Patent Description:
Lightning strikes to aircraft are frequent occurrences, which can generate high forces and temperatures that may cause structural damage and systems disruption if not appropriately designed for. Lightning strike protection of composite airframe structures often involves the use of one or more lightning strike protection layers, with these often including expanded metal foils (such as expanded copper foil) placed on the top of the composite aircraft skin. The expanded metal foil layer helps to dissipate the lightning strike energy over the surface, e.g. of an airframe component, since the electrical conductivity of metals is significantly larger than that of conventional composite laminates (such as glass fibre and carbon fibre composite laminates). As a result, electrical potential from the lightning strike flows through the lightning strike protection layer without directly affecting the composite structure beneath.

However, aircraft typically have complex geometries, involving three-dimensional curvature, and this can necessitate the use of multiple smaller, discrete, lightning strike protection layers that each cover a portion of the aircraft. It is important that these discrete lightning strike protection layers are adequately electrically connected to avoid problems such as a build-up of charge. Particularly as a significant proportion of repairs to aircraft wings are due to lightning strike damage, especially at the joins between the discrete lightning strike protection layers. <CIT> discloses an applique coating including a metal foil and a first polymer film underlying the metal foil. One embodiment shows abutting appliques.

A first aspect of the invention provides a lightning strike protection layer for laying on top of an aircraft structure, comprising: a first portion and a second portion, wherein the first portion comprises a first fibre reinforced polymer composite layer and a first electrically conductive metal layer and the second portion comprises a second fibre reinforced polymer composite layer and a second electrically conductive metal layer, wherein the first and second portions are joined at a butt joint, with the first and second fibre reinforced polymer composite layers abutting and the first and second electrically conductive metal layers abutting; and a butt-strap extending across the butt joint, the butt-strap comprising a third electrically conductive metal layer electrically connected to the first and second electrically conductive metal layers, wherein the first portion, second portion, and butt-strap are infused with a cured resin so as to form a unitary structure.

With this arrangement, thickness increase in the through-thickness direction of the lightning strike protection layer is minimised, whilst electrically connection between the electrically conductive metal layers of the first and second portions is ensured by the third electrically conductive metal layer. This provides for a simple join between adjacent and abutting portions of a lightning strike protection layer that reduces manufacturing costs and minimises lead time. Whilst also minimising disruption, wrinkles and other defects across the joint.

The lightning strike protection layer may comprise a third fibre reinforced polymer composite layer extending across the butt joint and adjacent the first and second fibre reinforced polymer composite layers.

With this arrangement, the joint between the abutting first and second portions is strengthened, with the third fibre reinforced polymer composite layer protecting the joint and the edges of the first and second portions from abrasion and micro-cracking.

Each fibre reinforced polymer composite layer may be a glass fibre reinforced polymer layer and/or a carbon fibre reinforced polymer layer.

Each electrically conductive metal layer may be an expanded metal foil layer.

Each electrically conductive metal layer may be an electrically conductive copper layer.

A second aspect of the invention provides an aircraft structure, comprising: an aircraft skin layer, and the lightning strike protection layer of the first aspect laid on top of the aircraft skin layer.

The aircraft skin layer may have curvature in two orthogonal directions, and the lightning strike protection layer may be laid on top of the curvature.

Joints may often be formed in a lightning strike protection layer due to geometrical restrictions. For example, a lightning strike protection layer may have difficulty in extending over curved parts without forming portions with wrinkles and portions in which the layer is stretched. The motivation for these curves may further drive the need for a join between these portions of the lightning strike protection layer, such as that of the first aspect, which has relatively little impact on the overall thickness and is easy to manufacture.

The aircraft structure may be a wing tip or wing tip device.

A third aspect of the invention provides an aircraft, the aircraft comprising the aircraft structure of the second aspect.

A fourth aspect of the invention provides a method of manufacturing a lightning strike protection layer for laying on top of an aircraft structure, the method comprising: providing a mould; providing a first portion and a second portion of a lightning strike protection layer, wherein the first portion comprises a first fibre preform layer and a first electrically conductive metal layer and the second portion comprises a second fibre preform layer and a second electrically conductive metal layer; laying the first portion and the second portion into the mould to form a butt joint, wherein the first and second fibre preform layers abut and the first and second electrically conductive metal layers abut; and laying a third electrically conductive metal layer across the butt joint to form a butt-strap so as to electrically connect to the first and second electrically conductive metal layers, wherein the first portion, second portion, and butt-strap are infused with a resin, and the method further comprises curing the resin to form a unitary lightning strike protection layer.

