Airframe Structures Research Articles

Development of high-toughness aerospace composites through polyethersulfone composition optimization and mass production applicability evaluation

Owing to the necessity of developing airframe materials for military unmanned aerial vehicles (UAVs), capable of operating in extreme environments, aerospace-grade composite materials were developed using a high-toughness epoxy–resin system, and were subsequently analyzed and evaluated. The phase transition behavior of a high-toughness epoxy–resin system was modeled and simulated as a function of the toughening agent, polyethersulfone (PES), and its content to optimize the resin system. Reliability was ensured through experimental validation. At the maximum PES content compatible with the base epoxy–resin and curing agent contents, the epoxy–resin system exhibited the highest tensile strength and toughness. However, results from preliminary tests performed using the pilot process revealed that an increase in viscosity beyond a certain level due to the addition of PES rendered it unsuitable for application in mass-production processes. The optimal composition of the high-toughness epoxy–resin system suitable for mass production was determined based on the results of the resin paper production using the pilot process. High-toughness carbon fiber-reinforced plastics (CFRPs) were then mass produced, and their characteristics were compared with those of conventionally toughened CFRPs and the pilot products. The results confirmed that compared with the conventionally toughened CFRPs, the mass-produced CFRPs containing the high-toughness resin system showed 246%, 728%, 392% and 480% improvement in compression-after-impact strength, Mode-I interlaminar fracture toughness (ILFT), Mode-II ILFT for crack initiation, and Mode-II ILFT for crack propagation, respectively. In addition, these composites were used to manufacture UAV wings to evaluate their applicability in airframe structures.

Multi-disciplinary optimization design method and application of BWB fuselage structure based on PRSEUS configuration

Aiming at the problems such as low structural bearing efficiency, structural weight gain, heavy load and cabin noise caused by high rear backbraced engine in BWB layout, based on novel materials and pultruded rod stitched efficient unitized structure(PRSEUS), a multi-disciplinary optimization design method for unconventional circular body structures with aerodynamic, noise, vibration and technological constraints was studied to improve the bearing efficiency and lightweight of structures. Based on the adaptive agent model construction technology, an agent model for the performance analysis of the aerodynamic and noise subsystem of the airframe structure is established to predict the aerodynamic load and noise performance of the airframe structure, and the basic requirements of the aerodynamic and noise performance are transformed into constraints such as geometric size and mechanical response, so that the multi-disciplinary problems are concentrated in a set of finite element analysis model with efficient mathematical programming method. The time-consuming problem of multi-disciplinary high-precision model analysis is solved, and the multi-disciplinary optimization design of the wing-body integrated body structure is realized. Through the finite element analysis of the body structure, the bearing efficiency of the structure has been greatly improved, and the weight loss rate of the central body structure has reached 8.7% under the premise of meeting the constraints of various disciplines.

Aircraft Skin Machine Learning-Based Defect Detection and Size Estimation in Visual Inspections

Aircraft maintenance is a complex process that requires a highly trained, qualified, and experienced team. The most frequent task in this process is the visual inspection of the airframe structure and engine for surface and sub-surface cracks, impact damage, corrosion, and other irregularities. Automated defect detection is a valuable tool for maintenance engineers to ensure safety and condition monitoring. The proposed approach is to process the captured feedback using various deep learning architectures to achieve the highest performance defect detections. Additionally, an algorithm is proposed to estimate the size of the detected defect. The team collaborated with TUI’s Airline Maintenance Team at Luton Airport, allowing us to fly a drone inside the hangar and use handheld cameras to collect representative data from their aircraft fleet. After a comprehensive dataset was constructed, multiple deep-learning architectures were developed and evaluated. The models were optimized for detecting various aircraft skin defects, with a focus on the challenging task of dent detection. The size estimation approach was evaluated in both controlled laboratory conditions and real-world hangar environments, providing insights into practical implementation challenges.

