RU2374145C1 - Method to protect composite-material fuel tank against electric discharges - Google Patents

Abstract

FIELD: aircraft engineering.

SUBSTANCE: invention relates to aircraft engineering, particularly to equipment intended for protecting fuel tank against electric discharges caused by lightning. In compliance with proposed method, insulating element is fitted at points where metal devices are attached to fuel tank to ensure electric insulation of aforesaid elements with respect to fuel tank. Line metal elements accommodate insulating inserts to divide them into isolated parts, each being jointed to metal substructures that feature extremely low resistance.

EFFECT: ruling out failures caused by lightning, expanded performances.

4 cl, 3 dwg

Description

FIELD OF THE INVENTION

The present invention relates to a method of protection against electric discharges caused by lightning striking into fuel tanks made of materials having low electrical conductivity and, in particular, to a method of protecting devices, such as pipes, valves and pumps, located inside the fuel tanks.

BACKGROUND OF THE INVENTION

Composites have greater electrical resistance compared to metallic materials. The latter were usually used in the field of aeronautics for the manufacture of structures designed to store fuel, taking into account their mechanical characteristics, due to the weight of such materials.

The high electrical resistance inherent in composites causes a very significant inductance effect on internal systems located in the fuel tank. This effect induces internal electric currents, which can lead to catastrophic accidents or phenomena associated with the overall integrity of the structure.

Phenomena associated with moderate or strong electric discharges arising in the event of a lightning strike, which should be prevented in a structure made of a material with low electrical conductivity, to ensure its structural integrity, prevent failures of the most important elements of electrical equipment and prevent the occurrence of an electric arc inside the tank, are :

superheat areas: high current density at certain specific places in the structure, such as joints or intersecting elements, can lead to hot spots. If the temperature exceeds 200 ° C (the self-ignition point of the fuel adopted by the FAA / JAA), then if the tank has the appropriate stoichiometric concentration, the fuel may heat up to the ignition temperature;

electric arcs (sparking): the current flow through materials having different resistances and in geometrically spaced positions can create a voltage drop at the junctions, leading to the appearance of discharges in the form of an electric arc and to ignition of the fuel and flammable liquid contained in the structure;

electrical equipment malfunction: electrical discharges caused by a lightning strike create strong currents circulating through the external structure, and can lead to the passage of current through internal systems as a result of bypass or induction. These effects can lead to critical failures, creating the danger of a catastrophic accident.

SUMMARY OF THE INVENTION

The aim of the present invention is to protect the fuel tanks of the aircraft located in the wings and / or stabilizers made of composite and equipped with various electrical equipment from strong electrical discharges. However, the present invention is applicable to any structure made of a material with low electrical conductivity and containing flammable liquids, as well as electrical and / or hydrodynamic systems.

In the construction of fuel tanks, a new generation of materials and, in particular, composites with low electrical conductivity has found application, has led to the appearance of designs of fuel tanks with other mechanical and electrical characteristics, due to the properties of the materials used for their manufacture. Although the tank is made of non-metallic materials with low electrical conductivity, the devices located inside are made of highly conductive materials, such as fuel pipe systems made of aluminum pipes and equipment in metal cases.

The disadvantages leading to catastrophic accidents caused by electric discharges in the fuel tank of the aircraft, in many cases that occurred during a lightning strike, are described above. When the structures used are made of non-conductive materials, there is a risk of current circulation through internal systems, usually made of metallic materials. Such a situation may lead to the occurrence of electric arcs, internal hot particles, hot spots or equipment malfunctions that are considered critical, creating potential sources of ignition, which can lead to an explosion and subsequent destruction of the structure.

According to the present invention, a method for protecting a group of metal devices located inside a fuel tank, fully or partially made of composites, from electrical discharges, according to which:

a) at the points where the metal devices are attached to or fixed on the fuel tank, an insulating element is installed, providing electrical insulation of these metal devices relative to the fuel tank;

b) insulating inserts are installed in linear metal devices so that these devices are divided into parts isolated from each other;

c) connection with metal substructures with very low resistance is provided in each of the parts isolated from each other in the group of metal devices.

Other features and advantages of the present invention will be apparent from the following detailed description of an illustrative embodiment with reference to the accompanying drawings.

DESCRIPTION OF DRAWINGS

Figure 1 is a schematic illustration of the application of the method of the present invention in a fuel tank equipped with several devices.

Figure 2 - pipe located in a fuel tank protected by the method of the present invention.

