Patent Publication Number: US-2020291896-A1

Title: Acoustic panel for an aircraft turbojet nacelle, and method for manufacturing an element of a nacelle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/FR2018/053036, filed on Nov. 29, 2018, which claims priority to and the benefit of FR 17/61415 filed on Nov. 30, 2017. The disclosures of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to an acoustic panel for an aircraft turbojet engine nacelle as well as a method for manufacturing such an acoustic panel. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     It is known to implement acoustic panels, commonly called “sandwich panels,” which are panels adapted to attenuate the noise emitted from the interior of a turbojet engine nacelle, for example. 
     Such an acoustic panel typically comprises an outer skin, an inner skin, and a cellular core which is interposed between the outer skin and the inner skin. 
     The cellular core comprises a plurality of corrugated strips, each corrugated strip extending longitudinally in length by forming a succession of bumps and hollows. 
     The corrugated strips are arranged so as to delimit cells which extend perpendicularly to each skin. 
     The cells form Helmholtz cavities which are adapted to attenuate the sounds. 
     Also, the inner skin which is intended to be oriented towards the source of noise is permeable to air in order to absorb the acoustic energy within the cellular core. 
     This type of panel makes it possible in particular to manufacture an ejection nozzle or an inner fixed structure of a nacelle. 
     There is known in the prior art a method for manufacturing an element of a nacelle comprising an acoustic panel of the previously described type, the element of the nacelle possibly being an ejection nozzle or an annular inner structure of a nacelle. 
     The manufacturing method according to the prior art successively comprises a step of flat manufacturing the acoustic panel which consists in fixing the outer skin and the inner skin on the cellular core, and a shaping step which consists in giving a shape to the previously made panel, as a revolution shape. 
     This type of method has several drawbacks. 
     Indeed, this type of method does not allow the manufacture of a nacelle element of a complex shape. The term “complex shape” means here an element which has a portion of a revolution shape and a flat portion for example. 
     Similarly, the minimum radius of curvature of a nacelle element produced by this type of method is limited. 
     In addition, according to this type of method, the outer skin and the inner skin are fixed on the cellular core by brazing. 
     Brazing consists of heating a filler metal to its melting temperature, which can take the shape of a brazing sheet. 
     Typically, the brazing sheet is placed between a skin and the cellular core of the panel. 
     The high cost of the brazing sheets makes brazing expensive. 
     SUMMARY 
     This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features. 
     The present disclosure relates to an acoustic panel which is adapted to be manufactured by a method which allows the production of complex shapes and at lower cost. 
     The present disclosure concerns more particularly an acoustic panel for an aircraft turbojet engine nacelle, of the type including: an outer skin, an inner skin, and a cellular core which is interposed between the outer skin and the inner skin and which comprises a plurality of corrugated strips, each of the corrugated strips extending longitudinally in length by forming a succession of bumps and hollows, and the corrugated strips being arranged so as to delimit cells which extend generally perpendicular to the skins, wherein that each corrugated strip extends in width from a first end forming an outer folded edge which is bearing on the outer skin, to a second end forming an inner folded edge which is bearing on the inner skin. 
     The folded edges make it possible to increase the contact surface between the cellular core and the associated skins, in order to promote the fixing of the cellular core on the associated skins. 
     According to another characteristic, at least one portion of the folded edges has a truncated portion to inhibit excess thickness by superposition of two folded edges. 
     This characteristic makes it possible to inhibit excess thickness by the overlap of two folded edges, which could harm the fixing of the cellular core on the associated skins. 
     According to another characteristic, the corrugated strips are offset in the direction of their length so that the bumps and the hollows of two adjacent corrugated strips are in contact to form the cells. 
     The present disclosure also concerns an element of a turbojet engine nacelle which includes at least one acoustic panel of the previously described type. 
     According to another characteristic, the nacelle element forms an ejection nozzle or an inner structure of a turbojet engine nacelle. 
     The present disclosure also concerns a method for manufacturing an element of a turbojet engine nacelle comprising at least one acoustic panel of the previously described type, characterized in that it comprises: a step of manufacturing the cellular core, a step of shaping the cellular core, a step of shaping the outer skin, a step of shaping the inner skin, and an assembly step which is carried out following the steps of shaping the skins, and which comprises assembling the cellular core, the outer skin and the inner skin in order to produce the element of the nacelle. 
