Contoured composite stringers

Aircraft that incorporates a rounded-hat composite stringer connected to an inner side of the skin of the aircraft to form an elongate conduit that defines a conduit axis, where the conduit axis includes at least one curving portion. The rounded-hat composite stringer can be manufactured by constructing a lower forming die and an upper forming die, each forming die having a length and defining a curve along at least a portion of the length of the die, cutting a pre-cured flat composite charge dimensioned to form the rounded-hat composite stringer, pressing the flat composite charge between the lower and upper forming dies to shape the composite charge into a pre-formed stringer having an inner side between curved fillet portions, contacting a forming member against the inner side of the pre-formed stringer, applying radius fillers to the curved fillet portions of the pre-formed stringer, curing the pre-formed stringer, and removing the forming member from the cured stringer.

FIELD

This disclosure relates to aircraft structure, and more specifically the disclosure relates to rounded-hat composite stringers that define a curving conduit along an inner side of an aircraft skin.

INTRODUCTION

Aircraft generally include an airframe to which skin panels are attached in order to form a smooth aerodynamic outer surface. The skin panels are typically thin and light in order to minimize weight, and so can be somewhat flexible. In order to stiffen the panels and provide additional reinforcement, hat stringers are generally applied to fuselage sections and wing skins. Hat stringers have typically been formed of thin metal formed into the acute angles that confer stiffness and strength to the skin panels to which they are attached.

More recently, composite materials have been employed to form numerous aircraft components, due to possessing enhanced strength and stiffness while remaining light in weight, including composite materials prepared using organic polymeric materials and epoxy resins. As stringers for aircraft began to be manufactured using composite materials, the shape of the stringers evolved. Stiff metallic stringers having straight legs and acute angles have been replaced for example by stringers having a trapezoidal cross-section, either formed by a solid composite material, or defining an internal passage for the flow of liquids.

Such composite stringers, however, can require multiple plies of material to be layered by hand, either using a forming mandrel, vacuum hot drape forming, or similar technique. Forming stringers ply by ply is time-intensive, labor-intensive, and increases the risk of repetitive injury. Wrinkles may form in the ply during application, and consistency in size and thickness can be problematic. Trapezoidal hat stringers can also exhibit deficiencies related to radius thinning/thickening, Barely Visible Impact Damage (BVID), and internal fuel flow.

SUMMARY

The present disclosure provides rounded-hat composite stringers, aircraft that incorporate rounded-hat composite stringers, and methods of manufacturing rounded-hat composite stringers.

In some embodiments, the disclosure may provide an aircraft that includes a skin portion having an inner side, and a rounded-hat composite stringer connected to the inner side of the skin portion, so that in combination with the skin portion the composite stringer forms an elongate conduit having a conduit axis, where the conduit axis includes at least one curving portion.

In some embodiments, the disclosure may provide methods of manufacturing rounded-hat composite stringers, the methods including constructing a lower forming die and an upper forming die, each forming die having a length and defining a curve along at least a portion of the length of the die; cutting a pre-cured flat composite charge dimensioned to form a rounded-hat composite stringer; pressing the flat composite charge between the lower and upper forming dies to shape the composite charge into a pre-formed stringer, where the resulting pre-formed stringer has an inner side between curved fillet portions; contacting a forming member against the inner side of the pre-formed stringer; applying radius fillers to the curved fillet portions of the pre-formed stringer; curing the pre-formed stringer; and removing the forming member from the cured stringer.

In some embodiments, the disclosure may provide methods of manufacturing a complex-contoured rounded-hat composite stringer, the method including cutting a two-dimensional elongate piece from a flat composite charge so that the elongate piece has at least two curved edges, pressing the elongate piece between upper and lower forming dies to form a curved pre-formed stringer having a rounded-hat-shaped cross-section, and curing the resulting pre-formed stringer.

The disclosed features, functions, and advantages of the disclosed composite stringers, aircraft, and methods may be achieved independently in various embodiments of the present disclosure, or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings.

