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... particular, we are interested in the ratio between friction and pressure drag, and hence size the optimization grid such that this ratio is as close to the grid converged value as possible. An overview of the grids is provided in Table 1. The off-wall spacings in wall units, y + , are based on the flow solutions of the optimized geometries. ...
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Citations
... The fin and the unnecessary fuselage length extension lead to a significant wetted area and friction drag penalties, which hinder the induced drag benefits of the concept. Given Prandtl plane concepts are of particular interest for green transport aviation, most recent studies are dedicated to stability issues of large box-wing aircraft concepts [9][10][11][12][13], with extremely few articles about light box-wings. ...
... However, directional stability details are missing in this concept due to it being equipped with a conventional twin tail with large vertical surfaces that ensures enough stability margin at the cost of a significant wetted area. As can be noted from most recent studies [9][10][11][12][13][14][15][16][17][18], longitudinal stability of box-planes has been studied very thoroughly, in particular for heavy transport mission profile at transonic speeds. There is, however, a significant knowledge gap concerning directional stability, especially for a light subsonic box-wing lacking a conventional fin. ...
This study aimed to explore the directional stability issues of a previously studied light box-wing aircraft model with a pusher propeller engine in the fuselage aft section. Earlier configurations have included the use of fuselage together with a lifting system consisting of two wings joined together at their wingtips with vertical stabilizers. However, these side vertical surfaces failed to provide the aircraft with sufficient directional stability, thus prompting the quest in this study for novel solutions that would exclude the need for a fuselage extension and a typical fin. Solutions included the use of a ducted propeller and few configurations of small "fishtail" vertical fins, which formed part of the aft fuselage itself and coupled with vortex generators on the fuselage surface to improve their interference and heal flow separation at the fuselage aft cone. The results of wind tunnel testing were supported with CFD simulations to explain the flow behavior of each of the studied solutions. Tuft visualization and computed flow patterns allowed identification of the sources of the observed low efficiency in terms of directional stability of the fishtail against a simple idle duct without a propeller. A final configuration with a duct and a modified version of the fuselage fins was achieved that provides enough yaw stability margins for a safe flight.
... In this case, for a fixed lift coefficient, the viscous drag of a box-wing is 26% higher than the one of the elliptical wing, whereas a benefit of 46% in terms of induced drag is obtained. This result is in line with previous aerodynamic shape optimization studies of box-wings, in which Euler-based [14,15] and RANS-based [16] approaches have been implemented to minimize drag considering prescribed design variables and constraints. In both cases, the box-wings were able to redistribute the total lift from one wing to the other, satisfying several design constraints, while simultaneously improving aerodynamic performance. ...
... In this case, the fore and aft wings are characterized by wash-out (i.e., decreasing the incidence angle from the wing root out to the wing tip) and wash-in (i.e., increasing the incidence angle from the root to the tip), respectively. The optimized results are consistent with those of past studies and with theoretical results, providing a reliable estimate of the performance of the BW aircraft [14,16]. Figures 10 and 11 illustrate the surface pressure contours and pressure distributions, respectively, across the vertical wing. ...
This paper presents an exploration of the aerodynamic design of a box-wing aircraft concept through high-fidelity aerodynamic shape optimization. The optimization framework consists of B-spline parameterization surfaces, an integrated mesh parameterization and deformation scheme based on the theory of linear elasticity , free-form and axial deformation geometry control, a Newton-Krylov-Schur flow solver for the Reynolds-averaged Navier-Stokes equations, a gradient-based optimizer, and the discrete-adjoint method for gradient evaluation. These modules are integrated in an aerodynamic shape optimization framework called Jetstream, which is used to minimize the cruise drag of a single-aisle, medium range box-wing aircraft at transonic flight conditions (Mach = 0.78) with lift, trim, and minimum wing volume constraints. The drag of the optimized box-wing configuration is about 10% lower than the initial concept due to reduced wave drag from the near elimination of shocks and reduced induced drag from an optimal spanwise lift distribution. These results demonstrate the potential aerodynamic advantages of the box-wing concept, and the importance of high-fidelity analysis and optimization tools towards the development and evaluation of novel configurations that can lead to reduced fuel burn and emissions.
