EP2318683A2

EP2318683A2 - Acoustic attenuation panel for aircraft engine nacelle

Info

Publication number: EP2318683A2
Application number: EP09784309A
Authority: EP
Inventors: Guy Bernard Vauchel; Guillaume Ruckert
Original assignee: Aircelle SA
Current assignee: Safran Nacelles SAS
Priority date: 2008-07-30
Filing date: 2009-07-28
Publication date: 2011-05-11
Also published as: CN102301122A; US8899512B2; BRPI0916330A2; WO2010012900A2; CA2731974A1; WO2010012900A3; US20110133025A1; CN102301122B

Abstract

This acoustic attenuation panel for an aircraft engine nacelle comprises a structuring skin (1) and, by way of acoustic absorption material, a porous material (5) attached to this skin (1).

Description

Sound attenuation panel for aircraft engine nacelle

The present invention relates to an acoustic attenuation panel for an aircraft engine nacelle, and to nacelle elements equipped with such a panel.

The use of sound attenuation panels in aircraft engine nacelles to reduce noise emissions from jet engines is known from the state of the art.

These acoustic attenuation panels generally have a sandwich structure comprising a structuring skin, a honeycomb-type honeycomb structure, and a resistive layer generally formed by a perforated skin.

The realization of these sound attenuation panels is expensive especially because of the presence of the honeycomb structure, and the need to fix this honeycomb structure on the structuring skin and perforated.

The present invention is thus particularly intended to provide an acoustic attenuation panel with a simplified design compared to the state of the art, which can be manufactured cheaply.

This object of the invention is achieved with an acoustic attenuation panel for an aircraft engine nacelle comprising a structuring skin and, as acoustic absorption material, a porous material attached to this skin.

By "porous material" is meant, in the context of the present invention, an open material (that is to say having many communicating cavities) in the form of foam, or in expanded form, or in the form of felt, or in the form of an aggregate of small elements such as marbles.

Due to its porous nature, such a material has good acoustic attenuation properties.

Such a material, formed from metallic materials, polymers, ceramics or composites available on the market, generally has a cost significantly lower than that of a honeycomb structure, and its implementation on the structural skin is significantly more simple.

In some cases, sound attenuation panels must be designed to be installed in a hot-air zone of a turbojet engine nacelle aircraft, and in particular in the downstream part of this nacelle through which are expelled exhaust gases whose temperature is typically greater than 600 ⁰ C.

The use of acoustic attenuation panels in this exhaust zone substantially reduces noise emissions in the high frequency range.

For these particular high-temperature applications, acoustic attenuation panels are generally used, the structuring skin of which is formed by a metal sheet, the honeycomb structure is metallic, and the resistive layer is a perforated metal sheet.

The cellular honeycomb structure is soldered (that is to say by assembling two materials using a filler metal having a melting temperature lower than that of the base metal) to the structuring metal sheet and the perforated metal sheet.

The use of metal alloys for all the elements forming this sandwich structure, and the implementation of brazing to connect them, are particularly expensive.

In addition, the panel obtained from all these metal elements is relatively heavy.

The present invention therefore also has the more particular object of providing an acoustic attenuation panel adapted to be installed in a nacelle hot zone, which is less expensive and heavy than those of the prior art.

This more particular object of the invention is achieved with an acoustic panel of the aforementioned type, remarkable in that said porous material is selected from the group consisting of materials resistant to temperatures up to 200 ^° C., materials resistant to temperatures up to 400 ⁰ C, materials resistant to temperatures up to 600 ⁰ C, and materials resistant to temperatures up to 800 ° C.

Depending on the applications provided in the hot zone, the porous material may have a greater or lesser heat conductivity.

In the particular case where this panel is intended to equip the air inlet lip of a pneumatic defrosting air inlet structure, the porous material will be selected so as to withstand a maximum temperature of the order 400 ⁰ C, and to have a high conductivity of heat. The material forming such a porous material for a hot zone may be chosen from the group comprising foams made of metallic materials, and in particular foams based on aluminum alloys and / or copper and / or nickel and / or of chrome, or carbon foams.

