CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/FR2008/052269 International Filing Date, 10 Dec. 2008, which designated the United States of America, and which International Application was published under PCT Article 21 (s) as WO Publication No. WO2009/080991 A1 and which claims priority from, and the benefit of, French Application No. 200759727 filed on 11 Dec. 2007, the disclosures of which are incorporated herein by reference in their entireties. 
     The aspects of the disclosed embodiments concern an anti-lightning system for protecting a composite structure, in particular panels comprising a light central core such as a honeycomb or foam core. It is especially suited to protecting an aircraft radome or the aircraft fairing comprised of panels made of composite structures. More precisely, the aspects of the disclosed embodiments concern an anti-lightning system comprising electrically conductive strips positioned on the external surface of these panels for attaching the lightning and allow the discharging of the currents due to lightning strikes on the external wall of the radome towards an aircraft ground. 
     BACKGROUND 
     The composite structures are widely utilized because of their high mechanical resistance together with a very low mass. These structures are therefore widely used by the aeronautics industry for equipping the aircraft&#39;s external wall such as the radome located at the front of the aircraft and the aircraft fairing. However, these structures which are electrically insulating do not make it possible to establish an evacuation of the electrical currents during lightning strikes on the external wall, locally generating an area of high-density charges that can, on the one hand, damage the aircraft&#39;s external wall and, on the other hand, perturb the electrical installations contained under this external wall. The case of the radome is even more critical since, by definition, it cannot integrate the standard lightning current discharge devices, such as a metal grid inserted in the structure&#39;s surface that, because of the requirements for transparency with regard to radar waves, are not allowed. 
     It is known to equip the external wall of aircraft with conducting elements extending along the wall to attach the lightning and allow the discharging of the currents due to lightning strikes on the external wall of the radome towards an aircraft ground, which is generally the fuselage. 
       FIGS. 1A and 1B  show such a conducting element presented in the form of an electrically conductive strip  1  typically in aluminum or copper installed on the external surface  9  of the radome  3  of an airplane. The radome&#39;s main function is to protect a radar, consequently it is made of an electrically insulating composite material permeable to electromagnetic waves. The strip is fitted on the radome by fastener means  8  such as a screw. Each lightning conductor strip  1  is electrically linked to the external surface  9  of the radome  3  to allow the electrostatic charges that build up there to be discharged. The part  5  in which the screw is fitted to fasten the strip  1  is in insulating plastic. 
     This strip is electrically connected to the junction of the airplane&#39;s fuselage to ensure that it is grounded individually. Thus the conductive strip makes it possible to attach the lightning and establish the discharging of the currents due to lightning strikes on the external wall of the radome towards the ground without affecting any other element of the radome. 
     Such an anti-lightning system generally gives good results for external walls that do not have requirements in terms of aerodynamics. Nevertheless, it has been observed that the unevenness between the external surface  9  of the radome  3  and the conductive strip  1  that forms a step  20  are sources of discontinuity in the airflow  23  over the external surface of the aircraft, and as a result of turbulences  4  that lead to a reduction in the aircraft&#39;s aerodynamic performance ( FIG. 1   b ). 
     However the generally conically-shaped radome located on the aircraft nose constitutes a main contribution to the aircraft&#39;s aerodynamics, thus it is essential to avoid these perturbations in order that the aircraft&#39;s aerodynamic performance is not penalized. 
     In effect, the consequence of these aerodynamic perturbations is to generate drag, increased by the triggering of the laminar/turbulent transition, and consequently a very noticeable rise in the airplane&#39;s consumption of fuel, which is incompatible with the economic requirements of the airlines. 
     It would therefore be beneficial to have a system protecting an airplane&#39;s radome against lightning allowing drag to be reduced so as to produce a significant saving in fuel weight. 
     SUMMARY 
     The objective of disclosed embodiments is therefore to propose an anti-lightning system for a composite structure, the external surface of which is subjected to an air flow, that is simple in its design and in its method of operation, making it possible to ensure an effective dissipation of the energy of a lightning impact without damaging the structure and without perturbation in the airflow. 
     For this, the disclosed embodiments concern an anti-lightning system for a composite structure having an external surface likely to be subjected to an airflow, said system comprising at least one electrically conductive strip and fasteners to fasten this strip to the structure, said strip being electrically connected to a ground permitting the currents to be evacuated. 
