Improved method for repairing an aircraft acoustic structure

Abstract

Improved method for repairing an aircraft acoustic structure Method for repairing an aircraft acoustic structure (10), having a honeycomb structure comprising acoustic cells (16), the method comprising detecting (S1) an impacted area (R) of the acoustic structure (10), machining (S2) the acoustic structure (10) on a surface encompassing the impacted area (R) to remove a portion (P) of the acoustic structure (10), producing (S3) a model of said portion (P) of the acoustic structure removed by machining, comprising determining a geometry of said removed portion (P), manufacturing (S4) an insert (50) from the model produced in the preceding step (S3), the insert (50) having the geometry of said removed portion (P) and comprising a plurality of replacement acoustic cells (56), fixing (S5) the insert (50) to the acoustic structure (10),comprising the insertion (S52) of the insert (50) in place of the portion (P) of the acoustic structure (10) removed in the machining step (S2). Figure for the abstract: Fig. 7.,

Description

Title of the invention: Improved method for repairing an aircraft acoustic structure Technical field

[0001] The present disclosure relates to the field of acoustic structures, in particular acoustic panels used in aircraft propulsion systems. More specifically, the present disclosure relates to a method for repairing an acoustic structure. Prior art

[0002] In the aeronautical industry, it is common to use laminated parts, particularly made of thermoplastic matrix composite. For example, in the context of insulating aircraft engines and reducing the noise emitted by these engines, current solutions use acoustic panels, known as "sandwich panels", using such materials. However, the nacelles, the fuselage or the interior of the cabin can also be equipped with these acoustic panels.

[0003] The structure of these panels consists of a stack of skins and honeycomb structures, also called acoustic complexes. These structures can comprise a single level of honeycombs (or acoustic complex), in which case we speak of an "SDOF" structure (from the English "single degree offreedom") or two levels of honeycombs, in which case we speak of a "2DOF" structure (from the English "2 degrees offreedom"). In a single-level honeycomb structure, for example, one of the skins is porous to sound (the acoustic skin), and the other is hermetic (the closing skin), the whole forming a Helmholtz cell (or resonator).

[0004] [Fig. 1] represents a partial perspective view of an acoustic panel 10 “SDOF” comprising an acoustic skin 12, a closing skin 14, and an acoustic complex 16, which is a core with a honeycomb structure sandwiched between these two skins. The acoustic complex 16 is made up of a network of honeycomb-shaped cells 18, in this example with a hexagonal base. The acoustic skin 12 is perforated by a plurality of orifices 20, each orifice 20 opening onto a cell 18 of the acoustic complex 16, several orifices 20 being able to open onto the same cell 18.

[0005] In the past, the acoustic behavior of these structures was adjusted by adapting the height of the honeycomb. In particular, the higher the cells, the more the low frequencies are attenuated.

[0006] The so-called new generation acoustic structures now make it possible to adjust the acoustic behavior of these structures by other means. By For example, a known solution for targeting the right frequencies while limiting the size consists of adding a conical element in each cell, the top of the cone having an orifice. [Fig.2] represents a sectional view of a new generation acoustic panel 10, comprising acoustic cones 17 with a wide inlet into which the orifices 20 open, and a narrower outlet.

[0007] Other solutions consist of creating labyrinth-type structures to extend the projected cavity length (S-shaped structures, spirals, channels), or creating lattice structures. Documents EP3775524 and FR3074444 describe examples of such new generation acoustic structures.

[0008] However, these acoustic panels may be subject to impacts leading to their degradation, and consequently to the degradation of their acoustic properties. These impacts may be due to tool handling during maintenance operations for example, or impacts during operation, by ingestion of foreign bodies in the engine for example. An impact during operation occurs mainly on the acoustic skin in contact with the air flow vein. An impact due to the fall of a tool during maintenance operations may occur both on the acoustic skin and on the closing skin.

[0009] Currently, the solutions for repairing a damaged area, particularly by impact, consist of machining (known as a trepanning operation) the impacted and therefore damaged part of the acoustic structure and filling the void (typically cylindrical in shape) thus left with densifying resin, an operation commonly referred to as "potting". However, this repair does not restore the acoustic performance in the area concerned. Over time, and with the accumulation of successive repairs, the acoustic performance of the acoustic panel is further degraded. This drawback is even more critical on new generation acoustic structures.

