Document
United States Patent [191 [111 4,352,142
Olson
[45] Sep. 28, 1982
[54] COMPOSITE AIRCRAFT STRUCTURE 3,989,984 11/1976 Amason et al. ..................... 361/218 HAVING LIGHTNING PROTECTION Primary Examiner-J. D. Miller [75] Inventor: Glenn 0. Olson, Seattle, Wash.
Assistant Examiner-L. C. Schroeder Attorney, Agent, or Firm-Conrad 0. Gardner; B. A.
[73] Assignee: The Boeing Company, Seattle, Wash.
Donahue [21] Appl. No.: 254,799 [571 ABSTRACT [22] Filed: Apr. 15, 1981 A lightning protection system for advanced composite [51] Int. C1.3 ............................................... H05F 3/00 aircraft structures consisting of a sandwich structure [52] U.S. CI. ..................................... 361/218; 361/117 including two layers of aluminum foil separated by a
[58] Field of Search ................ 361/218, 117, 217, 212
layer of dielectric material. The sandwich structure is c 5 g References Cited applied to the surface of the composite aircraft structure desired to be protected from lightning strike damage U.S. PATENT DOCUMENTS thereby confining damage to the sandwich structure 3,480,231 11/1967 Lumn .................................. 361/218 which can be removed and replaced.
3,482,802 12/1969 Lumn .................................. 361/218 3,498,572 3/1970 Lumn .................................. 361/218 3,906,308 9/1975 Amason et al. ..................... 361/218 4 Claims, 2 Drawing Figures
U.S. Patent Sep. 28, 1982 4,352,142
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surface member 14. It will be recognized that graphite COMPOSITE AIRCRAm STRUCTURE HAVING epoxy composite structures and other composite struc- LIGHTNING PROTECTION tures utilized on current and future aircraft will be sub- jected to lightning strike discharges. This is particularly The invention described herein was made in the per- 5 the case if the exterior surface structure is utilized at or formance of work under a NASA contract and is sub- near the aircraft protuberances such as wing tip, stabi- ject to the provisions of Section 305 of the National lizer tip, vertical fin tip, rudder, elevator, aileron, etc.
Aeronautics and Space Act of 1948, Public Law 85-568 Such locations will be subjected to the initial attach- (72 Stat. 435; 42 USC 2457). ment lightning strike. The initial attachment lightning The present invention relates to aircraft lightning 10 strike as characterized by high peak current Amp) and large energy transfer (2(10)6 Amp* Sec) is protection systems and, more particularly, to a lightning protection system for advanced composite aircraft capable of creating severe structural damage to unpro- tected graphite epoxy structures. Accordingly, such structures such as graphite epoxy and the like. Light- ning strikes involving aircraft are a common occurrence composite structures which are exposed to lightning and therefore aircraft lightning protection systems are 15 strikes at the exterior of the aircraft, particularly those needed for the prevention of structural failures, flight near aircraft protuberances, will necessitate lightning control system damage, fuel explosion, and electrical or protection, preferably a lightning protection system as electronic systems malfunctions caused by lightning shown at 100 in FIG. 1 which will substantially confine strikes. damage to the lightning protection system 100 and facil- In the patent literature, U.S. Pat. No. 3,906,308 is 20 itate removal and replacement thereof without costly illustrative of a lightning protection system for ad- repair to the underlying composite external surface vanced composite aircraft structures such as boron and member 14.
graphite epoxy composites. In accordance with the Turning now to FIG. 2, it will be seen that lightning protection sandwich structure 100 for underlying com- system of U.S. Pat. No. 3,906,308, the critical compo- nent desired to be protected such as a composite skin 25 posite external surface member 14 includes a pair of panel, composite fuel tank access door or other compo- outer layers of aluminum foil 20 and 28 between which nent located at the aircraft external surface, is covered is sandwiched an intermediate dielectric layer 24. Lay- by a thin dielectric coating and/or films of the appropri- ers of adhesive material 22 and 26 are utilized to sand- ate dielectric strength and thickness to provide a com- wich and affix dielectric layer 24 between outer sand- plete dielectric surface upon which is disposed a foil or 30 wich aluminum foil layers 20 and 28. The further adhe- metal strips of appropriate dimension affixed to the sive layer 30 is utilized between inner aluminum foil layer 28 and composite external surface member 14 to dielectric material thereby providing dwell points for adhere the sandwich structure 100 to the outer surface the sweptstroke and restrike lightning current channel.
