1. Field of the Invention
This invention relates to a scalable-thickness acoustic treatment panel.
2. Description of the Related Art
An aircraft propulsion system comprises a nacelle in which a power plant that is connected by the means of a mast to the rest of the aircraft is arranged in an essentially concentric manner.
As illustrated in FIG. 1, at the front, the nacelle comprises an air intake 10 that makes it possible to channel an air flow in the direction of the power plant 12, with a first portion of the incoming air flow, called primary flow, passing through the power plant to take part in the combustion process, whereby the second portion of the air flow, called secondary flow, is entrained by a fan and flows into an annular pipe that is bordered by the inside wall of the nacelle and the outside wall of the power plant.
The air intake 10 comprises a lip 14 whose surface that is in contact with the aerodynamic flows is extended inside the nacelle by an inside pipe 16 with essentially circular cross-sections and outside of the nacelle by an outside wall 18 with essentially circular cross-sections.
Techniques have been developed to reduce the noise emitted by an aircraft, and in particular the noise that is emitted by the propulsion systems. They consist in placing—in particular at the level of the wall of the inside pipe 16—a panel or covering 20 whose purpose is to take up a portion of the sound energy, in particular by using the Helmholtz resonator principle.
In a known manner, an acoustic treatment panel 20, also called an acoustic attenuation covering, comprises—from the outside to the inside—an acoustically resistive porous layer 22, at least one alveolar structure 24, and a reflective or impermeable layer 26.
The pipe 16 is to provide the uptake of at least a portion of the mechanical stresses between the lip 14 and the power plant 12. To limit the constraints at the acoustically resistive porous layer 22, stresses are transmitted between the lip 14 and the power plant 12 essentially via the reflective layer 26.
For this purpose, a flange 28 provides the connection between the power plant 12 and the reflective layer 26.
To provide the connection between the pipe 16 and the lip 14, the reflective layer 26 is flattened against the acoustically resistive porous layer 22, with the two layers 22 and 26 being made integral by any suitable means with the lip 14.
For this purpose, close to its end that is oriented toward the lip 14, the reflective layer comprises a slightly inclined face 30 (an angle of 45 to 60° relative to the acoustically resistive layer) over a length L, on the order of several centimeters, very significantly less than the remaining length A of the pipe 16 (with L representing on the order of 1% of A). To maintain this geometric shape, an alveolar structure 32 is provided in the space that is delimited by the acoustically resistive porous layer 22 and the inclined face 30. To provide the uptake of the compression stresses, the alveolar structure 32, in general in the form of a honeycomb, comprises cells with relatively small diameters that are not effective on the plane of acoustic treatment unlike the cells of the alveolar structure 24 whose cells are sized for acoustic treatment.
According to another aspect, the acoustic treatment panel 16 comprises an offset 34 at the surface that is in contact with the aerodynamic flows providing the housing of the end of the lip and optionally a flange of the front frame, facing which the connecting means are arranged.
Taking into account this offset 34, according to a first variant that is illustrated in FIG. 2, the acoustically resistive layer 22 comprises reinforcement folds 36 to provide the transmission of stresses that extend from the zone that is downstream from the offset 34 up to the point facing said offset.
The presence of these reinforcement folds 36 affects the porosity of the acoustically resistive layer 22, although the panel 16 no longer provides the acoustic treatment function facing the zone of these folds 36, which corresponds essentially to the non-treated zone because of the small dimensions of the cells of the alveolar structure 32.
As a variant, as illustrated in FIG. 3, the acoustic treatment panel 16 comprises a block 38 with an offset 34 and a beveled shape 40 at which the acoustically resistive layer 22 is made integral. With this block 38 not being porous, the acoustic treatment panel 16 no longer provides the acoustic treatment function opposite the zone of the block 38, which corresponds essentially to the non-treated zone because of the small dimensions of the cells of the alveolar structure 32.
Thus, according to the variants of the prior art, the junction zone between the pipe 16 and the lip 14, and in particular the zone that corresponds to the length L, is not treated on the acoustic plane.
The document U.S. Pat. No. 5,160,248 describes an acoustic treatment panel that at one end comprises a reinforcement for withstanding an impact from the breaking of a fan's blades. Inserted between two solid plates, this reinforcement comprises a stack of alveolar structures, with the rear solid plate being slightly tilted. The alveolar structures of the reinforcement are sized to withstand the impact and not to treat the acoustic waves. According to another aspect, the solid plate that is in contact with the aerodynamic flows is not porous but solid and does not make possible the passage of waves in the direction of the cavities of the reinforcement.