Patent Publication Number: US-2017368769-A1

Title: Method of forming a structural portion of a fuel tank for an aircraft

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of forming a structural portion of a fuel tank for an aircraft, in particular a method of forming a structural portion of a fuel tank having a sensor integrated in the structural portion. The present invention also relates to a fuel tank for an aircraft, a fuel quantity indicating system, and an aircraft. 
     BACKGROUND OF THE INVENTION 
     Aircraft have fuel quantity measuring systems for measuring the quantity of fuel in their fuel tanks. A typical fuel quantity measuring system includes fuel probes in the fuel tank to directly determine the fuel level in the fuel tank. 
     By placing fuel probes in a fuel tank, the probes are necessarily exposed to fuel in the fuel tank. As such the probes must be capable of long term exposure the fuel environment in the tank. Such an arrangement also requires the provision of electrical wiring in the fuel tank. The presence of electrical wiring in the fuel tank requires additional precautions and requirements, such as explosion prevention requirements, and also increases manufacturing and maintenance times due to the complexity and difficult accessibility of the interior of the fuel tank. 
     One possible solution is the provision of wireless sensors which are mountable to the outside of a fuel tank but are able to determine the quantity of fuel in the fuel tank. U.S. Pat. No. 7,814,786 B1 describes a fuel quantity measuring system having a wireless sensor assembly. The sensor assembly comprises a sensor and a substrate which is used for mounting the sensor. The sensor comprises an electrically conductive trace disposed on the substrate. The electrically conductive trace is a spiral winding of conductive material. The spiral winding of conductive material acts as an open-circuit magnetic field response sensor. 
     An interrogation system is used to interrogate the sensor. The interrogation system includes a broadband radio frequency (RF) antenna configured to transmit and receive RF energy to and from the sensor. The interrogation system is then able to determine the quantity of fuel in the fuel tank based on the sensor. 
     With such a sensor assembly, the accuracy of the sensor is at least partially dependent on maintaining the path and spacing of the electrically conductive trace. As such, the substrate enables the sensor to be mounted to the outside of the fuel tank and ensures that the correct configuration of the electrically conductive trace is maintained. Such a sensor is exposed to the external environment of the fuel tank, and may become at least partially detached from the outer surface of the fuel tank. 
     WO 2013/188443 describes embedding a sensor system into the wall of a polyethylene fuel tank. However, a problem with embedding the sensor in a structural wall of a fuel tank is that stress concentration points may be generated at the periphery of the sensor substrate. This may reduce the structural integrity and reliability of the fuel tank. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a method of forming a structural portion of a fuel tank for an aircraft, the structural portion being formed from a fibre reinforced polymer and having a sensor integrated in the structural portion, the method comprising providing a fibre ply acting as a structural component, embroidering an electrically conductive wire in a predetermined pattern on the fibre ply to form the sensor so that the fibre ply acts as a sensor substrate, and applying a polymer matrix to the fibre ply, so that the fibre ply and electrically conductive wire are covered by the polymer matrix. 
     As such, it is possible to remove the need for a separate dedicated substrate, whilst providing for the necessary accurate arrangement and configuration of the electrical conductive wire. By embroidering the electrically conductive wire to the fibre ply, it is possible to fixedly configure the electrical conductive wire. 
     By covering the electrically conductive wire with the polymer matrix it is possible for the electrically conductive wire to be isolated from the environment external to the fuel tank and the internal environment of the fuel tank. As such the sensor is non-invasive, and is protected from the external environment. 
     The method may further comprise providing a magnetic field response sensor as the sensor. 
     The method may further comprise covering the sensor with the polymer matrix so that the sensor is electrically isolated. 
     The method may further comprise providing a structural fibre ply stack to form part of the structural portion, wherein the fibre ply is arranged in the structural fibre ply stack. 
     The method may further comprise providing the sensor between the fibre ply on which the sensor is embroidered and another fibre ply of the structural fibre ply stack. 
     With such an arrangement, the protection afforded to the sensor is improved. 
     The method may further comprise providing the sensor on an outer face of the structural fibre ply stack. 
