Acoustic panel for an aircraft turbomachine, and associated method and installation

Abstract

The invention relates to an acoustic panel (9) for an aircraft turbomachine (1), the acoustic panel (9) comprising: - a first skin (10) formed from a first composite material having a first polymer matrix and reinforcing fibres embedded in the matrix, - a second skin (11) formed from a second composite material having a second, thermoplastic polymer matrix and reinforcing fibres embedded in the second matrix, and - a core (12) having a honeycomb structure, the core (12) being arranged between the first skin (10) and the second skin (11), characterised in that the second skin (11) comprises holes (11a) having a diameter that is smaller than 1 mm.

Description

[0001] DESCRIPTION

[0002] TITLE: ACOUSTIC PANEL FOR AN AIRCRAFT TURBOMACHINE, ASSOCIATED METHOD AND INSTALLATION

[0003] Technical field of the invention

[0004] The invention relates to the field of acoustic panels for aircraft turbomachines, and in particular for aircraft turbomachine nacelles.

[0005] The invention also relates to the field of manufacturing these acoustic panels.

[0006] Technical background

[0007] The state of the art is illustrated by documents US-B2-6 609 592, EP- A1 -2 821 621, US-A1 -2019 / 337632, and US-A1 -2021 / 324794.

[0008] An aircraft turbomachine comprises, for example, from upstream to downstream in the direction of gas flow along a longitudinal axis, a fan, a low-pressure compressor and a high-pressure compressor, a combustion chamber, a high-pressure turbine and a low-pressure turbine and a gas exhaust nozzle.

[0009] The blower allows the suction of an air flow divided into a primary flow and a secondary flow. The primary flow passes through a primary vein of the turbomachine while the secondary flow is directed towards a secondary vein surrounding the primary vein.

[0010] The primary flow is compressed within the compressors. The compressed air is then mixed with fuel and burned within the combustion chamber. The gases from the combustion pass through the turbines and then escape through the nozzle, whose cross-section allows the acceleration of these gases to generate propulsion.

[0011] The fan typically comprises a movable disc rotating about the longitudinal axis and blades mounted on the disc. The blades are surrounded by a fan casing centered on the longitudinal axis and intended to retain the blades in the event of damage, for example, to the blades.

[0012] The fan casing is typically surrounded by a nacelle that protects the fan. Such a fan is called a shrouded fan, as opposed to unshrouded fans, whose blades are not surrounded by a shroud.

[0013] Turbomachinery, particularly fans, are a significant source of noise pollution, and there is a strong demand to reduce this type of pollution. To this end, it has been proposed to equip nacelles with acoustic panels to reduce the noise generated by turbomachine fans.

[0014] An acoustic panel typically comprises a sandwich structure comprising first and second skins, and between which is arranged a core having a honeycomb structure intended to absorb sound waves. The first skin is fixed to the blower casing and the second skin is in contact with the secondary flow. The second skin typically has holes allowing sound waves to propagate in the core through which they are absorbed. The holes formed in the second skin generally have a diameter greater than 1 mm.

[0015] Although significantly reducing the noise pollution generated by the turbomachine, this type of acoustic panel is not entirely satisfactory. Indeed, the second skin is the radially inner skin, i.e., the closest radially to the longitudinal axis, and the first skin is the radially outer skin, i.e., the furthest radially from the longitudinal axis. The second skin is therefore in contact with the secondary airflow and is a factor in the aerodynamic performance of the turbomachine. The holes made in this second skin tend to increase drag and reduce the aerodynamic performance of the turbomachine. However, these holes are necessary for the passage of sound waves into the acoustic panel for their absorption. Thus, at present, noise reduction is achieved at the expense of the aerodynamic performance of the turbomachine.

[0016] There is therefore a need to provide an acoustic panel for an aircraft turbomachine that exhibits good sound insulation performance while minimizing drag in order to preserve the aerodynamic performance of the turbomachine.