With this arrangement, and integral lightning strike protection layer can be formed in which the manufacturability is increased and the overall thickness is decreased.

Each fibre preform layer may be a glass fibre preform layer and/or a carbon fibre preform layer.

The first fibre preform layer may be laid between the first electrically conductive metal layer and the mould, and the second fibre preform layer may be laid between the second electrically conductive metal layer and the mould.

The method may further comprise providing a third fibre preform layer, and laying the third fibre preform layer in the mould prior to laying the first and second portions in the mould, and wherein the first and second portions are laid in the mould such that the third fibre preform layer extends across the butt joint and adjacent the first and second fibre preform layers.

The mould may have curvature in two orthogonal directions, and the method may comprise laying the lightning strike protection layer on the curvature of the mould.

<FIG> illustrates a typical fixed wing aircraft <NUM>. The aircraft <NUM> may have a port wing <NUM> and a starboard wing <NUM> that extend from a fuselage <NUM>. Each wing <NUM>, <NUM> may carry wing mounted engines <NUM>. The fuselage <NUM> has a nose <NUM> and a tail <NUM>. The tail <NUM> may have horizontal and vertical stabiliser surfaces <NUM>, <NUM>. The aircraft <NUM> may be a typical jet passenger transport aircraft although the invention is applicable to a wide variety of fixed wing aircraft types, including commercial, military, passenger, cargo, jet, propeller, general aviation, etc. with any number of engines attached to the wings or fuselage.

The wings <NUM>, <NUM> are similar in construction so only the port wing <NUM> will be described in detail with reference to <FIG>.

The wing <NUM> of the aircraft <NUM> may have a main wing portion <NUM> that is a cantilevered structure with a length extending in a span-wise direction from a wing root to a wing tip, the root being joined to the aircraft fuselage <NUM>. The wing <NUM> may have a plurality of flight control surfaces, such as slats <NUM>, ailerons <NUM>, air brakes/spoilers <NUM>, and flaps <NUM>. A wing tip device <NUM> may be provided on the tip end of each wing <NUM>, <NUM>.

The wing tip device <NUM> extends between a root end <NUM>, attached to a main wing portion of the wing <NUM>, and a tip end <NUM> defining an outermost spanwise position of the wing <NUM>.

An increasing proportion of aircraft <NUM>, such as shown in <FIG>, are being constructed from comparatively lighter weight materials than the metals historically used on aircraft <NUM> (e.g. aluminium). However, in some applications composite materials can be inadequate conductors of electricity, and this can necessitate the use of lightning strike protection layers <NUM> forming part of the outer skin of the aircraft <NUM>. Expanded metal foils, such as aluminium, brass and copper, are common choices to incorporate in the lightning strike protection layers.

<FIG> shows an example of a lightning strike protection layer <NUM> including a fibre reinforced composite layer <NUM> and an electrically conductive metal layer <NUM>. The electrically conductive metal layer <NUM> may be an expanded metal foil layer, such as a copper mesh, aluminium mesh, or bronze mesh. Alternatively, the electrically conductive metal layer <NUM> may be a solid sheet of metal <NUM>, or similar.

In some examples, the fibre reinforced composite layer <NUM> may form an outer layer of the aircraft <NUM> with respect to the electrically conductive metal layer <NUM>, or the lightning strike protection layer <NUM> may include one or more electrically conductive metal layers <NUM> in the absence of an outer fibre reinforced composite layer <NUM>. The fibre reinforced composite layer <NUM> may protect the electrically conductive metal layer <NUM>, for example the fibre reinforced composite layer <NUM> may provide structural protection or abrasion protection to the electrically conductive metal layer <NUM>.

As aircraft <NUM> typically have complex geometries, involving complex curvatures (i.e. having curvature in two orthogonal directions), there is often a need to manufacture the lightning strike protection layer <NUM> as multiple discrete portions 31a, 31b that each cover a portion of the aircraft <NUM>. The alternative is to introduce distortions or fold lines in the lightning strike protection layer <NUM>, however this can introduce discontinuities in the aircraft skin that are undesirable for reasons of aerodynamics and electrical conductivity.

