Acoustic panel for an aircraft propulsion assembly and method for producing same

Abstract

The invention relates to a method for producing an acoustic panel (24') for an aircraft propulsion assembly (10), the panel comprising first and second skins (26, 30) and a cellular layer (28) positioned between the first and second skins (26, 30), the first skin (26) being acoustically permeable and the cellular layer (28) comprising cells (32) that are in fluid communication with perforations (34) in the first skin (26), each of these cells (32) comprising a central well (38) surrounded by peripheral walls (36) of the cell (32), the method comprising a step of producing the cellular layer (28) and a step of assembling the panel (24'), characterised in that the step of producing the cellular layer (28) comprises preparing a sheet (50) and folding this sheet (50) so as to form the cell (32) and its well (38), or so as to form the cells (32) and their wells (38).

Description

[0001] DESCRIPTION

[0002] TITLE: ACOUSTIC PANEL FOR AN AIRCRAFT PROPULSION UNIT, AND ITS MANUFACTURING METHOD

[0003] Technical field of the invention

[0004] The present invention relates in particular to an acoustic panel for an aircraft propulsion unit, and its manufacturing method.

[0005] Technical background

[0006] The technical background includes in particular documents FR-A1- 3 093 374, FR-A1-3 107 856, US-A1 -2022 / 199064, FR-A1-3 123 385, FR-1- 3 123755, US-A1 -20201 / 65975, US-A1 -2020 / 017189 and US-B2-11 ,059,559. An aircraft propulsion unit generally comprises a turbomachine and / or a nacelle surrounding this turbomachine. In the present application, the term propulsion unit means a turbomachine (or engine), or a turbomachine nacelle, or the assembly formed by a turbomachine and its nacelle.

[0007] The nacelle surrounds a gas generator of the turbomachine, in particular a double-flow one, and forms around the gas generator an annular vein whose walls in contact with the secondary flow are equipped with acoustic panels in order to ensure both the rigidity of the nacelle, the channeling of the air passing through the nacelle and the absorption of the noise generated mainly by the fan.

[0008] The vein has a general shape of revolution but its section varies continuously from the front to the rear of the nacelle in order to achieve the optimum compromise between the flow constraints of the secondary flow and those of the nacelle's size.

[0009] The acoustic panels to be produced therefore have curved shapes resembling shell sectors.

[0010] Acoustic panels are well known in aeronautics. They are sandwich structures comprising a plurality of layers assembled together along the thickness. Traditionally, panels are used with three layers, namely: a microperforated skin called "acoustic", a honeycomb layer and a solid skin. The acoustic skin is permeable to sound waves, the solid skin is a skin for reverberating the waves and the honeycomb layer serves as a guide box and is generally made of honeycomb.

[0011] The layers are generally assembled together by gluing. One of the skins (acoustic or solid) can be glued to the honeycomb layer, or both skins can be glued to the honeycomb layer. Some metallic parts that are hot during operation can be assembled by brazing. The acoustic skin and the solid skin are, for example, made of an organic composite material comprising layers of reinforcing fabric embedded in a resin hardened by polymerization. The acoustic skin and the solid skin are produced separately by molding to the final shape of the panel using well-known techniques. The acoustic skin is then pierced and the solid skin + honeycomb layer + acoustic skin assembly is assembled by hot and pressure gluing in an autoclave to ensure the best possible contact between the three layers.In a variant of the process, during assembly, the solid skin can be directly made and molded onto the honeycomb layer that it covers. It is understood that the skins kept separated by the honeycomb layer ensure the strength and rigidity of the panel.

[0012] The thickness of the alveolar layer is sized by the wavelength range (or audible frequency) to be attenuated. In the case where the frequencies to be attenuated are low (less than 700 Hz), the thickness of the alveolar layer required becomes significant and is therefore bulky. Several technical solutions tend to reduce the necessary thickness of an alveolar layer.

[0013] One such solution is to use cone-shaped cells. A conventional cell has peripheral walls that form, for example, a hexagon, or more precisely a right prism with a hexagonal base. A cone-shaped cell has a central cone or well that is surrounded by the peripheral walls of the cell. The cone or well generally has a truncated conical or pyramidal shape, with the smaller end of the cone or well being open.

[0014] In the current technique, the peripheral walls and the wells / cones of the cells are in practice made on two separate boards, which are then associated with each other.

