Page 249 - Aircraft Stuctures for Engineering Student

230 Principles of stressed skin construction

Fig. 7.1 1 Wing ribs for the European Airbus (courtesy of British Aerospace).

The different structural requirements of aircraft designed for differing operational
roles lead to a variety of wing constructions. For instance, high-speed aircraft require
relatively thin wing sections which support high wing loadings. To withstand the
correspondingly high surface pressures and to obtain sufficient strength, much thicker
skins are necessary. Wing panels are therefore frequently machined integrally with
stringers from solid slabs of material, as are the wing ribs. Figure 7.11 shows wing
ribs for the European Airbus in which web stiffeners, flanged lightness holes and
skin attachment lugs have been integrally machined from solid. This integral
method of construction involves no new design principles and has the advantages
of combining a high grade of surface finish, free from irregularities, with a more
efficient use of material since skin thicknesses are easily tapered to coincide with
the spanwise decrease in bending stresses.
An alternative form of construction is the sandwich panel, which comprises a light
honeycomb or corrugated metal core sandwiched between two outer skins of the
stress-bearing sheet (see Fig. 7.12). The primary function of the core is to stabilize
the outer skins, although it may be stress-bearing as well. Sandwich panels are capable
of developing high stresses, have smooth internal and external surfaces and require
small numbers of supporting rings or frames. They also possess a high resistance to
fatigue from jet efflux. The uses of this method of construction include lightweight
‘planks’ for cabin furniture, monolithic fairing shells generally having plastic facing
skins, and the stiffening of flying control surfaces. Thus, for example, the ailerons