Patent Description:
A typical aircraft includes a mechanical structure formed of skin panels attached to an underlying airframe. The skin panels form a surface that is acted upon by aerodynamic forces during flight. As the skin panels may be made relatively light and thin to reduce the overall weight of the aircraft and increase its potential payload and range, the airframe includes structures configured to reinforce the skin panels, and also to impart aerodynamic forces acting upon the skin panels to load-bearing support structures. The airframe thus prevents undesired flexure, vibration, and other types of skin panel motion while distributing forces to locations configured to bear loads.

In some aircraft, structures referred to as "stringers" are used to stiffen skin panels and transmit aerodynamic forces acting upon the skin panels to load-bearing structures, such as spars and/or ribs. Stringers may assume various forms and material composition. In some examples, stringers are formed from a composite material (e.g. a carbon fiber-epoxy composite). Depending upon the cross-sectional shape of such a stringer, the stringer may be formed from two or more different parts that are fused or otherwise joined together. One example of such a composite stringer has a blade-shaped cross section with a flange and a web and is formed by fusing together two curved stringer portions, each comprising a web portion, a flange portion, and a radius between the web portion and the flange portion. Due to the radii, a space is formed in the flange portion of the stringer where the two stringer portions meet. This space may be filled with a filler material, referred to herein as a filler or radius filler, to further strengthen the composite stringer.

As stringers may be arranged longitudinally along portions of an aircraft body, such as along a wing, the length of the stringer, and thus the length of the filler added to the space, may be relatively long. For example, when positioned in the wing of a commercial aircraft, the length of a composite wing stringer may be in a range of eighty to one hundred feet. Fabricating a filler with a length in this range poses various challenges. For example, where the filler piece is formed by extruding the filler material into a mold formed in a die, a die of the necessary length will occupy significant valuable factory space. Further, as the length of filler piece increases, the number of people needed to handle the filler piece (e.g. to remove it from the die and transfer it to the stringer space) also increases. While an assisted lift may be used to extract fabricated filler pieces, such tools may impose production flow issues, and consume valuable space in clean room storage and also on a manufacturing floor. Thus, in view of the above, a challenge exists to fabricate filler segments for composite stringers and potentially other composite structural components.

<CIT> states, according to its abstract, that a thermoplastic composite structure is produced by extruding a bead of composite material to a desired cross sectional shape. An extruder extrudes the polymer bead containing reinforcing fibers, using a low compression first extruder stage where the polymer is mixed and de-gassed, and a high compression second stage where the polymer is Consolidated and extruded. The cross sectional profile of the polymer bead may be altered using a variable extruder gate. <CIT> discloses according to its abstract, a forming device for manufacturing of a fillet of plastic for aircraft assemblies.

According to the present disclosure, a die for forming a radius filler for a composite aircraft wing stringer as defined in the independent claims is provided. The below embodiments, examples, and aspects are present for aiding in understanding the background and advantages.

The features, functions, and advantages that have been discussed can be achieved independently in various examples or may be combined in yet other examples, further details of which can be seen with reference to the following description and drawings.

In view of the considerations discussed above, methods and apparatuses are provided that relate to fabrication of a radius filler in segments that are fused after being installed. Briefly, a filler material is extruded into multiple mold channels of a die to thereby form multiple filler segments. The filler segments are removed from the die and arranged in end-to-end contact in a space where the web and flange of a composite wing stringer meet, or in a space of another structural component. The filler segments are then cured to fuse the segments, which thereby form a unitary filler structure. The fabrication of a radius filler from multiple filler segments enables the use of a compact die and thus savings in space occupied by the die and other fabrication tools, a reduction in labor, and a reduction in the complexity of the filler fabrication process.

<FIG> illustrates an example aircraft <NUM> having a mechanical structure formed in part by a skin <NUM> attached to an underlying airframe <NUM>. Among other components, airframe <NUM> includes a fuselage <NUM> and a wing <NUM> extending from the fuselage <NUM>. <FIG> shows a portion of skin <NUM> removed from wing <NUM>, revealing various structural components of the wing <NUM>, including a wing stringer <NUM> extending from wing root to wing tip. Wing stringer <NUM> is attached to a plurality of ribs (e.g. rib <NUM>), and to the skin <NUM> of wing <NUM>. Accordingly, wing stringer <NUM> reinforces and stiffens the skin <NUM> of wing <NUM> as well as transferring aerodynamic forces acting on the wing skin to the plurality of ribs and to other load-bearing components of airframe <NUM>.

