 |
Description  |
|
|
TECHNICAL FIELD
This invention relates to lightning protection systems for aircraft and,
more particularly, to a metallic fastener having a dielectric cap which
has sufficient resilience and is dimensioned to fill a gap between the
fastener head and a counterbore in which the fastener is installed, to
provide a continuous flush surface for receiving a crack free coat of
paint.
BACKGROUND ART
In the manufacture of aircraft structures, composite materials, such as
graphite fiber reinforced epoxy resins, are being used increasingly in
place of metals such as aluminum. One of the major advantages of the use
of the composite materials is that they make it possible to significantly
reduce the weight of the aircraft structure and, therefore, result in a
more fuel efficient aircraft. The potential increase in fuel economy makes
it desirable to maximize the use of composite materials in the manufacture
of commercial aircraft. One type of structure that may be at least
partially fabricated from composite materials is a wing box structure
which has an outside skin that forms a portion of the wing skin and
internal portions defining a wing fuel tank. The wing box is normally
attached to the rest of the wing structure by means of fasteners of the
type having a shaft and a head which is countersunk into the outer surface
of the structure. Such fasteners are generally made from metal in order to
provide a sufficient amount of structural strength.
The use of metallic fasteners to secure composite material structures like
wing boxes presents a serious problem in that the difference in electrical
conductivity between the composite material and the fastener causes
lightning to be attracted to the fastener. When a lightning strike
attaches to the fastener head, the fastener can conduct current into the
interior of the wing box and cause internal arcing or sparks inside the
wing fuel tank. The presence of fuel vapors in the tank makes such arcing
and sparking highly dangerous. Therefore, it is necessary to provide some
means of preventing or minimizing the chance of a lightning strike
attaching to the fastener.
Another problem associated with the use of countersink fasteners is that a
normal countersink fastener installation results in a slight gap between
the fastener head and the portion of the aircraft structure forming the
internal walls of the countersink bore. This gap is caused by
manufacturing tolerances. The gap is another cause of lightning strike
attachment to a metallic fastener securing composite material structures
and also can cause lightning strike attachment to a metallic fastener even
when the structure being fastened is aluminum. In either case, the
attachment is caused by a difference in electrical properties between the
area at the outer edge of the fastener head and the remainder of the
structure.
After the structure is attached by means of the appropriate number of
fasteners, a coat of paint normally is applied over the outer surfaces of
the fastener heads and the surrounding structure. The gap between a
fastener head and its counterbore causes the paint to thin or crack around
the outer edge of the fastener head. This thinning or cracking of the
paint in turn causes a difference in electrical conductivity, which causes
electrical streamers to form around the edges of the fastener heads. These
streamers create an electrical field and increase the chance of a
lightning strike hitting a fastener head.
There have been a number of proposals for providing protection against
lightning strikes attaching to metallic fasteners in a composite material
aircraft structure. One type of proposal is the use of plastic fasteners.
This approach can be effective in the limited situations in which plastic
fasteners have sufficient strength, but in most situations the structural
requirements of the aircraft necessitate the use of fasteners that are at
least partially metallic. Other solutions that have been suggested include
the use of dielectric tape or tank sealant to cover the fastener heads.
This kind of approach has failed to produce reliable protection against
lightning strikes and has proved to be quite costly.
The patent literature includes a number of examples of systems that have
been proposed for protecting a composite material aircraft structure from
lightning. U.S. Pat. No. 3,906,308, granted Sept. 16, 1975, to Amason et
al., discloses a system in which a dielectric coating is placed over
critical components of the structure and, for large span components,
spaced metallic strips are affixed to the dielectric outer surface to
provide dwell points for the lightning current channel. U.S. Pat. No.
3,989,984, granted Nov. 2, 1976, to Amason et al., discloses an outer
grounded perforated metal layer on the aircraft structure with a bonded
dielectric layer beneath the metal layer. At joints in the skin of the
structure, exterior surfaces of metallic fasteners are exposed and the
conductivity of the fasteners is enhanced by providing the fasteners with
suitable coatings. U.S. Pat. No. 4,382,049, granted May 3, 1983, to
Hofmeister et al., discloses a dielectric "barrier" around the dome
portion of a dome nut fastener that projects from the outside skin of an
aircraft structure. The barrier is formed after the fastener is in place
by molding a layer of dielectric material around the fastener dome. The
molding process includes the use of a cap which forms a cavity around the
fastener dome to control the thickness of the dielectric barrier.
