Airframe Structures Research Articles

Fatigue life prediction of aircraft gun cabin structure under impact

The aircraft gun firing will cause serious impact on airframe structure. According to the gun firing principle, the muzzle blast wave is simulated by chemical explosion. ALE method is used to deal with the fluid-structure interaction between high-pressure flowing air and the surface of the cabin. Then the expression model of the impact load is established. The numerical calculation is completed to analyse the dynamic response of the cabin structure under the impact load. The parameters of load and cabin model are modified by using the experiment results. Comparing the calculated dynamic response with the experiment results, the error is within acceptable range. Multiaxial stress equivalence and rain-flow counting method are used to process the simulation impact response data to estimate the impact fatigue life. In conclusion, the paper provides a feasible method for analysing dynamic response and fatigue life of gun cabin structure.

Bonded and additively manufactured crack retarders: A comparative study of damage tolerance properties

Fatigue crack grows faster in the integral structure than in the riveted structure. This study compares the bonded, cold-sprayed and ultrasonically consolidated crack retarders to increase the fatigue life of integral airframe structure. The 2024-T351 aluminum M(T) specimen reinforced with crack retarders made of AISI 301 and 316L steels, Ti–6Al–4V titanium alloy and unidirectional CFRP laminate are evaluated in terms of fatigue life, delamination resistance and weight efficiency. Specimens were subjected to constant amplitude loading up to the final failure. A substantial decrease in fatigue crack growth rate was achieved using the ultrasonically consolidated AISI 301 crack retarder.

Establishing cutting conditions for dry drilling of aluminium alloy stack materials

Aside from their remarkable lightness, high strength and corrosion resistance, aluminium alloys have dominated aircraft manufacturing for decades. The combination of aluminium alloys 2024 and 7150 is widely used in the fabrication of airframe structures. Numerous holes must be drilled through the materials in order for them to be connected. Due to structure size and use of mobile drilling machines, lubricant oil is released during the drilling operation and either becomes airborne or accumulates on the floor. The primary motivation for dry drilling development is to avoid this oil discharge. A significant disadvantage of the drilling process for aluminium alloys is the workpiece’s proclivity to stick to the cutting tool, especially when temperatures are high. This research investigated the selection of cutting conditions that enable dry drilling of stack aluminium panels. The selection of cutting parameters for experiment use was made based on the assessment of literature pertaining to drilling with carbide cutting tools. Apart from literature, the assessment of cutting parameters also took into consideration existing practices in Airbus UK. Results indicated that optimum cutting performance was achieved by drilling at higher feedrates and lower interaction time compared to existing fluid based processes. In addition, this paper outlines the aspects of energy and cutting forces in current cutting processes as well as focuses on determining optimum conditions that minimise energy input.

Structural design of the light UAV airframe

In this paper, the process of designing the airframe structure of light UAVs is described in detail, and the results of the airframe structural design of a complete UAV product (with a maximum take-off weight of 10 kg) are fully described, including material selection. The design process of the airframe structure complies with the standards for the design of light UAVs: STANAG 4703 and STANAG 4738. This design process is carried out in a step-by-step sequence: model building, construction, specification development, structural design (including endurance test simulation), fabrication, load testing and actual flight. During the design process, professional software, such as Solidword or Ansys, is used to construct models and simulate durability tests. The ground load test results are also compared with the simulation results to verify the method. The final process is a version of the airframe design that meets the requirements for strength, weight, material selection and machining methods.

On the issue of reducing the temperature heating of the airframe structure of a supersonic aircraft

In this paper, the influence of heating of the structure on its strength is considered in three aspects: a decrease in the mechanical characteristics of structural materials at high temperature, heating of the structure can lead to temperature stresses in its elements and elements of aircraft systems and the occurrence of the phenomenon of creep of structural materials - continuous plastic deformation of structural elements under constant load. There are several options for reducing heat flows due to modification of the basic aerodynamic layout: increasing the sweep angles of the bearing surfaces and the radius of the wing tip and the front edges of the inflows, reducing the area of the washed surface due to the “flattening” of the initial aerodynamic contours. Each of the considered existing and promising methods of thermal protection separately will not allow to fully develop thermal protection of promising reusable supersonic aircraft. A comprehensive approach is needed. At the same time, it is necessary to consider both the replacement of structural materials of panels and cladding with titanium or steel alloys, as well as design solutions for the use of fuel lines.