According to other optional features of the acoustic attenuation panel according to the invention:

said porous material is bonded to said structuring skin: this is a very simple means of fixing the porous material to the structuring skin;

said structuring skin comprises perforations: this arrangement is suitable when it is desired that the structuring skin be disposed on the side of the flow of the exhaust gases;

stiffeners are fixed on said structuring skin: these stiffeners make it possible to give the panel a rigidity comparable to that which is not provided by the honeycomb structure of the panels of the prior art;

- A resistive layer is reported (e) on the stiffeners: this resistive layer allows in particular to protect the porous material vis-à-vis the impacts;

- This resistive layer is formed by a mesh or a perforated skin, or a combination of these two elements;

said structuring skin and / or said stiffeners and / or said perforated skin and / or said resistive layer are formed in materials selected from the group consisting of metal alloys, ceramics, metal matrix composites, ceramic matrix composites: the choice of these materials is related to the constraints of weight and temperature and the mechanical stresses to which the acoustic panels must be subjected.

The present invention is also more particularly intended to provide a panel whose characteristics perfectly meet the conditions of temperature, geometry, frequency and frequency distribution of sound emissions, etc., in which it will be used ("custom" panel ).

This more particular object of the invention is achieved with a panel according to the foregoing, wherein said porous material comprises cavities: the presence of these cavities makes it possible to optimize the characteristics of weight and acoustic absorption of the panel according to its destination.

According to other optional features of this optimized panel, allowing to adapt it perfectly according to its destination:

at least a portion of said cavities are through;

at least a part of said cavities are blind;

at least a portion of said cavities have walls oriented substantially perpendicularly to the mean plane of said panel;

at least a portion of said cavities have walls inclined with respect to the mean plane of said panel;

said porous material is formed of a superposition of layers of porous materials of different characteristics, in the direction of the thickness of the panel;

said porous material is formed of a juxtaposition of porous material loaves of different characteristics, in the direction parallel to the mean plane of the panel.

The present invention also relates to an aircraft turbojet engine nacelle air intake structure, remarkable in that it comprises an air intake lip provided with at least a first acoustic attenuation panel. consistent with the above.

According to optional features of this air intake structure:

this air intake structure comprises a pneumatic deicing compartment delimited in particular by said lip and by an internal partition, and said first acoustic attenuation panel is of the type comprising a porous open-cell material capable of withstanding temperature up to 400 ⁰ C and having a high conductivity of heat;

said first acoustic attenuation panel is fixed inside said air inlet lip by an upstream holding plate and a downstream holding plate, and said internal partition is fixed on said downstream holding plate, preferably by riveting;

said air intake structure comprises a second acoustic attenuation panel fixed inside said air intake lip downstream from said internal partition, separated from said first panel by a material seal porous open cell capable of withstanding a temperature of up to 400 ° C and having low heat conductivity;

- said second acoustic attenuation panel is selected from the group comprising a porous material and open-cell panel in accordance with the foregoing, capable of withstanding a temperature up to 120 ⁰ C, and a structural panel in Honeycomb ;

said first panel, said seal made of porous material and said second panel are covered with a common sheet on which said internal partition is fixed, preferably by riveting;

said air inlet structure is of the type in which the air inlet lip forms a one-piece assembly with the external wall of the air inlet structure, this one-piece assembly being able to slide with respect to the fan case of the turbojet, as described for example in the document FR 2 906 568;

- said air inlet structure comprises centering members fixed on said common sheet.

The present invention also relates to a fixed internal structure of an aircraft turbojet engine nacelle, remarkable in that it comprises at least one sound attenuation panel in accordance with the foregoing. According to optional features of this fixed internal structure:

said acoustic attenuation panel is situated at least partly in the zone of said fixed internal structure intended to be subjected to high temperatures generated by said turbojet engine, and the porous material of this panel is of the open cell type and is suitable for to withstand a temperature of up to 800 ⁰ C and has a high conductivity of heat;

- said porous material is on the inner face of said fixed inner structure, the latter being provided with perforations on at least a portion of its surface covering said porous material;

said porous material is held by returns formed in said fixed internal structure;

said porous material is on the outer face of said fixed internal structure, inside a recessed zone formed inside this structure;

said porous material is covered, at least in the upstream part, with a perforated resistive layer; said resistive layer is formed in the same material as that of the fixed internal structure.

