Abstract:
A lightning protection appliqué incorporates a plurality of conductive plies adhesively affixed to a composite surface, at least a first one of the plies providing conductive characteristics sufficient to divert electrical energy from a lightning strike and at least a second one of the plies comprising operational circuitry. A dielectric ply is fixed to the composite surface over and completely covering at least one metal surface feature between the plurality of conductive plies and the composite surface.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 11/277,888 filed on Mar. 29, 2006 entitled Lightning Protection System for Composite Structure having a common assignee with the present application and is related to copending U.S. application Ser. No. 10/941429 published as U.S. Patent application No. 2005/0181203 A1 entitled Appliqué and having a common assignee with the present application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to the field of protecting composite structures from lightning strikes, and more particularly, to an appliqué having multiple conducting layers including one or more layers for protecting composite aircraft from lightning strikes and one or more layers for operational circuit functionality in aircraft systems. 
         [0004]    2. Description of the Related Art 
         [0005]    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. 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 are increasingly replacing aluminum structural components. 
         [0006]    Unfortunately, typical state of the art composites, such as Carbon Fiber Reinforced Plastic (CFRP), are approximately 2000 times more resistive than aluminum. A lightning strike to unprotected components, embedded in, passing through or attached to the CRFP aircraft skin that is generally 35-40% resin, does not dissipate as readily as in metal. Typically, temperature drives the damage level created by a lightning strike. The discharge current through composite skin resistance dissipates a burst of energy that causes the carbon fibers to become very hot, so much so, that the skin temperature becomes much hotter in a CRFP structure than in a state of the art aluminum skin. A lightning strike that may have little or no effect on an aluminum structure may damage unprotected CFRP components. For adequate lightning protection for a composite wing structure, the exterior CFRP structure must withstand an initial lightning strike of at least one hundred kiloamperes (100 kA) of discharge current without adverse affects or impact to safety-of-flight. 
         [0007]    Furthermore, skin fasteners at an exposed surface are most vulnerable to a direct strike and such a strike is most likely to cause sparking at structural joints. Accordingly, composite structure aircraft must have some protection, especially at exposed skin fasteners. 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 approaches to minimizing the possibility ignition and reducing damage are complicated and difficult to implement in CFRP. 
         [0008]    One approach involves selectively integrating metal (e.g., copper foil) into or onto the composite laminate at the fastening areas. However, this approach 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 resulted in 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. 
         [0009]    Numerous electrical circuits on aircraft and other vehicles require electrical conductors on or near the skin surface for operation such as deicing systems and certain antenna/sensor applications, structure health monitoring systems. Additionally, general electrical wiring for additional circuit elements, while not necessarily required to be on or near the surface require electrical conductors for operation and placement of those conductors is not critical to the operation of the system. 
         [0010]    Application of lightning strike protection to composite structures typically requires the addition conductive layers to the surface of the structure. To enhance the usefulness of these conductive layers, incorporation of additional functionality to the layers provides economic and design efficiency benefits. 
         [0011]    It is therefore desirable to provide lightning strike protection appliqués for aircraft which incorporate additional conductive layers for operational functionality in electrical system associated with the aircraft. 
       SUMMARY OF THE INVENTION 
       [0012]    The embodiments disclosed herein for the present invention provide a lightning protection appliqué incorporating a plurality of conductive plies adhesively affixed to a composite surface, at least a first one of the plies providing conductive characteristics sufficient to divert electrical energy from a lightning strike and at least a second one of the plies comprising operational circuitry. A dielectric ply is fixed to the composite surface over and completely covering at least one metal surface feature between the plurality of conductive plies and the composite surface. 