     According to the disclosed embodiments, said strip has a shape intended to match a recess created in the external surface such that the external surface of the strip is flush with the external surface of the structure and the fasteners comprise an upper part flush with the external surface of the strip. 
     In this manner, the anti-lightning system according to the disclosed embodiments makes it possible to ensure a perfect evenness between the external surface of the conductive strip and the external surface of the composite structure, thus making it possible to eliminate the turbulences and preserve the aircraft&#39;s aerodynamic performance. 
     “Evenness of the external surface of the conductive strip and the external surface of the structure” means that the external surface of the conductive strip is at the same level as the structure&#39;s external surface to close assembly tolerances. In particular, the external surface of the strip can jut out up to 0.2-0.3 mm, taking into account the acceptable tolerances with respect to aerodynamics. 
     In addition the anti-lightning system of the disclosed embodiments does not require a modification of the structure of the strip&#39;s electrical connection to the ground with respect to the state of the art by retaining the exterior strip technology. 
     Advantageously, said external surface of the composite structure with which the upper surface of the fasteners is flush is non-metallic. 
     This system can advantageously be implemented on any type of composite structure, whether monolithic, single sandwich or multiple sandwich. 
     The fasteners used to fit the strip on the structure comprise at least one fastener element having an upper part flush with the external surface of the strip and a lower part passing through the strip and the depth of the recess, and at least one holding element tightened on a lower part of the through fastener element on the internal surface of the structure so as to hold the strip in the recess. In this manner, the evenness between the external surface of the strip and the upper part of the fastener element also allows the risks of aerodynamic perturbations to be reduced as much as possible. 
     According to an embodiment of the disclosed embodiments, these fasteners to fasten said electrically conductive strip are arranged in the form of a strip. 
     Preferably, these fasteners are aligned and regularly spaced or not so as to closely follow the narrow shape of the electrically conductive strip, thus making it possible to limit the surface area of elements likely to form a radio barrier with regard to radar waves for a radome equipped with such an anti-lightning system. 
     Preferably, said holding element is made from an insulating material such as a Delrin resin or equivalent provided with a hollow able to fully receive said lower part of the fastener element, which is generally a threaded metal rod. Thus this holding element makes it possible to ensure the conductive strip&#39;s electrical insulation from the radar antenna located inside the radome. 
     In different particular embodiments of this anti-lightning system, each having its specific advantages and capable of numerous possible technical combinations: 
     the system comprises a composite member inserted in the depth of the structure and forming an integral part of the composite structure, said member having a hollow profile intended to match the shape of the electrically conductive strip. 
     “Integral part” means a composite member whose presence in the composite structure does not penalize the radome&#39;s radio performance and makes it possible to preserve the mechanical performance equivalent to a standard panel used for exterior strip technologies. 
     This composite member, which is a female strip imprint, is inserted between the layers of the composite structure during layup and cured together with the latter. Its installation is carried out in a mold equipped with a male strip imprint after layup of the composite structure&#39;s first skin. 
     In addition this member made of composite material also takes part in electrically insulating the anti-lightning strip with regard to the antenna contained inside the radome: 
     the composite member can be a strip of high-density semi-rigid foam 
     the composite member can be a strip of high-density honeycomb 
     the member can also be a strip made of a monolithic composite structure. 
     Advantageously, it is possible to select the material of the composite member to suit the material of the composite structure so as to preserve the integrity of the structure. 
     Alternatively, the anti-lightning system does not comprise a profiled composite member. The recess is formed directly by a groove located on the external surface of the composite structure. This groove is obtained by a layup of the composite structure&#39;s layers via a male imprint. The anti-lightning strip is thus profiled to the shape of this groove. 
     Preferably, the anti-lightning system comprises retaining layers made from an insulating material installed between the edges of the conductive strip and the edges of the recess to adapt the profile of the strip&#39;s external surface to the recess in order to obtain a perfect evenness between the external surface of the strip and the external surface of the structure. 
     The disclosed embodiments also concern a panel comprising at least one monolithic composite structure equipped with an anti-lightning system as described above. 
