Note: Descriptions are shown in the official language in which they were submitted.
CA 02628670 2010-12-24
ACOUSTIC NACELLE INLET LIP HAVING COMPOSITE CONSTRUCTION AND
AN INTEGRAL ELECTRIC ICE PROTECTION HEATER DISPOSED THEREIN
This application claims priority based on United States Patent Application
11/733,628 entitled "ACOUSTIC NACELLE INLET LIP HAVING COMPOSITE
CONSTRUCTION AND AN INTEGRAL ELECTRIC ICE PROTECTION HEATER
DISPOSED THEREIN" filed April 10, 2007.
Field of the Invention
The invention relates to ice protection systems for aircraft. More
specifically, the
invention relates to an aircraft equipped with a composite nacelle inlet lip
having both an
embedded electrical ice protection system and acoustic treatment.
Background of the Invention
Aircraft engine nacelles are prone to ice buildup. Fig. 1 shows a schematic
representation of a typical high-speed jet engine assembly 1400. Air enters
through inlet
section 1414, between fan blade spinner 1416 and the annular housing 1418,
which
constitutes the forward most section of the engine nacelle 1420, and includes
nacelle inlet lip
1421. Hot, high-pressure propulsion gases pass through the compressor section
1417 and the
exhaust assembly (not shown) out the rear of the engine. An annular space or D-
duct 1430 is
defined by bulkhead 1428 and annular housing 1418. Bulkhead 1428 separates D-
duct 1430
from the interior portion 1431 of the inner barrel 1412. In flight, under
certain temperature
and humidity conditions, ice may form on the nacelle inlet lip 1421, which is
the leading
edge of annular housing 1418, and on the fan blade spinner 1416. Accumulated
ice can
change the geometry of the inlet area between annular housing 1418 and fan
blade spinner
1416, and can adversely affect the quantity and flow path of intake air. In
addition, pieces of
ice may periodically break free from the nacelle 1420 and enter the engine
1450, potentially
damaging fan/rotor blades 1460 and other internal engine components.
Engine nacelles also channel fan noise from the engines, which can be a prime
source
of aircraft noise. As is known to those skilled in the art, aircraft engine
fan noise can be
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suppressed at the engine nacelle inlet 1414 with a noise absorbing inner
barrel liner 1440,
which converts acoustic energy into heat. The liner 1440 normally consists of
a porous face
skin supported by an open cell backing to provide required separation between
the porous
face sheet and a solid back skin. This arrangement provides effective and
widely accepted
noise suppression characteristics. Aircraft engines with reduced noise
signatures are
mandated by government authorities, and as a result, are demanded by aircraft
manufacturers,
airlines and local communities.
The prior art includes designs for combating both noise and ice buildup on
nacelle
surfaces, and on nacelle inlets, in particular.
Others have developed an acoustically treated nacelle inlet having a hot air
ice
protection system. An acoustic liner positioned forward of the inlet throat
has a perforated
face skin, a perforated back skin, and an acoustic core between the face skin
and the back
skin. The openings through the face skin are sized to allow acoustic energy to
be transmitted
to and dissipated in the acoustic core, and the openings in the back skin are
sized to channel
hot gas from the engine through the acoustic liner to the surface of the inlet
to heat the inlet
and prevent and/or restrict ice formation on the inlet.
U.S. Published Patent Application No. 2005/006529, assigned to Rohr Inc.,
discloses
an acoustically treated nacelle inlet having a low power electric heat ice
protection system.
As used herein, the term "low power" is intended to mean average electric
power
consumption less than about 1 watt per square inch (W/sq. in.). The electric
power supply
may be a conventional source such as batteries, or it may be the engine or an
auxiliary power
unit (APU), or a combination thereof.
Fig. 2 shows a schematic cross-sectional view of an inlet lip 1521 like that
described
in the above-identified published application. The bulkhead 1528 and the inlet
lip 1521
define a D-duct 1530. The inlet lip 1521 includes a noise abatement structure,
which in this
embodiment is an acoustic panel 1504 comprising an open cell core 1508, a
solid back skin
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1509, and an acoustically permeable front skin 1510. The acoustic panel 1504
may be
extended around the leading edge of the nacelle 1520 (as shown in dashed lines
1514 in Fig.
2), rather than ending at or near the leading edge 1505 of the nacelle 1520,
as shown. A low
power electric ice protection system (referred to herein by the acronym IPS)
1512 overlays
the outer surface of the front skin 1510, in the manner described below.
Fig. 3 shows an exploded view of the acoustic panel 1504 shown in Fig. 2. The
acoustic panel 1504 comprises a single degree-of-freedom open cell core 1508,
a solid back
skin 1509, and a perforated front skin 1510. Panels of this sort are well
known to those
skilled in the art. The perforations or openings in the front skin 1510 permit
interaction
between the open cell core 1508 and sound waves generated during operation of
the gas
turbine engine surrounded by the nacelle 1520. The open cell core 1508 is
affixed via epoxy
or other adhesive bonding to each of the skins 1510 and 1509. The sandwich
structure
defined by the core 1508, back skin 1509, and front skin 1510 can be made of
either a
metallic material, such as aluminum, a non-metallic material, such as a
graphite/epoxy
laminate, or a combination thereof.
The low power IPS 1512 is affixed using conventional bonding techniques (e.g.,
adhesive bonding) to the outer surface of the front skin 1510. The IPS 1512 is
connected to
an electric power supply or source (not shown in Figs. 2 or 3) by wiring. The
IPS 1512
comprises an electrically conductive material that is permeable to sound
waves, and can be a
fine grid stainless steel wire mesh adhesively bonded to the outer surface of
the perforated
skin 1510. The fine grid wire mesh typically has a Rayl value between about 50-
150, and
preferably between about 70-110. The IPS is affixed to the skin 1510 of the
acoustic panel
1504 in such a manner that it does not substantially block or otherwise
interfere with a
substantial number of openings in the skin 1510 of the acoustic panel 1504.
This goal may be
achieved by, e.g., selecting the size, shape and configuration of the wire
mesh comprising the
IPS 1512 vis-a-vis the size, shape and configuration of the perforation
pattern in the skin
1510; and/or by using well-established bonding methodologies sufficient to
minimize
blocking the openings with wire mesh and the adhesive used to affix the mesh
to the skin
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1510. In prior art systems, typically no more than about 1-2% of the openings
are completely
blocked, although this figure may range as high as 5% or even 10%.
The prior art system of Fig. 2 also includes a parting strip heater 1507. The
parting
strip heater 1507 is adhesively bonded to the front skin 1510 at or near the
highlight 1505 of
the nacelle (and away from the IPS 1512), where the highlight 1505 is the peak
of the curved
nose of the nacelle. Parting strip heater 1507 comprises an electrifiable grid
material
preferably made of heavier gage wire elements as compared to the IPS 1512 wire
mesh, in
order to conduct a higher power electrical current.
Fig. 4 depicts a perspective view of a portion of a prior art aircraft nacelle
1520
comprising inlet lip 1521. Bulkhead 1528 and inlet lip 1521 define the nacelle
interior
chamber or D-duct 1530. Bulkhead 1528 also separates the D-duct 1530 from the
interior
portion 1531 of inner barrel 1512. An acoustic panel 1504 forms the interior
portion of the
inlet lip 1521. An IPS 1512 and its associated thermal insulation layer (not
shown) are
affixed upon the surface of the acoustic panel, and extend around inlet lip
1521,
approximately to the highlight 1505. In the prior art system of Fig. 4, the
inner barrel 1512,
which is joined to the inlet lip 1521 by joint 1514, comprises one or more
acoustic open cell
panels 1506 for noise abatement. A second joint 1515 joins the nacelle inlet
lip 1521 to the
nacelle outer barrel 1516. The prior art system shown in Fig. 4 further
comprises a parting
strip 1507 at or near the highlight 1505 of the nacelle, depending upon the
location of the
stagnation point of the nacelle (i.e. the point on the nacelle inlet lip at
which the free stream
air impacts directly upon the nacelle inlet lip, and where the impacting air
is stagnant). The
IPS 1512 and parting strip 1507 are electrically connected (by means not
shown) to power
supplies of the type previously described.
