Duct stringer with bulkhead

A duct stringer has duct walls providing a duct with a closed cross-section; and a bulkhead in the duct. The duct is adapted to transport fluid, and the bulkhead is adapted to block the flow of fluid along the duct. The bulkhead is adhered to the duct walls by one or more co-cured or co-bonded joints. The bulkhead includes a pair of bulkhead parts, each with a web and one or more flanges. The duct stringer is manufactured by positioning the mandrels end-to-end with the bulkhead parts back-to-back between them; wrapping or laying-up the duct walls around the bulkhead parts and the mandrels; co-curing or co-bonding the flanges of the bulkhead parts to the duct walls; and after the bulkhead has been adhered to the duct walls, removing the mandrels from opposite ends of the duct.

RELATED APPLICATION

This application claims priority to United Kingdom Patent Application GB 1810730.0 filed Jun. 29, 2018, which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a duct stringer adapted to transport fluid, and a method of manufacturing a duct stringer.

BACKGROUND OF THE INVENTION

US 2013/0316147 discloses a stringer adapted to transport fluid in an aircraft wing. The stringer may be adapted to provide venting to one or more fuel tanks in the aircraft wing, or it may be adapted to provide fuel to the one or more fuel tanks. A downpipe is provided for providing a fluidic connection between the stringer and a fuel tank.

US 2015/0239570 discloses a vent dam for use in a vent stringer in a fuel vent system. The vent dam is configured to mount to and within the vent stringer. The vent dam has a contoured guiding surface for guiding fuel flow into and out of an interior of the vent stringer. The vent dam, the vent stringer, and a tube attached to the vent stringer are in fluid communication with one or more fuel tanks. The vent dam further has one or more side flanges extending from the contoured guiding surface for providing attachment of the vent dam to one or more interior portions of the vent stringer. The contoured guiding surface and the one or more side flanges are formed as one piece. The one or more side flanges may be mechanically attached to the vent stringer via one or more fastener elements. In another embodiment, the one or more side flanges may be bonded or adhered to the vent stringer via a bonding agent. In an embodiment, a two piece vent dam design is inserted through an access opening of the vent stringer.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a duct stringer comprising: duct walls providing a duct with a closed cross-section, wherein the duct is adapted to transport fluid; and a bulkhead in the duct, wherein the bulkhead is adapted to block the flow of fluid along the duct and the bulkhead is adhered to the duct walls by one or more co-cured or co-bonded joints.

In US 2015/0239570, the side flanges may be bonded or adhered to the vent stringer via a bonding agent, thereby forming a secondary-bonded joint. In such a secondary-bonded joint, the adhesive is a bonding agent made from a different material to the adherends. By contrast, in the present invention the bulkhead is adhered to the duct walls by one or more co-cured or co-bonded joints in which the bulkhead is adhered to the duct walls by an adhesive which is the same material as the bulkhead and/or the duct walls (the adherends).

Co-cured or co-bonded joints are advantageous compared with the secondary-bonded joints of US 2015/0239570 because they are simpler. They also enable the duct stringer to be assembled in an un-cured or partially cured state, making it easier to assemble.

A co-cured joint is a joint in which the adhesive and the adherends have been cured together in a single curing process. The co-cured joint(s) may be formed by providing the duct and the bulkhead as un-cured parts and then curing them in contact with each other. For instance the uncured parts may be made of pre-preg composite material comprising fibres impregnated with a matrix material such as epoxy resin. Alternatively the co-cured joint(s) may be formed by providing the duct and the bulkhead as dry-fibre parts, co-infusing them with matrix material, and then curing the matrix material to form the co-cured joint(s). In both cases the adhesive is the matrix material, which is the same as the matrix material of the bulkhead and/or the duct walls.

A co-bonded joint is a joint in which an uncured adherend has been placed in contact with a cured adherend, and then cured to form the joint. In this case the adhesive is the same material as the uncured adherend (which may be the bulkhead or the duct walls).

Optionally the duct stringer and the bulkhead are made of composite material, such as a fibre-reinforced composite material. The composite material of the duct stringer may be the same as the composite material of the bulkhead, or different.