Figure 3 is a support located in the wall of a fuel tank protected by the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a fuel tank 11 made of a material with low electrical conductivity, a device 13 consisting of a fuel pipe, and a device 15 consisting of a piece of equipment enclosed in a metal casing. The group of devices 13, 15 represents the metal nodes located inside the tank 11, necessary for pumping fuel.

According to the method of the invention, at the points where the devices 13 and 15 are attached to the tank 11, insulating elements 21 are used which can be made of plastic and which isolate these devices 13, 15 from the tank 11.

Figure 3 shows a specific example of an insulating element 21 at the attachment point on the tank 11.

In turn, several insulating elements 23 of non-conductive material, such as plastic, are included in the pipe 13, limiting its parts 31, 33 and 35, isolated from each other.

Figure 2 shows an example of an insulating insert 23 in a pipe 13 located between two connections with a substructure 25, the reference potential.

Finally, there are connections for parts 31, 33 and 35 of pipe 13 and device 15, which can be considered a part isolated from other devices, with a substructure 25 with a reference potential (0 V). Devices 13 and 15, which can be immersed in fuel, can accumulate a charge to a certain level as a result of generating a static charge after the tank 11 receives an electric discharge, but these compounds prevent the appearance of electric arcs and prevent this charge from rising to a certain acceptable level (200 μJ) due to removal of static charge and prevention of its accumulation on system components.

The basis of the method, which is the object of the present invention, is to prevent shunting of the current Ce circulating on the outer walls of the fuel tank 11 as a result of an electric discharge caused, for example, by a lightning strike, on the internal areas in which the devices 13 and 15 are located.

Since the entire installation, consisting of devices 13 and 15, isolated from the main structure of the tank 11, is inside the only path for the external current Ce, it forms the connection points with the substructure 25 with the reference potential (metal).

The inductance and small cross section of these electrical connection points, as well as the presence of insulating inserts 23 in the fuel pipe 13 cause the external current Ce to circulate inward through the shunts Cs at a very low level. Therefore, the only current exposed to the metal elements of the system inside the tank is the current Ci arising under the influence of the induction effect.

On the other hand, the insulating inserts 23 in the fuel pipe 13 or any other linear device prevent the occurrence of closed circuits for the current Ci with the repeated occurrence of high-density current circulation.

Insulating inserts 23 must maintain a certain length to prevent the occurrence of an electric arc. In particular, the range of applicable sizes tested and used in the fuel tanks of aircraft designed and manufactured by Airbus is 25-80 mm.

The insulating inserts 23 must be made of an insulating material capable of creating electrical insulation of more than 100 MΩ to counteract the potential difference that may arise due to a strong electrical discharge.

Determining the points at which insulating inserts 23 should be placed requires a detailed study of the resistances present in the system, as well as studying the levels of induced currents in each zone of the system in the most critical case of an external electric discharge. Nevertheless, it is believed that, as a general rule, an electrical resistance of less than 10 mOhm should exist between any metal section of the substructure 25 (reference potential) and each part 31, 33, 35 of the pipe 13 or any other linear device located inside the fuel tank 11 between two insulating inserts 23.

It should be noted that the main essence of the method proposed in this application is based on these insulating inserts 23, since they prevent the circulation of strong electric currents through metal parts related to the internal systems of the fuel tank. In some cases, these insulating components must have a completely new design, materials, geometry, because they must be placed in very small areas or in very short pipe sections.

The effectiveness of the claimed solution is supported by several tests conducted by the applicant company in relation to the fuel system located in the horizontal stabilizer of various aircraft models.

Modifications may be made to the preferred embodiment of the invention described above that are within the scope of the present invention as defined by the appended claims.

Claims (4)

1. A method of protecting a group of metal devices (13, 15) installed inside a fuel tank (11) made entirely or partially of composites from electrical discharges, characterized in that:
a) at the points where metal devices (13, 15) are attached to or fixed on the fuel tank (11), an insulating element (21) is installed, providing electrical isolation of these metal devices (13, 15) relative to the fuel tank (11);
b) insulating inserts (23) are installed in linear metal devices (13), so that these devices are divided into parts (31, 33, 35), isolated from each other;
c) in each of the parts (31, 33, 35), isolated from each other in the group of metal devices (13, 15), they provide a connection with metal substructures (25) with a very low resistance. 2. The method according to claim 1, characterized in that the insulating insert (23) creates an electrical resistance exceeding 100 MΩ. 3. The method according to claim 2, characterized in that the division of the linear devices (13) into parts (31, 33, 35) is carried out so that between each of them and the metal substructure (25) there is an electrical resistance of less than 10 mΩ. 4. The method according to any one of the preceding paragraphs, characterized in that the fuel tank is located in the wing and / or stabilizer of the aircraft.

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