Wing stringer <NUM> may be formed from a composite material (e.g. a carbon fiber-epoxy composite). Composite structural component, as used herein, refers to a structural component that is made of composite portions and combined to make a structural component, such as, for example a beam, spar, stringer, or any similar load bearing support structure. In such examples, wing stringer <NUM> is a composite structural component referred to as a "composite wing stringer". Composite wing stringer <NUM> may be fabricated by joining together a plurality of parts. As described above, some wing stringer parts comprise a radius that forms a space unoccupied by stringer material when joined together. This space thus may be filled with segments of a radius filler material fabricated, positioned in the wing stringer, and fused as described herein.

<FIG> show a cross-sectional view of wing stringer <NUM> attached to a skin panel <NUM> that forms a portion of skin <NUM> (<FIG>). In the depicted example, wing stringer <NUM> comprises a composite blade stringer formed by a first stringer portion 202A joined with a second stringer portion 202B. Composite wing stringer <NUM> includes a flange <NUM> and a web <NUM> formed by joining first stringer portion 202A and second stringer portion 202B. As shown in <FIG>, both stringer portions <NUM> include a radius (shown at <NUM> for second stringer portion 202B) where the flange and web meet, which together define a space <NUM>. The space <NUM> may run along a length of the stringer <NUM>. As described in further detail below, after joining stringer portions 202A and 202B, space <NUM>, as it extends along the length of the stringer, is filled with a plurality of radius filler segments <NUM> forming a unitary radius filler <NUM>, as shown in <FIG>. The stringer assembly is then cured to fuse the various components to form stringer <NUM>. Curing the assembly fuses the radius filler segments to form a radius filler in space <NUM>, as indicated by the cross-hatching in space <NUM>. In this process, the radius filler segments also fuse to the stringer portions 202A and 202B. The radius filler may thereby increase the structural integrity of composite wing stringer <NUM>, and its ability to stiffen skin panel <NUM> and transmit forces acting upon the skin panel to load-bearing structures in airframe <NUM>. The approaches described herein for forming a radius filler for a wing stringer also may be applicable to other types stringers, to non-stringer structural components (e.g. longerons), and to wing stringer geometries other than that depicted in <FIG>.

<FIG> depicts an apparatus <NUM> including a rotary die <NUM> for fabricating radius filler segments for composite wing stringer <NUM> (<FIG>). Rotary die <NUM> includes a plurality of mold channels <NUM> including mold channel 304A arranged around a circumference <NUM> (best shown in <FIG>) of a surface <NUM> (best shown in <FIG>) of the rotary die. Each mold channel is shaped to form a radius filler segment that may be arranged with other radius filler segments in an end-to-end manner and then fused to form a radius filler for composite wing stringer <NUM>. As such, each mold channel has a cross-sectional shape that matches the geometry of space <NUM> (<FIG>), such that a radius filler segment <NUM> formed in a mold channel <NUM> fills the space <NUM> when inserted therein.

An example radius filler segment fabrication process includes extruding a filler material <NUM> from an extrusion orifice <NUM> into a mold channel 304B. For example, by moving one or both of rotary die <NUM> and the orifice <NUM>, as indicated by arrow <NUM>, the filler material <NUM> is deposited into a length of the mold channel 304B. Optionally, a compactor such as compaction wheel <NUM> applies pressure to the filler material <NUM> to cause the filler material <NUM> to take a shape of the mold channel. Compaction wheel <NUM> is shown schematically in <FIG>, however, any other suitable structure may be used to apply pressure to filler material <NUM> to press the extruded filler material <NUM> firmly into the mold channel. When extruded into a mold channel <NUM>, the filler material <NUM> forms a radius filler segment <NUM> configured to fill space <NUM> of composite wing stringer <NUM> (<FIG>) and thereby reinforce the composite wing stringer <NUM>. The filler material <NUM> may comprise any suitable material(s), including but not limited to a discontinuous carbon (e.g. graphite) fiber material comprising fiber segments and an epoxy (e.g. thermoset) matrix that can be fused to other stringer components in a curing process.

