IFASD2024 Paper Submission & Registration
17 – 21 June 2024, The Hague, The Netherlands

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16:00   Aeroelastic design 1
Chair: Bart Eussen
30 mins
From Multivariate Random Loads To Deterministic Load Distributions: An Exact Method For Aeroelastic Design
Carlo Aquilini, Gabriele Grasso, Cyrille Vidy
Abstract: Multivariate random loads may arise from either a stationary Gaussian process, like stochastic or continuous gust, or from a random process, such as buffet. Buffet phenomena are important when the separated air flow induces strong fluctuating pressures on aircraft components. These loads need careful assessment during aircraft design, especially for fighters flying at high angles of attack and in the transonic regime, as well as for general aviation and transport aircraft. This paper introduces an exact method for Aeroelastic design, specifically focusing on determining distributions of deterministic quasi-static nodal loads from integrated load cases obtained from the stochastic problem. Quasi-static load distributions are required by the stress office in order to size the affected structural components. The methodology outlined in this paper builds on the work by Aquilini & Parisse [1], which provides a comprehensive method for predicting n-dimensional combined loads in the presence of massively separated flows. The methodology in [1] obtains a finite number of design load cases, by discretizing the n-dimensional design ellipsoid of equal probability with a semiregular polyhedron, a transformed small rhombicuboctahedron. The present paper extends and concludes the work in [1] by determining distributions of deterministic nodal loads for each selected load case. The exactness of this method allows it to be applied to any load case that satisfies the equation of the multidimensional design load envelope. Thus, the load case selection is not confined to the vertices of the small rhombicuboctahedron but can utilize any discretisation of the ellipsoid. Moreover, this approach is versatile applicable not only to buffeting but also to various stochastic problems, like continuous turbulence. To manage load case complexity, a reduction strategy is suggested, contributing to obtaining a reasonable and meaningful number of design load cases. The paper concludes with examples showcasing the successful application of this method in real-world scenarios and its impact on the traditional Aeroelastic design. [1] Aquilini, C. and Parisse, D. (2017). A Method for Predicting Multivariate Random Loads and a Discrete Approximation of the Multidimensional Design Load Envelope. IFASD 2017
30 mins
The Effect of Aspect Ratio Variation on the Preliminary Aeroelastic Assessment of a Mid-Range Transport Aircraft
Matthias Schulze, Florian Müller, Sunpeth Cumnuantip, Vega Handojo, Tobias Hecken, Thomas Klimmek, Markus Ritter, Markus Zimmer
Abstract: The wing aspect ratio (AR) is one of the main planform parameters to estimate the aerodynamic efficiency of a wing. A higher AR relates to a reduction in induced drag and consequently to an improved fuel economy. Since wings on civil transport aircraft are complex geometrical components with kinks, varying sweep, twist and dihedral angles over the span, mostly equipped with a heavy engine, the simplified assumptions made within the analytical formulations for wings conceptual design might be untrustworthy for modern aircraft with high AR wings. This paper evaluates the impact of the aspect ratio on the aeroelastic design of a medium-range transport aircraft using physics-based simulation at preliminary design stage. The research is conducted using the DLR-F25 configuration, developed in the German Aviation Research Program (LuFo-Project VirEnfREI). It features a baseline aspect ratio of 15.6. Since the AR of a wing is the ratio of the square of its span divided by its wing area, the AR can be varied in different ways. Within this paper, three different approaches to alter the AR are analyzed. All concepts utilize the same approach in which the center portion of the wing is kept constant until the kink. In this way, the wing-fuselage mount and the engine-wing integration is unaffected. Moreover, the leading-edge sweep angle has been kept unchanged to have comparable performance of the resulting aircraft. The baseline twist distribution at the corresponding sections of the wing is kept unchanged for the variants resulting in different lift distributions throughout the variations. Compared to other publications on the field of AR variations, where an increase of AR leads to an over-proportional ascending of wing-box mass, two variation approaches investigated in this paper show a passive load alleviation effect for higher AR leading to an even lower wing mass compared to the baseline wing. This effect is evoked by the bell-shaped spanwise lift distribution. These two variation methods result in a reduction in taper ratio with growing AR, accompanied in a reduction in wing mass. The third variation, on the other hand, increases in mass with respect to the AR, as the taper ratio is kept constant.