[0017] Summary of the invention

[0018] To this end, the invention proposes an acoustic panel for an aircraft turbomachine, the acoustic panel comprising:

[0019] - a first skin formed from a first composite material having a first polymer matrix and reinforcing fibers embedded in the matrix,

[0020] - a second skin formed from a second composite material having a second thermoplastic polymer matrix and reinforcing fibers embedded in the second matrix, and

[0021] - a core having a honeycomb structure, the core being arranged between the first skin and the second skin, the second skin comprising holes having a diameter of less than 1 mm.

[0022] The acoustic panel is remarkable in that it comprises a coating comprising a layer comprising a ceramic material and deposited by thermal spraying, the first skin being arranged between the coating and the core.

[0023] According to the invention, the acoustic panel comprises a second skin having holes with a diameter of less than 1 mm.

[0024] It has been found that a second skin having holes with such a diameter significantly reduces the aerodynamic drag of the turbomachine without impacting the acoustic insulation quality of the acoustic panel. Thanks to the invention, it is therefore possible to reduce noise pollution from the turbomachine while preserving the aerodynamic performance of the turbomachine. Furthermore, according to the invention, the second skin is made of a thermoplastic polymer matrix composite material. The polymer matrix makes it possible to make holes with a diameter of less than 1 mm without damaging the second skin. Indeed, it has been found that certain drilling processes for making holes with small diameters are not compatible with all materials. For example, making holes by laser drilling requires the use of a matrix resistant to temperatures exceeding 300°C.Thermosetting polymer matrices do not withstand such temperatures and making holes with a diameter of less than 1 mm on such matrices tends to degrade the second skin around the drilling area, reducing the quality of the acoustic panel.

[0025] Thanks to the invention, it is therefore possible to provide an acoustic panel with good acoustic and aerodynamic properties.

[0026] The acoustic panel according to the invention may comprise one or more of the following characteristics, taken in isolation from one another or in combination with one another:

[0027] - the diameter of the holes is between 0.1 mm and 0.8 mm, preferably between 0.3 mm and 0.8 mm,

[0028] - the second skin has a thickness of less than 1 mm, advantageously between 0.5 mm and 0.8 mm,

[0029] - the second polymer matrix has a melting temperature greater than 300°C, preferably greater than 400°C,

[0030] - the coating further comprises an additional layer of glass fibers, arranged between the layer and the first skin,

[0031] - the additional layer comprises a first underlayer of non-woven glass fibers and a second underlayer of woven glass fibers,

[0032] - a stiffener,

[0033] - the stiffener comprises a frame comprising walls arranged on edges of the second skin and / or studs arranged on the second skin. The invention also relates to a method of manufacturing an acoustic panel according to any one of the preceding characteristics, the method comprising the following steps:

[0034] - (100) form the second skin by automatic placement of fibers,

[0035] - (102) make the holes on the second skin,

[0036] - (104) form the core by additive manufacturing on the second skin, and

[0037] - (106) form the first skin by additive manufacturing on the core.

[0038] - (108) deposit the coating layer on the first skin by thermal projection.

[0039] The method may comprise one or more of the following features, taken in isolation from each other or in combination with each other:

[0040] - the holes are made by laser drilling.

[0041] The invention also relates to an installation for manufacturing an acoustic panel according to any one of the preceding characteristics, comprising:

[0042] - a first second skin manufacturing station including:

[0043] - a strip unwinder comprising the fibers impregnated in the thermoplastic polymer matrix,

[0044] - a roller for compacting the strips,

[0045] - a strip heating device, and

[0046] - a second skin piercing organ,

[0047] - a second core and first skin manufacturing station comprising an additive manufacturing device,

[0048] - a third coating manufacturing station, the third station comprising a thermal spraying device. Brief description of the figures

[0049] Other characteristics and advantages will emerge from the following description of a non-limiting embodiment of the invention with reference to the appended drawings in which: Figure 1 is a schematic representation in longitudinal section of an aircraft turbomachine, Figure 2 is a schematic representation in perspective of an acoustic panel according to the invention, Figure 3 is a schematic representation in perspective of an exemplary embodiment of the core of the acoustic panel, Figure 4 is a block diagram of a method for manufacturing an acoustic panel according to the invention, Figure 5 is a schematic representation of an installation for manufacturing an acoustic panel according to the invention, Figure 6 is a partial schematic representation of a station for manufacturing the second skin.

