Part I: Model Uncertainties

1.2 Introduction

To obtain a high level of accuracy for a structural model one possible approach is to reproduce the real structure with a high level of geometric detail. This approach implies two general drawbacks: it is connected with high modeling effort on the one hand and requires fine discretization of the wing box geometry on the other hand (see Fig. 4, on the left hand side), resulting in high model size and numerical costs. To demonstrate the dimension of complexity connected with a detailed model the reference structure described in section 1.1 is considered. The FE model has 740 design variables and is realized by 24900 shell and 10700 bar elements having in total 260000 degrees of freedom.

For coupled aeroelastic analysis, requiring a high number of iterations the min­imization of the finite element model size could be of high priority. As first ap­proach to reduce the number of degrees of freedom, reduction techniques are used to condense a 3D wing box structural model into a 1D-beam stick model. In the other case, if a parametric FE wing model with variable geometry should be op­timized, the amount of design variables associated with high level of modeling de­tail is undesirable. To reduce the number of design variables a simplified structural model is preferred which is composed only of main components of the wing struc­ture, like top and bottom covers, ribs and spars, including spar caps. Within such simplified models stringer stiffeners are commonly idealized by an additional layer with orthotropic material properties. The next sections deal with the effect of the simplifications within the 3D FE models.