[0010] There is therefore a need for a method of repairing aircraft acoustic structures which makes it possible to at least partially overcome the aforementioned drawbacks. Description of the invention

[0011] The present disclosure relates to a method of repairing an aircraft acoustic structure, the acoustic structure having a honeycomb structure comprising a plurality of acoustic cells extending in a thickness direction of the acoustic structure, the method comprising: - detection of an impacted area of ​​the acoustic structure, - machining the acoustic structure on a surface encompassing the impacted area, and on at least part of the thickness of the acoustic structure, so as to remove a portion of the acoustic structure, - producing a model of said portion of the acoustic structure removed by machining, comprising determining a geometry of said removed portion, - manufacturing an insert from the model produced in the previous step, the insert having the geometry of said removed portion and comprising a plurality of replacement acoustic cells, - fixing the insert to the acoustic structure, including inserting the insert in place of the portion of the acoustic structure removed during the machining stage.

[0012] It is understood that the impacted area corresponds to the point of impact of the foreign body having damaged the acoustic structure, as well as to the area of ​​propagation of the degradation from this point of impact. By way of comparison, the impact of an object on a window or a windshield causes degradation at the point of impact and on a region surrounding this point of impact, by the propagation of cracks.

[0013] It is therefore understood that machining the acoustic structure on a surface encompassing the impacted area makes it possible to remove the entire area of ​​the acoustic structure damaged by an impact. The machining can be carried out by milling the acoustic structure on the impacted area, leaving a void, that is, a hollow in the acoustic structure, at the location where the impacted area was.

[0014] The production of a model, typically a digital model, of the portion of the acoustic structure removed by machining, in other words of the hollow formed in the acoustic structure following machining, then the manufacture of an insert from this model, makes it possible to obtain a part having the shape and geometry of the portion of the acoustic structure removed by machining. In particular, the insert thus obtained comprises acoustic cells similar to the cells initially present in the acoustic structure, before machining.

[0015] The method according to the invention thus makes it possible to repair an acoustic structure damaged by an impact, while maintaining the acoustic performance in the area concerned, and consequently over the entire structure. It is in particular possible to manufacture an insert having cells each comprising an acoustic cone for example, which makes it possible to maintain the acoustic performance of new generation acoustic structures.

[0016] In some embodiments, the insert is manufactured by additive manufacturing.

[0017] In some embodiments, making the model of the portion of the acoustic structure removed by machining comprises determining an external geometry of said portion comprising the dimensions and shape of the exterior surfaces of said portion, then modifying the model to add the plurality of replacement acoustic cells from said external geometry.

[0018] In some embodiments, determining the external geometry of the portion of the acoustic structure removed by machining comprises a 3D scan (three di- (mensions) of said removed portion.

[0019] In some embodiments, the insert comprises a belt comprising a wall defining a contour of the insert, the replacement acoustic cells being disposed within the belt.

[0020] In some embodiments, the belt has a honeycomb or lattice structure.

[0021] In some embodiments, the acoustic structure is an acoustic panel comprising a solid closure skin and a perforated acoustic skin, the acoustic cells extending between the closure skin and the acoustic skin, the manufacture of the insert comprising the manufacture of a replacement acoustic skin and / or a replacement closure skin.

[0022] In some embodiments, the acoustic panel includes, in each acoustic cell, an acoustic cone, the fabricated insert including a plurality of replacement acoustic cells and a replacement acoustic cone in each thereof.

[0023] In some embodiments, during the manufacture of the insert, when the impacted area of ​​the acoustic panel is the acoustic skin, the replacement acoustic skin is manufactured first on a support, the replacement acoustic cells being manufactured on the replacement acoustic skin.

[0024] In some embodiments, the fixing of the insert comprises, before the insertion of the insert in place of the portion of the acoustic structure removed in the machining step, the application of an adhesive to a contour of the insert, then, after the insertion of the insert, a heat treatment making it possible to solidify the adhesive.

[0025] In some embodiments, the method comprises, after attaching the insert to the acoustic structure, applying at least one retaining ply by draping it over the acoustic structure, such that the at least one retaining ply covers both the insert and a portion of the surface of the acoustic structure extending around the insert.

[0026] In some embodiments, the method comprises, after applying the retaining ply, machining the retaining ply on said portion of the surface of the acoustic structure extending around the insert.

[0027] In some embodiments, the model of the portion of the acoustic structure removed by machining is made such that, after the manufacture of the insert and its attachment to the acoustic structure, a first thickness of the acoustic structure at the insert is less than a second thickness on the remainder of the acoustic structure, the difference between the first and the second thickness being substantially equal to a thickness of the at least one retaining ply draped over the acoustic structure.