[0015] One of the problems with a conventional honeycomb-type cellular layer is that the cell module is not perfect, due to small offsets and variations in the geometry of the cells. The pitch between the cells is therefore not perfect and repetitive within a panel and from one panel to another. This pitch is further modified by the forming of the cellular layer on the nacelle panel geometries, which are generally double-curved (around the longitudinal axis of the nacelle and from upstream to downstream). This configuration makes it even more difficult to combine a cone board with a cell board.

[0016] The aim is to find a production technique that allows the creation of an acoustic panel with, for example, a double-curved shape.

[0017] The problem to be solved is also to simplify manufacturing and reduce the manufacturing cost of an acoustic panel.

[0018] The invention aims in particular to provide a simple, effective and economical solution to at least some of the problems of the prior art.

[0019] Summary of the invention

[0020] The invention provides a method for manufacturing an acoustic panel for an aircraft propulsion unit, this panel comprising first and second skins and a cellular layer interposed between the first and second skins, the first skin being acoustically permeable and the cellular layer comprising cells in fluid communication with perforations of the first skin, each of these cells comprising a central well surrounded by peripheral walls of the cell, the method comprising a step of manufacturing the cellular layer and a step of assembling the panel, characterized in that the step of manufacturing the cellular layer comprises at least, for one or more of the cells of this cellular layer: a) a sub-step of preparing a sheet, and b) a sub-step of folding this sheet so as to form the cell as well as its well, or to form the cells as well as their wells.

[0021] The invention thus proposes to produce one or more cells of the alveolar layer from a sheet, and more particularly to produce both the peripheral walls and the central wells of this cell or these cells with the same sheet. The invention also proposes to produce these walls and these wells by folding the sheet. In the manner of origami, the cell or cells of the alveolar layer is / are obtained by folding the sheet. The advantage of this construction is that the wells are directly associated with the peripheral walls of the cells and therefore integral with these walls. Furthermore, even constructed in this way, the cells can be inserted between two skins and can adopt any general shape between the skins, for example a double-curved shape as mentioned above.Advantageously, such a honeycomb layer made up of the juxtaposition or co-folding of several cells can easily be integrated between two skins to constitute an acoustic panel.

[0022] The method according to the invention may comprise one or more of the characteristics or steps below, taken in isolation from one another or in combination with one another:

[0023] - during sub-step a), the sheet is cut;

[0024] - the sheet is cut along cutting lines previously added to the sheet, preferably before sub-step a) or at the start of sub-step a);

[0025] - cutting the sheet allows one or more patterns to be defined, the or each pattern corresponding to a cell to be formed; - the cut sheet includes several patterns arranged next to each other in a matrix manner;

[0026] - the patterns are connected to each other by said sectors;

[0027] - cutting the sheet includes cutting and removing unnecessary portions of the sheet;

[0028] - non-useful portions are portions located inside the patterns and / or between the patterns;

[0029] - the non-useful portions are central portions of the patterns and portions located between the first and second pieces

[0030] - the or each pattern has a circular or polygonal peripheral outline;

[0031] - the or each pattern comprises a star with at least five branches and a ring extending around the star and connected to these branches, the star being intended to form the central well of a cell, and the ring being intended to form the peripheral walls of the cell;

[0032] - during sub-step a), the sheet is pre-folded after being cut;

[0033] - the sheet is pre-folded along fold lines previously added to the sheet, preferably before sub-step a) or at the start of sub-step a);

[0034] - during sub-step b), the sheet is folded further;

[0035] - the method comprises, after sub-step b), an additional sub-step c) of gluing or welding certain parts of the sheet together, and in particular the first and second pieces;

[0036] - during sub-step c), sides of the branches of each star are glued or welded together, and / or sides of the peripheral walls of each cell are glued or welded together;

[0037] - only one of the first and second skins is fixed to the alveolar layer

[0038] - the first and second skins are both fixed to the alveolar layer;

[0039] -- said stiffening folds are formed by folding said pieces; -- each of said pieces is folded to form one of said stiffening folds;

[0040] -- one of the pieces at the star level and one of the pieces at the crown level are folded along the same fold line, to form at least one fold and preferably to form two separate folds;

[0041] -- the or each motif includes a star with at least six points;

[0042] -- the central shaft has a general polygonal shape in section with at least five or six sides;

[0043] -- the number of sides of the central well is equal to the number of branches of the star;

[0044] -- the method comprises, during or after sub-step c), a sub-step of fixing the cells together, this fixing step being carried out by stapling, riveting, punching, or gluing or welding;

[0045] -- the method comprises a sub-step of drilling drainage holes in the central well of the cell or of each cell;

[0046] -- the sheet is a metal strip, for example aluminum, titanium or Inconel®;

[0047] -- the sheet is an organic foil or a polymer or thermoplastic film chosen from: PAI, PI, PEI, PET, PEEK, etc.;

[0048] -- the sheet is made of paper, for example Nomex® or Kevlar®;

[0049] -- the sheet is coated with a product or resin, for example phenolic, this coating can take place before or after sub-steps a), b), or even c);

[0050] -- an acoustically permeable skin is a microperforated or multiperforated skin.