U.S. Pat. No. 3,592,100, granted July 13, 1971, to Mackiewicz et al.,
discloses a screw designed for use with grouped electrical switches to
prevent arcing between adjacent screw heads. The screw has an exposed
metal shank and threaded portion and an insulated head with a slot
therethrough which exposes the base metal.
The patent literature also includes a number of examples of plastic or
other corrosion resistant caps for fasteners. U.S. Pat. No. 3,425,313,
granted Feb. 4, 1969, to J. P. Villo, discloses such a cap for countersink
socket head screws. The cap includes an annular skirt portion and a
circular top, and is placed in position over the screw head after the
screw head has been positioned in a counterbore. The cap is forced into
position, such as by a hammer blow. The cap is provided with some
resiliency so that it will prevent loosening of the screw by vibrations
and seal the counterbore against water and other contaminants. The top of
the cap is flush with the surrounding surface when the cap is installed.
The socket in the head of the screw remains unfilled but is covered by a
portion of the top of the cap that may be broken away to insert a wrench
and remove the screw.
U.S. Pat. No. 3,494,243, granted Feb. 10, 1970, to W. H. Kleinhenn,
discloses a self-sealing screw with a coating of a material such as Teflon
(trademark) on the underside of the head and sometimes also on the shank
and threads. U.S. Pat. No. 3,620,119, granted Nov. 16, 1971, to King, Jr.,
et al., discloses a method and apparatus for making a fastener with an
anticorrosive material on the underside of the head and the unthreaded
shank portion and, apparently, sometimes on the top of the head.
Corrosion resistant caps for fasteners of a type having a protruding head
are disclosed in U.S. Pat. Nos. 3,470,787, granted Oct. 7, 1969, to W. L.
Mackie; 3,557,654, granted Jan. 26, 1971, to H. C. Weidner, Jr.;
3,618,444, granted Nov. 9, 1971, to W. Kay et al.; 3,693,495, granted
Sept. 26, 1972, to D. P. Wagner; 3,885,492, granted May 27, 1975, to C. E.
Gutshall; 3,897,712, granted Aug. 5, 1975, to D. A. Black; 4,154,138,
granted May 15, 1979, to R. R. Melone; 4,316,690, granted Feb. 23, 1982,
to R. L. Voller; and 4,373,842, granted Feb. 15, 1983, to J. E. Bettini et
al. W. H. Burleson discloses, in U.S. Pat. No. 2,253,264, granted Aug. 19,
1941, a tubular post-type electric insulator filled with a dielectric
liquid and having ends capped with metal and a dielectric.
The known proposals and patents discussed above and the prior art that is
discussed and/or cited in the patents should be studied for the purpose of
putting the present invention into proper perspective relative to the
prior art.
DISCLOSURE OF THE INVENTION
A subject of the invention is the combination, in an aircraft, of a first
structure with an external skin made from a plastic material, a second
structure, and at least one fastener for attaching the first structure to
the second structure. The fastener comprises a shaft and a head, each of
which is made from a metal that is more electrically conductive than the
plastic material and that has sufficient strength to carry loads of a
predetermined magnitude. The fastener also includes a cap of dielectric
material secured to and covering a top portion of the head to prevent
lightning strikes from attaching to the fastener. The external skin of the
first structure has a countersink hole therein for receiving the head and
the cap of the fastener. The cap is shaped and dimensioned and has
sufficient resilience to essentially fill gaps between the top portion of
the head and inner sidewall portions of the countersink hole, and to form,
along with the external surface of the external skin, an aerodynamic
surface which is sufficiently continuous to receive and maintain a
crack-free coat of paint having a uniform thickness.