A physics-informed machine learning model for global-local stress prediction of open holes with finite-width effects in composite structures

Fast and accurate methods are required to predict stresses in the vicinity of open and closed holes in composite structures, especially in a global-local modelling context as applied during the design of airframe structures. Fast analytical solutions for infinite-width anisotropic plates with open holes do not consider finite-width effects. Heuristic methods and semi-analytical solutions can be used to towards addressing such effects. To improve the accuracy and speed of these respective methods, we use machine learning (ML) methods trained on high-fidelity finite element analyses to make finite-width corrections. However, such methods require large amounts of training data to reduce errors to satisfactory levels. Therefore, in this study, the fusion of analytical solutions with machine learning is performed. We develop an analytical solution-informed ML model that is as fast as an analytical solution and superior in accuracy to analytical solutions with heuristic finite-width scaling. Our informed ML model offers accuracies equal to analytical solutions for the infinite-width case, and it is capable for use in a global-local modelling context, under uniaxial and biaxial loading. Our informed ML model outperforms prediction accuracy across all cases compared to uninformed ML models and requires a significantly lower size training dataset size.

Superior high-cycle fatigue property through in-situ precipitation of α' martensite in additively repaired titanium alloy

TC4-DT titanium alloy is commonly used in aircraft airframe structures due to its high strength-to-weight ratio. However, these parts are susceptible to fatigue failure during their service life. In this study, the damaged TC4-DT titanium alloy components were repaired using laser directed energy deposition (LDED) by depositing TC4 powders with a side repairing method, and the effects of the repair process on microstructural evolution, residual stress together with high-cycle fatigue strength were comprehensively investigated through multiscale characterization techniques. Results showed that the high-cycle fatigue strength of the repaired specimen was significantly increased by 11.9% compared with the substrate material, from 404.3MPa to 452.6MPa, and this was ascribed to its unique microstructural characteristics, especially the in-situ precipitation of α' martensite resulting from the rapid cooling involved in the LDED process. Furthermore, the dense repaired microstructure, the surface residual compressive stress, and the numerous dislocations induced by phase transformation, combined with the Hall-Petch effect caused by the formation of α lath, hindered the dislocation motion and delayed the strain localization, which significantly inhibited crack initiation and greatly contributed to the fatigue performance. This work demonstrates the potential of additive manufacturing to repair alloys with superior mechanical properties for structural applications.

An Analysis of Chip Formation and Hole Circularity in Drilling Applications: An Aircraft Components Perspective

Drilling plays a critical role in the production of precise holes for aircraft components. However, drilling aluminum alloys poses challenges, leading to poor hole quality and potential defects in the airframe structure. This study focused on investigating chip formation, hole circularity, and drilling parameters, specifically feed rate and spindle speed. Dry drilling trials are conducted using high-speed steel drill bits and the Al6061-T6 alloy. The CIMCO MDC-MAX software is employed to monitor machine performance and accumulate comprehensive data. The study investigates the effect of varying feed rates on hole circularity, chip development, and chip thickness. Findings indicate that higher feed rates result in increased circularity error and chip thickness. The circularity graph shows inconsistencies due to workpiece vibration during drilling. Chip thickness rises with the feed rate, particularly with an increased number of drilled holes, attributed to factors such as tool rubbing and improper cutting. Additionally, the study highlights the correlation between machine performance and product quality. The CIMCO MDC-MAX data reveals that a feed rate of 0.260 mm/rev corresponds to high machine performance and low circularity. Conversely, Drill 6, operating at a feed rate of 0.230 mm/rev, exhibits poor machine performance and higher average circularity. The research enhances understanding of chip formation and hole circularity in drilling applications for aircraft components. The results emphasize the importance of optimizing drilling parameters to achieve superior hole quality. Practical implications are provided for enhancing the drilling process in aerospace manufacturing.

Quasi-static tensile properties of DMLS TI6AL4V(ELI) specimens exposed to two different heat treatment processes and tested at different elevated temperatures

