The exponential growth of the aviation sector combined with environmental and energy supply challenges have led companies to innovate to reduce aircraft fuel consumption. Among the many areas currently under research, this thesis investigates the possibility of making aircraft wings lighter thanks to flight load alleviation systems and more accurate modelling methods. The development process of an aircraft involves using low-fidelity aerodynamic models at the early stage of the design process to rapidly compute the loads acting on the airframe, and to evaluate the efficiency of wing control surfaces. These models are, however, limited to linear flow conditions, and transonic shock or flow separation cannot be simulated with such methods. This requires important safety factors and leads to generally heavier designs. Computational Fluid Dynamic with Reynolds-Averaged Navier-Stokes (CFD-RANS) analysis is capable of better aerodynamic predictions, but the computational time required for such simulations is too long to be efficiently included in the sizing process of the airframe. The approach proposed in this thesis aims to combine the accuracy of CFD with fast linear loads estimation. This is achieved by deriving reduced-order models (ROM) of the aircraft control surfaces and manoeuvre loads from rigid CFD analysis to improve the accuracy of faster but lower-fidelity results where needed. These fast aerodynamic models for the control surfaces also allow rapid control optimisation to evaluate their load alleviation potential. The thesis starts by introducing and validating the unsteady and non-linear models with 2D examples. Then, it covers the application of these models to a flexible 3D wing. The models are validated against high-fidelity steady and dynamic Fluid-Structure Interaction simulations and show good agreement with a 5% to 10% error margin in loads and deformations in most of the cases. Finally, a wingbox sizing optimization is performed with active load alleviation. Choosing to either use the linear or the non-linear aileron model for the GLA alone leads to a 2.5% difference in the wingbox structural weight. @en

In this paper, we present a data-driven method to model the unsteady non-linear response of aircraft control surfaces. This method relies on aerodynamic reduced-order models (ROM) derived from computational fluid dynamics with Reynolds averaged Navier-Stokes (CFD-RANS) analysis in the transonic domain. The ROM consists of a combination of look-up tables and transfer functions, with which we can capture the incremental unsteady loads from aileron and spoiler large deflections. The ROM can replicate transient CFD results with a 5% margin of error in most scenarios using a realistic 3D wing model. We also investigate a hybrid approach to calculate aeroelastic wing deformations. To do so, we simulate the control loads with our the aforementioned ROM, while we rely on a fast but robust low-fidelity method to model the wing aeroelastic response. We compared this method against high-fidelity analysis and estimated an average error of 5% to 10% in most of the cases with a three orders of magnitude decrease in simulation time. The rapidity of such load estimation technique makes it suitable for wing sizing and flight control optimisation problems.@en

In aircraft design, proper tailoring of composite anisotropic characteristics allows to achieve weight saving while maintaining good aeroelastic performance. To further improve the design, dynamic loads and manufacturing constraints should be integrated in the design process. The objective of this paper is to evaluate how the introduction of continuous blending constraints affects the optimum design and the retrieval of the final stacking sequence for a regional aircraft wing. The effect of the blending constraints on the optimum design (1) focuses on static and dynamic loading conditions and identifies the ones driving the optimization and (2) explores the potential weight saving due to the implementation of a manoeuvre load alleviation (MLA) strategy. Results show that while dynamic gust loads can be critical for wing design, in the case of a regional aircraft, their influence is minimal. Nevertheless, MLA strategies can reduce the impact of static loads on the final design in favour of gust loads, underlining the importance of considering such load-cases in the optimisation. In both cases, blending does not strongly affect the load criticality and retrieve a slightly heavier design. Finally, blending constraints confirmed their significant influence on the final discrete design and their capability to produce more manufacturable structures.

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In this paper, a methodology is presented to size an aircraft wing-box accounting for steady and dynamic loads combined with active control. Several aerodynamic corrections are used and benchmarked to ensure a consistent level of fidelity during the load analysis. Reduced order models (ROM) of the aircraft movables, gust loads and maneuvers loads are derived from rigid CFD analysis and used as substitutes for the loads in the aeroelastic simulation.@en

The proposed approach in this paper aims to bring the accuracy of CFD to quick linear aeroelastic simulations for structural sizing. This is achieved by deriving reduced-order models (ROM) of the aircraft movables, gust loads and manoeuvres loads from rigid CFD analysis and to use these as substitutes for the loads in the aeroelastic simulation. The corrected loads can then be incorporated into the wing-box sizing process which relies on a gradient-based optimizer that will eventually determine the optimal stiffness and thickness distribution of the wing structure.@en

