METHOD FOR MANUFACTURING AN ACOUSTIC PANEL WITH OBLIQUE CAVITIES

Abstract

A method for manufacturing an acoustic panel including a first skin, a second skin and an intermediate structure enclosed between these skins and forming oblique cavities with respect to the latter. The method includes an intermediate polymerization step for fastening the intermediate structure to the first skin in order to increase the compressive strength thereof and in particular to support an automated operation of draping the second skin.

Claims

1. A method for manufacturing an acoustic panel, comprising: a step (DR1) of arranging on a mould one or more first layers of a material comprising a resin, so as to form a first skin of the panel, a first step of heating (CH1) the first layers so as to polymerize the resin, a step (CO1) of arranging on the first skin an adhesive material, a step (SI1) of arranging on the adhesive material an intermediate structure comprising cavities which each open out on the one hand on a first surface of the intermediate structure and on the other hand on a second surface of the intermediate structure, the cavities being oblique with respect to the first and second surfaces, such that the adhesive material is enclosed between the first skin and the first surface of the intermediate structure, a step (DR2) of arranging on the second surface of the intermediate structure one or more second layers of a material comprising a resin, so as to form a second skin of the panel, a third step of heating (CH3) the assembly formed by the first skin, the intermediate structure and the second skin, so as to polymerize the resin of the second layers, wherein the method comprises, before arranging the second layers, a second step of heating (CH2) the assembly formed by the first skin, the adhesive material and the intermediate structure, so as to polymerize the adhesive material.

2. The method according to claim 1, wherein one or more of said first layers and/or one or more of said second layers are arranged with a draping robot.

3. The method according to claim 1, wherein the first and third heating steps CH1 and CH3 comprise pressurizing the heated elements with gas, the maximum pressure applied during the third heating step being lower than the maximum pressure applied during the first heating step.

4. The method according to claim 3, wherein the maximum pressure applied during the first heating step CH1 is comprised between six and ten bar and wherein the maximum pressure applied during the third heating step CH3 is comprised between two and four bar, and the second heating step CH2 is carried out at a reduced pressure relative to that of the first and third heating steps CH1 and CH3, typically between 0.2 bar and 1.5 bar.

5. The method according to claim 1, wherein one or more among the first, second and third heating steps are carried out within an autoclave, and/or in that the second heating step is carried out under a bladder in an oven or a furnace.

6. The method according to claim 1, comprising a step (PE1) of piercing the first skin carried out after the first heating step and/or a step (PE2) of piercing the second skin carried out after the third heating step.

7. The method according to claim 1, comprising a step (PE1) of piercing the first skin carried out after the first heating step, wherein cross-linking of the adhesive material is carried out between this piercing step (PE1) and the second heating step (CH2).

8. The method according to claim 1, wherein each of the cavities of the intermediate structure extends along an axis (32) forming an angle (33) comprised between 30 degrees and 60 degrees, for example equal to 45 degrees, relative to the first surface of the intermediate structure.

9. The method according to claim 1, wherein the intermediate structure comprises a metallic material such as aluminium and wherein the first layers and the second layers comprise an organic matrix composite.

10. The method according to claim 1, wherein the intermediate structure has a density of less than 130 kg/m.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The detailed description that follows makes reference to the appended drawings in which:

[0043] FIG. 1 is a schematic longitudinal section view of an aircraft propulsion unit fitted with acoustic panels;

[0044] FIG. 2 is a schematic cross-sectional view of an acoustic panel according to the invention, comprising an intermediate structure having oblique acoustic absorption cavities;

[0045] FIG. 3 is a flow chart of the production of an acoustic panel according to a first embodiment;

[0046] FIG. 4 is a flow chart of the production of an acoustic panel according to a second embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

[0047] FIGS. 1 and 2 comprise a frame of reference D1, D2 and D3 defining respectively a longitudinal/axial direction, a circumferential/tangential direction and a radial direction which are orthogonal to each other.

