In the frame of the Clean Sky 2 European program, ONERA participated to the AIRFRAME Technology Stream A1 (investigation of innovative architectures together with DLR, Airbus and Dassault Aviation) and to the U-HARWARD Thematic Topic (dedicated to high aspect ratio wings), which were aiming at studying innovative aircraft architectures via a Multi-Disciplinary Analysis & Optimization process. To select the innovative configurations of interest, a step-by-step down-selection process was defined and applied to two aircraft missions (SMR and Low-Speed Business Jet - LSBJ). Both the aircraft configuration and the aero-propulsive integration were investigated, requiring improvements in the multi-fidelity Overall Aircraft Design process to be able to discriminate promising solution. This paper describes both the models development and their integration in the process, and the performance of the innovative configurations selected for the SMR mission. In terms of innovative rear fuselage engine integration configurations, the Common Inlet concept was selected as the most promising one for both missions and a first aerodynamic design was proposed to reduce the uncertainty on the potential of this concept. Regarding aircraft architectures, the Large Fuselage configuration was selected for the final performance assessment. For the SMR mission, the BWB concept was selected during the final down-selection workshop, and potential synergies between the BWB concept and BLI engine integration were identified and assessed at the end of the project. Moreover, in the U-HARWARD project, the Strut-Braced Wing configuration with high aspect ratio wing was investigated by ONERA together with ISAE-Supaero, enabling to quantify the potential fuel burn savings of such configuration. This work paves the way for the Clean Aviation projects in which ONERA is actively participating, especially in the SMR pillar. In the ACAP project, ONERA will re-use and consolidate the OAD process and models developed in Clean Sky 2 to assess Ultra-Efficient SAF configurations and H2-enabled configurations. In the UPWING and AWATAR projects, ONERA will contribute to the design and assessment of a high-aspect ratio dry wing suitable for H2 aircraft, making use of the tools and know-how acquired in the CS2 U-HARWARD project.

In the wake of ”flygskam” movement that emerged in Sweden a couple of years ago many voices recently raised denounce the environmental footprint of aviation. Even if the real impact of the sector could appear rather limited the critics reveal the necessity to propose cleaner aircraft to both meet public expectations and environmental goals. Because the classical wing-tube configuration seems to have reached its limits, disruptive designs must be considered. Among the perspectives to reduce emissions, high-aspect ratio wings represent a promising path to be explored within the European CleanSky2 project U-HARWARD. Indeed, substantial diminution of induced drag are expected from those new configurations resulting in fewer fuel consumption. To achieve high-aspect ratio without compromising the structural weight strut can be introduced. They allow for an alleviation of the bending moment at the wing root and therefore lighter structures. However, the consideration of those new wing configuration at early design stages is not straightforward and new methods have to be introduced. In this paper, we present three different fidelity approaches to tackle with (ultra) high-aspect ratio strut-braced wings sizing and weight estimation in preliminary design context. Already existing analytical formulations for the wings are extended, intermediate fidelity aero-structural coupling has been developed and high-fidelity structural representation are considered. Depending on the maturity of the concept these methods could be used to explore the design space, to refine the optimum or to analyse the final concept. Validation with respect to reference configurations is provided. Then the methods are applied to the analysis of strut-braced wings.

View Video Presentation: https://doi.org/10.2514/6.2022-0726.vid Increasing the wing aspect ratio appears as a straightforward way to improve aerodynamic performance of transport aircraft by reducing the lift-induced drag component. However, it comes at the price of a direct negative impact on the wing structural weight which is necessary to sustain aerodynamic loads in the case of a conventional cantilever wing. The strut-braced wing concept allows to reduce the flexural moment to be carried out by the inner-wing structure and therefore limits the weight penalty as aspect ratio is increased. A multidisciplinary evaluation of the potential benefits at aircraft level of High Aspect Ratio, Strut Braced Wing concept is presented. It relies on a multi-fidelity design approach in which an Overall Aircraft Conceptual Design framework is combined with high-fidelity aerodynamic and structural analyses to provide accurate physical information to the conceptual design process. This paper describes the tools, framework and approach used to combine OACD with high-fidelity CFD and CSM analyses and illustrates the first results of its application to design a HAR-SBW aircraft concept which are compared to a conventional tube-and-wing aircraft designed for the same mission.