DESCRIPTION

Various aspects and examples of composite material stringers and methods for their manufacture are described below and illustrated in the associated drawings. Unless otherwise specified, the disclosed apparatus and/or their various components may, but are not required to, contain one or more of the structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein. Furthermore, unless specifically excluded, the process steps, structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein in connection with the present teachings may be included in other similar devices and methods, including being interchangeable between disclosed embodiments. The following description of various examples is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. Additionally, the advantages provided by the examples and embodiments described below are illustrative in nature and not all examples and embodiments will necessarily provide the same advantages or the same degree of advantages.

FIG.1depicts an illustrative example of an apparatus10that may incorporate rounded-hat composite stringers as provided in the present disclosure to strengthen and/or stiffen the apparatus construction. Apparatus10may correspond to an aircraft12, and may incorporate a plurality of rounded-hat composite stringers14throughout the aircraft, including but not limited to wing structures16, fuselage structures18, or empennage structures20(i.e. the tail assembly) of aircraft12.

Although the present disclosure specifically relates to the use of rounded-hat composite stringers in aircraft manufacture, a variety of other apparatus10are within the scope of the present disclosure, and the present disclosure is not limited to aircraft and aircraft applications. For example, as illustrative, non-exclusive examples, other apparatus10that may include rounded-hat composite stringers14include (but are not limited to) spacecraft, watercraft, land vehicles, wind turbines, structural towers and masts, roofing material, and the like. Additionally, where apparatus10includes an aircraft, the aircraft may be any version or variation of aircraft that can benefit from incorporating the rounded-hat composite stringers14, such as commercial aircraft, military aircraft, private aircraft, or any other suitable aircraft. WhileFIG.1illustrates aircraft12in the form of a fixed wing aircraft, other types and configurations of aircraft are within the scope of aircraft12according to the present disclosure, including (but not limited to) helicopters.

Apparatus10generally includes a structure that can incorporate one or more rounded-hat composite stringers14associated with an inner surface of a portion of a skin of the apparatus. Where apparatus10includes aircraft12, the rounded-hat composite stringers are then typically associated with an inner surface of a portion of the skin of the aircraft. Although typically associated with skin portions, the rounded-hat composite stringers of the disclosure may alternatively or in addition be utilized in association with any other suitable structural element of apparatus10, such as for example a wall, a floor, a frame, a column, and the like.

FIG.2depicts a cross-sectional view of a representative rounded-hat composite stringer14. Stringer14is associated with a surface21of a base22. Base22can be a composite panel that is applied to stringer14to form a complete stringer assembly. Alternatively, surface21is an inner surface of a base that is a skin portion24of apparatus10. Where stringer14is associated with skin portion24, stringer14provides support to, increases the strength of, and enhances the stiffness of skin portion24.

Stringer14can include one or more plies of a composite material that form a body26of the stringer. Body26of stringer14defines a central ridge28that protrudes and/or extends in an inboard direction with respect to base22, and which extends substantially and/or completely along the longitudinal length of stringer14. Central ridge28generally is centered with respect to the lateral dimension (width) of stringer14. The shape of central ridge28is configured to stiffen the overall composite structure, relative to a similar composite structure lacking any longitudinal ridge. Composite stringer14can therefore be described as stiffened even when the stringer structure is in a pliable state, such as when the stringer structure is uncured and/or partially cured.

Central ridge28includes sidewalls30and a rounded hat portion32supported by sidewalls30. Rounded hat portion32transitions to a straight sidewall30at tangent points33, and then sidewalls30extend downwardly in a straight line until transitioning at a smooth but narrow radius curve34to stringer legs36, which extend laterally outward along surface21. Curve34corresponds to a smooth fillet portion that is devoid of any sharp turns or inflection points, and legs36extend from curve34to each side of stringer14to form longitudinal flanges. The continuous curvature from stringer sidewall30to stringer leg36serves to minimize or eliminate any potential for ply breakage, separation from a main body of filler (e.g., woven fabric or tape) and also to minimize excess resin accumulation.

Legs36can end in tapered portions38in order to provide a smooth transition from stringer14to surface21of base22when stringer14is associated with surface21. Legs36additionally serve as flanges to seal stringer14to surface21of base22, either by co-curing and consolidation, or with an adhesive suitable for use with the composite material making up stringer14and the material of base22.