[0050] Detailed description of the invention

[0051] An example of a turbomachine 1 for an aircraft is shown in Figure 1. The turbomachine 1 extends around and along a longitudinal axis X.

[0052] In the present application, the terms “upstream”, “downstream”, “axial”, “axially”, are defined in relation to the direction of circulation of the gases in the turbomachine along the longitudinal axis X.

[0053] The terms "radial", "radially", "internal", "inner", "external", "outer", "externally", are defined with respect to a radial axis Z which is perpendicular to the longitudinal axis X and with respect to the distance from the longitudinal axis X along the radial axis Z.

[0054] The turbomachine 1 comprises, from upstream to downstream, a fan 2, at least one compressor such as a low-pressure compressor 3 and a high-pressure compressor 4, a combustion chamber 5, at least one turbine 6 such as a high-pressure turbine and a low-pressure turbine, and a nozzle. The fan 2 allows the suction of an air flow dividing into a primary flow F1 and a secondary flow F2. The primary flow F1 passes through the engine of the turbomachine 1 while the secondary flow F2 is directed towards a secondary vein.

[0055] The primary flow F1 is compressed within the low-pressure compressor 3 and then the high-pressure compressor 4. The compressed air is then mixed with fuel and burned within the combustion chamber 5. The gases formed by the combustion pass through the high-pressure and low-pressure turbines. The gases finally escape through the nozzle, the cross-section of which allows the acceleration of these gases to generate propulsion.

[0056] The fan 2 comprises a disk rotatable about the longitudinal axis X and blades 7 mounted on the disk. The blades 7 extend radially from the disk. The blades 7 are surrounded by a casing 8. The casing 8 is centered on the longitudinal axis X. In addition, a nacelle (not shown) is arranged around the casing 8, coaxially and fixed thereto.

[0057] In order to reduce the noise pollution generated by the turbomachine 1, the turbomachine 1 further comprises at least one acoustic panel 9 fixed inside the casing 8. Generally, several acoustic panels 9 are fixed inside the casing 8. The acoustic panels 9 have a complex shape which makes it possible to adapt to the shape of the casing 8. Each acoustic panel 9 extends for example over an angular sector.

[0058] Each acoustic panel 9 is capable of absorbing acoustic energy over a frequency range between 100 Hz and 1500 Hz.

[0059] Each acoustic panel 9 advantageously forms a monolithic piece.

[0060] With reference to Figure 2, each acoustic panel 9 has a thickness e of between 10 mm and 50 mm. Such a thickness provides good acoustic insulation without increasing drag. Each acoustic panel 9 has a sandwich structure. Each acoustic panel 9 comprises a first skin 10, a second skin 11 and a core 12 arranged between the first skin 10 and the second skin 11. The second skin 11 and the first skin 10 are advantageously substantially parallel.

[0061] When the acoustic panel 9 is mounted in the casing 8, the first skin 10 is located outside the second skin 11. The second skin 11 advantageously forms the innermost skin.

[0062] The first skin 10 is formed from a first composite material having a first polymer matrix and reinforcing fibers embedded in the matrix. The polymer material of the first matrix is ​​for example chosen from thermoplastics such as polyolefins, for example polyethylene, polypropylene, or thermosets such as epoxies.

[0063] Reinforcing fibers are chosen, for example, from carbon, glass, and polyamide fibers.