The combination of features of the apparatus of the invention solves the
problems discussed above in relation to the use of plastic materials, such
as graphite fiber reinforced epoxy resin composite materials, in aircraft
structural components that must be attached by metallic fasteners in order
to meet the structural strength requirements of the aircraft. The
provision of a dielectric cap to cover the top portion of the head of such
a fastener, and the provision of such cap with characteristics that cause
it to fill gaps between the top portion of the fastener head and inner
sidewall portions of the countersink hole into which the head and the cap
are received effectively protect the structure and surrounding structures
from internal arcing and sparking caused by lightning strikes attaching to
the fastener. The shaping and dimensioning of the cap to form an
aerodynamic surface with the external surface of the structure provides
the additional benefit of improving the aerodynamic efficiency of the
structure. The filling of the gaps between the fastener head and the
sidewalls of the countersink hole, in combination with the forming of an
aerodynamic surface which is sufficiently continuous to receive and
maintain a crack-free coat of paint having a uniform thickness, eliminates
the problems of such gaps or the thinning or cracking of paint causing
differences in electrical properties which attract lightning strikes to
the fastener. The overall combination of the apparatus of the invention
provides reliable and effective protection against lightning strikes that
is relatively inexpensive and easy to install and maintain and, thus,
makes it possible to maximize fuel economy by maximizing use of
lightweight composite materials.
A preferred feature of the invention is a cap which has a top portion that
covers the top of the head of the fastener to form a part of the
aerodynamic surface, and an annular side portion that covers side portions
of the head and extends inwardly into the countersink hole from the top
portion of the cap. This arrangement ensures that any portion of the
fastener which is exposed to the outside environment is protected by a
dielectric material and that the upper portions of the gaps are
effectively filled to eliminate the undesirable electrical properties of
the gaps. The arrangement permits the cap to extend only partway down the
side surfaces of the head and only partially into the countersink hole.
This in turn helps to reduce the cost of the fastener and to insure easy
installation of the fastener.
In the combination of the invention, the fastener may have either an axial
tool receiving recess in the end of its shaft opposite its head, or a tool
receiving recess in the top surface of its head. In the former case,
preferably, the top surface of the head substantially conforms to the
aerodynamic surface, and the cap includes a continuous integral layer of
dielectric material that covers the top surface of the head and has an
essentially uniform thickness to form a part of the aerodynamic surface.
This arrangement helps to minimize the cost of manufacturing the fastener,
to maximize the smoothness and conformity of the aerodynamic surface, and
to ensure that there are no voids or pits in the top portion of the cap
which could affect its electrical properties.
In the latter case, the cap preferably includes a layer of dielectric
material of essentially uniform thickness covering the top surface of the
head and surrounding the recess in such top surface, and a body of
dielectric material that is shaped to fill the recess and to form, along
with the layer, a part of the aerodynamic surface. This body of dielectric
material is positioned in the recess after the fastener has been
positioned to attach the structures. This arrangement has the advantage of
providing effective protection against lightning strike attachment in
situations in which it is impractical or impossible to use an internally
wrenched fastener; i.e., a fastener having a tool receiving recess in the
end of its shaft. The provision of the body of dielectric material ensures
that there is lightning protection over the entire top portion of the
fastener and also improves the aerodynamic efficiency of the structure.
The positioning of such body after the structures are attached allows easy
installation of the fastener. The dielectric material that fills the
recess is relatively thick since it extends all the way into the recess,
and the body of dielectric material does not have to bridge any interface
between the fastener and the structure. Thus, the body of dielectric
material, along with the layer of dielectric material, forms a continuous
protective cap over the fastener that is not subject to cracking.