Sidewalls30, rounded hat portion32, and surface21of base22, when considered in combination, form an elongate internal conduit40that extends beneath/within the length of stringer14as it extends longitudinally along base22. The cross-sectional shape of conduit40defines an elongate internal space suitable for the flow of fluids within stringer14. Stringer14can further include a pair of elongate radius fillers42that similarly extend longitudinally along the length of the stringer, and which are configured to be inserted into the niches44formed where each curve34transitions from sidewall30to leg36along surface21. As shown inFIG.2, radius fillers42in combination with stringer14and surface21create the smooth cross-sectional profile of conduit40, which in turn permits the transport of fluids within stringer14with smooth flow and minimal turbulence. In one aspect of the present disclosure, stringer14is well-suited for the transport of fluids, such as but not limited to, jet fuel.

Conduit40defines an internal conduit axis46which is substantially centered within and aligned with elongate internal conduit40. That is, as stringer14extends along surface21of base22, conduit axis46similarly extends along the path of conduit40, and therefore along the path formed by stringer14on base22.

As is the case with previous stringers, the composite stringers14of the present disclosure may incorporate one or more joggles, corresponding to two bends in the stringer that are opposite each other and relatively close to each other. However, due to their composition and their method of manufacture, the composite stringers of the present disclosure provide exceptional flexibility in conforming to a desired curve profile, unlike previous types of stringers, including previous composite stringers. Typically, the composite stringers of the present disclosure are configured so that elongate internal conduit40defines a path that includes at least one curving portion.

Where the stringer path for a stringer of the present disclosure includes a curving portion, that curving portion can correspond to a simple curve, having a fixed radius. A simple curve is also a curve that can be completely defined without leaving a two-dimensional plane. The composite stringers of the present disclosure may also be configured to follow a path that includes a curving portion including a compound curve, or a curve having two or more arc sections of differing radii, joined tangentially. Alternatively, or in addition, the composite stringer of the present disclosure may be configured to follow a stringer path that corresponds to a complex curve, where the curve cannot be contained within a two-dimensional plane, but extends into three dimensions.

For example, as shown inFIG.3, rounded-hat composite stringer14extends along surface21of skin portion24, where skin portion24exhibits an overall curve, such as may be present in panels for the fuselage of an aircraft. While portions A and C of stringer14extend linearly along surface21, stringer14jogs to one side along skin portion24. As a result, in addition to the two opposing curves25forming the jog, portion B of stringer14necessarily also follows the curve of surface21of skin portion24, and therefore defines a complex curved path for conduit axis46. The conduit path followed by stringer14ofFIG.3cannot be defined within a two-dimensional plane.

Any given portion of a composite stringer14according to the present disclosure may incorporate one or more curving portions, and may include one or more curves that may be any combination of simple, compound, or complex curves. Composite stringer14can follow a pathway that includes any necessary joggles and/or curved portions so as to accommodate any local variations in skin portion24, skin ramps, and/or lap joints at the junction of individual sections of skin portion24sections.

FIG.4depicts a simplified cross-section of the stringers of the present disclosure. As shown, sidewall30of stringer14can form an acute angle with surface21of base22, forming a curved fillet portion. Typically, sidewall30forms an angle of between 90 degrees and 45 degrees. In another aspect of the disclosure, sidewall30forms a sidewall angle48of at least 70 degrees with surface21of base22.

The cross-section of the disclosed stringers14may additionally be characterized as having a cap radius50(i.e. the radius of rounded-hat portion32). In one aspect of the present disclosure, cap radius50has a value of between 0.5 inches (13 mm) and 1.5 inches (38 mm).

The cross-section of the disclosed stringers14may additionally or alternatively be characterized as having a hat height52of the rounded-hat portion32of the stringer, as well as a hat width54, and a stringer width56. Each of hat height52and hat width54can have a value between 1.5 inches (38 mm) and 3.5 inches (89 mm). In some aspects of the present disclosure, stringers14can be characterized as having a hat height52to hat width54ratio of at least 0.5. In another aspect of the present disclosure, stringers14can be characterized as having a hat height52to hat width54ratio of at least 1.0. Alternatively, or in addition, the cross-section of the disclosed stringers14may be characterized as having a hat width54to a stringer width56ratio of less than 0.5.

