ACOUSTIC TREATMENT DEVICE FOR AN AIRCRAFT PROPULSION ASSEMBLY AND METHOD FOR ITS PRODUCTION

Description

Domaine technique de l'invention

[0001] The invention relates to the field of acoustic treatment in aircraft and relates, in particular, to an acoustic treatment device for an aircraft propulsion unit and to its manufacturing method. Arrière-plan technique

[0002] The prior art includes in particular documents US-A1-2020 / 265821, US-A1-2015 / 0027629, US-A1-2018 / 0230905 and US-A1-2013 / 0133977.

[0003] For acoustic treatment in aircraft, and in particular to attenuate the acoustic waves which propagate in the different structures of an aircraft, it is known to use acoustic treatment devices which are in the form of a "sandwich" in which two skins enclose a cellular structure, typically a honeycomb structure, the properties of which are adapted to attenuate the acoustic waves in a given frequency range.

[0004] This type of acoustic treatment device is typically intended to be mounted on parts of a propulsion system comprising a nacelle and an aircraft engine. It can be mounted, for example, on nacelle panels, on engine panels or even on platforms located between stator blades or rotor blades.

[0005] There figure 1A illustrates an example of an acoustic treatment device 101 in which two plates 103 and 105 made of composite material, i.e. the skins of the acoustic treatment device, are assembled by gluing to a honeycomb structure 107.

[0006] In a known manner, the honeycomb structure 107 is composed of hollow cells, of hexagonal section, arranged in a periodic arrangement. In addition, the skin 103 of the acoustic treatment device, generally called acoustic skin, is porous to improve its acoustic performance while the other skin 105 is not and is simply a so-called closing skin.

[0007] The acoustic performance of such a device is mainly related to its alveolar structure and, in particular, the frequency range in which the acoustic waves are attenuated by the device results in particular from the geometry and dimensions of the cells of the alveolar structure.

[0008] In particular, in conventional acoustic treatment technologies, the length of the neck is small compared to the height of the cavity of the honeycomb structure. More precisely, in conventional technologies, the length of the neck is equal to the thickness of a composite sheet wall (carbon + resin) which constitutes the wetted surface of the treatment, because the neck is obtained by simple perforation of this wall.

[0009] The operation of the Helmholtz resonator thus constituted is optimized by sizing the air cavity so as to obtain the maximum acoustic speed at the neck. This optimization imposes cavity heights of the order of a quarter of the wavelength of the main frequency to be treated. This represents for a sound wave at a frequency of 100 Hz in ambient air having a wavelength of 3.4 meters, a necessary cavity height of: λ 4 = 3 , 4 4 = 0 , 85 m

[0010] It also offers very interesting properties in terms of the significant frequency bandwidth covered.

[0011] However, current trends in optimizing propulsion systems are oriented towards reducing the number of blades and reducing the rotational speed of rotating assemblies such as the fan in ducted architectures (e.g., a turbofan) and / or the propellers in unducted architectures (e.g., an unducted fan engine, in particular a contra-rotating open rotor). This results in a lowering of the acoustic radiation frequencies.

[0012] Optimizing the treatment panels then requires increasing their thickness to increase the cavity height and thus reduce the tuning frequency of the resonant cavities of the panels. This makes the panels incompatible with the mass and size constraints associated with the new UHBR (Ultra High Bypass Ratio) type architectures, i.e. with a very high dilution rate.

[0013] Indeed, the installation of conventional acoustic treatments without cones and tuned to these frequencies requires cavities with thicknesses of 20 to 25 cm. And, with so-called thin nacelle sizes and an acoustic signature starting at particularly low frequencies, turbojets with very high bypass ratios cannot use such acoustic treatment panels and therefore require the use of specific acoustic coatings. More precisely, in addition to conventional absorbent structures treating medium and high frequencies, there is a need for acoustic treatment that can effectively attenuate low frequencies in the order of 150 to 800 Hz with acoustic treatment panels having a reduced footprint.

[0014] It is always possible to size the Helmholtz resonator so that it is efficient at lower frequencies, for reduced radial size, for example, by varying both the height of the neck and the volume of the resonant cavity.

[0015] The downside, for this type of sizing under the constraint of a given reduced footprint, is that the frequency band on which the treatment acts optimally is drastically reduced when the frequency decreases. Indeed, the insertion of a cone in the honeycomb resonant cavities makes it possible to increase the height of the neck and therefore to reduce the height of the cell to 7 or 8 cm rather than the 20 to 25 cm height for a cell without an internal cone.