These metallic strips are installed perpendicularly to the of composite external surface member 14.
or at an angle from the airstream direction and 35 A primary advantage of the lightning protection airstream sandwich structure 100 is that the thermal and right- are grounded to metallic airplane structure at one or both ends. In contrast, the present aircraft lightning angle-effect of the lightning arc attachment to graphite epoxy skin 14 of the aircraft is nullified. Substantiallyall protection system for a composite aircraft structure utilizes a thin metal-dielectric-metalsandwich which is damage is confined to the protection system 100 which utilized to cover the region to be Drotected and which 40 may 6e easily removed and replaced: Lightning tests can be easily removed-and repla'ced in the event of accomplished on several prototype systems appear to damage to the sandwich structure. demonstrate the aforementioned theory behind the fea- It is accordingly an object of the present invention to sibility and performance advanced for the present light- provide in a composite aircraft structure, a two-metal ning protection system 100 for composite external sur- sandwich structure for providing surface protection of 45 face member 14.
the composite aircraft structure with respect to light- The right-angle-effect is herein defined as the interac- tion of forces between the current paths taken by the ning strikes.
It is a further object of the present invention to pro- lightning. That is, if the arc attachment is normal to the vide means for nullifying thermal and right-angle-effect aircraft surface, the lightning current flows away from of lightning arc attachment to an advanced composite 50 the attachment point along the direction of the graphite aircraft structure. fibers. Such graphite fibers in advanced composite A full understanding of the invention, and of its fur- graphite epoxy structures comprise angle ply laminates ther objects and advantages and the several unique (e.g., o " , +45", 90"). Therefore, in addition to the right- aspects thereof, will be had from the following descrip- angle-effect, buckling forces are also created between 5 5 the plies of the laminate when the current is forced to tion when taken in conjunction with the accompanying drawings in which: flow in different directions in each ply at the arc attach- FIG. 1 is a side view in section of an aircraft structure ment point. Both of these effects are herein termed having an advanced composite structural member; and, right-angle-effects.
FIG. 2 is a sectional view taken along the lines 2-2 Three-inch-wide, 5.5 mil thickness aluminum foil of the composite structural member shown in FIG. 1. 60 with nonconductive adhesive backing was utilized for Turning now to FIG. 1, an exterior aircraft structural aluminum foil members 20 and 28, requiring about one- 14 made of an advanced composite material fourth inch overlap as shown at 66 in FIG. 2. With member such as graphite epoxy will be seen to be supported by wider aluminum foil nonconductive adhesive backed typical rib and/or spar support members 10 and 12 and tape, a lesser number of overlaps would be required include typical caps, leading edges and/or access panel 65 and, in fact, if the aluminum foil members with noncon- members 16 and 18. Lightning protection sandwich ductive adhesive backing were of substantial width they structure 100 in accordance with the teachings of the could be cut to the width of the composite member 14 present invention is seen applied to composite exterior without requiring an overlap as shown at 66. Aluminum
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said sandwich structure comprising a pair of layers of foil members 20 and 28 at the outer layers of sandwich structure 100 may have a thickness from about two to aluminum foil having a layer of dielectric material disposed between said pair of layers of aluminum about six mils. Dielectric layer 24 may comprise a di- foil.
electric film, e.g., Tedlar film, a bondable polyvinyl
2. * lightning Protection system for Protecting a
fluoride (PVF) available in one-half to three mil thick- 5 lightning accessible surface of an aircraft comprising: ness. The thickness of dielectric layer 24 may be from a sandwich structure disposed adjacent said lightning about one to about Seven mils thickness. Adhesive lay- accessible surface; ers 22,24, and 30 may comprise a nonconductive adhe- said sandwich structure having a pair of outer layers sive such as type AF-143 as available from 3M of electrically conductive material and an inner 10 pany. Test results indicated that the present surface layer of electrically nonconductive material.
lightning protection sandwich structure 100 substan- wherein said 3. The invention according to claim tially eliminated thermal damage and inter-intra-lamina outer layers of electrically conductive material have a buckling damage characterized by respective burning thickness of between 2 and 6 mils, and said inner layer Of resin and twisted’torn Of the laminae at the 15 of electrically nonconductive material has a thickness of lightning attachment point of a graphite epoxy laminate between 1 and 7 mils.
14 utilized in an aircraft advanced composite surface or 4. ln combination in an aircraft surface member: skin structure.
a layer of composite laminate material; and, What is claimed is: a sandwich structure attached to said layer of com- 1. An aircraft structure comprising in combination: posite laminate material for substantially nullifying 20 a graphite epoxy laminate layer; right-angle-effects of lightning arc attachment to a sandwich structure applied to one major surface said layer of composite laminate material.
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area of said graphite epoxy laminate layer;