Though such prior art nacelle inlet lips may be effective in attenuating
engine noise
and electro-thermally eliminating or minimizing ice buildup on engine
nacelles, such prior
art devices have at least some shortcomings. First, a heating element that is
externally
mounted on a nacelle inlet lip may be susceptible to damage from impacts by
objects striking
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the inlet lip. In addition, externally applied heating elements may delaminate
from the inlet
lip outer skin during prolonged service. In addition, the adhesives used to
bond porous,
externally applied heating elements can at least partially block the acoustic
openings in the
heaters, thereby reducing the percentage of open area ("POA") of the heaters,
and decreasing
the sound-attenuation capabilities of the inlet lip. Accordingly, there is a
need for an
acoustically treated nacelle inlet lip having integrally formed, embedded
electro-thermal
heating elements, and a sufficiently large POA to provide a substantial degree
of engine
noise-attenuation.
SUMMARY OF THE INVENTION
The invention includes an acoustically treated aircraft engine nacelle inlet
lip. The
inlet lip can include an acoustic cellular core arranged along an inner barrel
portion of the
inlet lip, the acoustic core comprising a laminated composite outer skin and
an inner skin.
The inlet lip also can include an erosion shield arranged over at least a
portion of the
laminated composite outer skin, and overlying at least a portion of the
acoustic core. The
laminated composite outer skin can include at least one electrically
conductive heater layer
disposed therein, and can include a first set of openings extending
therethrough, the first set
of openings communicating with the acoustic cellular core. The erosion shield
can include a
second set of openings extending therethrough, the second set of openings
communicating
with the first set of openings.
The invention also includes a noise-attenuating, selectively heatable nacelle
inlet lip
for an aircraft engine. The inlet lip can include a contoured outer skin
structure including a
plurality of composite layers. The inlet also can include at least one
electrically conductive
heater layer disposed between at least two of the composite layers. The inlet
lip can further
include a plurality of openings extending through the plurality of composite
layers and the
electrically conductive sheet.
The invention further includes a nacelle inlet lip having an acoustic cellular
core, and
a plurality of composite outer skin layers covering an outer face of the
acoustic cellular core.
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At least one electrically conductive heater element can be disposed between at
least two of
the composite outer skin layers. At least some of the outer skin layers and
the heater element
can include a plurality of openings extending therethrough, the openings
forming acoustic
pathways to the acoustic cellular core.
The invention also includes a nacelle inlet lip having an acoustic cellular
core, and an
outer skin layer including a plurality of composite outer skin layers, and
covering an outer
face of the acoustic cellular core. The inlet lip also can include means for
heating at least a
portion of the outer skin layer, the heating means being disposed between at
least two of the
composite outer skin layers. The inlet lip can further include means for
permitting sound
waves to pass through at least a portion of the outer skin and the heating
means to the
acoustic cellular core.
In addition, the invention includes a method of forming a composite nacelle
inlet lip.
The method can include providing a tool having a contoured channel, and
placing a porous
erosion shield in the channel. The method also can include placing a composite
outer skin on
the erosion shield, the composite outer skin including a plurality of
composite outer skin
layers and at least one heater element disposed between at least two of the
composite outer
skin layers, and including a plurality of openings extending through the
composite outer skin
layers and the heater element. The method can further include placing an
acoustic cellular
core on the composite outer skin, placing at least one composite inner skin on
the acoustic
cellular core, and heat curing the erosion shield, composite outer skin,
acoustic cellular core,
and composite outer skin to form a unitary composite structure.
These and other aspects of the invention will be understood from a reading of
the
following detailed description together with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a side cross-sectional view of a typical engine nacelle attached to
the forward
portion of an engine assembly.
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Fig. 2 is a cross-sectional view of a prior art nacelle inlet having acoustic
treatment
and an electrical heating system.
Fig. 3 is an exploded view of the acoustic treatment of Fig. 2.
Fig. 4 is a detailed perspective view of a prior art nacelle inlet having
acoustic
treatment and an electrical heating system.
Fig. 5 is a cross-sectional view of a portion of a nacelle inlet lip in
accordance with
the present invention.
Fig. 6 is a detail perspective view of a portion of an outer skin structure
including an
acoustic cellular core of a type that may be employed in the inlet lip of
Figure 5.
Fig. 7 shows the layers of an outer skin bounded on opposite sides by an
erosion
shield and an acoustic core.
Fig. 8 illustrates the inlet lip of the present invention mounted on a
nacelle.
Figs. 9A-9E show other embodiments of an inlet lip in accordance with the
present
invention.
Fig. 10 is an elevation view of an end portion of a tool used to form a
contoured
composite laminate inlet lip according to the invention.
Fig. 11 shows a process flow for forming an inlet lip using the tool of Figure
10.
Fig. 12 is a perspective view of one embodiment of a composite structure for
the
leading edge of an aircraft that includes a composite ice protection heater
apparatus according
to the invention.
Fig. 13 is a perspective view of one embodiment of the composite ice
protection
heater apparatus portion of the composite structure shown in Fig. 12.
Fig. 14 is a cross section of the composite heater apparatus of Fig. 13 as
taken along
line 14-14 in Fig. 13.
Fig. 15 is a cross section of the composite heater apparatus of Fig. 13 as
taken along
line 15-15 in Fig. 13.
Fig. 16 is a perspective view of a portion of the composite heater apparatus
of Fig. 13
showing layers of one embodiment of the composite structure.
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Fig. 17A is an exploded cross sectional view of one embodiment of the
composite
structure of Fig. 1 as taken along line 17-17 in Fig. 12, showing details of
the composite
construction.
Fig. 17B is an exploded cross sectional view of another embodiment of the
composite
structure of Fig. 12 as taken along line 17-17 in Fig. 12, showing details of
the composite
construction.
Fig. 18A is an exploded cross sectional view of the embodiment of the
composite
structure of Fig. 17A as taken along line 18-18 in Fig. 12, showing layers of
the composite
construction.
Fig. 18B is an exploded cross sectional view of the embodiment of the
composite
structure of Fig. 17B as taken along line 18-18 in Fig. 12, showing layers of
the composite
construction.
Fig. 19 is an exploded perspective view of the composite heater apparatus of
Figs. 13-
17A and 18A showing a lay-up sequence for the composite structure.
Fig. 20 is an exploded perspective view of the composite heater apparatus of
Figs. 13,
17B and 18B showing a lay-up sequence for the composite structure.
Fig. 21 is a perspective view of the composite structure shown in Figs. 19-20
with
sheets of maskant applied before perforating the composite structure.
Fig. 22 is a perspective view of a portion of a perforated composite heater
apparatus
according to the invention assembled over an open-cell matrix.
Fig. 23 is a plan view of one embodiment of a composite heater element
according to
the invention.
Fig. 24 is a plan view of another embodiment of a composite heater element
according to the invention.
Fig. 25 is a plan view of a further embodiment of a composite heater element
according to the invention.
Fig. 26 is a plan view of another embodiment of a composite heater element
according to the invention.
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DETAILED DESCRIPTION
Fig. 5 shows a cross-section of a portion of one embodiment of a nacelle inlet
lip 800
in accordance with the present invention. The inlet lip 800 includes an inner
skin generally
designated by reference numeral 810, and an outer skin generally designated by
reference
numeral 820. Both the inner skin 810 and outer skin 820 are connected to a
bulkhead 802.
The inner skin 810 and the outer inner skin 820 each comprises a multi-layer
structure
formed at least in part of a cured composite material, such as layers of
graphite-epoxy fabric.
The inner skin 810 includes a plurality of sections, including a leading edge
inner
skin section 812 (extending along the region designated by reference numeral
812A in Fig.
5), a closeout section 814 (extending along the region designated by reference
numeral 814A
in Fig. 5), and a doubler section 816 (extending along the region designated
by reference
numeral 816A in Fig. 5).
The leading edge inner skin section 812 comprises a plurality of graphite-
epoxy plies,
and includes four plies in one embodiment. During manufacture of the leading
edge inner
skin section 812, the weave pattern of each layer of graphite-epoxy cloth
material preferably
is arranged on a bias relative to adjacent layers. For example the weave
patterns of the four
layers may be laid at 0 , 45 , -45 and 0 relative to a horizontal frame of
reference of a mold
in which the layers are stacked.
The multi-ply (e.g., 3-ply in one embodiment) closeout section 814 connects
the
leading edge inner skin section 812 to an outer barrel portion 822 of outer
skin 820. The
closeout section 814 comprises a closeout core material 855 formed from
fiberglass for
structural support.
In one embodiment, the multi-ply doubler section 816 comprises, in part, a
stepped or
tapered 4-ply reinforced portion having a thickness that is sufficient to
provide an anchor for
attachment to an inner edge of bulkhead 802. For example, in one embodiment,
the 4-ply
construction has a thickness of about 0.25-0.35 cm.
