FR3059300A1 - TURBOREACTOR SUSPENSION PYLONE REAR FAIRING - Google Patents

Abstract

The rear pylon fairing includes two side walls, and a heat shield floor or ceiling wall (13), wherein said shield comprises an acoustically absorbing portion (15).

Description

059 300

61714 ® FRENCH REPUBLIC

NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY © Publication number:

(to be used only for reproduction orders)

©) National registration number

COURBEVOIE © Int Cl ⁸ : B 64 D 29/06 (2017.01)

PATENT INVENTION APPLICATION

A1

©) Date of filing: 30.11.16. (© Applicant (s): SAFRAN NACELLES Company by (© Priority: simplified actions - FR. @ Inventor (s): DELEHOUZE ARNAUD, SENTIS SYLVAIN, DESJOYEAUX BERTRAND and VERSAEVEL (Zp Date of public availability of the MARC. request: 01.06.18 Bulletin 18/22. ©) List of documents cited in the report preliminary research: Refer to end of present booklet (© References to other national documents (® Holder (s): SAFRAN NACELLES Joint-stock company related: simplified. ©) Extension request (s): © Agent (s): CABINET GERMAIN & MAUREAU.

Pty REAR FAIRING OF TURBOJECTOR SUSPENSION PYLON.

thT) This rear pylon fairing comprises two side walls, and a floor or ceiling wall forming a heat shield (13), in which said shield comprises an acoustically absorbing part (15).

FR 3 059 300 - A1 / 3 z

The present patent application relates to a rear fairing of an aircraft engine suspension pylon.

Such a fairing, known for example from EP2583900, and frequently designated by the acronym APF (Aft Pylon Fairing), can be installed in the downstream extension of the pylon and of the hot gas ejection nozzle of a turbojet double flow.

This fairing fulfills several functions, in particular an aerodynamic function making it possible to regulate the air flow in the downstream area of the pylon, and a thermal function, making it possible to protect the pylon and the wing of the aircraft from the heat emitted by the hot gases. expelled by the turbojet engine.

As indicated by EP2583900, when the turbojet engine is suspended under the wing of the aircraft, such a fairing is typically in the form of a box comprising two side walls which meet in a tapered manner downstream of the box, a wall of floor forming a heat shield, and an appropriate internal ribbing giving the assembly the required rigidity.

In more rare cases where the turbojet is carried above the wing, the heat shield is formed by the ceiling wall of the fairing.

The object of the present invention is in particular to provide a rear pylon fairing which makes it possible to reduce the noise generated by the turbojet engine.

Research has been carried out in order to reduce the noise linked to the interaction between the air ejected by the turbojet and the pylon: this research has led to placing acoustically absorbent materials on the side walls of the rear pylon fairing, as is explained by the prior document FR2957894.

The object of the present invention is in particular to provide a rear pylon fairing having improved sound absorption characteristics.

This object of the invention is achieved with a rear casing fairing comprising two side walls, and a floor or ceiling wall forming a heat shield, remarkable in that said shield comprises an acoustically absorbent part.

By "acoustically absorbent" is meant a part of the heat shield provided with a capacity to reduce the noise emitted in particular by the exit of hot gases from the turbojet engine, and in particular the high frequencies (of the order of a few kHz).

Unexpectedly, we realized that an acoustic treatment of this particular part of the pylon rear fairing, which had never been envisaged because of the particularly aggressive environment of this part, contributed significantly the overall reduction of the noise emitted by the turbojet engine.

Advantageously, the shield of the rear casing fairing can include one of the following characteristics, taken alone or in combination:

- Said acoustically absorbing part is arranged in the upstream part of the shield;

- Said acoustically absorbent part comprises an internal structuring skin, an external skin provided with perforations, and sound absorption means sandwiched between the internal wall and the external wall;

- The diameter of said perforations is preferably less than 1.5 mm;

- Said sound absorption means comprise cavities such as cells;

- The height of said cavities is between 5 mm and 20 mm;

- Said sound absorption means comprise a porous material;

- The ratio between the volume without material and the overall volume of said porous material is between 74% and 99%, and preferably between 85% and 95%;

the length of the ligaments between two network junction points of the porous material is between 100 μm and 500 μm, and preferably between 200 μm and 400 μm;

- The length of the ligaments can have a gradient in the thickness and / or in the plane of the porous material;

- Said internal skin comprises ribs and / or lugs for fixing to the internal structure of the rear pylon fairing;

- said internal and / or external skins, said sound absorption means and / or said ribs and / or said fixing lugs are formed from ceramic matrix composite (CMC);

- Said heat shield is manufactured entirely in CMC, preferably in one piece.