     The structure is made of insulating material and comprises only the fasteners and the electrically conductive strip. Thus when the composite structure is hit by lightning, the current travels on the surface, without passing through this structure, until it meets the closest strip and the associated fasteners. 
     “Monolithic” means a set of layers associated or secured directly to each other, without interposition of a core in an alveolar material, as in the case of a composite sandwich structure. 
     In a particular embodiment, this monolithic composite panel comprises a first skin forming the external wall of the panel and a second skin forming the internal wall of the panel. The skins constituting the panel are, for example, made of composite materials. 
     The disclosed embodiments also concern a panel comprising a core and at least two walls arranged on either side of the core, this panel being equipped with an anti-lightning system as described above. 
     The core can be made of an alveolar material such as a honeycomb or foam material. 
     Advantageously, each of the strips is fastened in a recess installed in the external surface of this panel by at least two fasteners comprising a screw and a cap nut. 
     Clearly, other means of fastening the strip onto a composite structure exist, in particular it is possible to use inserts embedded in the composite structure. The insert is a hollow body intended to receive a fastener unit such as a bolt with a nut. 
     The disclosed embodiments also concern a radome equipped with at least one panel as described above. In this case, preferably, each of the strips is electrically connected to fastener elements made from an electrically conducting material located at the base of the radome, which are intended to fasten the radome to the aircraft fuselage. In this way, the currents due to lightning strikes are evacuated by the strips directly towards the fuselage. 
     Finally, the disclosed embodiments concern an aircraft equipped with at least one panel as described above. 
     More generally, the disclosed embodiments concern a method of fitting an anti-lightning system on a composite structure having an external surface intended to be subjected to an airflow, said system comprising at least one electrically conductive strip and fasteners to fasten this strip to said structure. 
     According to the disclosed embodiments, the method comprises the following steps: 
     at least one recess is realized in the external surface of the composite structure, 
     the electrically conductive strip is placed in each of the recesses, the shape of the strip being intended to match the shape of the recess such that the external surface of the strip is flush with the external surface of the structure, 
     the strip is then fastened to the structure, the upper part of the fasteners being flush with the external surface of the structure. 
     In order to produce the recess, a composite member is inserted in the depth of the composite structure, the member having a hollow profile intended to match the shape of the strip. 
     In another particular embodiment, a groove is formed directly in the external surface of the composite structure in which the strip is to be placed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed embodiments will be better understood after reading the following description and examination of the figures included in an appendix, in which: 
         FIG. 1A  shows a cross-section view of a radome equipped with an anti-lightning system according to the prior state of the art and  FIG. 1B  a top view of this radome and the airflow over this external surface; 
         FIG. 2  represents, schematically and in cross-section, a panel equipped with an anti-lightning system according to a particular embodiment of the disclosed embodiments, the anti-lightning system comprising a composite member made of foam, the member having a hollow profile matching the shape of the anti-lightning conductive strip; 
         FIG. 3  represents the anti-lightning system according to the particular embodiment comprising a profiled composite member made of high-density honeycomb; 
         FIG. 4  represents the anti-lightning system according to the particular embodiment comprising a member made of monolithic composite material, the hollow profile of which is obtained by a layup of layers in the form of the strip by a successive stacking of layers of pre-impregnated fibers; 
         FIG. 5  schematically represents the anti-lightning system according to the particular embodiment of the disclosed embodiments comprising a monolithic composite member, the hollow profile of which is obtained by machining a solid monolithic member after polymerization; 
         FIG. 6  schematically represents the anti-lightning system according to the particular embodiment of the disclosed embodiments comprising a monolithic member, the hollow profile of which is obtained by a layup in the contour of the conductive strip via a male imprint in the mold; 
         FIG. 7  schematically represents the anti-lightning system according to another particular embodiment of the disclosed embodiments not having a composite member, the recess for receiving the conductive strip being formed by a groove obtained by a layup via a male imprint of the layers of the panels; 
         FIG. 8  shows a top view of a radome and the airflow over this external surface equipped with an anti-lightning system according to a particular embodiment of the disclosed embodiments; 
         FIG. 9  represents, schematically and in cross-section, a panel equipped with an anti-lightning system according to a particular embodiment of the disclosed embodiments, the anti-lightning system comprising a composite member made of foam, the member having a hollow profile matching the shape of the anti-lightning conductive strip. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a panel  3   a  equipped with an anti-lightning system according to a first embodiment of the disclosed embodiments. The panel  3   a  comprises a composite sandwich structure comprising a core  18  and two walls  22 ,  12  arranged respectively on either side of the core on the external surfaces of the core. 
