Multifrequency absorption acoustic panel for an aircraft nacelle

Abstract

An acoustic panel for an aircraft nacelle includes, from a central axis of the nacelle to the exterior thereof, a resistive skin perforated with sound-absorbing micro-perforations, a first attenuation stage, a septum perforated with holes, a second attenuation stage, and a back skin configured to provide the mechanical strength of the acoustic panel. The septum is a planar wall having a thickness greater than that of the resistive skin, preferably greater than 4 mm. Such a panel is configured to attenuate several frequency ranges one of which being a low-frequency range, while optimizing the weight, the cost and the air intake functions. The thickness of the septum, the dimensions of the holes in the septum, the OAR of the septum and the height of the second attenuation stage are adjusted to match the mean attenuated low frequency to the vibration frequency of the aircraft engine.

Claims

1. An acoustic panel for an aircraft nacelle, comprising, from a central axis of the nacelle toward an exterior of the nacelle: a resistive skin that forms a visible face of an interior duct of the nacelle, wherein the resistive skin is microperforated to have sound absorbing holes; a core adapted to damp sound waves, the core comprising a first damping stage and a second damping stage which are separated from each other by a septum, wherein the septum is a planar wall that is formed with communication holes that extend between the first and second damping stages; and a back skin configured to provide mechanical strength for the acoustic panel; wherein a thickness of the septum is greater than a thickness of the resistive skin.

2. The acoustic panel of claim 1, wherein the thickness of the septum is greater than or equal to 4 mm.

3. The acoustic panel of claim 1, wherein the thickness of the resistive skin is between 0.6 mm and 1.5 mm, inclusive.

4. The acoustic panel of claim 1, wherein the sound absorbing holes of the resistive skin have a circular shape with a diameter between 0.8 mm and 1.6 mm, inclusive.

5. The acoustic panel of claim 1, wherein the first and second damping stages are honeycomb cellular structures.

6. The acoustic panel of claim 1, wherein the first damping stage has a height between 25 mm and 45 mm, inclusive.

7. The acoustic panel of claim 1, wherein the second damping stage has a height between 15 mm and 45 mm, inclusive.

8. The acoustic panel of claim 1, wherein the resistive skin and the septum have each have a respective open area ratio (OAR), as a percentage, such that a product of the OAR of the resistive skin by the OAR of the septum is greater than or equal to 50%.

9. The acoustic panel of claim 1, wherein an open area ratio (OAR) of the resistive skin is between 5% and 50%, inclusive, and an OAR of the septum is between 1% and 10%, inclusive.

10. The acoustic panel of claim 1, wherein the septum has an open area ratio (OAR) as a percentage, that is substantially equal to the thickness, in millimeters, of the septum.

11. The acoustic panel of claim 1, wherein the resistive skin has an open area ratio (OAR) of an order of 8% and wherein the septum has an OAR of an order of 7% and a thickness of an order of 7 mm.

12. The acoustic panel of claim 1, wherein the communicating holes have a diameter greater than or equal to 0.8 mm.

13. An aircraft nacelle comprising acoustic panels of claim 1.

14. An aircraft propulsion unit comprising the aircraft nacelle of claim 13.

15. An aircraft comprising at least one aircraft nacelle of claim 13.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) One embodiment of the disclosure herein will be clearly understood and its advantages will become more clearly apparent on reading the following detailed description given by way of nonlimiting illustration with reference to the appended drawings, in which:

(2) FIG. 1 is an exploded perspective view of a prior art air intake.

(3) FIG. 2 is a profile view of an aircraft on which is seen a nacelle 1 that may be equipped with acoustic panels in accordance with the disclosure herein.

(4) FIG. 3 is a view in cross section of an acoustic pan& portion in accordance with the disclosure herein.

(5) Identical elements represented in the aforementioned figures are identified by identical reference numbers.

DETAILED DESCRIPTION

(6) The air intake represented in FIG. 1 and the nacelle represented in FIG. 2 are elements known to the person skilled in the art; the air intake from FIG. 1 has been described in the introduction.

(7) FIG. 3 shows the structure of an acoustic panel 10 in accordance with the disclosure herein, which structure may be used to form the panels 103 of the air intake from FIG. 1.

(8) The acoustic panel 10 from FIG. 3 includes a resistive skin 11, a first damping stage 12, a septum 13, a second damping stage 14 and a back skin 15.

