FR3100918A1 - ACOUSTIC PANEL FOR AN AIRCRAFT PROPELLER ASSEMBLY, AND ITS MANUFACTURING PROCESS - Google Patents

Abstract

Acoustic panel (40) for an aircraft propulsion unit (10), this panel comprising a first acoustic skin (42) multiperforated, a first cellular layer (44), and a second skin (46), the first layer being interposed between the first and second skins and comprising first cells (48) in fluid communication with first perforations (50) of the first skin, characterized in that it comprises at least one primary zone (Z1) with a single layer of resonators in which the second skin is solid, and at least one secondary zone (Z2) with multiple layers of resonators in which the second skin is multi-perforated to form a first septum and comprises second perforations (52) in fluid communication with said first alveoli as well as with second alveoli (54) of at least one other alveolar layer (56), this second alveolar layer being carried by at least one box (58) attached and fixed to the second skin in said at least one secondary zone. Figure for abstract: Figure 4

Description

ACOUSTIC PANEL FOR AN AIRCRAFT PROPULSION ASSEMBLY, AND METHOD OF MANUFACTURING THEREOF

Technical field of the invention

The present invention relates in particular to an acoustic panel for an aircraft propulsion assembly, and its method of manufacture.

Technical background

The technical background includes in particular the document WO-A1-03/107325.

An aircraft propulsion assembly generally comprises a turbine engine and/or a nacelle surrounding this turbine engine. In the present application, the term propulsion assembly is understood to mean a turbomachine (or engine), or a turbomachine nacelle, or the assembly formed by a turbomachine and its nacelle.

The nacelle surrounds a gas generator of the turbomachine, in particular a turbomachine, and forms around the gas generator an annular vein whose walls in contact with the secondary flow are equipped with acoustic panels in order to ensure both the rigidity of the nacelle, the channeling of the air passing through the nacelle and the absorption of the noise generated mainly by the fan.

The vein has a general shape of revolution but its section varies continuously from the front to the rear of the nacelle in order to achieve the optimum compromise between the flow constraints of the secondary flow and those of the size of the nacelle.

The acoustic panels to be produced therefore have curved shapes resembling sections of shells.

Acoustic panels are well known in aeronautics. These are sandwich structures comprising in the direction of the thickness a plurality of layers assembled together. Traditionally, so-called "single resonator" panels are used, comprising three layers, namely: a so-called "acoustic" multi-perforated skin, a honeycomb and a solid skin. The layers are generally assembled together by gluing. Some metal parts that are hot in operation can be joined by brazing. The acoustic skin and the solid skin are for example made of organic composite material comprising from 3 to 10 layers of reinforcing fabric embedded in a resin hardened by polymerization. The acoustic skin and the solid skin are made separately by molding to the final shape of the panel using well-known techniques. The acoustic skin is then pierced and the solid skin + honeycomb + acoustic skin assembly is assembled by hot bonding under pressure in an autoclave in order to ensure the best possible contact between the three layers. In a variant of the method, during assembly, the solid skin can be made directly and molded onto the honeycomb that it covers. It is understood that the skins kept separated by the honeycomb ensure the strength and rigidity of the panel.

There are also known more efficient acoustic panels called "double resonator". These are sandwich structures comprising successively in the direction of the thickness: a multi-perforated acoustic skin, a first so-called "primary" honeycomb, a septum also multi-perforated, a second so-called "secondary" honeycomb, and a skin full.

The septum is made of organic composite comprising 1 to 3 layers of reinforcing fabric embedded in a resin hardened by polymerization. Unlike the skins, the septum is thin and flexible because it contributes little to the strength and rigidity of the panel, its function therefore being essentially acoustic. The manufacture of the skins and of the septum, the drilling of the acoustic skin and of the septum as well as the assembly of the panel are carried out using the techniques already described for a single resonator panel.

The problem to be solved for these acoustic panels is to simplify their manufacture and reduce their manufacturing cost.

A solution has already been proposed in the PCT application cited above. Although this solution is satisfactory, it is however limited to the manufacture of thin double resonator panels, generally less than 50 mm.

