Design optimization offers the potential to develop lightweight aircraft structures with reduced environmental impact. Due to the high number of design variables and constraints, these challenges are typically addressed using gradient-based optimization methods to maintain efficiency, however overlooking the global design space. Moreover, gradients are frequently unavailable. Bayesian optimization presents a promising gradient-free alternative, enabling sample-efficient global optimization through probabilistic surrogate models. Although Bayesian optimization has shown its effectiveness for problems with a small number of design variables, it struggles to scale to high-dimensional problems, particularly when incorporating large-scale constraints. This challenge is especially pronounced in aeroelastic tailoring, where directional stiffness properties are integrated into the structural design to manage aeroelastic deformations and enhance both aerodynamic and structural performance. Ensuring the safe operation of the system requires simultaneously addressing constraints from various analysis disciplines, making global design space exploration even more complex. This study seeks to address this issue by employing high-dimensional Bayesian optimization combined with dimensionality reduction to tackle the optimization challenges in aeroelastic tailoring. The proposed approach is validated through experiments on a well-known benchmark case, as well as its application to the aeroelastic tailoring problem, demonstrating the feasibility of Bayesian optimization for high-dimensional problems with large-scale constraints.

Cantilevers find a wide range of applications in the design of scientific equipment and large-scale engineering structures such as aircraft wings. Analysis techniques based on linearization approximations are unable to capture the large amplitude oscillation behaviour of such structures and thus, necessitates development of dedicated nonlinear methods. In this work, the recent developments in the Koiter-Newton model reduction method are utilized to obtain nonlinear reduced order models (ROMs) from full finite element structural models in order to simulate large amplitude dynamics of cantilevers. The method describes a nonlinear system of governing equations comprising quadratic and cubic terms which are obtained as higher order derivatives of the in-plane strain energy. To ensure that the large rotations in cantilevers and the resultant foreshortening effect is also accounted for, a ROM updating algorithm is adopted where the ROM parameters are varied with the structural deflections. Linear eigenmodes of the structure are utilized to form the reduction subspace. To validate the methodology, the ROM solution is compared against experimental results and a convergence study is conducted to identify the number of modes needed to replicate the nonlinear response. Finally, a composite wingbox structure is considered for which time domain simulations are conducted and frequency response curves, obtained through a frequency sweep, are presented.

Bayesian Optimisation (BO) is a sample-efficient method for optimising expensive black-box functions, making it particularly suitable for engineering problems where gradients are unavailable and evaluating the objective or constraints is computationally costly. However, such problems often involve high-dimensional inputs and a large number of constraints, posing significant challenges for standard BO frameworks. While prior research has addressed scalability with respect to high-dimensional inputs in constrained settings, efficiently handling large numbers of constraints, i.e. high-dimensional outputs, remains an open problem. This work introduces Autoencoder-Enhanced Joint Dimensionality Reduction for Constrained BO (AERO-BO), a framework that performs dimensionality reduction in both the input (design variable) and output (objective and constraint) spaces via autoencoders. These autoencoders are trained online, requiring no pre-training, and their respective latent representations are connected through Gaussian Processes, which serve as surrogate models during optimisation. By operating in a joint latent space, AERO-BO enables scalable and efficient optimisation in settings with hundreds of design variables and thousands of black-box constraints.

Correction notice The CL in the title of Fig. 7(b) was corrected from 0.4 in the original version to CL=1.0. (a) Climb local lift spanwise distribution at CL=1 0 0.2 0.4 0.6 0.8 1 0 5 10 10-3 0 0.2 0.4 0.6 0.8 1 0 5 10 10-3 (b) Solid line (suction side)-dashed line (pressure side) Fig. 7 Local lift coefficient distribution with a selected friction coefficient of one section.

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.