[0028] In certain embodiments, the insert has a circular shape with a diameter of between 3 and 20 cm. Brief description of the drawings

[0029] The invention and its advantages will be better understood upon reading the detailed description given below of different embodiments of the invention given as non-limiting examples. This description refers to the appended pages of figures, in which:

[0030] [Fig-1] [Fig. 1] represents a partial perspective view of an acoustic panel according to the prior art,

[0031] [Fig.2] [Fig.2] schematically represents a side and sectional view of a new generation acoustic complex, comprising acoustic cones,

[0032] [Fig.3] [Fig.3] represents a sectional view of a turbojet engine comprising acoustic panels repaired by a method according to the invention, in a longitudinal plane of the turbojet engine,

[0033] [Fig.4] [Fig.4] schematically represents a perspective view of a portion of an impacted acoustic panel, in a first step of a repair method according to the invention,

[0034] [Fig.5] [Fig.5] schematically represents a perspective view of the portion of the acoustic panel of [Fig.4], in a machining step of the repair method according to the invention,

[0035] [Fig.6] [Fig.6] schematically represents a perspective view of an insert obtained by a manufacturing step of the repair method according to the invention,

[0036] [Fig.7] [Fig.7] schematically represents a perspective view of the portion of the acoustic panel of [Fig.5], in a fixing step of the repair method according to the invention,

[0037] [Fig.8] [Fig.8] schematically represents a perspective view of the portion of the acoustic panel of [Fig.7], at the end of the repair method according to the invention,

[0038] [Fig.9] [Fig.9] schematically represents the different stages of the repair process according to the invention. Description of the embodiments

[0039] In [Fig. 3] is shown a sectional view of a turbojet engine 1 comprising an acoustic structure, in this example an acoustic panel 10, in a longitudinal plane of the turbojet engine 1. The turbojet engine 1 comprises a nacelle 2, an intermediate casing 3 and an internal casing 4. The nacelle 2 and the two casings 3 and 4 are coaxial. The nacelle 2 defines at a first end an inlet channel 5 for a fluid flow and at a second end, opposite the first end, an exhaust channel 6 for a fluid flow. The nacelle 2 and the inter casing The intermediate casing 3 delimit between them a primary fluid flow vein 7. The intermediate casing 3 and the internal casing 4 delimit between them a secondary fluid flow vein 8. The primary vein 7 and the secondary vein 8 are arranged in an axial direction of the turbojet between the inlet channel 5 and the exhaust channel 6.

[0040] The turbojet engine 1 further comprises a fan 9 configured to deliver an air flow F as a fluid flow, the air flow F being divided at the outlet of the fan into a primary flow Fp circulating in the primary duct 7 and into a secondary flow Fs circulating in the secondary duct 8. The turbojet engine 1 further comprises at least one acoustic panel 10 as described previously with reference to FIGS. 1 and 2, configured to attenuate the acoustic waves emitted by the turbojet engine before these waves escape radially outside the nacelle 2 of the turbojet engine 1. The acoustic panel 10 is configured to attenuate acoustic waves whose frequency belongs to a predetermined frequency range. In the embodiment illustrated in [Fig. 3], the panel 10 can be integrated with the intermediate casing 3, the internal casing 4, and the nacelle 2.

[0041] During its operation, the acoustic panel 10 may be exposed to impacts of different natures depending on its position in the turbojet engine 1 and depending on the context. During maintenance operations for example, tools may fall onto one or other of the acoustic or closing skins 12, 14. During its operation, moreover, the turbojet engine 1 may ingest foreign bodies which impact the acoustic skin 12, which is the skin in contact with the air flows in the vein. Whatever the origin, these impacts may degrade the structure of the acoustic panel 10, and therefore its properties.

[0042] [Fig. 4] represents such an impact on the acoustic skin 12 of an acoustic panel 10. Such an impact includes an impact point 30, which is the precise point where the object (tool or other foreign bodies) impacts the panel 10, as well as the cracks 32 propagating from this impact point 30.

[0043] To repair the acoustic panel 10 thus damaged, without losing or degrading the acoustic properties thereof, a method according to the invention is described as follows, with reference to FIGS. 4 to 9.