Another subject of the invention is a method of attaching an aircraft
structure with an external skin of a plastic material to another aircraft
structure, and of preventing lightning strikes from causing internal
sparking or arcing in the structures. According to an aspect of the
invention, the method comprises providing a fastener having a shaft and a
head, each of which is made from a metal that is more electrically
conductive than the plastic material and that has sufficient strength to
carry loads of a predetermined magnitude, and a resilient cap of
dielectric material secured to and covering a top portion of the head to
prevent lightning strikes from attaching to the fastener. The method also
includes forming a countersink hole in the external skin, and positioning
the fastener to attach the structures together. The positioning of the
fastener includes positioning the head and the cap in the countersink
hole. While the fastener is being positioned, the cap is allowed to
essentially fill gaps between the top portion of the head and inner
sidewall portions of the countersink hole. An aerodynamic surface is
formed from the cap and external surface portions of the external skin
surrounding the hole. The aerodynamic surface is sufficiently continuous
to receive and maintain a crack-free coat of paint having a uniform
thickness. The method has the advantages discussed above in connection
with the apparatus of the invention.
These and other advantages and features will become apparent from the
detailed description of the best modes for carrying out the invention that
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings, like element designations refer to like parts throughout,
and:
FIG. 1 is a sectional view of an internally wrenching fastener in a prior
art installation.
FIG. 2 is a sectional view of the same type of fastener in another prior
art installation.
FIG. 3 is a sectional view of a first preferred embodiment of the apparatus
of the invention.
FIG. 4 is an elevational view of the fastener shown in FIG. 3.
FIG. 5 is a pictorial view looking toward the outer surfaces of the first
structure and a second preferred embodiment of the fastener of the
invention.
FIG. 6 is a sectional view taken along the line 6--6 in FIG. 5.
FIG. 7 is an elevational view of the head and part of the shaft of the
fastener shown in FIGS. 3 and 4, with the cap shown in section.
FIGS. 8-11 are similar to FIG. 7, except that they show alternative
configurations of the cap of the fastener.
BEST MODES FOR CARRYING OUT THE INVENTION
The drawings show apparatus that is constructed according to the invention
that also constitutes the best modes of the invention currently known to
the applicant. The drawings also illustrate the best modes for carrying
out the method of the invention currently known to the applicant.
Each of FIGS. 1-3, 5, and 6 illustrates a plastic material external skin
portion 2 of a first aircraft structure attached by means of a fastener to
a second aircraft structure 4. The plastic material of the external skin 2
would most usually be a graphite fiber reinforced epoxy resin composite
material or a similar composite material. However, it is intended to be
understood that the term "plastic material", as used herein, includes
other plastic materials having strength and weight characteristics
appropriate for use in aircraft as well as such composite materials. The
second structure 4 might be a flange of an internal supporting structure
or some other type of structure, depending on the requirements of the
particular installation, and might be made from metal, as shown in the
drawings, or any other suitable material.
FIG. 1 shows, for the purpose of comparison, an internally wrenching
countersink fastener 10 being used to attach an external skin 2 of
composite material to a metal structure 4. The fastener 10 is of a known
type and includes a head 12 and a shank 14. The head 12 is received into a
countersink hole 6 in the skin 2. A layer of paint 8 has been applied to
the external surfaces of skin 2 and head 12. Because of manufacturing
tolerances, there is a gap 16 between the fastener head 12 and the inner
sidewalls of the countersink hole 6. Reference numeral 18 indicates an
electric field created by lightning induced streamers. As discussed above,
the formation of the streamers and the electric field is a result of
differences in electrical properties in the vicinity of the gap 16 caused
by the gap 16 itself and by cracking 20 and thinning 22 of the layer of
paint 8 in the area of the gap 16. The situation in FIG. 1 would, of
course, be unacceptable in a commercial aircraft in areas where fuel
vapors and the like are likely to be present inside the structures 2,4.
FIG. 2 illustrates a modification of the apparatus shown in FIG. 1 that
includes previously proposed means for providing protection against
lightning strike attachment to the fastener 10. The countersink hole 6' in
FIG. 2 has been modified to include a thin annular portion 26 above the
frustoconical portion that receives the fastener head 12. After the
structures 2,4 have been secured together by the fastener 10, the annular
portion 26 is filled with a tank sealant 28. The sealant 28 extends over
the top of the fastener 10 and radially outwardly therefrom. The layer of
sealant 28 is relatively thin and must bridge the gap 16 between the
fastener head 12 and the sidewalls of the hole 6'. These characteristics
make the layer of sealant 28 subject to cracking, indicated at reference
numeral 30, and thus make the protection provided by the sealant 28
ineffective and unreliable.
