The aeroelastic loads and design processes at the German Aerospace Center, Institute of Aeroelasticity in the framework of multi-disciplinary optimization are constantly evolving. New developments have been made in the in-house model generation tool ModGen, which allow us to create detailed fuselage models for preliminary design. As a part of the subsequent developments to integrate the fuselage structure in our aeroelastic design process, a new framework for optimizing the fuselage structure has been developed. The process is based on a bi-level optimization approach which follows a global–local optimization methodology to simplify a large optimization problem. A sub-structuring procedure is used to define stiffened panels as independent structures for local optimization. The panels are sized with stress and buckling constraints with consideration of several aeroelastic load cases. Furthermore, in this paper, we present a physical sub-structure grouping process which enables reduced number of panel optimizations and saves considerable computational effort with little compromise in the solution accuracy.

Stiffness directions of wing structures are already part of the optimisation in aircraft design. Aircraft like the A350 XWB and the Boeing 787 mainly consist of such composite material, whose stiffness directions can be optimised. To proceed with this stiffness optimisation, the aim of this work is to modify and optimise also the linear stress-strain relation. On that account, the Hooke’s law is exchanged by a multi-linear formulation to analyse any nonlinear elastic structural technology on wing structures. The wing structures, which are used to investigate the nonlinear behaviour, are deduced from a mid-range and a long-range aircraft configuration. These wings are analysed with an extended beam method and coupled with a VLM solution to calculate the aeroelastical loading. The proposed beam method is capable of analysing any multi-linear wing structure technology. A degressive structural behaviour shows up a good potential to reduce the bending moment which is one of the main drivers of the structural weight.

A Nastran Bulk Data Deck of a miniature wing is provided. The wing is a small-scale representation of the DLR ISTAR research aircraft wing. The model consists of quadrilateral CQUAD4 shell elements with multi-layer GFRP composite properties. It has been built using the DLR in-house software ModGen which enables the parametric design and optimization of full aeroelastic analysis models. The input ".bdf"-file was used in MSC Nastran version 2018.2 to generate the output files ".h5, .f06, .xdb". By default a normal modes analysis is conducted with the wing being clamped at the symmetry plane of the aircraft. https://www.dlr.de/en

Composite structures have shown a prominent impact in the aircraft structural design. With an increasing shift towards incorporating more composite materials in the primary aircraft structure it is imperative to have corresponding design tools to simplify the design process. In the present work, a simplified implementation for composite optimization has been developed within the DLR-AE (German Aerospace Centre, Institute of Aeroelasticity) automated aeroelastic structural design framework cpacs-MONA. This paper presents the results of structural optimization of a high aspect ratio composite wing aircraft model developed in the DLR project ATLAs. The generation of almost all involved simulation models for this study is done using the in-house DLR tool ModGen. An aeroelastic trim analysis is conducted for various manoeuvre and gust conditions. A load selection process is used to determine the most relevant sizing load cases. A comparison is made between the optimization results of a composite wing and an aluminium wing to demonstrate the more favourable strength to weight ratio of the composite wing. A manoeuvre load alleviation procedure has been introduced in the load calculation process. The results show further weight savings in the design process when load alleviation is utilized due to reduction in the span wise bending moment.

This chapter presents the conceptual loads analysis and the structural design of the unmanned cargo aircraft concepts within the Automated Low Altitude Air Delivery (ALAADy) project of the German Aerospace Center, DLR. Three concepts of a gyroplane, a fixed high-wing with a twin boom v-tail aircraft and a box wing aircraft are concerned in the project. The main focus is on the estimation of the structural weight of each unconventional aircraft configuration. However, the application of empirical formulations based on conventional aircraft configurations is not suitable for this task. Instead, the task is performed with a parametric design process which is designed to be used for the structural design of an unconventional aircraft. In this process, the structural models are set up as finite element models in a parametric manner. The loads analysis and the aeroelastic structural dimensioning are then performed using these finite element models. This process is developed by the DLR and it is called the MONA process. The major result of the process is the structural weight of each unmanned aircraft (UA) concept. Finally, the structural weights of all aircraft are compared. The weight comparison contributes to the global evaluation of the aircraft concepts within the ALAADY project. Based on the considered aircraft requirements, the considered load cases and the considered system masses, the gyroplane has the minimum structure weight of 2,720 kg.