Optionally the duct stringer comprises: a structural member with a hat-shaped cross-section, the structural member comprising a crown, a pair of webs and a pair of feet; and a duct member adhered to the crown and to opposed inner faces of the webs, wherein the duct walls are provided by the duct member.

Optionally the bulkhead comprises a pair of C-section bulkhead parts which are positioned back-to-back to form an I-section, each C-section bulkhead part comprises a web and one or more flanges, and each flange is adhered to the duct walls by a respective co-cured or co-bonded joint.

Optionally the bulkhead divides the duct into a first portion on a first side of the bulkhead and a second portion on a second side of the bulkhead. Optionally the first portion is adapted to transport fluid via an interconnecting pipe.

Optionally the first portion is adapted to transport fluid; and the second portion is not adapted to transport fluid.

Optionally the bulkhead is adhered to the duct walls around a full periphery of the bulkhead, so that the bulkhead substantially prevents the flow of fluid between the first and second portions of the duct.

Optionally the duct is in fluid communication with a tank, such as a fuel tank or surge tank, via an interconnecting pipe.

The fluid transported by the duct may be air, fuel, or any other fluid.

The duct stringer may be adhered to a skin to form a structure such as a stiffened panel structure. The skin may be a skin of a vehicle such as a boat or aircraft. For instance the skin may be a boat hull, an aircraft wing skin or an aircraft fuselage skin.

Optionally the skin is made of a composite material, such as a fibre-reinforced composite material.

Optionally the duct stringer is adhered to the skin by one or more co-cured or co-bonded joints.

A further aspect of the invention provides a fuel system comprising a first tank; a second tank; and a duct stringer according to the first aspect, wherein the duct is in fluid communication with the first and second tanks.

Optionally the first tank is a fuel tank, and the second tank is a surge tank.

Optionally one or both of the tanks are in an aircraft wing.

Optionally the duct is in fluid communication with the first or second tank via an interconnecting pipe.

Optionally the bulkhead divides the duct into a first portion on a first side of the bulkhead which is in fluid communication with the first and second tanks; and a second portion on a second side of the bulkhead, which may not be adapted to transport fluid.

A further aspect of the invention provides an aircraft comprising a duct stringer according to the first aspect.

A further aspect of the invention provides a method of manufacturing a duct stringer, the method comprising: forming duct walls around a bulkhead, wherein the duct walls provide a duct with a closed cross-section, and the duct is adapted to transport fluid; and after the duct walls have been formed around the bulkhead, curing the bulkhead or the duct walls, thereby co-curing or co-bonding the bulkhead to the duct walls so that the bulkhead is adapted to block the flow of fluid along the duct.

Optionally the bulkhead and the duct walls are cured together in a single curing process, thereby co-curing the bulkhead to the duct walls. The duct walls and the bulkhead may be provided as un-cured parts and then cured in contact with each other to co-cure the bulkhead to the duct walls. Alternatively the duct walls and the bulkhead may be provided as dry-fibre parts, co-infused with matrix material, and then cured together to co-cure the bulkhead to the duct walls.

Alternatively the duct walls or the bulkhead may be provided as un-cured part and then cured in contact with the other (cured) part to co-bond the bulkhead to the duct walls. The un-cured part may be uncured pre-preg material, or dry-fibre material which is infused then cured to form the co-bonded joint(s). In one embodiment the duct walls are provided as an un-cured part and then cured in contact with the bulkhead to co-bond the bulkhead to the duct walls. In this case the bulkhead may be formed from a thermoplastic material.

Optionally, forming the duct walls around the bulkhead comprises wrapping or laying-up a duct member around the bulkhead so that the duct member has a closed cross-section and surrounds the bulkhead.

Optionally the method comprises: providing a pair of mandrels; positioning the mandrels end-to-end with the bulkhead between them; wherein forming the duct walls around the bulkhead comprises wrapping or laying-up the duct walls around the bulkhead and the mandrels; and after the bulkhead has been adhered to the duct walls, removing the mandrels from opposite ends of the duct.