The evolving designs and requirements of aircraft structural components has recently created an increased interest in application of nonlinear modelling techniques. While the finite element (FE) methods already incorporate the necessary mechanics to model nonlinear behavior in structures, a major drawback is the considerably higher computation cost in comparison to the linear counterparts. Reduced order modelling (ROM) techniques offer a solution to counter this limitation. The work presented here is focused on the Koiter-Newton (K-N) model reduction technique which is based on a cubically nonlinear mechanical model. The K-N method utilizes existing FE models as a starting point to generate equivalent ROM parameters and thus, can be applied to obtain ROMs for generic structures. The model validity is assessed by conducting nonlinear dynamic analyses of two models with different boundary conditions. Nonlinear frequency response analyses are conducted to demonstrate hardening effects in both the test cases. Comparisons to full FE analyses show significant reduction in computational times.

This study investigates the aerodynamic benefits of integrating trailing edge camber morphing on the strut of a regional strut-braced wing aircraft designed to cruise at Mach number of 0.5. Strut-braced wings are recognized for their weight advantages in high aspect ratio designs compared to the equivalent cantilever wings since the strut decreases the main wing’s bending moment. Hence, the induced drag component can be reduced due to the high aspect ratio without increasing the weight of the main wing. However, the strut increases the parasite drag component highlighting the need for innovative methods to improve the strut-braced wing overall aerodynamic efficiency. Recent studies have shown the significance of strut shape in the overall drag reduction and the necessity of maintaining high aerodynamic efficiency in off-design conditions. In this work, a genetic algorithm was utilized in conjunction with a mid-fidelity aerodynamic model to optimize the morphing strut trailing edge geometry across a range of climb and cruise conditions. The optimization objective was the minimization of drag and the design variables were the equivalent trailing edge deflection angles in seven sections of the strut. The results demonstrate a drag reduction of 0.5% to 3% both in climb and cruise. For lift coefficients below 0.8, the drag reduction is mainly attributed to the redistribution of the loading and the induced drag component reduction. In contrast, at lift coefficients above 0.8, the parasite drag component decreases due to the increased region of laminar flow over the upper wing surface.

In the development of electric aircraft, due to the use of Distributed Electric Propulsion (DEP), not only the classic wing flutter but also the propeller whirl flutter needs to be considered for wing structural design. To this end, this paper proposes an aeroelastic optimization method within the framework of an in-house tool named PROTEUS, which enables the preliminary design of DEP wing laminates including propeller whirl flutter effect. In this method, a new aeroelastic model is developed for the coupled propeller-wing system, based on a classic whirl flutter analysis model and the wing aeroelastic model implemented in PROTEUS. Further, the required sensitivities of aeroelastic stability constraints are derived and implemented by making use of these implemented in PROTEUS for conventional wing design. The objective of the optimization is to minimize wing mass by aeroelastically tailoring the lamination parameters and thickness of wing laminates, subject to given aerostructural design constraints. The features and usefulness of the proposed optimization approach are demonstrated through two numerical case studies (with and without whirl flutter constraints) focused on sizing the wing structure of a reference DEP aircraft. The necessary inputs regarding propeller mounting stiffness and damping for the case studies are determined through parametric studies of isolated propellers. The results indicate that including whirl flutter effect in wing sizing slightly increases wing mass, and introducing a flexible-mount-propeller leads to the decrease in wing flutter speed. Additionally, a parametric study of investigating propeller mounting stiffness is conducted, which confirms that the propeller mounting properties have a large influence on aeroelastic instability of the coupled propeller-wing system.

This paper aims to develop a reduced-order modelling methodology for nonlinear, unsteady, aerodynamic loads for active control transonic aeroelastic instabilities. To this end, a NACA0012 airfoil equipped with a flap is chosen as the test configuration. The aim here is to understand the interaction between the transonic shock dynamics and flap actuation at various amplitudes and frequencies. The high-fidelity simulations are carried out for two angles of attack, i.e. a = 0.0°, 4.0°. It is found that transonic buffet characteristics significantly change with airfoil geometry. Additionally, the flap is seen to be ineffective in the separated flow regions, thereby making the Ci-fi slopes highly nonlinear. However, increasing the frequencies of flap oscillations, increases flap effectiveness, increases control over buffet motion and moves towards linear lift responses. Furthermore, we also evaluate the performance of several Bayesian Filters that are crucial in the state-estimation process of the active control of nonlinear systems. It is observed that nonlinear filters such as Unscented Karman Filter perform better than the traditional linear Kalman Filter as system response to flap actuation becomes nonlinear in the presence of separated boundary layer.

