The aim of the paper is to experimentally validate a numerical design methodology for optimizing composite wings subject to gust and fatigue loading requirements and to assess the effect of fatigue on the aeroelastic performance of the wing. Traditionally, to account for fatigue in composite design, a knockdown factor on the maximum stress allowable is applied, resulting in a conservative design. In the current design methodology, an analytical fatigue model is used to reduce the conservativeness and exploit the potential of composite materials. To validate the proposed analytical model, a rectangular composite wing is designed and manufactured to be critical in strength, buckling and fatigue. An experimental campaign comprising wind tunnel and fatigue tests is performed. In the wind tunnel, both static and dynamic aeroelastic experiments are conducted to validate the numerical dynamic aeroelastic model. The fatigue test is used to validate the analytical fatigue model and to understand the effect of fatigue on aeroelastic properties of the wings. The results from experimental campaign validated both the aeroelastic predictions as well as fatigue predictions of the numerical design methodology. However the fatigue process resulted in degradation of the wing stiffness leading to change in the aeroelastic response of the wing. 

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The COVID-19 pandemic had a significant impact on the aviation industry with more than 60% reduction in the passenger traffic in the year 2020 compared to year 2019. The passenger traffic which is now expected to reach the 2019 level only around the year 2024 will still continue to grow but at a lower pace compared to the pre pandemic levels. The environmental issues which were a major concern for the aviation industry before pandemic will still be relevant. The objective of achieving an environmentally friendly zero emission aircraft will not be met only with alternative fuels and propulsion concepts but also require advanced material technologies and novel designs. A promising technology having the potential to improve the performance of an aircraft by improving the structural efficiency is the application of aeroelastic tailoring with the help of composite materials. However, incorporating aeroelastic tailoring with composite materials in the design process is not a trivial task. In the traditional design process, knowledge about the design increases, while the design freedom decreases as one goes from conceptual to preliminary and finally to the detailed design. For conventional designs, the lack of knowledge during the initial stages is compensated through empirical knowledge. However, the lack of such empirical knowledge for novel design and advanced technology, results in the need for increased physics-based knowledge during the initial design process. In the research presented in this dissertation, the focus was on increasing knowledge in the early stages of the aeroelastic design process of a composite wing. As current state of art in aeroelastic tailoring does not include critical gust and fatigue loads, this thesis is focused on including critical gust loads and fatigue loading requirements in the preliminary aeroelastic optimization framework…@en

Aeroelastic tailoring offers an effective way to exploit the anisotropic properties of the composite materials used in lightweight aerospace structures. This paper presents a design methodology for experimental wings that employ a typical wingbox structure. The proposed methodology combines three different analysis tools: Proteus, a low-fidelity aeroelastic framework with tailoring capabilities, OptiBLESS, an open-source toolbox for optimization of blended stacking sequences and MSC Nastran, a commercial software commonly used in industry for aeroelastic analyses. An experimental wing is designed using the proposed framework. The developed design is manufactured using carbon fibre pre-preg and hand layup technique. Finally, the manufactured wing is tested in the wind-tunnel at speeds up to 25 m/s. Both static and dynamic tests are performed, where for the latter a gust generator is used. The experimental results provide a source of comparison for the numerical models used in the proposed design methodology.@en

More then 10 years ago a large investment was made in extending the NSMB Navier Stokes Multi Block (NSMB) Computational Fluid Dynamics (CFD) towards Fluid Structure Interaction (FSI) simulations (Guillaume et al. in Fluid structure interaction simulation on the F/A-18 vertical tail, 2010 [1], Guillaume et al. in Aeronaut J 115:285–294, 2011 [2]). At that time a segregated approach was adopted using a loosely coupled approach. More recently NSMB was coupled to the open-source Finite Element Analysis environment B2000++ (http://www.smr.ch/products/b2000/ [3]) in a strongly coupled approach. This has led to the possibility to perform both static and dynamic FSI simulations using either a modal or a FEM approach without the need to interrupt the simulation. Results of aero-elastic simulations for the MDO-aircraft, the AGARD445.6 wing and for a Strut Braced Wing configuration will be presented.@en

Including a gust analysis in an optimization framework is computationally expensive as the critical load cases are not known a priori and hence a large number of points within the flight envelope have to be analyzed. Model order reduction techniques can provide significant improvement in computational efficiency of an aeroelastic analysis. In this paper, after thorough analysis of 4 commonly used model order reduction methods, balanced proper orthogonal decomposition is selected to reduce the aerodynamic system which is based on potential flow theory. The reduced aerodynamic system is coupled to a structural solver to obtain a reduced-order aeroelastic model. It is demonstrated that the dominant modes of the aerodynamic model can be assumed to be constant for varying equivalent airspeed and Mach number, enabling the use of a single reduced model for the entire flight envelope. A dynamic aeroelastic optimization method is then formulated using the reduced-order aeroelastic model. Results show that both dynamic and static loads play a role in optimization of the wing structure. Furthermore, the worst case gust loads change during the optimization process and it is important to identify the critical loads at every iteration in the optimization.@en