Method of Manufacture

The composite stringers of the present disclosure can be manufactured by shaping one or more plies of composite material to form a composite charge, placing the composite charge in a forming press, and using an appropriate form or forms to shape the composite charge into a stringer precursor that exhibits the desired rounded-hat profile as well as the desired three-dimensional contour. A forming member, such as a mandrel or bladder can be placed within the rounded-hat profile to maintain the desired profile, and the stringer precursor can then be consolidated with a surface such as an inner side of a skin portion by the application of one or more of pressure and temperature to cure the composite material and consolidate the cured composite material with the skin portion. Upon removal of the forming remember, the result is a rounded-hat composite stringer of the disclosure.

The process of manufacturing composite material stringers, and detailed descriptions of apparatus for forming composite material stringers, have been previously disclosed, for example by U.S. Pat. No. 8,465,613 to Rotter et al., U.S. Pat. No. 9,561,602 to Jones et al., and U.S. Pat. No. 9,649,817 to Robins et al., each of which is hereby incorporated by reference in its entirety for any and all purposes.

An illustrative method of manufacturing a complex-contoured rounded-hat composite stringer is described by flowchart60ofFIG.5, which includes cutting a two-dimensional elongate piece from a flat composite charge, where the elongate piece has curved edges, at step62of flowchart60; pressing the elongate piece between upper and lower forming dies to form a curved pre-formed stringer having a rounded-hat-shaped cross-section, at step64of flowchart60; and curing the resulting pre-formed stringer, at step66of flowchart60.

A flat composite charge70is depicted inFIG.6. Composite charge70can include a composite prepreg material that is not fully cured, and may incorporate one or more plies of a selected composite material, where the plies that may be the same or different. Typically, composite charge70includes a carbon fiber reinforcing fabric and/or tape saturated with an appropriate epoxy resin or other matrix material.

As shown inFIG.6, composite charge70ofFIG.6is flat, and is shown aligned horizontal within or parallel to the x-y plane. However, the outline of composite charge70is typically cut to at least substantially reflect the desired contour of the final stringer with respect to the x-y plane. That is, the desired stringer contour in three-dimensions is converted into a flat outline by ignoring the position of the desired stringer contour along the z-axis. Put another way, the outline of composite charge70may correspond substantially to the projection of the desired three-dimensional stringer contour onto the x-y plane. In this manner, composite charge70is cut so as to already incorporate the components of any desired curves that lie in the x-y-plane. The formation of curve components that extend along the z-axis can then be formed during pressing.

Contoured composite charge70can then be positioned between a lower forming die72and an upper forming die74, as shown inFIG.7for a representative short segment of composite charge70, and dies72and74. Lower forming die72and upper forming die74are shaped appropriately that pressing flat composite charge70between dies72and74shapes composite charge70into a pre-formed stringer14that incorporates the desired cross-sectional profile. That is, the dies72and74are shaped to produce a pre-formed stringer14that incorporates the rounded-hat profile discussed previously and shown inFIGS.2and4. In particular, the resulting pre-formed stringer14has an inner side between curved fillet portions that defines desired central ridge28of stringer14.

In one aspect of the present disclosure, rather than being manufactured to form a specific stringer contour, lower forming die72and upper forming die74are prepared from elastomeric materials, so that the dies themselves exhibit sufficient flexibility to define the desired stringer contour in three dimensions. Such elastomeric forming dies can be subsequently reshaped and reused manufacture a different stringer having a different shape. That is, the use of elastomeric forming dies permits the creation of a variety of stringers having individual customized contours in three dimensions, conserving materials and simplifying manufacturing.

As discussed above, the shape of composite charge70with respect to the x-y plane is established by cutting the composite charge appropriately. While the forming dies shape the profile of composite charge70to match a desired cross-sectional profile, the lower forming die72and upper forming die74when placed in an appropriate forming press can additionally shape composite charge70to incorporate a desired contour along the z-axis.

Forming presses capable of forming a substantially flat composite charge into a fully contoured part are described in detail in U.S. Pat. No. 9,561,602. An appropriate forming press76can mount elastomeric dies such as lower forming die72and upper forming die74so that individual segments of each die are independently displaceable relative to each other, as shown semi-schematically inFIG.8. Using forming press76, lower forming die72can be supported by a flexible die tray78that is in turn coupled to a set of lower anvils80. A set of contour control actuators82can be used to sequentially position lower anvils80so as to shape lower forming die72to achieve the desired stringer contour in three dimensions.

