DEVICE FOR DIVERTING STATIC ELECTRICITY

Abstract: Here described is a device for diverting or distributing static electricity, particularly from surfaces on elements of plastic and/or composite material, particularly on aircraft. The device is characterized primarily in that it is formed as a course of stretch metal (4, 7), i.e. a metal foil (1) attached to said surface, the metal foil having been perforated (2) in such a manner that when stretched in its plane it admits plastic deformation essentially in optional direction while maintaining the form of an integral net.

Description: DEVICE FOR DIVERTING STATIC ELECTRICITY The present invention relates to a device for diverting or distri¬ buting static electricity from surfaces where such tends to accumulate, for instance under the effect of atmospherical light¬ ning activity, particularly surfaces of elements of plastic and/ or composite material, preferably vehicles such as aircraft etc. In aircraft technique development has gone towards the use of more and more plastic and composite material in the construction of aircraft fuselage and control and brake surfaces such as rudders and flaps. At the same time electronic equipment for communi- cation, navigation and control in relation to aircraft for civil and military use has become more extensive and sensitive to disturbance. Since static electricity tends to accumulate on surfaces of elements of plastic and/or composite material in connection with thunder, i.e. atmospherical lightning activity, but also under the effect of friction between aircraft and the atmosphere during flight through the atmosphere there is a great and increasing need for effective devices of the kind mentioned in the introduction. Such a device is usually connected with the metallic body part of the vehicle through conductors with low electric resistance. Devices of the kind mentioned introductorily are since long known. They function well, in a greater or less degree, but they all have drawbacks from different aspects. Common to all hitherto known devices of this kind is that in one way or another they consist of conductors arranged in a thin layer at the surface of said elements. Metal foil has then been used, which is glued onto the surface in the manufacturing process. It is a difficult manufacturing- technical problem how to effect the gluing without blisters appearing between the foil and the surface, which is often multi- dimensionally curved, especially in connection with aircraft. As the metal foil is practically undeformable in every direction, i.e. it cannot be stretched plastically in any direction, it must be slit or folded in order to be applied on such a surface, which entails extensive manual work. Metal nets have also been used in devices of the kind mentioned in the introduction. These are built of straight wires composed principally in the same way as a textile fabric, i.e. with warp and weft essentially at right angles to each other. The wires have relatively good resistance, particularly in one of the two directions, and carry structural loads in the same manner as for instance glass fibres in glass-fibre-reinforced plastic. A metal net can be applied in a somewhat simpler manner than a metal foil for the present purpose, but entails nevertheless relatively extensive manual work for application on multi-dimensionally curved surfaces owing to its relative undeformability. There are per se methods to coat surfaces of plastic and composite with metal that has in different ways been vaporized or atomized. A surface that has been metallized in such a manner will have good quality in several aspects, but it requires a very expensive equipment and extensive development work for such a method to work well. It should also be mentioned that an element surface of the kind mentioned above can be coated with a lacquer in which metal pig- ments have been dispersed. The metal pigments can be of nickel, silver or other metals. The actual application of such a lacquer presents a great many difficulties, because during this process the metal pigment must be distributed evenly in the lacquer, since the metal pigment has a strong tendency to settle due to the difference in density compared with the lacquer. In addition, it is difficult to distribute the metal pigment so evenly on the surface in question that good conductivity is attained. Further- more, this kind of lacquer is very expensive, with prices of about 1,000-10,000 SwKr/kilo. The object of the present invention is to achieve a device of the kind mentioned in the introduction, which does not have the drawbacks of hitherto known devices. It must therefore be easily applicable on multi-dimensionally curved surfaces, have good electrical conductivity and entail a low cost, totally. Such a device is characterized according to the invention pri¬ marily in that it is formed as a course of "stretch metal", i.e. a metal foil, attached to said surface, the metal foil having been perforated so that when stretched in its plane it admits plastic deformation in essentially optional direction while main¬ taining the form of an integral net. Stretch metal is usually manufactured from a metal foil that is perforated with mutually parallel cuts in rows with unperforated spaces inbetween, which spaces are short relative to the length of the cuts. The rows are made with approximately as long a dis¬ tance from each other as said spaces, and are longitudinally displaced so that the unperforated spaces will be right in front of the cuts. When the metal foil has been provided with a regular pattern of cuts in this manner, the foil is stretched in a direc¬ tion perpendicular to the longitudinal direction of the cuts, so as to form a net with rhombic meshes. The stretch metal can easily be deformed plastically in any direction. The stretch metal can naturally be manufactured in manners other than the one just mentioned, although the latter is the most common. Instead of perforating - usually punching cuts - one can for instance punch holes and then stretch. The invention relates to all types of stretch metal, i.e. metal foils, that has been perforated in various manners and stretched so as to form an integral net that admits plastic deformation in every possible direction. What is important is that good conductivity in every direction is main¬ tained also after the deformation. The stretch metal can if desired be rolled in order to reduce its extension perpendicular to the plane of the metal foil after the stretching, i.e. in order to reduce the thickness of the metal. The stretch metal can consist of different metals suitable for the current use, such as aluminium or copper. Because it is de¬ sirable to keep down the weight of certain vehicles such as air¬ craft, aluminium is particularly suitable, as it has a low dens¬ ity and in addition a relatively good conductivity. If the stretch metal is soft-annealed its plastic characteristics are improved. It has been shown that a suitable thickness of the metal foil, which is the original material in the manufacturing of the stretch metal, is about 0.1 to 0.5 mm, of which the least thick is pre¬ ferred. The net pattern of the stretch metal shall show holes, usually rhombic with a relatively small maximum dimension less than about 10 mm, preferably about 1-2 mm. The stretch metal can be applied in different manners on said surface e.g. by gluing or countersinking in unhardened plastic, i.e. polymer, which thereupon is set so as to fix the stretch metal nearest to the surface. It is suitable to provide an electrically insulating course immediately under the stretch metal. The invention will now be described in more details with reference to the accompanying figures of which: Figure 1 shows an enlarged view of a portion of a metal foil, provided with perforations in the form of straight cuts, for the manufacturing of stretch metal; Figure 2 shows an enlarged view of a portion of a stretch metal; Figure 3 shows a cut along the markings III - III in figure 2, the cut corresponding to the markings A - A in Figure 1; Figure 4 shows a plan view of an aircraft with markings of the surfaces to which the device according to the invention is applied, whereas; Figure 5 shows a view of the construction of a detail of the device according to the invention, i.e. stretch metal with under- layers. In Figure 1, 1 designates a metal foil and 2 designates linear perforations made therein, i.e. cuts, which are parallel and evenly distributed over the surface of the foil. A foil thus prepared is subject to a controlled drawing force prependicular to said cuts 2, whereupon stretch metal according to Figure 2 is formed. The stretch metal have in this case a number of essentially rhombic holes 3 in a netting of metal 4. The characteristic dimen¬ sions of the stretch metal are evident from the mesh length B and the mesh hight C. The breadth or thickness of the stretch metal is D and the thickness of the metal therein is E. In figure 4 the control surfaces 6 of an aircraft 5 are marked, which are particularly relevant for the application of the device according to the invention. The construction of the device according to the invention is evident from Figure 5, which shows as an example of embodiment the parts contained in the device. The stretch metal is designated by 7, while 8 is an electrically insulating course, for instance p of KEVLAR , glass fibre or a foil of polyethylene or other plastic. The supporting composite layers are designated by 9-11 and can be constituted of carbon-fibre-reinforced epoxy plastic. The different parts are put together in vacuum and are then heated until the comprised material is melted, whereupon the stretch metal sinks into the underlying layer and the parts together form an integral structure.