... From the point of view of academic projects, many authors have used low-fidelity [22][23][24]26], medium fidelity [27,28] and high-fidelity [29,30] methodologies to compare the aerodynamic benefits of BW configurations against CTW designs. Despite their obvious differences, mainly regarding aerodynamic modeling and optimization algorithms, the studies have offered useful insight into the basic patterns and trade-offs of the large-scale geometric parameters of a BW design. ...
... For the BW case, sweep values ( Λ ) are determined by the stagger variation so that the fore wing has an aft-swept and the aft wing has a forward-swept while keeping their tips aligned, i.e., the vertical tip fin [3] has zero cant angle. Since the sweep angle has a significant Previous studies on BW concepts have shown the Lift-to-Drag (L/D) ratio improves when height-to-span and staggerto-span ratios increase [16,28,30]. Therefore, the fore wing leading edge station was chosen as a design variable, enabling the BW concept to be compatible with both the fuselage length of the reference aircraft and the current airport infrastructure. ...
Unconventional configurations and innovative propulsion technologies have been continuously developed for reducing both fuel-burn and global net carbon emissions. This article describes an advanced civil transport aircraft designed from the combination of a Box-Wing configuration with a Boundary Layer Ingestion (BLI) propulsion system. A conceptual-level Multidisciplinary Design Optimization strategy provided the main aerodynamic and performance characteristics of the aircraft, based on appropriate design requirements, variables and constraints. For direct performance comparison against a conventional aircraft, a single-point objective function based on minimum block fuel was evaluated by means of low-fidelity aircraft models. Subsequently, a back-to-back Computational Fluid Dynamics assessment of non-BLI and BLI versions of the aircraft was performed. Two major analyses comprised the aerodynamic evaluation: (i) quantification of the BLI benefit using the power balance method, (ii) performance evaluation of the propulsor inlet in terms of the total pressure recovery and the distortion index. The conceptual design results showed the box-wing configuration provided major fuel-burn savings compared to its conventional counterpart. On the other hand, the BLI version reduced engine power requirements at cruise in comparison with the non-BLI version, but decreased the total pressure recovery, resulting in more distortion at the aerodynamic interface plane. The main contribution of this study lies on the potential benefits of such an original unconven-tional configuration, whose technologies increased aerodynamic performance, which reduced fuel consumption and hence carbon emissions.
... The fin and the unnecessary fuselage length extension lead to a significant wetted area and friction drag penalties, which hinder the induced drag benefits of the concept. Given Prandtl plane concepts are of particular interest for green transport aviation, most recent studies are dedicated to stability issues of large box-wing aircraft concepts [9][10][11][12][13], with extremely few articles about light box-wings. ...
... However, directional stability details are missing in this concept due to it being equipped with a conventional twin tail with large vertical surfaces that ensures enough stability margin at the cost of a significant wetted area. As can be noted from most recent studies [9][10][11][12][13][14][15][16][17][18], longitudinal stability of box-planes has been studied very thoroughly, in particular for heavy transport mission profile at transonic speeds. There is, however, a significant knowledge gap concerning directional stability, especially for a light subsonic box-wing lacking a conventional fin. ...
This study aimed to explore the directional stability issues of a previously studied light box-wing aircraft model with a pusher propeller engine in the fuselage aft section. Earlier configurations have included the use of fuselage together with a lifting system consisting of two wings joined together at their wingtips with vertical stabilizers. However, these side vertical surfaces failed to provide the aircraft with sufficient directional stability, thus prompting the quest in this study for novel solutions that would exclude the need for a fuselage extension and a typical fin. Solutions included the use of a ducted propeller and few configurations of small “fishtail” vertical fins, which formed part of the aft fuselage itself and coupled with vortex generators on the fuselage surface to improve their interference and heal flow separation at the fuselage aft cone. The results of wind tunnel testing were supported with CFD simulations to explain the flow behavior of each of the studied solutions. Tuft visualization and computed flow patterns allowed identification of the sources of the observed low efficiency in terms of directional stability of the fishtail against a simple idle duct without a propeller. A final configuration with a duct and a modified version of the fuselage fins was achieved that provides enough yaw stability margins for a safe flight.
... Finally, these BW concepts share specific design characteristics such as a rear installation of the engines and fuselage-mounted main landing gear, which increase fuselage weight, as well as cost and integration complexity. • There are a few works focused on high-fidelity optimization of BW concepts [249,250]. Such works provided a more detailed perspective about its benefits in terms of the geometric arrangement. ...