As lower forming die72is gradually shaped by anvils80, upper forming die74can be mounted on a backing plate84, and can similarly be displaced sequentially via upper anvils86and actuators88. Forming press76can be controlled by a programmed controller which is configured to individually and gradually displace the lower and upper anvils80,86so as to incrementally form the flat composite charge70between lower forming die72and upper forming die74in a manner that reduces or eliminates wrinkles in the formed stringer. When positioned appropriately, lower forming die72and upper forming die74, in combination, form composite charge70to follow a desired stringer contour. That is, each of the upper and lower forming dies has a first curvature around a first axis and a second curvature around a second axis, the first and second axes being orthogonal to each other and distinct from the longitudinal axis of the upper and lower forming dies.

Once shaped by forming press76the resulting pre-formed stringer14, as shown inFIG.9, exhibits a longitudinal central ridge28that includes sidewalls30, and rounded hat portion32, as well as lateral legs36on each side of central ridge28. Stringer14additionally incorporates the desired degree and pattern of curvature around a vertical axis (z-axis), but in addition incorporates the desired degree and pattern of curvature around either horizontal axis (x-axis, y-axis).

Pre-formed stringer14can be prepared for attachment to base22by contacting a forming member90against the inner side of the pre-formed stringer, i.e. along and substantially within the interior of central ridge28, as shown inFIG.10. Forming member90can be a flexible elongated mandrel, elongated inflatable bladder, or any other suitable space-filling material configured to preserve the shape and configuration of central ridge28and therefore stringer14during curing.

In an alternative aspect of the present disclosure, before forming member90is positioning against the inner side of the pre-formed stringer, forming member90can be pre-wrapped with a polymeric film, such as a thermoset plastic or thermoplastic film. Forming member90can be wrapped with such a film in any suitable pattern, or patterns, for example in a helical pattern or, where two film strips are used, in opposing helical patterns. Pre-wrapping the forming member in this way can result in consolidation of the resulting inner laminate layer with the stringer shell, and in particular, helps associate stringer14with base22, improving structural performance and improving fuel flow through conduit40.

Once forming member90is in place, elongate radius fillers92are applied adjacent to forming member90and abutting curve34of the pre-formed stringer. Radius filler (or noodle)92serves to fill the space between curve34and surface21of base22when stringer14is installed, and create a smooth continuous inner surface for resulting conduit40. Radius filler92therefore should be reversibly or irreversibly deformable under pressure, such as for example a thermoset plastic, or thermoplastic, among others. The radius filler optionally can include one or more reinforcing fibers or threads, such as carbon fibers, polymer fibers, or other reinforcing materials to provide strength and resilience.

Once forming member90and radius fillers92are appropriately positioned, the resulting assembly94can be associated with a base22and consolidated by curing, as shown inFIG.11. Base22can include a composite charge that extends along and consolidates with stringer14, forming an integral and unitary stringer that defines inner conduit40. Alternatively, base22can include skin portion24, where stringer14is associated with surface21of skin portion24and cured in place. Alternatively, stringer14can be formed with a base22that is an elongate composite charge, and the integral resulting stringer14can in turn be associated with a skin portion24of apparatus10. In one aspect of the present disclosure, the curing of pre-formed stringer14is carried out with the pre-formed stringer disposed directly on a skin portion of an aircraft. In another aspect of the present disclosure, the curing of pre-formed stringer14is carried out with the pre-formed stringer disposed directly on a skin portion of an aircraft wing.

Once stringer14is appropriately cured, forming member90can be removed from the newly-formed elongate internal conduit40, leaving radius fillers92in place, as shown inFIG.12.