As each mold channel <NUM> is angularly offset from adjacent mold channels around circumference <NUM> of rotary die <NUM>, extruding filler material <NUM> into the mold channels includes rotating the rotary die <NUM> between each extrusion. <FIG> depict a partial view of rotary die <NUM> in a respective rotational orientation during the extrusion process. In <FIG>, a first mold channel 304A is filled with filler material <NUM> to thereby form a first radius filler segment 402A. In <FIG>, rotary die <NUM> is rotated, and a second mold channel 304B is filled with filler material <NUM> to thereby form a second radius filler segment 402B. In <FIG>, rotary die <NUM> has been rotated through eight extrusions, with eight radius filler segments formed in the mold channels. As described below, the radius filler segments <NUM> may be removed from the mold channels (e.g. after a cooling period in some examples), arranged in end-to-end contact of space <NUM> defined by the radius of composite wing stringer <NUM>, and fused together to form the unitary radius filler <NUM> filling the space <NUM> of composite wing stringer <NUM>. Further, in some examples, an interface material, such as tape comprising polytetrafluoroethylene (e.g. TEFLON tape, available from The Chemours Company of Wilmington, DE), may be arranged in each mold channel <NUM>, with filler material <NUM> deposited in each mold channel <NUM> over the interface material. The interface material may aid in unseating radius filler segments <NUM> formed by the filler material <NUM> from mold channels <NUM>. Alternatively, or additionally, rotary die <NUM> may be coated in a release agent (e.g. FREKOTE, available from Henkel AG & Company of Düsseldorf, Germany) to facilitate removal of radius filler segments <NUM>. Further, rotary die <NUM> may comprise a tooling foam, and/or any other suitable material(s).

In the example of <FIG>, where each mold channel has a length of ten feet, eight ten-foot radius filler segments may be formed, and later joined together to form an eighty-foot radius filler - e.g. for an eighty-foot long space of a composite wing stringer. In other examples, rotary die <NUM> may be configured with any other suitable length and/or number of mold channels. As additional examples, rotary die <NUM> may be rotated between six to ten extrusions, with filler material being extruded into mold channels each having a length between eight and twelve feet. In other examples, a rotary die may have any other suitable configuration, based upon the geometry of a space to be filled using the rotary die. Further, while the examples depicted in <FIG> show an area immediately adjacent to each mold channel that is substantially flat, such that the rotary die <NUM> comprises a polygonal circumference <NUM> with multiple flat surfaces <NUM>, each flat surface hosting a mold channel, in other examples the area adjacent to each mold channel may have a curved geometry in a circumferential direction, or may exhibit any other suitable geometry in a circumferential direction. Also, in some examples, a plurality of dies may be arranged (e.g. in parallel and/or in series) for filling by a same extrusion orifice, wherein each die of the plurality of dies comprises a plurality of mold channels, and each die is coupled to a motor to allow rotation of the die. The extrusion system then may be used to sequentially extrude the filler material into each mold channel of each die of the plurality of dies. This may help to increase throughput of a radius filler fabrication system. Further, in some examples, rotary die <NUM> may be removable from apparatus <NUM> (e.g. from a rotary mandrel in the apparatus), enabling different dies to be swapped into and out of the apparatus. This may help to increase fabrication throughput. Where dies of differing mold channel geometry are used, such dies may be swapped to manufacture filler segments potentially of differing profile with high throughput.

As shown in the example depicted in <FIG>, apparatus <NUM> may employ a motor <NUM> to rotate rotary die <NUM> (e.g. via a thrust bearing) between each extrusion. Further, apparatus <NUM> may use an optical sensing system <NUM> configured to image the mold channels (e.g. via one or more lasers, cameras, or other similar imaging devices), and a feedback mechanism configured to provide a signal to motor <NUM> prompting rotation of rotary die <NUM>, to control extrusion into the mold channels <NUM>, as examples. Further, in some examples, a parallel gripper <NUM> may be used to hold and/or guide radius filler material <NUM> as it is extruded to help seat the material. In other examples, any other suitable mechanisms may be used to control extrusion into rotary die <NUM>. For example, parallel gripper <NUM> may be omitted, and compaction wheel <NUM> may be used without the parallel gripper to seat radius filler material <NUM> within mold channels <NUM>. As another example not according to the invention, <FIG> shows a planar die <NUM> that may be used to form radius filler segments for composite wing stringers. Planar die <NUM> includes a plurality of mold channels (e.g. mold channel <NUM>) each laterally offset from one or more adjacent mold channels on a substantially planar surface <NUM> of the planar die. Here, extruding filler material into each mold channel may include changing a relative lateral position of planar die <NUM> and an extrusion orifice between each extrusion. In any example, each mold channel may have a same length, while in other examples one or more mold channels may have different lengths.