The present invention also relates to a heat shield for a rear pylon fairing, having the above-mentioned characteristics.

The present invention also relates to an aircraft turbojet suspension pylon, incorporating a rear pylon fairing in accordance with the above.

Other characteristics and advantages of the present invention will become apparent on reading the description which follows, and on examining the attached figures, in which:

FIG. 1 represents, in perspective, a rear pylon fairing according to the invention installed on the rear part of a propulsion unit composed of a turbojet engine and its nacelle;

Figure 2 shows, in perspective and in axial section, the assembly of Figure 1;

FIG. 3 represents, in perspective, a heat shield for a rear pylon fairing according to the invention;

FIG. 4 represents, in longitudinal section, this heat shield, and FIGS. 5 to 13 are views explaining different methods of obtaining materials in ceramic matrix composites (CMC) which can be used in the manufacture of the heat shield of the rear fairing pylon according to the invention.

FIGS. 1 and 2 show a propulsion unit for an aircraft, typically comprising a turbofan engine 1 enveloped by a nacelle 3, this assembly being intended to be suspended under the wing of an aircraft by means of a pylon 5 .

In operation, the turbojet engine 1 ejects in its downstream part on the one hand a hot flow C leaving the engine 7, and on the other hand a cold flow F generated by the fan, the latter typically representing around 80% of the thrust of the turbojet engine.

As explained above, in order on the one hand to streamline the downstream part of the pylon 5, and on the other hand to protect this pylon as well as the airfoil of the aircraft from the heat emitted by the flow of lime air C emitted downstream of the motor 7, a rear fairing of the pylon 9 is arranged in the downstream extension of this pylon, and above the flow C.

Such a fairing is typically in the form of a box comprising two lateral walls 11a, 11b defining a profile which tapers downstream, a floor forming a heat shield 13 vis-à-vis the hot flow C, and a ribbing internal 15.

The present invention relates more particularly to the heat shield 13, illustrated in more detail in FIGS. 3 and 4.

As can be seen in these figures, this heat shield is in the form of a plate configured substantially in horse saddle.

In the upstream part of this plate, an acoustic absorption zone 15 is provided having a thickness greater than the rest of the shield, typically comprising an internal structuring skin 17 intended to be placed inside the box of the rear pylon fairing, a external skin 19 intended to face the hot flow C leaving the motor 7 and provided with perforations 21, and acoustic absorption means 23 sandwiched between these two skins.

The diameter of the acoustic perforations is preferably less than 1.5 mm, and typically of the order of 1 mm.

In a preferred embodiment, the sound absorption means 23 include cavities such as cells, for example forming a honeycomb structure, the height of which can typically be between 5 mm and 20 mm.

In another embodiment, the sound absorption means 23 may comprise a porous material, the ratio of the volume without material to the overall volume is between 74% and 99%, and preferably between 85% and 95%.

In this porous material, the length of the ligaments between two network junction points of the porous material is between 100 μm and 500 μm, and preferably between 200 μm and 400 μm; the length of these ligaments can have a gradient in the thickness and / or in the plane of the porous material.

The internal wall includes ribs and / or fixing lugs (not shown) to the internal structure of the rear pylon fairing 5.

In a variant not shown, it is the entirety of the heat shield 13 which can have acoustic absorption characteristics.

The internal 17 and / or external 19 skins, said sound absorption means 23 and / or said ribs and / or said fixing lugs can be formed from a ceramic matrix composite (CMC).

In one embodiment, the entire heat shield 13 can be manufactured in CMC, preferably in one piece.

In the examples below, examples of manufacturing the acoustically absorbent part of the heat shield are provided with ceramic matrix composites (CMC).