The method described by flowchart60ofFIG.5can be described alternatively by reciting in greater detail a series of individual manufacturing steps as performed during the manufacture, or lay-up, of an illustrative rounded-hat composite stringer14as presently disclosed. This series of manufacturing steps can begin with preparing a stack of the desired number of layers, or plies, of the desired sheets of reinforcing material and impregnating the reinforcing material with the desired resin. Alternatively or in addition, a stack of sheets of prepreg composite material can be assembled. This full ply stack of composite sheets can then be trimmed so that the stack incorporates the desired curves along the plane of the flat stack. The contoured stack, or charge, can then be placed in a forming press, such as a punch former, that incorporates upper and lower forming dies. Placing the trimmed composite charge in the punch former can include indexing the charge for more accurate positioning of the charge with respect to the punch former and/or the forming dies. The punch former can then be adjusted so that the appropriate cross-sectional profile is formed in the charge, and also so that the charge is oriented into the desired three-dimensional contour, specifically by imposing the desired curvature on the composite charge along the vertical z-axis. Once the charge has been shaped into the appropriate contour, including shaping the central ridge, the charge is heated to a sufficient temperature to soften the composite material for forming, and the punch former applies pressure to the charge. Pressure is typically also applied to the sides of the charge. The shaped composite charge is then allowed to cool.

A mandrel (or forming body) can then be placed within the central ridge of the cooled and shaped pre-formed stringer. Noodles, or elongate radius fillers, can be installed alongside the mandrel, and the resulting stringer assembly can be compacted by a press. The compacted stringer assembly can then be transferred to a surface of a skin portion where the stringer is to be installed, and positioned on the skin as desired. The stringer assembly can then be bagged and autoclaved to fully cure the composite materials of the stringer in place to consolidate the stringer to the skin, permanently attaching it in place.

Another illustrative method of manufacturing a rounded-hat composite stringer is additionally described by flowchart100ofFIG.13. The illustrative method includes constructing a lower forming die and an upper forming die, each forming die having a length and defining a curve along at least a portion of the length of the die, as set out at step102of flowchart100; cutting a pre-cured flat composite charge dimensioned to form a rounded-hat composite stringer, as set out at step104of flowchart100; pressing the flat composite charge between the lower and upper forming dies to shape the composite charge into a pre-formed stringer, wherein the resulting pre-formed stringer has an inner side between curved fillet portions, as set out at step106of flowchart100; contacting a forming member against the inner side of the pre-formed stringer, at step108of flowchart100; applying radius fillers to the curved fillet portions of the pre-formed stringer, at step110of flowchart100; curing the pre-formed stringer, at step112of flowchart100; and removing the forming member from the cured stringer, at step114of flowchart100.

The following numbered paragraphs describe selected aspects of the disclosed rounded-hat composite stringers and methods of manufacturing rounded-hat composite stringers. The examples in these sections are intended for illustration and should not be interpreted as limiting the entire scope of the present disclosure. Each section may include one or more distinct embodiments or examples, and/or contextual or related information, function, and/or structure.

a skin portion having an inner side, and

a rounded-hat composite stringer connected to the inner side of the skin portion, that in combination with the skin portion forms an elongate conduit having a conduit axis;

wherein the conduit axis includes at least one curving portion.

A2. The aircraft of paragraph A1, where the conduit axis includes at least one portion tracing a complex curve.

A3. The aircraft of paragraph A1, where the skin portion is on a wing, a fuselage, or an empennage of the aircraft.

A4. The aircraft of paragraph A1, where the stringer has a cross-section characterized by a hat height to hat width ratio of at least 0.5.

A5. The aircraft of paragraph A1, where the stringer has a cross-section characterized by a hat height to hat width ratio of at least 1.0.

A6. The aircraft of paragraph A5, wherein each of the hat height and the hat width has a value between 1.5 inches and 3.5 inches.

A7. The aircraft of paragraph A1, where a side wall of the stringer forms an angle of at least seventy degrees with the inner side of the skin portion.

A8. The aircraft of paragraph A1, where the stringer has a cross-section characterized by a cap radius of between 0.5 inch and 1.5 inches.

A9. The aircraft of paragraph A1, where the stringer has a cross-section characterized by a hat width to stringer width ratio of less than 0.5.

B1. A method of manufacturing a rounded-hat composite stringer, comprising:

constructing a lower forming die and an upper forming die, each forming die having a length and defining a curve along at least a portion of the length of the die;

cutting a pre-cured flat composite charge dimensioned to form a rounded-hat composite stringer;

pressing the flat composite charge between the lower and upper forming dies to shape the composite charge into a pre-formed stringer, wherein the resulting pre-formed stringer has an inner side between curved fillet portions;

contacting a forming member against the inner side of the pre-formed stringer;

applying radius fillers to the curved fillet portions of the pre-formed stringer; and

curing the pre-formed stringer; and

removing the forming member from the cured stringer.