Research on Rigid–Elastic Coupling Flight Dynamics of Hybrid Wing Body Based on a Multidiscipline Co-Simulation

Due to the special aerodynamic layout and mass distribution, the natural frequency differences between the hybrid wing body (HWB)’s rigid-body motion modes and the fuselage structure elastic modes are smaller compared to conventional aircraft, resulting in the disappearance of the decoupling relationship between the HWB’s rigid-body motion and the elastic motion of the airframe structure. For the above reason, the traditional analysis approach based on the rigid-body assumption is no longer applicable when analyzing the flight dynamics of an HWB aircraft, and a shift must be made to an analysis method that takes into account aeroelastic effects. Therefore, in this paper, a time-domain co-simulation program combining the computational fluid dynamics (CFD) method with computational structure dynamics (CSD) and rigid-body dynamics (RBD) is developed to investigate the effect of a rigid–elastic coupling effect on the flight dynamics of the HWB and this co-simulation method is more advantageous in the calculation of unsteady aerodynamics compared to existing methods of rigid–elastic coupling dynamics analysis. By means of the co-simulation technology, this paper completed a series of simulations, based on which the influence of rigid–elastic coupling effect on the short-period dynamic characteristics of aircraft was studied.

Application of an advanced piezoelectric strain sensor for crack closure measurement

It is well known that fatigue crack growth in airframe structures is governed by sequence effects in aircraft load spectrums, which typically include millions of load cycles of varying amplitude. In predicting the fatigue life of airframe structures, one advanced method used to account for these effects considers the concept of crack closure. This concept requires the measurement or prediction of the load required to open the crack tip, otherwise known as the crack opening load. However, traditional methods used to directly measure the level of crack closure for each cycle tend to be prohibitive for large load sequences. This is mainly due to sensor limitations and large scatter using traditional measurement techniques. The current study proposes a new piezoelectric strain sensor for detection of the crack opening load to addresses these issues. In this work, a comparison is conducted using three sensors on a compact fatigue test specimen: an extensometer (clip gauge), a conventional back face strain gauge, and the piezoelectric strain sensor. It is shown that the piezoelectric strain sensor exhibits far superior performance in measuring the crack opening load relative to the back face strain gauge and the extensometer. At minimum, for all cases considered the piezoelectric strain sensor displayed 100% or better reduction in measurement scatter than the next best sensing method considered. In some cases where the stress ratio is high and the crack length is small, the improvement seen using the piezoelectric strain sensor approached 200%. Crack tip opening loads were detected in almost all cycles across a wide range of crack sizes and load ratios; while the efficiency of the traditional sensors reduced significantly for relatively short cracks and high load ratios. The outcomes of this work will help to better understand the load sequence effects on crack growth rates, improve fatigue life predictive methods and develop effective procedures of truncation/compression of load spectrums for accelerated testing of aircraft components.

COMPARISON OF MIG-29 AND F-16 AIRCRAFT IN THE FIELD OF SUSCEPTIBILITY TO DESTRUCTION IN COMBAT

Because the Polish Air Force currently uses F-16 and MiG-29 aircraft, the aim of the study was to conduct a comparison of the susceptibility to destruction in combat of these two aircraft. The first part of the work concerned the analysis of individual critical components, such as: general characteristics of the airframe structure, aircraft engine, flight control system, fuel system, aircraft weapons, radar system. The index of susceptibility to destruction in combat was defined considering the listed critical components and the following types of enemy weapon: aircraft gun, air-to-air missile, anti-aircraft gun, surface-to-air missile. The analysis proved that the aircraft have similar susceptibility to damage in combat, a slight advantage of the F-16 aircraft in this respect was determined. The presented scheme can be used to analyze other aircraft. Proposals of aircraft modifications, directions of further actions, possibilities of using the described method were presented. The method can be used to making decisions by governments regarding the purchase aircraft for their fleets and identify aircraft critical components with high susceptibility to destruction in order to introduce appropriate modifications by military aircraft manufacturers.