Example 1 - Figure 5

This process relates to the manufacture of an acoustic attenuation panel in CMC comprising the following steps:

To drape in a mold a plurality of plies consisting of fibrous reinforcements comprising fibers of a ceramic material defining a first skin 25 of the acoustic attenuation panel,

B depositing on the first skin a plurality of blocks 27 of a first material, called a fugitive material, so as to define at least one space between two of said blocks,

Draping over the surface formed by the blocks a plurality of plies constituted by fibrous reinforcements comprising fibers of a ceramic material so as to define a second skin 29 of the acoustic attenuation panel,

D to infiltrate, by means of a liquid medium, the precursor of the ceramic phase through said skins 25, 29 and into said at least one space (E) separating said blocks of fugitive material,

E remove the liquid medium by evaporation or polymerization,

F sinter the assembly at a temperature allowing consolidation of the oxide ceramic material and elimination of the fugitive material.

This manufacturing process makes it possible to obtain a panel of composite material with a ceramic matrix in which the cellular core is produced at the same time as the skins and their assembly, which makes it possible to avoid the prior manufacturing of the cellular structure, as well as its assembly with the skins of the composite panel.

According to other optional features of this process:

- two stages of blocks of fugitive material are deposited, connected and aligned or not aligned by means of a plurality of protuberances also made of fugitive material, creating after the sintering step a septum of ceramic material the thickness of which can be adjusted according to acoustic need;

- after step B, ceramic fibers are added to the interblock space in order to manufacture a composite matrix septum with a ceramic matrix;

- After step B there is at least one fibrous reinforcement woven or not, continuous or not of ceramic material between said blocks of fugitive material;

- Said blocks inserted in step B have various and varied geometries making it possible to guarantee an acoustic attenuation power;

- The corners of said blocks are rounded so as to allow radii for connection of said skins with the walls of the cellular core of the acoustic panel;

- Said blocks of fugitive material incorporate lateral protrusions of variable dimensions, which will define the spacing between the blocks and therefore the thickness of the walls after sintering;

- one of the said skins of the acoustic panel is perforated, so as to create acoustic holes;

- At least one block of fugitive material comprises a plurality of protrusions in fugitive material positioned on said block so as to form, after the sintering step of the assembly, a plurality of acoustic holes in the second skin of the acoustic panel;

- The method comprises, before step F and preferably before step D, an additional step in which a plate is deposited having a plurality of protrusions in fugitive material positioned so that the protrusions pass entirely through the plies of reinforcements fibrous of one of said skins to form, after the sintering step of the assembly, a plurality of acoustic holes;

- A plurality of protrusions in material capable of withstanding the sintering step, is integrated into one of the molding surfaces of the tool, in order to form a plurality of acoustic holes in one of said skins;

- At least one block of fugitive material has a shaped geometry to define, after the sintering step of the assembly, at least one communication channel between the cells of the acoustic attenuation panel, allowing the drainage of liquids which can penetrate into the panel when it is in service;

- Said skins include a fibrous metal oxide reinforcement and a metal oxide matrix;

- At least two different ceramic materials are used for the matrix and the fibrous reinforcement of said skins. The local characteristics of the acoustic panel are thus adapted according to the constraints.

Example 2 - Figure 6

This process relates to the manufacture of an acoustic attenuation panel in CMC comprising the following steps:

- deposit in a mold a temporary fugitive filling material, intended to make networks of cavities 31 coming into contact with a surface of the panel 33, as well as at least one outer fibrous reinforcement layer of ceramic material 35 on a surface of this panel;

- Filtering by means of a liquid vector a material intended to constitute the ceramic matrix through the interstices formed around the fugitive filling material and in the outer layer of fibrous reinforcement;

- if necessary depending on the type of liquid vector, its elimination; and to

- Sinter the ceramic matrix material at a temperature allowing the elimination of the fugitive filler material, to form a ceramic material matrix filling all the volumes left free.

An advantage of this manufacturing process is that by performing both the step of filtering the material of the ceramic matrix and the sintering step, for the complete matrix of the product forming both the porous body for sound absorption. and one or two surface skins with the outer reinforcing layers, the finished panel is quickly obtained with a limited number of operations.

In addition, very good cohesion is obtained between the porous body and the surface skins, which gives the panel significant mechanical strength.

This manufacturing process can include one or more of the following characteristics, which can be combined with one another.

