Lightning protection system for composite structure

A lightning protection system for protecting composite structures and a method of protecting composite structures from lightning strikes. A dielectric ply is fixed above and completely covers metal surface features, e.g., skin fasteners through a composite skin to a wing fuel tank. A conductive ply is fixed above and completely covers the dielectric ply and extends to an external connection to a platform ground. The conductive ply directs current from lightning strikes away from metal surface features, e.g., to the platform ground. Both plies may be adhesively backed and sequentially pressed into place.

CROSS REFERENCE TO RELATED APPLICATION

The present invention is related to Published U.S. Patent Application No. 20050181203, entitled “Appliqué” to Diane C. Rawlings et al., assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to protecting composite structures from lightning strikes, and more particularly, to an appliqué for protecting composite aircraft from lightning strikes.

2. Background Description

Since aluminum and other metals are highly conductive, a transient charge from a lightning striking a metal body discharges into the metal body with current from the discharge being distributed relatively evenly over the body. So, a typical lightning strike to a metal aircraft causes no or only minor damage to aircraft components. However, carbon fiber composites generally have a higher strength-to-weight ratio than aluminum, and so, are increasingly replacing aluminum structural components. Unfortunately, typical state of the art composites, such as Carbon Fiber Reinforced Plastic (CFRP), are approximately 2000 times more resistive than aluminum.

So, a lightning strike that may have little or no effect on an aluminum structure may affect unprotected CFRP components. For adequate lightning protection for a composite wing structure, the exterior CFRP structure must withstand not only the initial lightning strike, but also at least one hundred kiloamperes (100 kA) of discharge current without adverse affects or impact to safety. Furthermore, skin fasteners at an exposed surface are most susceptible to a direct strike. Accordingly, composite structure aircraft must have some protection, especially at exposed skin fasteners, fuel and hydraulic couplings. However, it is also important that this protection is economically feasible, in its initial application, in its effectiveness for minimizing resulting damage and, in subsequent consequent repair or replacement, both for continued aircraft flightworthiness and to meet economic repair targets.

Unfortunately, typical lightning strike protection approaches are complicated and difficult to implement in CFRP. One approach involves selectively integrating metal (e.g., copper foil) into or onto the composite laminate at the fastening areas. Moreover, this new approach has been expensive; is often difficult to implement/rework with labor intensive application processes both pre and post-assembly; and, has not consistently exhibited acceptable EME protection. Copper foils, for example, have been subject to wrinkling during lay-up/cure. Drilling the laminate for fastener installation may contaminate the fuel tank with copper. Even with this additional protection, in the absence of other supporting protection (e.g., fastener collar isolation, fillet/cap sealing), the structure may still have a low sparking threshold. In addition to added complexity, integrating a conductive surface protection layer into the composite wing skin may carry with it an unacceptable weight penalty.

Thus, there is a need for effective lightning protection for composite structures that is lightweight, relatively low-cost, as well as simple to apply and repair, and especially for such lightning protection for composite aircraft.

SUMMARY OF THE INVENTION

An embodiment of the present invention reduces lightning strike affects to aircraft and in particular, to composite surfaces. Thus, an embodiment of the present invention simplifies protecting composite aircraft and in particular wing fuel tanks from lightning strikes, and simplifies repairing of damage to the aircraft lightning strike protection system.

More particularly, embodiments of the present invention include a lightning protection system in a lightning protection appliqué, an aircraft including the lightning protection system and method of protecting an aircraft from lightning strikes. A dielectric ply is fixed (e.g., bonded) above and completely covers metallic skin fasteners. A conductive ply is fixed (e.g., bonded) above and completely covers the dielectric ply and extends to an external connection, e.g., to a platform ground. The conductive ply directs lightning discharge current away from critical areas. Both plies may be adhesively backed and sequentially pressed into place.