In recent decades, the environmental impacts of aviation have become a key challenge for the aeronautical community. Advanced and well-established technologies such as active flow control systems, wing-tip devices, high bypass ratio engines, composite materials, among others, have demonstrated fuel-burn benefits by reducing drag and/or weight. Nevertheless, aviation remains under intense pressure to become more sustainable. For this reason, there is a strong drive to explore unconventional aircraft with the aim of reducing both environmental emissions and Direct Operating Cost. This paper presents the current state-of-the-art in the development of future aircraft for civil aviation. The literature review is conducted through an appropriate search protocol to ensure the selection of the most relevant sources. After a brief historical background, progress in the design and development of several unconventional aircraft configurations is presented. Concepts such as Blended/Hybrid Wing Bodies, nonplanar wing designs, next-generation propulsion technologies that are tightly integrated with the airframe, among others, are reviewed. Special attention is given to design methodologies (level-of-fidelity), cruise altitude, aerodynamic performance, and fuel-burn benefits over conventional configurations. The primary contributions of this review are (i) a detailed survey of the design characteristics of unconventional aircraft for non-specialists, and (ii) a comprehensive review of the literature detailing past and current design trends of such configurations for specialists.
... Exploratory design may be considered as a third class of problem or as a subcategory within preliminary design, combining large geometric flexibility with often unknown design spaces, and searching for novel designs. Common examples of exploratory optimization include the study of hybrid wing body (HWB) geometries [3,12], box-wing aircraft [13] or other unconventional designs. In preliminary or exploratory design, aerodynamic shape optimization is often combined with other disciplines as in multidisciplinary optimization. ...
A gradient-based multistart method based on a set of 17 to 33 random initial geometries is used to examine the risk associated with multimodality when applying gradient-based optimization to aerodynamic shape optimization. Aerodynamic shape optimization problems typical of detailed, preliminary, and exploratory design are shown to consistently present design spaces with multiple local optima. In the case of detailed design, the risk of converging to a local optimum with performance significantly inferior to that of the best local optimum found is reduced due to the ability of a well-designed initial geometry, which is often available for such problems, to converge to a well-performing local optimum. In problems permitting increased geometric freedom typical of preliminary design, the risk associated with multimodality is much higher. This risk is further exacerbated in exploratory cases where high geometric freedom is combined with limited knowledge of the design space in question and hence greater differences between available initial geometries and the optimal geometry. Therefore, for preliminary and exploratory design, allocating resources toward addressing multimodality can significantly reduce the risk of overlooking a superior optimum.
... An induced drag evaluation in compressible flow at M ∞ 0.5 was performed in Ref. [14], where the optimal shape of the Box Wing was presented and Euler equations were numerically solved. In Ref. [16], the investigation was further refined by performing an aerodynamic shape optimization of a Box-Wing system for regional aircraft applications. Compressibility effects were included at M ∞ 0.78, and Reynolds-averaged Navier-Stokes equations were considered. ...
... The result indicated that the increase of vertical aspect ratio was preferred by the optimizer, which is a property found also in Ref. [8] for the incompressible inviscid case. Thus, the study [16] was an indirect proof that induced drag is one of the key aspects of the Box Wing for flight conditions well beyond the original ones adopted by Prandtl in 1924 [3], when he first introduced this concept. This is the main technical point addressed by the present contribution: the goal is to precisely assess the relative importance of the different drag components, and it will even go forward to demonstrate that the theoretical predictions made in Ref. [8] for incompressible and inviscid flow can actually be directly applied to the most general compressible and even transonic regimes. ...
... At M ∞ 0 (incompressible case), Eqs. (15)(16)(17) have been used with F rev given by Eq. (12) and F irr by Eq. (13). At M ∞ 0.3 (subsonic case), the aerodynamic force breakdown has been provided by Eqs. ...
Prandtl introduced the best wing system or Box Wing a century ago and showed its exceptional lift-induced drag performance with respect to wing systems having the same wingspan and lift (Prandtl, L., “Induced Drag of Multiplanes,” NACA TN-182, 1924.). In this work, and for the first time, their superior induced drag properties are verified in compressible and transonic regimes. The aerodynamic forces are obtained by analyzing computational fluid dynamics data, and the drag is subdivided in induced, viscous, and wave contributions. This decomposition is provided by a recently introduced aerodynamic force breakdown technique based on the vortex-force theory and allows an unambiguous definition of the lift-induced drag. The methodology is applicable to incompressible, compressible, and transonic flows. This paper will address whether the Best Wing system provides interesting induced drag performance also in transonic viscous flow.