Claims: Claims 1. A device for diverting or distributing static electricity from surfaces where such tends to accumulate, for instance under the effect of atmospherical lightning activity, particularly surfaces of elements (6) of plastic and/or composite material, preferably vehicles such as aircraft etc., c h a r a c t e¬ r i z e d in that it is formed as a course of "stretch metal" (4, 7), i.e. a metal foil (1) attached to said surface, the metal foil having been perforated (2) so that when stretched in its plane it admits plastic deformation in essentially op- tional direction while maintaining the form of an integral net. 2. A device according to claim 1, c h a r a c t e r i z e d in that the stretch metal (4, 7) consists of aluminium or an alloy thereof. 3. A device according to claim 1, c h a r a c t e r i z e d in that the stretch metal (4, 7) consists of copper or an alloy thereof. 4. A device according to claim 2 or claim 3, c h a r a c t e¬ r i z e d in that the stretch metal (4, 7) has been soft- annealed in order to improve its plastic characteristics. 5. A device according to any preceding claim, c h a r a c t e¬ r i z e d in that said metal foil (1) shows a maximum thick¬ ness of 0.5 mm, preferably 0.1 mm. 6. A device according to any preceding claim, c h a r a c t e¬ r i z e d in that the net of the stretch metal (4, 7) shows holes with a maximum dimension (B, C) of about 10 mm, preferably 1-2 mm. 7. A device according to any preceding claim, c h a r a c t e¬ r i z e d in that the stretch metal (4, 7) is glued onto said surface. 8. A device according to any one of claims 1 to 6, c h a r a c¬ t e r i z e d in that the stretch metal (4, 7) is countersunk in the portion of said element which is nearest to the sur¬ face, fixed through setting of this portion.

Inventors: ANDERSSON KARL-GUSTAV, ANDERSSON, KARL-GUSTAV
Applicants: SAAB SCANIA AB, SAAB-SCANIA AKTIEBOLAG
Application year: 1989
Main classification: H05F3
IPC codes: B64D45, H05F3