Fiber and Mylar Foam Cladding
Although hot-wire-cut foam can be used to make very light and cost-effective aerodynamic parts, the approach suffers from some drawbacks. First, the finished surface is not perfectly smooth; second, any local loading must be fed into the foam via load spreaders to avoid local crushing damage; and third, the foam is usually not resistant to fuels of any kind. To deal with all these issues, and to provide robustness against ground handling damage, we almost always clad our hot-wire-cut foam parts after manufacture. This also significantly strengthens the foam in bending and, because it closes off the cut lines needed to make interior lightening, access, and spar holes, significantly increases torsional rigidity. We do this either by applying very fine glass-fiber or carbon-fiber tissue using thin resins or by covering the foam with sheets of heat-shrinkable aero-modeler wing film or thin Mylar attached with spray-on contact adhesives. This is one of the few remaining manufacturing tasks in our UAV builds that relies on skilled workers to produce the best results, although with a little practice and some care, cladding parts is not that difficult. If fiber tissue has been used, the resulting surface, though tough and fuel proof, is not completely smooth. This can be overcome via judicious use of fillers, but this adds weight, noticeably so on large wings. Figure 18.10 shows a selection of aircraft wings with differing forms of cladding. Table 18.4 gives typical bulk properties of glass-fiber-reinforced materials; but note that our claddings ultimately fail by delamination of the bond between the foam and the cladding and not by failure of the cladding itself. Also note that, when applying very thin glass fiber layers, the ratio of fiber to resin is lower than for thicker parts, and consequently the stiffness and strength properties achieved in our glass-fiber claddings are typically less than half the values given in the table.