Fiber-reinforced composites are widely used in primary aircraft structures on account of their superior performance when compared to metallic structures. When buckling is a dominant driver of the structural design, the use of sandwich composites could potentially yield more efficient designs. This paper applies a recently developed approach for optimizing practical commercial-scale aircraft wings using sandwich composites in a preliminary design stage to perform design studies using the NASA Common Research Model (CRM) as a reference. The approach uses lamination parameters as design variables in a continuous optimization step. Structural constraints for classic composite laminate design, such as material failure and buckling, and for sandwich design, such as crimping, wrinkling, dimpling, and core shear failure, are accounted for using industrial-standard and empirical methods driven by finite element analyses. The optimization studies present comparisons in structural weight between sandwich composite designs and their monolithic counterparts. The studies present several cases where sandwich composites offer superior structural performance, as well as potential cost savings by affording a lesser number of stringers in the design.

This article presents the application of aeroelastic tailoring in the design of wings for a flying demonstrator, as well as the validation of the design methodology with flight test results. The investigations were performed in the FLEXOP project (Flutter Free Flight Envelope Expansion for Economical Performance Improvement), funded under the Horizon 2020 framework. This project aimed at the validation of methods and tools for active flutter control, as well as at the demonstration of the potential of passive load alleviation through composite tailoring. The technologies were to be demonstrated by the design, manufacturing and flight testing of an unmanned aerial vehicle of approximately 7 m wingspan. This article addresses the work towards the load alleviation goals. The design of the primary load-carrying wing-box in this task is performed using a joint DLR–TU Delft optimization strategy. Two sets of wings are designed in order to demonstrate the potential benefits of aeroelastic tailoring—first, a reference wing in which the laminates of the wing-box members are restricted to balanced and symmetric laminates; second, a tailored wing in which the laminates are allowed to be unbalanced, hence allowing for the shear–extension and bending–torsion couplings essential for aeroelastic tailoring. Both designs are numerically optimized, then manufactured and extensively tested to validate and improve the simulation models corresponding to the wing designs. Flight tests are performed, the results of which form the basis for the validation of the applied aeroelastic tailoring approach presented in the article.

Wings of modern aircraft have to be designed to give optimal response with respect to loads, comfort and performance. An essential part of the wing development is thus a design process which can take all these aspects into consideration. In the “Adaptive Wing” work package of the CleanSky “Smart Fixed Wing Aircraft” project, a multi-fidelity wing design method using aeroelastic tailoring has been developed. In the article, the process is presented in detail. The approach is based on a parametric wing design approach. Both beam models and shell models are derived and optimized in separate optimization environments. Investigations of the use of unbalanced laminates in aeroelastic tailoring are presented, employing the optimization of lamination parameters. The applications are demonstrated on two aircraft configurations, a long range and a short range transport aircraft. Further developments presented in the article include the introduction of CFD-based aerodynamics in the tailoring process, and a process extension to assess the influence of aeroelastic tailoring on fatigue.

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.

This paper presents an aeroelastic tailoring procedure used to design wing structures to meet strength, buckling, and flutter requirements simultaneously. The optimization process is divided into a continuous optimization step using lamination parameters, and a discrete optimization step, which produces detailed blended stacking sequences satisfying complex design and manufacturing guidelines. The implementation of the continuous optimization step benefits from recent advances in lamination parameter constraints, allowing for close approximation of realistic directional stiffness requirements. In the final discrete optimization step, a novel and efficient parametrization scheme called the Slice and Swap Method is developed. The scheme provides a design space that is simple to implement, allows for an ample family of blended stacking sequences to be explored, and includes a number of design requirements satisfied by suitable encoding. The efficiency of the aeroelastic tailoring procedure is demonstrated via a sample application to a realistic industry-scale regional jet wing.

This paper presents a semi-experimental method which can be used to predict the gust response of a wind tunnel model. The method incorporates transfer functions of the wind tunnel model – in this case a flexible forward swept wing – which are obtained using sweep excitations with a gust generator. The prediction results are then compared with those resulting from experimental measurements and numerical simulations. The observed quantities comprise accelerations and loads data. In the ideal case, the advantage of this semi-experimental prediction of the wing’s response to gusts lies mainly in the reduction of wind tunnel time. Instead of testing each gust profile at each frequency, a measurement of several sweep excitations would suffice.

This paper presents the application of aeroelastic tailoring in the design of flying demonstrator wings. The work is part of the Flutter Free Flight Envelope eXpansion (FLEXOP) project, funded under the Horizon 2020 framework. The project involves the design, manufacturing and flight-testing of a UAV toward two principle goals: i) to demonstrate the passive load alleviation potential through composite tailoring, ii) to validate methods and tools for flutter modelling and flutter control. The work presented here addresses the first of the above mentioned goals. The design of the primary load-carrying wing-box in this task is performed using a joint DLR – TU Delft optimization strategy. In total, two sets of wings are designed in order to demonstrate the potential benefits of aeroelastic tailoring – i) a reference wing wherein the laminates of the wing-box entities are restricted to balanced and symmetric laminates; ii) a tailored wing wherein the laminates are allowed to be unbalanced, hence enhancing the bending-torsion coupling essential for aeroelastic tailoring. The optimized design is then manufactured and extensively tested to validate and improve the simulation models corresponding to the wing design. Flight tests are scheduled to be performed in late 2018 to demonstrate the load alleviation capabilities attained through the applied aeroelastic tailoring.