The present invention relates to noise attenuation panels and is particularly, although not exclusively, concerned with noise attenuation panels for use in the attenuation of noise in aero engines.

In patent application publication No. GB-A-2223448 a noise attenuation panel is disclosed comprising a backing component part, a facing component part and a cellular component part having a multiplicity of open-ended juxtaposed cells. The backing component part extends across the ends of the cells of the cellular component part at the rear thereof and the facing component part extends across the ends of the cells of the cellular component part at the front thereof. The facing component part comprises or includes an outer facing sheet which is made of a porous permeable thermoplastics material and preferably the porous permeable thermoplastics material is produced by powder sintering of a thermoplastics material.

In a particular embodiment of the invention described in our GB-A-2223448 the cellular component part comprises a front cellular sub-component having a multiplicity of open-ended juxtaposed cells, a rear cellular sub¬ component having a further multiplicity of open-ended juxtaposed cells and a septum component which extends across the ends of the cells of the rear cellular sub-component at the front thereof and the ends of the cells of the front cellular sub-component at the rear thereof. The septum component is made of a porous permeable thermoplastics sheet material and is preferably produced by powder sintering of a thermoplastics material.

While the noise attenuation panel disclosed in GB-A- 2223448 has been found to be successful for use in aero engine environments, it suffers from a number of disadvantages. In particular, in order to include the septum component in the noise attenuation panel, it is necessary to construct two separate cellular sub¬ components and join them together by means of the septum component or to split or cut the cellular component into two sub-components and join them to the septum component. Where two septum components are required then three separate cellular sub-components need to be provided leading to complex manufacturing procedures. Furthermore, since the or each septum component is in the form of a continuous sheet of material, the configuration of the septum within the panel is limited insofar as it is required to lie in a continuous surface.

It is an object of the present invention to provide a noise attenuation panel which has one or more septum components and which does not suffer from the above mentioned disadvantage of requiring the provision of cellular sub-components, and to provide a method of manufacturing it which is cheaper and simpler for the manufacture of panels with septums having configurations not limited as hereinbefore set forth.

According to a first aspect of the present invention there is provided a noise attenuation panel comprising a cellular component which has a front face and a rear face and a multiplicity of cells each of which extends from the front face to the rear face and is formed by wall portions which extend from the front face to the rear face, characterised by the provision in each cell in a predetermined position therein where the wall portions forming the cell are continuous of a cell dividing septum

element which extends across the cell and at which it is secured to the wall portions to divide the cell into two subcells.

In an embodiment of the invention hereinafter to be described the septum elements are secured to the wall portions in positions in which they lie in a continuous surface extending through the cellular component. The wall portions may be arranged in a continuous planar surface or a continuous curved surface depending upon the structural form the panel is required to take or upon the acoustic design requirements for it.

In an alternative embodiment of the invention, the septum elements are secured to the wall portions in positions in which they lie in a surface which extends through the cellular component and which is not a continuous surface. Depending upon the design requirements for the panel, the septum elements may be secured to the wall portions at different depths within the cells to meet any special acounstic design requirements.

In embodiments of the invention hereinafter to be described the septum element in each cell is held in the predetermined position in the cell by securing it to the wall portions with an adhesive material. Alternatively, the wall portions and the septum elements may be made of compatible bonding thermoplastics materials and the septum element in each cell is held in the predetermined position by bonding the septum element to the wall portions without the use of an adhesive material.

Preferably the septum elements are made from a porous permeable thermoplastics material. The porous permeable thermoplastics material may be produced by powder

sintering a thermoplastics material.

In another embodiment of the invention hereinafter to be described each of the cells or each of predetermined ones of the cells of the cellular component is provided with one or more further septum elements to divide the cell into three or more subcells.

According to a second aspect of the present invention there is provided a method of manufacturing a noise attenuation panel according to the first aspect of the invention comprising the steps of first forming the cellular component without the provision in each cell of the cell dividing septum element, bringing a septum sheet into engagement with the cellular component such that the edges of the wall portions at a face of the cellulat component cut through the septum sheet to form a multiplicty of cell dividing septum elements and allowing or causing each cell dividing septum element formed from the septum sheet to advance in the cell to a predetermined position therein at which it is secured to the wall portions of the cell thereby to divide the cell into two subcells.