[0016] Furthermore, known manufacturing methods restrict the possibilities of usable geometries for the cells to honeycomb structures such as that shown in figure 1A . Therefore, the accessible frequency range (for acoustic attenuation) is inherently limited.

[0017] One possible approach to expand the frequency range in which acoustic waves are attenuated is the stacking of several honeycomb structures having cells of different dimensions and therefore intended to generate acoustic attenuation in different frequency ranges.

[0018] As illustrated by the figure 1B , the acoustic treatment device 109 is then made up of two skins 111 and 113 and two distinct alveolar structures 115 and 117 separated by an intermediate layer 119 which is a porous layer also called a “septum”. This type of stacking is called DDOF (from the English « Double Degree Of Freedom ”) as opposed to a simple structure, as shown in figure 1A , designated by the acronym SDOF (from the English “ Single Degree Of Freedom »).

[0019] However, these approaches are also limited in that, to generate acoustic attenuation in the low frequencies, they require significantly increasing the height of the hollow cells of the alveolar structure and therefore the total thickness of the acoustic treatment device. However, compliance with certain space constraints and in particular the fact of not impacting the drag of a propulsion unit prevents the acoustic treatment device from being enlarged beyond certain limits.

[0020] To address this problem, it is known to incorporate shapes specifically designed to attenuate low frequencies into the acoustic treatment device without significantly increasing the overall thickness of the device. Typically, protruding shapes, such as cones, are inserted into the cells of the honeycomb structure. The precise shape and dimensions of the shapes used are adapted to generate acoustic attenuation in a given frequency range.

[0021] A known approach is to insert cones into the cells of the honeycomb structure in a discreet manner. However, this approach is limited in terms of the useful surface area that can be treated and, consequently, in terms of acoustic performance, since the number of cones determines the level of acoustic attenuation obtained.

[0022] Another known approach also consists of the integration of cones (which are also inserted into alveoli) linked together by bars. In this case, in practice, the play existing between said cones and the cells of the alveolar structure tends to deteriorate the acoustic performance of the device. Indeed, it is necessary to position the bars in notches of an alveolar structure, which requires very complex geometric control.

[0023] A final approach involves introducing into the device a structure formed by a plate from which cones emerge. These cones are inserted into the alveoli (i.e., into the cells) of the alveolar structure when the device is assembled. This type of structure is manufactured by injection or compression and makes it possible to obtain a thin acoustic treatment device capable of attenuating low frequencies.

[0024] However, this latter approach is also limited by the fact that, to allow the acoustic treatment device to be fixed to a part of a propulsion unit, it is necessary to fill certain cells of the honeycomb structure in order to pass fixing means which allow assembly.

[0025] In particular, the assembly of the device requires filling certain cells of the honeycomb structure with a coating, a resin or even a putty. This operation, called « potting ", has several disadvantages: First, it is an additional operation in the manufacturing process. Second, it necessarily involves drying time. And, third, it suffers from inaccuracy, resulting in more cells than necessary being filled. in fine.

[0026] As a result, it generates additional production costs and a loss of treated surface area due to the imprecision in the application of the mastic, coating or resin. Résumé de l'invention

[0027] The present invention provides a solution to these drawbacks.

[0028] To this end, according to a first aspect, the invention relates to an acoustic treatment device for an aircraft propulsion unit, this device comprising a first skin and a second skin enclosing an acoustic structure, said acoustic structure comprising a honeycomb structure, comprising a plurality of hollow cells, and a shape structure, comprising a plurality of projecting shapes, and so that each of said projecting shapes is inserted into a different hollow cell of the honeycomb structure, said acoustic treatment device being characterized in that said shape structure further comprises at least one stud, of a shape distinct from that of the projecting shapes, this stud being designed to fit into at least one hollow cell of the honeycomb structure and to allow the acoustic treatment device to be fixed to a part of the aircraft propulsion unit.

[0029] The acoustic treatment device 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: the dimensions of each stud of the shaped structure are designed so that each stud fits into a different hollow cell of the honeycomb structure. the dimensions of each stud of the shaped structure are designed so that each stud fits into a hollow area of ​​the honeycomb structure formed of several neighboring hollow cells. each stud comprises a through hole designed to allow the passage of fastening means. each hole comprises machining, for example a bore, a milling or a counterbore, adapted to allow the insertion of a head of the fastening means so that said head of the fastening means does not emerge from said hole.