High aspect ratio strut-braced wing aircraft can significantly reduce the induced drag while limiting the weight penalty of increasing the wingspan. As part of the Hybrid Electric Regional Wing Integration Novel Green Technologies (HERWINGT) project, a multifunctional morphing strut is being investigated. In this study, an optimization framework is proposed to define the thickness distribution of the morphing trailing edge of the strut to achieve the desired operational shapes while considering laminate manufacturing guidelines and material allowables. The optimizer finds designs capable of achieving the objective shapes and provides load and mass estimations that can be used to make design decisions.

Incorporating sensors such as microelectromechanical system (MEMS)-based inertial measurement units (IMUs) and strain gauges into aircraft structures has the potential to complement ground vibration testing results and improve the tracking of structural modes and wing shape in flight, as well as structural health monitoring. This study evaluates the feasibility and accuracy of employing MEMS accelerometers and gyroscopes together with strain gauges to estimate the structural modes of an aircraft. For this purpose, a ground vibration test was carried out on a 1:3 scaled Diana 2 glider model from which the displacement, rotation, and strain modes were estimated. The estimated modal parameters were compared with traditional piezoelectric accelerometer results and Finite Element Method model predictions. The results showed that the modal frequencies, damping ratios, and mode shapes estimated using MEMS IMUs and strain gauges closely matched the reference accelerometer estimates. Furthermore, the combination of displacement, rotation, and strain mode shapes allowed for greater insight into the structural dynamics. The exploratory use of gyroscopes for aircraft GVT allowed the structural torsion to be captured directly, thereby potentially simplifying future GVT setups by eliminating the need for placing accelerometers in pairs across the structure.

In the development of large battery-electric aircraft, integrating the batteries into the wing is a crucial decision, as it results in a lighter wing structure due to bending relief. To understand the influence of wing-integrated batteries on wing structures, this paper presents a study on the wing structural design and sizing of the Elysian E9X aircraft configuration. In this study, the baseline wing design is defined, and the critical load cases for wing sizing are identified. Wing structural sizing is performed using an in-house aeroelastic optimization tool. The objective of the optimization is to minimize wing mass by adjusting the thickness of the wing design sections, subject to various design constraints, including structural strength, buckling, and aeroelastic instability. Sensitivity studies on wing mass with respect to key design parameters are conducted. The study results confirm that placing the batteries in the wing results in a significant wing structural mass reduction compared to housing the batteries in the fuselage. The wing mass sensitivities to other design parameters, such as the spanwise position of the main landing gear, may serve as input for the next round of aircraft design.

This paper presents an experimental investigation into the aeroelastic behavior of an innovative wind turbine design featuring a downwind two-blade rotor with a teetering hub mounted on a tower with adjustable tilt. The rotor model incorporates two sets of elastic blades—stiff and flexible—for scaling purposes, each instrumented with strain gauges and accelerometers. Ground and wind tunnel tests were conducted to analyze the aeroelastic response. Static tests exhibited discrepancies between measured and numerically predicted displacements, with maximum displacements near the tip exceeding numerical predictions by 14% and 31% for flexible and stiff blades respectively. Frequency differences between measured and numerically simulated elastic modes ranged from 0.5% to 18% for both blade sets, as determined by ground vibration tests. Wind tunnel tests revealed the dominance of rotational speed harmonics, particularly the second harmonic, in the blades’ periodic response. A sensitivity analysis was also carried out with respect to tower tilt angle, rotational speed and blade pitch angle, for both blade sets at a range of tip-speed ratio values. The static response of the system, as captured by the generated power and thrust, was primarily sensitive to tower tilt angle variation and to a lesser extent blade pitch angle. Conversely, the tip-speed ratio in conjunction with rotational speed were found to dictate the dynamic response, influencing the azimuthal position and magnitude of the maximum bending moment at the blade root. Finally, no dynamic aeroelastic instability was observed during wind tunnel tests.