Loads analysis and structural design are core steps of the aircraft design process. To reduce wing loads and with it the wing structural mass, methods of passive and active loads alleviation have been researched in recent years. However, those methods are currently implemented without the consideration of dynamic simulations or unsteady aerodynamics in the optimization process in aircraft predesign. The purpose of this research is to investigate the influence of passive and active loads alleviation methods on the structural mass in aircraft preliminary design. The methods have been applied on the wing of a Generic Mid-Range (GMR) aircraft configuration. The models have been created with ModGen, an in-house program at DLR Institute of Aeroelasticity. The models comprise FE-models for the structure and masses, as well as DLM (Doublet-Lattice-Method) model for the aerodynamics. For the investigation of the influence of the loads alleviation systems, a loop of loads analysis and subsequent structure optimization has been conducted. The loads analysis consists of gust and maneuver simulations. For the passive loads alleviation, an aeroelastic tailoring of the wing structure has been implemented, whereas for the active loads alleviation the ailerons are deflected to redistribute lift during maneuvers and to partially compensate lift increment during gust encounters. With the implemented methods, a first quantification of the influence of loads alleviation methods on the structural mass in aircraft predesign is possible.

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This paper summarises the design of a gust generator and the comparison between high fidelity numerical results and experimental results. The gust generator has been designed for a low subsonic wind tunnel in order to perform gust response experiments on wings and assess load alleviation. Special attention has been given to the different design parameters that influence the shape of the gust velocity profile by means of CFD simulations. Design parameters include frequency of actuation, flow speed, maximum deflection, chord length and gust vane spacing. The numerical results are compared to experimental results obtained using a hot-wire anemometer and flow visualisation by means of a tuft and smoke. The first assessment of the performance of the gust generator showed proper operation of the gust generator across the entire range of interest.

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In the present paper the authors want to investigate the effect of different load configuration in order to identify the ones driving the optimization. A set of static loads, gust loads and static loads with maneuver load alleviation (MLA) are tested. Gust loads have been included in the optimization via an equivalent static load (ESL). Composite blending is tackled by means of continuous constraints and a two phases approach is proposed to find a blended stacking sequence table. Results show that region of influence can be identified for specific loads and that MLA can be beneficial for structural weight reduction. Finally, the blending constraints prove their effectiveness by significantly reducing the error in retrieving a blended stacking sequence.

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A detailed performance investigation of the gust generator built for the Open Jet Facility wind tunnel at TU Delft is summarised in this paper. The influence of various parameters such as reduced frequency, measurement location, and excitation amplitude on the generated gust profile was quantified. In addition, unsteady lift measurements were performed using a rigid wing exposed to various gust profiles. In addition, unsteady incremental lift was compared with the DLM method which showed good agreement.

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This paper summarises a conceptual study regarding a passively actuated spoiler for gust load alleviation. The design of such system is intended to limit the use of computers, sensors and actuators to operate the device. The study mainly relies on using Theodorsen’s unsteady flow theory for a typical airfoil section. To cope with the limitations of this type of model for spoiler aerodynamic, corrections are brought from unsteady high fidelity flow simulations by means of transfer functions. The outcome is a 2D aeroelastic model with three degrees of freedom. It describes the spoiler contribution to the overall aeroelastic behaviour of the airfoil in the event of a gust encounter. Results show that the spoiler can help to reduce loads passively, but requires to be retracted with an active system to work properly.@en

This paper presents the results from the equivalent static load method applied to gust response optimisation of an aircraft wing. Through the different optimisation runs, it is assessed that gust load cases can be critical and are difficult to constrain with the sole use of static load cases. Several cases are evaluated with different gust parameters and wing boundary conditions. Effects of control efficiency and engine location are also studied.@en

The present paper proposes a set of blending constraints expressed in lamination parameter space, applicable during the continuous optimisation of composite structures. Thicknesses and ply orientations of large composite structures are often locally optimised in response to unequal spatial load distribution. During this process, ensuring structural continuity is essential in order to achieve designs ready to be manufactured. Single step stacking sequence optimisations relying on evolutionary algorithms to enforce continuity, through the application of blending rules, are prone to the curse of dimensionality. By contrast, multi-step optimisation strategies including a continuous sub-step can optimise composite structures with reasonable computational effort. However, the discrepancies between continuous and discrete optimisation step result in performance loss during stacking sequence retrieval. By deriving and applying blending constraints during the continuous optimisation, this paper aim is to reduce the performance loss observed between optimisation levels. The first part of this paper is dedicated to the derivation of blending constraints. The proposed constraints are then successfully applied to a benchmark blending problem in the second part of this paper. Numerical results demonstrate the achievement of near-optimal easy-to-blend continuous designs in a matter of seconds. Keywords: Composite materials, Blending, Lamination parameters, Optimisation, Variable @en