[0030] The invention also relates, according to a second aspect, to an aircraft propulsion assembly, comprising at least one acoustic treatment device according to the first aspect.

[0031] The invention finally relates, according to a third aspect, to a method of manufacturing an acoustic treatment device according to the first aspect, comprising: manufacturing the shaped structure; assembling the shaped structure with the honeycomb structure so as to form the acoustic structure; and, assembling the acoustic structure with the first skin and the second skin so as to form the acoustic treatment device.

[0032] The manufacturing method 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: the method further comprises drilling the acoustic treatment device at the pads of the shaped structure. during the manufacture of the shaped structure, holes are formed in the pads of said shaped structure from protruding parts of a mold or inserts positioned in a mold. during the manufacture of the shaped structure, the pads of said shaped structure are made with an insert positioned in a mold. Brève description des figures

[0033] The present invention will be better understood and other details, characteristics and advantages of the present invention will appear more clearly on reading the description of a non-limiting example which follows, with reference to the appended drawings in which: [ Fig. 1A ] there figure 1A is a schematic representation of an embodiment of an acoustic treatment arrangement according to the prior art; [ Fig. 1B ] there figure 1B is a schematic representation of an embodiment of an acoustic treatment arrangement according to the prior art; [ Fig. 2 ] there figure 2 is a schematic diagram of the assembly of the acoustic structure according to one embodiment of the invention; [ Fig. 3 ] there figure 3 is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 4 ] there figure 4 is a schematic diagram of the assembly of an acoustic treatment device according to one embodiment of the invention; [ Fig. 5A ] there figure 5A is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 5B ] there figure 5B is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 6A ] there figure 6A is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 6B ] there figure 6B is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 7A ] there figure 7A is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 7B ] there figure 7B is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 8A ] there figure 8A is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 8B ] there figure 8B is a schematic representation of an embodiment of a shape structure according to the invention; [ Fig. 9 ] there figure 9 is a schematic representation of an embodiment of a shape structure according to the invention; and, [ Fig. 10 ] there figure 10 is a step diagram of an embodiment of a method for manufacturing an acoustic treatment device according to the invention.

[0034] Elements having the same functions in different embodiments have the same references in the figures. Description détaillée de l'invention

[0035] In reference to the figures 2 à 5 , we will now describe an acoustic treatment device according to one embodiment of the invention.

[0036] The acoustic treatment device 401 is designed to be mounted on a part of an aircraft propulsion assembly. Such an assembly comprises an aircraft nacelle and an aircraft engine and such a part may be, for example, a nacelle panel, an engine panel, a platform between stator blades or between rotor blades.

[0037] The device 401 comprises a first skin 403 and a second skin 405 which enclose an acoustic structure 407. The first skin 403 and the second skin 405 may be, for example, plates made of composite material. Furthermore, as can be seen in the figure 4 , in the example shown, the first skin 403 is porous, that is to say that it is pierced by orifices 423 in order to improve its acoustic properties.

[0038] As is more particularly visible at the figure 2 , the acoustic structure 407 comprises a honeycomb structure 409 which comprises a plurality of hollow cells 411 and a shape structure 413 which comprises a plurality of protruding shapes 415.

[0039] In the example shown, the alveolar structure 409 is a honeycomb structure which comprises hollow cells 411 of hexagonal section arranged in a periodic arrangement. It is these hollow cells 401 (i.e. the alveoli) which are designed to generate acoustic attenuation in a given frequency range.

[0040] Furthermore, the protruding shapes 415 of the shape structure 413 are cones which are also arranged in a periodic arrangement, identical to that of the cells 411 of the honeycomb structure 409, and whose dimensions are adapted so that each cone fits into a different hollow cell 411 of the honeycomb structure 409 when the acoustic treatment device 401 is assembled. The cones are designed to generate, in association with the honeycomb structure, acoustic attenuation in a frequency range distinct from that of the honeycomb structure alone. In particular, these may be low frequencies which are not sufficiently attenuated by the honeycomb structure alone.

[0041] The invention is not limited to a shape structure in which the protruding shapes are cones. The geometry of the protruding shapes may be, in different embodiments, pyramidal, spiral, funnel-shaped, or even hopper-shaped. Those skilled in the art will be able to adapt this shape to a desired attenuation in a given frequency range and / or to given desired mechanical properties.