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The outer skin 820 comprises a plurality of graphite-epoxy plies, and
preferably is
formed as a continuous unit. However, this continuous unit can be considered
to have a
number of different portions, each of whose significance is described further
below. These
portions can include an outer barrel portion 822 (the extent of which is
indicated by reference
numeral 822A in Fig. 5), an inner barrel portion 824 (the extent of which is
indicated by
reference numeral 824A in Fig. 5), and a leading edge portion 826 (the extent
of which is
indicated by reference numeral 826A in Fig. 5) which connects the outer and
inner barrel
portions 822, 824. As discussed further below, the inner barrel portion 824 of
the outer skin
820 is acoustically permeable, and preferably includes a plurality of spaced
openings
extending through the skin 820 for acoustic treatment purposes. The leading
edge outer skin
portion 826 includes highlight 828, which is the forward most point in the
cross-section of
the inlet lip 800.
As shown in Fig. 5, an acoustic cellular core 840 is positioned along the
extent 824A
of the inner barrel inner skin portion 824 (i.e., between an inside edge 880
and boundary
842). In combination with the inner skin 810 and outer skin 820, the acoustic
cellular core
840 functions to attenuate noise within the inlet portion of the nacelle to
which the inlet lip
800 belongs. The acoustic cellular core 840 also provides strength and
rigidity to the inlet lip
800.
In one embodiment, the acoustic cellular core 840 has a double-degree-of-
freedom
honeycomb construction of a type known to those skilled in the art. The
acoustic cellular
core 840 extends between the inner barrel portion 824 of the outer skin 820 on
one side, and
a portion of inner skin section 812 and all of doubler section 816 of the
inner skin 810 on the
other side. In one embodiment, as shown in Fig. 5, no portion of the acoustic
cellular core
840 extends past the highlight 828. In other words, in the embodiment shown in
Fig. 5, all
portions of the acoustic cellular core 840 are coextensive with an inboard
surface of the inlet
lip 800.
CA 02628670 2008-04-09
Fig. 6 shows a detailed view of one construction of a portion of the inlet lip
that
includes the acoustic cellular core 840. The acoustic core 840 is disposed
between the
perforated inner barrel outer skin portion 824 and the imperforate inner skin
810, 812, 816.
In the embodiment shown, the acoustic cellular core 840 includes a first sheet
910 of open
cells 920, a second sheet 912 of open cells 922, and a septum 906 disposed
between the first
and second sheets 910, 912. The open cells 920, 922 may have a hexagonal or
honeycomb
shape as shown in Fig. 6, or may have any other desired shape. The open cells
920 of the
first sheet 910 and the open cells 922 of the second sheet 912 may be
substantially aligned
with one another as shown in Fig. 6. Alternatively, the first and second
sheets 910, 912 and
open cells 920, 922 may be laterally offset from one another (in a manner
known to persons
skilled in the art). In one embodiment, the cells 920 of the first sheet 910
are shallower than
the cells 922 of the second sheet 912. For instance, the cells 920 of the
first sheet 910 may
be about 0.5 cm deep, and the cells 922 of the second sheet 912 may be about
2.0 cm deep.
Alternatively, the cells 920, 922 may have equal depths, or other different
depths. In some
embodiments, the first and second open cell sheets 910, 912 are separately
formed, and each
sheet 910, 912 is bonded to one of the opposed sides of the septum 906, thus
forming the
acoustic cellular core 840. In other embodiments, the first and second sheets
910, 912 are
portions of a single open cell sheet, and the septum 906 is introduced within
the open cells of
the single open cell sheet to divide the cells into two separate cell portions
920, 922 in a
manner known in the art.
Regardless of how the acoustic cellular core 840 is formed, the septum 906
includes a
first set of openings 932. Alternatively, the septum 906 may be constructed of
a porous or
permeable material that permits sound waves to pass through the septum. In
such an
embodiment, the first set of openings 932 is a plurality of pores or other
open pathways that
extend through the thickness of the septum 906. In addition, the overlaid
inner barrel outer
skin portion 824 includes a second set of openings 930. The inner skin 810,
812, 816
backing the acoustic cellular core 840 is devoid of such openings, consistent
with the double-
degree-of-freedom construction. The first and second sets of openings 932, 930
may have
the same or different patterns, sizes and/or spacings. In one embodiment, each
set of
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openings 932, 930 is arranged in a uniform pattern, such that the openings 932
or 930 are
evenly spaced apart in a rectangular, hexagonal, staggered-row, or other
uniform distribution.
Returning to Fig. 5, a flexible structural cellular core 850 is positioned
along the
extent 826A of the leading edge portion 826 of the outer skin 820. In one
embodiment, the
structural cellular core 850 has a honeycomb structure formed from a non-
metallic material,
such as heat resistant phenolic-reinforced (HRP) fiberglass, of a type known
to those skilled
in the art. Such a flexible, non-metallic open cell matrix more easily
conforms to the acute
curvature of the leading edge of the nose lip 800 than metallic open cell core
materials. In
the embodiment shown, the boundary 842 between the inner barrel outer skin
portion 824
and the leading edge outer skin portion 826 coincides with the boundary
between the
acoustic cellular core 840 and the structural cellular core 850. Along the
leading edge, outer
skin portion 826 forms a face sheet of the structural cellular core 850, and
the inner skin 812
and closeout section 814 form a backing sheet for the structural cellular core
850. Like the
acoustic cellular core 840, the structural cellular core 850 provides the
inlet lip 800 with
structural rigidity. However, unlike the acoustic core 840, the structural
cellular core 850
does not play a role in noise attenuation.
The boundary 852 between the leading edge outer skin portion 826 and the outer
barrel outer skin portion 822 is defined by the rearward extent of the
structural cellular core
850 and aft edge 882. In the outer barrel portion 822, the outer skin 820
comprises, in part, a
multi-ply composite construction having a thickness sufficient to provide
adequate structural
integrity to the outer portion of the inlet lip 800, and to anchor an outer
edge of the bulkhead
802. In one embodiment, the outer barrel portion 822 of the multi-ply outer
skin 820 has a
thickness of about 0.3-0.4 cm.
At least part of the outer barrel portion 822 of the outer skin 820 may be
covered by a
lightning shield 860. In some embodiments, the entire outer barrel outer skin
portion 822 is
covered by the lightning shield 860. In one embodiment, the lightning shield
860 comprises
a thin conductive mesh formed of copper, and having a thickness between about
0.015 and
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about 0.020 cm. Alternatively, the lightning shield 860 may include a
conductive surfacing
film comprising a non-woven metallic copper mesh impregnated with toughened
epoxy resin
of a type well known to those skilled in the art.
Portions of the surface of the outer skin's inner barrel portion 824 and
leading edge
portion 826 are covered by an erosion shield 870. In some embodiments, the
entire extent of
the inner barrel portion 824 and leading edge portion 826 of the outer skin
820 is covered by
the erosion shield 870. The erosion shield 870 in these regions protects the
outer skin 820
against erosion by particulates that may otherwise damage the composite
materials of the
outer skin 820. In one embodiment, the lightning shield 860 and the erosion
shield 870 are
spaced apart from one another on the outer skin 820 such that they are in non-
overlapping
arrangement with one another. Alternatively, the lightning shield 860 and the
erosion shield
870 may at least partially overlap, and have an electrically insulating
material disposed
therebetween (not shown). In one embodiment, the erosion shield 870 includes
eight
separate circumferential sections, each section spanning a 45-degree segment
of the inlet lip
800. The erosion shield segments are bonded to the outer skin 820 using
methods well
known in the art. In one embodiment, the erosion shield 870 comprises a
titanium alloy
sheet (e.g., Ti-6A1-4V , a titanium alloy which is well-known to persons
skilled in the art)
that is about 0.015 - 0.25 cm thick. In another embodiment, the erosion shield
870
comprises an aluminum sheet of a similar thickness. In yet another embodiment,
the erosion
shield 870 comprises a stainless steel screen.
Returning briefly to Fig. 6, inner barrel outer skin portion 824 includes a
second set
of spaced openings 930. As described below, after the composite outer skin 820
is laid up
and cured, the openings 930 can be formed in a conventional manner using
erosive blasting
and a perforated mask having a pattern of holes corresponding to those
locations where
openings 930 are to be eroded in the composite outer skin 820. Other portions
of the outer
skin 820 may be covered with a non-perforated mask, or otherwise protected
from erosive
blasting.
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The erosion shield 870 includes a third set of openings (not shown in the
figures) in
that portion that coincides with the acoustic cellular core 840; i.e., in that
portion that
coincides with the inner barrel portion 824 of the outer skin 820. This third
set of openings
in the erosion shield 870 communicates with the second set of openings 930 in
the composite
outer skin 824 in this region. As a result, acoustic pathways extend through
the erosion
shield 870, through the outer skin 824, into the outer cells 922 of the
acoustic core 840,
through the first set of openings 932 in the septum 906 of the acoustic core
840, and into the
inner cells 920, thereby effecting noise attenuation via Helmholtz resonance.