[0048] FIG. 1 shows in a simplified manner an aircraft propulsion unit 1 comprising a turbomachine 3 and a nacelle 4 extending around a central longitudinal axis 2. In this example, the turbomachine 3 is a turbofan engine.

[0049] Hereinafter, the terms front and rear are considered according to a main gas flow direction S1 in the propulsion unit 1 along the axis 2, which is parallel to the direction D1.

[0050] In a manner known per se, the propulsion unit 1 comprises, from the front to the rear, an air inlet 5, a fan 6, a secondary duct 7 delimited radially towards the inside by an internal fairing 8 surrounding a gas generator 9 formed by the turbojet 3 and radially towards the outside by elements of the nacelle 4, and a nozzle 10 for ejecting a primary flow leaving the gas generator 9. The nozzle 10 comprises an ejection cone 11 and an ejection nozzle 12.

[0051] The invention relates more specifically to acoustic panels 20 intended to be fitted to such a propulsion unit 1.

[0052] In this in no way limiting example, the propulsion unit 1 comprises several acoustic panels 20 as described below, illustrated by bold lines in FIG. 1. These panels 20 include panels 20A forming an inner wall of the air inlet 5, panels 20B forming part of the internal fairing 8 and panels 20C delimiting the secondary duct 7, panels 20D forming an outer wall of the ejection cone 11 and panels 20E forming an inner wall of the ejection nozzle 12.

[0053] In this example, each of the panels 20A to 20E extends circumferentially around the axis 2, forming a ring sector. Thus, in particular, each of the panels 20A extends over a respective circumferential sector so as together to form an axis 2 ring. What has just been described for panels 20A applies mutatis mutandis to panels 20B to 20E.

[0054] A panel 20 according to the invention is shown in cross-section in FIG. 2, in a sectional plane parallel to the directions D1 and D3.

[0055] The panel 20 comprises a first skin 21, a second skin 22 and an intermediate structure 23.

[0056] The intermediate structure 23 has an inner surface 26 and an outer surface 27 which are, in the cross-section in FIG. 2, shown flat and parallel to the direction D1. Of course, the inner surface 26 and/or the outer surface 27 can have a curved geometry, both in the cross-section shown in FIG. 2 and/or in other cross-sections.

[0057] In this example, the intermediate structure 23 extends in the circumferential direction D2 so as to form a ring sector.

[0058] The intermediate structure 23 comprises partitions 30 delimiting cavities 31 which extend both radially and axially.

[0059] In this example, the majority of the cavities 31 open out on the one hand on the inner surface 26 and on the other hand on the outer surface of the intermediate structure 23.

[0060] Each of the cavities 31 extends along an axis 32 oblique to the inner 26 and outer 27 surfaces.

[0061] In this example, the axis 32 of each of the cavities 31 forms an angle 33 of around 45 with the surfaces 26 and 27.

[0062] In terms of the geometry of the cavities 31, the partitions 30 are designed in this example such that each of the cavities 31 has a hexagonal cross-section in a plane normal to the axis 32. Alternatively, one or more of the cavities 31 can comprise a triangular, square or other cross-section. The cavities 31 can have any other shape to avoid telegraphing in particular.

[0063] The first skin 21, also known as the inner skin, is arranged radially on one side of the intermediate structure 23 so as to cover the inner surface 26 of this structure 23. As the inner skin 21 is flush with the inner surface 26 of the intermediate structure 23, the aforementioned angle 33 is also formed between the axis 32 of the cavities 31 and the inner skin 21.

[0064] The second skin 22, also known as the outer skin, is arranged radially on the other side of the intermediate structure 23 so as to cover the outer surface 27 of the intermediate structure 23.

[0065] In this example, the outer skin 22 is solid, while the inner skin 21 comprises openings (not shown) designed to guide air into the cavities 31 in order to absorb acoustic energy.