Advantageously, the manufacturing process uses one or more materials chosen from thermoplastic and thermosetting plastics for the fugitive filler material. These materials are economical and simple to use.

In particular, the matrix and the fibrous reinforcements may be made of ceramic material of the metal oxide type.

In particular, the matrix or the fibrous reinforcements can comprise at least two different ceramic materials. The local characteristics of the matrix or of the fibers are thus adapted according to the constraints.

Advantageously, the aggregates of filling materials have a diameter of between approximately 100 and 500 μm.

Advantageously, the panel has a volume porosity rate greater than 74%.

In particular, the panel may include substantially spherical cavities whose diameter is between approximately 200 and 400 μm, this panel having a volume porosity rate of between 80 and 98%.

Advantageously, the aggregates of filling material have a substantially spherical shape comprising planar facets. It is thus easy to stack the aggregates with both a large contact surface between them giving good communication between the porosities, and a high porosity rate.

In particular, the manufacturing process may locally include the deposition of aggregates according to various characteristics, including in particular variations in size, density or distribution.

In addition, the manufacturing process may include the deposition of fibrous reinforcements in the matrix, comprising a volume greater than 20% of the total volume of this matrix.

Advantageously, the manufacturing process comprises an additional step performing during the molding of the perforations of an outer layer of fibrous reinforcement. Acoustic holes are thus quickly made during the molding operation.

In this case, the additional step may include the production of spikes in a fugitive filling material, passing through the fibrous reinforcing layer. It's tips are automatically eliminated during the sintering step, which leaves the holes.

The additional step may alternatively include depositing on the outside of the fibrous reinforcing layer, a plate equipped with spikes passing through this layer. After removing this plate, the holes are obtained.

Alternatively, the perforations can be produced by any other method, such as mechanical drilling, or laser beam drilling.

In addition, the manufacturing process may include a step of depositing substantially vertical fibrous reinforcing layers passing through the panel, intended to form vertical ribs. One thus obtains in the same molding operation vertical ribs in the thickness of the panel, providing significant mechanical reinforcement.

Example 3 - Figure 7

This method relates to the manufacture of an acoustic attenuation panel comprising two outer skins 37, 39 in CMC containing a fibrous reinforcement, assembled on each side of a central cellular core 41 comprising walls 43 forming acoustic cavities, produced in a metal or a metal alloy having a melting temperature higher than the sintering temperature of the composite material.

This method includes a step of inserting a fugitive filler material into acoustic cavities leaving an annular space around each cavity on each side against the skin, and a step of sintering the composite material. ceramic eliminating the fugitive material, with a filling by the composite material of the spaces around the cavities.

An advantage of this manufacturing process is that by adapting the material as well as the shapes of the fugitive material, one obtains during sintering a protection preserving the internal volume of the cells, avoiding a deformation of the skins towards this volume as well as a flow of the matrix inside, which would reduce the volume of the cells by reducing the acoustic performance of the panels.

At the same time, by filling with the composite material, spaces around the cavities, larger adhesion surfaces between the skins and the central core greatly increasing the mechanical strength of the panel.

This process can include one or more of the following features, which can be combined with each other.

Advantageously, the manufacturing process comprises an additional step intended to produce, during molding, perforations in one of the skins made of composite material. A large number of perforations are obtained quickly.

This additional step may include the production of spikes on the fugitive filling material, in this same material, passing through a fibrous reinforcement of a skin.

As a variant, the additional step may include the deposition on the outer side of a fibrous reinforcement provided for a skin, of a plate equipped with spikes passing through this reinforcement. These tips are made of fugitive material, or of a material capable of withstanding the sintering step, in which case the inserts have a demouldable shape.

The manufacturing process can use to make the skins of dry fibrous reinforcements then receiving the matrix by filtration, or of fibrous reinforcements pre-impregnated with a matrix.

Advantageously, the fugitive material can be any material capable of disappearing during the sintering operation: the fugitive material can comprise ίο one or more materials chosen from thermoplastic and thermosetting materials.

Example 4 - Figure 8

This process relates to the manufacture of an acoustic attenuation panel in CMC.

