New aircraft configuration D2AE - from the Institute of Aeroelasticity

For this reason, the DLR Institute of Aeroelasticity has begun designing the single-aisle short to medium-haul aircraft configuration D2AE. The D2AE will be used in particular to analyse the aeroelastic properties of future aircraft configurations. The D2AE configuration is based on the so-called D239+ configuration, which was developed by the DLR Institute of System Architectures in Aeronautics for a maximum of 239 passengers in the single-class variant [1]. The main difference between the D239+ and the D2AE configuration is the wing planform.  While the D239+ configuration with a wingspan of approximately 36 m can easily fit into a category C parking box according to the regulations of the International Civil Aviation Organisation (ICAO). Such is a typical gate for short and medium range (SMR) configurations. The D2AE configuration with a wingspan of 42.5 m can only achieve this by using a folding wing The wing area of the D2AE is 131 m² and the aspect ratio is 12.5 (see Figure 1). The wing area also enables the potential for a variant with a longer fuselage, i.e. an increase in the number of passengers to over 250.

For the initial design of the D2AE configuration, the computer programme openAD for the conceptual design of aircraft from DLR [2] and the parametric aeroelastic design process cpacs-MONA from the DLR Institute of Aeroelasticity were used [3]. With openAD, the typical concept design parameters were determined on the basis of the specified TLARs (Top-Level Aircraft Requirements) using statistics-based methods (e.g. geometry, masses, flight performance, etc.). The cpacs-MONA design process is based on the estimated results and combines the determination of flight and ground loads as well as structural dimensioning with structural optimisation methods using physics-based methods and typical simulation models (see Figure 2). Due to the parametric approach within cpacs-MONA, adjustments to the sometimes complex simulation models, e.g. for the structure, are easy to implement within the design process.

The structural model used in WISDOM for the entire aircraft was specifically modified with regard to the mass model and the stiffness distribution in order to achieve a wing flutter case between limit of the flight envelope at dive speed (VD/MD) and the 1.15 VD/MD limit of the aeroelastic stability envelope. In this flight range, flutter is then to be suppressed by a control system with the aid of the three ailerons.

Conceptual design loads are used in the early phase of an overall aircraft design (OAD) to consider structural dimensioning in a multidisciplinary process. Due to novel configurations such as the D2AE with a high aspect ratio wing, analytical methods are no longer fully applicable. As an example, a comparison between conceptual loads from the application of such methods with the computer programme LOADzero [6] and the loads calculated with higher accuracy in the fully automated design process cpacs-MONA using MSC Nastran is shown below for the D2AE configuration. Figure 4 shows section loads for the wing and fuselage for both methods. The differences between the two approaches are clearly visible.

With regard to load analysis, for example, it was investigated how different simulation methods for gust load analysis affect structural dimensioning. The investigation shows that the loads from the more conservative Pratt gust analysis are dimensioning for larger areas of the structure compared to those from the 1-cos gust load analysis (see Figure 5).

The increased use of fibre composites in load-bearing structural components requires new analysis and optimisation methods for the exact calculation of material requirements and manufacturing boundary conditions. It also offers a much larger parameter and optimisation space compared to traditional, homogeneous materials. The targeted optimisation of main stiffness directions, also known as aeroelastic tailoring, enables deformation-dependent couplings that can have a significant influence on the global load distribution. The possible load reduction at the limits of the permissible flight envelope can contribute significantly to a reduction in the weight of the load-bearing structural components. The possible potential is to be demonstrated for the D2AE configuration (see also [8]). Figure 6 shows the wall thickness distribution and the stiffness distribution for the D2AE wing box.

Geometrically non-linear analyses of wing structures are playing an increasingly important role due to the development of wings with high aspect ratio and flexible structure. The usual linear analysis methods use assumptions for small deformations that are no longer valid for large deflections. The investigations carried out here concentrate on showing how the non-linearities influence the kinematics of the wing of the D2AE configuration. To this end, the deflection profile and modal properties of the linear and non-linear simulations are compared. The non-linear structural simulation for the clamped wing and fixed loads was performed with MSC Nastran. The results for the strains are shown in Figure 7 and for the deformation and frequencies in Figure 8. DLR-AE is also developing its own methods for non-linear structural simulation [9].