One way to optimize the performance of an acoustic panel for high bypass ratio engines is to increase its thickness and this results in an increase in the thickness of the honeycomb(s). Honeycombs are made of different materials than skins and therefore have different thermal behaviors. During its manufacture, the panel is heated in an autoclave to solidify the assembly and the differential thermal expansions between the honeycombs and the skins are amplified when the thicknesses of the honeycombs are significant, which generates significant deformations of the sign. It is this phenomenon that explains why today, only panels with a thickness of less than 50mm are manufactured with the process described in the PCT document mentioned above.

The aim of the invention is in particular to provide a simple, effective and economical solution making it possible to manufacture more easily and therefore at lower cost acoustic panels of the "double resonator" type, even of considerable thickness.

The invention proposes an acoustic panel for an aircraft propulsion assembly, this panel comprising a first multiperforated acoustic skin, a first honeycomb layer, and a second skin, the first layer being inserted between the first and second skins and comprising first cells in fluid communication with first perforations of the first skin, characterized in that it comprises at least one primary zone with a single layer of resonators in which the second skin is solid, and at least one secondary zone with multiple layers of resonators in which the second skin is multi-perforated to form a first septum and comprises second perforations in fluid communication with said first cells as well as with second cells of at least one other cell layer, this second cell layer being carried by at least one insert box and attached to the second skin in said at least one secondary zone.

In the present application, the term “single layer of resonators” is understood to mean a part of a panel with a single resonator, that is to say comprising a single cellular layer, the cells of which form resonators. By multiple layer of resonators is meant a part of a panel with double, triple or more resonators, that is to say comprising two or more alveolar layers, the cells of which form resonators.

A resonator within the meaning of the invention must be interpreted as a Helmholtz resonator whose operation is well known to those skilled in the art.

The invention thus proposes to manufacture the first layer of resonators independently of the other layers. The invention is thus not limited by the thickness of these layers. The first layer of resonators can be fabricated as in the prior art. The other layers are for their part assembled with a box which allows their support and their fixing vis-à-vis the first layer.

The panel is also innovative insofar as only secondary zones of the panel comprise a box and therefore at least two layers of resonators.

The panel according to the invention may include one or more of the characteristics below, taken separately from each other or in combination with each other:

- the or each box comprises a third skin comprising at least one recess in which said at least one other alveolar layer is housed;

- said at least one other alveolar layer is interposed between the second and third skins, the third skin comprising flanges applied and fixed to the second skin;

- said at least one other alveolar layer is separated by a set of predetermined values from said second skin;

- said at least one other cellular layer comprises a second cellular layer and a third cellular layer, a second perforated septum being interposed between the second and third cellular layers;

- the panel has an annular or curved sector shape around an axis;

-- the first honeycomb layer has a thickness between 20 and 40mm;

-- the first skin has an open area ratio between 8 and 20%; the open area ratio of a wall is the ratio of the total sum of the sections of the openings of this wall to the total area of the wall;

-- the first perforations have a diameter between 1.5 and 2mm;

-- said at least one further layer has a thickness less than a depth of said recess;

-- said third skin is made of composite material or metal alloy;

-- the second perforations have a smaller diameter than the first perforations;

-- the second perforations have a diameter of between 0.5 and 1.5mm, and for example of the order of 1mm;

-- the second skin has, in said at least one secondary zone, a lower open area ratio than that of the first skin;

-- the second skin has, in said at least one secondary zone, an open surface rate of between 2 and 5%;

-- the second skin has a thickness between 1 and 2mm;

-- the second septum has a thickness between 0.1 and 0.5mm;

-- the panel has a thickness less than or equal to 50mm in said at least one primary zone, and a thickness greater than 50mm in said at least one secondary zone; And

-- Said clearance J is between 1mm and 5mm.

The present invention also relates to an aircraft propulsion assembly, comprising at least one panel as described above.

The present invention further relates to a method of manufacturing a panel comprising the steps of:

a) production of a first assembly comprising the first and second skins as well as the first cellular layer, the first cellular layer being glued to the first and second skins, the first skin comprising the first perforations,

b) production of at least a second assembly comprising a box carrying the second honeycomb layer, and

c) making the second perforations in the second skin, only in said at least one secondary zone,

d) fixing the or each box to the second skin, in said at least one secondary zone. Step c) can be carried out before or after step b).

In step d), the or each box is preferably fixed by gluing or riveting to the second skin.

In step b), the second alveolar layer preferably comprises only one of its two faces coated and fixed, for example by gluing, to the third skin.