In an embodiment of the invention hereinafter to be described the method according to the second aspect of the invention comprises the further step of bringing one or more further septum sheets into engagement with the cellular component such that the edges of the wall portions at a face of the cellular component cut through the or each further septum sheet and allowing or causing each cell dividing septum element formed from the or each further septum sheet to advance in the cell to a predetermined further position at which it is secured to the wall portions of the cell thereby to divide the cell

into three or more subcells.

According to a third aspect of the present invention there is provided an aero engine including as a structural part of the engine a panel according to the first aspect of the invention or produced by a method according to the second aspect of the invention.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figure 1 is a schematic isometric view from above of a noise attenuation panel according to a first embodiment of the invention;

Figure 2 is a schematic cross section of an end region of the panel shown in Fig. 1, secured to a supporting channel member;

Figure 3 is a schematic cross section of a noise attenuation panel according to a second embodiment of the invention; and

Figure 4 is a schematic cross section of an aero engine embodying noise attenuation panels according to the invention.

Referring first to Figs 1 and 2, the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 12 and a facing component part 13 comprising outer and inner facing sheets 131 and 132.

As will be seen from Figs 1 and 2, the cellular component 12 comprises a multiplicity of open ended juxtaposed

cells 15 of hexagonal cross section to provide a honeycomb configuration. The walls of the cells 15 extend from the front face of the cellular component 12 to the rear face. Each cell is, however, divided into an upper subcell 151 and lower subcell 152 by a septum element 14 introduced as hereinafter to be more fully described.

The walls of the cells 15 of the cellular component 12 are preferably made from a non-porous impermeable sheet of any of the following materials:-

(i) A thermoplastic such as polyether ether ketone.

(ii) A polyester f bric/[phenolic resin.

(iii) A fibreglass/phenolic resin.

(iv) A NOMEX/phenolic resin (NOMEX being a registered trade mark for an aramid fibre paper impregnated with various resins to produce a structural material and by "aramid is meant an aromatic polyamide polymer.)

(v) An aluminium alloy.

The outer facing sheet 131 of the facing component part 13 comprises a sheet of a porous permeable thermoplastics material produced by powder sintering the thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polya ide- imide, thermoplastic polyimide, polyether-imide,

polyurethane and polyethylene.

The inner sheet 132 of the facing component part 13 is made from an open square weave fabric formed from a carbon fibre/resin matrix composite material, the weave being such as to provide apertures constituted by the openings between adjacent warp and weft threads of the fabric. The fabric is preferably so woven as to produce a proportion of open aperture area relative to the total surface area of the sheet of 30% or substantially 30%. The fabric is also so woven that a relatively large number of its apertures are contained within the bounds of each cell 15 of the cellular component 12.

The outer facing sheet 131 is bonded to the inner facing sheet 132 and the inner facing sheet 132 is, as shown in Fig. 2, secured to the upper face of the cellular component 12 by means of an epoxy resin adhesive El.

The backing sheet 11 is unperforated and made from a non- porous impermeable sheet material and, as shown in Fig 2, is secured by an epoxy resin adhesive E2 to a lower face of the cellular component 12. The backing sheet 11 may be made of from any the following materials:-

(i) A carbon/thermoplastic composite where for example the thermoplastic is polyether ether ketone, the material being automatically tape wound or hand laid.

(ii) A carbon/epoxy resin.

(iii) An aluminium alloy.

The cells 15 are preferably provided with drainage slots

16 to allow for condensates to drain from the panel 10.

In the embodiment of the invention illustrated in Figs 1 and 2 the septum elements 14 are fixed in position in the cells 15 using adhesive E3 and as shown lie in a planar surface extending throughtout the component 12.

The septum elements 14 are made from a porous permeable thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide, thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.

The epoxy adhesives El, E2 and E3, may for example be obtained from Ciba-Geigy Plastics & Additives Company Limited of Cambridge, England. Adhesives and resins need not however be epoxy resin adhesives, but could for example be a phenolic, polyimide or thermoplastics resin.

In alternative embodiments of the invention, the septum elements 14 and the walls of the cells 15 may be made from compatible bonding thermoplastics materials and the septum elements 14 bonded to the walls without the use of an adhesive.