The third set of openings in the erosion shield 870 may be micro-perforations,
which
typically are formed by laser drilling. In one embodiment, the micro-
perforations in the
titanium alloy erosion shield 870 have a hole density of about 2,800 - 3,300
holes/cm2, and
have a diameter equal to about 0.005 cm. Lower perforation densities (and thus
lower POAs)
may be acceptable when the erosion shield 870 comprises an aluminum sheet, and
still lower
perforation densities may be acceptable when the erosion shield 870 comprises
a stainless
steel screen.
Fig. 7 shows a cross-sectional view of the inlet lip 800 taken through inner
barrel
portion 824A of the nose lip (as indicated by section line 7-7 in Fig. 5). It
is understood that
the thicknesses of the various layers shown in Fig. 7 are not necessarily to
scale. The outer
skin 824 includes, a first (outer) electrically insulating layer 820A, an
electrically conductive
layer 820B and associated electrical bus strips 521A, 521 B, a second (inner)
electrically
insulating layer 820C, and a structural support layer 820D. Accordingly, the
outer skin 824
is a laminated composite structure having an integrated, electrically
conductive layer 820B
embedded therein. The construction of such a laminated structure and a method
of
producing such a perforated composite outer skin 824 with an embedded
conductive layer
820B are described in detail below. In an alternative embodiment having an
erosion shield
870 with sufficient thickness to structurally support the outer skin 824, the
structural layer
820D shown in Fig. 7 can be omitted from the outer skin 824.
14
CA 02628670 2008-04-09
1
Within the outer skin 824, the electrically conductive layer 820B forms an
electric ice
protection heater. The electrically conductive layer 820B may include a single
electrothermal heating element, or a plurality of spaced, independently
operable heating
elements. If desired, the electrically conductive layer 820B may include a
parting strip heater
element located proximate to the leading edge of the inlet lip 800 (not shown
in the figures).
Fig. 8 shows an inlet lip 800 installed on a nacelle body 1000. A junction
electrical
pylon box 1002, working in tandem with a control unit 1004, provides the
necessary
connections and switches to distribute power to the conductive layer 820B (not
seen in Fig.
8) via wiring 1006. In one embodiment, the wiring 1006 passes through openings
formed in
the inner skin 810 to reach the electrically conductive heater layer 820B. In
other
embodiments, wiring 1006 is attached to the electrically conductive heater
layer 820B either
at a rearward edge 880 of the inner barrel portion 824, or proximate to a
rearward edge 882
of the outer barrel portion 824.
While the foregoing discussion has been directed to the inlet lip 800 shown in
Fig. 5,
it should be understood that other variations of the described inlet lip 800
are within the
scope of the present invention, at least one common theme being that the outer
skin is a
laminated composite structure having an embedded conductive layer which is
operable as an
electric ice protection heater.
In the variation shown in Fig. 9A, an inlet lip 1102 includes an integral
bulkhead
1104 connecting an inner barrel side 1105 of the inlet lip 1102 to an outer
barrel side 1106 of
the inlet lip 1102, thereby formed a D-duct 1103. In the shown embodiment, the
integral
bulkhead 1104 has an open cell or honeycomb structure. Other constructions of
the bulkhead
1104 also are possible. Like the inlet lip 800 described above, the inlet lip
1102 includes at
least one electrically conductive heater layer embedded within its composite
outer skin 1124
(not shown in Fig. 9A).
CA 02628670 2008-04-09
In another variation shown in Fig. 9B, a forward-most portion of an inlet lip
1112
includes a section of structural foam 1114 positioned between an acoustic core
1116 and a
structural core 1118 in one direction, and between a structural outer skin
1120 and an inner
skin 1122 in the other direction. Again, like the inlet lips 800, 1102
described above, the
inlet lip 1112 includes at least one electrically conductive heater layer
embedded within its
composite outer skin 1120 (not shown in Fig. 9B).
In a further variation shown in Fig. 9C, an inlet lip 1150 is selectively
attachable and
detachable to a first flange portion 1152A and a second flange portion 1152B
of a nacelle
body 1156. In particular, the inlet lip 1150 is provided with means to attach
the inlet lip
1150 to the flange portions 1152A, 1152B of the nacelle body 1156. These means
may
include bolts 1154, mating bolt holes 1158 formed in an aft flange area of the
inlet lip 1150,
and aligned complementary bolt holes formed in the flange portions 1152A,
1152B. Thus,
the nacelle 1156 comprises an inlet lip 1150 including a composite outer skin
1158 having an
embedded conductive sheet therein (not shown in Fig. 9C), the inlet lip 1150
being
selectively attachable to, and detachable from, the remainder of the nacelle
1156.
Another variation can be seen by comparing Figs. 9D and 9E. In Fig. 9D, the
outer
barrel portion of the outer skin 1132 has a short axial length D1 (axial
length D1 taken in a
rearward direction from the highlight 1147) relative to the axial length D2 of
the balance
1133 of the nacelle 1134. In contrast, as shown in Fig. 9E, the outer barrel
portion of the
outer skin 1142 has a longer axial length D3 (axial length D3 taken in a
rearward direction
from the highlight 1148) compared to axial length DI, and in relative
proportion to the axial
length D4 of the balance 1143 of the nacelle 1144. In Fig. 9E, the outer
barrel portion 1142
also has a greater axial length D3 than the axial length D5 (axial length D5
being taken from
the highlight 1148) of the inner barrel portion 1146 (whose axially rearward
most extent is
depicted in phantom). In one embodiment, the axial length of the outer barrel
portion 1142 is
more than twice as great as that of the inner barrel portion 1146 (i.e., D3/D5
> 2). The
significance of the embodiment shown in Fig. 9E is that the aft edge of the
nacelle is a
substantial distance D3 from the leading edge of the inlet lip 1140. This
placement of the aft
16
CA 02628670 2008-04-09
h
I T
edge of the outer barrel portion 1142 of the inlet lip facilitates desirable
laminar airflow over
the surface of the nacelle 1144, thereby reducing drag.
The assembly of the inlet lip 800 of Fig. 5 is described below.
First, it should be evident from the foregoing description that a number of
component
layers must be bonded together to form the inlet lip 800. For instance, when
present, the
erosion shield 870 and the lighting prevention sheet 860 are bonded to the
outer skin 820.
The outer skin 820, in turn, is bonded at its inner barrel portion 824 to the
core assembly 840,
and at its leading edge portion 826 to the structural cellular core 850.
Finally the acoustic
cellular core 840 and the structural cellular core 850 are bonded to the inner
skin 810, and the
closeout section 814 is bonded to the outer skin 820 and to the inner skin
810.
Fig. 10 shows a lay-up tool 1200 into which a number of component layers have
been
positioned during a lay up process. In one embodiment, the lay-up tool 1200 is
a
semicircular member having a generally V-shaped or generally U-shaped cross-
section
defining an open-topped channel 1202. The tool 1200 is used to form a first
circumferential
segment of the inlet lip 800. In one embodiment, the first circumferential
segment subtends
about 180 of the total circumference of the inlet lip 800. Similarly, a
second tool (not
shown), may be used to form a second circumferential segment of the inlet lip
800, the
second circumferential segment also subtending about 180 . Once formed, each
end of the
two composite circumferential segments (each of which is an inlet lip half) is
trimmed, and
the two half-segments are mated to form a complete 360-degree unit. The mating
ends of
each half-segment can be joined in a conventional manner to form the complete
inlet lip 800.
It is understood that the first and second lay-up tools, though at least
similar, are not
necessarily identical, since the inlet lip 800 may or may not be rotationally
symmetric about a
central axis in all inlet lip designs.
Fig. 11 presents a process diagram 1300 detailing the principal steps in
forming an
inlet lip 800. In step 1302, a lightning shield 860 and a perforated erosion
shield 870 are
17
CA 02628670 2008-04-09
placed in the channel 1202. In one embodiment, the lightning shield 860 and
erosion shield
870 are placed in the channel 1202 in a non-overlapping arrangement. Together,
the
lightning and erosion shields 860, 870 occupy substantially the entire extent
of the channel
1202. At this stage, the erosion shield 870 includes the third set of openings
preformed
therein.