This panel includes the successive steps consisting of:

- depositing in a mold 45 a lower fibrous reinforcement layer 47 to form a lower skin, comprising fibers of ceramic material;

- depositing above the lower fibrous reinforcement a set of layers alternately comprising a temporary fugitive filling material, forming a grid 49 comprising series of protrusions, and an intermediate fibrous reinforcement 51 comprising fibers of ceramic material;

- depositing an upper fibrous reinforcement layer 53 to form an upper skin, comprising fibers of ceramic material;

- And to proceed to sintering to obtain a matrix of composite material which permeates the fibrous reinforcements of the assembly, at a temperature allowing the elimination of the fugitive filling material, and allowing the consolidation of the matrix with all of the fibrous reinforcements .

An advantage of this manufacturing process is that by adapting the shapes of the protrusions of the grids, in particular by superimposing the different grids so that the protrusions touch each other, one obtains in a single simple and economical operation, after the disappearance fugitive filling material, networks of cavities each comprising a series of interconnected cavities formed by the protrusions in contact with each other.

In addition, by providing spikes passing through the upper fibrous reinforcement of fugitive filling material, a porosity of the upper skin is obtained, forming perforations connecting the internal cavities to the external environment, which provides acoustic attenuation.

This manufacturing process can include one or more of the following characteristics, which can be combined with one another.

Advantageously, the fugitive filler material has spikes passing through the upper fibrous reinforcement. Thus, in the same sintering operation, the porosity of the upper skin opening the acoustic cavities on the external environment is achieved.

Advantageously, the method uses one or more materials chosen from thermoplastic plastics and thermosetting plastics for the fugitive filler material. These materials are economical and simple to use.

Advantageously, the fibrous reinforcements and the matrix include ceramic metal oxides.

In particular, the fibrous reinforcements or the material forming the matrix can comprise at least two different ceramic materials. The local characteristics of the material are thus adapted according to the constraints.

The grids of fugitive filler material may comprise in a plane longitudinal and transverse rods, as well as protuberances forming inserts penetrating into the adjacent fibrous reinforcing layers.

According to one embodiment, the fibrous reinforcements deposited being dry, the precursor of the ceramic matrix filters these fibrous reinforcements by means of a vector in liquid form, the method comprising a following step of drying the temperature of the liquid vector before the step sintering, or polymerization of a resin in the case of a pre-ceramic resin, at a temperature below the melting point of the temporary molding material.

Advantageously, each protuberance of a grid of fugitive filler material is in contact with several protrusions of the grid opposite.

In particular, the distances between the axis of two adjacent protrusions of the same grid are between 100 and 4000 micrometers.

In particular, the grids can be obtained by weaving fibers, by stamping strips, by expansion or by continuous molding of a film.

Advantageously, the grids comprise in their flat parts rods having a cross-sectional area of between 5 μm ² and 250,000 μm ² .

Advantageously, the cross-sectional area of the rods is between 20 pm ² and 100 pm ² .

Advantageously, the protrusions have a cross-sectional area of between 10,000 pm ² and 16,000,000 pm ² , and a height of between 100 pm and 4,000 pm.

Advantageously, the grids have protrusions arranged on its two faces.

Example 5 - Figure 9

This process relates to the manufacture of an acoustic attenuation panel in CMC.

This process comprises the successive stages consisting in:

- deposit in a mold 55 a lower fibrous reinforcing layer of ceramic material 57 to form a lower skin;

- depositing above the lower fibrous reinforcement 57 a set of fibrous reinforcements made of ceramic material forming intermediate layers 59, 60 and an upper layer 62, these various layers previously containing or then receiving a dispersion of aggregates 64 formed by a temporary material fugitive filling, these aggregates forming groups of aggregates in contact with each other which extend over all of these layers;

- And proceed to sintering to obtain a matrix of ceramic composite material 66 which permeates the fibrous reinforcements of the assembly, at a temperature allowing the elimination of the fugitive filler material.

An advantage of this manufacturing process is that by adapting the size, density and distribution of the aggregates, with aggregates on the surface to form a porosity of the upper surface, and with the contact between them of aggregates of a group, there is obtained in a single simple and economical operation, after the elimination of the fugitive filling material during the sintering of the matrix, a porous body with a ceramic matrix forming an acoustic attenuation panel, comprising networks of cavities opening out on the top formed by each group of aggregates, and a waterproof bottom skin.