B2. The method of paragraph B1, where each of the upper and lower forming dies incorporates a complex curve.

B3. The method of paragraph B1, where the pre-formed stringer has a cross-section characterized by a hat height to hat width ratio of at least 0.5.

B4. The method of paragraph B1, where the pre-formed stringer has a cross-section having a base plane and a side wall, where the side wall forms an angle of at least seventy degrees with the base plane of the pre-formed stringer.

B5. The method of paragraph B1, where the pre-formed stringer has a cross-section characterized by a cap radius of between 0.5 inches to 1.5 inches.

B6. The method of paragraph B1, where cutting the pre-cured flat composite charge includes forming at least one curved border on the pre-cured flat composite charge.

B7. The method of paragraph B1, where curing the pre-formed stringer is carried out with the pre-formed stringer disposed directly on a wing skin.

B8. The method of paragraph B1, where the forming dies are comprised of an

reshaping the upper and lower forming dies into different shapes for use in subsequently manufacturing a different stringer having a different shape.

C1. A method of manufacturing a complex-contoured rounded-hat composite stringer, comprising:

cutting a two-dimensional elongate piece from a flat composite charge, the elongate piece having at least two curved edges,

pressing the elongate piece between upper and lower forming dies to form a

curved pre-formed stringer having a rounded-hat-shaped cross-section, and

curing the resulting pre-formed stringer.

C2. The method of paragraph C1, where the rounded-hat-shaped cross-section is characterized by a hat height to hat width ratio of at least 0.5.

C3. The method of paragraph C2, where each of the upper and lower forming dies has a first curvature around a first axis and a second curvature around a second axis, the first and second axes being orthogonal to each other and distinct from a longitudinal axis of the upper and lower forming dies.
C4. The method of paragraph C1, wherein the pre-formed stringer has an inner side between curved fillet portions, further comprising:

contacting a forming member against the inner side of the pre-formed stringer, and

applying radius fillers to the curved fillet portions of the pre-formed stringer prior to curing the pre-formed stringer.

C5. The method of paragraph C1, wherein the upper and lower forming dies comprise an elastomeric material, the method further comprising reshaping the upper and lower forming dies for use in subsequently manufacturing a different stringer having a different complex-contoured shape.

The different embodiments and examples of the apparatus and methods described herein provide several advantages over earlier stringer implementations.

The rounded-hat composite stingers of the present disclosure can be formed to follow curved, compound curved, and/or complex curved pathways from a composite laminate charge.

They presently disclosed stringers can be manufactured at the same facility and/or location where aircraft assembly is performed.

They presently disclosed stringers can be manufactured using reconfigurable tooling, decreasing production costs and increasing production rates.

The rounded-hat profiles of the presently disclosed stringers help resist Compression After Impact (CAI) damage, as well as Barely Visible Impact Damage (BVID), or damage suffered by composite materials due to low-velocity impacts that is typically not readily detectably by visual inspection, but that can result in internal structural damage (delamination, matrix cracking, fiber fracture, and fiber pullout, among others). The presently disclosed stringers are less prone to failure due to BVID than conventional trapezoid-hat stringer, which are susceptible to impacts having lower energies, and which suffer larger damage areas along their flat surfaces. The stringers of the present disclosure can withstand higher compressive loads than conventional stringers even after BVID, and due to the rounded-hat profile, the disclosed stringers provide a consistent resistance to impact damage regardless of the angle of impact on the upper rounded portion of the stringer.

The presently disclosed stringers provide better liquid flow via the internal conduit. In particular, the rounded geometry of the conduit helps create better and less turbulent fuel flow through the stringer, compared to conventional trapezoidal-hat shaped stringers.

Stringers may be constructed with radius fillers positioned at the intersection of the flange to web. These radius fillers can be made from multiple thermoset/composite fiber combinations, as well as a thermoplastic/fiber combinations, and the radius fillers can be extruded and installed via make-on-assembly (phantom assembly) processes.

Make-on-assembly stringer construction processes can include pre-wrapping the forming body with a thermoset plastic or thermoplastic material. In doing so, the resulting continuous inner laminate layer helps tie the stringer body to the base upon curing. In addition, the inner laminate additionally improves fuel flow and overall structural performance.

CONCLUSION