In conventional aircraft, the outer surface of the wing, the so-called wing cover, is often added to the completed internal structure of the wing, typically by riveting. On model aircraft, this covering was originally provided by doped tissue paper stretched over a balsa wood structure. After experimenting with many types of wing structure, we have finally settled on using closed-cell foam to form the aerodynamic shape to which we add a covering of either a thin Mylar film (held in place with spray adhesive) or fine glass fiber tissue (adhered with a water based resin). The covering adds to torsional and bending strength and at the same time gives
Figure 3.5 A SPOTTER UAV wing spar under static sandbag test.
resilience to ground handling. In either case, great care is taken to use the minimum amount of adhesive or resin so as to control weight build-up. All of the wings illustrated in this chapter have been built following this approach.
To help join the foam cores to the main spar and to provide hard points for features such as servos or hinges, we typically add a few SLS nylon ribs to the wing build-up, see Figure 3.6 for an example. The rib has large attachment areas for bonding to the foam wing core, a hard point for a trailing edge flap, and a hole that is a push-fit to the main wing spar. A wing might have four or five such ribs depending on the number of individual control surfaces in use and the number of pieces the foam core has been made from.