[0064] Carbon fibers are, for example, polyaryletherketone (PAEK) fibers such as polyetherketone (PEK), polyetheretherketone (PEEK) or polyetherketoneketone (or PEKK) or polyacrylonitrile (PAN) fibers such as HexTow® AS4, AS7 or IM7 fibers marketed by the company Hexcel.

[0065] Advantageously, the fibers are continuous fibers, that is to say strands made up of sets of continuous parallel or twisted filaments.

[0066] Advantageously, the fibers are high strength and / or intermediate modulus fibers, for example a modulus between 200 GPa and 250 GPa.

[0067] The first skin 10 has, for example, a thickness of between 0.5 mm and 2 mm, preferably between 0.8 mm and 2 mm.

[0068] The first skin 10 is advantageously produced by additive manufacturing. The second skin 11 is formed from a second composite material. The second composite material comprises a second polymer matrix chosen from thermoplastics and reinforcing fibers embedded in the second matrix. The thermoplastic material of the second matrix may be identical to or different from the thermoplastic of the first polymer matrix. The thermoplastic of the second matrix is, for example, chosen from polyolefins such as polyethylene, polypropylene, or fluorinated polymers or polyamides or polyaryletherketones such as polyetheretherketone (PEEK).

[0069] Advantageously, the second matrix has a melting temperature greater than 300°C, preferably greater than 400°C and even more preferably between 400°C and 500°C, preferably between 420°C and 480°C.

[0070] Reinforcing fibers are chosen, for example, from carbon, glass, and polyamide fibers.

[0071] Carbon fibers are, for example, polyaryletherketone (PAEK) fibers such as polyetherketone (PEK), polyetheretherketone (PEEK) or polyetherketoneketone (or PEKK) or polyacrylonitrile (PAN) fibers such as HexTow® AS4, AS7 or IM7 fibers marketed by the company Hexcel.

[0072] Advantageously, the fibers are continuous fibers, that is to say strands made up of sets of continuous parallel or twisted filaments.

[0073] Advantageously, the fibers are high strength and / or intermediate modulus fibers, for example a modulus between 200 GPa and 250 GPa.

[0074] The reinforcing fibers may be the same or different from the reinforcing fibers of the first skin 10.

[0075] The second skin 11 is formed by the automatic fiber placement method. This allows automated manufacturing of the second skin 11.

[0076] The second skin 11 has holes 11 a. The holes 11 a are through holes. They thus pass through the second skin 11 in its entire thickness. According to the invention, the holes 11 a have a diameter less than 1 mm, and advantageously between 0.1 mm and 0.8 mm and even more advantageously between 0.3 mm and 0.8 mm. Such a diameter of the holes 11 a makes it possible to reduce the aerodynamic drag of the turbomachine 1. Indeed, the second skin 11 is in contact with the secondary flow F2 and its presence causes aerodynamic drag reducing the performance of the turbomachine 1. It has been found that holes 11 a of such dimensions make it possible to considerably reduce this drag.

[0077] The holes 11a have a section that is, for example, circular.

[0078] The holes 11a are made by laser drilling or by mechanical drilling. Laser drilling has the advantage of allowing holes with a diameter of less than 1 mm to be made on the second skin 11 shaped and in an automated manner, thus considerably reducing the time and costs of manufacturing the acoustic panel 9. Also, since the second matrix is ​​made of thermoplastic, laser drilling does not damage the second skin 11, unlike thermosetting materials which cannot withstand the temperatures used during laser drilling.

[0079] Advantageously, the second skin 11 has a thickness of less than 1 mm, advantageously between 0.5 mm and 0.8 mm.

[0080] The core 12 is arranged between the first skin 10 and the second skin 11. The core 12 has a honeycomb structure. The core 12 comprises in particular honeycomb cells 12a. Each honeycomb cell 12a has, for example, a hexagonal or triangular section. The section of the honeycomb cells 12a may differ from one honeycomb cell 12a to another. Each honeycomb cell 12a is hollow and has an internal cavity 12b which communicates with at least one hole 11a. This allows each honeycomb cell 12a to absorb part of the acoustic energy emitted by the blower 2.