[0042] The shaped structure can be manufactured, for example, from thermoplastic materials such as PAEK (polyaryletherketone), PEI (polyetherimide), PC (polycarbonate), PPS (polyphenylene sulfide) or PESU (polyethersulfone). It can have variable dimensions, for example, of the order of 1000 by 500 square millimeters and several shaped structures can be assembled together to form a larger one if necessary, for example, by gluing or welding (by ultrasound or laser in particular).

[0043] Furthermore, by way of example, the height of the protruding shapes of the shape structure may be between 5 and 100 millimeters and the dimensions of the base (i.e. the area located at the plate) of these protruding shapes may be included in a circle with a diameter of between 5 and 50 millimeters. Finally, the thickness of the material constituting the protruding shape may be between 0.3 and 0.5 millimeters.

[0044] As is more particularly visible at the figure 5A and to the figure 5B , the shape structure 413 also comprises studs 417, which have a shape distinct from that of the protruding shapes 415.

[0045] The studs 417 are designed to fit into the hollow cells 411 (positioned opposite each other at the time of assembly) of the honeycomb structure 409. Thus, in the example shown, the studs 417 have a hexagonal section and dimensions which allow this fitting.

[0046] Generally speaking, the shape and arrangement of the pads 417 are adapted to allow their embedding in the hollow cells, even in the case where the shapes of the cells and the pads differ.

[0047] Thus, the honeycomb structure 409 and the shape structure 413 are assembled to form the acoustic structure 407 by inserting the protruding shapes 415 into certain hollow cells 411 and by embedding the studs 417 into other hollow cells 411 of the honeycomb structure 409. In particular, during the assembly of the honeycomb structure 409 and the shape structure 413, the studs 417 embedding into hollow cells 411 guide the shape structure 413 with at least a portion of their external dimensions which are designed to fit exactly to the internal dimensions of the hollow cells 411 opposite each other.

[0048] As described further in more detail with reference to the figure 10 , it is this honeycomb structure which is assembled with the two skins to form the complete acoustic treatment devices. For example, the total height of the acoustic treatment device, once assembled, can be around 30 millimeters.

[0049] Furthermore, the pads 417 are designed to allow the acoustic treatment device 401 as a whole to be attached to a part of the aircraft propulsion assembly. In particular, the pads may be solid or sufficiently filled so that the material of each pad constitutes a support element for attachment means.

[0050] For example, as it is visible at the figure 8A and to the figure 8B , which represent a particular embodiment, each stud 417 may comprise a through hole 419 designed to allow the passage of fixing means such as for example a screw.

[0051] Furthermore, in this example, the hole 419 comprises a machining 421, which may be for example a bore, a milling or a counterbore, and which is adapted to allow the insertion of a head of the fixing means so that this head does not emerge from the hole 419. Thus, advantageously, no part of the fixing means risks hindering an aerodynamic flow when the acoustic treatment device 401 is mounted near a flow circulation vein.

[0052] In a variant not shown, each stud 417 may also include a threaded insert which is designed to allow its attachment to the part of the aircraft propulsion assembly to which it is intended to be attached. In this case, the attachment to the part of the propulsion assembly is done from the rear. The screw head is then on the side of a casing for example.

[0053] There figure 6A and to the figure 6B represent an embodiment in which several individual pads 417 are arranged so that they fit into neighboring cells 411 of the honeycomb structure.

[0054] Generally, in the embodiments shown in figures 5A et 5B on the one hand and, 6A and 6B on the other hand, the dimensions of each pad 417 of the shape structure 413 are designed so that each pad 417 fits into a different hollow cell 411 of the honeycomb structure 409. In other words, all the hollow cells 411 of the honeycomb structure 409 have the same dimensions and each pad 417 has dimensions adapted to fit into a single cell.

[0055] In an alternative embodiment, as shown in figure 7A and to the figure 7B , the dimensions of each pad 417 of the shape structure 413 are designed so that each pad 417 fits into a hollow area of ​​the honeycomb structure 409 formed of several neighboring hollow cells 411. Typically, an area formed of several identical hollow cells 411 of the honeycomb structure 409 must have been cut out beforehand to allow the fitting of a single pad 417.

[0056] Furthermore, in this case as in the one where each plot fits into a single cell, as can be seen in the figure 9 , each stud 417 may include a through hole 419 designed to allow the passage of fixing means such as for example a screw.