Additionally, as described above, the lightning strike protection layer <NUM> is often formed of multiple layers stacked in the through-thickness direction (e.g. a fibre reinforced composite layer <NUM> and an electrically conductive metal layer <NUM>), and there may be a reduction in the manufacturing lead times if those multiple layers are stacked as a single unit prior to assembly to the aircraft <NUM>. However, this advantage is often counteracted by the reduced drapability of thicker lightning strike protection layers <NUM> during manufacture, such that the lightning strike protection layer <NUM> often requires splitting into further discrete portions 31a, 31b to compensate for its reduced drapability (especially when draped upon complex geometries), and this can mitigate many of the advantages mentioned above.

These discrete portions 31a, 31b of the lightning strike protection layer <NUM> introduce discontinuities in the lightning strike protection layer <NUM>, and it is therefore important to ensure the portions 31a, 31b of the lightning strike protection layer <NUM> are electrically connected.

One approach is to individually lay up the layers <NUM>, <NUM> such that the layers <NUM>, <NUM> of each portion 31a, 31b overlap and interlock with the layers <NUM>, <NUM> of the other portion 31a, 31b. However, such overlapping is time consuming. An alternative approach is to pre-form the layers <NUM>, <NUM> into a portions 31a, 31b, and to overlap the portions 31a, 31b to form a lap joint. However, this can create an excessive build-up of thickness at the join, which can have knock-on effects such as wrinkles in the lightning strike protection layer <NUM>, as well as variations in fibre volume fraction and part thickness variations (among others). Such a join in the lightning strike protection layer <NUM> may also lead to increased build up of current, and increased dissipation of current between the overlapping layers <NUM>, <NUM>, particularly when the highly conductive electrically conductive metal layers <NUM> are separated by the comparatively low conductivity fibre reinforced composite layers <NUM>.

According to an example of the invention, there is provided a lightning strike protection layer <NUM> comprising a first portion 31a and a second portion 31b. The first portion 31a comprises a fibre reinforced composite layer 32a and an electrically conductive metal layer 34a, such as an expanded metal foil layer 39a. Similarly, the second portion 31b comprises a fibre reinforced composite layer 32b and an electrically conductive metal layer 34b, such as an expanded metal foil layer 39b.

The first and second portions 31a, 31b are joined at a butt joint <NUM>, with the fibre reinforced composite layer 32a of the first portion 31a abutting the fibre reinforced composite layer 32b of the second portion 31b, and the electrically conductive metal layer 34a of the first portion 31a abutting the electrically conductive metal layer 34b of the second portion 31b. It should be noted that abutting refers to the respective layers touching one another or being next to one another (i.e. directly adjacent).

An example of this arrangement is shown in <FIG>. In this example, the first portion 31a and the second portion 31b directly contact, such that at least some direct electrical contact may be made between the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b.

To electrically connect the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b across the butt joint <NUM> (or at least improve the electrical connection), an additional (overlapping) electrically conductive metal layer <NUM> overlaps a portion of the electrically conductive metal layer 34a, 34b of the first and second portions 31a, 31b and extends across the butt joint <NUM>. In other words, the additional electrically conductive metal layer <NUM> is physically and electrically connected to the electrically conductive metal layer 34a, 34b of the first and second portions 31a, 31b. The overlapping metal layer <NUM> forms part of a butt-strap joint that extends across the butt joint <NUM>. An example is shown in <FIG>.

Whilst conventionally an additional component (in this case the additional electrically conductive metal layer <NUM>) is considered to increase manufacturing lead times, and thereby slow production, the increased speed at which a butt joint <NUM> may be created compensates for the additional time required to add the overlapping electrically conductive metal layer <NUM>.

As a result, the first and second portions 31a, 31b of the lightning strike protection layer <NUM> are electrically connected, whilst the thickness of the lightning strike protection layer <NUM> is minimised and the discontinuities across the butt joint <NUM> are minimised. The manufacturing process of forming the lightning strike protection layer <NUM> is also simplified, reducing manufacturing lead times.

The fibre reinforced composite layers 32a, 32b of the first and second portions 31a, 31b may form an outer face of the lightning strike protection layer <NUM>, such that the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b form an inner face of the lightning strike protection layer <NUM>.