     As such, ease of inspection of the sensor may be improved. 
     The method may further comprise providing the predetermined pattern as a spiral arrangement. 
     The method may further comprise providing the predetermined pattern on a plane. 
     With such an arrangement, the accuracy of the sensor is improved, and the sensor is provided in a two dimensional orientation. 
     The method may further comprise forming the fibre ply from carbon fibre. 
     The method may further comprise forming the polymer matrix from a resin. 
     The method may further comprise embroidering the electrically conductive wire to the fibre ply by tailored fibre placement. 
     As such, it is possible to reliably and quickly embroider the electrically conductive wire to the fibre ply. 
     The method may further comprise embroidering the electrically conductive wire to the fibre ply using the double lock stitch technique. 
     Therefore, the ease of manufacturing using an embroidering technique is increased. 
     The fibre ply may extend significantly from the boundary of the sensor. 
     With such an arrangement, there are no ramped plies in the vicinity of the sensor. As such, the stresses acting on the structural portion in the vicinity of the fuel tank and on the sensor itself may be minimised and so the reliability of the system is maximised. 
     The method may further comprise applying the polymer matrix so that the electrically conductive wire is electrically isolated. 
     As such, the sensor is a passive sensor and so does not comprise any moving or electrically connected elements. The reliability of a system comprising such a sensor is therefore maximised. 
     According to another aspect of the invention, there is provided a fuel tank for an aircraft comprising: a structural portion of the fuel tank, the structural portion being formed from a fibre reinforced polymer comprising a fibre ply acting as a structural component and a polymer matrix, a sensor integrated in the structural portion, the sensor comprising an electrically conductive wire embroidered to the fibre ply in a predetermined pattern in which the fibre ply acts as a sensor substrate, wherein the fibre ply and electrically conductive wire embroidered to the fibre ply are covered by the polymer matrix. 
     According to another aspect of the invention, there is provided a fuel quantity indicating system comprising a fuel tank as recited above and an interrogation system spaced from the sensor which is configured to interrogate the sensor to determine a fuel quantity in the fuel tank. 
     According to another aspect of the invention, there is provided an aircraft comprising a fuel quantity indicating system as recited above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an aircraft with a fuel tank; 
         FIG. 2  is a schematic perspective view of the fuel tank of the aircraft of  FIG. 1  with a sensor of a wireless sensing system integrated in a wall of the fuel tank; 
         FIG. 3  is a schematic cross-sectional view of a portion of the fuel tank shown in  FIG. 2  with the integrated sensor; 
         FIG. 4  is a schematic partial view of part of a fibre ply with an electrically conductive wire forming the integrated sensor partially embroidered to the fibre ply during manufacture; 
         FIG. 5  is a schematic partial view of part of a fibre ply with an electrically conductive wire embroidered to the fibre ply; and 
         FIG. 6  is a flow diagram showing a method of forming a structural portion of the fuel tank for the aircraft of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     Referring to  FIG. 1 , an aircraft  10  is shown. The aircraft  10  has a fuselage  11  and wings  12 . The wings  12  extend from the fuselage  11 . An engine  13  is mounted to each wing  12 . 
     Fuel for each engine  13  is stored in one or more aircraft fuel tanks  20 . Fuel for each engine is stored in a centre tank within the fuselage  13  and one or more wing tanks within the wings  12 . The description below refers to the aircraft fuel tank  20 , which could equally refer to the centre tank, any of the wing tanks, or an alternative fuel tank arrangement. 
     Referring to  FIG. 2 , a schematic view of a fuel tank  20  is shown. The fuel tank  20  includes a number of fuel tank structural portion  21 , such as a base, outer walls and an upper side. The fuel tank  20  defines a fuel receiving space  22 . The fuel tank structural portions  21  may be formed from other structural parts of the aircraft  10 , such as skins, ribs and spars. The structural portion  21  is a portion that contributes to the structure of the fuel tank. 
     In  FIG. 2 , one structural portion  21 , in this case a wall, has a sensor  30 . The sensor  30  is integrated in the structural portion  21 . It will be understood that the configuration of the sensor  30  and the fuel tank  20  is shown schematically in  FIG. 2  and so the configuration may vary. 