[0051] According to a preferred embodiment of the invention, the cutting of the sheet in step a) makes it possible to define one or more patterns, the or each pattern corresponding to a cell to be formed and comprising a multi-branched star as well as a ring extending around the star and connected to these branches, the star being intended to form the central well of a cell, and the ring being intended to form the peripheral walls of the cell. The branches of the cell are connected to each other by first pieces, and the ring comprises sectors which are connected to each other by second pieces. The first and second pieces are crossed by folding lines to form stiffening folds.

[0052] These folds are particularly useful for reinforcing the cells and thus form stiffening elements for these cells.

[0053] Preferably, each first piece is located between two adjacent branches, and each branch is located between two adjacent first pieces.

[0054] Preferably, each second piece is located between two adjacent sectors, and each sector is located between two adjacent second pieces.

[0055] Each of the first and second pieces is preferably intended to be fixed, preferably by gluing, at least on itself.

[0056] Each of the first pieces is preferably aligned with one of the second pieces and crossed by one of the fold lines which passes through this second piece.

[0057] Advantageously, the folds of each of the first and second pieces comprise facing faces which are applied and glued to each other.

[0058] The folds of each of the first and second pieces may project into the cell or cells. Alternatively, the folds of each of the first and second pieces are folded against faces of the cell or cells.

[0059] The number of points of the star is not limited and is preferably between three and eight. For example, it is three, five or six.

[0060] The present invention also relates to an acoustic panel for an aircraft propulsion unit, this panel comprising first and second skins and a cellular layer interposed between the first and second skins, the first skin being acoustically permeable and the cellular layer comprising cells in fluid communication with perforations of the first skin, each of these cells comprising a central well surrounded by peripheral walls of the cell, one or more of the cells of this cellular layer being formed by preparing and folding a sheet which forms both the cell and its well, or the cells and their wells.

[0061] Only one of the first and second skins may be attached to the honeycomb layer. Alternatively, both skins are attached to the honeycomb layer.

[0062] The present invention also relates to an acoustic panel for an aircraft propulsion unit, this panel being manufactured by a method as described above and comprising stiffening folds which extend at the sides of the central well of the cell or of each of the cells, and stiffening folds which extend at the peripheral walls of the cell or of each of the cells.

[0063] Brief description of the figures

[0064] The invention will be better understood and other details, characteristics and advantages of the invention will appear more clearly on reading the following description given by way of non-limiting example and with reference to the appended drawings in which:

[0065] [Fig.1] Figure 1 is a very schematic axial sectional view of an aircraft propulsion unit,

[0066] [Fig.2] Figure 2 is a schematic perspective view with partial tearing of an acoustic panel,

[0067] [Fig.3] Figure 3 is a schematic perspective view with partial tearing of an acoustic panel whose cells include central wells,

[0068] [Fig.4] Figure 4 is a schematic view of a sheet for producing a cell and its central well, in accordance with an embodiment of a method according to the invention,

[0069] [Fig.5] Figure 5 is a schematic view of a sheet similar to that of Figure 4, this sheet being cut and pre-folded, [Fig.6] Figure 6 is a schematic perspective view from the inner face of the well of a cell with a central well formed by folding a sheet, [Fig.7a-7b] Figure 7a is a schematic perspective view from the outer face of the well of a cell with a central well formed by folding a sheet, and Figure 7b is a schematic perspective view from the outer face of the well of a cell with a central well formed by another folding of a sheet,

[0070] [Fig.8] Figure 8 is a schematic view of a sheet for producing several cells and their wells, in accordance with an embodiment of a method according to the invention,

[0071] [Fig.9] Figure 9 is a schematic view of a sheet similar to that of Figure 8, this sheet being cut out,

[0072] [Fig.10] Figure 10 is a schematic view of the sheet of Figures 8 and 9, after cutting and pre-folding this sheet,