Computational assessment of disbond growth behaviour in adhesively bonded wide joints or patch repairs for primary airframe structures

This paper evaluates the applicability of a damage slow growth management strategy to patch repairs or bonded joints of primary aircraft structures established earlier by the authors utilising wide bonded metal joints through a computational study using MSC Marc software. The adhesive element failure criteria was applied to establish the residual static strength of the joint as a function of disbond length. A cohesive zone element model implemented in a Ucohesive subroutine was used to evaluate the strain energy release rates (SERRs) as a function of disbond crack extent and predict the disbond growth in the joint. Similar to the 2D analysis conducted in the past, the results showed that for a wide joint with sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would be stable within a particular length range. Therefore, the slow growth approach would be viable when the patch is modelled to be large enough to allow expanded damage growth. Furthermore, numerical results on the load redistribution effect indicate an overall significantly slower disbond growth and longer fatigue life of the joint with part width disbond than that with full-width disbond.

Unsteady Aeroelastic Analysis of the Semi Aeroelastic Hinge Including Local Geometric Nonlinearities

A recent consideration in aircraft design is the use of folding wing tips, with the aim of enabling higher aspect ratio wings with less induced drag but also meeting airport gate limitations. Of particular interest is the concept of using in flight free-floating wing tips in order to reduce aircraft gust and maneuver loads. This study investigates the effects of local geometric nonlinearities on the dynamic stability, postflutter behavior and gust response of floating wing tips. A multibody formulation is introduced to account for finite rotations of rigid folding wing tips attached through hinges on a flexible airframe structure including also aerodynamic follower forces for the folding wing tip components. It is found that the postflutter behavior is characterized by super-critical limit cycle oscillations and no subcritical instability was observed for the analyzed cases. Moreover, the wing tip gust response can vary significantly when geometric nonlinearities are accounted for whereas a small impact was observed on the main airframe structure.

Uncertainty quantification of large-scale dynamical systems using parametric model order reduction

The finite element method is a widely used tool for the discretization and modeling of complex engineering structures. The output quantities of analyses conducted with finite element models are given as sharp values. This misleads about the fact that model input parameters used to define the finite element model are usually only imprecisely known, either due to aleatory uncertainty and/or due to epistemic uncertainty. Therefore, a meaningful quantification and analysis of these polymorphic uncertainties is indispensable to obtain robust simulation results. Very often, an uncertainty quantification (UQ) scheme involves multiple simulation model evaluations where the imprecisely known model parameters are varied in order to rate the impact of the uncertain model parameters on the simulation results. However, the underlying systems of ordinary differential equations of industrial finite element models usually exhibit hundreds of thousands degrees of freedom making repeated model evaluations in dynamical analyses computationally infeasible. This article proposes a novel simulation workflow for the efficient UQ of large-scale dynamical systems. Its fundamental idea is to combine parametric modeling and parametric model order reduction in order to drastically reduce the number of degrees of freedom of the underlying systems of ordinary differential equations. This allows for a very efficient UQ of large-scale finite element models with almost no loss of accuracy. The workflow is successfully applied to an industrial finite element model of a helicopter airframe structure in a possibilistic UQ. In this contribution, numerical speedups by a factor of 1630 compared to a conventional simulation workflow can be reached.

Design methods and manufacturing techniques applicable in constructing dynamically scaled airplane models