By virtue of using efficient methods to compute the design sensitivities, such as the coupled adjoint methods, gradient-based optimization techniques allow aircraft designers to efficiently obtain an optimum design, that satisfies all considered constraints. The more constraints and disciplines engaged, the higher the reliability of the optimization output. Decision makers in aircraft industry, however, prefer to look at Pareto fronts rather than one optimum design before making their critical decisions, since such diagrams provide better understanding of the main trade-offs and compromises between the different targets that the aircraft is supposed to satisfy. Gradient-free algorithms have better reputation for generating Pareto fronts because these fronts result naturally. These algorithms are, however, relatively inefficient, even for single disciplinary high-fidelity optimizations. They are restricted by the number of design parameters and the size of the design space. Engaging more disciplines, more design parameters and higher fidelity computational models, to increase the reliability of the outputs, can only increase the computational cost of gradient-free algorithms, and quickly announce them to be not usable for the generation of Pareto fronts. This study investigates the generation of Pareto fronts efficiently, using gradient-based algorithms on the industry-relevant aircraft; AIRBUS XRF-1. The approach starts several optimizations with different weighting of objectives, in parallel. The multidisciplinary design chain, which is used in this study, was connected by experts in aerodynamics, structure and loads, propulsion and overall aircraft design. The generation of Pareto front was completed successfully for a multi-point multidisciplinary optimization task, where the use of gradient-free algorithms would have been computationally infeasible. As expected and mentioned in the literature, some regions of the Pareto front were not satisfactorily covered by employing this approach, which shows the necessity to look into different gradient-based approaches that search for optimal designs along the Pareto front.

The purpose of this paper is to investigate the influence of the engine position and mass as well as the pylon stiffness on the aeroelastic stability of a long-range wide-body transport aircraft. As reference configuration, DLR’s (German Aerospace Center/Deutsches Zentrum für Luft und Raumfahrt) generic aircraft configuration DLR-D250 is taken. The structural, mass, loads, and optimization models for the reference and a modified configuration with different engine and pylon parameters are set up using DLR’s automatized aeroelastic design process cpacs-MONA. At first, the cpacs-MONA process with its capabilities for parametric modeling of the complete aircraft and in particular the set-up of a generic elastic pylon model is unfolded. Then, the influence of the modified engine-wing parameters on the flight loads of the main wing is examined. The resulting loads are afterward used to structurally optimize the two configurations component wise. Finally, the results of post-cpacs-MONA flutter analyses performed for the two optimized aircraft configurations with the different engine and pylon characteristics are discussed. It is shown that the higher mass and the changed position of the engine slightly increased the flutter speed. Although the lowest flutter speeds for both configurations occur at a flutter phenomenon of the horizontal tail-plane outside of the aeroelastic stability envelope.

Within the DLR project VicToria various high fidelity-based MDO processes were set-up as applicalble methods for aircraft design. Apart from aerodynamic optimization using high fidelity-based CFD analysis, the sub-processes overall aircraft design synthesis, loads analysis, and structural optimization were part of the MDO processes. The presented paper expounds such MDO sub-processes in order to exhibit their contributions and capabilities for the respected MDO process.

Das Paper gibt eine Ubersicht uber die Arbeiten und Ergebnisse im Projekt KonTeKst. Im Projekt wurden im ersten Hauptarbeitspaket zwei Flugzeugkonfigurationen mit einer konfigurativen Larmreduktion im Vergleich zu einem Referenzflugzeug entworfen sowie hinsichtlich Leistung, Larmabstrahlung und Emissionen bewertet. Das zweite Hauptarbeitspaket befasste sich mit ausgewahlten Technologien, die im Bereich des Kurzstreckenflugzeugs die Larmreduktion am Flugzeug zum Ziel haben. Dazu gehorten Untersuchungen an Hochauftriebshilfen sowie Masnahmen zur ihrer konstruktiven Gestaltung sowie Analysen des Larmverhalten unterschiedlicher Treibwerkskonfigurationen. Im dritten Hauptarbeitspaket wurde der DLR-ubergreifende Lastenprozess um spezifische Funktionalitaten erweitert. Die Verfahren wurden durch experimentelle Untersuchungen, so z. B. Auswertungen von Flugversuchen sowie Durchfuhrung eines Windkanalversuches fur aktive und passive Lastabminderung von Boenlasten, validiert. Im Hauptarbeitspaket vier wurden die Auswirkungen des Einsatzprofils von Kurzstreckenflugzeugen, d. h. eine hoheren Anzahl von Starts- und Landungen pro Tag, auf den Flughafen, die Prozesse fur die Abfertigung von Passagieren und Fracht sowie fur die Fuhrung des Flugzeugs am Boden und in der Luft untersucht. Zunachst wird die Motivation des Projekts herausgearbeitet, vor allem vor dem Hintergrund der Leitkonzepte des DLR. Anschliesend werden die Arbeiten im Projekt kurz vorgestellt und projektubergreifende Ergebnisse prasentiert. Schlieslich wird ein Ausblick auf die weitere Arbeit im neuen Leitkonzept 'Transportflugzeug' gegeben.

