Note: Descriptions are shown in the official language in which they were submitted.
Docket 6~34 ~ ~ 6~
~ackyround of the Invention
This invention relates to aircraft lightning
protection and more particularly to an improved lightning
diverter strip for aircraft.
It is well known that aircraft, when flying in the
vicinity of thunderstorms, are subject to lightning strikes.
Various points and regions of the aircraft extremities are
subject to the formation of streamers prior to the actual
lightning strike. It is also known that the lightning strike
will occur when a leader connects up with one of the streamers,
and the resulting current can reach as high as 200,000 amperes.
While the aluminum aircraft skin is seldom subject to severe
lightning damage by reason of its electrical conductivity,
lightning attachments at certain critical points, for example,
the radome section, are to be avoided due to the likelihood of
damage to the electrical equipment.
Lightning diverter strips have been devised in the
past for application to aircraft str~ctural surfaces for the
purpose of providing a non-destructive, electrically-conducted
path to protect the underlying structure from direct lightning
attachments. For example, thin metal foil strips and solid
metal bars have been used to divert the charge. In addition,
a series o~ closely spaced metal disks or dots have been
applied to flexible strip material. In such instances, metal
disks of 1/10 inch diameter have been bonded to a substrate,
and the latter has been fixed by an epoxy adhesive to the
outer sur~ace of the radome to provide an ionized conductive
pa~h. While such an arrangement has been successful, it iF
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of relatively high cost, and the relatively large disks
absorb a great deal of heat occasionally these disks
have been known to explode and shoot out like pellets,
especially if they are spaced too far apart. Also, the
size of the disks adversely affect the radio frequency
absorption of the strip.
Summary_of the Invention
The present invention is directed to a low-cost
non-destructive lightning diverter strip which may readily
be applied to radomes and the like. Accordingly, the invention
in one aspect provides a lightning diverter strip for
conducting lightning induced electrical currents and thereby
protecting vulnerable aircraft components such as radomes
and the like, comprising a substrate of flexible dielectric
tape having a lower surface adapted to be applied to the
aircraft component to be protected thereby, and having an
upper surface, a flexible epoxy binder on said upper surface,
a conductive metal powder of finely divided aluminum uniformly
deposited along said tape and bound thereto by said binder,
said powder having a density such as to provide a very high
DC resistance which reads esse~tially as an open circuit to
DC voltages while forming a discrete non-destructive con-
ductive path for electrical currents induced by lightning.
A further aspect of the invention provides the
method of manufacturing a lightning diverter strip comprising
the steps of:
a) providi.ng a length of thin dielectric plastic
tape,
b) applying a curable epoxy to one surface of the
tape,
c) uniformly applying finely divided aluminum
powder to the epoxy wîth a density such as to provide a very
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high DC resistance which reads essentially as an open circuit
to DC voltages while forming a discrete non-destructive
conductive path for lightning-induced currents, and
d) curing said epoxy to bind the aluminum
particles to the tape.
A preferred embodiment of the invention includes
a light polyester substrate and a uniformly dispersed
powdered metal applied to the upper surface thereof and
bonded thereto by a suitable flexible epoxy. Finely powdered
aluminum is used as the conductor metal and is distributed
uniformly along the surface of the substrate, in such a
manner that a direct current conductive path is not formed.
Thus, the finished strip will have a very high resistance so
that it will read essentially as an open circuit to DC
voltage. The particles are dispersed in sufficiently close
fashion that the strip is essentially a short circuit to
lightning currents and is also transparent to rf energy. '
Since the aluminum particles are of small size, very little
energy is absorbed in the particles and physical damage
due to lightning attachments is held to a minimum. The
particles themselves r since they are very close together,
appear to the lightning as a large number of series-connected
tiny capacitors which guide the lightning strike to the
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Docket 6434
The supporting film strip or substrate is prefer-
ably formed of a thin dielectric plastic of the polyester
type, preferably polyethylene terephalate. The strip is
conveniently applied to the aircraft surface to be protected
by a suitable epoxy cement.
It is accordingly an important object of this
invention to provide a low-cost, light-weight lightning
diverter strip which is essentially transparent to rf
er.ergy. It has low aerodynamic drag, is capable of absorbing
repeated lightning strikes, and is easy to apply and maintain.
Another object of the invention is the provision
of a lightning diverter strip, as outlined above, including
a dispersion of finely divided metal particles bonded to a
thin supporting dielectric strip.
Another, and more particular, object of the inven-
tion is the provision of the lightning diverter strip in
which finely powdered aluminum is finely dispersed along an
epoxy binder on a thin dielectric substrate.