The combination of fibre reinforced composite layers 32a, 32b with the electrically conductive metal layers 34a, 34b assists in compensating for the increased thermal expansion coefficient of the electrically conductive metal layers 34a, 34b, as the increased thermal expansion coefficient of the electrically conductive metal layers 34a, 34b is at least partially balanced out by the thermal expansion coefficient of the fibre reinforced composite layers 32a, 32b so as to more closely align with the overall composite component expansion ratios of the aircraft structure (such as the aircraft skin beneath the lightning strike protection layer <NUM>, discussed below).

Each of the electrically conductive metal layers 34a, 34b, <NUM> may comprise any suitable metal, such as copper, brass, aluminium, or bronze. The electrically conductive metal layers 34a, 34b, <NUM> of a lightning strike protection layer <NUM> may be the same metal, or one or more of the electrically conductive metal layers 34a, 34b, <NUM> may be a different metal to the other electrically conductive metal layers 34a, 34b, <NUM>. Each electrically conductive metal layer 34a, 34b, <NUM> may have a thickness of between <NUM> and <NUM> microns. The electrically conductive metal layers 34a, 34b, <NUM> may each be an expanded metal foil layer. Alternatively, the electrically conductive metal layer <NUM> may be a solid sheet of metal <NUM>, or similar.

As shown in <FIG>, the lightning strike protection layer <NUM> may include one or more additional (overlapping) fibre reinforced polymer composite layers <NUM>. The overlapping fibre reinforced polymer composite layers <NUM> may extend across the butt joint so as to be adjacent the fibre reinforced polymer composite layers 32a, 32b of the first and second portions 31a, 31b. The overlapping fibre reinforced polymer composite layer <NUM> forms part of the butt-strap. The overlapping fibre reinforced polymer composite layer(s) <NUM> may assist in abrasion protection of the butt joint <NUM> and/or a reduction of micro-crack propagation due to thermal cycling, which may occur due to the temperature variations experienced towards the outer skin of the aircraft <NUM>.

In addition, or alternatively, the lightning strike protection layer <NUM> may include one or more fibre reinforced polymer composite layers <NUM> adjacent an internal side, such that the one or more fibre reinforced polymer composite layers <NUM> are adjacent the overlapping electrically conductive metal layer <NUM> and/or the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b. An example in shown in <FIG>.

It will be understood that the lightning strike protection layer <NUM> is formed from a composite preform assembly. "Preform" refers to the fact the fibre reinforced composite layers 32a, 32b are not cured until after final assembly of the lightning strike protection layer <NUM>, such that the fibre reinforced composite layers <NUM> may be referred to as fibre preform layers 132a, 132b.

The fibre preform layers 132a, 132b may comprise any suitable reinforcing fibres, such as glass fibres and/or carbon fibres, with the fibres of the layers <NUM> pre-impregnated with a resin (i.e. a pre-preg preformed fibre composite material) or dry fibre preforms to which a resin is impregnated after assembly (e.g. by resin transfer moulding, RTM), as known in the art. It will be understood that the fibre reinforced composite layers 32a, 32b will similarly comprise these reinforcing fibres once the fibre preform layers 132a, 132b are cured.

<FIG> schematically show the method of a composite preform assembly, and in particular a method of manufacturing a lightning strike protection layer for an aircraft.

<FIG> show a mould <NUM>. The mould <NUM> may have curvature in two orthogonal directions, as shown most clearly in <FIG>.

A fibre preform layer <NUM> may be laid into the mould <NUM>, such as shown in <FIG>.

A first portion 31a and a second portion 31b of a composite preform assembly may be laid into the mould <NUM>, such as shown in <FIG>.

The first portion 31a may comprise a first fibre preform layer 132a and a first electrically conductive metal layer 34a and the second portion may comprise a second fibre preform layer 132b and a second electrically conductive metal layer 34b.

The first portion 31a and the second portion 31b meet in the mould <NUM> to form a butt joint <NUM>, such that the first and second fibre preform layers 132a, 132b abut and the first and second electrically conductive metal layers abut 34a, 34b. The first portion 31a and the second portion 31b may be arranged in to mould <NUM> such that the fibre preform layer <NUM> is located over at least a portion of the butt joint <NUM>, forming an overlapping fibre preform layer <NUM> with respect to the first portion 31a and second portion 31b.