The DLR project VicToria brings together disciplinary methods and tools of different fidelity for collaborative multidisciplinary design optimization (MDO) of long-range passenger aircraft configurations, necessitating the use of high-performance computing. Three different approaches are being followed to master complex interactions of disciplines and software aspects: an integrated aero-structural wing optimization based on high-fidelity methods, a multi-fidelity gradient-based approach capable of efficiently dealing with many design parameters and many load cases, and a many-discipline highly-parallel approach, which is a novel approach towards computationally demanding and collaboration intensive MDO. The XRF-1, an Airbus provided research aircraft configuration representing a typical long-range wide-body aircraft, is used as a common test case to demonstrate the different MDO strategies. Additional results are presented for the NASA Common Research Model (CRM) to show their flexibility. Parametric disciplinary models are used in terms of overall aircraft design synthesis, loads analysis, flutter, structural analysis and optimization, engine design, and aircraft performance. The different MDO strategies are shown to be effective in dealing with complex, real-world MDO problems in a highly collaborative, cross-institutional design environment, involving many disciplinary groups and experts and a mix of commercial and in-house design and analysis software.

The paper addresses the application of a parametric design process for a flying wing configuration. The multi-disciplinary configuration (MULDICON) is a generic unmanned combat air vehicle (UCAV) developed for research purposes, a further development of the DLR-F19 configuration, which was used for research activities in the scope of the DLR project Mephisto and its predecessors FaUSST and UCAV2010. For the MULDICON, the DLR parametric design process MONA is applied. Special emphasis is placed on the structural modeling with composite material where each layer is modeled and analyzed. Various failure criteria are compared to define suitable constraints for the optimization of the load carrying structure. In contrast to optimize the thickness of composites with global allowable strains, such strategy allows for a detailed analysis of every layer. The number of constraints due to the set-up of every ply is substantially increased compared to the strain allowables but the structural optimization is still applicable. The detailed structural and mass models represent the global stiffness and structural dynamic characteristics of the aircraft. For the loads analysis part of the design process, 9 different mass configurations with a total of 306 maneuvering load cases as well as 336 1-cos gust load cases are taken into account. Furthermore, a new simplified landing impact simulation is introduced to consider 12 landing load cases. All load cases are defined according to regulations like CS-25. Such number of load cases is necessary to cover a sufficient number of flight conditions. For the selection of the design loads for the structural optimization, the essential loads are analyzed for a subset of locations. Together with a parametrized optimization model, the structural optimization is conducted. The result is a weight-optimized structural model for the MULDICON. This entire model allows for the investigation of physics-based effects already at an early stage of the design process.

Over the past decade, profound attention was given to exploring the benefits of engaging numerical multidisciplinary design optimization in aircraft design. Due to its importance, aerostructural wing design optimization is the most visited multidisciplinary problem in research institutes. To deal with this problem efficiently, gradient-based algorithms are popularly used. The complexity of the gradient-based aerostructural optimization, however, forced researchers to apply several simplifications to the problem formulation, such as neglecting engine effects or oversimplifying the loads process into few predefined load cases. The authors of this paper aim at running a gradients-based multidisciplinary design optimization of a commercial aircraft while including a powered engine and engaging a comprehensive, multi-fidelity loads process, subject to flutter as well as overall aircraft design constraints. The work, done by experts in the mentioned fields, is performed on a commercial aircraft provided by Airbus with many industry-relevant constraints. The results, showed the necessity to include a comprehensive loads process during the optimization. Additionally, it was concluded that engaging powered engines during the optimization is inevitable to come up with realistic designs; significantly different design geometries resulted when engaging a powered engine than when running the optimization with a flow-through nacelle.

