This invention relates in general to methods and fiber reinforced resin matrix aircraft structures and, more particularly, to methods and structures for protecting such structures from damage due to lightning strikes.
In recent years, composite materials made up of graphite or other high strength fibers embedded in a resin matrix have come into increasing use in aircraft. The very high strength-to-weight-ratios and high stiffness of these composites make them superior to metal structures for aircraft structures such as engine cowls. However, these materials have low conductivity making them susceptible to damage due to lightning strikes. Fiber layers may shatter, delaminate or peel when struck by lightning, causing a serious loss of structural integrity.
Many different schemes have been devised to increase the surface conductivity of composite materials to rapidly dissipate the energy of a localized lightning strike to prevent local damage. Problems remain, however, since these conductive surface materials tend to be heavy, reducing the weight advantage of the composites over metal structures, difficult to apply to complex curved surfaces, prone to peeling corrosion or other degradation in use and not fully protective against lightning strikes.
Typical of the prior lightning protection methods if that proposed in Propp's U.S. Pat. No. 4,186,237. There, a coating is applied to the low-conductivity aircraft surface which release a cloud of ions when exposed to an immediately pre-lightning strike electrical field. Typically, the coating consists of a metal layer covered with a layer of subliming metal salts in a lacquer coating. While somewhat helpful, the coating is complex, difficult to apply to curved surfaces and adds undesirable weight.
In U.S. Pat. No. 4,448,838, McClenahan discloses a lightning strike protection method in which metal fibers are woven in both the warp and fill directions in a woven graphite fiber sheet. The metal fibers are closer to the outer surface in some areas than others, diverting the lightning to those areas, causing a number of separate strikes so that instead of one heavily damaged area there are a number of less damaged areas. Because of the relatively wide spacing of the "small" strikes, significant damage may still occur. This method requires a 90.degree. or similar weave, requiring timeconsuming and expensive hand lay-up of the conductive fibers in manufacturing the aircraft structure.
At least partially conductive surface layers or coatings have also been disclosed by King in U.S. Pat. No. 4,429,341 (surface coating of metal powder in a binder), Sanders in U.S. Pat. No. 4,349,859 (metallic tin surface coating), Paszowski in U.S. Pat. No. 3,755,713 (metal mesh embedded in surface coating) and Brick in U.S. Pat. No. 4,445,161 (metal particles embedded in an adhesive tape.) While each of these gives some degree of protection against lightning strikes, all are slow and difficult to apply and add undesirable weight. Satisfactory adhesion to the underlying composite is difficult to obtain. The added surface layers are subject to peeling, rain erosion and corrosion.
While many prior aircraft components, such as couling and nacelles, have in the past been fabricated from sheet metal and have little, if any, problem with lightning, those materials are going out of favor due to the relatively low strength, greater thickness, greater weight and difficulty of fabricating complex surface of rotation shapes from metal when compared to advanced fiber reinforced resin matrix materials. While it is possible to form surface of rotation shapes form metal filaments or strips in a resin matrix, as described, for example, by Eldred in U.S. Pat. No. 3,189,510, those structures are excessively heavy for aircraft purposes, generally inferior to sheet aluminum. Eldred teaches the use of metal wire wrapped structures for high strength applications, such as pressure vessels, with no suggestion that such structures could be used in applications, such as aircraft, in which light weight is essential.
Thus, there remains a need for improved means for protecting low conductivity composite aircraft structures against lightning strikes which overcomes the above noted problems. The protective system needs to be capable of simple and rapid application to curved surfaces, such as surfaces of revolution, to be variable in response to varying degrees of protection required, to resist peeling, erosion and corrosion and to not excessively increase the weight of the structure.