Advantageously, a preferred lightning protection appliqué provides flexibility in lightning protection design without increasing aircraft weight appreciably and with superior performance and protection. Appliqué dielectric and conductor layers are simply sequentially pressed in place on the skin to avoid adding an embedded conductive layer or requiring a conductive surface protection layer for a CFRP skin. Thus a preferred lightning protection system simplifies aircraft skin design while reducing weight. In addition to weight, cost savings are realized from simple dielectric and conductor layer manufacturing requirements, as well as maintenance simplicity. Although age may make removal increasingly difficult, an entire preferred appliqué may be replaced by peeling the old ply(plies) off and pressing a new one(s) in place.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to the drawings and more particularly,FIG. 1shows an example of a lightning protection system according to a advantageous embodiment of the present invention in a cross section100of an aircraft. In this example, the cross section100is taken through the composite skin102, e.g., a Carbon Fiber Reinforced Plastic (CFRP) skin, of the aircraft wing, protected by a preferred embodiment Lightning Protection Appliqué (LPA)104. The composite skin102is fastened to a rib (metal or CFRP)106or similarly to a spar (CFRP), by skin fasteners108extending through the skin102and shear tie flange107, held in place by collars or nuts110. In this example, the cross section100is part of a fuel tank of a wing section. Although shown in this example protecting skin fasteners108at a wing fuel tank, this is for example only. A preferred LPA104may be used to protect any composite structure surface area where metal is exposed at the skin surface and so, is exposed to similar lightning threat levels, including other areas of a composite aircraft.

Preferably, the lightning protection appliqué102includes a dielectric ply112electrically isolating and insulating the skin fasteners108, e.g., from a lightning strike, and a conductive ply114diverting electrical energy from such a lightning strike away from the isolated skin fasteners108. Further, the conductive ply114extends at least 1.0″ (2.54 cm) beyond the dielectric ply112at the skin fasteners108, in this example to a ground contact116, e.g., a bolt or rivet, that is separated from the skin fasteners108and located away from the fuel tank. The ground contact116is connected to a platform ground118and held in place by suitable attachment120, e.g., a nut. Alternately, the conductive ply114is selected large enough that the lightning strike current dispersed around the surface of the structure to what are non-critical areas and without connecting the conductive ply114to platform ground118.

The dielectric ply112may be, for example, a suitable electrically insulating or dielectric film112D of an appropriate thickness, e.g., 0.003″ to 0.010″ (0.076-0.254 mm) and an attachment backing112A, e.g., of pressure sensitive adhesive, preferably, 0.002″ (0.050 mm) thick. The specific material selected for electrically insulating dielectric film112D depends on the intended installation conditions and the system's design. For demanding environments or designs where paint is not intended over the appliqué, the selected insulating material may be a film of a fluoropolymer such as for example polytetrafluoroethylene (PTFE); or a terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride (THV); or fluorinated ethylene propylene (FEP); or perfluoroalkoxytetrafluoroethylene (PFA). For applications where painting the appliqué may be desired or necessary, polyester, polyimide, or polyurethane films may be more appropriate to facilitate paint adhesion.

The conductive ply114in this example includes a conductive center layer114C sandwiched between and encapsulated by a protective surface layer114S and an attachment backing114A. Preferably, the conductive center layer114C includes a 0.001″ to 0.004″ (0.025-0.102 mm) thick metallic layer (solid or mesh) incorporated in an adhesive. The preferred protective surface layer114S provides the conductive center layer114C with partial environmental protection and promotes primer/top coat adhesion for subsequent painting, if necessary. Also, the preferred protective surface layer114S is 0.002″ to 0.004″ (0.051-0.102 mm) thick electrically insulating film, that may be the same material as electrically insulating film112D in the dielectric ply112. The attachment backing114A may be a layer of pressure sensitive adhesive, 0.002″ to 0.008″ (0.051-0.203 mm) thick. Both attachment backing112A and114A provide adhesion for attaching the respective ply112,114to the underlying structure, i.e., CFRP skin, and/or the dielectric ply112. The metallic layer in conductive center layer114C may be incorporated in the same adhesive material. Also, a sealant may be applied along the edges of the plies112,114, e.g., to prevent chemical/environmental erosion.