The article describes methods and techniques intended for designing and manufacturing scaled airplane models applicable in experimental flight testing. Reduced model with resized geometric dimensions should be constructed in such way to preserve similarity of its structural properties with properties of the real aircraft. Having kept the similarity of weight, stiffness and aerodynamic load distribution and maintaining properly scaled values of thrust-to-weight and wing loading ratios we can prepare scaled model showing in-flight features identical to the real airplane.A series of measuring and technological activities have to be undertaken to develop full CAD model mapping airframe geometry and structure of the real aircraft. The process chain includes measuring, design and technological activities. The development of the geometry model includes: scanning outer geometry of the real airframe, development of the surface geometry model in the CAD environment and then introducing inner geometry items which together with outer skin face make a completely assembled model of the aircraft. For the needs of constructing scaled model the full-size CAD model has to be reduced. The structure of that resized model do not have to be the same as the real one. Materials and structural elements can be quite different but after assembling them the similarity of static and dynamic structural effects should be proved. Within the manufacturing phase there are some necessary stages like preparing molds for laminating components, milling solid elements, assembly of structural components, surface finishing of the model-body surface. The final model was subjected to a load test in a special test stand to check its deformability under specific load. In the same way the numerical static analysis of the full-size FEM model was conducted. The distribution of static displacement results obtained in both cases were compared. The other way of checking structural properties of the scaled model is comparable analysis of resonant vibrations. However, in this case, vibration measurements were not carried out and model verification stopped on a comparative analysis of construction mass distribution.The procedure and results were elaborated on the example of the Tu-154M aircraft, of which the reduced model was developed in the Faculty of Mechatronics, Armament and Aerospace of the Military University of Technology (FMA MUT Warsaw, PL). Constructing and testing dynamically scaled model is one of many tasks solved in the framework of complex research project, of which the general purpose was to give numerical and experimental results useful in explaining the circumstances of the Smolensk disaster.

Experimental and computational assessment of disbond growth and fatigue life of bonded joints and patch repairs for primary airframe structures

The applicability of a damage slow growth management strategy to bonded joints/patch repairs of primary aircraft structures was evaluated through an experimental and computational study. Fatigue tests were conducted to investigate the entire process of disbond growth from initiation up to joint ultimate failure. The residual static strength of the joint as a function of disbond length was established using finite element modelling, in which the mesh size was calibrated using the static strength of the specimens measured in room temperature and dry (RD) and hot-wet (HW) conditions, based on the characteristic distance approach. A virtual crack close technique (VCCT) approach was utilised to assess the strain energy release rates (SERRs) as a function of disbond crack length. The measured disbond growth rates were correlated with the SERRs using a modified Paris law that enabled prediction of joint fatigue life. The fatigue test results indicated that for a joint having a sufficient static strength safety margin under a typical fatigue loading that would propagate disbond, the disbond growth would be stable in a particular length range. Thus, the slow growth approach would be feasible for a bonded joint/patch repairs if the patch is designed to be sufficiently large to allow extended damage propagation (whilst in the case when patch size must be limited, safe-life design for the patch termination region in critical repairs must be considered. Should disbond growth occur in this case, the joint must be repaired or replaced). The work presented in this paper validated the framework/procedure proposed previously by the authors (Tanulia et al., 2020) for managing damage slow growth in bonded joints/patch repairs. In the last part of this paper the planned follow-on research is briefly described.

Improvement of European thin haul mobility: the role of small Green commuter aircraft

In the last years, transportation demand has grown up as consequence of the increase in leisure, and business for both heavily travelled routes (between major cities) and thin routes (between remote areas, and remote areas with major cities).In parallel, aviation sustainability issues have become more and more important, being aviation pollution an important source of global environmental pollution. Moreover, several States, as well as private Companies travelling for business, have introduced bans and/or special taxes on short flights covering distances that could be travelled by train in less than 3 hours. These regulations have a major impact on short haul thin routes, which could be covered by small air transport (up to 19 seats). Thus, dedicated research and technology innovation effort has been put in place over the last years to develop eco-friendly, and economically viable, aircraft able to answer sustainability needs, market demand, and cost-effectiveness.In particular, Clean Sky 2 Joint Undertaking, a public-private partnership between the European Commission and the European aeronautics industry, has co-financed the SAT (Small Air Transport) TA (Transverse Activity) initiative. SAT TA main goal is to address technology innovations to reduce environmental impact as well as operational cost of small commuters, ensuring improved levels of operational safety. SAT TA is a Project with an overall EU (European Union) funding of about 68 mln€, including small aircraft OEMs (Original Equipment Manufacturers), major aircraft suppliers, SMEs (Small and Medium Enterprises), research centres and academia (more than 80 Partners involved).The research activity has focused on the following main technology streams:•New generation turboprop engine with reduced fuel consumption, emissions, noise and maintenance costs for 19 seats aircraft.• More electric digital systems including:• Affordable fly-by-wire architecture for small aircraft (CS23 certification rules).• More electric systems replacing pneumatic and hydraulic aircraft systems (hybrid de-ice system, landing gear and brakes, high voltage EPGDS –Electrical Power Generation and Distribution System –).• Advanced avionics for small aircraft, to reduce pilot workload, paving single pilot operations for 19 seats.• Affordable airframe structures including:• Low cost composite wing box and nacelle using OoA (Out of Autoclave) technology, LRI (Liquid Resin Infusion) and automated deposition process.• Affordable small aircraft manufacturing of metallic fuselage by means of FSW (Friction Stir Welding) and LMD (Laser Metal Deposition).• Advanced cabin comfort with new materials and more comfortable seats.To address the abovementioned technologies, two different platforms have been designed: a Reference and a Green aircraft. Reference aircraft is a virtual aircraft designed considering 2014 technologies with an existing engine assuring requested take-off power. Green aircraft is designed integrating the technologies developed within Clean Sky 2.The present work describes designed aircraft, and preliminary mission analyses, showing the achievement of Project goals (20% CO2, 20% NOx reduction for the design mission).

