Design of the Wing Structure

The structure of a common transport aircraft wing is composed of the following components (see fig. 2):

• Spars with spar webs carrying shear load and spar caps which resist tension or compression normal loads. Generally, the wing box is composed of a front and rear spar. The rear spar is of special importance for the mounting of movables as well as for systems integration. As shown in fig. 2, additive components, like a mid-spar, or a false rear spar can be integrated in the wing structure as well.

• Top and bottom covers which have a contouring as well as a load carrying func­tion. These components carry both, normal and shear stresses. Along with main spars, the wing skin forms the box structure of the wing. The skin parts are stiffened by stringers to prevent buckling failure

• Ribs which can be oriented perpendicular to the wing box axis or parallel to the aircraft symmetry plane. Ribs are used to keep the aerodynamic shape of the wing cross-sections under aerodynamic load and for insertion of concentrated loads in the wing box structure caused by engine mountings or landing gear.

In figure 2 different levels of modeling details for each structural component are depicted. The stiffening components, like stringers, spar – or rib caps can be realized by beam elements with defined cross-sections, by rod elements neglecting the bend­ing stiffness of the stiffener or can be "smeared" over the area of the correspondent thin-walled structural component. The smeared stiffening component in turn can be realized as an additional orthotropic material layer, which is reasonable when modeling the skin-stringer panels or taken into account in the wall thickness of the thin-walled components what is commonly done by ribs and spars.

In the current study the structural model is realized by shell and bar elements. The inclusion of element offsets was used as a reference with highest grade of modeling detail limited to the main stiffening components within the present work.

The different levels of detail, shown in figure 2 are employed within the test wing model. The idealized FE models are derived from the reference geometry by repla­cing the built-up structure, realized with shell and bar elements, by a. m. simplified structural design. To estimate the deviations in the deformation behavior caused by modeling simplifications the structural response of both the reference and simplified wing box models is compared for a reference loading. To ensure the comparability of these results the volume of the wing structure was kept constant for all derivations with varying grade of the detail.

Uncertainties caused by different levels of approximation are discussed in the following section. The intent of this overview is not to enable the general prediction
of the modeling error of the structural and thus of the aeroelastic response resulting from distinct simplification. Due to the individual design and stiffness properties of wing structures realized in different aircraft types the contribution of structural components to the bending, shear, and torsional stiffness as well as to the warp­ing characteristics varies depending on a given structural design. Instead of that, a series of calculations is performed to estimate the dimension of the deviations resulting from different levels of simplification using the parametric finite element model. The effect of uncertainty on the stiffness properties of the wing structure is considered only in the frame of static aeroelastic analysis under assumption of lin­ear elastic structural behavior. Non-linear effects as well as dynamic properties or effects caused by the usage of non-isotropic materials are not in the objective of the present work.