     These walls  22 ,  12  form the external skins of the composite panel, generally made of high-resistant fibers such as glass, quartz, aramid or basalt fibers. The core  18  can be made from a foam, honeycomb or yarn material. The skins are secured to the core via polymerization of a resin. 
     An airplane&#39;s radome  3  is generally equipped with such a panel  3   a  made of an electrically insulating composite sandwich structure permeable to electromagnetic waves. The upper wall  22  of the panel  3   a  forming the external wall of the radome  3  is subjected to an airflow during flight. 
     The panel  3   a  is equipped with an anti-lightning system comprising an electrically conductive strip  1   a  fastened to the panel via fasteners  8 . Generally, this strip  1   a  extends along the external wall  22  of the panel  3   a  on a longitudinal axis  19  from one end of the panel to another end ( FIG. 8 ). For a large surface area, the panel  3   a  can also be provided with a plurality of strips  1   a  installed at regular intervals, wherein the number and sizing of the strip are therefore advantageously provided so as to allow the discharging of the currents due to lightning strikes towards the ground. The cross-section of these electrically conductive strips  1   a  is conditional on two criteria, namely the choice of material used and the impact of the presence of these strips on radio performance. Purely for purposes of illustration, a minimum requirement of the state of the art permitting an effective evacuation of the lightning current in the strip is 20 mm 2  for copper strips and 30 mm 2  for aluminum strips. 
     The anti-lightning strip  1   a  shown in  FIGS. 2 to 6  and  8  to  9  presents a substantially semi-circular cross-section, the circular part being in contact with the bottom of a recess  6 .  FIG. 7  presents another form of the anti-lightning strip  1   b  having a rectangular cross-section with a very small depth. The strip can have various geometrical shapes, with a rectangular, trapezoidal, triangular or other cross-section. 
     The conductive strip  1   a  shown in  FIG. 2  is placed in the recess  6  made at in the external surface  9  of the panel intended to be subjected to the airflow, the shape of the strip  1   a  being intended to match the shape of the recess  6  such that the external surface  21  of the strip  1   a  is flush with the external surface  9   a  of the panel  3   a . In this way the assembly realizes an aerodynamic continuity allowing aerodynamic turbulences to be eliminated. 
     This anti-lightning strip  1   a  forms both a lightning receiver and a conductor of currents towards the ground. 
       FIG. 2  shows a particular embodiment of the fitting of the strip  1   a , which is fastened in the recess  6  by fasteners  8  each comprising a fastener element having a lower part  10  passing through the strip  1   a  and the depth of the recess  6  and the part  5  comprising a holding element tightened on the lower part  10  of the through fastener element  8  on the internal surface  12  of the panel  3   a.    
     Advantageously, the fastening is carried out such that an upper part  2  of this fastener element  8  is flush with the external surface  21  of the strip  1   a  so as not to create a discontinuity in order to avoid generating turbulences. 
     The lower part  10  of the fastener element  8  is generally a metal rod equipped over at least one portion of its external surface with an external threading that in part passes through a drilled hole in the conductive strip  1   a  and a through hollow realized in the depth of the panel  3   a.    
     In order to avoid any perturbation from the strip  1   a  on the radar antenna, the holding element  5 , a cap nut that has to entirely cover the lower through part of the rod  10 , is preferably made from an insulating material such as a Delrin® resin or equivalent to form a shielding to the electrical field created by the circulation of currents in the strip so that there is no electrical arc attachment on the antenna. 
     The fastener elements  8  are preferably placed along the strip  1   a  on the axis  19  that extends from one end of the panel  3   a  to the opposite end (see  FIG. 8 ). 
     Each of the strips  1   a  is electrically and individually connected to fastener elements  25  made from an electrically conducting material located at the base of the radome  3 , which are intended to fasten the radome to the aircraft fuselage  24  as shown in  FIG. 9 . 