In step 1304, a first layer of adhesive is applied on the inner surfaces of
the lightning
shield 860 and the perforated erosion shield 870. A heat source, such as a
heat gun, may be
used to soften the first layer of adhesive. In step 1306, the composite outer
skin 820, already
having the second set of openings formed in its inner barrel portion, is
placed on the
lightning shield 860 and the perforated erosion shield 870.
In step 1308, a second layer of adhesive is applied on the exposed inner
surface of the
outer skin 820. A heat source, such as a heat gun, may used to soften the
second layer of
adhesive.
In step 1310, consistent with the embodiment shown in Fig. 5, the acoustic
cellular
core 840, the structural cellular core 850, and the closeout core 855 are
positioned atop the
composite outer skin 820. In one embodiment, a foaming or foamable adhesive is
applied
between adjoining edges of the open cell core sections 840, 850 in a manner
well known in
the art. Alternatively, other methods may be employed to join the adjoining
edges of the
cellular cores 840, 850.
In step 1312, a third layer of adhesive is applied on the exposed inner
surfaces of the
cores 840, 850 in a manner known to those skilled in the art. A heat source,
such as a heat
gun, may be used to soften the third adhesive layer.
In step 1314, the composite inner skin 810 is placed on the third layer of
adhesive.
18
CA 02628670 2008-04-09
In step 1316, the tool 1200 and the composite assembly therein are placed in
an
autoclave to cure the various layers together and form a circumferential inlet
lip segment.
Preferably, the laid up layers are pressed in the channel 1202 of the tool
1200 during heating
in the autoclave such that the layers substantially conform to the contour of
the channel 1202
as they cure.
In step 1318, the tool 1200 and cured circumferential inlet lip segment is
removed
from the autoclave, allowed to cool, and removed from the tool 1200. The ends
and edges of
the inlet lip segment are trimmed to provide a desired configuration and
finish. If desired,
the inlet lip segment can be acoustically tested to check the noise
attenuation properties of
the inlet lip segment.
In step 1320, the mating ends of two circumferential segments are joined
together to
form a complete inlet lip 800. The adjoined ends of the two inlet lip segments
are spliced or
otherwise joined together using methods known to those of skill in the art.
Finally, in step 1322, any necessary fittings are installed on the inlet lip
800 for use in
attachment to a nacelle body.
It is understood that a number of conventional steps have been omitted from
process
diagram 1300, for the sake of simplicity. These omitted steps include
intermediate tests and
quality checks, conditioning of the various composite layers, the use of
intermediate release
layers, the use of vacuum curing bags, and the like, all of which are known to
those skilled in
the art.
Various configurations and production methods of a composite aircraft surface
structure having at least one embedded electric heating element and acoustic
treatment for
use in a composite nacelle inlet lip like that described above are described
below.
19
CA 02628670 2008-04-09
ti
Figure 12 shows a composite surface structure 200, 300 for the leading edge of
an
aircraft. In one embodiment, the composite surface structure 200, 300 is a
segment of an
aircraft engine nacelle inlet lip. For example, the composite surface
structure 200, 300 can
be at least a portion of nacelle inlet composite outer skin 820 like that
described in detail
above. In the embodiment shown, the surface structure 200, 300 includes a
composite ice
protection heater portion 10, 100. In the embodiment of Fig. 12, the composite
heater
portion 10, 100 is integrally incorporated into the composite surface
structure 200, 300. As
shown in Figure 12, the heater portion 10, 100 may include a plurality of
spaced electrical
heater elements 18A-18F. The heater elements 18A-18F may be collectively or
individually
energized to prevent and/or eliminate ice formation on the leading edge of the
structure 200,
300 during service.
Fig. 13 shows one embodiment of a composite heater portion 10, 100 of the
invention. The generally thin and generally flexible heater portion 10, 100
forms a moldable
sheet capable of conforming to at least a portion of a surface contour of an
external surface of
an aircraft. The composite heater portion 10, 100 can be constructed such that
the heater
portion 10, 100 is substantially flat in an unrestrained state. Alternatively,
the heater portion
10, 100 can be constructed such that the heater 10, 100 has a desired three-
dimensional, non-
flat shape in an unrestrained state (like that shown in Figure 13, for
example). In either
embodiment, the composite heater portion 10, 100 is capable of conforming to
an underlying
aircraft support surface or structure, such as an inlet lip of an aircraft
engine nacelle. For
example, with reference to Fig. 5, the composite heater 10, 100 may form at
least a portion of
the outer skin 820, 822, 824, 826 of inlet lip 800.
As shown in Fig. 13, the composite heater portion 10, 100 can include a
plurality of
spaced openings 30 that extend through the entire thickness of the heater. The
composite
heater 10, 100 may also include at least some openings 32 that extend only
partially through
the thickness of the heater 10, 100. The spaced openings 30, 32 can serve two
functions.
First, the spaced openings 30, 32 may provide each heater element 18A-18F with
a desired
degree of electrical resistance, such that when energized, each heater element
imparts a
CA 02628670 2008-04-09
desired level of resistance heating to an associated surface of an aircraft.
In addition, the
spaced openings 30, 32 may act to attenuate at least some aircraft noise by
absorbing or
dissipating at least some acoustic energy at or near the surface of the heater
10. The spaced
openings 30, 32 may have any desired size or shape, and may be arranged in any
desired
array or pattern in the composite heater portion 10, 100. In addition, the
openings 30, 32
may be spaced over substantially the entire extent of the heater 10, 100, or
may be provided
in only select portions of the heater portion 10, 100. In the embodiment shown
in Figure 13,
the heater portion 10, 100 includes six span-wise heating elements 18A-18F
(indicated by
dashed lines). In this embodiment, the full openings 30 are spaced over
substantially all of
heating elements 18A-18D, and the partial openings 32 are provided in heating
elements 18E
and 18F. As described in detail below, the full openings 30 can be provided in
those heater
elements 18 that are located in surface regions of the heater structure 10,
100 where at least
some noise attenuation is desired. Conversely, partial openings 32 can be
provided in those
heater elements that are located in surface regions where noise attenuation is
significant.
In one embodiment of the invention, the openings 30, 32 are holes that are
about 2.5
mm in diameter, and are substantially equally spaced on about 3.8 mm centers.
Accordingly,
in this embodiment, the openings 30, 32 consume slightly less than about 30
percent of the
total surface area of the heater assembly 10, 100. In other words, the
openings 30, 32 define
a POA of nearly 30 percent. Smaller or larger hole diameters and center
spacings, as well as
percentages of POA also may be used, as desired.
Figures 14-16 show enlarged details of one representative laminated composite
construction of a heater portion 10 like that shown in Fig. 13. In this
construction, the heater
portion 10 includes at least one outermost electrically insulating layer 60
covering at least
one underlying electrically conductive layer 50. The outermost insulating
layer 60 may
include one or more plies of low dielectric glass cloth that are pre-
impregnated with a
suitable curable resin. Suitable resins include, but are not limited to, epoxy
resins, cynate
esters, phenolic resins, bismaleimide (BMI) resins, polyimide resins, and the
like. The type
of curable resin used may be based upon the maximum anticipated service
temperature of the
21
CA 02628670 2008-04-09
heater portion 10. For example, phenolic resins may be used for maximum
service
temperatures up to about 107 C., cynate esters for temperatures up to about
121 C., epoxy
resins for temperatures up to about 149 C., BMI resins for temperatures up to
about 204 C.,
and polyimide resins for temperatures up to about 288-343 C. For example, the
insulating
layer 60 may include one or more plies of Style 120 pre-impregnated woven E-
glass fabric of
a type that is well known in the art. Alternatively, the insulating layer 60
may include one or
more plies of Style 7781 E-glass woven fabric prepreg, of a type that is well
known in the art.
Alternatively, the electrically insulating layer 60 may be constructed of any
other suitable
electrically insulating material. Suitable electrically insulating layers 60
preferably have a
dielectric constant less than or equal to about 7, and a dielectric tangent
less than or equal to
about 12x10 at a frequency of about 1 MHz at room temperature.
Preferably, the electrically conductive layer 50 is a sheet that includes a
carbon-based
material such as graphite fibers. For example, the sheet 50 may be a single
layer of an
electrically conductive woven or unidirectional graphite fabric or tape
impregnated with a
suitable curable resin. Suitable resins include, but are not limited to, epoxy
resins, cynate
esters, phenolic resins, bismaleimide (BMI) resins, polyimide resins, and the
like. The type
of resin used may be selected based upon the maximum anticipated service
temperature of
the heater 10, as described above regarding the insulating layers 60.
