AU7925198A

AU7925198A - Spark-resistant structure, in particular for aircraft

Info

Publication number: AU7925198A
Application number: AU79251/98A
Authority: AU
Inventors: Luc Andrivet; Jean-Pierre Avenet
Original assignee: Aerospatiale Matra
Current assignee: Aerospatiale Matra
Priority date: 1997-06-20
Filing date: 1998-06-10
Publication date: 1999-01-04
Anticipated expiration: 2018-06-10
Also published as: FR2765066B1; WO1998058840A1; CA2285102A1; DE69801752T2; EP0988228B1; ES2163281T3; AU729113B2; FR2765066A1; CA2285102C; EP0988228A1; DE69801752D1

Description

WO 98/58840 PCT/FR98/01182 Spark-resistant structure, in particular for aircraft The present invention relates to a spark-resistant structure, particularly for aircraft. 5 It is known that numerous parts of aircraft are now made of composite materials, particularly materials based on carbon fiber, and that in order to avoid these composite parts being destroyed, or at least damaged, in the event of lightning, electricity conducting means are 10 provided for dissipating the energy of the lightning. Such electrically conducting means are gene ally conducting layers, for example made of metal mesh, latticework or fabric, arranged on the surface of said composite parts. Thus, the known spark-resistant structures, particularly 15 for aircraft, generally comprise: - an external element, in the form of a sheet, made of a composite material and having an exterior face bearing an electrically conducting surface layer for dissipating lightning energy, and an interior face; 20 - an internal element in the form of a sheet having an exterior face and an interior face, said external and internal elements being arranged in such a way that the interior face of the external element is at least partially superposed with the exterior face of said 25 internal element, and being provided with holes which are aligned opposite each other; and - fixing means passing through said aligned holes opposite each other and forming an alignment of fixing means for holding said elements together in a fixed position, said 30 fixing means being electrically in contact with said conducting surface layer. Thus, when lightning strikes said fixing means (metallic screws or rivets for example), the lightning current is transmitted to said electrically conducting 35 surface layer, where it dissipates.