The present invention also relates to an aircraft engine nacelle, remarkable in that it is equipped with at least one acoustic attenuation panel according to the foregoing.

Other features and advantages of the present invention will emerge in the light of the description which follows, and on examining the appended figures, in which:

FIG. 1 schematically and in section shows an embodiment of an acoustic panel according to the invention, and

- Figures 2 to 5 show optimized variants of the acoustic panel of Figure 1;

- Figures 6 and 7 show, in schematic longitudinal sectional view, two variants of a nacelle air inlet incorporating at least one acoustic attenuation panel according to the invention;

FIG. 8 represents, in schematic longitudinal sectional view, a nacelle of the prior art, enclosing a conventional turbofan jet engine, and

- Figures 9 to 13 show, in partial view and in section, different variants of a fixed internal structure nacelle, equipped with at least one acoustic attenuation panel according to the invention.

In all of these figures, similar or identical references designate organs or sets of similar or identical members.

As can be seen in Figure 1, an acoustic panel according to the invention comprises, on the opposite side to the origin of the sound excitation, a structuring skin 1, formed in a sheet.

On this structuring skin 1 are reported a plurality of stiffeners 3, which can be formed for example by longitudinal members having a section i, arranged parallel to each other.

Between these stiffeners 3 is disposed a porous material 5 that is to say a material having an open structure, that is to say open cells, able to absorb the energy of acoustic waves. This porous material, which may be in the form of foam, or in expanded form, or in the form of felt, or in the form of an aggregate of small elements such as beads, may be fixed by sticking to the skin structurantel.

A resistive layer 7, formed by a perforated plate or a grid, or by a combination of these two elements, can be attached to the stiffeners 3, so as to encapsulate the porous material 5.

The stiffeners 3 can be fixed on the structuring skin 1 by brazing or riveting.

The resistive layer 7 can be fixed on the stiffeners 3 by gluing, brazing or welding.

As previously indicated, the porous material can be formed from commercially available metallic, polymeric, ceramic or composite materials.

The porous material is chosen according to the conditions of use of the acoustic panel.

The table below gives, by way of example, various types of foams that may be suitable as porous material for different conditions of use of the acoustic panel:

δ

In the particular case where the acoustic attenuation panel is intended to be installed in high temperature areas of an aircraft nacelle (particularly in the turbojet engine exhaust zone), it is expected that the porous material 5 is formed in a material capable of withstanding temperatures up to 800 ⁰ C: carbon foam may for example be suitable. Regarding the materials used for the other elements of the acoustic attenuation panel, namely the structuring skin 1, the stiffeners 3 and the resistive layer 7, the choice will be made according to the constraints of weight, temperature and mechanical stress.

As indicated above, these materials may be selected from metal alloys, ceramics, metal matrix composites (CMMs) and ceramic matrix composites (CMCs). The mode of operation of the benefits of the sound attenuation panel just described result directly from the foregoing explanations.

The structuring skin 1 is fixed against a wall of a nacelle element, such as an exhaust gas ejection nozzle.

The resistive layer 7 is in this way exposed to the sound excitation whose intensity is to be reduced.

The acoustic waves emitted by this sound source pass through the resistive layer 7 and penetrate inside the cavities of the porous material 5, which causes the reduction of the energy of these acoustic waves.

Several panels similar to the one shown in the attached figure can be joined edge to edge to cover the desired surface.

It is understood that the implementation of the porous material 5 between the structuring skins 1 and perforated 7, is much simpler and therefore less expensive than the implementation of a honeycomb structure.

This is particularly true in the case of an acoustic attenuation panel intended to be used in a high temperature zone: where it was necessary to use a metallic honeycomb structure fixed by brazing on a structuring skin and a metallic resistive layer, a simple placement of the porous material between these two skins achieves the desired result.

It should further be noted that the use of a porous material which is currently on the market, in itself reduces the manufacturing costs compared with the use of honeycomb honeycomb structure.

It should also be noted that the use of a porous material generally makes it possible to obtain a substantial reduction in weight compared to the use of a honeycomb structure, especially when the latter is metallic for high temperature applications.