As described above, radius filler segments fabricated according to the described approaches may be arranged in space <NUM> (<FIG>) and cured to thereby fuse the filler segments to each other and to other stringer portions. <FIG> and <FIG> respectively illustrate schematically the arrangement of a radius filler in space <NUM> defined by composite wing stringer <NUM> before and after curing of the radius filler. In <FIG>, two radius filler segments 600A and 600B are arranged in end-to-end contact (e.g., abutingly directly contact) in space <NUM>. In <FIG>, the radius filler segments have been cured to form a unitary radius filler <NUM>. Further, the curing process also bonds radius filler segments <NUM> to other portions (e.g. portions 202A and 202B of <FIG>) of composite wing stringer <NUM> to form a unitary structure <NUM> comprising the composite wing stringer <NUM> and radius filler <NUM>. In this way, a plurality of composite wing stringers may be formed that each comprises a corresponding radius filler having a plurality of filler segments each fused with one or more adjacent filler segments in an end-to-end arrangement.

In some examples, curing radius filler segments 600A and 600B may include heating the radius filler segments <NUM> to form radius filler <NUM>. For example, radius filler segments 600A and 600B may be heated to approximately <NUM> to initiate and perform a curing process, thereby chemically fusing and cross-linking a polymer resin material or other suitable material that forms composite wing stringer <NUM> and radius filler segments 600A and 600B. In other examples, any other suitable curing process may be used, such as a photo-initiated curing process.

<FIG> shows a flowchart illustrating a method <NUM> of fabricating a composite structural component for an aircraft. Method <NUM> may be performed to fabricate composite wing stringer <NUM>, as one example.

At <NUM>, method <NUM> includes extruding a filler material (e.g., filter material <NUM>) into each mold channel of a plurality of mold channels (e.g., mold channels <NUM>) of a die (e.g., rotary die <NUM>) to form a plurality of filler segments (e.g., filler segments <NUM>). In some examples, extruding the filler material may include rotating <NUM> a rotary die between each extrusion. In other examples, extruding the filler material may include changing <NUM> the relative lateral position of a planar die (e.g. planar die <NUM>) and an extrusion orifice (e.g. extrusion orifice <NUM>) between each extrusion. Further, in some examples, method <NUM> may include arranging a plurality of dies, each die of the plurality of dies comprising a plurality of mold channels, and sequentially extruding the filler material into each mold channel of each die of the plurality of dies, as indicated at <NUM>.

At <NUM>, method <NUM> may include applying pressure to the filler material while extruding the filler material into each of the plurality of mold channels (e.g. by a compression wheel that follows the extrusion orifice). At <NUM>, method <NUM> includes removing the plurality of filler segments from the plurality of mold channels of the die. The plurality of filler segments may be removed from the plurality of mold channels after a cooling period <NUM>. In some examples, the plurality of mold channels may be covered with an interface material (e.g. Teflon tape) or a coating (e.g. FREKOTE) to facilitate removal of the plurality of filler segments.

At <NUM>, method <NUM> includes arranging the plurality of filler segments in a space in the composite stringer, the space being defined by a radius of the composite stringer, such that the filler segments are in end-to-end contact. The plurality of filler segments may be arranged in a space <NUM> where a web and a flange of the composite stringer meet in some examples, or in any other suitable space in other examples.

At <NUM>, method <NUM> includes curing the plurality of filler segments in the space to fuse the plurality of filler segments. Curing the plurality of filler segments may include heating <NUM> the plurality of filler segments. Curing the plurality of filler segments forms a unitary structure <NUM> with the composite structure.

Claim 1:
A die (<NUM>) for forming a radius filler (<NUM>) for a composite aircraft wing stringer (<NUM>), the die (<NUM>) comprising a plurality of mold channels (<NUM>) arranged across a surface (<NUM>) of the die (<NUM>), each mold channel (<NUM>) shaped to form a radius filler segment (<NUM>) for a radius filler (<NUM>), wherein the mold channels (<NUM>) are angularly offset around a polygonal circumference (<NUM>) of the die (<NUM>).