     The sensor  30  is isolated from the fuel receiving space  22  of the fuel tank  20 . Therefore, the sensor  30  cannot come into contact with fuel received therein. The sensor  30  is also isolated from the external environment outside the fuel tank. 
     The sensor  30  is part of a fuel quantity measuring system  40 . The fuel quantity measuring system  40  comprises the sensor  30  and an interrogation system  50 . The fuel quantity measuring system  40  is non-invasive. That is, none of the components of the fuel quantity measuring system  40  are exposed to the fuel receiving space  21  of the fuel tank  20 . 
     Details of the arrangement and method of operation of the fuel quantity measuring system  40  are disclosed in detail in U.S. Pat. No. 7,814,786 B1, filed 17 Jan. 2008, the contents of which are hereby incorporated by reference. A detailed description of the fuel quantity measuring system  40  and its method of operation will only be described briefly herein. 
     The fuel quantity measuring system  40  is a wireless sensing system. That is, the sensor  30  is electrically isolated from the remainder of the fuel quantity measuring system  40 . The sensor  30  is formed of an electrical conductor  31  shaped to form a spiral arrangement. The electrical conductor  31  is arranged on a plane, that is in a two-dimensional geometric pattern. The electrical conductor  31  has an open-circuit arrangement, having inductance and capacitance. The sensor  30  is able to store and transfer electrical and magnetic energy. The spiral arrangement of the electrical conductor  31  is shown in  FIG. 2 . 
     The electrical conductor  31  of the sensor  30  is formed from an electrically conductive wire. By using an electrically conductive wire, the reliability of the sensor is maximised. The method of forming the sensor  30  will be described below. 
     The electrically conductive wire has a uniform cross section along its length. The electrically conductive wire is arranged, upon assembly, to have uniform spacing between adjacent sections of the electrically conductive wire. Adjacent sections extend parallel to each other. However, the sensor  30  is not limited to a uniformly spaced, spiral arrangement, and may be another geometrically shaped conductor arrangement. However, it will be understood that the specific arrangement and geometry of the electrical conductor  31  should be predefined and maintained throughout forming of the sensor and structural portion  21  of the fuel tank  20 . 
     The sensor  30  in which inductance and capacitance are operatively coupled defines a magnetic field response sensor. 
     The interrogation system  50  is a magnetic field response recorder. Referring to  FIG. 3 , the interrogation system  50  includes an antenna  51  and a control unit  52 . The antenna  51  is a broadband radio frequency (RF) antenna configured to transmit and receive RF energy. It will be understood that the antenna  52  may be a single antenna, or separate transmission and receiving antennas. 
     The control unit  52  comprises a processor and a memory. The control unit  52  is configured to control the antenna  51  to transmit RF energy. The control unit  52  is also configured to determine the response received by the antenna  51 . 
     The sensor  30  resonates in the presence of a time-varying magnetic field to generate a harmonic response having a frequency, amplitude and bandwidth. The interrogation system  50 , acting as a magnetic field response recorder, wirelessly transmits the time-varying magnetic field to the sensor and wirelessly detects the sensor&#39;s response frequency, amplitude and bandwidth. 
     The antenna  52  is disposed proximate to the sensor  30 , within range to reliably transmit and receive RF energy to the sensor  30 . The antenna  52  is disposed external to the fuel tank  30 . As such, the interrogation system  52  is more easily accessible for installation and maintenance. The interrogation system  52  is in spaced relationship to the sensor  30 . 
     The structural portion  21  of the fuel tank  20  is shown schematically in  FIG. 3 . The structural portion  21  is formed from a carbon fibre reinforced polymer, such as a carbon fibre reinforced plastic (CFRP). The structural portion  21  comprises carbon fibres acting as a reinforcement. The carbon fibres are arranged in a plurality of fibre plies  23 . Each ply  23  is formed from a plurality of fibres arranged in a weave, although the specific arrangement may vary. In  FIG. 3 , two fibre plies  23  are shown, however the number of plies is not limited to two plies. The structural portion  21  also comprises a resin acting as a polymer matrix  27 . The polymer matrix  27  covers the fibre plies and is dispersed between the fibre plies. 