[0044] It will be noted that, although the following description is based on a single acoustic panel (“SDOF”), the method according to the invention also applies to two-story honeycomb structures (“2DOF”), and to all types of acoustic structures having acoustic functions, in particular nacelle parts such as the air intake, the internal or external structure, etc.

[0045] In a non-limiting manner, the acoustic cells are hexagonal mesh, their circumscribed diameter being typically 3 / 8”, 1 / 2”, 'A” and up to 1”. The height of the 10 single acoustic panel, in other words its thickness, is typically between 10 and 30 mm.

[0046] Furthermore, the following example relates to a new generation acoustic panel 10 comprising acoustic cones 17. This example is not limiting, the invention also applying to all types of acoustic panels, in particular new generation, having for example labyrinth-type structures (S-shaped structures, spirals, channels), or lattice structures. Furthermore, in order to simplify the description, the example illustrated is a flat acoustic panel 10. The invention, however, applies to structures of more complex geometries, having at least one curvature.

[0047] Furthermore, in the following example, the damaged part is the acoustic skin 12, on which the orifices 20 are not shown in order to simplify the description. However, this example could also apply to the opposite closing skin 14.

[0048] A first step of the method (step S1) comprises the detection of an impacted zone R of the acoustic structure, here of the acoustic panel 10. [Fig. 4] represents a perspective view of a portion of the damaged acoustic panel 10, and comprising said impacted zone R. Typically, but in a non-limiting manner, the impacted zone R is a circle comprising the point of impact 30, and all of the cracks 32 propagating from this point of impact 30. It is therefore understood that the dimensions of the impacted zone R considered must be sufficiently large to encompass all of the damage caused by the impact, namely the point of impact 30 and the cracks 32.

[0049] It will also be noted that the damage may be visible (in particular the cracks 32), and also barely or not at all visible, for example in the case of delamination phenomena between the upper layer of the skin 12 and the non-visible core of the skin 12, due to the impact. It is therefore understood that the detection of the impacted zone R includes both visible damage and non-visible damage. Such detection, in particular for non-visible damage, may be carried out by ultrasonic testing, or by a tap test to detect defects and anomalies of the skin 12.

[0050] The next step (step S2) comprises the machining of the acoustic panel 10 ([Fig.5]). Typically, this machining is a trepanning operation using a milling cutter 40 which removes material from the acoustic panel 10 at the impacted zone R detected in the previous step.

[0051] More precisely, the machined surface encompasses the impacted zone R, and is preferably at least equal to the surface area of ​​said impacted zone R, or even slightly greater. In addition, the material of the acoustic panel 10 is removed over a certain thickness of the panel 10, considered in the thickness direction Z, which is the stacking direction of the different layers (skins 12, 14 and the acoustic complex 16 comprising the cells) of the panel 10.

[0052] Typically, this thickness is equal to the added thicknesses of the acoustic skin 12 and the acoustic complex 16. In other words, the acoustic panel 10 is machined up to the opposite skin, here the closing skin 14, without machining the latter. Alternatively, the acoustic panel 10 can be machined over its entire thickness, including the closing skin 14. This machining makes it possible to remove the portion P of the acoustic panel 10 which has been damaged. This portion P is typically circular, in order to facilitate the machining operation by means of the milling cutter 40, and corresponds to the volume of material removed by machining.

[0053] The following step (step S3) comprises the production of a model, in particular a digital model, of the portion P of the acoustic panel 10 removed by machining in step S2. This step S3 comprises the determination of a geometry of said removed portion P.

[0054] More precisely, step S3 comprises on the one hand (step S31) the determination of an external geometry of the portion P (step S31), and on the other hand the modification of the model to add the acoustic complex 16 comprising the plurality of acoustic alveoli (step S32).

[0055] By "external geometry" is meant the overall external shape of the portion P, including the dimensions and the shape of the external surfaces of said portion P, without the acoustic part. In the example described relating to a flat acoustic panel 10, the external geometry of the removed portion P is a cylinder having a given diameter (typically the diameter of the impacted zone R) and a given thickness (typically the thickness of the panel 10, less the thickness of the closing skin 14)

[0056] In the case of a part of simple geometry, as in the present example, or for a part having a simple curvature, the determination of the external geometry can be carried out from the known three-dimensional drawing (3D, in other words the "CAD", for "computer-aided design" of the part). It is sufficient to model a geometry taking the diameter of the machined zone, in particular the diameter of the impacted zone R.