It will be understood that the fibre preform layer <NUM> first laid into the mould <NUM> is optional, such that the first portion 31a and second portion 31b may be laid directly in the mould <NUM> without a fibre preform layer <NUM> between any of the first and second portions 31a, 31b and the mould <NUM>.

In either case, the first and second portions 31a, 31b are laid such that the first and second fibre preform layers 132a, 132b are adjacent a surface of the mould <NUM>, with the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b comparatively distal from the surface of the mould <NUM>.

An additional electrically conductive metal layer <NUM> is then laid across the butt joint <NUM> of the first and second portions 31a, 31b, such as shown in <FIG>, with the additional electrically conductive metal layer <NUM> located adjacent the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b so as to electrically connect to the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b.

As additionally shown in <FIG>, the composite preform assembly may also include one or more fibre preform layers <NUM> adjacent the overlapping electrically conductive metal layer <NUM> and/or the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b.

The composite preform assembly may then be manufactured so as to produce a lightning strike protection layer <NUM>, such as shown in <FIG>.

This may involve consolidating the layers in the mould <NUM>, for example by forming a vacuum seal. As shown in <FIG>, a plastic film or sheet <NUM> may be placed over the mould <NUM>, so as to cover the first and second portions 31a, 31b, the electrically conductive metal layer <NUM>, the fibre preform layer <NUM> and any other layers of the composite preform assembly. A vacuum pressure may then be applied via a valve <NUM>. Consolidating the layers helps to form electrical contact between the layers, and in particular the overlapping electrically conductive metal layer <NUM> with the electrically conductive metal layers 34a, 34b of the first and second portions 31a, 31b.

After consolidating the layers, a resin is introduced into the fibre preform layers 132a, 132b, <NUM>, <NUM>. For example, the resin may be infused through the fibre preform layers 132a, 132b, <NUM>, <NUM> via a second valve <NUM> whilst still under the vacuum pressure. The resin may also infuse the other layers of the composite preform assembly, such as the electrically conductive metal layers 34a, 34b, <NUM>. Alternatively, in some examples, the fibre preform layers 132a, 132b may be composite pre-preg layers (i.e. fibre layers pre-impregnated with resin) in which case infusion of a resin via the second valve <NUM> is not necessary.

The resin is then cured to form the lightning strike protection layer <NUM> described previously, and as shown for example in <FIG>, such that the lightning strike protection layer <NUM> is a unitary structure for forming the outer layer of an aircraft <NUM> or similar structure. Curing the resin may involve applying heat and/or pressure to the composite preform assembly.

It will be clear to the skilled person that the examples described above may be adjusted in various ways. For example, the lightning strike protection layer <NUM> is referred to as forming part of the outer layers of an aircraft <NUM>, however it will be understood that further layers (e.g. binder layers, adhesive layers and/or paint layers) may be laid on top or within the lightning strike protection layer <NUM>.

The lightning strike protection layer <NUM> may subsequently be laid on top of a corresponding aircraft skin layer <NUM>, such as shown schematically in <FIG>.

The examples referred to above are described in relation to their use as the lightning strike protection layer <NUM> on an aircraft <NUM>, such as part of a wing tip or wing tip device <NUM> on an aircraft <NUM>. However, the described lightning strike protection layer <NUM> is suitable for any location on an aircraft <NUM>, or any other structure in which lightning strike protection on an underlying structure may be required.

Claim 1:
A lightning strike protection layer (<NUM>) for laying on top of an aircraft structure, comprising:
a first portion (31a) and a second portion (31b), wherein the first portion (31a) comprises a first fibre reinforced polymer composite layer (32a) and a first electrically conductive metal layer (34a) and the second portion (31b) comprises a second fibre reinforced polymer composite layer (32b) and a second electrically conductive metal layer (34b), wherein the first and second portions (31a, 31b) are joined at a butt joint (<NUM>), with the first and second fibre reinforced polymer composite layers (32a, 32b) abutting and the first and second electrically conductive metal layers (34a, 34b) abutting; and
a butt-strap extending across the butt joint (<NUM>), the butt-strap comprising a third electrically conductive metal layer (<NUM>) electrically connected to the first and second electrically conductive metal layers (34a, 34b);
characterised in that
the first portion (31a), second portion (31b), and butt-strap are infused with a cured resin so as to form a unitary structure.