This paper contributes towards the development of a reduced-order modelling methodology for nonlinear, unsteady aerodynamic loads for the active control of transonic aeroelastic flutter. To this end, a 1-DOF torsional NACA0012 airfoil is chosen as the test configuration. The aim is to develop the reduced-order model in nonlinear state-space form to be used in active control scenarios. Hence, a nonlinear coupled differential equation that captures the shock dynamics. The underlying hypothesis of this work is that, once these aerodynamic effects are included in the low-order model, the nonlinear trend in the flutter stability boundary, specifically in the transonic regime, will be predicted purely based on first principles, without the need for numerical or experimental corrections. In this work, we observe that the aeroelastic system could become prematurely unstable as soon as the aerodynamic flow field undergoes a Hopf bifurcation. For low amplitude airfoil pitching below a certain threshold, the aerostructural system is seen to exhibit a coupled oscillator behaviour that has an exact linear analytical formulation. The analytical formulation thus produces an accurate prediction whilst being orders of magnitude faster than the numerical simulation.

Hybrid-electric aircraft represent a promising solution for the urgent need to decarbonize short-haul flights and bolster aviation sustainability. Nevertheless, the realization of hybrid-electric aircraft demands rigorous environmental impact analysis, given the substantial investments, time, and research required for technology development. This study offers a comprehensive life cycle inventory spanning the years 2030, 2040, and 2050 for both conventional and hybrid-electric aircraft configurations. Our inventory datasets are meticulously constructed through a systematic approach, ensuring data harmonization by drawing upon scientific literature, industry expertise, and primary data sources. This extensive dataset encompasses all pertinent systems necessary to model the environmental footprint of flights covering distances ranging from 200 to 600 nautical miles, utilizing a 50-passenger aircraft with the ATR42 as a reference model. Additionally, we furnish supplemental data for end-of-life considerations and uncertainty analysis. The systems under examination include the airframe, powertrain, power electronics and drives, batteries, fuel cells, hydrogen onboard storage, airport infrastructure, and battery charging stations. Notably, the carbon footprint of conventional aircraft aligns with data from the ecoinvent v3.8 database; however, our provided datasets are more than tenfold more detailed and incorporate a forward-looking perspective. These meticulously curated life cycle inventories can be amalgamated to simulate the potential environmental ramifications of conventional aircraft powered by kerosene or alternative aviation fuels, hybrid-electric aircraft utilizing battery technology, and hybrid-electric aircraft employing hydrogen as a fuel in conjunction with batteries. In this context, our findings play a pivotal role in nurturing the development of technology roadmaps that prioritize environmental sustainability within the realm of regional aviation.

This research investigates the application of aeroelastic tailoring to enhance the post-control surface reversal regime on a mid-range aircraft. Conventionally, active Maneuver Load Alleviation (MLA) is achieved through control surface actuation, while passive MLA utilizes structural modifications at the material or layout level to exploit wing wash-out deformation. Previous studies have demonstrated the significance of high control effectiveness in active MLA and the limitations of composite tailoring in passive MLA due to roll control authority constraints which typically result in stiffer wings with moderate mass savings. The aeroelastic optimization framework PROTEUS, developed at TU Delft, is employed to enhance operation in the post-control surface reversal regime. This is done to capitalize on increased control authority and thus promote load alleviation. The approach taken in this study is to identify critical constraints and assess the advantages of this strategy while acknowledging the technology’s immaturity, particularly its challenges in maintaining roll control effectiveness in certain flight envelope sectors. The results demonstrate significant mass savings in active MLA within the post-control surface reversal regime compared to conventional active MLA and highlight the substantial impact of the cruise-twist constraint on enhancing this regime.

Design optimisation potentially leads to lightweight aircraft structures with lower environmental impact. Due to the high number of design variables and constraints, these problems are ordinarily solved using gradient-based optimisation methods, leading to a local solution in the design space while the global space is neglected. Bayesian Optimisation is a promising path towards sample-efficient, global optimisation based on probabilistic surrogate models. While for problems with a low number of design variables, Bayesian Optimisation methods have demonstrated their strength, the scalability to high-dimensional problems while incorporating large-scale constraints is still lacking. Especially in aeroelastic tailoring where directional stiffness properties are embodied into the structural design of aircraft, to control aeroelastic deformations and to increase the aerodynamic and structural performance, the safe operation of the system needs to be ensured by involving constraints resulting from different analysis disciplines. Hence, a global design space search becomes even more challenging. The present study attempts to tackle the problem by using high-dimensional Bayesian Optimisation in combination with a dimensionality reduction approach to solve the optimisation problem occurring in aeroelastic tailoring, presenting a novel approach for high-dimensional problems with large-scale constraints. Experiments on well-known benchmark cases with black-box constraints show that the proposed approach can incorporate large-scale constraints.