[0081] Advantageously, the core 12 comprises a superposition of alveolar cells 12a. The core 12 comprises, for example, a first layer of alveolar cells 12a and a second layer of alveolar cells 12a (not shown). The first and second layers are separated by a septum. In the present technical field, a septum is a perforated membrane which is transparent to certain frequency ranges and impermeable to other frequency ranges. Such a configuration of the core 12 allows the acoustic panel 9 to absorb sound energy in a wider frequency range. Indeed, the first layer makes it possible to absorb sound energy in a first frequency range while the second layer makes it possible to absorb sound energy in another frequency range.Furthermore, thanks to the diameter of the holes 11a, it is possible to increase the height of the acoustic panel 9 for absorption of acoustic energy in a wide range of frequencies without increasing the drag. Indeed, generally speaking, the greater the height of the panel, the greater the aerodynamic drag.

[0082] The core 12 is for example made of a metallic material such as aluminum or of a polymeric or composite material. The core 12 is for example made of the same material as the first skin 10.

[0083] The core 12 is advantageously produced by additive manufacturing directly on the second skin 11. Thus, the manufacture of the acoustic panel 9 according to the invention can be fully automated without a manual assembly step.

[0084] With reference to Figure 3, advantageously, the acoustic panel 9 comprises a stiffener and preferably a plurality of stiffeners. The stiffeners make it possible to stiffen the acoustic panel 9. This makes it possible to limit the risks of collapse of the acoustic panel 9.

[0085] The stiffeners comprise, for example, a frame 13 and / or pads 14. Advantageously, the frame 13 is arranged on the second skin 11. The frame 13 has, for example, a square or rectangular shape. The frame 13 comprises walls 13a arranged on edges, and advantageously the four edges of the second skin 11 and spacers 13b arranged inside the walls 13a. The frame 13 has, for example, a height substantially equal to the height of the honeycomb cells 12a.

[0086] The pads 14 are arranged on the second skin 11 and between the honeycomb cells 12a. The pads 14 have, for example, a circular section. The height of the pads 14 is advantageously equal to the height of the honeycomb cells 12a. The pads 14 have, for example, an internal bore. Thus, in addition to giving stiffness to the acoustic panel 9, the pads 14 allow the acoustic panel 9 to be fixed to the casing 8 by means of fixing screws (not shown) passing through the pads 14 and fixed to the casing 8.

[0087] Preferably, the acoustic panel 9 further comprises a coating 15 arranged on the first skin 10. The first skin 10 is thus arranged between the core 12 and the coating 15. The coating 15 comprises a layer 15a comprising a ceramic material and preferably comprising a majority of a ceramic material and even more preferably consisting of a ceramic material. The ceramic material comprises, for example, yttria-containing zirconia, silica, alumina. The layer 15a makes it possible to improve the fire resistance of the acoustic panel 9 as well as its thermal insulation.

[0088] Layer 15a is for example deposited by thermal spraying.

[0089] In order to further improve the insulating character of the coating 15, the latter further comprises an additional layer of glass fibers. The additional layer is arranged between the layer 15a of ceramic material and the first skin 10.

[0090] The additional layer comprises for example a first sub-layer 15b for example made of non-woven glass fibers and a second sub-layer 15c for example made of woven glass fibers or thermoplastic to promote adhesion to the first skin 10. The second sub-layer 15c is arranged between the first sub-layer 15b and the first skin 10. The second sub-layer 15c is for example applied by wet surface treatment such as by dipping or spraying.

[0091] The coating 15 has, for example, a thickness of between 0.2 mm and 5 mm.

