A typical built up structure consists of longitudinal ribs, which are attached to a leading edge box and to a trailing edge box. The leading edge box usually also houses the main wing spar. While the boxes are covered with wood, the surface of the wing between them covered with a flexible material, which only supported by the ribs. In reality, the shape of the surface between neighbouring ribs, and the leading and trailing edge boxes is also controlled by the mechanical properties of the cover material. The real surface geometry could be calculated by using a finite element membrane model, but it will be very difficult to find the correct tension forces acting on the membrane. They depend on the amount of dope used to paint the surface, or the amount of heat applied to shrink a plastic film cover and on the aerodynamic forces acting on it.
To simulate the effect of a the cover material sagging between the ribs, a simple model was used for the present investigation (see figure 2). On the two dimensional airfoil two points were marked: one point at x/c=25%, representing the end of the leading edge 3D box, and one point at 85% chord, corresponding to the beginning of the trailing edge box. Then, a straight line, connecting these two points, was said to represent 100% sag (sag factor). This would be the shape of the cover material, if there were no ribs between the leading and the trailing edge boxes.
Fig. 2: Wing section, showing various degrees of the cover material
sagging between ribs.
Due to the ribs, which add a spanwise component to the stress in the membrane, the true shape will be somewhere in between the 100% and the 0% shape. For the following results, it was assumed, the a maximum of 60% sag occurs between two ribs. The spanwise distribution of the sag factor was represented by a quadratic curve.
Fig. 3: Rear view of the wing, illustrating the spanwise sag distribution
as well as the rib spacing.
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