The use of Ti6Al4V components in the aerospace industry has increased in the past years s. Jet engine components made of Ti6Al4V such as compressor blades and exhaust ducts, as well as landing gear, hydraulic systems, and many airframe parts, are exposed to different temperatures while in service, which may affect their mechanical properties. Compressor blades are exposed to temperatures up to 400 °C, exhaust ducts up to 300 °C, and hydraulic systems in the range of −50 °C–135 °C, while landing gears and many airframe structures are exposed to temperatures in the range of −50 °C–70 °C. Therefore, it is of great importance to determine their stability at different temperatures. The results of a study aimed at investigating the effect of elevated temperatures on the mechanical properties and microstructures of DMLS Ti6Al4V(ELI) alloy are presented and discussed in this paper. A DMLS EOSINT M290 machine was used to manufacture test specimens, which were then machined to comply with ASTM E8 for tensile tests. Thereafter, tensile tests were carried out at different test temperatures till fracture. Fractography was then used to study the characteristics of the fracture surfaces. The results obtained in this work showed that temperatures between 175 °C and 325 °C have a significant effect on the mechanical properties and microstructures of the alloy.

Fixed-wing UAV based on vortex lattice method modular technology research

The so-called modularity refers to dividing the system into several modules from top to bottom, layer by layer when solving a complex problem with various properties reflecting its internal characteristics. The main driving force for the demand for UAV modular technology comes from the requirements of UAV large-scale industrialization for UAV generalization, serialization, application scenario richness, and economy. The modular design of UAV is based on standardized software and hardware interfaces to carry out UAV aerodynamic shape module design, modular design of airframe structure, modular design of application load, module design of flight control system, module design of power system, etc.

An experimental and parametrical study on repair of cracked titanium airframe structures with single-side bonded carbon fiber-reinforced polymer prepreg patches

The present research aims to investigate efficient repair techniques of cracked Ti-alloy aircraft structures with adhesively bonded carbon fiber-reinforced polymer prepreg patches. The repaired specimens in the configuration of a Ti-alloy butt joint with one-side bonded composite patch were prepared under multiple repair factors including patch thickness, patch length, adhesive thickness, cure pressure, patch layup and surface treatment. The repair efficiency was evaluated by loading behavior, bonded interface microstructure and failure mode. The results reveals that the geometric factors affect the loading performance and alter failure modes by adjusting stress distribution in the repair system, whereas the cure pressure and surface treatment act on the bondline and change interfacial properties. A sensitivity-optimization model based on analysis of variance was established for parametrical study to quantify the contribution of repair factors and obtain optimal values. The optimum parameters were validated by repaired central-cracked specimens via static and fatigue tests, which proved that the repaired structure could restore 90.7% loading capacity of intact ones and endure more than 106 fatigue cycles of 25% ultimate failure load level of center-cracked ones. The proposed experimental and parametrical study possessed good efficacy in refurbishing strength and stiffness of cracked metallic structures.

Virtual Full Scale Static Test of a Civil Tilt Rotor Composite Wing in Non-Linear Regime

This study addresses the crucial role of post-buckling behavior analysis in the structural design of composite aeronautical structures. Traditional engineering practices tend to result in oversized composite components, increasing structural weight. EASA AMC 20-29’s Building Block Approach suggests phased testing, but its time and cost challenges necessitate a shift to high-fidelity post-buckling analyses, exemplified by MSC NASTRAN SOL 400. This approach, showcased in the analysis of the Next Generation Civil Tilt Rotor Technology Demonstrator’s wing (NGTCTR-TD), effectively de-risks static tests, contributing to a more efficient certification process. The study demonstrates how advanced simulations provide detailed insights into local buckling phenomena, allowing precise stress distribution analysis. These analyses eliminate the risk of structural failure, paving the way for safer, more efficient, and cost-effective airframe structures. Future developments aim to validate numerical analyses with experimental data, further emphasizing the reliability and benefits of high-fidelity simulations.