- 2 However, dissipation of energy of this kind assumes that there is good electrical contact between said fixing means and said conducting surface layer, because otherwise the lightning current causes damage to the 5 structure, in the region of said fixing means and said internal and external elements. So, to ensure satisfactory electrical contact between the fixing means and said conducting surface layer, provision has already been made for the interposition of electrically conducting washers, 10 for example made of foil, associated with each of said fixing means whose task is to make sure that these fixing means make electrical contact with said surface conducting layer. However, the fitting and fixing of these washers are difficult, lengthy and complex operations which are 15 further complicated by the fact that, in order for the exterior surface of the surface to be smooth, it is usual for said fixing means to have a countersunk head housed in countersinks made in the exterior face of said external element. 20 The use of such conducting washers is therefore expensive. The object of the present invention is to overcome these drawbacks. To this end, according to the invention, the 25 spark - resistant structure mentioned hereinabove is noteworthy: - in that said external element comprises, under said conducting surface layer, an electrically conducting strip extending longitudinally in the direction of said 30 alignment of the fixing means, through which strip said fixing means pass, and which is in electrical contact with said means; and - in that the electrical contact between each of said fixing means, on the one hand, and the conducting surface 35 layer and said conducting strip, on the other hand, is - 3 obtained by means of an amount of electrically conducting paste introduced into said holes in said external element. Thus, because of the presence of said conducting 5 strip and because of the use of amounts of a conducting paste which, for example, consists of an elastomer filled with conducting particles, for example made of aluminum or carbon, it is possible to avoid the use of the contact washers and the associated fitting operations mentioned 10 hereinabove, while at the same time obtaining excellent protection against lightning. Lightning tests have demonstrated that the structure of the invention was able, without damage, to withstand lightning currents of the order of 150 to 200 kA. In addition, such a conducting 15 paste also seals the structure at the fixing means and affords protection against corrosion of the galvanic type. The amount of electrically conducting paste introduced into said holes is sufficient to cover the entire shank of each fixing means. 20 Furthermore, it will be noted that said conducting strip may easily be incorporated into said external element, at its time of production. As a preference, said electrically conducting strip extends transversely symmetrically with respect to 25 said alignment of fixing means and has an electrical conductivity similar to that of said conducting surface layer. In order to further improve the ability of the structure to dissipate lightning energy, said external 30 element may comprise, under said conducting strip, at least one additional conducting strip extending longitudinally in the direction of said alignment of the fixing means, through which strip said fixing means pass, and which is in contact- with said fixing means and 35 separated from said conducting strip by a thickness of -4 composite material. Electrical contact between said additional conducting strip and said fixing elements may also be provided by the conducting paste. 5 As was the case with the conducting strip, it is advantageous for said additional conducting strip to extend transversely symmetrically with respect to said alignment of fixing means. To facilitate implementation, it is preferable for 10 the outline of said additional conducting strip to be vertically aligned with the outline of the conducting strip. As a preference, at the opposite end to the free edge of said external element, the edges of said 15 conducting strip and of said additional conducting strip protrude beyond a position of vertical alignment with the free edge of said internal element. When, as is usual, said fixing means have a countersunk head, said conducting strip and said 20 additional conducting strip are, just like the conducting surface layer, in contact with said countersunk heads. In the structure according to the present invention, said internal element may be metallic, for example made of aluminum, or alternatively may be a part 25 made of composite material. The figures of the appended drawing will make it easy to understand how the invention may be carried out. In these figures, identical references denote similar elements. 30 Figure 1 is a cross section, passing through the axis of a fixing bolt, of a spark-resistant structure according to the present invention. Figure 2 is a plan view, with partial cutaway, of the spark-resistant structure of Figure 1. 35 Figures 3 and 4 illustrate two applications of the -1-_4 7xi - 5 structure of Figures 1 and 2. The spark-resistant structure according to the present invention and depicted in Figures 1 and 2 comprises two elements 1 and 2 in the form of sheets which 5 partially overlap. The external element 1 is made of composite material, for example based on carbon fiber, and has an exterior face 3 and an interior face 4. Said exterior face 3 bears, as is known, an electrically conducting surface 10 mesh 5, for example of bronze wires, intended to dissipate lightning energy when lightning strikes the external element 1. The surface mesh 3, the mass of which may be of the order of 80 g/m 2 , is itself covered with a protective layer of paint 6. 15 The internal element 2, which may be metallic or made of composite material, has an exterior face 7 and an interior face 8. The elements 1 are 2 are superposed in such a way that the interior face 4 of the external element 1 at 20 least partially covers the exterior face 7 of the internal element 2, in such a way that the edge 9 of the external element 1 is vertically aligned with said exterior face 7 of said internal element 2 and that the edge 10 of the internal element 2 is vertically aligned with said 25 interior face 4 of said external element 1. In their zone of overlap, said elements 1 and 2 are respectively provided with alignments of holes 11 and 12, said alignments being, for example, parallel to the edges 9 and 10 respectively. When said elements 1 and 2 30 are in their overlapping position of Figures 1 and 2, each hole 11 of the element 1 faces a hole 12 on the element 2. Furthermore, on the exterior face 3 of the external element 1, each hole 11 is widened by a countersink 13. A metallic screw 14 with a countersunk head 15 and 35 an at least partially threaded shank 16 is introduced into - 6 each pair of holes 11, 12 facing each other. The countersunk heads 15 of the metallic screws 14 rest in the countersinks 13, with the interposition of an amount 17 of electrically conducting paste, and the shank 16 of each 5 screw 14 collaborates with a nut 18. The nuts 18 bear, for example via washers 19, on the interior face 8 of the internal element 2, to press the countersunk heads 15 of the screws 14 into the countersinks 13 so that said elements 1 and 2 are held together in a fixed position, as 10 depicted in Figures 1 and 2 and so that said countersunk heads 15 compress said amounts 17 of paste and cause the conducting paste to flow along the shank of the metallic screws 14. Thus, the surface conducting grating 5 is in electrical contact with the countersunk heads 15 of said 15 metallic screws 14 via the electrically conducting paste 17. The countersunk heads 15 are covered on the outside with the protective paint 6 and, as shown in Figure 2, the screws 14 form an alignment of direction L 20 L. According to the present invention, the spark resistant structure of Figures 1 and 2 additionally comprises: - a first strip 20 of electrically conducting mesh, for 25 example made of bronze wires and with a mass of 230 g/m 2 , arranged under the surface mesh 5 and separated therefrom by a film of adhesive or resin 21 fixing said surface mesh in place, said first strip of mesh 20 extending longitudinally in the direction L-L of the alignment of 30 metallic screws 14, having the latter passing through it and being in electrical contact with their countersunk heads 15 via the amounts 17 of electrically conducting paste; and - a second strip 22 of electrically conducting mesh, for 35 example made of bronze wires and with a mass of 80 g/m 2 , L Ji: C )'! -7 arranged under the first strip of mesh 20 and separated therefrom by a thickness 23 of composite material, said second strip of mesh 22 extending longitudinally in the direction L-L of the alignment of metallic screws 14, 5 having the latter passing through them and being in electrical contact with their countersunk heads 15 via the amounts 17 of electrically conducting paste. Said first and second strips 20 and 22 extend transversely and symmetrically with respect to the 10 alignment L-L of the screws 14. One of their longitudinal edges, 20G and 22G, respectively, lies flush with the edge 9 of the external element 1. Their opposite edges, 20D and 22D, respectively, are vertically aligned with one another and preferably protrude beyond a position of vertical 15 alignment with the edge 10 of the internal element 2, extending further than this position of vertical alignment in the direction of the central part of the external element 1. The internal element 2 may be metallic, for 20 example made of aluminum. It may therefore act as a fishplate 2A for assembling the external element 1 with a similar element 1A, by means of fixing means 14, 18 and 14A, 18A (see Figure 3). This internal element 2 may, as an alternative, 25 itself be a composite element 2B assembled with the external element 1 using the fixing means 14, 18.