Of course, the invention is not limited to the embodiment just described.

For example, it is possible to envisage an extremely simplified structure, comprising neither stiffeners 3 nor resistive layer 7: such a structure would therefore be formed solely by bonding a layer of porous material 5 on the structuring skin 1, as shown in Figure 1a.

One could consider the structuring skin 1 is placed on the side of the flow F of the exhaust gas, in which case this skin would include perforations 8 for acoustic absorption, as shown in Figure 1 ter.

In another simplified world of realization, one could predict the presence of stiffeners 3 without resistive layer 7: such a structure would be formed solely by the structuring skin 1 on which the stiffeners 3 would be attached and between which strips of porous material would be arranged 5 fixed by gluing on the skin 1.

It should be noted, however, that these simplified structures would not benefit from the protection function with respect to mechanical impacts, provided by the resistive layer 7.

Thus it is also possible to envisage that the porous material 5 is not homogeneous, but on the contrary has zones of different acoustic absorption characteristics.

These different zones may be zones of absence of porous material (cavities), and / or zones of porous materials of different natures (different densities of foams).

Such heterogeneity of the porous material 5 can be obtained by superposing layers of different porous materials in the thickness of the panel, and / or by juxtaposing loaves of porous materials in the direction of the average plane of the panel.

Such heterogeneity of the porous material 5 makes it possible to produce a custom acoustic absorption panel, that is to say perfectly adapted to the conditions (geometry, temperature, nature of the noise emissions, weight constraints, etc.) in which it is intended to be used.

By way of non-limiting example, there is shown in Figures 2 to 5 different variants envisaged panel layer of heterogeneous porous material.

In the example of FIG. 2, the layer of porous material 5 is provided with through cavities 9, the walls 11 of these cavities being substantially perpendicular to the average plane M of the acoustic panel. These cavities 9 may be made by perforation of the porous material 5, or by provision of breads of porous materials at regular intervals or not.

Note that the shape of these cavities 9 may be arbitrary: these cavities may be cylindrical, parallelepipedic, or may have a scalable section in the thickness of the panel.

In the variant of Figure 3, the walls 11 of the cavities 9 are inclined relative to the average plane M of the panel.

In the variant of FIG. 4, the cavities 9 are blind, that is to say that they open only on one side of the panel: on the side of the structuring skin 1 (cavities 9a) or of the layer resistive 7 (cavities 9b, 9c).

In the variant of Figure 5, the layer of porous material 5 is in fact formed of a superposition of porous material layers 5a, 5b of different characteristics, in the direction of the thickness of the panel.

It should be noted that the number of superposed layers is not limited, and that each layer may itself consist of several densities of foams, in order to produce a distributed treatment.

In a particular variant (not shown), it is conceivable to place an intermediate layer (solid or recessed) between the two attenuation layers 5a, 5b, to act as septum or wedges in order to control the play of these layers. 5a, 5b with respectively the structuring skin 1 and the resistive layer 7.

Two examples of application of panels in accordance with the above will now be described.

In these examples, the panels are placed in relatively hot areas: temperatures up to 400 ⁰ C in the first example, and temperatures up to 800 ⁰ C in the second example.

Referring to FIG. 6, an air intake structure of an aircraft turbojet engine nacelle corresponding to the zone V1 of FIG. 8 can be seen.

As is known per se, such an air intake structure 13 comprises an outer panel 15, that is to say located at the outer periphery of the nacelle, and an air intake lip. 17, forming the leading edge of the nacelle, and located in the extension of an annular inner portion 18, often referred to by the term "ferrule", this ferrule may have sound absorption properties. In the operating situation, the air flow F runs along the lip 17 and the shell 18 before passing inside the engine 19 (see FIG. 8) disposed inside the nacelle.

In what follows, the terms "upstream" and "downstream" must be understood in relation to the direction of air circulation, as indicated by the arrow F.

The air intake structure 13 may be of the type in which the air intake lip 17 and the outer panel 15 form a one-piece assembly, able to slide relative to the shell 18 during maintenance operations, such as this is taught for example in document FR 2 906 568: in this case, it is commonly referred to as a "CFL" structure, that is to say "Laminar Forward Cowl".