     For forming the recess  6 ,  FIG. 2  also shows that the anti-lightning system comprises a member  11  made in an insulating composite material having a hollow profile  6   a  intended to match the shape of the anti-lightning conductive strip  1   a . This profiled composite member  11  is inserted in the depth of the panel  3   a  and forms an integral part of the composite structure of the panel. This member  11 , which is a female strip imprint, is inserted between the layers of the composite structure during layup and cured together with the latter. Its installation is carried out in a mold equipped with a male strip imprint after layup of the first skin forming the interior surface  12  of the composite structure. In this way, its presence does not modify the composite structure, and influences as little as possible the radome&#39;s radio performance. In addition this member  11  made of composite material also makes it possible to ensure the electrical insulation of the anti-lightning strip  1   a  with regard to the antenna contained inside the radome  3 . It also makes it possible to preserve the mechanical performance equivalent to a standard panel made from a composite sandwich structure. 
     Another advantage is to be able to provide a profiled composite member  11  that allows an interface to be realized between the fasteners and the sandwich structure, thus allowing the linkage forces, which are local, to be spread over a larger volume of the composite sandwich structure. 
       FIGS. 2 to 6  present the different embodiments of this profiled composite member  11 . 
     In  FIG. 2 , the profiled composite member  11  is a strip  11   a  of high-density semi-rigid foam, expanded, extruded or stamped in the shape of the conductive strip  1   a . This strip  11   a  is inserted in the depth of the panel  3   a , more precisely between the external skin  9  and the internal skin  12  of the panel during the layup of the panel. 
       FIG. 3  shows another example of the profiled member  11 , which is a strip  13  of high-density honeycomb. This strip  13  is machined in advance in the shape of the conductive strip  1   a , then inserted in the depth of the panel  3   a  between the skins  12 ,  22 . 
       FIG. 4  shows another example of a profiled member  11  that is a monolithic strip  14 , the hollow profile  6   a  of which is obtained by a layup in the form of the strip  1   a  by a successive stacking of layers of pre-impregnated fibers. 
       FIG. 5  shows an example of a profiled member  11  that is a monolithic strip  15 , but different from the example in  FIG. 4  in that the hollow profile  6   a  is obtained by machining the solid monolithic member  15  after the polymerization of panel  3   a.    
       FIG. 6  shows a last example of a profiled member  11  that is a monolithic strip  16  stacked in the contour of the strip  1   a  via a male imprint in the mold of the strip. 
     Whatever the type of profiled member  11 , the means of fastening the strip  1   a  to the panel  3   a  are unchanged with respect to those presented in  FIG. 2 . 
     In another embodiment of the anti-lightning system as illustrated in  FIG. 7 , it does not comprise a profiled composite member  11 . The recess is formed directly by a groove  17  located on the external surface  9  of the panel  3   a . This groove  17  is obtained by a layup of the composite structure&#39;s layers via a male imprint. Generally, in order not to cause a significant modification in the structure of the panel  3   a , this is a shallow groove  17  in which a thin anti-lightning strip  1   b  is received. Consequently, in order to preserve the effective cross-section for discharging currents, this requires an increase in the width. To ensure the fastening of such a conductive strip  1   b , it is glued in advance on the external surface  9  in the groove  17 . The securing of the strip  1   b  to the panel  3   a  is then completed by mechanical fasteners  8  already used for the other embodiments. 
     Advantageously, the anti-lightning system comprises retaining layers  7  made from an insulating material installed between the edges of the conductive strip  1   a ,  1   b  and the edges of the recess  6 ,  17  to adapt the profile of the external surface  21  of the strip  1   a ,  1   b  to the recess  6 ,  17  in order to obtain a perfect evenness between the external surface  21  of the strip  1   a ,  1   b  and the external surface  9   a  of the structure  3   a.
An anti-lightning system for a composite structure having an external surface liable to be subjected to an air flow, the aforementioned system including at least one electrically conducting strip and fasteners fastening the aforementioned strip to the structure, the aforementioned strip being electrically connected to an electrical earth for removing current. According the disclosed embodiments, the aforementioned strip has a shape intended to collaborate with a housing created at the aforementioned external surface so that the external surface of the strip lies flush with the aforementioned external surface of the structure, and the aforementioned fasteners include an upper part flush with the external surface of the strip.