[0057] Likewise, each stud 417 may include machining 421, which may be for example a bore, a milling or a counterbore, and which is adapted to allow the insertion of a head of the fixing means so that this head does not emerge from the hole 419. With reference to the figure 10 , we will now describe a method of manufacturing an acoustic treatment device such as that described with reference to figures 2 à 9 .

[0058] Step 901 consists of manufacturing the shaped structure. In particular, in different embodiments of the manufacturing method, the shaped structure can be manufactured, for example, by injection or by stamping and overmolding or by additive manufacturing. Injection designates the injection of a thermoplastic resin, filled or not, into a closed mold. Stamping and overmolding designates the use of a thermoplastic sheet, filled or not, which is stamped to form the pads.

[0059] Step 903 consists of assembling the shaped structure with the honeycomb structure to form the acoustic structure. This assembly can be carried out, for example, by gluing or welding.

[0060] Step 905 consists of assembling the acoustic structure with the first skin and the second skin to form the complete acoustic treatment device. In the same way as for step 903, this assembly can be carried out, for example, by gluing or by welding.

[0061] Those skilled in the art will appreciate that the operations of assembling the shaped structure with the honeycomb structure and the acoustic structure with the first skin and the second skin can be interchanged in the manufacturing process.

[0062] Furthermore, in the non-limiting example shown, the manufacturing method also comprises a step 907 of drilling the acoustic treatment device at the level of the pads of the shaped structure in order to form the holes which will allow the fixing means to pass through.

[0063] Alternatively or in a complementary manner, it is possible that, during step 901 of manufacturing the shape structure, the holes are formed in the studs of the shape structure either from projecting parts of a mold which is used for said manufacturing or from inserts which are positioned in the mold.

[0064] Finally, in a particular embodiment of the manufacturing method, during step 901 of the manufacturing of the shaped structure, the pads can be made with an insert positioned in a mold. Advantageously, it can thus be a mold “conventionally” used for the manufacturing of a shaped structure without a pad in which it is sufficient to place inserts to allow the manufacturing of the pads.

Claims

1. An acoustic treatment device (401) for an aircraft propulsion assembly, this device comprising a first skin (403) and a second skin (405) enclosing an acoustic structure (407), said acoustic structure (407) comprising a cellular structure (409), comprising a plurality of hollow cells (411), and a structure of shapes (413), comprising a plurality of protruding shapes (415), and so that each of said protruding shapes (415) engages in a different hollow cell (411) of the cellular structure (409), said acoustic treatment device (401) being characterised in that said structure of shapes (413) further comprises at least one stub (417), of a shape distinct from that of the protruding shapes (415), this stub (417) being designed to fit into at least one hollow cell (411) of the cellular structure (409) and to enable the acoustic treatment device (401) to be fixed to a component of the aircraft propulsion assembly.

2. The acoustic treatment device (401) according to claim 1, wherein the dimensions of each stub (417) of the structure of shapes (413) are designed so that each stub (417) fits into a different hollow cell (411) of the cellular structure (409).

3. The acoustic treatment device (401) according to claim 1, wherein the dimensions of each stub (417) of the structure of shapes (413) are designed so that each stub (417) fits into a hollow area of the cellular structure (409) formed by several adjacent hollow cells (411).

4. The acoustic treatment device (401) according to any one of the preceding claims, wherein each stub (417) comprises a through hole (419) designed to allow fastening means to pass through.

5. The acoustic treatment device (401) according to claim 4, wherein each hole (419) comprises a machined portion (421), for example a bore, milling or counterbore, adapted to allow the engagement of a head of the fastening means so that said head of the fastening means does not emerge from said hole (419).

6. An acoustic propulsion assembly comprising at least one acoustic treatment device according to one of the preceding claims.

7. A method of manufacturing an acoustic treatment device (401) according to any one of claims 1 to 5, comprising: - manufacturing (901) the structure of shapes; - mounting (903) the structure of shapes with the cellular structure to form the acoustic structure; and, - mounting (905) the acoustic structure with the first skin and the second skin so as to form the acoustic treatment device.

8. The manufacturing method according to any one of the preceding claims, further comprising: - drilling (907) the acoustic treatment device at the level of the stubs of the structure of shapes.

9. The manufacturing method according to any one of the preceding claims, wherein, during the manufacture (901) of the structure of shapes, holes are formed in the stubs of said structure of shapes from protruding parts of a mould or from inserts positioned in a mould.

10. The manufacturing method according to any one of the preceding claims, wherein, during the manufacture (901) of the structure of shapes, the stubs of said structure of shapes are made with an insert positioned in a mould.