[0057] Nevertheless, preferably, for more complex geometries of acoustic panel 10 and impacted zone R, the determination of the external geometry can be carried out from a 3D scan of the portion P. Such a scan can for example be carried out by known technologies of structured light projection or stereo-correlation.

[0058] After determining the external geometry (step S31), step S32 of modifying the model consists of modeling an acoustic complex analogous to the complex acoustic 16 of the panel 10, from this external geometry, that is to say respecting the limits, the shape and the dimensions of the external geometry. In particular, the modeled acoustic complex comprises replacement cells 56 and replacement acoustic cones 57.

[0059] The next step (step S4) comprises the manufacture of an insert 50 from the model produced in the previous step S3, having the geometry of the portion P removed by machining.

[0060] The insert 50 is preferably manufactured by additive manufacturing. This manufacturing can include all types of 3D printing such as filament deposition known as “FFF” (for “Fused Filament Fabrication” in English) or “FDM” (for “Filament Deposit Molding” in English), powder sintering “SLS” (for “Selective Laser Sintering” in English), or photopolymerization.

[0061] The materials used are preferably compatible with the constraints of the application, in particular environmental temperatures and exposure to hot fluids. For example, the filaments deposited during 3D printing are preferably made of polymer, for example polyetherimide (PEI), polyphenylene sulfide (PPS), or the polyaryletherketone family (PAEK).

[0062] The use of an additive manufacturing process is made possible by the fact that the manufactured part is small, the manufactured insert 50 typically having a diameter of between 3 and 20 cm. This process is all the more advantageous as it allows the shape of the insert 50 to be adapted on a case-by-case basis, its shape being unique and specific to each repair operation following an impact.

[0063] Indeed, additive manufacturing is generally not very suitable for directly manufacturing new generation acoustic structures in their entirety, because it is not very suitable for manufacturing large uniform surfaces, in particular because of the long printing times. On the other hand, for small parts such as the insert 50, having a specific shape defined on a case-by-case basis according to the impact and degradation conditions, printing by additive manufacturing is suitable, and advantageous because it can be carried out on site, and it constitutes a flexible solution which allows a local and specific solution.

[0064] It will be noted that this additive manufacturing process for insert 50 can also be used for old-generation acoustic panels, although it is also possible in this case to use conventional honeycomb inserts, not manufactured by additive manufacturing, machined and then glued to the panel.

[0065] [Fig.6] represents in perspective an insert 50 obtained at the end of step S4 by fa additive manufacturing, from the model produced in step S3. It is understood that the insert 50 corresponds substantially to the negative of the portion P removed by machining, and that it has substantially the same shapes and dimensions as the latter.

[0066] It will be noted that the insert 50 shown in [Fig.6] also makes it possible to illustrate the model digital of the insert 50, produced in the previous step S3, their shapes and geometries being identical.

[0067] It is understood that in [Fig. 6], the visible face of the insert 50 corresponds to the lower side, intended to come to bear on the closing skin 14, and the visible part of the replacement acoustic cones 57 is the narrower end of said cones. It will be noted in this regard that the terms “lower” or “upper” are considered according to the thickness direction Z, in other words according to the vertical direction in the reference frame of the figures (the acoustic skin 12 then being the upper skin, and the closing skin 14 being the lower skin).

[0068] The insert 50 comprises a belt 53, also modeled in step S3 of producing the digital model, the belt 53 comprising a wall inside which the replacement cells 56 and the replacement acoustic cones 57 are manufactured and extend.

[0069] In the illustrated example, the wall of the belt 53 has a solid structure. However, said wall of the belt 53 could have a honeycomb or lattice structure, making it easier to anchor in the next fixing step and allowing drainage of fluids with the rest of the undamaged structure of the acoustic panel 10.

[0070] On the upper side of the insert 50 (not visible in [Fig.6]), the acoustic skin can either be manufactured at the same time as the rest of the insert 50 (and therefore also added to the model in step S3), or added later by draping during the assembly described later. It will be noted that when the acoustic panel 10 is machined over its entire thickness, including the closing skin 14, the latter can also be manufactured at the same time as the rest of the insert 50 (and therefore also added to the model in step S3).

[0071] It is therefore understood that the insert 50 may comprise a replacement acoustic skin 52. In this case, during the manufacture of the insert 50, the replacement acoustic skin 52 is manufactured first on a support (not shown), and the replacement acoustic cells 56 are manufactured on the replacement acoustic skin. This makes it possible to reduce the roughness of the surface of the replacement acoustic skin, and therefore of the acoustic panel 10 thus repaired, which makes it possible to improve the aerodynamics of the surface.