As a variant, in step b), the box and the second honeycomb layer can be produced in one go by injection of thermoplastic material.

In a particular embodiment, in step b), the second alveolar layer is formed by stacking at least two alveolar layers, preferably in honeycombs, separated from each other by a perforated septum, an adhesive being crosslinked on one face of this stack before being glued to the septum.

Brief description of figures

The invention will be better understood and other details, characteristics and advantages of the invention will appear more clearly on reading the following description given by way of non-limiting example and with reference to the accompanying drawings in which:

Figure 1 is a very schematic view in axial section of an aircraft propulsion assembly,

Figure 2 is a schematic perspective view with partial cutaway of an acoustic panel with a single resonator,

Figure 3 is a schematic perspective view with partial cutaway of a double resonator acoustic panel,

Figure 4 is a schematic view in axial section of an acoustic panel according to a first embodiment of the invention,

Figures 5a to 5e are very schematic sectional views of a panel according to the first embodiment, and illustrate steps in its manufacturing process,

Figure 6 is a graph showing noise attenuation as a function of frequency for different acoustic panels,

Figure 7 is a schematic view in axial section of an acoustic panel according to a second embodiment of the invention,

FIGS. 8a to 8e are very schematic sectional views of a panel according to the second embodiment, and illustrate steps in its manufacturing process, and

Figure 9 is a graph showing noise attenuation as a function of frequency for different acoustic panels.

Detailed description of the invention

FIG. 1 very schematically shows a propulsion assembly 10 for an aircraft, such as an airplane, this propulsion assembly 10 comprising a turbomachine and a nacelle. The turbomachine is for example dual-flow and comprises a gas generator 12 surrounded by the nacelle 14. A turbine of the gas generator 12 rotates a fan 16 inside the nacelle 14.

The air flow 17 which enters the propulsion assembly 10 and crosses the fan 16 is channeled by the nacelle 14 and is then divided into a primary flow 18 which flows into the gas generator 12, and into a secondary flow 20 which flows in an annular vein 22 formed between the gas generator 12 and the nacelle 14.

To limit the propagation of noise from the propulsion assembly 10, in particular from the fan 16, outwards, and thus reduce noise pollution, it is known to provide acoustic panels 24 on the walls in contact with the flow of air inlet 17 and the secondary flow 20. Such panels 24 can thus be mounted on the nacelle 14 around the fan 16 or the gas generator 12, on the gas generator 12, etc.

Figures 2 and 3 show known technologies of acoustic panels 24, 24'.

Figure 2 shows an acoustic panel with a single resonator, that is to say with a single honeycomb layer, each honeycomb-shaped cell, generally forming a Helmholtz resonator, the operation of which is well known to the public. skilled in the art. In known manner, the perforations 34 form necks of the resonant cavities formed by the cells 32.

The panel 24 of Figure 2 comprises a first multi-perforated acoustic skin 26, a honeycomb layer 28, and a second solid skin 30. Layer 28 is interposed between skins 26, 30 and includes cells 32 in fluid communication with perforations 34 of first skin 26.

The panel 24 'of Figure 3 comprises, in addition to the elements referenced 26, 28 and 30, a second multi-perforated acoustic skin 32 and a second honeycomb layer 34. The layer 34 is inserted between the skins 32 and 26 and comprises cells 36 in fluid communication with the perforations 34, 38 of the skins 26, 32.

The present invention proposes an acoustic panel 40 which comprises two types of zones, called primary and secondary, the or each primary zone being of the single resonator type, that is to say with a single alveolar noise attenuation layer, and the or each secondary zone being of the multiple resonator type (at least double resonator), that is to say with two or more alveolar noise attenuation layers.

The first embodiment illustrated in Figures 4 and 5a-5e shows an acoustic panel 40 whose or each secondary zone is a double resonator, and the second embodiment illustrated in Figures 7 and 8a-8e shows an acoustic panel 40' whose the or each secondary zone has a triple resonator.

Panel 40 in Figure 4 includes:

- a first multi-perforated acoustic skin 42,

- a first alveolar layer 44, and

- a second skin 46.

The first layer 44 is inserted between the skins 42, 46 and comprises first cells 48 in fluid communication with first perforations 50 of the skin 42.

In the or each primary zone, denoted Z1, the skin 46 of the panel is solid and this panel is here of the single resonator type.