In addition, by depositing layers organized according to the thickness of the panel of aggregates of fugitive filling materials adapted in size and density, a network of structured cavities is obtained making it possible to control the porosity of the panel in thickness, in order to optimize the mechanical and acoustic characteristics.

This process can include one or more of the following features, which can be combined with each other.

Advantageously, the upper fibrous reinforcement layer has a fine mesh making it possible to insert the smallest aggregates inside. With these fine aggregates arranged on the surface of the panel, the porosities of the upper face are produced.

Advantageously, the method uses one or more materials chosen from thermoplastic plastics and thermosetting plastics for the fugitive filler material. These materials are economical and simple to use.

Advantageously, the fibrous reinforcements or the matrix include ceramic metal oxides.

In particular, the matrix and the fibrous reinforcement may comprise at least two different ceramic materials. We thus adapt the local characteristics of the matrix according to the constraints.

According to one embodiment, the fibrous reinforcements deposited being dry, the matrix infiltrates these fibrous reinforcements by means of a vector in liquid form, the method comprising a following stage of temperature drying of the liquid vector or of polymerization of a resin after the filtration step and before the sintering step, at a temperature below the melting point of the fugitive filler material.

Alternatively, aggregates may be provided by the liquid carrier.

As a variant, aggregates can be deposited by addition between layers of fibrous reinforcement.

As a variant, aggregates can be placed beforehand in fibrous reinforcing layers.

Advantageously, the aggregates have diameters substantially between 100 and lOOOpm.

Preferably, the aggregates have diameters substantially between 200 and 400 μm.

Advantageously, the manufacturing process uses several different aggregate dimensions.

In particular, the manufacturing process may include a step of mechanically drilling the upper surface to open the cavities to the outside.

Example 6- Figure 10

This method relates to the manufacture of an acoustic attenuation panel comprising two outer skins 68, 69 of composite material with a ceramic matrix containing a fibrous reinforcement, assembled on each side of a central cellular core 71 comprising walls 73 forming acoustic cavities produced by at least partial electrochemical conversion of aluminum to aluminum oxide.

This method includes a step of inserting a fugitive filler material into acoustic cavities leaving an annular space around each cavity on each side against the skin, and a step of sintering the composite material. ceramic eliminating the fugitive material, with a filling by the composite material of the spaces around the cavities.

An advantage of this manufacturing process is that by adapting the material as well as the shapes of the fugitive material, one obtains during sintering a protection preserving the internal volume of the cells, avoiding a deformation of the skins towards this volume as well as a flow of the matrix inside, which would reduce the volume of the cells by reducing the acoustic performance of the panels.

At the same time, by filling with the composite material, spaces around the cavities, larger adhesion surfaces between the skins and the central core greatly increasing the mechanical strength of the panel.

This process can include one or more of the following features, which can be combined with each other.

Advantageously, the manufacturing process comprises an additional step intended to produce, during molding, perforations in one of the skins made of composite material. A large number of perforations are obtained quickly.

This additional step may include the production of spikes on the fugitive filling material, in this same material, passing through a fibrous reinforcement of a skin.

As a variant, the additional step may include the deposition on the outer side of a fibrous reinforcement provided for a skin, of a plate equipped with spikes passing through this reinforcement. These tips are made of fugitive material, or of a material capable of withstanding the sintering step, in which case the inserts have a demouldable shape.

The manufacturing process can use to make the skins of dry fibrous reinforcements then receiving the matrix by filtration, or of fibrous reinforcements pre-impregnated with a matrix.

Advantageously, the fugitive material can be any material capable of disappearing during the sintering operation: the fugitive material can comprise one or more materials chosen from thermoplastic and thermosetting materials.

Advantageously, the production of the acoustic cavities comprises a step of assembling between them aluminum strips by means of work hardening, crimping, welding or bonding with a pre-ceramic adhesive.

Example 7 - Figure 11

This process relates to the manufacture of an acoustic attenuation panel in CMC.

