhesives. Room-temperature curing epoxy adhesives EC-9323 and EA-9395 were used for bonding. The specimens’ shear strength and failure modes were evaluated under four different environmental conditions from −72 °C up to 70 °C unconditioned and at 70 °C after humidity conditioning. The results show that EC-9323 performed excellently at room temperature, but very poorly at elevated temperatures after hot–wet conditioning. Adhesive EA-9395 performed consistently well across all tested conditions. The failure mode analysis explained the performance trends and the effect of the environment on the fractured surface. This study will support proper repair design and verification of numerical simulations. The novelty of this article lies in its combined analysis of multiple environmental factors, providing a more realistic assessment of joint performance.

Integrated Optical Coherence Tomography and Hyperspectral Imaging for Automated Structural Health Monitoring of Carbon Fibre Aircraft Structures

Structural health monitoring of carbon fiber components is critical in high-stakes applications such as aerospace and prosthetics. Carbon fiber’s exceptional mechanical properties demand precise defect detection to ensure safety and longevity. This paper reviews recent advancements in monitoring carbon fiber aircraft structures using a custom optical coherence tomography (OCT) imaging system. This innovative system integrates hyperspectral imaging with automated classifiers to detect and classify both surface and subsurface defects, including roughness and cracks. By employing OCT with magnitude and quantitative phase imaging algorithms, the study introduces methods for detailed three-dimensional visualization of material defects. The high-resolution capabilities of the OCT system enable accurate and automated crack detection, enhancing reliability in critical applications. The paper also addresses challenges in deploying these advanced systems in practical scenarios, such as integration with existing maintenance protocols and data interpretation. It explores the potential of combining OCT with other non-destructive evaluation techniques to improve monitoring accuracy. These advancements contribute to more reliable, non-invasive monitoring of carbon fiber structures, with significant implications for safety and performance in various industries.

Influence of the combined effect of long hygrothermal aging and thermal cycling on mode I and II interlaminar fracture toughness of carbon/epoxy composites

Carbon fiber/epoxy composites (CFRP) are nowadays extensively used in aircraft structures. During the lifecycle of an aircraft, composites are subjected to temperature and humidity variation over time and that can lead to performance loss. Efforts to understand the environmental conditioning impact on CFRP properties have been widely investigated, leading to the main objective of this study, which was investigating whether damages caused by different environmental conditioning individually are intensified when they are combined. During an aircraft lifetime, several different environmental conditions may occur, and to simulate these effects, this research tested coupons that were exposed to long-term hygrothermal conditioning, drying and thermal cycling and their combined effects on fracture toughness in mode-I and mode-II. Dynamic mechanical and fractographic analysis were also performed on the specimens. Thermal cycling on unaged samples (reference) resulted in average reductions of 5% and 15%, for mode-I and mode-II fracture toughness, respectively, compared to the reference samples. Furthermore, combined effect of thermal cycling and hygrothermal conditionings resulted in reductions around 44% (mode-I) and 15% (mode-II) compared to the reference. The combination of different environmental conditionings was more detrimental to fracture toughness in Mode-I, as confirmed by fractographic analysis. In contrast, Mode-II showed no change in results when exposed to a combination of conditionings, suggesting that isolated analysis of the results can be misinterpreted. Combined effects of thermal cycling and hygrothermal conditioning affected fracture toughness more than either effect alone. Mode-I testing proved to be more sensitive to identifying environmental exposure effects on properties than mode-II.

Advanced Defect Detection on Curved Aeronautical Surfaces Through Infrared Imaging and Deep Learning

Detecting defects on aerospace surfaces is critical to ensure safety and maintain the integrity of aircraft structures. Traditional methods often need more precision and efficiency for effective defect detection. This paper proposes an innovative approach that leverages deep learning and infrared imaging techniques to detect defects with high precision. The core contribution of our work lies in accurately detecting the size and depth of defects. Our method involves segmenting the size of the defect and calculating its centre to determine its depth. We achieve a more comprehensive and precise assessment of defects by integrating deep learning with infrared imaging based on the U-net model for segmentation and the CNN model for classification. The proposed model was rigorously tested on both a simulation dataset and an experimental dataset, demonstrating its robustness and effectiveness in accurately identifying and assessing defects on aerospace surfaces. The results indicate significant improvements in detection accuracy and computational efficiency, showing advancements over state-of-the-art methods and paving the way for enhanced maintenance protocols in the aerospace industry.