Numerical simulation is already an important cornerstone for aircraft design, although the application of highly accurate methods is mainly limited to the design point. To meet future technical, economic and social challenges in aviation, it is essential to simulate a real aircraft at an early stage, including all multidisciplinary interactions covering the entire flight envelope, and to have the ability to provide data with guaranteed accuracy required for development and certification. However, despite the considerable progress made there are still significant obstacles to be overcome in the development of numerical methods, physical modeling, and the integration of different aircraft disciplines for multidisciplinary analysis and optimization of realistic aircraft configurations. At DLR, these challenges are being addressed in the framework of the multidisciplinary project Digital-X (4/ 2012-12/2015). This paper provides an overview of the project objectives and presents first results on enhanced disciplinary methods in aerodynamics and structural analysis, the development of efficient reduced order methods for load analysis, the development of a multidisciplinary optimization process based on a multi-level/variable-fidelity approach, as well as the development and application of multidisciplinary methods for the analysis of maneuver loads.
Gust load analysis is a relevant part of the certification process of aircraft. In need of low computing times, industrial analysis procedures often rely mainly on low-fidelty numerical aerodynamics methods, such as the Doublet Lattice method (DLM). However, their accuracy with respect to loads has not been assessed sufficiently in comparison to high-fidelity methods in the past. In this paper, simulation results of a classical DLMbased gust load analysis process are compared to the results of an alternative process in which the DLM solver is replaced by the CFD solver TAU. The investigation is performed with a realistic modern passenger aircraft. It is studied how the consideration of different levels of multidisciplinarity in the simulations affects the overall loads. The simulation methodologies exploited in the CFD-based analysis process are outlined. It is shown that the gust load factors predicted with CFD are significantly lower than those of the classical DLM-based process-not only in the transonic flow regime, where benefits from the CFDbased analysis are expected, but also in the subsonic flow regime.
A recently proposed modal-based method that captures the geometric nonlinear effects in the regime of large deformations is applied to the modeling of University of Michigan's X-HALE UAV. The method is an extension of the classical modal approach towards large geometric deflections and uses higher-order stiffness terms and mode components to account for nonlinear force-displacement relationships and geometrically nonlinear displacement fields. In this paper, the method is further extended to enable rigid body motions of an aircraft in free flight. The verification of this extension is done using the X-HALE model which is considered as a challenging test case due to pronounced nonlinearities in steady and unsteady flight conditions. A structural and an aerodynamic model of the X-HALE UAV were built and aeroelastic simulations of the model were done and validated with UM/NAST results.
A new modal-based method that captures the geometric nonlinear effects that arise in the regime of large deformations of wing-like structures is presented. The most limiting factors of the modal approach are the linear force-displacement relationship and the representation of the nodal displacement field based on normal modes. The proposed extension includes stiffness terms that cubically depend on the generalized coordinates. The structural deformation is calculated not only by normal modes but also by higher order mode components that account for the foreshortening effect at beam-type structures. The approach is applied to a cantilever slender wing. Static and dynamic results are presented together with results from a commercial finite element solver and from the UM/NAST aeroelastic solver from the University of Michigan. The numerical study highlights the capability of the new approach to capture nonlinear effects while keeping the simplicity of the modal approach.
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