Conceptual design and comparative study of strut-braced wing and twin-fuselage aircraft configurations with ultra-high aspect ratio wings

Sustainable and fuel-efficient next-generation air transportation demands a step change in aircraft performance. The ultra-high aspect ratio wings (UHARW) configuration is one key enabling strategy for improving aircraft aerodynamic efficiency and reducing fuel consumption and emissions. Unconventional aircraft configurations and advanced airframe technologies are required to address the large bending moment and shear stresses in the UHARW structure. This paper considers two promising unconventional configurations for adopting UHARW design, including strut-braced-wing (SBW) and twin-fuselage (TF), with advanced airframe technologies, i.e., active flow control, active load alleviation, and advanced airframe structures and materials. Three typical missions, including short-range (SR), medium-range (MR), and long-range (LR), are considered for aircraft design. A conceptual design and performance analysis framework for SBW and TF configurations is developed in this paper. According to the mission profile and top-level requirements proposed for each mission, an SBW and a TF configuration are designed, respectively. A comparative study is carried out to determine the best-in-class configuration of the corresponding mission to evaluate the potential of SBW and TF configurations for next-generation sustainable aviation applications. The results showed that the TF configuration has a better wing weight reduction effect than the SBW configuration, and the MR-TF and LR-TF aircraft have lower takeoff weight and fuel weight than those of the SBW aircraft for the same mission. However, due to the adjustment of the cabin dimensions for the SR-TF aircraft, the SBW configuration outperforms the TF configuration in this mission.

FEM simulation of stiffened structures with pultruded carbon fibre profiles

Simulation of composite structures through Finite Elements Method (FEM) is widely used in the aeronautics sector to reliably predict its performance. The incorporation of new materials with enhanced properties, and also new manufacturing processes usually require the use of such tools for validation. This work has been focussed on studying an emerging technology such as Pultrusion for stiffeners manufacturing. For this, it has been used a demonstrator based on a common airframe structure, panels stiffened with profiles. This time, the classic stiffener manufactured using prepreg and autoclave cured has been replaced by stiffeners manufactured by Pultrusion. For that purpose, it was manufactured flat panels using prepegs and autoclave curing, for subsequently bond L shaped high resistance carbon fibre profiles manufactured by Pultrusion. Furthermore, FEM simulations were conducted over these coupons for three different mechanical testing conditions, tensile, compression and shear, aiming to determine the capability of these methods for simulating reliably the mechanical performance of pultruded materials on such above-mentioned structures. The results obtained in this work have been used for model parameter adjustments to obtain reasonable correlation levels, enabling the future usage of Pultrusion technology in the stiffener profiles manufacturing.

BEDYN: development of a methodology to characterize the behaviour of composite structures under dynamic loading

The methodology of design of polymer-based composite structures under static loadings, and even fatigue loads, is quite mature and well stablished. However, for intermediate and high dynamic loading conditions, the methods are still under development and often limited to academic research levels, without any type of standardization. It is crucial to understand how the materials used in the aerospace sector behave under dynamic loadings. Composite materials may exhibit strain rate effects and the analysis tools based on static formulations could be far away from the actual material and structural response, therefore robust and industrial dedicated dynamic tests, analysis and modelling methods are then necessary to design and certify composite airframe structures. This is what the proposed project will deal with. The aim of BEDYN project is to address a methodology to properly characterize the dynamic behaviour up to rupture of thermoset polymer-based composite structures submitted to dynamic loading. BEDYN is a European granted Clean Sky 2 Joint Undertaking (JU). 