Alternatively, the
electrically conductive layer 50 may include plural layers of electrically
conductive woven
and/or unidirectional graphite fabrics or tapes. For example, the electrically
conductive layer
50 may include a first layer of a woven graphite fabric, and a thinner second
layer of
unidirectional graphite tape. A combination of plural layers of woven and/or
unidirectional
non-woven graphite fabric sheets or tapes may be used to yield an electrically
conductive
layer 50 having desired electrical characteristics, such as electrical
resistance.
Alternatively, the electrically conductive layer 50 may be any substantially
continuous
conductive material that is capable of conducting an electric current when
subjected to an
electric potential, and that is capable of receiving a plurality of spaced
openings therethrough
22
CA 02628670 2008-04-09
without adversely affecting the material's ability to conduct an electric
current. Other
materials with these characteristics are known to persons skilled in the art.
As shown in Figs. 14-16, 17A, 18A and 19, at least one first electrically
conductive
bus strip 40 is positioned in electrical contact with at least a portion of
the electrically
conductive sheet 50 proximate to one edge of the sheet 50. As described in
detail below, at
least one second electrically conductive bus strip 40 is in electrical contact
with an opposed
portion of the electrically conductive sheet 50 proximate to an opposed edge
of the sheet 50.
When the electrically conductive sheet 50 includes at least one non-woven
electrically
conductive fabric sheet having unidirectional electrically conductive threads,
the first and
second bus strips 40 preferably are placed in electrical contact with opposed
edges that
correspond to opposed ends of the unidirectional threads. Preferably, the bus
strips 40
extend along substantially the full length of the respective opposed edges of
the conductive
sheet 50. The opposed bus strips 40 permit an electric potential to be
substantially uniformly
established across the electrically conductive sheet 50 by connecting the bus
strips 40, 42 to a
suitable power source. Preferably, the bus strips 40 are highly conductive
metal strips, such
as thin strips of copper or the like. As shown in Figs. 14-16, 17A, 18A and
19, at least one
second electrically insulating layer 62 underlies the conductive sheet layer
50 and the bus
strips 40. The second insulating layer 62 may be a layer of pre-impregnated
low-dielectric
glass fabric such as a single ply of Style 120 or Style 7781 E-glass/epoxy
fabric, or any other
suitable electrically insulating material. Accordingly, the conductive sheet
50 and bus strips
40 are encapsulated between the insulating layers 60, 62. As shown in Figures
14-17A, and
in order to minimize the possibility of delamination during service, strips of
adhesive
material 82 may be disposed between the bus strips 40 and the second
insulating layer 62.
The strips of adhesive material 82 enhance bonding between the bus strips 40
and the
insulating layer 62 after curing. For example, the strips of adhesive material
82 may be strips
of FM-300 epoxy adhesive film, available from Cytec Industries, Inc.
Hereinafter, the
combination of the insulating layers 60, 62, conductive layer 50, bus strips
40, and adhesive
strips 82 are collectively referred to as the heater element layers 14 (as
shown in Figs. 17A,
18A and 19).
23
CA 02628670 2008-04-09
As shown in Figs. 18A and 19, the electrically conductive layer 50 may include
a
plurality of spaced conductive sheets 50. Each of the spaced conductive sheets
50 may form
one of a plurality of separate heating elements, such as heating elements 18A-
18F as shown
in Figs. 12 and 13. Preferably, adjacent edges of adjacent conductive sheets
50 are
sufficiently spaced apart to prevent electrical current from passing between
adjacent
conductive sheets 50 during service. Alternatively, or in addition, as shown
in Figs. 18A and
19, inter-heater insulating strips 65 may be positioned between adjacent edges
of adjacent
conductive sheets 50 to electrically isolate adjacent conductive sheets 50
from each other.
As shown in Fig. 18A, one edge of each inter-heater insulating strip 65 may
extend beneath
an edge of a first conductive sheet 50, and an opposed second edge of each
insulating strip 65
may extend over an adjacent edge of an adjacent conductive sheet 50. The inter-
heater
insulating strips 65 preferably are strips of low dielectric glass prepreg
fabric, such as Style
120 or Style 7781 fabric. Alternatively, other electrically insulating
materials may be used
for the insulating strips 65.
As shown in Figs. 14-15, 16, 17A, 18A and 19, the composite heater portion 10
may
further include one or more structural layers 70 beneath the heating element
layers 14. The
structural layers 70 support and reinforce the heating element layers 14, and
help to maintain
the heater portion 10 in a desired contour or shape. The structural layers 70
may be a
plurality of stacked pre-impregnated glass/epoxy fabric layers, for example.
The structural
layers 70 may be adhered to the heating element layers 14 by a suitable layer
or film of
adhesive material 80. One suitable low-flow adhesive that may be used to form
the adhesive
layer 80 is a nitrile phenolic adhesive available from 3M Co., for example.
Alternatively, the
structural layers 70 may be adhered to the heating element layers 14 by
bonding together pre-
impregnated resins within the insulating layer 62 and within at least one of
the structural
layers 70 during an elevated-temperature curing cycle.
As shown in Figs. 13, 15 and 16, the heater portion 10 further includes a
plurality of
spaced openings 30 that extend through the first insulating layer 60, the
conductive sheet
layer 50, the second insulating layer 62, and the structural layers 70. Though
openings also
24
CA 02628670 2008-04-09
can be provided through the bus strips 40, the bus strips 40 preferably are
non-perforated.
The openings 30 may provide the conductive layer 50 with a desired degree of
electrical
resistance, such that when an electrical potential is established across the
opposed bus strips
40, a desired degree of thermal energy is emitted from the conductive sheet
50. In addition,
as further discussed below, the openings 30 can provide the heater portion 10
and an aircraft
surface structure 200 incorporating the heating device 10 with desirable noise
attenuation
characteristics. As shown in Figs. 13-16, one or more attachment openings 20
may be
provided to permit electrical connection of the bus strips 40 to a power
source in a
conventional fashion. In one embodiment, the aircraft surface structure 200
forms at least a
portion of the outer skin 820 of a composite nacelle inlet lip 800 like that
shown in Fig. 5.
The invention also includes a method of producing the heater portion 10
described
above. The process includes assembling the layers of the composite heater
structure 10 as
shown in Fig. 19, for example. The composite lay-up and curing steps and
processes
generally described herein are well known in the art. In a process according
to the invention,
the first insulating layer 60 can be laid over a layer of suitable peel ply
material 92. The peel
ply material 92 may be Code 60001 Peel Ply by Richmond Aircraft Products,
Inc., for
example. At least one sheet of conductive material 50 can be laid on the first
insulating layer
60. Preferably, the first insulating layer 60 is oversized, such that excess
material extends
beyond the outer edges of the conductive sheets 50. When the heater apparatus
includes
plural sheets of conductive material 50 forming separate heater elements, the
sheets 50
should be sized and spaced such that adjacent conductive sheets 50 do not
contact each other.
For single-phase heaters 10, pairs of opposed bus strips 40 can be placed
along edges of the
conductive sheets 50 as shown in Fig. 19. Preferably, the bus strips 40 are
sized such that
they extend along substantially the full lengths of the opposed edges of their
respective
conductive sheets 50. Alternatively, for three-phase systems, four separate
bus strips 40 may
arranged such that one bus strip forms a common ground on a first edge of a
conductive sheet
50, and the other three "hot" bus strips 40 are spaced along an opposed second
edge of the
conductive sheet 50. The bus strips 40 may be overlaid with adhesive strips 82
to enhance
bonding with adjacent layers. A second insulating layer 62 can be laid over
the conductive
CA 02628670 2008-04-09
sheets 50, bus strips 40, and adhesive strips 82, thus completing lay-up of
the heater element
layers 14.
In one embodiment of the process, in order to prevent the second insulating
layer 62
from adhering to the structural layers 70, a release layer 90 can be laid over
the second
insulating layer 62. The release layer 90 may be a layer of porous ArmalonTM
by Du Pont,
for example. Next, structural layers 12 comprising one or more reinforcement
layers 70 can
be laid over the heating element layers 14 and the release layer 90. The
stacked layers 12, 14
then can be prepared for curing at an elevated temperature using methods known
in the art.
Preferably, the stacked layers 12, 24 are placed inside a vacuum bag to
extract entrapped air
from the lamination. Once the air has been excluded, pressure is applied to
compress the
stack, and the stack is subjected to elevated temperatures to cause the pre-
impregnated epoxy
resins to meld and cure.
As discussed above, the heater portion 10 may be generally flat in shape, or
may have
a desired three-dimensional contoured shape like that shown in Fig. 13. When a
generally
flat shape is desired, the stacked layers may be compressed between
substantially flat platens
during curing, for example. Similarly, when a non-flat, contoured shape is
desired, the stack
may be laid up and pressed within a suitably shaped mold 1200 like that shown
in Fig. 10 to
impart the desired three-dimensional shape to the lamination during curing.