View Video Presentation: https://doi.org/10.2514/6.2022-0082.vid Multidisciplinary design optimization methods aim at adapting numerical optimization techniques to the design of engineering systems involving multiple disciplines. In this context, a large number of mixed continuous, integer and categorical variables might arise during the optimization process and practical applications involve a large number of design variables. Recently, there has been a growing interest in mixed variables constrained Bayesian optimization but most existing approaches severely increase the number of the hyperparameters related to the surrogate model. In this paper, we address this issue by constructing surrogate models using less hyperparameters. The reduction process is based on the partial least squares method. An adaptive procedure for choosing the number of hyperparameters is proposed. The performance of the proposed approach is confirmed on analytical tests as well as two real applications related to aircraft design. A significant improvement is obtained compared to genetic algorithms.

Multidisciplinary design optimization methods aim at adapting numerical optimization techniques to the design of engineering systems involving multiple disciplines. In this context, a large number of mixed continuous, integer and categorical variables might arise during the optimization process and practical applications involve a large number of design variables. Recently, there has been a growing interest in mixed variables constrained Bayesian optimization but most existing approaches severely increase the number of the hyperparameters related to the surrogate model. In this paper, we address this issue by constructing surrogate models using less hyperparameters. The reduction process is based on the partial least squares method. An adaptive procedure for choosing the number of hyperparameters is proposed. The performance of the proposed approach is confirmed on analytical tests as well as two real applications related to aircraft design. A significant improvement is obtained compared to genetic algorithms.

To face the increasing environmental footprint of commercial aviation, industrial and research efforts have been focusing on exploring unconventional configurations and new propulsion paradigms, mostly based on electric technology. Such explorations require Overall Aircraft Design that has to be performed in an integrated multidisciplinary design environment. Such design environments are often limited to multidisciplinary analysis, adapted for a single aircraft configuration or require an important effort to be mastered. FAST-OAD is a software program developed by ONERA and ISAE-SUPAERO for aircraft sizing analysis and optimization that aims at user friendliness and modularity. It is an aircraft sizing code based on multidisciplinary design optimization techniques and the point mass approach to estimate the required fuel and energy consumption for a given set of TLARs. This paper presents the motivations for moving from the original software program, called FAST, to the open source code FAST-OAD based on OpenMDAO.

Multidisciplinary Design Optimization (MDO) methods aim at adapting numerical optimization techniques to the design of engineering systems involving multiple disciplines or components. Among MDO architectures, various ones are considering the resolution of the Multidisciplinary Design Analysis (MDA). In our study, the system of interest being an aircraft, the resolution of the MDA will be provided by the Future Aircraft Sizing Tool with Overall Aircraft Design (FAST-OAD), a point mass approach that estimates the required fuel and energy consumption for a given set of top-level aircraft requirements. In this context, a large number of mixed continuous, integer and categorical variables that arise from aircraft design has to be tackled by the optimization process. Recently, there has been a growing interest in mixed variables constrained Bayesian optimization based on Gaussian process surrogate models. In this setting, most existing approaches severely increase the dimension of the covariance matrix related to the surrogate. In fact, the construction of the Gaussian process model may not be scalable to practical applications involving a large number of mixed variables. In this paper, we address this issue by constructing a covariance kernel for the surrogate model that depends on only a few hyperparameters. The new kernel is constructed based on the information obtained from the partial least squares method. The obtained numerical results lead to interesting results for the optimization of a baseline aircraft and to reduce the fuel consumption of DRAGON, a new hybrid electric propulsion aircraft, with a high number of mixed variables and for a small budget of time-consuming evaluations.

During flight, emergency descent situations are part of those extreme conditions that can lead the empennage of an aircraft to vibrate. These vibrations are mainly due to the separated flow on the upper surface of the structure which increases the pressure fluctuations on the empennage, sometimes leading to buffeting. This situation can cause structural fatigue and can induce certification and design constraints on the structure. Hence, an accurate prediction of the unsteady loads is needed to take these forces into account in the early phase of the empennage design. This paper presents a new approach to accurately model the unsteady aerodynamic loads resulting from the interaction between the horizontal tail plane and the wing wake. The method is based upon the coherence method and is compared to the method developed by Soumillon [2], based upon the correlation method. The results obtained by this new model show good agreements with the experimental data.