Note however that the invention is not limited to this particular type of air intake structure.

Inside the air intake lip 17 is a hot air collector 21 of substantially annular shape, powered by at least one hot air supply duct 23 itself in relation to the zones hot engine 19.

The hot air distributed by the collector 21 inside the air intake lip 17 makes it possible to defrost this lip.

An internal partition 25 closes the defrost compartment 26, and thus prevent hot air from escaping into other areas of the air intake structure.

In order to reduce noise emissions from the nacelle, the air inlet lip 17 is equipped with an acoustic attenuation panel P in accordance with the foregoing.

More specifically, the skin of the lip 17 forms the structuring skin 1 of this panel P, which is provided with perforations 8.

Inside this structuring skin 1 is the porous material 5, fixed by an upstream retaining plate 27 and by a downstream retaining plate 29.

The internal partition 25 has a return 31 which is preferably riveted to the downstream plate 29.

At its other end 32, the internal partition 25 is riveted inside the outer panel 15. Given the high temperatures reigning inside the defrosting compartment, the porous material P acoustic attenuation panel is selected so as to withstand temperatures up to 400 ⁰ C.

Furthermore, it will be ensured that this porous material has a high heat conductivity so as to allow the heat of the hot air inside the deicing compartment 26 to radiate to the surface of the lip. air inlet 17, thus enabling effective defrosting.

In the variant shown in Figure 7, there is a panel P1 similar to the panel P of the variant of Figure 6, downstream of which is a P2 panel according to the invention, and the porous material 5 is chosen so to withstand a temperature up to 120 ⁰ C.

Between these two panels P1 and P2 is a substantially annular shaped seal 33, preferably formed of a porous material capable of withstanding temperatures up to 400 ⁰ C.

As can be seen in FIG. 7, the seal 33 and the acoustic attenuation panel P2 are situated downstream of the internal partition 25. More precisely, a sheet 35 may cover the downstream part of the panel P1, the seal 33 and the P2 panel, the return 31 of the inner partition 25 being fixed preferably by riveting on the downstream part of the sheet 35.

In the particular case where the air intake structure 13 is of the aforementioned "LFC" type, it is possible to provide centering members 37 fixed to the sheet 35, making it possible to center the air intake structure 13 relative to to the ferrule 18.

As in the case of FIG. 6, the skin of the air intake lip 17 forms the structuring skin of the panels P1 and P2, this structuring skin being provided with perforations 8.

Of course, it is possible to choose different acoustic properties for each of the panels P1 and P2, and it is possible to form all the panels P, P1, P2 according to the precepts of the embodiments of FIGS. 2 to 5 in particular (porous material formed of the juxtaposition and / or the superposition of foam loaves, possibly provided with cavities).

Of course, it is also possible to envisage replacing the acoustic attenuation panel P2 made of porous material in accordance with the invention with a conventional acoustic attenuation panel, of the type comprising a honeycomb structure: the zone in which it is located the P2 panel being significantly less hot than the area in which the P1 panel, the use of a conventional acoustic attenuation panel is indeed possible.

It will also be noted that, for the seal 33, a porous material having a low heat conductivity is preferably chosen, so as to isolate the panel P2 correctly from the panel P1: a ceramic foam could, for example, be suitable for this seal.

We will now refer to Figures 8 to 13, which illustrates a second example of application of an acoustic panel of the invention.

FIG. 8 shows a nacelle 39 of the prior art, surrounding a turbofan engine including engine 19.

As is known per se, the air intake structure 13 of this nacelle makes it possible to capture an air flow F coming from the outside, which passes inside the fan of the turbojet and is divided into a cold air flow FF flowing to the periphery of the engine 19, and a flow of hot air FC circulating inside this engine.

More specifically, the circulation flow of the cold flow FF is delimited on the one hand by an outer structure 45 of the nacelle 39, and on the other hand by a fixed internal structure 47 (often referred to as "IFS" (for "Inner Fixed Structure "), which carries out the fairing of the engine 19.

In order to reduce the acoustic emissions inherent in the circulation of this cold flow, acoustical attenuation panels 49 are conventionally placed at the periphery of the fixed internal structure 47.