This method includes the steps of: depositing in a mold one or more lower fibrous reinforcements 75, comprising fibers of a ceramic material;

depositing a plurality of intermediate fibrous reinforcements 77, comprising fibers of a ceramic material;

inserting through the intermediate fibrous reinforcements 77 elements of a molding material, said fugitive material, in the form of pins 79, said pins being inserted perpendicularly and / or obliquely with respect to the layers of fibrous reinforcements;

depositing one or more upper fibrous reinforcements 81, comprising fibers of a ceramic material;

sinter the assembly at a temperature allowing the elimination of the fugitive material.

Thus, this method makes it possible to simply obtain an acoustic attenuation panel comprising a solid skin, a core having a plurality of internal channels, and an acoustic skin, these elements being of composite material with ceramic matrix. The use of a fugitive material in the form of pins makes it possible to obtain a network of internal channels passing through the core of the attenuation panel. The method according to the invention also makes it possible to precisely control the characteristics of the network of internal channels formed within the body constituting the central core of the acoustic panel, and therefore the characteristics of acoustic attenuation of this panel.

In one embodiment, the method comprises the step of inserting into the upper fibrous reinforcements, prior to the sintering step, a plurality of inserts passing through said upper fibrous reinforcements right through.

In one embodiment, the plurality of inserts is made of fugitive material, and is preferably made integrally with a support, such as a plate.

In one embodiment, the plurality of inserts is a material capable of withstanding the sintering step, the inserts then having a demouldable shape, for example slightly conical.

In one embodiment, the pins inserted in the intermediate fiber reinforcements have a larger cross-sectional area than the inserts passing through the upper fiber reinforcements.

In one embodiment, the inserts have a density greater than the density of the pins, passing through the intermediate fibrous reinforcements.

In one embodiment, the pins, inserted in the intermediate fibrous reinforcements also pass through the upper fibrous reinforcements.

In one embodiment, secondary pins are inserted through only part of the intermediate fibrous reinforcements.

In one embodiment, the pins of fugitive material have cross sections whose area is between 5 pm ² and 250,000 pm ² , preferably between 20 pm ² and 100 pm ² .

In one embodiment, the density of pins ensures a final porosity rate in the central core of the laminated panel, after sintering, greater than 50%, preferably greater than 75%.

In one embodiment, at least part of the fibrous reinforcements is, before being placed in the mold, impregnated with a liquid containing a powder of a suspended ceramic material, or impregnated with a preceramic resin.

In one embodiment, prior to the sintering step, the step of infiltrating through part or all of the layers of fibrous reinforcements is carried out, a liquid containing a powder of a suspended ceramic material, or a pre-ceramic resin.

In one embodiment, the method comprises, after the infiltration step and before the sintering step, a drying and / or polymerization step.

In one embodiment, the fugitive material is a plastic material of the thermoplastic or thermosetting type, having a melting point higher than the drying and / or polymerization temperature.

In one embodiment, the fibrous reinforcements comprise a metal oxide type material.

Example 8 - Figure 12

This process relates to the manufacture of a porous CMC material, having sound absorption properties.

This process includes the steps of:

- depositing in a mold 83 aggregates formed by a temporary fugitive filling material, these aggregates constituting one or more groups of aggregates in contact with each other;

- Filtering by means of a liquid vector the material intended to form the ceramic matrix through the interstices formed between the aggregates of fugitive filler material;

- if necessary depending on the type of liquid vector, its elimination; and

- Sintering the material of the ceramic matrix at a temperature allowing the elimination of the fugitive filling material, to form a matrix of ceramic material 85 filling all the volumes left free between the aggregates.

An advantage of this manufacturing process is that by adapting the size, density and distribution of the aggregates, with aggregates on the surface to form a porosity of the upper surface of the panel, and with the contact between them of aggregates of a group, a porous body with a ceramic matrix forming an acoustic attenuation panel is obtained in a single simple and economical operation, after at least partial, and preferably total, elimination of the fugitive filling material during the sintering of the matrix. comprising networks of cavities opening onto the top formed by each group of aggregates.

In addition, by depositing aggregates locally adapted in size and density, a network of structured cavities is obtained, making it possible to control the porosity of the panel in thickness, in order to optimize its mechanical and acoustic characteristics.

Advantageously, the manufacturing process uses one or more materials chosen from thermoplastic and thermosetting plastics for the fugitive filler material from the aggregates.

In particular, the matrix can comprise at least two different ceramic materials. We thus adapt the local characteristics of the matrix according to the constraints.