축소 기체 구조 모델을 이용한 고속 비행 복합형 무인 회전익기의 능동 진동 제어 시험

Active vibration control for a 25 %-scale model of a high-speed compound unmanned rotorcraft was experimentally studied. The compound rotorcraft uses wings, propellers, and a lift-offset coaxial rotor system with two blades per rotor. A small-scale airframe structure was manufactured using aluminum and steel and appropriately represents the dynamic similarity of the full-scale model. A ground vibration test (GVT) was used in this experimental work. An active vibration control system (AVCS) using two force generators and three accelerometers was applied for the active vibration control of the small-scale airframe model. Three accelerometers that measure acceleration signals in the vertical direction of the airframe are located at specific airframe locations: the remote cockpit unit, wing root, and wing tip. A 2P rotor hub pitch moment that produces a linear force generator was used to excite the airframe structure. The AVCS using the Fx-LMS algorithm was implemented using MATLAB Simulink, dSPACE ControlDesk, and MicrolabBox. This experimental study shows that the vibration responses of the airframe in the vertical direction at the specific airframe positions are reduced by 47.54 % ~ 84.67 %, and the test results compare reasonably with the simulation results.

Internal Damage Detection of Composite Structures Using Passive RFID Tag Antenna Deformation Method: Basic Research

This manuscript deals with the detection of internal cracks and defects in aeronautical fibreglass structures. In technical practice, it is problematic to accurately determine the service life or MTBF (Mean Time Between Failure) of composite materials by the methods used in metallic materials. The problem is mainly the inhomogeneous and anisotropic structure of composites, possibly due to the differences in the macrostructure during production, production processes, etc. Diagnostic methods for detecting internal cracks and damage are slightly different, and in practice, it is more difficult to detect defects using non-destructive testing (NDT). The article deals with the use of Radio frequency identification (RFID) technology integrated in the fibreglass laminates of aircraft structures to detect internal defects based on deformation behaviour of passive RFID tag antenna. The experiments proved the potential of using RFID technology in fibreglass composite laminates when using tensile tests applied on specimens with different structural properties. Therefore, the implementation of passive RFID tags into fibreglass composite structures presents the possibilities of detecting internal cracks and structural health monitoring. The result and conclusion of the basic research is determination of the application conditions for our proposed technology in practice. Moreover, the basic research provides recommendations for the applied research in terms of the use in real composite airframe structures.

A Simplified FE Modeling Strategy for the Drop Process Simulation Analysis of Light and Small Drone

The numerical accuracy of drop process simulation and collision response for drones is primarily determined by the finite element modeling method and simplified method of drone airframe structure. For light and small drones exhibiting diverse shapes and configurations, mixed materials and structures, deformation and complex destruction behaviors, the way of developing a reasonable and easily achieved high-precision simplified modeling method by ensuring the calculation accuracy and saving the calculation cost has aroused increasing concern in impact dynamics simulation. In the present study, the full-size modeling and simplified modeling methods that are specific to different components of a relatively popular light and small drone were analyzed in an LS-DYNA software environment. First, a full-size high-precision model of the drone was built, and the model accuracy was verified by performing the drop tests at the component level as well as the whole machine level. Subsequently, based on the full-size high-precision model, the property characteristics of the main components of the light and small drone and their common simplification methods were classified, a series of simplified modeling methods for different components were developed, several single simplified models and combined simplified models were built, and a method to assess the calculation error of the peak impact load in the simplified models was proposed. Lastly, by comparing and analyzing the calculation accuracy of various simplified models, the high-precision simplified modeling strategy was formulated, and the suggestions were proposed for the impact dynamics simulation of the light and small drone falling. Given the analysis of the calculation scale and solution time of the simplified model, the high-precision simplified modeling method developed here is capable of noticeably reducing the modeling difficulty, the solution scale and the calculation time while ensuring the calculation accuracy. Moreover, it shows promising applications in several fields (e.g., structure design, strength analysis and impact process simulation of drone).