Other objects and advantages of the invention will
be apparent from the following description, the accompanying
drawings and the appended claims.
Brief Description of the Drawings
Fig. 1 is a perspective view of the nose of the
aircraft showing a radome as having diverter strips applied
thereto;
Fig. 2 is a perspective view, partially broken
away, of a strip according to this invention;
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Docket 6434
Fig. 3 is a transverse section through the strip;
and
Fig. 4 shows the strips as applied to the canopy
and vertical stabilizer surfaces of an aircraft.
S Description of Preferred Embodiment
Referring to the figures of the drawing which
illustrate a preferred embodiment, a typical aircraft is
shown at 10 as having a radome 11 which is to be protected
by the lightning diverter strips 12 of this invention. As
shown, the strips 12 will commonly lead along the surface
of the part to be protected and will terminate at or near
the skin of the aircraft.
The strip of this invention is illustrated in
Figs. 1 and 2 as including a base or substrate 15 formed
of polyester tape having a lower surface 16 adapted to be
applied directly to the aircraft component to be protected
and bonded thereto by suitable epoxy. The base lS is pre-
ferably a dielectric material such as thin polyethylene
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this material is in the range of 4 mils in thickness,
although the thickness,-width, and length are not critical.
A heat curable flexible epoxy 18 is uniformly
applied to the upper surface of the substrate 15.
A conductive metal powder 20 is uniformly deposited
along the upper surface of the epoxy 18. The epoxy forms a
binder to bind the metal powder to the base 15. While many
different conductive metal powders may be used, it is
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Docket 6434
preferable to employ finely divided aluminum. The aluminum
particles are deposited sparsely along the binder 18 on the
base 15.
Preferably, an atomizer grade powder is used which
is 99.5% pure aluminum with a particle size of 13 + 3 microns.
The powder is uniformly dispersed along the binder or epoxy 18
with an approximate density of 300,000 particles per square
centimeter. The actual density is not critical and may be
100,000 particles per square centimeter, or lower. The indi-
vidual particles are of such a small physical mass ~hat verylittle energy is absorbed within the particles. Thus, physical
damage to the diverter strip 12 due to lightning attachment is
held to a minimum. For the best results, the aluminum particles
are deposited sparsely along the binder 18 on the base 15. Pre- -
ferably, the powdered aluminum particles are deposited on thebinder 18 before it is cured and thus become embedded into the
exposed surface of the binder. An excess of particles may be
deposited on the uncured binder, and the excess particles
removed by physically shaking or brushing the strip after curing
so that only the attached or embedded particles remain. While
the individual aluminum particles may touch each other, never-
theless a DC conductive path is not formed in view of the fact
that the aluminum particles have a microscopically thin coating
of aluminum oxide on their outer surfaces, and aluminum oxide
is essentially a non-conductive or a dielectric material. There-
fore, the strip will read, after curing, as an essentially open
DC circuit or will present a very high resistance to DC voltage.
While the strip reads as an open circuit to DC vol-
tage~ it will be substantially tr~lsparent to rf frequencies,
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Docket 6434
- and provides a low impedence path at the frequencies of the
lightning strike. Since the particles are close together, the
strip appears to the lightning as a large series of very small
capacitors guiding the strike to the conductive skin of the
aircraft.
The finished diverter is very thin, is flexible, and
is easy to apply. The tape base 15 may for example be 3/8" wide,
and any length. It is easily cut by scissors to a desired length.
For the purpose of illustration, the thicknesses shown in Fig.
3 have been greatly exaggerated. Further, the particles 20 are
actually slightly embedded within the epoxy forming the binder
18 and are permanently attached to the base 15. The diverter
stxip 12 has high resistance to errosion and damage due to air
currents and the impact of rain on the exposed surface. Since
15 the strip is flexible and thin, it is easy to apply and it is
relatively inexpensive to manufacture as compared to prior devices.
It does not adversely affect the operation of radar and has low
aerodynamic drag. It h~s little or no maintenance requirements
; and is capable of taking repeated lightning strikes without
severe degradation.
Of course, the strip 12 may be used wherever lightning
protection on an aircraft is desired. In Fig. 4~ the jet air-
craft 30 is shown as having a plastic canopy 32 protected by a
strip 12. A vertical stabilizer has fiber glass leading edges
35 which are also protected by strips 12.
While the method herein described, and the form of
apparatus for carrying this method into effect constitute pre-
ferred embodiments o this invention, it is to be understood
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that the invention is not limited to this precise method and
form of apparatus, and that changes may be made in either
without departing from the scope of the invention.