These conventional acoustic panels 49 are generally of the honeycomb structure type, and to prevent their destruction by the heat emitted by the engine 19, thermal protection mattresses 50 are conventionally used which are placed on the inner face of the acoustic panels 49, that is to say on the face of these acoustic panels which is opposite the engine 19.

Indeed, in zones Z1, 22, Z3 shown in FIG. 8, typically corresponding to the zones of compression, combustion and expansion of the engine 19, the temperatures can typically and respectively be between 120 and 150 ^° C., 150 and 400 ° C, and 400 and 800 ⁰ C. Under these conditions, the use of an acoustic attenuation panel according to the invention, with a porous material capable of withstanding high temperatures, that is to say up to 800 ^° C., is particularly indicated.

FIGS. 9 to 13 show different ways of integrating one or more acoustic attenuation panels according to the invention into the fixed internal structure 47, the latter being generally formed of composite material, typically based on carbon fibers. .

In the variant shown in Figure 9, the wall of the fixed inner structure 47 has perforations 8, and a porous material 5 adapted to high temperatures, that is to say up to 800 ⁰ C, is fixed on the internal face of the fixed internal structure 47, that is to say on the face of this internal structure which is opposite the motor 43.

In the variant shown in FIG. 10, it can be noted that the porous material 5 is maintained by an upstream return and by a downstream return 53, preferably formed in the same material as the wall of the fixed internal structure 47.

In the variant of FIG. 11, the wall of the fixed internal structure 47 defines a recessed zone 1, inside which the porous material 5 is located, which is therefore directly exposed to the circulation of the cold flow FF ( see Figure 8).

The variant of FIG. 12 differs from that of FIG. 11 in that a resistive layer 7 of the type indicated above covers the porous material 5 in the area of the fixed internal structure 47 that is most exposed to erosion caused by the flow of cold flow FF, that is to say in the case in the upstream zone of this porous material.

In the variant shown in FIG. 13, it can be seen that the downstream portion of the porous material 5 is partially covered by an advance 47 'of the wall forming the fixed internal structure 47.

For this particular example of application of an acoustic attenuation panel according to the invention, a porous material having a good heat conductivity will be chosen, so as to allow the heat emitted by the motor 19 to escape. towards the cold flow FF.

Claims

1. A sound attenuation panel for an aircraft engine nacelle comprising a structuring skin (1) and, as acoustic absorption material, a porous material (5) attached to this skin (1).

2. Panel according to claim 1, wherein the structure of said porous material (5) is selected from the group consisting of foams, expanded materials, felts, aggregates of small elements.

3. Panel according to one of claims 1 or 2, wherein the material forming said porous material (5) is selected from the group consisting of metal materials, polymers, ceramics or composites.

4. Panel according to any one of the preceding claims, wherein said porous material (5) is selected from the group consisting of materials resistant to temperatures up to 200 ⁰ C, materials resistant to temperatures up to 400 ^° C., materials resistant to temperatures up to 600 ^° C., and materials resistant to temperatures up to 800 ^° C.

.

5. Panel according to claim 4, wherein the material forming said porous material (5) is selected from the group consisting of metallic or ceramic materials.

The panel of claim 5, wherein said ceramic material is carbon foam.

7. Panel according to any one of the preceding claims, wherein said porous material (5) is bonded to said structuring skin (1).

8. Panel according to any one of the preceding claims, wherein said structuring skin comprises perforations

(8).

9. Panel according to any one of the preceding claims, wherein stiffeners (3) are fixed on said structuring skin (1).

10. Panel according to claim 9, comprising a resistive layer (7) attached (e) on the stiffeners (3).

11. Panel according to claim 10, wherein said resistive layer (7) comprises a mesh or perforated skin, or a combination of these two elements.

12. Panel according to any one of the preceding claims, wherein said structuring skin (1) and / or said stiffeners (3) and / or said resistive layer (7) are formed in materials selected from the group consisting of metal alloys. , ceramics, metal matrix composites, ceramic matrix composites.

13. Panel according to any one of the preceding claims, wherein said porous material comprises cavities (9).

14. Panel according to claim 13, characterized in that at least a portion of said cavities (9) are through.

15. Panel according to one of claims 13 or 14, characterized in that at least a portion of said cavities (9a, 9b, 9c) are blind.

