U.S. patent application number 11/277888 was filed with the patent office on 2007-10-11 for lightning protection system for composite structure.
Invention is credited to James F. Kirchner, Quynhgiao N. Le, Ron Murakami, Eric R. Steele.
Application Number | 20070236855 11/277888 |
Document ID | / |
Family ID | 38349516 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070236855 |
Kind Code |
A1 |
Le; Quynhgiao N. ; et
al. |
October 11, 2007 |
LIGHTNING PROTECTION SYSTEM FOR COMPOSITE STRUCTURE
Abstract
A lightning protection system for protecting composite
structures and a method of protecting composite structures from
lightning strikes. A dielectric ply is fixed above and completely
covers metal surface features, e.g., skin fasteners through a
composite skin to a wing fuel tank. A conductive ply is fixed above
and completely covers the dielectric ply and extends to an external
connection to a platform ground. The conductive ply directs current
from lightning strikes away from metal surface features, e.g., to
the platform ground to preventing sparking in the fuel tank. Both
plies may be adhesively backed and sequentially pressed into
place.
Inventors: |
Le; Quynhgiao N.; (Bellevue,
WA) ; Murakami; Ron; (Edgewood, WA) ; Steele;
Eric R.; (Kent, WA) ; Kirchner; James F.;
(Renton, WA) |
Correspondence
Address: |
LAW OFFICE OF CHARLES W. PETERSON, JR. -- BOEING
435B Carlisle Dr.
Herndon
VA
20170
US
|
Family ID: |
38349516 |
Appl. No.: |
11/277888 |
Filed: |
March 29, 2006 |
Current U.S.
Class: |
361/212 |
Current CPC
Class: |
B64D 45/02 20130101 |
Class at
Publication: |
361/212 |
International
Class: |
H02H 1/00 20060101
H02H001/00 |
Claims
1. A lightning protection system comprising: a dielectric ply fixed
to a composite surface above and completely covering at least one
metal surface feature, each said at least one metal surface feature
being a protected feature; and a conductive ply fixed to said
composite surface and electrically connected to any conductive
unprotected feature, said conductive ply completely covering said
dielectric ply, wherein current from lightning strikes is dispersed
by said conductive ply.
2. A lightning protection system as in claim 1, wherein said
dielectric ply and said conductive ply are fixed to a composite
wing of an aircraft and said at least one metal surface feature
includes a skin fastener.
3. A lightning protection system as in claim 2, wherein said
conductive ply comprises: a protective surface layer; a conductive
center layer; and an attachment backing layer adjacent to said
conductive center layer and conductively attaching said conductive
center layer to said composite wing, said conductive center layer
being sandwiched between said protective surface layer and said
attachment backing layer.
4. A lightning protection system as in claim 3, wherein said
conductive ply consists of said protective surface layer, said
conductive center layer and said attachment backing layer.
5. A lightning protection system as in claim 3, wherein said
protective surface layer is a dielectric film, said conductive
center layer includes a metallic layer and said attachment backing
layer is layer of pressure sensitive adhesive.
6. A lightning protection system as in claim 5, wherein said
metallic layer comprises a 0.001 '' to 0.004'' (0.025-0.102 mm)
thick metal mesh incorporated in an adhesive.
7. A lightning protection system as in claim 5, wherein said
dielectric ply comprises: a layer of said dielectric film; and a
pressure sensitive adhesive layer attaching said layer of
dielectric film to said composite wing.
8. A lightning protection system as in claim 7, wherein said
dielectric film is a film of dielectric material selected from a
group of materials consisting of a fluoropolymer, a polyester, a
polyimide, and polyurethane.
9. A lightning protection system as in claim 7, wherein said
dielectric film in each said ply comprises a glass filler.
10. A lightning protection system as in claim 1, further
comprising: a platform ground; and an external connection to said
platform ground, said conductive ply contacting said external
connection, current from lightning strikes dispersing through said
platform ground.
11. A method of protecting a composite structure from lightning
strikes, said method comprising the steps of: fixing a dielectric
ply to a composite skin, said dielectric ply completely covering at
least one metal surface feature, each said at least one metal
surface feature being a protected feature; and fixing a conductive
ply to said composite skin, said conductive ply covering said
dielectric ply and electrically connected to any conductive
unprotected feature, conductively connected to said composite skin
and providing a ground path for current from lightning strikes.