[0073] [Fig.11] Figure 11 is a schematic view of the sheet of Figures 8 to 10, after cutting and folding this sheet,

[0074] [Fig.12] Figure 12 is a schematic top view of the well cells obtained from the sheet of Figures 8 to 11,

[0075] [Fig.13] Figure 13 is a schematic bottom view of the well cells obtained from the sheet of Figures 8 to 12,

[0076] [Fig.14] Figure 14 is a schematic sectional view of several cells having wells of different shapes, which can be produced by the method according to the invention,

[0077] [Fig.15] Figure 15 is a schematic view from below of a well cell, and shows the bonding of the central well of this cell,

[0078] [Fig.16a-16c] Figure 16a is a schematic view from below of a well-celled cell, and shows the bonding of the peripheral walls of this cell, with peripheral folds from the inside of the cells, like those of Figure 7a, and Figures 16b and 16c are sectional views of a set of cells without the wells with peripheral folds from the outside as in Figure 7b, [Fig.17] Figure 17 is a view similar to that of Figure 4 and shows an alternative embodiment of the sheet, and

[0079] [Fig.18] Figure 18 is a view similar to that of Figure 4 and shows another alternative embodiment of the sheet.

[0080] Detailed description of the invention

[0081] Figure 1 shows very schematically a propulsion assembly 10 for an aircraft, such as an airplane, this propulsion assembly 10 comprising a turbomachine and a nacelle. The turbomachine is for example a double flow turbomachine and comprises a gas generator 12 surrounded by the nacelle 14. A turbine of the gas generator 12 rotates a fan 16 inside the nacelle 14.

[0082] The air flow 17 which enters the propulsion unit 10 and passes through the fan 16 is channeled by the nacelle 14 and then divides into a primary flow 18 which flows into the gas generator 12, and into a secondary flow 20 which flows into an annular vein 22 formed between the gas generator 12 and the nacelle 14.

[0083] To limit the propagation of noise from the propulsion unit 10, in particular from the fan 16, to the outside, and thus reduce noise pollution, it is known to provide acoustic panels 24 on the walls in contact with the air inlet flow 17 and the secondary flow 20. Such panels 24 can thus be mounted on the nacelle 14 around the fan 16 or the gas generator 12, on the gas generator 12, etc. Figure 2 shows a structure of acoustic panel 24. The panel 24 comprises a first acoustic skin 26 which is acoustically permeable and preferably microperforated (i.e. comprising perforations or microperforations), a honeycomb layer 28, and a second solid skin 30. The layer 28 is interposed between the skins 26, 30 and comprises cells 32 in fluid communication with perforations 34 of the first skin 26.

[0084] The cells 32, generally in the form of a honeycomb, form resonant cavities of the Helmholtz resonator type, the operation of which is well known to those skilled in the art. In a known manner, the perforations 34 form necks of the resonant cavities formed by the cells 32. In a also known manner, the cells 32 are only connected to one of the two skins 26 or 30. The unconnected skin is in the immediate vicinity of the face of the cells 32 opposite each other.

[0085] The present invention provides a method for manufacturing an acoustic panel. The particularity of this panel is that the cells of its alveolar layer comprise central cones or wells.

[0086] Figure 3 shows an acoustic panel 24' of this type. Each of the cells 32 comprises peripheral walls 36 which define the resonant cavity C of the cell 32. Inside this cavity C is arranged a central well 38 which divides the resonant cavity C into two distinct spaces, respectively upper 40 and lower 42. These two spaces 40, 42 communicate with each other via the smaller (transverse) end 38a of the well 38, which is open. The well 38 thus has a truncated shape and has for example a general truncated cone or truncated pyramidal shape. The larger (transverse) end 38b of the well 38 is connected to the upper end 36a of the peripheral walls 36 of the cell 32. In the case of a sandwich panel, the upper ends 36a of the cells 32 are connected to the skin 26 and their lower ends 36b are connected to the skin 30, the lower ends 38a of the wells 38 being at a distance from the skin 30.This distance is for example between 1 and 5mm.

[0087] The manufacturing method according to the invention comprises a step of manufacturing the alveolar layer 28 whose cells 32 comprise central wells 38, and a step of assembling the acoustic panel 24' in which at least one of the skins 26, 30 is fixed to the alveolar layer 28.

[0088] The step of manufacturing the alveolar layer 28 comprises at least, for one or more of these cells 28: a) a sub-step of preparing a sheet 50, and b) a sub-step of folding this sheet 50 so as to form the cell 32 as well as its well 38, or to form the cells 32 as well as their wells 38.