30 mins
Flexible aircraft conceptual co-design based on the RCE framework
Béla Takarics, Bálint Patarics, Bálint Vanek, Fanglin Yu, Yasser M. Meddaikar, Matthias Wuestenhagen, Thiemo Kier
Abstract: In a traditional aircraft design process, the airframe is designed first, followed by synthesizing the control system. This sequential method does not guarantee the best possible closed-loop performance. The goal of the paper is to propose automatic control design methods for flexible aircraft. Such approach enables the inclusion of the control design algorithms into the multidisciplinary design optimization (MDO) of aircraft design. In such an extended MDO framework, called co-design, the sizing, structural dynamics, aerodynamics and the controllers of the aircraft are optimized in one single step. Co-design allows the usage of control technologies early in the conceptual preliminary design stage of aircraft design. A demonstrator aircraft T-Flex serves as a test bed for the conceptual co-design. DLR’s Remote Component Environment (RCE) framework is used as the backbone for the MDO toolchain implementation. The control design algorithms of the co-design considered in the paper are the baseline and the flutter suppression controllers, which require a control oriented aeroservoelastic model. The modeling is done via the bottom-up modelling approach and the resulting control oriented models are given in the linear parameter-varying (LPV) framework. The baseline control system features a classical cascade flight control structure with scheduled control loops to augment the lateral and longitudinal axis of the aircraft. The control loops use scheduled elements of proportional-integral-derivative (PID) controller structures. The flutter controller aims to mitigate the undamped oscillations of the wings that occur if the aircraft is flying beyond the flutter speed. Parametric and dynamic uncertainties are accounted for in the control design. The objective of the design is to minimize the sensitivity function of the closed-loop while limiting the bandwidth of the controller to prevent the excitation of high-frequency dynamics. The effect of the following parameters on the conceptual co-design is investigated: wing sweep angle between 0 and 30 degrees, flutter mass between 0 and 0.4 kg and ply angle between -45 and 45 degrees. It is examined how these parameters affect the flutter modes of the aircraft and the performances of the baseline and flutter suppression controllers and the results can be used for conceptual aircraft design.
30 mins
Wing mounted hydrogen fuel tanks: mitigating the aeroelastic penalties of dry wing configurations?
Fintan Healy, Huaiyuan Gu, Djamel Rezgui, Jonathan Cooper
Abstract: The aviation industry’s desire to mitigate its environmental impact has re-invigorated research into hydrogen-powered aircraft concepts. The transition to liquid hydrogen (LH2) fuel results in large, cryogenic fuel tanks that cannot be accommodated within the wingbox structure, leading to fuel-free or ‘dry’ wings. In traditional kerosene powered configurations fuel stored in the wings provides inertial relief, reducing the loads experienced during flight and, therefore, the required structural mass. Wing mounted fuel tanks could be used to regain this inertial relief, and this paper investigates the aerodynamic and structural implications of integrating wing-mounted hydrogen fuel tanks into medium-sized commercial aircraft with high aspect ratio wings. A multidisciplinary conceptual aircraft sizing methodology is used to explore the effect of different fuel tank configurations - where LH2 is stored within the fuselage, or in external wing-mounted tanks - on an aircraft’s geometry and performance metrics, such as fuel efficiency. The sizing of the wingbox structure includes the numerical simulation of manoeuvre, gust and turbulence loads using an aeroelastic model. The findings suggest that while wing-mounted tanks offer inertial relief, reducing wing mass by over 20%, the increased parasitic drag from the external fuel tanks outweighs the reduction in lift-induced drag. This conclusion was observed between aspect ratios of 8 and 20, suggesting that permanently attached wing-mounted fuel tanks are not viable for hydrogen-powered aircraft with high aspect ratio wings.

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