Low-cost development of a fully composite fixed-wing hybrid VTOL UAV

Fixed-wing hybrid vertical take-off and landing (VTOL) unmanned aerial vehicles (UAV) are popular due to their interoperability in the military and civilian domains, primarily where significant terrain difficulties exist for humans. Additionally, they can operate without requiring any runway infrastructure and have extended air endurance and efficiency. Since the hybrid UAV operates in distinct flight modes, viz., (a) VTOL and (b) fixed-wing cruise, carrying different payloads, the airframe structure requires careful design and manufacturing to realize sufficient strength. This experimental study aimed to identify the best combinations of various composite materials for manufacturing a lightweight, low-altitude long endurance (LALE) hybrid VTOL UAV. Primary materials include carbon fiber, Kevlar, fiber-reinforced plastic (FRP), resins, etc. Different rectangular test specimens of 120 × 5 mm size were made from ten different grades of carbon fiber, FRP, and resins using vacuum bagging. After properly curing these test specimens, we quantified their dynamic mechanical characteristics using various bending load experiments on a universal testing machine (UTM). An analysis of the experimental data facilitated the identification of the best composite combinations that provide maximum strength while reducing overall weight. Thus, we could understand the dynamic interplay between peak stress and test specimen weight. We also manufactured a UAV prototype using the identified combination and instrumented and flight-tested it to substantiate the experimental findings.

Study on the Energy Absorption Characteristics of Different Composite Honeycomb Sandwich Structures under Impact Energy

A honeycomb structure is a sandwich structure widely used in fuselage, among which the hexagonal honeycomb core is the most widely used. The energy absorption characteristics and impedance ability of the structure are the main reasons that directly affect the energy absorption characteristics of the honeycomb sandwich structure. Therefore, it is necessary to study the out-of-plane mechanical properties of the composite honeycomb sandwich structure. Based on the numerical simulation results, the energy absorption characteristics of several composite honeycomb sandwich structures are verified by drop hammer impact experiments. The research shows that the transient energy absorption characteristics of the composite honeycomb sandwich structure are mainly related to the cell size of the honeycomb structure. The smaller the size of the front cell, the stronger the overall impact resistance; the strength of the composite honeycomb sandwich structure exceeds that of 7075 aluminum alloy-NOMEX and carbon fiber-NOMEX honeycomb sandwich structures. In this paper, the energy absorption characteristics of composite honeycomb sandwich structures under different impact energy are compared and studied. The displacement, force and energy curves of energy absorption characteristics related to time variables are analyzed. The difference in protective performance between the composite honeycomb sandwich structure and existing airframe structure is compared and studied. The optimal structural design parameters of composite honeycomb sandwich under low-speed impact of drop hammer are obtained. The maximum energy absorption per unit volume of the designed honeycomb sandwich structure is 171.7% and 229.8% higher than that of the NOMEX-AL and NOMEX-C structures. The 6.4 mm and 3 mm cell sizes show good characteristics in high-speed buffering and crashworthiness. The composite honeycomb sandwich airframe structure can improve the anti-damage performance of the UAV airframe structure, ensure the same thickness and lightweight conditions as the existing honeycomb sandwich airframe structure, and improve the single-core bearing mode of the existing airframe structure.

Industrial application of a low-cost structural health monitoring system in large-scale airframe tests

A small, novel, integrated SHM system has been deployed during full-scale testing of a wing fatigue test for several weeks and a fuselage pressurisation test for several months. Complementary NDE measurement techniques were combined, with inputs from visible and infrared optical sensors, as well as resistance strain gauges. Sensor units were deployed at regions of interest and integrated board computers permitted near real-time data processing. The outputs were full-field measurement datasets from digital image correlation and thermoelastic stress analysis systems. Changes in these datasets in the regions of interest were successfully quantified using orthogonal decomposition and were indicative of changes in the condition of the structure. The results from these case studies demonstrate that this system can be successfully deployed in spatially restricted areas within airframe structures to monitor crack growth. The low cost and small footprint of the system presents the opportunity for installation of arrays of similar sensors for both test and in-service data collection. Near real-time data processing would allow timely reporting to service engineers, informing maintenance or operational decisions.

Tensile Properties of Aircraft Coating Systems and Applied Strain Modeling

In this work, we develop a structural model for the fracturing of an aircraft coating system applied to a complex airframe structure that includes aluminum panels and stainless-steel fasteners. The mechanical properties of the coating system, which consisted of an MIL-PRF-85582E, Type II, Class C1, two-part epoxy primer and an MIL-PRF-85285 Rev E, Type IV, Class H, two-part polyurethane topcoat, were measured before and after 8 months of atmospheric exposure. The loads applied to the coating occurred from local deformations of the fastener-panel system in response to flight stresses. Two types of flight stresses, compression dominated and tension dominated, were modeled. The degradation of the mechanical properties of the coating after atmospheric exposure increases the severity of cracking of the coating at a critical fastener–skin interface.