Optionally the bulkhead comprises a pair of C-section bulkhead parts, and the method comprises: providing each C-section bulkhead part on a respective mandrel, wherein each bulkhead part comprises a web and one or more flanges; positioning the mandrels end-to-end with the bulkhead parts back-to-back between them; wherein forming the duct walls around the bulkhead comprises wrapping or laying-up the duct walls around the bulkhead parts and the mandrels; wherein the flanges of the bulkhead parts are co-cured or co-bonded to the duct walls; and after the bulkhead has been adhered to the duct walls, removing the mandrels from opposite ends of the duct.

The duct stringer may be adhered to a skin. Optionally adhering the duct stringer to the skin comprises curing the duct stringer and the skin together in a single curing process, thereby co-curing the duct stringer to the skin.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1is a plan view of an aircraft1comprising a pair of wings2and a fuselage3.FIG. 2is a plan view of certain elements of the aircraft's fuel tank system, including a centre fuel tank10inside the fuselage, and wing tanks11-13inside the wing2. The wing tanks comprise an inboard fuel tank11, an outboard fuel tank12and a surge tank13. The tanks10-13are separated by ribs14-16.

The wing tanks11-13are bounded fore and aft by spars17,18. The upper and lower boundaries of the wing tanks11-13are provided by upper and lower wing skins, not shown inFIG. 1. Each wing skin is stiffened by stringers running span-wise along the length of the wing. Two of the stringers attached to the upper wing skin are so-called “duct stringers”20,30which are shown inFIG. 2. The other stringers attached to the upper wing skin are not shown in order to simplify the drawing.

Each duct stringer20,30includes a duct portion20a,30aoutboard of an internal bulkhead21,31; and a structural portion20b,30binboard of the internal bulkhead21,31.

The duct portion20aof the duct stringer20is in fluid communication with the outboard fuel tank12via an interconnecting pipe22and with the surge tank13via an outlet23at the end of the stringer20. The duct portion30aof the duct stringer30is in fluid communication with the inboard fuel tank11via an interconnecting pipe33and with the surge tank13via an outlet34at the end of the stringer30. The duct portion30aof the duct stringer30is also in fluid communication with the centre fuel tank10via an interconnecting pipe32. Optionally, the distal end of each interconnecting pipe22,32,33is fitted with a bell mouth or float valve, not shown.

During operation of the aircraft, the duct stringers20,30are used to transport air in either direction between the surge tank13and the fuel tanks10-13in order to manage the air pressure within the tanks. So each duct stringer20,30has two functions: stiffening the upper wing skin; and providing a duct which transports air to and from the surge tank13.

FIG. 3is a transverse cross-sectional view of the duct stringer20and the upper wing skin46which carries it. The other duct stringer30has an identical construction so will not be described in detail. The duct stringer20comprises a structural member25with a hat-shaped cross-section, a duct member26with a closed trapezoidal cross-section; and a pair of noodles62. The structural member25has a crown40, a pair of webs41and a pair of feet42. The crown40, webs41and feet42are formed as a single laminar composite part, with plies of the composite material running from one foot42to the other via the webs41and the crown40. The duct member26is adhered to the crown40and to the opposed inner faces of the webs41by respective co-cured joints. The duct member26and the feet42of the structural member are adhered to the upper wing skin46by co-cured joints. The duct member26provides duct walls which fully enclose a duct45a,bwith a closed trapezoidal cross-section which is adapted to transport air between the tanks12,13.

The bulkhead21divides the duct into a first portion45aon a first side of the bulkhead and a second portion45bon a second side of the bulkhead as shown inFIG. 4. The first portion45aof the duct (which runs along the duct portion20ashown inFIG. 2) is adapted to transport air; and the second portion45bof the duct (which runs along the structural portion20bshown inFIG. 2) is not adapted to transport air so may optionally be blocked at its inboard end.

The interconnecting pipe22is connected to the first portion45aof the duct45aby cutting a hole in the crown40, and attaching a downpipe fitting (not shown) such as the fitting described in US2013/0316147, the disclosure of which is incorporated herein by reference.