The dielectric ply112is applied in a strip to the skin102covering all skin fasteners108in the area of exposure. If applicable, the dielectric ply112also covers any other exposed surface metal features. Since the dielectric ply strip112may be limited to only the area around surface metal such as skin fasteners108, the strip112provides a significant weight saving over a more encompassing approach. Examples of such encompassing approaches include, for example, Published U.S. Patent Application No. 20050181203, entitled “Appliqué” to Diane C. Rawlings et al., and Published U.S. Patent Application No. 20050150596 entitled “Methods and Materials for Reducing Damage from Environmental Electromagnetic Effects” to Terrence G. Vargo et al., both of which are assigned to the assignee of the present invention and incorporated herein by reference. The overlap distance that the dielectric layer strip112must overlap surface metal depends on the skin resistance and the level of desired protection. However, preferably, for a state of the art CFRP and for a one hundred kiloampere (100 kA) lightning strike, the overlap is at least 1.8″ (4.57 cm) to sufficiently isolate metallic surface features.

The conductive ply114has significantly greater area coverage than the dielectric layer strip112and is applied directly over the dielectric layer strip112and CFRP skin102. Depending on design requirements, the preferred conductive ply114may cover the entire structure (e.g., aircraft or other composite structure) or only selected sections of the structure (e.g., selected sections of a composite wing or fuselage), e.g., for weight reduction. The conductive ply114provides a high current path to the platform ground118that directs current from lightning strikes away from the isolated skin fasteners108and through grounding studs116and nuts120. So, the grounding studs116also must be spaced adequately away from the skin fasteners108, depending upon skin102resistance and desired protection level. Thus, the conductive ply114overlaps and completely covers a significantly greater area than the dielectric ply112. The much lower resistance of the conductive ply114assures that very little current, if any, flows through the isolated skin fasteners108. So instead, substantially all of the effects of a lightning strike are directed well away the critical wing box section and so, away from the fuel tank. Accordingly, a preferred lightning protection appliqué104is capable of successfully meeting the lightning protection requirements for lightning strike zone2(100 kA) as set forth in SAE International standard No. ARP5412.

FIG. 2shows an example of an aircraft120with a preferred lightning protection appliqué (e.g., the LPA104ofFIG. 1) fixed to the skin102of the wing, e.g., over metal features on the wing. As noted above, the metal features may be fasteners at the fuel tank. Moreover, since the preferred conductive ply provides this external high current path, it is unnecessary to add weight to the CFRP skin by including a conductive surface protection layer. Thus, the preferred embodiment lightning protection appliqué104, which contains the lightning discharge protection, avoids the substantial weight of including a conductive surface protection layer. In particular, a preferred lightning protection appliqué104may be applied post-assembly after fastener installation and is easily inspected, maintained and replaced as necessary. Further, for a selective implementation, the preferred lightning protection appliqué104may also be uniquely configured/designed to satisfy the Electromagnetic Effect (EME) requirements for a particular lightning zone. Generally, a higher expected lightning discharge carries a higher current level and requires greater conductor thickness. So, the dielectric layer (112D,114D inFIG. 1) thicknesses and the conductive element (114C inFIG. 1) thickness can be selected accordingly to satisfy the particular lightning protection requirement level of each particular lightning zone.

Advantageously, the preferred lightning protection appliqué provides flexibility in lightning protection design without increasing aircraft weight appreciably and with superior performance and protection. Instead of adding an embedded conductive layer or requiring a conductive surface protection layer for a CFRP skin, the dielectric and conductor layers are simply sequentially pressed in place on the skin. Thus the present invention simplifies aircraft skin design while reducing weight. In addition to weight, cost savings are realized from simple dielectric and conductor layer manufacturing requirements, as well as maintenance simplicity. An entire appliqué may be replaced by just peeling the old ply(plies) off and pressing a new one(s) in place.

While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. It is intended that all such variations and modifications fall within the scope of the appended claims. Examples and drawings are, accordingly, to be regarded as illustrative rather than restrictive.