An innovative and low-cost system for in situ and real-time cure monitoring using electrical impedancemetry for thermoset and CFRP laminate

This paper is divided into five sections. The first one introduces the context of our study dealing with the monitoring of the cure of structural composites made of carbon fibers and thermoset matrix (CFRPs). The second one presents a brief state of the art. The third one deals with the materials characterized in this study, and the design and the production of a new bench dedicated to electrical impedance (EI) and thermal measurements. The fourth one compares the results obtained on CFRP and unreinforced matrix samples using conventional differential scanning calorimetry and EI measurements. A conclusion and some prospectives close the paper. In a recent previous work, we studied phases transitions in CFRP during their curing by EI showing that changes in the electrical complex impedance Z can easily be related to those of conventional parameters such as the thermoset matrix degree of cure (DOC) α. This work was carried out using a commercial single-channel impedance analyzer: HIOKI. In addition to its high acquisition cost, this device presented certain limitations in terms of measurement channels (only one) and data acquisition rate (60 s for a frequency sweep). These facts led us to develop a new EI measurement bench for monitoring the impedance changes in CFRP part for aeronautical applications (airframe structures). The innovative new multi-channel (8) bench we designed and manufactured is based on the Digilent PmodIA module (never used previously for this purpose) and an analog-front-end developed for EI measurements. It costs only 15% of the HIOKI analyzer and has a data acquisition rate 48 times higher (1,23 s versus 60 s). All this while maintaining an equivalent impedance measurement range: 100 mΩ to 1 MΩ. The proposed approach makes it possible not only to monitor the DOC α, but also to detect potential cure cycle issues. This latter demonstration was carried out on a new composite material (vacuum-bag oven-cured), NC66/1808NA (supplied by CTMI), which, to our knowledge, had never been characterized in the literature. Compared with preliminary results, similar behavior is obtained on two different CFRPs using two different benches. This clearly underlines the value and quality of the study. Experimental validation of our approach will contribute to CFRP structural health monitoring.

МЕТОД ПРОЕКТУВАННЯ ТА ПАРАМЕТРИЧНОГО МОДЕЛЮВАННЯ СИЛОВОЇ НЕРВЮРИ КРИЛА ЛІТАКА ТРАНСПОРТНОЇ КАТЕГОРІЇ

The creation of any structural element begins with the design of basic geometric parameters. In the article, the method of designing and determining the main elements of the structure of the main ribs of the wing with conditions of static strength were reviewed, and a method of three-dimensional parametric modeling was developed. The rib is one of the most important elements of the wing, which act significant flight loads, and the main ribs additionally act concentrated forces. The article analyzed the design features of the rib design of the wing of the transport category aircraft. An analysis of the current loads on the main rib was performed, shear and bending diagrams were determined. During realize high-lift devices, the aerodynamic flow of the wing changes significantly, which leads to a change in the stress-strain state of the wing. This relates not only to the increase in lift due to a change in the curvature of the wing and an increase in the wing area, but also due to a change in the position of the center of pressure relative to the chord of the wing. Therefore, to determine the loads, the aerodynamic characteristics, obtained by the numerical method in the ANSYS program, were used. In order to remain competitive, the aviation company needs to ensure the high quality of the manufactured equipment, its quick modernization and modification or change of the model series. The use of CAD/CAM/CAE/PLM systems at all stages of the life cycle of aviation parts, including the design and production stages, can significantly improve the quality of the created parts and reduce the cost of performing work related to design and production, while maintaining high rates of work. The method of three-dimensional parametric modeling of the main machined wing rib was developed using the Siemens NX computer integrated system, which allows to significantly reduce the stage of modeling typical elements of the aircraft airframe structure using the created three-dimensional parametric model of a typical structural element

Design of Dual-tiltable Ducted-Fan Aircraft Based on Cross-coupling Network Control

In recent years, with the rapid development of drone technology, unmanned aerial systems have been widely applied in various fields. Although the flight control and airframe structure of traditional multirotor aircraft are fairly mature, they still face challenges such as low aerodynamic efficiency, low safety, and complex structure. In order to solve these issues, a dual-tiltable-ducted-fan aircraft system is designed. The system is equipped with an STM32F407 processor as the main controller, which receives motion control information from the remote control module. By employing cross-coupling network control methods, it coordinates the output values and tilt angles of the two channeling fans to achieve stable control of the aircraft's flight attitude.