12. A method of protecting a composite structure from lightning
strikes as in claim 11, wherein the step fixing said dielectric ply
comprises: selecting an exposed surface area on said composite
skin, said exposed surface area encompassing said at least one
metal surface feature; providing an adhesive backed dielectric
film, said adhesive backed dielectric film being larger than said
exposed surface area by a selected overlap amount; and applying
said adhesive backed dielectric film to said exposed surface area,
said adhesive backed dielectric film overlapping each said at least
one metal surface feature by said selected overlap amount.
13. A method of protecting a composite structure from lightning
strikes as in claim 12, wherein said composite structure is an
aircraft, said composite skin is at a wing fuel tank and said at
least one metal surface feature includes a plurality of skin
fasteners, and the step of selecting said exposed surface area
comprises determining a perimeter defined by said plurality of skin
fasteners, said perimeter defining said exposed surface area.
14. A method of protecting a composite structure from lightning
strikes as in claim 11, wherein the step of fixing said conductive
ply comprises: providing an adhesive backed conductive layer, said
adhesive backed conductive layer being larger than said dielectric
ply by a selected amount and large enough to extend to an external
connection to a platform ground; and applying said adhesive backed
conductive layer over said dielectric ply and said external
connection, wherein lightning striking said conductive ply is
insulated from protected metal surface features by said dielectric
ply and current from a lightning strike is diverted through said
conductive ply to said platform ground.
15. A lightning protection system attached to a composite skin
above metal surface features and outboard of a fuel tank, said
lightning protection system protecting said fuel tank from
lightning strikes, said lightning protection system comprising: at
least one dielectric ply fixed above and completely covering one or
more metal surface features; a platform ground path; a surface
connection to said platform ground path; and a conductive ply fixed
above and completely covering one said at least one dielectric ply
and extending to said surface connection, said conductive ply
directing current from lightning strikes to said platform ground
path.
16. A lightning protection system as in claim 15, wherein said
conductive ply comprises: a protective surface layer; a conductive
center layer; and an attachment backing layer adjacent to said
conductive center layer and attaching said dielectric material
layer to said composite skin, said conductive center layer being
sandwiched between said protective surface and said attachment
backing layer.
17. A lightning protection system as in claim 16, wherein said
protective surface layer is a dielectric film, said conductive
center layer includes a metallic layer and said attachment backing
layer is layer of pressure sensitive adhesive.
18. A lightning protection system as in claim 17, wherein said
dielectric film is a film of dielectric material selected from a
group of materials consisting of a fluoropolymer, a polyester, a
polyimide, and polyurethane.
19. A lightning protection system as in claim 17, wherein said
metallic layer comprises a 0.001'' to 0.004'' (0.025-0.102 mm)
thick metallic mesh incorporated in a 0.002'' to 0.008''
(0.050-0.210 mm) adhesive layer.
20. A lightning protection system as in claim 19, wherein said
dielectric ply comprises: a layer of said dielectric film; and a
pressure sensitive adhesive layer attaching said layer of
dielectric film to said composite wing.
21. A lightning protection system as in claim 20, wherein each said
dielectric film comprises a glass filler, said dielectric film in
said conductive ply is 0.002'' to 0.0035'' (0.050-0.089 mm) thick,
and said dielectric film in said dielectric ply is 0.003'' to
0.010'' (0.076-0.254 mm) thick.
22. A lightning protection system as in claim 21, wherein glass
filler comprises a glass weave cloth.
23. A lightning protection system as in claim 21, wherein glass
filler comprises glass particles.
24. A lightning protection system as in claim 19, wherein said
composite skin is a portion of the wing of a composite aircraft
outboard of a fuel tank and ones of said one or more metal surface
features are skin fasteners penetrating said composite skin to said
fuel tank and holding said composite skin in place.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present invention is related to Published U.S. Patent
Application No. 20050181203, entitled "Applique" to Diane C.
Rawlings et al.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to protecting
composite structures from lightning strikes, and more particularly,
to an applique for protecting composite aircraft from lightning
strikes.
[0004] 1. Background Description
[0005] Since aluminum and other metals are highly conductive, a
transient charge from a lightning striking a metal body discharges
into the metal body with current from the discharge being
distributed relatively evenly over the body. So, a typical
lightning strike to a metal aircraft causes no or only minor damage
to aircraft components. However, carbon fiber composites generally
have a higher strength-to-weight ratio than aluminum, and so, are
increasingly replacing aluminum structural components.
Unfortunately, typical state of the art composites, such as Carbon
Fiber Reinforced Plastic (CFRP), are approximately 2000 times more
resistive than aluminum.