An analytical model to predict the fatigue life of a composite laminate is formulated. The model calculates stresses in each ply using classical lamination theory, degrades the residual strength using the linear wear-out law and predicts failure based on Tsai Wu failure theory. The cycles to failure are predicted using the updated cycle-by-cycle probability of failure. The predictions are validated for both a constant amplitude and a variable amplitude loading on a Glass/Epoxy laminate. Additionally the analytical model is extended to work with laminates described using lamination parameters instead of ply angles and stacking sequence. The analytical fatigue model is then integrated in the TU Delft aeroelastic and structural optimization tool PROTEUS. A thickness and stiffness optimization of the NASA Common Research Model (CRM) wing has been carried out. Results show that fatigue, strength and stiffness are the design drivers in the aeroelastic optimization of a composite wing. Furthermore, by including the analytical fatigue model instead of using a traditional knockdown factor to account for fatigue, a lighter wing is obtained.

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The research and innovation AGILE project developed the next generation of aircraft Multidisciplinary Design and Optimization processes, which target significant reductions in aircraft development costs and time to market, leading to more cost-effective and greener aircraft solutions. The high level objective is the reduction of the lead time of 40% with respect to the current state-of-the-art. 19 industry, research and academia partners from Europe, Canada and Russia developed solutions to cope with the challenges of collaborative design and optimization of complex products. In order to accelerate the deployment of large-scale, collaborative multidisciplinary design and optimization (MDO), a novel methodology, the so-called AGILE Paradigm, has been developed. Furthermore, the AGILE project has developed and released a set of open technologies enabling the implementation of the AGILE Paradigm approach. The collection of all the technologies constitutes AGILE Framework, which has been deployed for the design and the optimization of multiple aircraft configurations. This paper focuses on the application of the AGILE Paradigm on seven novel aircraft configurations, proving the achievement of the project’s objectives.

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High aspect ratio strut braced aircraft can significantly reduce the induced drag. The inherent anisotropic behaviour of the composite material along with their weight saving potential can improve the performance of the aircraft during the flight. Thus, a composite strut braced aircraft is one of the promising candidates to achieve the targets set by the European Commission in Flightpath 2050 report. In their previous works, authors have developed methodologies to include gust loads using a reduced order model and account for fatigue loads through an analytical model. In this paper, previously developed methodologies are used, to carry out a stiffness and thickness optimization of a composite strut braced wing which includes critical gust loads as well as fatigue loads. The results show that a composite strut braced wing is sized by both dynamic as well as static load cases. Additionally, by accounting for fatigue through analytical model instead of a knockdown factor, a lighter wing can be obtained.@en

An analytical model to predict the fatigue life of a composite laminate is discussed. It is based on the method developed by Kassapoglou to predict fatigue failure. The analytical model calculates stresses in each ply using classical lamination theory, degrades the residual strength using the linear degradation law and predicts failure based on Tsai Wu failure theory. The cycles to failure are predicted using the updated cycle-by-cycle probability of failure. The predictions are validated for both a constant amplitude and a variable amplitude loading on a Glass/Epoxy laminate. Furthermore the analytical model is extended to work with laminates described using lamination parameters instead of ply angles and stacking sequence. The analytical fatigue model is then integrated in the TU Delft aeroelastic and structural optimization tool PROTEUS. A thickness and stiffness optimization of the NASA Common Research Model (CRM) wing has been carried out. Results show that fatigue may play an important role in the aeroelastic optimization of a composite wing.@en

High aspect ratio strut braced aircraft can significantly reduce the induced drag. The inherent anisotropic behaviour of the composite material along with their weight saving potential can improve the performance of the aircraft during the flight. Thus, a composite strut braced aircraft is one of the promising candidates to achieve the targets set by European commission in Flightpath 2050 report. In this paper, multidisciplinary design analysis and optimization framework for strut braced aircrafts, is set-up involving tools provided by AGILE partners distributed worldwide. In the workflow, composite aeroelastic analysis and tailoring capability has been integrated with use of surrogate modelling. A design of experiment of the workflow with wing planform parameters as design variables is performed and a surrogate model is build. The optimization with an objective to reduce the fuel mass is performed using the surrogate of the workflow. @en

Including a gust analysis in an optimization framework is computationally inefficient as the critical load cases are not known a priori and hence a large number of points within the flight envelope have to be analyzed. Model order reduction techniques can provide significant improvement in computational efficiency of an aeroelastic analysis. In this paper a reduced order aeroelastic model is formulated by reducing the aerodynamic system with a balanced proper orthogonal decomposition and coupling it to a structural solver. It is demonstrated that the dominant modes of the aerodynamic model can be assumed to be constant for varying equivalent airspeed and Mach number, enabling the use of a single reduced model for the entire flight envelope. Comparison of the results from the full and reduced order aeroelastic model shows a high accuracy of the latter and a large saving in computational cost. A dynamic aeroelastic optimization framework is then formulated using the reduced order aeroelastic model. Results show that both dynamic and static loads play a role in optimization of the wing structure. Furthermore, the worst case gust loads change during the optimization process and hence it is important to identify the critical loads at every iteration in the optimization.@en