In the apparatus combination of the invention, the fastener comprises a
shaft and a head similar to the shaft 14 and head 12 of the known
fasteners shown in FIGS. 1 and 2, and a cap of dielectric material. The
shaft and head are made from a metal that has sufficient strength to carry
loads of a predetermined magnitude, which is determined on the basis of
the structural requirements of the particular installation. The metal is
more electrically conductive than the plastic or composite material which
forms the external skin 2 of the first structure being attached by the
fastener. The cap is secured to and covers a top portion of the fastener
head to prevent lightning strikes from attaching to the fastener. The
external skin 2 has a countersink hole 6 therein for receiving the head
and the cap of the fastener. The cap is shaped and dimensioned and has
sufficient resilience to essentially fill the gap 16 between the top
portion of the head and the inner sidewall of the countersink hole 6. The
shaping and dimensioning and the resilience of the cap also enable it to
form, along with the external surface of skin 2, an aerodynamic surface
which is sufficiently continuous to receive and maintain a crack-free coat
of paint having a uniform thickness.
FIGS. 3, 4, and 7 illustrate a first preferred embodiment of the fastener
of the invention. The fastener 36 is of the internally wrenching type and
includes a metal head 44 and a metal shaft 38. The outer end of the shaft
38 opposite the head 44 has threads or lockbolt pull grooves 40 on its
circumferential surface and a tool receiving socket or recess 42 extending
axially inwardly from its radial end surface. See FIG. 4. The head 44 is
generally frustoconical in shape with a cylindrical portion 46 at its top
end of relatively small axial dimension. This cylindrical portion 46 is
most clearly shown in FIG. 7.
The dielectric cap 50 of the fastener 36 shown in FIGS. 3, 4, and 7 has a
top portion 52 which covers the top of the metal head 44, and an annular
side portion 54 which covers side portions of the head 44. Preferably, the
top portion 52 and the side portion 54 are integrally formed in a single
molding operation. The frustoconical side surface of the cap 50 extends
inwardly from the top surface into the countersink hole 6 essentially
parallel to the sidewalls of the hole 6. The covered side portions of head
44 include the outer circumferential surface of the cylindrical portion 46
and a portion of the frustoconical outer surface adjacent to cylindrical
portion 46. The cap 50 extends down the frustoconical surface of the head
44 an amount sufficient to ensure secure attachment between the metal head
44 and the cap 50 and to ensure that the top of the gap 16 is completely
filled. The inner radial surface of the cap 50 is substantially parallel
to the outer top surface. Some advantages of the configuration of cap 50
are that it is relatively simple and it positively locks onto head 44.
As can be seen in FIGS. 3, 4, and 7, the top surface of the head 44
substantially conforms to the external surface of skin 2. The top portion
52 of the cap 50 is formed by a continuous integral layer of dielectric
material that has an essentially uniform thickness. The top or outer
surface of cap portion 52 generally conforms to the external surface of
skin 2 to form therewith an aerodynamic surface. As can be seen in FIG. 3,
the cap 50 effectively fills the upper portion of the gap 16 so that the
coat of paint 8 applied over the skin 2 and cap 50 may be made of
essentially uniform thickness and be maintained crack-free.
FIGS. 5 and 6 illustrate a second preferred embodiment of the fastener of
the invention which is of the externally wrenching type. The fastener 58
has a shaft 60 with threads (not shown) similar to the threads 40 of
fastener 36, and a head 62 with a tool receiving recess 66. The recess 66
shown in FIGS. 5 and 6 is designed for receiving a Phillips head screw
driver, but it is of course to be understood that other types of recesses,
such as an allen wrench recess might also be provided. The cap 70 of
fastener 58 is similar to the cap 50 of fastener 36 except that the top
portion 72 of the cap 70 is formed differently to accommodate the recess
66 in the head 62. The top portion 72 includes a layer of dielectric
material of essentially uniform thickness covering the top surface of the
head 62 and surrounding the recess 66. The top portion 72 also includes a
| | |