Advantageously, the aggregates of filling materials have a diameter of between approximately 100 and 500 μm.

Advantageously, the panel has a volume porosity rate greater than 74%.

In particular, the panel may include substantially spherical cavities whose diameter is between approximately 200 and 400 μm, this panel having a volume porosity rate of between 80 and 98%.

Advantageously, the aggregates of filling material have a substantially spherical shape comprising planar facets. It is thus easy to stack the aggregates with both a large contact surface between them giving good communication between the porosities, and a high porosity rate.

In particular, the manufacturing process may locally include the deposition of aggregates according to various characteristics, including in particular variations in size, density or distribution.

In addition, the manufacturing process may include the deposition of fibrous reinforcements in the matrix, comprising a volume greater than 20% of the total volume of this matrix.

Example 9- Figure 13

This example relates to a porous CMC material having sound absorption properties.

This porous material 87 in CMC for acoustic attenuation panel, comprises a plurality of interlaced ceramic fibers and a metal oxide matrix 89.

This material comprises a plurality of channels 91 interlaced with said ceramic fibers and interconnected with each other, said channels defining at least one cavity 93, 95.

According to optional characteristics of this porous material:

the channels are oriented in different directions, in particular in the weft direction and / or in the warp direction of the ceramic fibers;

- at least one channel is wrapped around a ceramic fiber;

- at least one ceramic fiber and at least one channel are twisted together;

- At least one ceramic fiber has a titer between 50 grams / 1000 meters and 2500 grams / 1000 meters for densities between 2.2 and 4;

- At least one channel has an ovoid section with a small axis between 0.05 mm and 5 mm and a long axis between 0.05 mm and 10 mm;

- The volume ratio of the channels of the porous body is between 2% and 95% of said body, preferably between 50% and 90%;

- The porous material according to the invention comprises at least one aero-acoustic surface and at least one channel interconnected to at least one cavity with said at least one aero-acoustic surface.

Claims (15)

1. Rear fairing (9) of pylon (5) comprising two side walls (11a, 11b), and a floor or ceiling wall forming a heat shield (13), in which said shield comprises an acoustically absorbent part (15). 2. Fairing (9) according to claim 1, wherein said acoustically absorbent part (15) is disposed in the upstream part of the shield (13). 3. Fairing (9) according to one of claims 1 or 2, wherein said acoustically absorbent part (15) comprises an internal structuring skin (17), an external skin (19) provided with perforations (21), and means sound absorption (23) sandwiched between the inner skin (17) and the outer skin (19). 4. Fairing (9) according to claim 3, wherein the diameter of said perforations (21) is less than 1.5 mm. 5. Fairing (9) according to claim 3, wherein said sound absorption means (23) comprise cavities such as cells. 6. Fairing (9) according to claim 5, wherein the height of said cavities is between 5 mm and 20 mm. 7. Fairing (9) according to claim 3, wherein said sound absorption means (23) comprise a porous material. 8. Fairing (9) according to claim 7, wherein the ratio between the volume without material and the overall volume of said porous material is between 74% and 99%, and preferably between 85% and 95%. 9. Fairing (9) according to one of claims 7 or 8, in which the length of the ligaments between two junction points of the network of the porous material is between 100 μm and 500 μm, and preferably between 200 μm and 400 μm. 10. Fairing (9) according to claim 8, wherein the length of the ligaments can have a gradient in the thickness and / or in the plane of the porous material. 11. Fairing (9) according to any one of claims 3 to 10, wherein said internal skin (17) comprises ribs and / or lugs for fixing to the internal structure of the rear pylon fairing. 12. Fairing (9) according to any one of claims 3 to 11, in which said internal skins (17) and / or external (19), said sound absorption means (23) and / or said ribs and / or said fixing lugs are formed from ceramic matrix composite (CMC). 13. Fairing (9) according to any one of claims 3 to 12, wherein said heat shield (13) is made entirely of CMC, preferably in one piece. 14. Heat shield (13) for rear pylon fairing (9), having the characteristics indicated in any one of the preceding claims. 15. An aircraft turbojet suspension pylon (5), incorporating a rear pylon fairing (9) according to any one of claims 1 to 13. 1/5