     This characteristic, which comprises forming the skins before they are assembled on the cellular core, makes it possible to limit the deformation of the cellular core. 
     According to another characteristic, the assembly step is carried out by diffusion welding. 
     Diffusion welding makes it possible in particular to limit manufacturing costs. 
     According to another characteristic, each shaping step comprises giving a final shape to the shaped element. 
     According to another characteristic, the method includes an acoustic treatment step which comprises perforating the outer skin. 
     According to another characteristic, the step of manufacturing the cellular core comprises: a shaping phase which comprises forming the corrugated strips, each corrugated strip forming a succession of bumps and hollows, and each corrugated strip extending in width from a first end forming an outer folded edge which is configured to be bearing on the outer skin, up to a second end forming an inner folded edge which is configured to be bearing on the inner skin, and a phase of assembling the corrugated strips together. 
     According to another characteristic, the step of manufacturing the cellular core comprises a punching phase, which is carried out following the shaping phase of the cellular core and which comprises truncating a portion of at least one portion of the folded edges to inhibit an excess thickness by superposition of two edges folds. 
     According to another characteristic, the shaping phase which comprises forming the corrugated strips is carried out by stamping. 
     According to another characteristic, the phase of assembling the corrugated strips to manufacture the cellular core is carried out by welding. 
     According to another characteristic, the method comprises a step of protection against oxidation of the cellular core which comprises protecting all the surface of the corrugated strips by an anticorrosion protective layer, with the exception of the face of each folded edge which is configured to be bearing on one of the skins of the acoustic panel. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which: 
         FIG. 1  is an exploded perspective view which illustrates a nozzle of a nacelle which comprises an outer skin, a honeycomb cellular core and an inner skin, according to the present disclosure; 
         FIG. 2  is a perspective view which illustrates an inner fixed structure of a nacelle for a turbojet engine, comprising an acoustic panel, according to the present disclosure; 
         FIG. 3  is a schematic view in radial section which illustrates the acoustic panel of  FIG. 2 ; 
         FIG. 4  is a detailed perspective view which illustrates a corrugated strip with folded edges belonging to the cellular core of  FIG. 2 ; 
         FIG. 5  is a schematic view of a portion of the cellular core of  FIG. 2  comprising a plurality of corrugated strips with punched folded edges; 
         FIG. 6  is a schematic view which illustrates the outer skin of the acoustic panel arranged in a half-tool of tooling for implementing a manufacturing method according to the present disclosure; 
         FIG. 7  is a schematic view which illustrates the outer skin, the inner skin and the cellular core of the acoustic panel arranged in the half-tool of  FIG. 6 ; and 
         FIG. 8  is a schematic view similar to that of  FIG. 6 , which illustrates the outer skin, the inner skin and the cellular core of the acoustic panel arranged in the two half-tools of the tooling of  FIG. 6 . 
     
    
    
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     There is represented in  FIG. 2  an acoustic panel  10  which forms a first inner half-structure  14   a  of an aircraft turbojet engine nacelle. 
     The first inner half-structure  14   a  has a complex shape which comprises a portion of a revolution shape  16  about the axis A whose generatrix is curved and two generally planar radial portions  18 . 
     The first inner half-structure  14   a  is designed to be assembled with a complementary  20  second inner half-structure  14   b,  visible in  FIG. 8 , to form an element  41  (an inner structure) of the turbojet engine nacelle. 
     With reference to  FIG. 3 , the acoustic panel  10  comprises an outer skin  22 , an inner skin  24  and a cellular core  26 . 
     According to the form described here, the outer skin  22  and the inner skin  24  are made of titanium. 
     Without limitation, the outer skin  22  and the inner skin  24  can be made with different types of alloys/materials, in particular a nickel alloy known under the trade name “INCONEL®.” 
     The cellular core  26  is interposed between the outer skin  22  and the inner skin  24  radially with respect to the central axis A. 
     There is represented in  FIG. 4  a corrugated strip  28  which extends in length by forming a succession of bumps  30  and hollows  32 . 