[0072] Furthermore, the orifices 20 may be provided during the manufacture of the insert 50, or drilled subsequently after the assembly described below.

[0073] The next step (step S5, [Fig.7]) comprises fixing the insert 50, manufactured in step S4, on the acoustic panel 10, in place of the portion P of the acoustic panel 10 removed in the machining step S2. This step S5 may comprise firstly the application of an adhesive 60 on the insert 50 (step S51), the insertion of the insert 50 into the housing of the panel 10, corresponding to the portion P, left at the end from machining step S2 (step S52), then a heat treatment to crosslink and harden the glue (step S53).

[0074] In step S51, the glue 60 is applied to the walls of the insert 50 intended to come into contact with the acoustic panel 10, in particular the contour of the insert 50 on the belt 53, and its lower wall. The glue 60 may be in paste or film form, for example epoxy resin.

[0075] In step S52, the insert 50 is inserted into the acoustic panel 10 in place of the housing, corresponding to the portion P, formed by the machining of the panel 10, until it comes into abutment against the bottom, that is to say against the closing skin 14, and thus fills the void left by the machining. The heat treatment carried out in step S53 consists, for example, of placing the acoustic panel 10 thus reconstituted in an oven, in order to harden the glue 60.

[0076] The part thus obtained is shown in [Fig.8]. The circle on the acoustic skin 12 represents the junction between the insert 50 and the rest of the acoustic panel 10. The acoustic panel 10 thus reconstituted and repaired comprises an acoustic complex 16, including at the level of the repaired zone, that is to say at the level of the insert 50 thanks to the presence of the replacement acoustic cells 56 and the replacement acoustic cones 57. The acoustic properties of the acoustic panel 10 are thus preserved, despite the repair to which it has been subjected.

[0077] It will be noted that, as mentioned above, a replacement acoustic skin 52 can be provided during the production of the model (step S3) and the manufacture of the insert 50 (step S4), or added after the fixing of the insert 50 on the acoustic panel 10 (step S5), by automated draping of a thermoplastic or thermosetting matrix composite.

[0078] This draping can be carried out by deposition tools (not shown) known per se, such as so-called "AFP" robots, for "automated fiber placement" in English, successively depositing wicks, or thermoplastic or thermosetting pre-impregnated strips, parallel to each other and on several layers, called "plies", or even by "ATL" for "automated tape layer" in English, by depositing pre-impregnated sheets of greater width than the strips deposited in the "AFP" technique. These sheets are deposited one after the other. The skin is then polymerized in an autoclave for several hours.

[0079] Furthermore, after step S5 of inserting and fixing the insert 50 on the acoustic panel 10, one or more additional retaining plies (not shown) may be applied to the acoustic panel 10, such that the retaining plies cover both the insert 50 and a portion of the surface of the acoustic panel 10 extending around the insert 50. This makes it possible to improve the retention and mechanical performance of the repair.

[0080] In this case, after the application of the retaining fold(s), the latter can be machined at the level of the portion of the surface of the acoustic panel 10 surrounding the insert 50. This makes it possible to eliminate the excess thickness generated by these retaining folds.

[0081] Furthermore, when these retaining folds are applied, it is preferable to adapt the model of the portion P in step S3 accordingly, and therefore the manufacture of the insert 50 (step S4). More precisely, in order to anticipate the excess thickness generated by the subsequent application of the retaining folds, it is preferable to slightly reduce the thickness of the insert 50, by a thickness corresponding to the thickness of the retaining folds to be applied.

[0082] In other words, when the insert 50 is fixed to the acoustic panel 10 (step S5), the thickness of the resulting acoustic panel 10 is slightly less at the level of the insert 50 than on the rest of the acoustic panel 10. Thus, after the application of the retaining folds on the acoustic panel 10, the thickness of the acoustic panel 10 at the level of the insert 50 will have the same value as on the rest of the acoustic panel 10.

[0083] In addition, preferably, the retaining folds comprise the same material as the rest of the acoustic panel 10.

[0084] Furthermore, as mentioned above, the insert 50 preferably has a circular shape with a diameter of between 3 and 20 cm. These dimensions are advantageous in that, below 3 cm, the gains obtained by the method according to the invention, in terms of acoustic performance, are low in comparison with the solution according to the prior art. Above 20 cm, the method according to the invention remains applicable, but the capacity of the structure of the insert 50 to withstand mechanical forces could be reduced.