(9) The resistive skin 11 is of small thickness, between 0.6 and 1.5 mm inclusive, and is pierced by a multitude of sound absorbing holes 16 of small diameter, for example between 0.8 and 1.6 mm inclusive, which diameter is advantageously slightly greater than the thickness of the resistive skin 11. The sound absorbing holes 16 are considered to be microperforations that generate little drag.

(10) In the example illustrated the sound absorbing holes 16 are circular. But this is not limiting on the disclosure herein; they could have any shape, geometrical or otherwise.

(11) The first damping stage 12 consists of or comprises a honeycomb cellular structure; it has a height between 25 and 45 mm inclusive. Sized in this way and associated with a resistive skin of small thickness (less than 2 mm) it contributes to absorbing a first range of sound waves at “high” frequencies, between 1000 Hz and 4000 Hz inclusive.

(12) The septum 13 is a thick plane wall, in that it has a greater thickness than the resistive skin and preferably greater than 4 m for example of the order of 7 mm. The thickness of the septum is selected as a function of the mean low frequency to be damped (which depends on the engine). It is also the result of a compromise between the benefits obtained by a greater thickness of the septum on the acoustic plane and the undesirable consequences of too thick a septum in terms of weight. From an acoustic point of view a septum thickness up to 20 mm could be justified but in practice a smaller thickness will be selected for reasons of weight.

(13) The second damping stage 14 consists of or comprises a honeycomb cellular structure; it has a height between 15 and 45 mm inclusive. Dimensioned in this way and associated with a thick septum it contributes to absorbing a second range of sound waves at “low” frequencies, between 300 Hz and 600 Hz inclusive. The height of the second damping stage is selected as a function of the thickness of the septum and of the mean frequency that it is wished to damp.

(14) In the example illustrated the cells of the two damping stages have identical (hexagonal) cross sections and are aligned in the radial direction (the panel being observed as if positioned in an air intake). But this is not limiting on the disclosure herein; the first damping stage could for example feature smaller cells than the second stage.

(15) The septum is pierced by communication holes 17 between the two sound absorbing stages. The diameter of the communicating holes 17 may be greater than the diameter of the sound absorbing holes 16, which is limited by the negative influence of the holes on drag. The diameter (or the greatest front dimension if the holes of the latter are not circular) is therefore selected to be greater than 0.8 mm to enable the evacuation of water and thus to provide a drainage function. The maximum diameter is fixed to obtain the required OAR with at least one hole per cell of the honeycomb of the second damping stage.

(16) The OAR of the septum is preferably between 1% and 10% inclusive, for example of the order of 7%. The inventors have demonstrated the importance of the ratio between the OAR and the thickness of the septum. The OAR of the septum as a percentage is advantageously substantially equal to the thickness of the septum in millimeters.

(17) This ratio makes it possible to “fix” the damped frequencies. The mean low frequency to be attenuated is determined by the engine in question. To attenuate a given mean low frequency it is possible to act on the diameter of the communicating holes, the OAR of the septum, the thickness of the septum and the height of the second damping stage. Each of these parameters is limited upward or downward by the appearance of various disadvantages. For example, the thickness of the septum 13 is therefore limited (upward) by the maximum weight that is fixed for the panel, the diameter of the communicating holes 17 is limited (downward) by the occurrence of the problem of condensation and accumulation of water in the panel. The height of the second damping stage 14 is also variable (upward) only to a small degree because it influences the overall size of the acoustic panel. For the same damping height, if the thickness of the septum and the OAR of the septum are increased, whilst maintaining the equality between these two parameters, the frequency damped by the panel is lowered. It is therefore possible to adjust the mean frequency damped by the panel to make it correspond to the vibration frequency of the engine.

(18) The back skin is a solid airtight wall. Its thickness is selected so as to confer on the panel the required mechanical strength, also taking account of its influence on the weight of the panel. It is for example between 1 mm and 8 mm inclusive.

(19) The disclosure herein extends to all variants that may occur to the person skilled in the art falling within the scope of the appended claims. For example, the honeycomb structure of the first or second damping stage may therefore be replaced by some other known type of damping structure. The septum may be less thick than that illustrated provided that it remains thicker than the resistive skin. In this case, to maintain a range of low frequencies damped as low as that of the panel illustrated (the septum of which is thicker), it will be necessary to increase the height of the second damping stage.

(20) While at least one exemplary embodiment of the invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms “comprise” or “comprising” do not exclude other elements or steps, the terms “a”, “an” or “one” do not exclude a plural number, and the term “or” means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.