[0066] In this example, the panel 20 has a thickness 50, or radial dimension, comprised between 30 mm and 40 mm and the skins 21 and 22 each have a thickness comprised between 0.2 mm and 2 mm, for example 1.0 mm.

[0067] The panel 20 is preferably manufactured using a method comprising the steps described below.

[0068] With reference to FIG. 3, a draping step DR1 is carried out in which layers are arranged on a mould (not shown) so as to form the inner skin 21 of the panel 20.

[0069] The layers comprise an organic matrix composite, for example carbon fibres, and are pre-impregnated with a polymerizable resin, for example epoxy.

[0070] In one alternative embodiment, the layers each comprise juxtaposed strips. In this example, the layers are arranged on the mould by means of a draping robot (not shown) which comprises in particular a roller to press the layers against the mould and against each other.

[0071] The layers arranged in this way are placed in an autoclave (not shown) with the mould in order to undergo a first heating step CH1.

[0072] The autoclave chamber is pressurized with gas, in this example to a maximum pressure comprised between six and ten bar, and the temperature is increased to reach a level comprised between 100 C. and 250 C., and more preferably comprised between 15 and 220 C., so as to polymerize the resin contained in the layers forming the inner skin 21.

[0073] Once the inner skin 21 has cooled and hardened, a piercing step PE1 is carried out so as to form the aforementioned openings.

[0074] A layer of a polymerizable adhesive material, for example epoxy, is then placed on the inner skin 21 (step CO1), followed by the intermediate structure 23 (step SI1) such that the adhesive material is interposed between the inner skin 21 and the inner surface 26 of the intermediate structure 23. The adhesive layer is cross-linked (for example by blowing hot air) before the intermediate structure 23 is removed, so as not to obstruct the acoustic openings made in step PE1.

[0075] In this example, the intermediate structure 23 comprises aluminium and has a density of around 86 kg/m.sup.3.

[0076] The assembly formed by the inner skin 21, the adhesive material and the intermediate structure 23 is then placed in the autoclave chamber in order to carry out a second heating step CH2.

[0077] The autoclave chamber is pressurized with gas, in this example to a reduced maximum pressure which is in this example comprised between 0.2 and 1.5 bar, and the temperature is increased to reach a level comprised between 150 C. and 230 C., so as to at least partly polymerize the layer of the adhesive material. Given that this pressure is reduced, this step CH2 can alternatively be carried out outside of the autoclave, for example under a bladder in a furnace or an oven.

[0078] This second heating step CH2 enables the intermediate structure 23 to be fixed to the inner skin 21.

[0079] A second draping step DR2 is then carried out in which layers similar to those described above are arranged on the outer surface 27 of the intermediate structure 23, so as to form the outer skin 22 of the panel 20.

[0080] The panel 20 is then placed in the autoclave chamber in order to undergo a third heating step CH3.

[0081] The autoclave chamber is pressurized with gas, in this example to a maximum pressure comprised between two and four bar, and the temperature is increased to reach a level comprised between 100 C. and 250 C., and preferably comprised between 150 C. and 230 C., so as to polymerize the resin of the layers forming the outer skin 22.

[0082] Of course, the invention is not limited to the description set out above and the method can in particular comprise additional steps and/or steps carried out in a different order.

[0083] For example, with reference to FIG. 4, the step of piercing PE1 the inner skin 21 can be dispensed with and a step of piercing PE2 the outer skin 22 can be carried out once the outer skin 22 having undergone the third heating step CH3 has cooled and hardened.

[0084] Furthermore, all or part of the draping can alternatively be carried out manually and/or the heating steps CH1, CH2 and/or CH3 can be carried out using a heating means other than an autoclave and/or by using any appropriate tooling and pressurization means.

[0085] Moreover, the layers of the inner 21 and/or outer 22 skins can also be made of a stack of different materials, such as glass fibre and carbon fibre layers.