Spark resistant structure, in particular for aircraft

Details Spark resistant structure, in particular for aircraft Spark resistant structure, in particular for aircraft

1999-10-08

2001-12-04

Fleming, Fritz (Department: 2836)

Electricity: electrical systems and devices

Discharging or preventing accumulation of electric charge

Structurally combined with building or vehicle

C361S217000, C361S212000, C361S216000, C361S117000, C361S220000, C244S00100R

Reexamination Certificate

active

06327132

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a spark-resistant structure, particularly for aircraft.
BACKGROUND OF THE INVENTION
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 provided for dissipating the energy of the lightning. Such electrically conducting means are generally 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 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;
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 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 fixing means being electrically in contact with said conducting surface layer, for example by means of amounts of electrically conducting paste introduced into said holes of said external element, as described in document EP-A-0248122.
Thus, when lightning strikes said fixing means (metallic screws or rivets for example) the lightning current is transmitted to said electrically conducting surface layer, where it dissipates.
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 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, for example made of foil, associated with each of said fixing means and 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 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.
SUMMARY OF THE INVENTION
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 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 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 and said conducting strip is obtained by means of said amount of electrically conducting paste introduced into the hole in said external element through which said fixing means passes.
Thus, because of the presence of said conducting 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 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 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.
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 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 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, composite material.
Electrical contact between said additional conducting strip and said fixing elements may also be provided by the conducting paste.
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 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 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 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 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.


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