This paper presents the aeroservoelastic modelling toolchain established for the aircraft design exercise within the European research project, FLEXOP. The FLEXOP project aims to develop and apply active flutter suppression and load alleviation techniques on an unmanned flying demonstrator. The developed methods are then to be applied in the design of a commercial-scale wing derivative in a scale-up task. A high-fidelity finite element (FE) structural model is the first block in the modelling process. A condensed FE model together with aerodynamic models based on the doublet-lattice (DLM) and vortex-lattice (VLM) methods represent the aeroelastic system. The aerodynamics represented by the afore-mentioned panel methods is complemented by results from higher-fidelity computational fluid dynamics (CFD) simulations. Reduced-order aeroservoelastic models suitable for control-synthesis are then generated using a “bottom-up” modelling approach. The aim of the paper is to present an overview of the different models encountered during such a design process and their domains of application. � 2019 by German Aerospace Center (DLR). Published by the American Institute of Aeronautics and Astronautics, Inc.

The highly parameterized process cpacs-MONA for structural and aeroelastic design of aircraft configurations is presented. The parameterized design process is based on the Common Parametric Aircraft Configuration Schema (CPACS), developed by DLR, where almost all aircraft parameters like mission, geometry, structure, and material are defined. The process stands out for its independent use for structural and aeroelastic design on the one hand and its integration into high fidelity based multidisciplinary optimization tasks on the other hand. The process consists of preliminary mass and loads estimation based on conceptual design methods followed by a parameterized set-up of simulation models and an optimization model. They are used for a comprehensive loads analysis followed by a component wise structural optimization. The latter takes stress, strain, buckling and control surface efficiency as constraints into account. The structural simulation model consist of shell and beam elements for the wing-like component and the fuselage in order to represent the construction of the load carrying structure of the complete aircraft appropriately. Such depth of modelling allows also for the use of well-established structural optimization methods. MONA stands for ModGen, the in-house parametric model generation computer program, and MSC Nastran, used for loads and aeroelastic analysis as well as for structural optimization. cpacs-MONA is also integrated in MDO tasks, where other disciplines like aerodynamics and overall aircraft design are involved. Therein aerodynamic data from high fidelity CFD calculations are used to improve the vortex lattice based aerodynamic method that is applied of the aeroelastic loads analysis. The MDO tasks where cpcas-MONA is integrated can be gradient-free and gradient-based. For the gradient-based MDO task cpacs-MONA delivers to the system optimization level the structural variables, the structural responses the corresponding sensitivities. As structural optimization task alone can deal with a high number of variables and constraints, for the MDO task with other disciplines, like aerodynamics, methods were developed where the number of constraints is reduced that are taken into account on the system level. Two applications are presented for cpacs-MONA. In the first application cpacs-MONA is applied as independent and stand-alone structural and aeroelastic design process. For a long range wide body transport aircraft configuration the design loads case are estimated after a comprehensive loads analysis campaign. For the structural design especially the structural requirements and alternative strategies to achieve sufficient control surface effectiveness within the flight envelope are investigated. In the second application cpacs-MONA is part of a gradient-free optimization task with aerodynamics and the structure to be optimized simultaneously. The presented results show that the aerodynamic optimum lead on the one hand to a heavier wing mass, but investigating the structural optimization results more closely, also local benefits of the new aerodynamic design regarding the wing structure can be found.

Aircraft design under the terms of regulations postulates load cases between 2.5g and ‑1.0g. According to statistical load data, high manoeuvre and gust loads (-1.0g and 2.5g) occur seldom, but they determine aircraft structures. In contrast to linear aircraft materials (e.g. aluminium, carbon), rubber like materials show nonlinear elastic behaviour. Hence, the aim is to create passive load alleviation with nonlinear elastic materials. The idea is to increase the performance during low load cases around cruise and to decrease the loading for high load cases. In other words, we want to create a wing which is stiff during cruise and which gets more flexible at high loading cases. To examine this, an iterative process computes three trimmed quasi steady manoeuvres (1.0g, 2.5g, -1.0g). In doing so, the process considers the aeroelastic coupling. It iteratively calculates loads and deformations, using the Vortex Lattice Method (VLM) for the aerodynamic analysis and the Nonlinear Solution of MSC Nastran (SOL400) for the structural analysis. Also, Solution 400 computes the deformation due to the loads iteratively. The process utilises a rectangular 30° aft swept wing with a wingspan of 60 m which is close to common long range aircraft. A finite element model with beam elements represents the load carrying structure. A vortex lattice simulates the aerodynamics. The reference wing structure with the linear material is untwisted. To compare two different approaches of nonlinear elastic materials, the corresponding wing structures are pretwisted. This ensures an equal lift distribution of the 1g load cases. The root bending moment decreases of about 3.98% for the material with the stiffer area at cruise conditions.