[0089] Figure 4 shows an embodiment of a sheet 50 prepared in sub-step a). By definition, the sheet 50 is a thin, flat element. This sheet must be folded and therefore has a thickness allowing it to be folded.

[0090] Advantageously, the sheet 50 has a thickness less than or equal to 300 pm, preferably less than 150 pm, and more preferably less than 90 pm.

[0091] The sheet 50 is for example a metal strip made of aluminum, titanium or Inconel®. The sheet 50 is for example an organic strip or a polymer or thermoplastic film chosen from: PAI, PI, PEI, PET, PEEK, etc. The sheet 50 is for example made of paper, for example Nomex® or Kevlar®, or even very fine fabric (for example glass fibers).

[0092] The sheet can be coated with a product or resin, for example phenolic.

[0093] A sheet 50 can be used to manufacture a cell 32 and therefore each cell 32, or a sheet 50 can be used to manufacture several cells 32 or even all cells 32 of a 24' acoustic panel.

[0094] In the case of Figure 4, the sheet 50 is used to make a single cell 32.

[0095] The sheet 50 of Figure 4 may initially have the particular shape shown in this figure. Alternatively, this particular shape may be obtained by cutting the sheet 50, and in particular the periphery of the sheet 50.

[0096] The sheet 50 has a particular pattern M which is specific to the shape of the cell 32 which must be obtained, and of the well 38 which must be obtained.

[0097] The shape or pattern M of the sheet 50 has a circular or polygonal peripheral contour. This pattern M essentially comprises two parts, a central part 52 having a central axis A, and a peripheral part 54 extending around the axis A and the central part 52. The sheet 50 of FIG. 4 may also comprise cutting and folding lines 56, 58. The lines 56 are cutting lines and form the peripheral contour of the pattern M as mentioned above. The lines 58 are folding lines which may be present on the sheet 50 originally or drawn or printed on this sheet 50. This is also the case for the cutting lines 56.

[0098] It can be seen that the central part 52 comprises a star 60 with several branches 62 and preferably at least five branches 62. It has six in the example shown. The branches 62 are distributed radially around the axis A and are identical. They are at a circumferential distance from each other. In the example shown, each of the branches 62 has a generally trapezoidal shape, the small base of which is located on the side of the axis A and the large base is located on the radially opposite side.

[0099] At the axis A, the sheet 50 comprises a central zone 68 which is connected to the small bases of the branches 62. In the example shown, this central zone 68 is hexagonal. Alternatively, it could have a different shape, for example circular.

[0100] The peripheral part 54 of the pattern M forms a ring 63 around the star 60. This ring 63 comprises sectors 64 which extend in the radial extension of the branches 62 of the star 60. These sectors 64 are distributed radially around the axis A, like the branches 62, and there are as many sectors 64 as there are branches 62.

[0101] Between the branches 62 and the sectors 64, the sheet 50 comprises portions 66 of generally triangular shape, the point of which is oriented towards the axis A.

[0102] In general, the sheet 50 therefore comprises, in addition to the central zone 68, two other zones. An internal annular zone Zint (intended to form the well 38 of the cell 32) extends around the zone 68 and comprises the six branches 62 which are inside the dodecagon formed by the lines 52. An external annular zone Zext (intended to form the walls 36 of the cell 32) extends around the aforementioned internal zone. It can be seen in FIG. 4 that the fold lines 58 extend all around the branches 62 and the sectors 64. It can also be seen that each of the portions 66 is separated in two by a fold line 58 which has a radial orientation with respect to the axis A.

[0103] Figure 5 shows the sheet 50 of Figure 4 which has undergone cutting and pre-folding. A first cut here has made it possible to remove the central area 68. This central area thus becomes a hole. In a variant not shown, the central part 68 can be retained and micro-perforated to form an acoustically permeable septum (micro-perforated or porous). In yet another variant not shown, the central part 68 is removed and a porous septum is installed fixed to the walls of the well.

[0104] A second set of cuts (optional) also made it possible to remove a part 70 of the portions 66 which are each separated into two pieces 66a, 66b. The parts 70 removed from the portions 66 are located on a circumference centered on the axis A and passing substantially at the junctions between the sectors 64 and the branches 62. That is to say that the parts 70 are located at the junction between the aforementioned zones Zint and Zext. The pieces 66a are thus located between the branches 62 of the star 60, and the pieces 66b are located between the sectors 64 of the ring. The pieces 66a, 66b are each crossed by one of the fold lines 58, 59.