     The cellular core  26 , illustrated in  FIG. 5 , comprises a plurality of corrugated strips  28  which are arranged so as to delimit cells  34  which extend radially perpendicular to the outer skin  22  and to the inner skin  24 . 
     To this end, the corrugated strips  28  are offset in the direction of their length so that the bumps  30  and the hollows  32  of two adjacent corrugated strips  28  are in contact so as to form the cells  34 . 
     The corrugated strips  28  are fixed to each other, for example by welding. 
     As can be seen in  FIG. 3 , the corrugated strips  28  which form the cellular core  26  extend along their length about the axis A of the acoustic panel  10 . 
     With reference to  FIG. 4 , each corrugated strip  28  extends in width from a first end forming an outer folded edge  36  which is bearing on the outer skin  22 , up to a second end forming an inner folded edge  38  which is bearing on the inner skin  24 . 
     The outer folded edge  36  and the inner folded edge  38  are provided for increasing the contact surface between each corrugated strip  28  and the outer skin  22  and the inner skin  24 , to promote the fixing of the corrugated strips  28  on the associated skins. 
     According to  FIG. 5 , the outer folded edge  36  and the inner folded edge  38  of each corrugated strip  28  have reduced portions  40  to inhibit excess thickness by superposition of two folded edges. 
     In fact, it can be seen that the absence of excess thickness by superposition of two folded edges promotes the fixing of the folded edges  36 ,  38  on the associated skin  22 ,  24 . 
     The term “reduced portion”  40  means a portion whose thickness is reduced so that the reduced portion has a thickness comprised between zero millimeter and half the thickness of the corrugated strip  28 . 
     According to one form, the thickness of the reduced portion  40  is zero, so it is a truncated portion, for example by punching or by laser cutting. 
     According to a variant, the thickness of the reduced portion  40  is equal to or less than half the thickness of the corrugated strip  28 . Such a reduced portion  40  is obtained by crushing during the stamping of the corrugated strip  28  for example. 
     According to the described form, each reduced portion  40  of a folded edge is arranged in the vicinity of each bump  30  of each corrugated strip  28 . 
     The present disclosure also applies to the production of a nacelle element which comprises an acoustic panel  10  and which forms an annular nozzle  12  of a general barrel shape about a central axis A of revolution, as can be seen in  FIG. 1 . 
     The annular nozzle  12  comprises an outer skin  48 , an inner skin  50  and a cellular core  52  similar to the cellular core  26  previously described. 
     The present disclosure also concerns a method for manufacturing an element  41  of a turbojet engine nacelle. 
     According to the selected form, the element  41  is an inner structure which comprises a first inner half-structure  14   a  of the previously described type, and a second inner half-structure  14   b  identical to the first, as can be see  FIG. 8 . 
     For the sake of clarity, only the steps of manufacturing the first inner half-structure  14   a  are described below, the second inner half-structure  14   b  being identical to the first one. 
     The manufacturing method comprises a step of manufacturing the cellular core  26  which is followed by a step of shaping the cellular core  26 . 
     The step of manufacturing the cellular core  26  comprises a reducing phase which includes reducing a portion of each corrugated strip  28  to inhibit an excess thickness by superposition of two folded edges. 
     The reducing phase comprises locally truncating each corrugated strip  28  to form the reduced portions  40 , so that following the shaping phase described below, so that the outer folded edge  36  and the inner folded edge  38  of each corrugated strip  28  has a reduced portion  40  in the vicinity of each bump  30 . 
     Without limitation, it is also possible to locally crush each corrugated strip  28  to form the reduced portions  40 . 
     The step of manufacturing the cellular core  26  comprises a shaping phase, which is carried out following the reducing phase, and which comprises forming the corrugated strips  28 , that is to say forming the bumps  30 , the hollows  32 , the outer folded edge  36  and the inner folded edge  38  of each corrugated strip  28 . 
     The corrugated strips  28  are formed by stamping a sheet, for example. 
     It is possible to carry out the reducing phase by local crushing and the shaping phase simultaneously, during the stamping of the corrugated strips  28 . 