16. Panel according to any one of claims 13 to 15, characterized in that at least a portion of said recesses (9) have walls (11) oriented substantially perpendicular to the mean plane (M) of said panel.

17. Panel according to any one of claims 13 to 16, characterized in that at least a portion of said cavities (9) have walls inclined relative to the mean plane (M) of said panel.

18. Panel according to any one of the preceding claims, characterized in that said porous material (5) is formed of a superposition of layers (5a, 5b) of porous materials of different characteristics, in the direction of the thickness of the sign.

19. Panel according to any one of the preceding claims, characterized in that said porous material (5) is formed of a juxtaposition of breads of porous materials of different characteristics, in the direction parallel to the mean plane of the panel.

20. Air intake structure (13) nacelle (39) of aircraft turbojet engine, characterized in that it comprises an air intake lip (17) provided with at least a first panel d acoustic attenuation according to any one of the preceding claims.

21. Air intake structure (13) according to claim 20, characterized in that it comprises a pneumatic deicing compartment (26) delimited in particular by said lip (17) and by an internal partition (25), and in that said first acoustic attenuation panel (P; P1) is of the type comprising a porous open-cell material suitable for withstand a temperature of up to 400 ⁰ C and having a high conductivity of heat.

Air intake structure (13) according to claim 21, characterized in that said first acoustic attenuation panel (P) is fixed inside said air intake lip (17) by an upstream retaining plate (27) and a downstream retaining plate (29), and in that said internal partition (25) is fixed on said downstream retaining plate (29), preferably by riveting.

23. Air intake structure (13) according to one of claims 21 or 22, characterized in that it comprises a second acoustic attenuation panel (P2) fixed inside said inlet lip of air (17) downstream of said internal partition (25), separated from said first panel (P1) by a seal (33) of porous open cell material able to withstand a temperature of up to 400 ⁰ C and having a low heat conductivity.

The air inlet structure (13) according to claim 23, characterized in that said second acoustic attenuation panel (P2) is selected from the group consisting of a porous material and open cell panel in accordance with which preceding, able to withstand a temperature of up to 120 ⁰ C, and a honeycomb structural panel.

25. Air intake structure (13) according to one of claims 23 or 24, characterized in that said first panel (P1), said seal of porous material (33) and said second panel (P2) are covered a common sheet (35) on which said internal partition (25) is fixed, preferably by riveting.

26. Air intake structure (13) according to any one of claims 20 to 25, characterized in that it is of the type in which the air inlet lip (17) forms a one-piece assembly with the outer wall (15) of the air inlet structure (13), this one-piece assembly being slidable relative to the fan casing of the turbojet engine

27. Air intake structure (13) according to claim 26, characterized in that it comprises centering members (37) fixed on said common sheet (35).

28. Fixed internal structure (47) of an aircraft turbojet engine nacelle, characterized in that it comprises at least one panel acoustic attenuation device (49) according to any one of claims 1 to 19.

29. fixed internal structure (47) according to claim 28, characterized in that said sound attenuation panel is located at least partly in the zone of said fixed internal structure intended to be subjected to high temperatures generated by said turbojet, and the porous material of this panel is of the open cell type and is able to withstand a temperature of up to 800 ^° C. and has a high conductivity of heat.

30. fixed internal structure (47) according to claim 29, characterized in that said porous material is on the inner face of said fixed internal structure (47), the latter being provided with perforations (8) on at least a portion of its surface covering said porous material.

31. Fixed internal structure (47) according to claim 30, characterized in that said porous material is maintained by returns (51, 53) formed in said fixed internal structure (47).

32. Fixed internal structure (47) according to claim 29, characterized in that said porous material is on the outer face of said fixed internal structure (47), inside a recessed area formed inside. of this structure.

33. Fixed internal structure according to claim 32, characterized in that said porous material is covered, at least in the upstream part, by a perforated resistive layer (7).

34. Fixed internal structure according to claim 33, characterized in that said resistive layer (7) is formed in the same material as that of the fixed internal structure.

35. Aircraft engine nacelle equipped with at least one acoustic panel according to any one of claims 1 to 19.