FIGS. 4-6show the structure of the bulkhead21. The bulkhead21comprises a pair of C-section bulkhead parts21a,21bwhich are positioned back-to-back to form an I-section; and a noodle61, e.g., a fillet. Each C-section bulkhead part comprises a web50a, band a flange51a,b. Each flange51a,bis adhered to the duct member26by a respective co-cured or co-bonded joint.

As shown inFIGS. 3 and 6, each flange51a,bhas a closed trapezeoidal cross-section and runs around the bulkhead. Each flange51a,bis adhered to the duct member26around its full periphery so that the bulkhead substantially prevents the flow of fluid between the two portions of the duct.

FIGS. 8-23show a method of manufacturing the structure ofFIGS. 3 and 4.

A pair of mandrels60a,bare provided. Both mandrels60a,bare shown inFIG. 12. The mandrels are identical so only one will be described in detail with reference toFIGS. 8 to 11.

The mandrel60bmay be formed of silicone rubber, and has a recess61bat one end. The bulkhead part21bis fitted in the recess61bas shown inFIGS. 10 and 11. This may be achieved by laying-up the bulkhead part21bply-by-ply directly into the recess61b, or by laying it up ply-by-ply on a male tool (now shown) then transferring it onto the mandrel60b.

The bulkhead part21bat this stage is a dry-fibre preform, comprising dry carbon fibre plies and a binder. The plies are formed with darts to enable them to be laid up without wrinkling to form the corners of the flange51b.

The mandrels are then positioned end-to-end with the bulkhead between them as inFIGS. 12 and 13. Next, a noodle61is fitted as shown inFIGS. 14 and 15

The duct member26is then formed by wrapping or laying-up around the bulkhead21and the mandrels60a,bas shown inFIGS. 16 and 17. This may be achieved by laying-up the duct member26ply-by-ply directly onto the bulkhead21and the mandrels;60a,b; or by laying it up as a flat or C-section charge and then wrapping the charge around the bulkhead21and the mandrels;60a,b. The duct member26at this stage is a dry-fibre preform, comprising dry carbon fibre plies and a binder.

Note that the duct member26is formed around the bulkhead21as a dry-fibre preform, rather than inserting the bulkhead21into a previously formed duct. This makes the structure easy to assemble.

Next the assembly is placed onto the skin46as shown inFIGS. 18 and 19, and noodles62are fitted as shown inFIGS. 20 and 21. The skin46at this stage is a dry-fibre preform, comprising dry carbon fibre plies and a binder.

Next the hat-section structural member25is laid-up over the assembly as inFIGS. 22 and 23. The structural member25at this stage is a dry-fibre preform, comprising dry carbon fibre plies and a binder.

Next the assembly is covered with a vacuum bag (not shown) and the composite parts21,25,26,46,61,62co-infused with epoxy resin matrix material. After infusion, the matrix material cures to form the various co-cured joints in a single curing process. After the parts are fully cured, the mandrels are removed from opposite ends of the duct as shown inFIGS. 24 and 25.

In the method described above, the structure is formed by infusion of resin into dry fibre preforms, but alternatively some or all of the carbon-fibre parts21,25,26,46,61,62may be laid up as “pre-preg” laminates, where each ply in the laminate is a fibre ply pre-impregnated with thermosetting epoxy resin which is cured by heating after the structure has been assembled.

In the method described above, the bulkhead is a two-part bulkhead which is adhered to the duct walls by co-cured joints, but in an alternative embodiment the bulkhead may be a pre-manufactured thermoplastic single piece bulkhead which is co-bonded to the duct walls.

FIGS. 26aand 26bschematically show the formation of a co-cured joint between two elements of the structure described above. In this example, the adherends are the flange51aand the duct member26, but the other co-cured joints are similar.

The adherends are first placed together as dry-fibre preforms as inFIG. 26a, with no epoxy resin. After being co-infused with epoxy resin matrix material, the epoxy resin is cured to form the co-cured joint70ofFIG. 26b. The adhesive at the bond line71is the same material as the epoxy resin matrix material in the adherends51a,26.

Where the word or appears this is to be construed to mean ‘and/or’ such that items referred to are not necessarily mutually exclusive and may be used in any appropriate combination.