In the or each secondary zone, denoted Z2, the skin 46 is multi-perforated to form a first septum and comprises second perforations 52 in fluid communication with the cells 48 as well as with the second cells 54 of a second cell layer 56.

To facilitate the manufacture of the panel 40, the alveolar layer 56 is initially carried by a box 58 which is then attached and fixed to the skin 46 in the secondary zone Z2.

The box 58 preferably comprises a third skin 59 comprising at least one recess 60 in which the alveolar layer 56 is housed. This third skin 59 is preferably made of a composite material or a metal alloy. This third skin is preferably solid.

The recess 60 can have a constant or changing depth, as in the example shown. This makes it possible in particular to adapt the overall thickness of the panel 40 to its mounting environment. In the example shown in the drawing, the panel 40 is intended to be fixed to a tapered wall 62 of a nacelle.

The alveolar layer 56 can be formed by a one-piece assembly or by the assembly of partitions (for example spacers made of injected thermoplastic).

The alveolar layer 56 is fixed, for example by gluing one of its faces to the bottom of the recess 60, and preferably has a thickness less than that of the recess. This makes it possible to maintain a clearance J of a predetermined value between the alveolar layer 56 and the skin 46 and thus to prevent this layer from interfering with the fixing of the box 58 on the skin 46.

The alveolar layer 56 is inserted between the skins 59, 46, the skin 59 comprising flanges 59a applied and fixed to the skin 46. This fixing can be achieved by gluing or riveting for example.

Figures 5a to 5e show steps in a process for manufacturing panel 40.

A first step of the method comprises the production of a first assembly comprising the skins 42, 46 as well as the alveolar layer 44, the alveolar layer 44 being glued to the skins 42, 46, and the skin 42 comprising the perforations 50 (step a) – Figure 5a).

A second step of the method comprises the production of at least a second assembly comprising the box 58 bearing the alveolar layer 56 (step b) – FIG. 5c and 5d). This can be done for example by sticking a honeycomb (layer 56) on a composite or metal skin 59 or by injecting the recess and the partitions in one piece with a thermoplastic material.

A third step of the method comprises the making of the perforations 52 in the skin 46, only in the zone Z2 (step c) – figure 5b).

Finally, a fourth step of the method comprises fixing the box 58 to the skin 46, in the zone Z2, by gluing or riveting for example as mentioned above (step d) – figure 5e).

In a particular embodiment of the invention:

- the alveolar layer 44 has a thickness of between 20 and 40mm,

- skin 42 has an open surface rate of between 8 and 20%,

- the perforations 50 have a diameter of between 1.5 and 2mm,

- the perforations 46 have a diameter smaller than that of the perforations 50, and which is for example between 0.5 and 1.5 mm,

- the skin 46 has, in the or each zone Z2, an open surface rate lower than that of the skin 42, and for example between 2 and 5%,

- the skin 42 has a thickness of between 1 and 2mm,

- the panel has a thickness less than or equal to 50mm in zone Z1 and a thickness greater than 50mm in zone Z2.

FIG. 6 is a graph illustrating and making it possible to compare the acoustic performances of three acoustic panels, all of the same total thickness close to 100mm. Curve 70 illustrates the acoustic performance of a single resonator panel (figure 2). Curve 72 illustrates the acoustic performance of a double resonator panel (Figure 3) and curve 74 illustrates the acoustic performance of panel 40 (Figure 4).

The conclusions we can draw are that:

- the three panels have similar efficiencies at low frequencies (around 200 to 500Hz - part 1 of the graph),

- panel 40 has a slightly degraded performance at medium frequencies (around 500 to 1200Hz - part 2 or middle of the graph), and

- panel 40 has a performance equivalent to a double resonator and much better than that of a single resonator at high frequencies (from 1200 Hz - part 3 of the graph.

These frequency ranges can be adapted by modifying the total thickness of the panels (by decreasing the thickness, these values can be shifted towards higher frequencies and vice versa).

Panel 40' in Figure 7 has the same features as Panel 40 except for the following. The single honeycomb layer 56 of the panel 40 is here replaced by a stack of two honeycomb layers 56 and 64. A multi-perforated skin forming a septum 66 is inserted between the two layers 56, 64.