... The current literature features many box wing aerodynamic studies using simulation, analytical and parametric methods [7][8][9][10]. Research has shed light on the aerodynamic benefits of the box-wing design. ...
Highly non-planar aircraft configurations promise vast improvements concerning aircraft efficiency, but their flight mechanical properties are currently not well understood. Using reference wind tunnel data and RANS CFD (STAR-CCM+) results, a vortex lattice solver (VSPAERO) and a surface vorticity flow solver (FlightStream) are being evaluated concerning their ability to model joined and box wing longitudinal aerodynamic characteristics. It is shown that FlightStream is able to provide results of CFD-like quality, as long as flow separation effects do not become prominent. VSPAERO delivers good lift and pitching moment results, but lacks in drag modelling. STAR-CCM+’s solutions come very close to wind tunnel results.
... Due to the fluid nature of the initial design process, it is not recommended to use high fidelity analysis design tools, such as Computational Fluid Dynamics (CFD), at this stage as they can be unnecessarily costly [4,54]. What is required is a tool that strikes a balance between sufficient accuracy and computational cost [55][56][57][58]. Piperni et al. [59] and Zhang et al. [60] have argued that the level of fidelity that should be delivered by the models is mostly determined at the development stage, which the design process aims to enhance. ...
In this work, an interactive optimisation framework—a combination of a low fidelity flow solver, Athena Vortex Lattice (AVL), and an interactive Multi-Objective Particle Swarm Optimisation (MOPSO)—is proposed for aerodynamic shape design optimisation of any aerial vehicle platform. This paper demonstrates the benefits of interactive optimisation—reduction of computational time with high optimality levels. Progress towards the most preferred solutions is made by having the Decision Maker (DM) periodically provide preference information once the MOPSO iterations are underway. By involving the DM within the optimisation process, the search is directed to the region of interest, which accelerates the process. The flexibility and efficiency of undertaking optimisation interactively have been demonstrated by comparing the interactive results with the non-interactive results of an optimum design case obtained using Multi-Objective Tabu Search (MOTS) for the Aegis UAV. The obtained results show the superiority of using an interactive approach for the aerodynamic shape design, compared to posteriori approaches. By carrying out the optimisation using interactive MOPSO it was shown to be possible to obtain similar results to non-interactive MOTS with only half the evaluations. Moreover, much of the usual complexity of post-data-analysis with posteriori approaches is avoided, since the DM is involved in the search process.
... In [43,44], the authors reported developing a strategy that used non-interactive and interactive techniques, respectively, to formulate the design problem and improve the optimisation speed. To deliver a sufficient level of fidelity to the DM at very fast computational times, the low fidelity flow solver Athena Vortex Lattice (AVL) was used to capture the physics of the problem [45][46][47][48]. Improving the ability of the developed method to accelerate the search while retaining all the useful information in the design space was the main area of work. ...
... The flow solver used to evaluate worthwhile solutions is the Athena Vortex Lattice (AVL) [51,52]. Many codes utilize Vortex Lattice Method (VLM) for aerodynamic characteristics calculations, but AVL is the most well-known and provides the most accurate and efficient results when compared with other aerodynamic analysis software employing the same method [45][46][47][48]52]. In addition, AVL code is easy to use and capable of manipulating a large number of design parameters within a short computational time and limited cost. ...
This article presents an optimisation framework that uses stochastic multi-objective optimisation, combined with an Artificial Neural Network (ANN), and describes its application to the aerodynamic design of aircraft shapes. The framework uses the Multi-Objective Particle Swarm Optimisation (MOPSO) algorithm and the obtained results confirm that the proposed technique provides highly optimal solutions in less computational time than other approaches to the same design problem. The main idea was to focus computational effort on worthwhile design solutions rather than exploring and evaluating all possible solutions in the design space. It is shown that the number of valid solutions obtained using ANN-MOPSO compared to MOPSO for 3000 evaluations grew from 529 to 1006 (90% improvement) with a penalty of only 8.3% (11 min) in computational time. It is demonstrated that including an ANN, the ANN-MOPSO with 3000 evaluations produced a larger number of valid solutions than the MOPSO with 5500 evaluations, and in 33% less computational time (64 min). This is taken as confirmation of the potential power of ANNs when applied to this type of design problem.