This paper presents an aero load correction strategy applicable to the static aeroelastic optimization of composite wings. The optimization framework consists of a successive convex subproblem iteration procedure, in which a gradient-based optimizer consecutively solves a local approximation problem. Responses are approximated as a linear and/or reciprocal function of the laminate membrane and bending stiffness matrices. Together with the laminate thicknesses h, they constitute the design variables in the optimization process. Internally, the design space is transformed from stiffness matrices to lamination parameters, resulting in a continuous and convex optimization problem. Structural responses considered in the stiffness optimization are strength, local buckling and mass; aileron effectiveness, divergence, and twist constitute the aeroelastic responses. Steady aeroelastic loads are calculated with a doublet lattice method (DLM) embedded in the applied finite element solver, allowing for the generation of response sensitivities that incorporate the effects of displacement-dependent aeroelastic loads. To incorporate flow phenomena that cannot be reproduced with DLM, a higher order aerodynamic method is considered. The developed correction methods and their application are presented in this paper. The correction is twofold, first, aiming at a correction of DLM by means of camber and twist modifications applied directly to the doublet lattice mesh and second, by employing the capabilities of a higher order computational fluid dynamics (CFD) solver, like the DLR-based TAU code. To this end, DLM loads transferred to the structure are rectified by means of the supposedly superior CFD results. The aero load correction method is applied in the stiffness optimization of a forward swept wing. First, a trim application without structural optimization is discussed, to demonstrate the convergence behavior of the correction forces. The results of a wing skin mass minimization with balanced and unbalanced laminates are presented. In particular, the differences between optimizations with and without aero correction are highlighted. Eventually, a stacking sequence optimization based on the continuous optimization results is demonstrated.

The DLR project VicToria deals also with high fidelity-based MDO with various approaches. Aside from aerodynamic optimization, further sub-processes are part of the developed MDO processes. They belong to overall aircraft design synthesis, loads analysis, and structural sizing and optimization. The Paper lays out the mentioned MDO sub-processes in order show their contribution and capabilities for the selected MDO process, but also their complexity when dealing with a high fidelity based MDO approach.

This paper provides an overview of current activities of DLR (German Aerospace Center) with respect to stability and control investigations in the context of early stages of aircraft design. For this purpose, DLR follows an interdisciplinary and multi-level design approach. Using an integration framework in combination with a central data exchange format, largely automated process chains are set up that combine calculation and simulation capabilities of the multitude of disciplines required in early aircraft design. Rather than using empirical relations and assumptions based on experience, the underlying methods applied by the tools are mainly based on physical model representations. The major aim of this design approach is to generate all relevant data needed for stability and control investigations, including aerodynamic damping derivatives and to assemble them within a flight dynamics model. Not only does this approach allow for an early consideration of stability and control characteristics, but it also respects interdisciplinary effects and enables automated design changes. This paper describes the infrastructure used for setting up the described process. It presents disciplinary tools used to calculate engine performance maps, calculate aerodynamic performance maps and structural properties, generate flight dynamics models with associated control laws and to assess aircraft handling qualities. Furthermore, this paper provides application examples of early stability and control considerations, using integrated interdisciplinary process chains. This comprises a handling qualities assessment under uncertainty considerations and vertical tailplane sizing for a blended wing body. In addition, engine and split flap sizing processes for an unmanned combat aerial vehicle are shown. The interdisciplinary design approach presented here, serves to find a well justified early configuration and reduces the risk of later design changes.

Background: The consideration of composite design in an early design phase has become more and more necessary in aircraft design. In the scope of multidisciplinary optimisation, a composite aeroelastic reference model is often desired to investigate multidisciplinary effects on a near-industrial application. Objective: The aim is to generate a benchmark aeroelastic model by developing a robust design process for composite configurations. Method: This is done by using a continuous gradient based optimisation with lamination parameters as design variables followed by a discrete stacking sequence retrieval combined with a comprehensive load analysis routine comprising manoeuvre, gust, landing loads and a manoeuvre loads alleviation system. Simulation Models: The process utilises a GFEM/Dynamic, a DLM and a condensed FEM model based on the reference XRF1 configuration. Results: Results presented show the optimised wing-box design, comparing the effect of a manoeuvre load alleviation system, the effect of including blending constraints in the optimisation, and the change of the structrual design when moving to a stacking sequence design.