         [0013]    In exemplary embodiments the operational circuitry comprises heater elements for deicing, antenna/sensor elements or health monitoring circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    These and other features and advantages of the present invention will be better understood for exemplary embodiments by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0015]      FIG. 1  is a cross section of an exemplary multiple functionality appliqué incorporating the elements of the present invention; 
           [0016]      FIG. 2   a  is a cross sectional profile view of an appliqué according to the present invention having two conductive layers; 
           [0017]      FIG. 2   b  is a cross sectional profile view of an appliqué according to the present invention having 4 conductive layers; 
           [0018]      FIG. 3  is a peeled sectional view of an appliqué according to the present invention incorporating a deicing heating element as an outer layer of the flex circuit; and, 
           [0019]      FIG. 4  is an exemplary diagram of an application of the inventive system to an aircraft. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The disclosure of parent application Ser. No. 11/277,888, previously referenced, is incorporated herein as though fully set forth. An example of a lightning protection system according to an advantageous embodiment of the present invention is shown in  FIG. 1  in a cross section  100  of an aircraft wing section. In this example, the cross section is taken through a composite skin  102 , e.g., a Carbon Fiber Reinforced Plastic (CFRP) skin, of the aircraft wing, protected by an exemplary embodiment of a Flex-Circuit Lightning Protection Appliqué (FCLPA)  104 . Thicknesses of the FCLPA are exaggerated for clarity. The composite skin  102  is fastened to a rib (metal or CFRP)  106  or similarly to a spar, by metallic skin fasteners  108  extending through the skin and shear tie flange  107 , held in place by metallic collars or nuts  110 . In this example, the cross section is part of a fuel tank housed in the wing section. Although shown in this example as protecting skin fasteners at a wing fuel tank, this is only one exemplary embodiment. FCLPA  104  is applicable for use in protecting any composite structure surface area where metal is exposed at the skin surface and thereby exposed to similar lightning threat levels, including other areas of a composite aircraft. For the embodiment shown, the lightning protection appliqué  102  includes a dielectric ply  112  electrically isolating and insulating skin fasteners  108  from a lightning strike, and multiple conductive plies  114 A,  114 B and  114 C. The multi-layered system of alternating conductor and isolator layers is specifically designed and fabricated to provide unique electrical routing functions in addition to lightning protection with each of the conductors having separate functions. 
         [0021]    At least one of the conducting layers has conductive characteristics sufficient to divert electrical energy from such a lightning strike away from isolated skin fasteners  108 . Further, the conductive ply having these desired conductive characteristics extends at least 1.0″ (2.54 cm) beyond dielectric ply  112  at the skin fasteners  108 , in this example to a ground contact  116 , e.g., a bolt or rivet that is separated from skin fasteners  108  and located away from the fuel tank. The ground contact  116  is connected to a platform ground  118  and held in place by a nut  120  or other suitable attachment. Alternately, the conductive ply employed for lightning current dissipation is 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 ply  114  to platform ground  118 . In an exemplary embodiment, the conductive layer may be connected to the leading/trailing edge interface segments outside the composite wing box fuel tank. 
         [0022]    The dielectric ply  112  may be, for example, a suitable electrically insulating or dielectric film  112 D of an appropriate thickness, e.g., 0.003″ to 0.010″ (0.076-0.254 mm) and an attachment backing  112 A. In exemplary embodiments, a pressure sensitive adhesive approximately 0.002″ (0.050 mm) thick is employed. The specific material selected for electrically insulating dielectric film  112 D depends on the intended installation conditions and the system design. Fluoropolymer films such as for example polytetrafluoroethylene (PTFE); or a terpolymer of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride (THV); or fluorinated ethylene propylene (FEP); or perfluoroalkoxytetrafluoroethylene (PFA) are exemplary choices for the dielectric layer in various embodiments. For embodiments intended for applications in areas with lower lightning threat levels, the dielectric ply, as a discrete element is omitted. With the Flex Circuit Lightning Protection Appliqué design, the dielectric layer may be fully integrated into the flex circuit product form. For applications involving high lightning threat levels where thicker isolation is needed, the use of a separate dielectric layer  112  is still advisable. 
         [0023]    Multiple conductive plies  114 A-C in this example are separated by dielectric layers  115  to insulate the various conductors from one another to allow proper functionality. A protective surface layer  117  (for the embodiment shown an additional dielectric layer comparable to the internal separation layers) covers the outer conductive ply and an attachment backing  119  is employed to attach the multi-conductor flex circuit elements over protective dielectric ply  112 . 
         [0024]    In the exemplary embodiment, there is at least one conductive center layer  114 B that provides the lightning protection function. For the exemplary embodiment, this conductive layer has a conductor pattern of 0.0007 inch thickness minimum in solid or mesh configuration. The conductor material is aluminum, copper or a derivative alloy. Placement of the lightning protection layer in various embodiments depends on the overall function of the flex circuit. If the flex circuit contains surface sensors/features, then those features are expected to be placed in layer  114 A with the lightning protection layer being in layer  114 B. However, if there are not surface sensors, then the lightning protection feature would be at layer  114 A. 