     In addition, the step of manufacturing the cellular core  26  comprises a phase of assembling the corrugated strips  28  which is carried out following the shaping phase previously described, and which comprises fixing the corrugated strips  28  in view of fabricating the cellular core  26 . 
     The fixing of the corrugated strips  28  is for example carried out by welding of the edges which delimit the bumps  30  and the hollows  32  of each corrugation of each corrugated strip  28 . 
     Furthermore, the method comprises a step of protection against oxidation of the cellular core  26  which comprises protecting all the surfaces of the corrugated strips  28  with an anticorrosion protective layer, with the exception of the face of each folded edge  36 ,  38  which is configured to be bearing on one of the skins  22 ,  24  of the acoustic panel  10 . 
     The protection step can be carried out by protecting the folded edges  36 ,  38  by a masking element, then by depositing the anticorrosion protective layer on each corrugated strip  28 . 
     Alternatively, it is also possible to deposit an anticorrosion protective layer over the entire surface of the corrugated strips  28 , then to remove the protective layer deposited on the face of each folded edge  36 ,  38  which is configured to be bearing on one of the skins  22 ,  24 . 
     In one form, the anticorrosion protective layer is a nickel and chromium-based layer. 
     Following the step of manufacturing the cellular core  26 , the step of shaping the cellular core  26  comprises giving the cellular core its final shape, i.e. a shape of an inner half-structure  14  according to the described exemplary embodiment. 
     The shaping of the cellular core  26  can be carried out hot or cold. 
     Also, the method comprises a step of shaping the outer skin  22  and a step of shaping the inner skin  24 , which comprises giving the outer skin  22  and the inner skin  24  its final shape, for example by stamping a sheet. 
     The method also includes an acoustic treatment step which comprises perforating the outer skin  22 . 
     Finally, the method according to the present disclosure comprises an assembly step which is carried out following the steps of shaping the skins  22 ,  24  and which comprises assembling the cellular core  26 , the outer skin  22  and the inner skin  24  with a view to produce the first inner half-structure  14   a.    
     With reference to  FIG. 6 , the assembly step comprises arranging the outer skin  22  in a first half-tool  42   a  of a tooling  44 , the half-tool  42   a  having a shape which matches the shape of the outer skin  22 . 
     The assembly step then includes a phase of arranging the cellular core  26  against the outer skin  22 , then a phase of arranging the inner skin  24  on the cellular core  26 , as can be seen in  FIG. 6 . 
     The assembly step is repeated to form a second inner half-structure  14   b  by a second half-tool  42   b  of the tooling  44 . 
     The assembly step comprises a diffusion welding phase under gas pressure which makes it possible to fix the outer skin  22  and the inner skin  24  on the cellular core  26  of each inner half-structure  14   a,    14   b.    
     To carry out the diffusion welding phase, it is necessary to increase the gas pressure within the tooling  44 . 
     For this purpose, the ends of the tooling  44  are each closed by a plug (not represented). 
     According to a non-represented variant, each inner half-structure  14   a,    14   b  is separately assembled by diffusion welding by two distinct tools. 
     According to this variant, each tool comprises a counter-shape which matches the shape of the inner half-structure  14   a,    14   b  and a cover which cooperates with the counter-shape to allow increasing the gaseous pressure in the tooling during the diffusion welding. 
     The method according to the present disclosure aims to separately shape the outer skin  22  and inner skin  24  firstly, then to assemble the skins  22 ,  24  with the cellular core  26  in a second step, unlike a method according to the prior art which comprises assembling the skins and the flat cellular core, then forming the whole. 
     The method according to the present disclosure makes it possible to limit, or reduce, the unwanted deformations of the cellular core  26 . 
     Particularly, the method according to the present disclosure makes it possible to do little or no deformation of the folded edges of the corrugated strips  28  of the cellular core  26 . 
     The present description is given by way of non-limiting example. 
     According to a non-represented variant, only the inner skin  24  is assembled on the cellular core by diffusion welding during the assembly step. 
     According to this variant, the outer skin  22  can be made of composite or aluminum for example and assembled on the cellular core  26 , for example by bonding. 
     Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word “about” or “approximately” in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, material, manufacturing, and assembly tolerances, and testing capability. 
     As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” 
     The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.