[0006] So, a lightning strike to unprotected components, embedded
in, passing through or attached to the CRFP aircraft skin that is
generally 35-40% resin, does not dissipate as readily as in metal.
A lightning strike can cause considerable damage to the CRFP
aircraft skin, even destroying critical portions of the aircraft.
Typically, temperature drives the damage level. The discharge
current through composite skin resistance dissipates a burst of
energy that causes the carbon fibers to become very hot, so much
so, that the skin temperature becomes much hotter in a CRFP
structure than in a state of the art aluminum skin. The temperature
may exceed the CRFP resin pyrolization temperature, causing the
resin to vaporize, turning some of it from a solid to a gas. Some
of the gas may be trapped inside the CFRP skin layer. Consequently,
internal gas pressure from the vaporized resin can damage the
structure causing de-lamination, and possibly, puncture the
underlying systems or structure. Further, as the vaporized resin
escapes explosively from the skin, hot particles (sparks) eject
(e.g., from fastener interfaces and CFRP composite joints) from the
CFRP.
[0007] So, a lightning strike that may have little or no effect on
an aluminum structure may be disastrous to unprotected CFRP
components. For adequate lightning protection for a composite wing
structure, the exterior CFRP structure must withstand not only the
initial lightning strike, but also at least one hundred kiloamperes
(100 kA) of discharge current without adverse affects or impact to
safety-of-flight. Furthermore, skin fasteners at an exposed surface
are most vulnerable to a direct strike and such a strike is most
likely to cause sparking at structural joints and, more
importantly, in the fuel tank. Accordingly, composite structure
aircraft must have some protection, especially at exposed skin
fasteners, and especially at fuel and hydraulic couplings in the
fuel tank to minimize this sparking. However, it is also important
that this protection is economically feasible, in its initial
application, in its effectiveness for minimizing resulting damage
and, in subsequent consequent repair or replacement, both for
continued aircraft flight worthiness and to meet economic repair
targets.
[0008] Unfortunately, typical approaches to minimizing the
possibility of fuel ignition and reducing damage are complicated
and difficult to implement in CFRP. One approach involves
selectively integrating metal (e.g., copper foil) into or onto the
composite laminate at the fastening areas. Moreover, this new
approach has been expensive; is often difficult to implement/rework
with labor intensive application processes both pre and
post-assembly; and, has not consistently exhibited acceptable EME
protection. Copper foils, for example, have been subject to
wrinkling during lay-up/cure. Drilling the laminate for fastener
installation may contaminate the fuel tank with copper. Even with
this additional protection, in the absence of other supporting
protection (e.g., fastener collar isolation, fillet/cap sealing),
the structure may still have a low sparking threshold. In addition
to added complexity, integrating a conductive surface protection
layer into the composite wing skin may carry with it an
unacceptable weight penalty.
[0009] Thus, there is a need for effective lightning protection for
composite structures that is lightweight, relatively low-cost, as
well as simple to apply and repair, and especially for such
lightning protection for composite aircraft.
SUMMARY OF THE INVENTION
[0010] An embodiment of the present invention reduces lightning
strike damage to aircraft and in particular, fuel tank sparking
from lightning strikes to composite surfaces. Thus, an embodiment
of the present invention simplifies protecting composite aircraft
and in particular wing fuel tanks from lightning strikes, and
simplifies repairing of damage to the aircraft lightning strike
protection system.
[0011] More particularly, embodiments of the present invention
include a lightning protection system in a lightning protection
applique, an aircraft including the lightning protection system and
method of protecting an aircraft from lightning strikes. A
dielectric ply is fixed (e.g., bonded) above and completely covers
metallic skin fasteners. A conductive ply is fixed (e.g., bonded)
above and completely covers the dielectric ply and extends to an
external connection, e.g., to a platform ground. The conductive ply
directs lightning discharge current away from critical areas. Both
plies may be adhesively backed and sequentially pressed into
place.
[0012] Advantageously, a preferred lightning protection applique
provides flexibility in lightning protection design without
increasing aircraft weight appreciably and with superior
performance and protection. Applique dielectric and conductor
layers are simply sequentially pressed in place on the skin to
avoid adding an embedded conductive layer or requiring a conductive
surface protection layer for a CFRP skin. Thus a preferred
lightning protection system simplifies aircraft skin design while
reducing weight. In addition to weight, cost savings are realized
from simple dielectric and conductor layer manufacturing
requirements, as well as maintenance simplicity. Although age may
make removal increasingly difficult, an entire preferred applique
may be replaced by peeling the old ply(plies) off and pressing a
new one(s) in place.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The foregoing and other objects, aspects and advantages will
be better understood from the following detailed description of a
preferred embodiment of the invention with reference to the
drawings, in which:
[0014] FIG. 1 shows an example of an aircraft skin cross section
through the wing with lightning protection applique (LPA) according
to an advantageous embodiment the present invention.