     Although, in the present embodiment, the structural portion  21  is formed from carbon fibre reinforced polymer, it will be understood that alternative reinforcement materials may be used. For example, an alternative fibre reinforcement material may be used. Furthermore, a combination of carbon fibres and/or alternative fibre materials may be used, for example kevlar, aluminium and fibreglass. 
     Although, in the present embodiment, the polymer matrix is a resin, it will be understood that materials for the polymer matrix may include an epoxy, polyester, vinyl ester or nylon, or another polymerized resin. 
     The structural component  21  comprises the sensor  30 . The sensor  30  is mounted to one of the fibre plies  23 . An embroidered arrangement  24 , acting as a mounting arrangement, mounts the sensor  30  to the fibre ply  23 . The embroidered arrangement  24  directly mounts the electrically conductive wire to the fibre ply  23 . Therefore, the sensor  30  is mounted to a mounting face  25  of the fibre ply  23 . 
     The electrically conductive wire is mounted in a fixed relationship on the mounting face  25 . The embroidered arrangement  24  comprises a thread  26  which is mounted to the fibre ply  23  by tailored fibre placement. In the present embodiment, the thread  26  is a carbon or Kevlar material, although alternative materials are possible. The thread may be formed from the same material as the fibre ply  23 . 
     The method of forming the fuel tank  20 , and in particular the structural portion  21 , will now be described with reference, in particular, to  FIG. 4 ,  FIG. 5  and  FIG. 6 . Although a number of method steps are described below, it will be understood that in embodiments, one or more method steps may be omitted, and/or one or more method steps additionally included. It will also be understood that the order of one or more method steps may be altered. 
     In step  101 , the fibre ply  23 , which acts as a structural component of the structural portion, is provided. 
     At step  102 , the electrically conductive wire is mounted to the fibre ply  23  to form the sensor  30 . The electrically conductive wire is mounted in place by the embroidered arrangement  24 . 
     The embroidered arrangement  24  is formed by a double lock stitch technique, although the forming of the embroidered arrangement  24  is not limited thereto. The electrically conductive wire is positioned on the fibre ply  23  and fixedly mounted in a predetermined arrangement by the tailored fibre placement, in which the wire acting as the fibre is fed and held in position by a guiding element  61  of an embroidering machine  60 . A needle  62  of the embroidering machine  60  stitches the thread  26  through the fibre ply  23  and over the electrically conductive wire in a predetermined pattern. Therefore, the electrically conductive wire is mounted in situ. 
     Once the desired length of electrically conductive wire is mounted to the fibre ply  23 , the embroidering machine  60  is removed. The fibre ply  23  therefore acts as a structural component. 
     At step  103 , the fibre ply  23  on which the electrically conductive wire is mounted is disposed in a stack of fibre plies  23 . The fibre ply  23  on which the electrically conductive wire is mounted may be an outer ply, or an inner ply. If positioned as an outer ply, the mounting face  25  on which the sensor  30  is mounted may be exposed or face inwardly. In a situation when the mounting face  25  faces inwardly, or the fibre ply  23  on which the electrically conductive wire is mounted is an inner ply, then the sensor  30  is disposed between two plies. 
     As one of the fibre plies  23  of the ply stack forming structural components acts as the substrate for the sensor, it has been found by the inventor that it is possible to dispose the sensor  30  in the structural portion without causing a ramped ply arrangement which would result from use of a sensor substrate, whilst enabling the sensor to be fixedly mounted in a predetermined geometric pattern. 
     The ply stack is disposed in a mold. 
     At step  104 , the polymer matrix  27  is applied to the ply stack. The polymer matrix  27  covers the fibre plies  23  and the sensor  30 . When the polymer matrix  27  has hardened, the structural portion  21  is removed from the mold. As such, the sensor  30  is isolated from the fuel receiving space  22  and the environment external to the fuel tank  20 . 
     At step  105 , the fuel tank  20  is assembled together with the interrogation system  50  so that the interrogation system  50  is in spaced relationship with the sensor  30 . 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.