After the lamination has been suitably cured, the cured composite can be
removed
from the mold and vacuum bag, and prepared for perforating. In a preferred
process, sheets
of perforated maskant 94 can be selectively placed over those portions of the
stacked layers
that are to be perforated, as shown in Fig. 21. Non-perforated sheets of
maskant 96 and non-
perforated strips of maskant 98 can be applied to those portions of the
stacked layers that do
not receive openings. The maskant sheets 94, 96 and maskant strips 98 may be a
vinyl
masking material available from Diamond Manufacturing, Co., or any other
suitable masking
material. Once the composite structure has been suitably masked, the masked
surface is
blasted with conventional techniques using an erosive media such as metal or
ceramic
26
CA 02628670 2008-04-09
particles, or another suitable erosive media. The erosive blasting is
continued until the
openings 30 extend through the full thickness of the stack at all exposed, non-
masked
locations. Though erosive blasting is a preferred method of forming the
openings 30 in the
stacked layers, other suitable perforation processes also may be used. For
example, the
openings 30 may be formed by mechanical drilling, laser drilling, electron
beam drilling,
chemical etching, or the like.
After blasting, the maskant 94, 96, 98 is removed, and the edges of the
stacked layers
can be trimmed to remove any excess material. Where a release layer 90 is
included between
the heater element layers 14 and the support layers 12, the release layer 90
is removed.
Those portions of the heater element layers 14 protected by the non-perforated
maskant 96
remain non-perforated after erosive blasting. When desired, a non-perforated
region of the
heater element layers 14 may be separately masked with a perforated maskant
and blasted
with an erosive material to perforate that region only with partial openings
32. In this way, at
least some portions of the heater element layers 14 may include partial
openings 32 that have
no corresponding openings in matching portions of underlying support layers
12. These
partial openings 32 may be desirable to modify the electrical resistivity of
the conductive
heating layer 50, without affecting the noise attenuation aspects of the
invention. As shown
in Figure 13, such partial openings 32 may be provided in portions of a
composite heater 10
where electrical resistance modification is required, but sound attenuation is
less important or
not required. For example, in the heater portion 10 shown in Fig. 13 for use
in the nacelle
nose lip segment 200 shown in Fig. 12, partial openings 32 may be provided in
outermost
heater elements 18E and 18F, since these outermost heater elements correspond
to portions
of the nacelle inlet lip that are relatively distant from and shielded from
the noise-generating
turbine blades of an associated aircraft engine.
As shown in Figures 17A and 18A, after perforating the stacked layers and
trimming
away any excess material, a layer of adhesive material 80 can be applied
between the heater
element layers 14 and the support layers 12 in such a manner that the adhesive
material 80
does not substantially block the full openings 30. Corresponding openings 30
in the heating
27
CA 02628670 2008-04-09
element layers 14 and support layers 12 are re-aligned with each other when
the two sets of
layers 12, 14 are bonded together by the adhesive 80. The layers 12, 14 are
again placed in a
suitable vacuum bag, and the adhesive 80 is cured at an elevated temperature
to form a
unitary heater structure 10. After the adhesive 80 is cured, the heater device
10 is finally
trimmed of any remaining excess material.
Alternatively, where no release layer 90 is included between the heater
element layers
14 and the support layers 12, no adhesive 80 is required, and the epoxy resins
of the second
insulating layer 62 and the adjacent support layer 70 can be bonded together
during the initial
curing cycle. Accordingly, the heater device 10 can be finally trimmed after
the assembly
has been perforated, thereby completing the heater device 10.
Another embodiment of a heater portion 100 according to the invention is shown
in
Figs. 13, 17B, 18B and 20. In this embodiment, the heater portion 100 includes
plural layers
of electrically conductive sheets 150, 152 separated by one or more
electrically insulating
layers 160, 162. Though only two layers of conductive sheet layers 150, 152
are shown in
Figs. 17B, 18B and 20, the heater device 100 may include two or more layers of
conductive
sheets 150, 152, each separated by one or more insulating layers 160, 162 as
desired. The
overlapping conductive sheet layers 150, 152 may form redundant heating
elements to
provide backup heaters in the event one or more of the heating elements formed
by one of the
conductive sheets 150, 152 becomes inoperative. Alternatively, the heating
elements formed
by overlapping conductive sheet layers 150, 152 may be selectively energized
in any desired
combination to generate a desired level of heating from a particular region of
the device 100.
In addition, the overlapping conductive heater layers 150, 152 may be
identically sized and
positioned within the heater structure 10 as shown in Figure 20, or may have
different sizes
and positions in the structure 10.
The heater portion 100 otherwise may be substantially similar to the heater
portion 10
having a single conductive layer 50 as described above. As shown in Figures
17B and 20,
electrically conductive bus strips 140, 142 are placed in contact with opposed
portions of the
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CA 02628670 2008-04-09
conductive layers 150, 152 and permit an electrical voltage to be induced
across the
conductive heater layers 150, 152. As shown in Fig. 20, inter-heater
insulating strips 165,
167 may be provided between adjacent edges of adjacent conductive sheets 150,
152 to
minimize the possibility of an electric current passing between adjacent
conductive sheets
150, 152 when the sheets are energized. As shown in Figures 17B and 20, and in
order to
minimize the possibility of delamination, strips of adhesive material 182, 184
may be
disposed between the bus strips 140, 142 and the adjacent insulating layers
162, 164. The
strips of adhesive material 182, 184 enhance bonding between the bus strips 40
and the
insulating layer 62 during curing. Hereinafter, the combination of the
insulating layers 160,
162, and 164, conductive sheet layers 150, 152, bus strips 140, 142, and
adhesive strips 182,
184 are collectively referred to as the heater element layers 16 (as shown in
Fig. 20).
The composite heater assembly 100 includes a plurality of spaced full openings
30
therethrough like those described above for heater portion 10. The composite
heater
assembly 100 also may include a plurality of spaced partial openings 32 like
those described
above for heater portion 10. As shown in Fig. 13, one or more attachment
openings 20 may
be provided in composite heater assembly 100 to permit electrical connection
of the bus
strips 140, 142 to a power source.
The invention also includes a method of producing the multi-layer heater
portion 100
described above. In one embodiment, the process includes assembling the layers
of the
composite heater structure 100 as shown in Fig. 20. A first insulating layer
160 can be laid
over a layer of peel ply material 192. At least one sheet of conductive
material 150 can be
laid over the first insulating layer 160. Preferably, the first insulating
layer 160 can be sized
such that excess material extends beyond the outer edges of the conductive
sheets 150.
When the heater portion 100 includes plural conductive sheets 150 forming
separate heater
elements, the sheets 150 can be sized and spaced such that the conductive
sheets 150 do not
contact each other. Alternatively, or in addition, inter-heater insulating
strips 165 can be
placed between adjacent conductive sheets 150 as shown in Figure 18B. The
inter-heater
insulation strips 165 may be strips of pre-impregnated dielectric glass
fabric, or any other
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CA 02628670 2008-04-09
suitable electrically insulating material. Pairs of opposed bus strips 140 can
be placed along
opposed edges of the conductive sheets 150 as shown in Fig. 20, for example.
Preferably,
the bus strips 140 are sized such that they extend along substantially the
full lengths of the
opposed edges of their respective conductive sheets 150. In order to enhance
the bond
between the bus strips 140 and an overlaid adjacent layer 162, adhesive strips
182, 184 may
be placed over the bus strips 140, 142 as shown in Figs. 17B and 20. Next, a
second
electrically insulating layer 162 can be laid over the layers of conductive
sheets 150, bus
strips 140, and adhesive strips 182. The lay-up process is continued by adding
one or more
additional insulating layers 162, one or more additional layers of conductive
sheet layers 152,
one or additional layers of inter-heater insulating strips 167, one or more
additional layers of
bus strips 142, one or additional layers of adhesive strips 184, one or more
additional
insulating layers 164, and so on. A release layer 190 and one or more
structural support
layers 170 can be laid over the final insulating layer 164. The stacked layers
are placed
inside a vacuum bag, and compressed and cured at an elevated temperature in
the manner
described above.
