## 6 Gap Closure  
The three most prominent configurations of the MLE are shown in Figure 24. After assembly of the skin and actuation system and mounting on the front spar of the wing, the MLE is in the intermediate baseline shape shown in grey. For cruise flight, the skin is tilted upwards by the actuation system to assume the contour shown in blue (S4). For take-off and landing, the skin is tilted downward into the high-lift configuration shown in orange (S5).  
The deformation from the cruise- to the high-lift configuration not only increases the wing profile's camber, but also its chord-length. Thus, and as mentioned in Section 3.1, the contour of the high-lift configuration is significantly longer than that of the cruise configuration. To avoid excessive membrane strains in the skin, the lower edge of the leading-edge skin is allowed to move forward and aft. The formation of a gap between it and the front spar, however, needs to be avoided to prevent the ingress of dirt and foreign objects, and also because the airflow interacting with it could cause significant noise emission. Therefore, the CFRP-skin is extended aft to overlap the lower surface of the front spar (and the parts of the wing-skin attached to it in this region). The laminate lay-up and shape of the extension are shape- and stiffness-tailored so that installation of the skin on the front spar results in a bending deformation and thus a tensioning of the overlapping part against the contact surface. To reduce wear, a thin profile running in the span-wise direction and made of an appropriate low friction material is attached to the overlapping part of the skin. When the MLE transitions from one shape to another, this slides along the lower surface of the front spar.  
At this point, the exact amount of pre-tension necessary to avoid the loss of contact between the skin-extension and the front spar in all situations, e.g., also in the presence of local turbulence, has not yet been analysed in detail. For the cruise and high-lift conditions investigated so far, Figure 25 shows the contact force per metre span at the sliding contact for the load cases given in Table 4 computed with the FE model described in Section 4.2.  
This figure shows that the aerodynamic pressure distribution does not cause opening of the contact. As shown in Table 4, the load cases 4, 5, and 6 are with different pressure fields, whereas the load cases 1, 2, and 3 represent cases without aerodynamic loads. The pre-tension reached with the current design is therefore sufficient. Whether this holds true for all possible flight conditions--e.g., also for negative angles of attack, which might cause suction on the wing's lower surface--needs to be investigated in the future.  
Figure 24: Schematic view of cruise-configuration (S4, blue), intermediate baseline shape (grey), and high-lift configuration (S5, orange) of the MLE; CFRP-stringers are each stiffened by a pair of steel profiles with box- or C-shaped cross-sections, respectively—the figure shows their cross-section including the fittings for the kinematic system (see Figure 11 for more detail); the rearward extension of the MLE’s skin slides along the lower surface of the front spar.  
which requires accessibility from the sides, are best realised in a preassembly stage before mounting to the front spar. The preassembled system can then be attached as a single item.