[0092] A method of manufacturing the acoustic panel 9 will now be described with reference to FIG. 4. The method comprises a first step 100 of forming the second skin 11 by automatic placement of fibers. During this first step 100, several strips 210 of pre-impregnated fibers are deposited on a support 220. The strips 210 are then compacted and heated to promote adhesion of the strips 210 to each other. The heating temperature of the strips 210 is between 400°C and 500°C, preferably between 400°C and 450°C, even more preferably 420°C.

[0093] Then, in a second step 102, the holes 11a are made on the second skin 11. During this second step 102, the second skin 11 is pierced by mechanical or laser drilling.

[0094] Laser drilling is particularly advantageous in that it allows small diameter holes to be made on the second skin 11 shaped without degradation of the second skin 11, the matrix of which is made of thermoplastic polymer material.

[0095] Laser drilling is performed, for example, by a laser. The laser is, for example, a short-pulse or ultra-short-pulse power laser. The laser advantageously generates radiation with a wavelength between 1 nm and 1 mm. The radiation generated is therefore in the infrared, visible, or ultraviolet range.

[0096] Then, in a third step 104, the core 12 is formed by additive manufacturing on the second skin 11. The core 12 is therefore directly formed on the second skin 11. No additional assembly step is therefore necessary. The core 12 is for example formed by the fused filament deposition technique (known by the acronym FFF for “Fused Filament Fabrication” in English).

[0097] Then, in a fourth step 106, the first skin 10 is formed by additive manufacturing on the core 12. The first skin 10 is therefore directly formed on the core 12. No additional assembly step is therefore necessary. The first skin 10 is for example formed by the fused filament deposition technique (known by the acronym FFF for “Fused Filament Fabrication” in English). Then, in an optional fifth step 108, the coating 15 is deposited on the first skin 10. In a first sub-step, the additional layer is deposited on the first skin 10 and then the ceramic layer 15a is deposited by thermal spraying.

[0098] Finally, in a sixth optional finishing step 110, the acoustic panel 9 is machined.

[0099] According to the invention, the manufacturing process of the acoustic panel 9 is fully automated, which makes it possible to reduce the manufacturing time of the acoustic panel 9 and to considerably reduce costs. Furthermore, the quality of the holes 11a in terms of shape and dimensions thanks to the laser drilling method makes it possible to provide an acoustic panel 9 having remarkable acoustic performance while having low drag.

[0100] An installation for the manufacture of the acoustic panel 9 will now be described with reference to figures 5 and 6.

[0101] The installation comprises a first station 200 for manufacturing the second skin 11 and a second station 300 for manufacturing the core 12 and the first skin 10.

[0102] The first station 200 is for example shown in Figure 6. The first station 200 comprises an unwinder 230 of the strips 210 of fibers pre-impregnated in the second thermoplastic polymer matrix. The unwinder 230 makes it possible to deposit the strips 210 on the support 220.

[0103] The first station 200 further comprises a compacting roller 240. The compacting roller 240 makes it possible to apply pressure to the strips 210 deposited on the support 220 in order to promote adhesion between the strips 210. The compacting roller 240 is movable in translation along the strips 210 in a direction D1. This makes it possible to apply pressure over the entire length of the strips 210 to promote adhesion of the strips 210 to each other over their entire lengths.

[0104] The first station 200 further comprises a heating device 250 for the strips 210. The heating device 250 makes it possible to heat the strips 210 as they are deposited on the support 220. This makes it possible to promote the adhesion of the strips 210 to each other by melting the thermoplastic matrix. The combination of pressure and temperature therefore promotes the adhesion of the strips to each other. The heating temperature is between 400°C and 500°C, preferably between 400°C and 450°C, even more preferably 420°C. The heating device 250 is for example a laser such as an infrared laser.

[0105] The first station 200 further comprises a piercing member 260 for piercing the second skin 11. The piercing member 260 is, for example, a laser or a mechanical drill. The laser is, for example, a short-pulse or ultra-short-pulse power laser. The laser advantageously generates radiation with a wavelength between 1 nm and 1 mm. The radiation generated is therefore in the infrared, visible or ultraviolet range.