[0015] FIG. 2 shows an example of a preferred LPA attached to the
wing of an aircraft.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0016] Turning now to the drawings and more particularly, FIG. 1
shows an example of a lightning protection system according to a
advantageous embodiment of the present invention in a cross section
100 of an aircraft. In this example, the cross section 100 is taken
through the composite skin 102, e.g., a Carbon Fiber Reinforced
Plastic (CFRP) skin, of the aircraft wing, protected by a preferred
embodiment Lightning Protection Applique (LPA) 104. The composite
skin 102 is fastened to a rib (metal or CFRP) 106 or similarly to a
spar (CFRP), by skin fasteners 108 extending through the skin 102
and shear tie flange 107, held in place by collars or nuts 110. In
this example, the cross section 100 is part of a fuel tank of a
wing section. Although shown in this example protecting skin
fasteners 108 at a wing fuel tank, this is for example only. A
preferred LPA 104 may be used to protect any composite structure
surface area where metal is exposed at the skin surface and so, is
exposed to similar lightning threat levels, including other areas
of a composite aircraft.
[0017] Preferably, the lightning protection applique 102 includes a
dielectric ply 112 electrically isolating and insulating the skin
fasteners 108, e.g., from a lightning strike, and a conductive ply
114 diverting electrical energy from such a lightning strike away
from the isolated skin fasteners 108. Further, the conductive ply
114 extends at least 1.0'' (2.54 cm) beyond the dielectric ply 112
at the skin fasteners 108, in this example to a ground contact 116,
e.g., a bolt or rivet, that is separated from the skin fasteners
108 and located away from the fuel tank. The ground contact 116 is
connected to a platform ground 118 and held in place by suitable
attachment 120, e.g., a nut. Alternately, the conductive ply 114 is
selected large enough that the lightning strike current dispersed
around the surface of the structure to what are non-critical areas
and without connecting the conductive ply 114 to platform ground
118.
[0018] The dielectric ply 112 may be, for example, a suitable
electrically insulating or dielectric film 112D of an appropriate
thickness, e.g., 0.003'' to 0.010'' (0.076-0.254 mm) and an
attachment backing 112A, e.g., of pressure sensitive adhesive,
preferably, 0.002'' (0.050 mm) thick. The specific material
selected for electrically insulating dielectric film 112D depends
on the intended installation conditions and the system's design.
For demanding environments or designs where paint is not intended
over the applique, the selected insulating material may be a film
of a fluoropolymer such as for example polytetrafluoroethylene
(PTFE); or a terpolymer of tetrafluoroethylene,
hexafluoropropylene, and vinylidene fluoride (THV); or fluorinated
ethylene propylene (FEP); or perfluoroalkoxytetrafluoroethylene
(PFA). For applications where painting the applique may be desired
or necessary, polyester, polyimide, or polyurethane films may be
more appropriate to facilitate paint adhesion.
[0019] The conductive ply 114 in this example includes a conductive
center layer 114C sandwiched between and encapsulated by a
protective surface layer 1 14S and an attachment backing 114A.
Preferably, the conductive center layer 114C includes a 0.001'' to
0.004'' (0.025-0.102 mm) thick metallic layer (solid or mesh)
incorporated in an adhesive. The preferred protective surface layer
114S provides the conductive center layer 114C with partial
environmental protection and promotes primer/top coat adhesion for
subsequent painting, if necessary. Also, the preferred protective
surface layer 114S is 0.002'' to 0.004'' (0.051-0.102 mm) thick
electrically insulating film, that may be the same material as
electrically insulating film 112D in the dielectric play 112. The
attachment backing 114A may be a layer of pressure sensitive
adhesive, 0.002'' to 0.008'' (0.051-0.203 mm) thick. Both
attachment backing 112A and 114A provide adhesion for attaching the
respective ply 112, 114 to the underlying structure, i.e., CFRP
skin, and/or the dielectric ply 112. The metallic layer in
conductive center layer 114C may be incorporated in the same
adhesive material. Also, a sealant may be applied along the edges
of the plies 112, 114, e.g., to prevent chemical/environmental
erosion.