As illustrated in Figures 8a to 8e, the manufacture of the panel 40 'is similar to that of the panel 40. All the elements referenced 59, 56 and 64 can be fixed by gluing to achieve the second assembly (step b) and figures 8c and 8d). As mentioned above, the cumulative thickness Ha+Hb of the layers 56, 64 and of the septum 66 is preferably less than the depth Hbox of the recess 60 to have the aforementioned clearance J between the layers 56, 64 and the skin 46 (step d) and figure 8 ^e )).

The septum 66 can have a thickness comprised between 0.1 and 0.5 mm. It has an open surface rate for example between 2 and 5%

FIG. 9 is a graph illustrating and making it possible to compare the acoustic performances of three acoustic panels, all of the same total thickness close to 100mm. Curve 80 illustrates the acoustic performance of a single resonator panel (Figure 2).

Curve 82 illustrates the acoustic performance of a double resonator panel (figure 3) and curve 84 illustrates the acoustic performance of the 40' panel (figure 7).

The conclusions we can draw are that:

- the three panels have similar efficiencies at low frequencies (part 1 of the graph),

- the panel 40 has a performance equivalent to a double resonator at medium and high frequencies and significantly better than a single resonator.

The panel 40, 40' according to the invention can have an annular or curved shape around an axis.

The panel has the advantage of being relatively light, simple to make and resistant to stresses because these stresses pass through the first assembly of the panel, the added box(es) essentially having an acoustic function.

The shape of the cells of the alveolar layers is not limited to a honeycomb shape and can have any shape suitable for providing acoustic treatment.

Claims (12)

Acoustic panel (40, 40') for an aircraft propulsion assembly (10), this panel comprising a first multiperforated acoustic skin (42), a first cellular layer (44), and a second skin (46), the first layer being inserted between the first and second skins and comprising first cells (48) in fluid communication with first perforations (50) of the first skin, characterized in that it comprises at least one primary zone (Z1) with a single layer of resonators in which the second skin is solid, and at least one secondary zone (Z2) with multiple layers of resonators in which the second skin is multi-perforated to form a first septum and comprises second perforations (52) in fluid communication with said first cells as well as with second cells (54) of at least one other cell layer (56, 64), this second cell layer being carried by at least one box (58) attached and fixed to the second skin in the said at least one secondary area. Panel (40, 40') according to Claim 1, in which the or each box (58) comprises a third skin (59) comprising at least one recess (60) in which is housed the said at least one other alveolar layer (56, 64). Panel (40, 40') according to Claim 2, in which the said at least one other alveolar layer (56, 64) is interposed between the second and third skins (46, 59), the third skin comprising flanges (59a) applied and attached to the second skin (46). Panel (40, 40') according to one of the preceding claims, in which said at least one other cellular layer (56, 64) is separated by a clearance (J) of predetermined value from said second skin (46). Panel (40, 40') according to one of the preceding claims, in which the said at least one other honeycomb layer comprises a second honeycomb layer (56) and a third honeycomb layer (64), a second perforated septum (66) being interposed between the second and third alveolar layers. Panel (40, 40') according to one of the preceding claims, in which it has the shape of an annular or curved sector around an axis. Aircraft propulsion assembly (10), comprising at least one panel (40, 40') according to one of the preceding claims. Method of manufacturing a panel (40, 40') according to one of Claims 1 to 6, in which it comprises the steps of:
a) production of a first assembly comprising the first and second skins (42, 46) as well as the first cellular layer (44), the first cellular layer being glued to the first and second skins, the first skin comprising the first perforations (50 ),
b) production of at least a second assembly comprising a box (58) bearing the second alveolar layer (56), and
c) making second perforations (52) in the second skin, only in said at least one secondary zone (Z2),
d) fixing the or each box to the second skin, in said at least one secondary zone. Method according to claim 8, in which, in step d), the or each box (58) is fixed by gluing or riveting to the second skin (46). Process according to Claim 8 or 9, in which, in step b), the second cellular layer (56) comprises only one of its two faces coated and fixed, for example by gluing, to the box (58). Process according to Claim 8 or 9, in which, in step b), the box (58) and the second cellular layer (56) are produced in one go by injection of thermoplastic material. Method according to one of Claims 8 to 10, in which, in step b), the second alveolar layer (56) is formed by stacking at least two alveolar layers (56, 64), preferably in honeycombs, separated from each other by a perforated septum (66), an adhesive being crosslinked on one face of this stack before being glued to the septum (66).