The panel 10 is of arcuate form, possibly of double curvature, and is embodied as a structural part of a duct of a nose cowl of a turbofan aero engine, the panel 10 being one of several arcuate panels disposed just upstream of the fan of the engine. It is, of course, of vital importance that the panel does not deteriorate in use and, in particular, that no part of it becomes detached from its supporting structure. The structure

ANNEX TO THE INTERNATIONAL SEARCH REPORT

ON INTERNATIONAL PATENT APPLICATION NO. GB 9200120

SA 55655

This annex lists the patent family members relating to the patent documents cited in the above-mentioned international search report.

The members are as contained in the European Patent Office EDP file on

The European Patent Office is in no way liable for these particulars which are merely given for the purpose of information. 10/04/92

H For more details about this annex : see Official Journal of the European Patent Office, No. 12/82

- 9 - will usually include supporting channel members of which only one member 17 is shown in Fig 2. The panel 10 is secured to the member 17 by bonding the inner facing sheet 132 to an outer face of a flange 18 of the channel member 17 using a carbon to carbon bond 19 and by bonding the backing sheet 11 to the outer face of a flange 20 of the channel member 17 using a carbon to carbon bond 21. The gap between the panel 10 and the base 22 of the channel member 17 may be sealed or closed by use of a mastic 23.

In the embodiment illustrated in Figs 1 and 2, the septum elements 14 are fixed in position so as to lie in a plane extending throughout the cellular component 12. However, in alternative embodiments of the invention, the septum elements 14 may be fixed in position at predetermined other positions in the cells.

A panel having an outer facing sheet 131 made of a porous thermoplastics material as described with reference to Figs. 1 and 2 has been found to give rise to several advantages over the panels of the prior proposals, including the following:

(1) The cellular structure of the facing sheet when produced by the powder sintering technigue can be made to meet permeability requirements over a wide range. The cellular structure may be made permeable to gaseous flow over a wide range of tightly controlled flow and resistance requirements which will be engine dependent and non permeable to a wide range of liquids and solid contaminants.

(2) The cellular structure of the facing sheet when produced by the powder sintering technique provides a

- 10 - highly complex interference flow path as a result of which the noise attenuation properties are greatly enhanced over other forms of perforate and porous material.

(3) The very smooth surface of the facing sheet when produced by the powder sintering technique has substantial acoustic/air flow advantages over other perforate and porous forms. There is a lower flow resistance to high speed air flow, and therefore the overall aero engine power plant efficiency is improved over that obtained using the previously proposed panels;

(4) the sound attenuation is greater and covers a wider frequency range than that of the previously proposed panels;

(5) the thermoplastic component parts do not have the problem of metal corrosion;

(6) the panel is lighter than the previously proposed panels;

(7) there is a improved "blade-off" energy absorption compared with the previously proposed structures; and

(8) there is an improved appearance.

In addition to the above-mentioned advantages, a panel as described with reference to Fig 1 and 2 has the following advantages:-

(1) it is cheaper to manufacture since there is no need to provide two cellular sub-components to form the cellular component 12 or to split the cellular component

- 11 - 12 in order to insert a septum sheet or layer

(2) it allows a greater flexibility in configuration since the septum elements 14 can be fixed to the wall portions at any position within the cells.

Referring now to Fig 3, a panel according to a second embodiment of the invention is illustrated, in which the cellular component 12 of the embodiment of the invention described with reference to Figs 1 and 2 is replaced by a cellular component 32 in which each cell 35 contains three septum elements 34 to form four subcells 351,352,353 and 354. As in the embodiment illustrated in Figs 1 and 2, the cell walls extend continuously from the front face to the rear face of the cellular component 32. and the septum elements 34 are fixed in position within the cells using adhesive E3, but in an alternative embodiment of the invention where the septum elements 34 and the walls of the cells 35 are made of compatible bonding thermoplastics materials may be fixed in position in the cells by bonding them to cell walls.

A backing sheet 31 and inner and outer facing sheets 332 and 331 of the panel shown in Fig 3 are constructed and bonded or secured together in the same manner as the sheet 11 and inner and outer facing sheets 132 and 131 of the embodiment described with reference to Figs 1 and 2, with the backing sheet 31 being secured to the lower face of the cellular component 32 and the outer and inner facing sheets 331 and 332 secured to the upper face of the cellular component 32.

The septum elements 34 are constructed from a porous permeable thermoplastics material which may take the same form as that of the septum elements 14 described with