## 3 Design of the Skin and the Stringers  
### Material Selection and Design of the Skin  
A crucial task in designing a morphing leading-edge system is the design of the skin including material selection and stringer layout. On the one hand, the skin needs to provide high deformability in order to be able to morph into the positions S4 (cruise) and S5 (high-lift), see Figure 2. On the other hand, the skin has to meet competing requirements regarding strength and stiffness. As shown in Section 2.4, the leading edge is the region, where the highest aerodynamic loads occur, compared to other regions of the wing. Furthermore, the contour-length of S5 is 7.7% longer than that of S4. This elongation cannot be realised by elastic straining of conventional composite materials (or metals), especially since cyclic deformation has to be endured over the service life. Thus, an alternative solution using a sliding contact is selected in order to address the length difference, see Section 6. It is positioned at the lower side of the MLE so that C2-continuity at the upper surface of the MLE is not affected. The contact position is carefully designed so that it is in the region of the favourable pressure gradient where flow separation is less likely to occur, and thus does not deteriorate the aerodynamic performance. Morphing from S4 to S5 was therefore expected to be realised by pure bending of the skin and without any membrane strain. Under these conditions, one surface of the skin is compressed (negative strain) while the other is stretched (positive strain), and the absolute values of strain on both sides are equal so that the overall length in any direction does not change. The change of curvature of the MLE's skin due to morphing is given by the two target shapes S4 and S5. The bending stiffness of the skin should be as low as possible to limit the forces needed for actuation and maintain a low overall weight of the system. At the same time, however, its bending stiffness must be sufficient to limit the deviation from the target shapes under the relevant aerodynamic pressure distributions computed by 2D-CFD, see Section 2.4. While this is largely achieved by means of span-wise stringers, the areas between the stringers still require a certain bending stiffness so as not to bulge in- or outward under the pressure distribution. In a first step towards material selection, a simplified finite element model of the skin consisting of shell elements and assuming isotropic material was therefore used to identify a suitable distribution of the bending stiffness along the contour. The limit for acceptable displacement of the skin perpendicular to its original surface due to aerodynamic loads was set to 1 mm. The thickness in the different regions of the skin was adjusted to meet this requirement and to approximate the target shapes as closely as possible. After the distribution of the bending stiffness along the contour had thus been determined, generic composite laminates (different physicalists and lay-ups) were analysed by means of Classical Laminate Theory (CLT) and [PERSON]'s failure criterion [30] to determine their respective bending stiffness and bending limit, i.e., the allowable curvature change with respect to avoiding failure. Material data (elastic properties and uniaxial strengths) for typical glass- and carbon-fibre reinforced plies (unidirectional and woven; the database gave values for specific combinations of fibres, resins, and fibre-volume-fractions) were taken from the database of a commercial CLT-software (ESACOMP). Different laminates (all laminates symmetric and balanced, fibre orientations between +/\(-\)5\({}^{\circ}\) and quasi-isotropic) and with different thicknesses (achieved by repetition of the basic symmetric and balanced laminates) were defined for each ply material and thus analysed. The failure criterion considers the stresses occurring in each ply of the laminate using a failure envelope defined by interpolation of the uniaxial strength values.