This paper presents a method for identifying flight dynamics models for aircraft that includes effects from the flexible structure and the effects from unsteady aerodynamics. In the time domain, the unsteady aerodynamic effects are often modelled using aerodynamic lag states. The proposed method involves first determining the poles that govern the dynamics for these lag states from flight data. This is followed by reconstructing the time signal histories for these lag states so that they can then be used as part of the model fitting procedure. Flight tests were conducted using a scaled Diana2 glider unmanned aerial vehicle (UAV) in order to collect experimental data for modelling. To be able to measure the response of the aircraft and its structure, the glider was instrumented with a wide range of sensors including accelerometers, gyroscopes and strain gauges placed across the aircraft structure. During the flight, various excitation manoeuvres were conducted by the pilot while the aircraft responses were collected. From these measurements, a full flight dynamics model consisting of both lateral and longitudinal dynamics was then identified. Additionally, a model predicting the tail and wing root loads was also identified. First, a rigid aircraft model was fitted that was then extended with states corresponding to the flexible modes and aerodynamic lags. Comparison between the rigid and extended model showed that the addition of structural modes and aerodynamic lag states to the identified models can lead up to 30% improvement in predicting aircraft responses. In conclusion, the method developed and presented in this paper is able to identify flight dynamics models from flight data that more accurately capture the dynamics of flexible aircraft by including effects from the flexible structure and unsteady aerodynamics.

This research takes a further step towards the development of an autonomous aeroservoelastic wing concept with distributed flaps. The wing demonstrator, developed within the TU Delft SmartX project, aims to demonstrate in-flight performance optimization and multi-objective control using an over-actuated wing design. To address the challenges posed by the aeroelastic system’s nonlinearities and uncertainties, this paper employs an optimal control method relying on solving the State-Dependent Riccati Equation (SDRE). Geometrical nonlinearities, introduced in the form of plunge and torsion stiffness, make the system state-dependent and unsuitable for linear control methods. Additionally, a backlash model is incorporated to represent the uncertainty of the actuation system. The control strategy is implemented in a multi-objective manner to perform maneuver and gust load alleviation while accounting for the nonlinearities and uncertainties using the SDRE control. Firstly, a numerical sample case is investigated involving a state-dependent and highly non-linear canard aircraft configuration, to assess the ability of the SDRE control method. Then, in a numerical experiment, the effectiveness of the control strategy is evaluated through the nonlinear aeroelastic model. Evaluations are made on the practicality of the control approach, laying a foundation for future static and dynamic wind tunnel experiments with the SmartX-Neo demonstrator.

This study explores the implementation of aeroelastic tailoring in the design of a regional aircraft featuring a strut-braced wing (SBW). Making use of the aeroelastic optimisation framework from Delft University of Technology, PROTEUS, the research addresses two distinct cases. The first case involves a simplified SBW geometry to validate the modifications of PROTEUS, which were conducted to include the strut in the aeroelastic analysis. Static and dynamic load cases are compared with a NX Nastran aeroelastic model, showing good agreement in displacements, strains, and gust response. In the second case, the study investigates the weight-saving potential of aeroelastic tailoring in an SBW aircraft based on the ATR-72. Three optimisation scenarios, allowing various laminate types, are examined: unbalanced symmetric laminates, balanced symmetric laminates, and a thickness optimisation with a prescribed balanced symmetric stacking sequence. The results reveal that the prescribed stacking sequence limits stiffness tailoring, thereby also reducing potential weight savings. Moreover, the study shows how the presence of a strut reduces wing deflections, limiting the effectiveness of aeroelastic tailoring.