After curing, the composite structure is masked and perforated as described
above
regarding the single-layer heating device 10. After perforating, the release
layer 190, is
removed from the lamination, and the separate portions of the structure are
adhered together
by a suitable adhesive 180 as described above. Alternatively, the release
layer 190 may be
omitted during lay-up, thereby eliminating the need for adhesive. The heater
assembly 100 is
finally trimmed to remove excess material. Like the single layer heater device
10 described
above, the multi-layer heater assembly 100 may be formed in a substantially
flat state, or may
be laid up and cured in a suitable mold to impart a desired three-dimensional
shape to the
heater device 100. For example, the heater device 100 may be molded to have a
curved
shape that conforms to a nacelle inlet lip, as shown in Fig. 13.
A heater portion 10, 100 according to the invention may be incorporated into a
surface structure of an aircraft to provide ice protection, or to provide
noise attenuation in
addition to ice protection. In particular, the heater device 10, 100 can be
incorporated into an
CA 02628670 2008-04-09
aircraft engine nacelle inlet nose lip segment like that shown in Figs. 5 and
9A-9C, for
example. In one embodiment of the invention, the composite heater device 10,
100 is
mounted over an open-cell matrix 120 with a suitable adhesive 80, 180 as shown
in Figs.
17A-18B and 22. The open-cell matrix 120 may be an open-cell honeycomb
structure, any
other suitable open-cell structure, or any combination thereof. For example,
the open-cell
matrix layer 120 may include a layer of HexWeb HRP Flex-Core available from
Hexcel
Corporation. The open-cell matrix layer also may include a perforated septum
906 like that
shown in Fig. 6. With reference to Fig. 13, one or more non-perforated layers
124 may be
attached on the rear surface of the open-cell matrix 120 by a suitable
adhesive or adhesive
layer 80, 180. The full openings 30 in the heater assembly 10, 100 provide
passageways
between the exterior of the heater device 10, 100 and the cells 122 of the
open-cell matrix
120. Such a construction can provide substantial absorption of acoustic energy
by creating
Helmholtz resonance. Accordingly, such a structure 200, 300 is particularly
suited for use on
a nacelle inlet lip to attenuate engine fan noise, and to provide ice
protection at the nacelle
inlet.
The bus strips 40, 140, 142 of the heater 10, 100 are connected to a suitable
power
source, and operation of each resistance heating element 50, 150, 152 or
combination of
heating elements 50, 150, 152 is controlled by a suitable control device as is
known in the
art. Heat dissipated from the conductive layers 50, 150, 152 of the composite
heater 10, 100
can effectively minimize ice accumulation on the associated surface of the
aircraft, or can
melt or cause the delamination of ice that accumulates on the aircraft
surface.
As shown in Figures 17A-18B, the outermost surface of an aircraft surface
structure
that incorporates a composite heater 10, 100 according to the invention may
include a
durable, acoustically permeable erosion layer 35, 135. In a preferred
embodiment, the
erosion layer 35, 135 is a micro-perforated titanium foil. For example, the
erosion layer 35,
135 may be a 0.2 mm thick titanium alloy foil having a plurality of spaced
openings that are
about 0.25 mm in diameter, and are spaced apart by about 0.5 mm. The erosion
shield 35,
135 shields the composite structure 200, 300 from erosion and damage during
service, and
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CA 02628670 2008-04-09
provides a substantially smooth aerodynamic outer surface to the structure.
The micro-
perforations in the erosion layer 35, 135 permit at least some sound waves to
pass through
the outer surface structure 200, 300, travel through the openings 30 in the
underlying
composite heater 10, 100, and to enter the open cells 122 of the open-cell
layer 120.
Alternatively, the erosion shield 35, 135 may include a perforated portion or
portions that
coincide with an acoustically treated section or sections of the aircraft
surface structure, and a
non-perforated portion or portions that coincide with a non-acoustically
treated section or
sections of the structure. In addition, a layer of non-woven scrim cloth 37,
137 may be
sandwiched between the composite heater 10, 100 and the erosion layer 35, 135
as shown in
Figs. 17A-18B to further enhance the noise attenuation properties of the
structure 200, 300.
As described above, the conductive layers forming the resistance heating
elements 50,
150, 152 may be constructed of a woven or unidirectional pre-impregnated
fabric or tape
including threads containing electrically conductive graphite fibers or
another suitable
conductive component. As described above, and as shown in Fig. 23, the
electrical
resistance of a sheet of electrically conductive fabric 450 can be increased
by introducing a
plurality of spaced openings 430 through the fabric 450. The spaced openings
430 create
discontinuities in at least some of the woven threads, thereby interrupting
the flow of
electrical current through the affected threads when a voltage is applied
between the bus
strips 440. This interruption of current flow forces an electrical current to
seek a more
circuitous, less direct conductive path between the bus strips 440, thereby
generating
resistance heating in the conductive fabric 450.
Spaced, open perforations 430 are desirable when a composite heater 10, 100
according to the invention is incorporated into a composite aircraft surface
structure 200, 300
like that shown in Fig. 12 that attenuates aircraft noise. Other types of
discontinuities in an
electrically conductive woven composite fabric also may be used to provide a
desired rate of
resistance heating from the fabric. As shown in Fig. 24, for example, a
plurality of spaced
slits 530 may be provided in a woven conductive sheet 550. Like the spaced
perforations
430 discussed above, the slits 530 increase the effective electrical
resistance to current flow
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CA 02628670 2008-04-09
between opposed bus strips 540 when an electric potential is applied between
the bus strips
540. As shown Fig. 24, the spacing of slits 530 in woven fabric 550 can be
varied to provide
varying local electrical resistances across the extent of the woven fabric
550. For example,
parallel slits 530 may be closely grouped together in a local region 532 to
create an area of
relatively high electrical resistivity. This region 532 forms a "hot spot"
where the rate of
dissipated resistance heating is greater than other areas of the fabric having
more widely
spaced slits 530. Such a "hot spot" 532 may be desirable along a forward-most
portion of a
leading edge of an aircraft surface structure, for example, which is
susceptible to ice
accumulation. Accordingly, the unevenly spaced slits 530 create at least one
locally
discontinuous property in the weave pattern
The invention also includes a composite heater structure including a fabric
having a
plurality of conductive threads, but without openings such as holes,
perforations, slits, or
other such discontinuities. As shown in Fig. 25, a composite heater structure
600 according
to the invention can include a woven fabric 650 wherein the conductive threads
652
essentially extend in a single direction. The balance of threads forming the
woven fabric
structure 650 may be non-conductive threads, such as low dielectric glass
threads, for
example. In the embodiment shown in Fig. 25, conductive threads 652 extend in
a warp
direction between two opposed bus strips 640. The parallel conductive threads
652 may be
equally spaced, or the thread spacing may be closer in one or more regions 632
of the fabric
650 to create different effective local electrical resistances in different
portions of the fabric
650. In the embodiment of a composite heater 600 shown in Fig. 25, for
example, the
effective electrical resistance in that portion 632 of the woven fabric 650
having more closely
spaced conductive threads 652 is less than the local electrical resistance in
that portion of the
fabric 650 having more widely spaced conductive threads 652. Accordingly, when
an
electrical voltage is applied across opposed bus strips 640, the resistance
heating generated
from region 632 is less than the heating produced where the conductive threads
632 are more
widely spaced. Accordingly, the arrangement of the conductive threads 632
creates at least
one locally discontinuous property in the weave pattern.
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CA 02628670 2008-04-09
The invention also includes a composite heater apparatus 700 as shown in Fig.
26. In
this embodiment, the heater 700 includes a sheet of woven fabric 750 including
a first
plurality of conductive threads 752 extending in a warp direction, for
example, and a second
plurality of conductive threads 754 extending in a fill direction. As shown in
Fig. 26, the
spacing (threads per inch) of warp conductive threads 752 in the weave pattern
is greater than
the spacing (threads per inch) of conductive threads 754 extending in the fill
direction. The
balance of the weave pattern of the woven fabric 750 includes non-conductive
threads, such
as glass threads, for example. Because the fabric 750 includes fewer possible
conductive
paths for current than a composite fabric sheet woven entirely of conductive
warp and fill
threads 752, 754, the effective electrical resistance of the woven fabric
sheet 750 is greater
than the resistance of a composite fabric sheet woven entirely of conductive
threads 752,
754. Accordingly, when an electric voltage is applied across the opposed bus
strips 740, a
greater amount of heat is dissipated from the woven sheet 750 than would
result if the woven
sheet were constructed entirely of conductive threads 752, 754.
The above description of various embodiments of the invention is intended to
describe and illustrate various aspects of the invention, and is not intended
to limit the
invention thereto. Persons of ordinary skill in the art will understand that
certain
modifications may be made to the described embodiments without departing from
the
invention. All such modifications are intended to be within the scope of the
appended
claims.
34
