Acoustics – Sound-modifying means – Sound absorbing panels
Reexamination Certificate
2000-04-18
2002-04-16
Nappi, Robert E. (Department: 2837)
Acoustics
Sound-modifying means
Sound absorbing panels
C181S290000, C181S296000, C181S288000, C181S210000, C428S116000, C493S966000
Reexamination Certificate
active
06371242
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
As is schematically illustrated in
FIG. 1
a typical aero engine
25
includes a turbofan power unit
26
mounted within a nacelle
27
suspended from a pylon
32
. The nacelle
27
includes a nose cowl
28
having an outer wall
29
and an inner wall
30
. The inner wall
30
is in part formed by noise attenuation panels P. The panels P are arranged to form part of the inner wall of the nose cowl
28
in such disposition that the outer facing sheet of the panel forms the wall surface defining the air intake duct
31
for the power unit
26
, The panels P in this disposition serve to reduce noise created by the high speed flow of air passing though the duct
31
and into the power unit
26
, as well as to reduce noise generated by the fan blades of the unit
26
.
As shown in
FIG. 2
, a typical noise attenuation panel
10
comprises a backing sheet
11
, a honeycomb core
12
and a facing component part
13
comprising outer and inner facing sheets
131
and
132
.
The core
12
comprises a multiplicity of open ended juxtaposed cells
15
of hexagonal cross section. The walls of the cells
15
extend from the front face of the core
12
to the rear face. Each cell
15
is, however, divided into an upper subcell
151
and lower subcell
152
by a septum element
14
.
The outer facing sheet
131
of the facing component part
13
takes the form of a woven stainless steel mesh. The inner sheet
132
of the facing component part
13
is an open 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 say 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 honeycomb core
12
.
The outer facing sheet
131
is bonded to the inner facing sheet
132
and the inner facing sheet
132
is secured to the upper face of the honeycomb core
12
by means of an epoxy resin adhesive.
The backing sheet
11
is unperforated and made from a non-porous impermeable sheet material and is secured by an epoxy resin adhesive to a lower face of the honeycomb core
12
.
The walls of the cells
15
of the core
12
are made from a non-porous impermeable sheet. The cells
15
are preferably provided with drainage slots
16
to allow for condensates to drain from the panel
10
.
The panel
10
is typically of arcuate form, possibly of double curvature, and is embodied as a structural part of a duct of a nose cowl of the turbofan aero engine, the panel
10
being one of several arcuate panels P disposed just upstream of the fan of the engine.
Such noise attenuation panels when used in aeroengine nacelles are termed acoustic liners and absorb engine intake noise by allowing a controlled resonance to occur with partially closed honeycomb cells.
In a typical manufacturing procedure for such panels, the following steps are carried out:
(1) The backing sheet
11
is precured
(2) The open weave inner facing sheet
132
is precured to a predetermined profile
(3) The cured inner facing sheet
132
is bonded to the stainless steel mesh outer facing sheet
131
(4) Adhesive is reticulated onto the walls of the cells of the honeycomb core
12
.
(5) The above components are assembled and bonded together, that is to say, the backing sheet
11
, the honeycomb core
12
and the pre-bonded outer and inner sheets
131
,
132
.
The application of adhesive to the honeycomb core
12
is typically carried out as illustrated in FIG.
3
(A) to FIG.
3
(F) and comprises the following steps:
A) Adhesive film
101
is applied to the face of honeycomb core
12
, as illustrated in FIG.
3
(A),
B) The adhesive film
101
is heated so that it tacks to side walls of the cells
15
of the honeycomb core, as illustrated in FIG.
3
(B),
C) Hot air is applied to the adhesive film
101
in the direction of the arrows R to cause the adhesive film
101
to balloon while thinning the film at the mid point of each cell, as illustrated in FIG.
3
(C),
D) The adhesive film
101
is caused to burst and starts to reticulate, as illustrated in FIG.
3
(D),
E) The adhesive film fully reticulates to envelop the ends of the cell walls and form beads
102
, as illustrated in FIG.
3
(E), and
F) Special heat treatment is applied to improve containment of the reticulated adhesive, as illustrated in FIG.
3
(F).
During the final stage bonding of the pre-bonded sheets
131
and
132
to the honeycomb core
12
adhesive bleeds through the outer facing sheet
132
causing cosmetic spots
103
as illustrated in FIG.
4
. Furthermore, the volume of adhesive deposited around the honeycomb cell edges is non-uniform.
In an attempt to control adhesive flow, an oven stabilisation cycle was introduced after reticulation and prior to final stage cure. It causes adhesive to flow away from the cell edge and also introduces a degree of cure advance. In theory a more uniform adhesive bead
102
is formed with a higher initial viscosity which is less prone to excessive flow during final stage cure.
It has however been found that oven stabilisation is not always successful and that a critical size adhesive droplet
103
will flow through a typical open area intersection as shown in FIG.
4
.
The spotting effect is unpredictable and a function of reticulation, stabilisation and autoclave cure combined with adhesive chemistry/viscosity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a noise attenuation panel so constructed as not to give rise to the above-mentioned adverse spotting effects at the front surface of the outer facing sheet of a noise attenuation panel.
According to a first aspect of the present invention there is provided a method of manufacturing a noise attenuation panel which comprises:
a cellular component part which has a front face, a rear face and wall portions which extend from the front face to the rear face and which provide bounding surfaces for a multiplicity of cells which extend from the front face to the rear face, and
a facing component part which:
has a front face and a rear face,
extends across the ends of the cells of the cellular component part at the front face therof with the rear face of the facing component part adjacent the front face of the cellular component part,
is formed with a multiplicity of apertures which provide gaseous fluid communication between the cells of the cellular component part and the front face of the facing component part for the attenuation of noise generated by gaseous fluid flow over the surface of the front face of the facing component part, the method comprising:
bonding the facing component part to the cellular component part by the steps of:
applying an adhesive film to the front face of the cellular component part
causing the film to reticulate to the ends of the walls of the cells at the front face of the cellular component part,
introducing an adhesive flow control sheet between the front face of the cellular component part and the rear face of the facing component part,
bringing the two component parts together, with the interposition of the adhesive flow control sheet, and
causing the reticulated adhesive on the ends of the wall portions of the cells of the cellular component part to bond the two component parts together with adhesive flow to the facing component part under the control of the adhesive flow control sheet.
In an embodiment of the invention according to its first aspect, the rear face of the outer facing sheet and the front face of the inner facing sheet are bonded to form the facing component part prior to the step of bringing the facing and cellular component parts together for bonding
Braniff Mark
Parkes Richard
Wilson Robert Samuel
Martin Edgardo San
Nappi Robert E.
Short Brothers PLC
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