Claims

1. Spark - resistant structure, particularly for aircraft, comprising: 5 - an external element (1), in the form of a sheet, made of a composite material and having an exterior face (3) bearing an electrically conducting surface layer (5) for dissipating lightning energy, and an interior face (4); - an internal element (2) in the form of a sheet having an 10 exterior face (7) and an interior face (8), said external (1) and internal (2) elements being arranged in such a way that the interior face (4) of the external element (1) is at least partially superimposed on the exterior face (7) of said internal element (2), and being provided 15 with holes (11, 12) which are aligned opposite each other; and - fixing means (14, 18) passing through said aligned holes (11, 12) opposite each other and forming an alignment (L L) of fixing means (14, 18) for holding said elements (1, 20 2) together in a fixed position, said fixing means (14, 18) being electrically in contact with said conducting surface layer (5), characterized: - in that said external element (1) comprises, under said 25 conducting surface layer (5), an electrically conducting strip (20) extending longitudinally in the direction (L L) of said alignment of the fixing means (14, 18), through which strip said fixing means (14, 18) pass, and which is in electrical contact with said means; and 30 - in that the electrical contact between each of said fixing means (14, 18), on the one hand, and the conducting surface layer (5) and said conducting strip (22), on the other hand, is obtained by means of an amount of electrically conducting paste (17) introduced 35 into said holes (11) in said external element (1). -9

2. Structure according to Claim 1, characterized in that said electrically conducting strip (20) extends transversely symmetrically with respect to said alignment of fixing means (14, 18). 5

3. Structure according to one of Claims 1 and 2, characterized in that said electrically conducting strip (20) has an electrical conductivity similar to that of said conducting surface layer (5).

4. Structure according to one of Claims 1 to 3, 10 characterized in that said external element (1) comprises, under said conducting strip (20), at least one additional conducting strip (20) extending longitudinally in the direction of said alignment of the fixing means (14, 18), through which strip said fixing means pass, and which is 15 in electrical contact with said fixing means and separated from said conducting strip by a thickness of composite material (23).

5. Structure according to Claim 4, characterized in that said additional conducting strip 20 (22) extends transversely symmetrically with respect to said alignment of fixing means (14, 18).

6. Structure according to Claim 4 or 5, characterized in that the outline of said additional conducting strip (22) is vertically aligned with the 25 outline of said conducting strip (20).

7. Structure according to one of Claims 4 to 6, characterized in that, at the opposite end to the free edge (9) of said external element (1), the edges (20D and 22D) of said conducting strip (20) and of said additional 30 conducting strip (22) protrude beyond a position of vertical alignment with the free edge (10) of said internal element (2).

8. Structure according to one of Claims 4 to 7, in which said fixing means (14, 18) have countersunk heads (15) 35 housed in countersinks (13) made in the exterior face (3) - 10 of said external element (1), characterized in that said conducting surface layer (5) and said conducting strips (20, 22) are in contact with said countersunk heads (15). 5

9. Structure according to one of Claims 1 to 8, characterized in that said internal element (2) is metallic.

10. Structure according to one of Claims 1 to 8, characterized in that said internal element (2) is made of 10 a composite material.