Оптимизация профиля иглы жидкостно-газового амортизатора опоры шасси самолета

The gas-liquid damper is the main element in depreciation system of the aircraft landing gear. In this damper, the hydraulic resistance force to the rod motion depends on the working fluid flow rate through the plunger throttle hole, which area is regulated by a pin. If the passage hole area changes, the hydraulic resistance force also changes. Based on formulated criteria for the depreciation quality (energy intensity and dynamic coefficient), the problem of optimizing the pin profile was solved. To describe the damper operation, a mathematical model validated according to the landing gear testing results was used. Geometric parameters of the pin profile are described using the piecewise linear approximation or the cubic spline. To solve the problem of multicriteria optimization, the objective function was used covering a wide range of the aircraft landing weights. The solution was found using numerical integration of the damper mathematical model equations and the global optimization surrogate model. Analysis of the obtained results showed the possibility of reducing the load on the aircraft landing gear and the airframe structure.

Advanced Hypersonic Transportation Perspectives

High-speed transportation design concepts are examined for advanced civil aviation and space transportation applications. The design concept of transatmospheric vehicles, flying directly from earth to orbit, will enable launching and recovery of small telecommunication and remote sensing satellites, harvesting solar energy from space, and space tourism.Commonality of flight regimes in atmospheric flight, the similarity of fuels, propulsion technology, aerodynamic configuration design, and airframe structure materials used in these high-speed flight regimes for air and space transportation are discussed in detail.A single, Integrated Hypersonic Transport Programme, is recommended, based on small size, low risk, and low cost vehicles. A dual fuel, kerosene-hydrogen system meets the needs of such an Integrated Hypersonic Transportation Programme, with kerosene for take-off, powered landing and low speed ascent/descent flight. Hydrogen is used for high-speed cruise flight. India would greatly benefit by an Integrated Hypersonics Transportation Programme, with leading institutions like ISRO, NAL and ADA working together to jointly study, design and build a strong technology base for small, unmanned hypersonic flight demonstrators. This project could then lead on, safely and economically, to hypersonic Executive Jet aircraft, and small fully Reusable Trans-Atmospheric Vehicles for direct ascent of the hypersonic aerospacecraft to earth orbit.

Failure Mode Effect Analysis of Composites Prepreg Manufacturing Process for Airworthiness Certification - A Case Study

Airworthiness certification of aircraft structures requires a holistic approach ranging from material manufacturing process to design evaluation of airframe components. Present day airframe structures find greater applications of composite materials owing to their extraordinary mechanical properties on pro rata weight basis. In case of polymer based composites, the prepreg form of material is preferred for several applications due to matrix efficiency (high fiber-to-volume ratio) and ease of application. As part of composite material certification, the design audit of composite prepreg manufacturing process is carried out to assess the possible failure scenarios. Manufacturing Process Design and Control plays a significant role in satisfactory realization of composite parts right from raw material stage. Process variability/ scatter can lead to large variation in performance of the final product and thus stringent control of process parameters is essential to achieve aero-grade composite structures. This study provides basic understanding of composite prepreg manufacturing process and application of FMEA as a process design tool to detect and assess potential failures with an aim to devise preventive measures for improved process control. The complete process of manufacturing prepregs is divided into three stages viz.: Fabric weaving, Resin formulation and Prepregging operation. FMEA principles are used to identify the predominantly vulnerable operations and to quantify the inherent risk associated. Risk Priority Numbers (RPN) are evaluated for various operations in these three stages. Based on process technology and engineering expertise, suitable corrective and preventive measures are offered according to the potential risk factors and failure types. Emphasis is to prioritize the efforts and attention towards major preventive measures by targeting the least number of factors causing the maximum impact for failures (Pareto principle). Present study substantiates the significance of trained personnel, proper documentation of operating procedures and work instructions along with contamination control as critical factors in eliminating failures in composite prepreg manufacturing process.