[0105] The removal of the parts 70 can limit interference with the folded parts of the sheet 50 during folding. They can have any shape. Preferably, the contours of the removed parts 70 are quadrilaterals, two lines of which are the extensions (or extensions) of the lines which delimit the central part 52 or the internal zone Zint (between the branches 62 and the sectors 64) towards the folding line 58, 59 median to two adjacent sectors, and two lines have an angle corresponding to the angle formed by the internal branch 62 with the vertical after complete forming of the cell 32. This angle allows the folds between two adjacent branches 62 not to abut on the sectors 64. As an alternative to this removal, the material can be preserved, then requiring additional folds between the maximum contours.

[0106] The pre-folding makes it possible to begin folding the sheet 50 along the fold lines 58, 59. Certain fold lines 58, 59, as is the case with the lines 58 located at the portions 66, are intended to undergo a re-entrant fold, while the other fold lines 58, 59 which delimit the branches 62 and the sectors 64 are intended to undergo outgoing folds. The re-entrant folds are for example oriented downwards (or towards a lower face of the sheet 50), and the outgoing folds are oriented on the opposite side and for example upwards (or towards an upper face of the sheet 50).

[0107] Figures 6 and 7a show the cell 32 at the end of sub-step b), after folding. Figure 6 is a top view and the central well 38 can be seen, which is formed by the branches 62 of the star 60. The ends 38a, 38b of the central well 38 can also be seen, including the end 38b which is connected to the end 36a of the peripheral walls of the cell 32, at the junction between the branches 62 of the star 60 and the sectors 64 of the ring 63, that is to say at the level of the fold lines 58 between them.

[0108] Figure 7a is a bottom view and we can see the resonant cavity C defined by the peripheral walls 36 of the cell 32 and in which the central well 38 is located. The peripheral walls 36 of the cell 32 are defined by the sectors 64 of the ring 63.

[0109] The method can then comprise a step of bonding the cell 32 in order to fix its shape, in particular between the adjacent faces of the pieces 66a, as well as between the adjacent faces of the pieces 66b, or even of the latter with one of the sectors 64 of the ring 63.

[0110] In a variant (not shown), the folds formed by the pieces 66a, 66b can, in the case of a simple cone, be folded towards the inside of the well.

[0111] In the first or second case, they can also be folded against one of the faces 62 adjacent to the fold. In a variant illustrated in Figure 7b, the folds formed by the pieces 66a, 66b are made outside the walls 64.

[0112] Generally speaking, the folds help to stiffen the cells and therefore the acoustic panel.

[0113] A cell 32 can then be used alone or in combination with several cells, in particular by juxtaposing walls 36 of different cells against each other, to form a multi-cell plate suitable for being integrated between two skins and forming an acoustic panel.

[0114] Figure 8 shows another embodiment of a multi-cell cellular core from a sheet 50 which here comprises several patterns M linked to each other and organized for example in matrix form, in columns and rows. In the example shown, the sheet 50 is not entirely represented and comprises at least ten patterns M, some of which are partial. The sheet 50 is therefore suitable for being used to produce at least ten cells 32 in a single piece. The patterns M are integral with each other and therefore the cells 32 are intended to remain integral with each other. The patterns M of the sheet 50 are here all identical and are similar to those described above. The patterns M are integral with each other by the sectors 64 of the rings 63, each of the sectors 64 being connected by a fold line 72 to a sector 64 of an adjacent pattern M.It is therefore understood that the sectors 64 of a pattern M are all connected to adjacent patterns M and therefore that each pattern M is connected to six other patterns M. In other words, the number of branches 62 of the star 60 defines the number of patterns M to which each pattern M is connected.

[0115] Figure 9 shows a portion of the sheet 50 of Figure 8 after a cutting step comprising five entire M patterns and a portion of a sixth pattern, all connected together. The central areas 68 and the portions 70 mentioned above have been cut out. In this embodiment, portions 74 located between the portions 66 of M patterns have also been removed. These portions 74 have a generally triangular shape because they are found at the intersection of three adjacent M patterns. Alternatively, these portions 74 could have another shape, for example circular. As yet another alternative, it would be possible to limit oneself to making three cuts starting from each fold line 72. And joining, for example at the point of intersection between the extensions of the lines 72 surrounding a portion 74 (balanced three-branch cuts, but the intersection can be placed wherever one wishes in this area).The materials of each part 74 thus separated can be folded with the previous pieces 66b or in additional folds.