         [0025]    Protective surface layer  117  provides the FCLAP conductors with partial environmental protection and promotes primer/top coat adhesion for subsequent painting, if necessary. In the exemplary embodiment, the protective surface layer is a 0.0005″ to 0.005″ (0.013-0.13 mm) thick electrically insulating film, employing the same material as electrically insulating film employed for dielectric layers  115  placed alternately in between conductive plies  114 A-C. Attachment backing  119  is a layer of pressure sensitive adhesive, typically 0.002″ thick (0.051 mm) thick. A sealant may be applied along the edges of the plies  112 ,  114 , to prevent chemical/environmental erosion. 
         [0026]    The dielectric ply  112  is applied in a strip to the skin  102  covering all skin fasteners  108  in the area of exposure. If applicable, the dielectric ply  112  also covers any other exposed surface metal features. The overlap distance that the dielectric layer strip  112  must overlap surface metal depends on the skin resistance and the level of desired protection. However, 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. 
         [0027]    The flex circuit containing conductive plies  114 A-C has significantly greater area coverage than dielectric layer strip  112  at least for the ply specific to lightning protection and is applied directly over the dielectric layer strip and CFRP skin  102 . In certain embodiments, the dielectric layer strip is incorporated directly into the flex circuit. Conductive ply  114 B provides a high current path to the platform ground  118  that directs current from lightning strikes away from the isolated skin fasteners  108  and through grounding studs  116  and nuts  120 . The grounding studs  116  are strategically located outside the critical areas to allow current to safely transition from the conductive layer  114 B. The much lower resistance of conductive ply  114 B assures that very little current, if any, flows through the isolated skin fasteners  108 . 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 appliqué  104  is capable of successfully meeting the lightning protection requirements for lightning strike zone  2  (100 kA) as set forth in SAE International standard No. ARP5412. 
         [0028]      FIGS. 2   a  and  2   b  show exemplary flex circuit multilayer systems applicable to the present invention with  FIG. 2   a  demonstrating a 2 conductor device (incorporating only two of the exemplary conductor elements  114 A and  114 B) with  FIG. 2   b  demonstrating a 4 conductor device (including 4 th  conductor element  114 D). Depending of the complexity of the intended function, the flex circuit conductive layer may contain up to 16 layers. The lightning protection layer in the flex circuit is provided in alternative embodiments with the added current routing function by etching/patterning of the conductor material (e.g., copper). Vias  121  in the Flex Conductor element are plated through or selectively insolated from conductors to achieve desired functionality. For example, connection through the Flex Conductor to grounding studs  116  as previously described with the studs extending through appropriately plated vias for connection to the lightning protection layer. Connection to alternative layers such as the deicing conductor for attachment to necessary circuit connections is similarly accomplished. 
         [0029]    The additional layers provided by the Flex Circuit LPA allow addition of increased functionality. For example, for a dual-purpose integrated de-icing and lightning protection appliqué system, supplementing a conductor layer (solid or patterned for current routing) for diversion of the lightning current at the surface with an additional layer of copper foil conductor, etched to form specific heating element patterns for specific sections of composite structures, provides for de-icing.  FIG. 3  shows an exemplary Flex Circuit employing two layers for this configuration. The etched heater element conductor layer  114 A and the lightning protection conductor Layer  114 B are isolated by dielectric layers  115  made of polyimide or Teflon that have been previously demonstrated as effective isolating materials in lightning protection systems. In this example, the high temperature characteristics of polyimide system, combined with the flexibility offered by thin, etched copper foil heating element and the already demonstrated lightning protection appliqué technology enable implementation of novel, dual-application lighting protection, de-icing system based on proven, mature technologies. While conductor  114 A is described herein as a heating element for deicing, the etched copper foil element shown is applicable to complex antenna/sensor patterns. Similar applications are implemented for lightning protection appliqué system with health monitoring, communications and wiring capabilities in alternative embodiments. 
         [0030]      FIG. 4  shows an example of an aircraft  120  with an exemplary lightning protection appliqué comparable to the FCLPA  104  of  FIG. 1  fixed to the skin  102  of the wing 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 lightning protection appliqué, which contains the lightning discharge protection, avoids the substantial weight of including a conductive surface protection layer. In particular, the lightning protection appliqué may 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 appliqué  104  may 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 ( 112 D,  115  in  FIG. 1 ) thicknesses and the lightning protection conductive element ( 114 B in  FIG. 1 ) thickness can be selected accordingly to satisfy the particular lightning protection requirement level of each particular lightning zone. 
         [0031]    The example shown in  FIG. 4  also includes the deicing element in the FCLPA which is interconnected with current source and controller  122  in the aircraft for providing actuation current to the deicing element. 
         [0032]    Having now described exemplary embodiments of the invention as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present invention as defined in the following claims.