[0020] The dielectric ply 112 is applied in a strip to the skin 102
covering all skin fasteners 108 in the area of exposure. If
applicable, the dielectric ply 112 also covers any other exposed
surface metal features. Since the dielectric ply strip 112 may be
limited to only the area around surface metal such as skin
fasteners 108, the strip 112 provides a significant weight saving
over a more encompassing approach. Examples of such encompassing
approaches include, for example, Published U.S. Patent Application
No. 20050181203, entitled "Appliqu+E,acr e" to Diane C. Rawlings et
al., and Published U.S. Patent Application No. 20050150596 entitled
"Methods and Materials for Reducing Damage from Environmental
Electromagnetic Effects" to Terrence G. Vargo et al., both of which
are assigned to the assignee of the present invention and
incorporated herein by reference. The overlap distance that the
dielectric layer strip 112 must overlap surface metal depends on
the skin resistance and the level of desired protection. However,
preferably, for a state of the art CFRP and for a one hundred
kiloampere (100 kA) lightning strike, the overlap is at least 1.8''
(4.57 cm) to sufficiently isolate metallic surface features.
[0021] The conductive ply 114 has significantly greater area
coverage than the dielectric layer strip 112 and is applied
directly over the dielectric layer strip 112 and CFRP skin 102.
Depending on design requirements, the preferred conductive ply 114
may cover the entire structure (e.g., aircraft or other composite
structure) or only selected sections of the structure (e.g.,
selected sections of a composite wing or fuselage), e.g., for
weight reduction. The conductive ply 114 provides a high current
path to the platform ground 118 that directs current from lightning
strikes away from the isolated skin fasteners 108 and through
grounding studs 116 and nuts 120. So, the grounding studs 116 also
must be spaced adequately away from the skin fasteners 108,
depending upon skin 102 resistance and desired protection level.
Thus, the conductive ply 114 overlaps and completely covers a
significantly greater area than the dielectric ply 112. The much
lower resistance of the conductive ply 114 assures that very little
current, if any, flows through the isolated skin fasteners 108. So
instead, substantially all of the effects of a lightning strike are
directed well away the critical wing box section and so, away from
the fuel tank. Accordingly, a preferred lightning protection
applique 104 is capable of successfully meeting the lightning
protection requirements for lightning strike zone 2 (100 kA) as set
forth in SAE International standard No. ARP5412.
[0022] FIG. 2 shows an example of an aircraft 120 with a preferred
lightning protection applique (e.g., the LPA 104 of FIG. 1) fixed
to the skin 102 of the wing, e.g., over metal features on the wing.
As noted above, the metal features may be fasteners at the fuel
tank. Moreover, since the preferred conductive ply provides this
external high current path, it is unnecessary to add weight to the
CFRP skin by including a conductive surface protection layer. Thus,
the preferred embodiment lightning protection applique 104, which
contains the lightning discharge protection, avoids the substantial
weight of including a conductive surface protection layer. In
particular, a preferred lightning protection applique 104 may be
applied post-assembly after fastener installation and is easily
inspected, maintained and replaced as necessary. Further, for a
selective implementation, the preferred lightning protection
applique 104 may also be uniquely configured/designed to satisfy
the Electromagnetic Effect (EME) requirements for a particular
lightning zone. Generally, a higher expected lightning discharge
carries a higher current level and requires greater conductor
thickness. So, the dielectric layer (112D, 114D in FIG. 1)
thicknesses and the conductive element (114C in FIG. 1) thickness
can be selected accordingly to satisfy the particular lightning
protection requirement level of each particular lightning zone.
[0023] Advantageously, the preferred lightning protection applique
provides flexibility in lightning protection design without
increasing aircraft weight appreciably and with superior
performance and protection. Instead of adding an embedded
conductive layer or requiring a conductive surface protection layer
for a CFRP skin, the dielectric and conductor layers are simply
sequentially pressed in place on the skin. Thus the present
invention simplifies aircraft skin design while reducing weight. In
addition to weight, cost savings are realized from simple
dielectric and conductor layer manufacturing requirements, as well
as maintenance simplicity. An entire applique may be replaced by
just peeling the old ply(plies) off and pressing a new one(s) in
place.
[0024] While the invention has been described in terms of preferred
embodiments, those skilled in the art will recognize that the
invention can be practiced with modification within the spirit and
scope of the appended claims. It is intended that all such
variations and modifications fall within the scope of the appended
claims. Examples and drawings are, accordingly, to be regarded as
illustrative rather than restrictive.
* * * * *