[0116] Figure 10 shows the sheet 50 after pre-folding. Figure 11 shows the sheet 50 after folding. Figures 12 and 13 show the sheet 50 after complete folding and gluing of the folds, respectively seen from above and below. Figure 14 shows that the shapes of the cells 32 and in particular of their wells 38 are not limiting and can be any insofar as the folding according to the invention can make it possible to create numerous geometric shapes.

[0117] From left to right in Figure 14, there are respectively:

[0118] X1) a simple truncated central well 38, the truncated cone extending directly into the resonant cavity from the end of the cell 32, X2) a bi-truncated or double-sloped central well 38, and therefore having two successive truncated cones having, for example, respective angles of 30 and 60°,

[0119] X3) a central well 38 whose end 38a forms a right prism with a hexagonal or circular base (this pattern being that illustrated in figures 9 to 13,

[0120] X4) a central well 38 comprising a truncated cone connected to the end 36a of the cell 32 by a right prism with a hexagonal or circular base, thus the cone does not originate at the top of the cell but set back, X5) a central well 38 with an evolving slope and having for example a cone angle which varies from 15° to 90°, and X6) a central well 38 which combines the characteristics of the central wells X3 and X4 (this pattern being that defined by figure 18).

[0121] Figures 15 and 16 illustrate areas Z1, Z2 of bonding of the sheet 50 during the aforementioned sub-step c).

[0122] In Figure 15, the pieces 66a located between the branches 62 of the star 60 are folded along each fold line 58 and are folded down and glued to each other. These pieces 66a are then located in the lower space 42 of the resonant cavity C. In Figure 15, they are shown extending radially towards the outside of the well from the fold lines 58. Alternatively, they can extend radially towards the inside of the well from the fold lines 58, all the more easily since the fold lines 58 are straight.

[0123] In Figure 16a, the pieces 66b located between the sectors 64 of the ring 63 are folded along each fold line 58 and are folded down and glued to each other. These pieces 66b are also located in the lower space 42 of the inner cavity C of the cell 32. In Figure 16a, they are folded down and glued along each wall 64, as in the example unit cell of Figure 7a.

[0124] In Figures 16b and 16c, the pieces 66b located between the sectors 64 of the ring 63 are folded along each fold line 58 and are folded down and glued to each other. These pieces 66b are this time located outside the cavity C of each cell 32, as in the example of a unit cell in Figure 7b. They are folded along each wall 64 between two walls 64 of two adjacent cells.

[0125] To limit the size of the cavities C by the folded and glued pieces 66a, 66b, the latter can be cut before or after gluing. Furthermore, these pieces 66a, 66b could be folded and glued in the upper space 40 of the cavity C, rather than in its lower space 42.

[0126] The method may further comprise, during or after sub-step c), a step of fixing the cells 32 together, this fixing step being carried out by stapling, riveting, punching, gluing or welding. The peripheral walls of the cells 32 may for example be fixed to each other.

[0127] The pieces 66a can also be glued or even welded (if the material lends itself to it) against an adjacent branch 62.

[0128] The folded and glued pieces 66b can also be folded back and fixed to the peripheral walls of the cells 32 (i.e. the sectors 64 of the ring 63) by gluing, stapling or punching, or even welding.

[0129] The folded pieces 66b can be further folded additionally so that they take up less length along the peripheral walls of the cells 32 (i.e. the sectors 64 of the ring 63).

[0130] The method may also comprise a sub-step of drilling drainage orifices 80 in the central well 38 of the cell 32 or of each cell, for example near its end 38b, as illustrated schematically in FIG. 12, or even in the peripheral walls of the cells 32 (i.e. the sectors 64 of the ring 63).

[0131] The method may also include a sub-step of coating the sheet. This sub-step may be carried out before sub-step a), or after sub-step b) or c).

[0132] Figure 17 illustrates an alternative embodiment of the sheet 50, which makes it possible to produce cells 32 with central wells 38 of different shapes. Each central well 38 is a type of that illustrated in Figure 14 with reference X3 and comprises a lower end 38a of generally hexagonal shape. The peripheral partitions 38a1 of this lower end 38a are obtained thanks to radially internal ends of the branches 62 which each have a generally square or rectangular shape. The rest of the branches 62 have a generally trapezoidal shape whose small base is connected to this square or rectangular shape.