[0085] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated / mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

Claims

Claims

1. A method for repairing an aircraft acoustic structure (10), the acoustic structure (10) having a honeycomb structure comprising a plurality of acoustic cells (16) extending in a thickness direction (Z) of the acoustic structure (10), the method comprising: - detecting (SI) an impacted area (R) of the acoustic structure (10), - machining (S2) the acoustic structure (10) on a surface encompassing the impacted area (R), and on at least a portion of the thickness of the acoustic structure (10), so as to remove a portion (P) of the acoustic structure (10), - producing (S3) a model of said portion (P) of the acoustic structure removed by machining, comprising determining a geometry of said portion (P) removed, - manufacturing (S4) an insert (50) from the model produced in the preceding step (S3),the insert (50) having the geometry of said portion (P) removed and comprising a plurality of replacement acoustic cells (56), - fixing (S5) the insert (50) on the acoustic structure (10), comprising the insertion (S52) of the insert (50) in place of the portion (P) of the acoustic structure (10) removed in the machining step (S2).,

2. The method of claim 1, wherein the insert (50) is manufactured by additive manufacturing.

3. A method according to claim 1 or 2, wherein the production (S3) of the model of the portion (P) of the acoustic structure (10) removed by machining comprises the determination (S31) of an external geometry of said portion (P) comprising the dimensions and the shape of the external surfaces of said portion (P), then the modification (S32) of the model to add the plurality of replacement acoustic cells (56) from said external geometry.

4. Method according to claim 3, in which the determination (S31) of the external geometry of the portion (P) of the acoustic structure (10) removed by machining comprises a 3D scan of said portion (P) removed.

5. A method according to any one of claims 1 to 4, wherein the insert (50) comprises a belt (53) comprising a wall delimiting a contour of the insert (50), the acoustic alveoli of re- placement (56) being arranged inside the belt (53).

6. The method of claim 5, wherein the belt (53) has a honeycomb or lattice structure.

7. A method according to any one of claims 1 to 6, wherein the acoustic structure (10) is an acoustic panel comprising a solid closing skin (14) and a perforated acoustic skin (12), the acoustic cells (16) extending between the closing skin (14) and the acoustic skin (12), the manufacture of the insert (50) comprising the manufacture of a replacement acoustic skin (52) and / or a replacement closing skin.

8. The method of claim 7, wherein the acoustic panel (10) comprises, in each acoustic cell (16), an acoustic cone (17), the fabricated insert (50) comprising a plurality of replacement acoustic cells (56) and a replacement acoustic cone (57) in each of them.

9. A method according to claim 7 or 8, wherein, when manufacturing the insert (50), when the impacted area of ​​the acoustic panel (10) is the acoustic skin (12), the replacement acoustic skin (52) is manufactured first on a support, the replacement acoustic cells (56) being manufactured on the replacement acoustic skin (52).

10. Method according to any one of claims 1 to 9, in which the fixing of the insert (50) comprises, before the insertion (S52) of the insert (50) in place of the portion (P) of the acoustic structure (10) removed in the machining step (S2), the application (S51) of an adhesive (60) on a contour of the insert (50), then, after the insertion (S52) of the insert (50), a heat treatment (S53) making it possible to solidify the adhesive (60).

11. A method according to any one of claims 1 to 10, comprising, after fixing (S5) the insert (50) to the acoustic structure (10), applying at least one retaining ply by draping onto the acoustic structure (10), such that the at least one retaining ply covers both the insert (50) and a portion of the surface of the acoustic structure (10) extending around the insert (50).

12. A method according to claim 11, comprising, after applying the retaining ply, machining the retaining ply on said portion of the surface of the acoustic structure (10) extending around the insert (50).

13. Method according to claim 11 or 12, in which the model of the portion (P) of the acoustic structure (10) removed by machining is made such that, after the manufacture (S4) of the insert (50) and its fixing (S5) on the acoustic structure (10), a first thickness of the acoustic structure (10) at the insert (50) is less than a second thickness on the rest of the acoustic structure (10), a difference between the first and the second thickness being substantially equal to a thickness of the at least one retaining ply draped on the acoustic structure (10).

14. Method according to any one of claims 1 to 13, in which the insert (50) has a circular shape with a diameter of between 3 and 20 cm.

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