[0106] The second station 300 for manufacturing the core 12 and the first skin 11 comprises an additive manufacturing device (not shown). The additive manufacturing device comprises, for example, a first filament dispenser, a first extruder, and a tray for depositing the molten filaments. According to another exemplary embodiment and in particular when the core 12 and the first skin 10 are manufactured from distinct materials, the additive manufacturing device further comprises a second filament dispenser and a second extruder.

[0107] The installation may further comprise a third station 400 for manufacturing the coating 15. The third station 400 comprises a thermal spraying device. The thermal spraying device comprises, for example, a torch enabling the spraying of a powder comprising a ceramic material, and preferably predominantly ceramic, via a carrier gas.

[0108] The installation according to the invention therefore allows online manufacturing of the acoustic panel 9 without manual intervention.

Claims

DEMANDS 1. Acoustic panel (9) for an aircraft turbomachine (1), the acoustic panel (9) comprising: - a first skin (10) formed of a first composite material having a first polymeric matrix and reinforcing fibers embedded in the matrix, - a second skin (11) formed of a second composite material having a second thermoplastic polymer matrix and reinforcing fibers embedded in the second matrix, and - a core (12) having a honeycomb structure, the core (12) being arranged between the first skin (10) and the second skin (11), the second skin (11) comprising holes (11a) having a diameter of less than 1 mm, characterized in that it further comprises a coating (15) comprising a layer (15a) comprising a ceramic material and deposited by thermal spraying, the first skin (10) being arranged between the coating (15) and the core (12).

2. Acoustic panel according to the preceding claim, characterized in that the diameter of the holes (11 a) is between 0.1 mm and 0.8 mm, preferably between 0.3 mm and 0.8 mm.

3. Acoustic panel according to any one of the preceding claims, characterized in that the second skin (11) has a thickness of less than 1 mm, advantageously between 0.5 mm and 0.8 mm.

4. Acoustic panel according to any one of the preceding claims, characterized in that the second polymer matrix exhibits a melting point above 300°C, preferably above 400°C.

5. Acoustic panel according to any one of the preceding claims, characterized in that the coating (15) further comprises an additional layer of glass fibers, arranged between the layer (15a) and the first skin (10).

6. Acoustic panel according to the preceding claim, characterized in that the additional layer comprises a first sub-layer (15b) of non-woven glass fibers and a second sub-layer (15c) of woven glass fibers.

7. Acoustic panel according to any one of the preceding claims, characterized in that it comprises a stiffener.

8. Acoustic panel according to the preceding claim, characterized in that the stiffener comprises a frame (13) comprising walls (13a) arranged on edges of the second skin (11) and / or studs (14) arranged on the second skin (11).

9. A method for manufacturing an acoustic panel (9) according to any one of the preceding claims, the method comprising the following steps: - (100) form the second skin (11) by automatic placement of fibers, - (102) make the holes (11 a) on the second skin (11 ), - (104) form the core (12) by additive manufacturing on the second skin (11), - (106) form the first skin (10) by additive manufacturing on the core (12), and - (108) deposit by thermal projection the layer (15a) of the coating (15) onto the first skin (10).

10. Method according to the preceding claim, characterized in that the holes (11 a) are made by laser drilling.

11. Installation for manufacturing an acoustic panel (9) according to any one of claims 1 to 8, comprising: - a first station (200) for manufacturing the second skin (11) comprising: a strip unwinder (230) for strips (210) comprising the fibers impregnated in the second thermoplastic polymer matrix, a roller (240) for compacting the strips (210), a heating device (250) for the strips (210), and a drilling device (260) for the second skin (11), - a second station (300) for manufacturing the core (12) and the first skin (10) comprising an additive manufacturing device, and - a third station (400) for manufacturing the coating (15), the third station (400) including a thermal projection device.

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