[0133] Figure 17 also shows the aforementioned bonding zones Z1, Z2, respectively on the pieces 66a located between the branches 62 of the star 60, and on the pieces 66b located between the sectors 64 of the ring 63. Figure 18 illustrates another variant embodiment of the sheet 50, which makes it possible to produce cells 32 with central wells 38 of different shapes. The method according to the invention can be carried out entirely by hand by one or more operators. Alternatively, its sub-steps could be at least partly automated.

Claims

CLAIMS 1. Method for manufacturing an acoustic panel (24') for an aircraft propulsion unit (10), this panel comprising first and second skins (26, 30) and a cellular layer (28) interposed between the first and second skins (26, 30), the first skin (26) being acoustically permeable and the cellular layer (28) comprising cells (32) in fluid communication with perforations (34) of the first skin (26), each of these cells (32) comprising a central well (38) surrounded by peripheral walls (36) of the cell (32), the method comprising a step of manufacturing the cellular layer (28) and a step of assembling the panel (24'), the step of manufacturing the cellular layer (28) comprising at least, for one or more of the cells (32) of this cellular layer (28): a) a sub-step of preparing a sheet (50), during which the sheet (50) is cut,and b) a sub-step of folding this sheet (50) so as to form the cell (32) as well as its well (38), or to form the cells (32) as well as their wells (38), characterized in that the cutting of the sheet (50) in step a) makes it possible to define one or more patterns (M), the or each pattern (M) corresponding to a cell (32) to be formed and comprising a star (60) with several branches (62) as well as a ring (63) extending around the star (60) and connected to these branches (62), the star (60) being intended to form the central well (38) of a cell (32), and the ring (63) being intended to form the peripheral walls (36) of the cell (32), in that the branches (62) of the cell (32) are connected to each other by first pieces (66a), and the ring (63) comprises sectors (64) which are connected to each other by second pieces (66b), and in that the first and second pieces (66a) are crossed by fold lines (58,59) to form stiffening folds., 2. Method according to claim 1, in which the sheet (50) is cut along cutting lines (56) previously added to the sheet (50), preferably before sub-step a) or at the start of sub-step a).

3. Method according to one of the preceding claims, in which the cut sheet (50) comprises several patterns (M) arranged next to each other in a matrix manner.

4. Method according to the preceding claim, in which the patterns (M) are connected to each other by said sectors (64).

5. A method according to any preceding claim, wherein cutting the sheet (50) comprises cutting and removing unnecessary portions of the sheet (50).

6. Method according to claim 4, in which the non-useful portions are portions located inside the patterns (M) and / or between the patterns (M).

7. Method according to claim 6, in which the non-useful portions are central portions of the patterns (M) and portions (70) located between the first and second pieces (66a, 66b) 8. Method according to one of the preceding claims, in which the or each pattern (M) has a circular or polygonal peripheral outline.

9. Method according to one of the preceding claims, in which, during sub-step a), the sheet (50) is pre-folded after having been cut.

10. Method according to claim 8, wherein the sheet (50) is pre-folded along fold lines (58) previously added to the sheet (50), preferably before sub-step a) or at the start of sub-step a).

11. Method according to claim 8 or 9, wherein, during sub-step b), the sheet (50) is further folded.

12. Method according to one of the preceding claims, in which it comprises, after sub-step b), an additional sub-step c) of gluing and / or welding together certain parts of the sheet (50), and in particular the first and second pieces (66a, 66b).

13. Method according to claim 12, in which, during sub-step c), sides of the branches (62) of each star (60) are glued together, and / or sides of the peripheral walls (36) of each cell (32) are glued together.

14. Method according to one of the preceding claims, in which only one of the first and second skins (26, 30) is fixed to the cellular layer (28).

15. Method according to one of claims 1 to 13, in which the first and second skins (26, 30) are both fixed to the cellular layer (28).

16. Method according to one of the preceding claims, in which the folds of each of the first and second pieces (66a, 66b) comprise facing faces which are applied and glued to each other.

17. Method according to one of the preceding claims, in which the folds of each of the first and second pieces (66a, 66b) protrude inside the cell or cells (32).

18. Method according to one of claims 1 to 16, in which the folds of each of the first and second pieces (66a, 66b) are folded against faces of the cell or cells (32).

19. Acoustic panel (24') for an aircraft propulsion unit (10), this panel (24') being manufactured by a method according to one of the preceding claims and comprising stiffening folds which extend at the sides of the central well (38) of the cell or of each of the cells (32), and stiffening folds which extend at the peripheral walls (36) of the cell or of each of the cells

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