EP3119577A1

EP3119577A1 - Primary structure connecting element for aircraft and method for manufacturing the connecting element

Info

Publication number: EP3119577A1
Application number: EP15715492.3A
Authority: EP
Inventors: Jan Willem Gunnink; Jacobus Wilhelmus Cornelis Anna Gofers
Original assignee: Gofers Beheer BV; GTM Advanced Products BV
Current assignee: Gofers Beheer BV; GTM Advanced Products BV
Priority date: 2014-03-17
Filing date: 2015-03-17
Publication date: 2017-01-25
Also published as: WO2015142171A1

Abstract

The invention relaters to a connecting element for fastening an aircraft skin to an aircraft primary reinforcing element. The connecting element comprises a shear web and, integrally formed therewith, a frame stabilizing element. The connecting element comprises a fibre-reinforced composition of discontinuous fibres in a thermoplastic or thermoset matrix material, is configured to transfer primary loads between the aircraft skin and the primary aircraft frame, and is characterized in that at least part of the discontinuous fibres are intentionally and controllably oriented along paths of primary load transfer. An aircraft with a primary structure connecting element for fastening the aircraft skin to the primary structure of the aircraft is also specified. The primary structure connecting element is preferably produced from a fibre-reinforced composition by means of injection moulding. Used in particular for this are chopped graphite, carbon fibres and/or glass fibres embedded in a matrix material, for example in the form of pellets.

Description

Primary structure connecting element for aircraft and method for manufacturing the connecting element

TECHNICAL FIELD

The invention relates to connecting an aircraft skin to an aircraft (sub) structure, in particular to an aircraft reinforcing element, for example to a frame or to a rib. The invention further relates to a connecting element for fastening an aircraft skin to a primary aircraft reinforcing element, and to an aircraft comprising the connecting element.

BACKGROUND OF THE INVENTION

The primary structure of an aircraft consists, in case of a fuselage for instance, of multiple shape defining frames and stringers that typically extend between the frames. In primary structures such as wings, vertical and horizontal tail planes, ribs are used as reinforcing elements. The aircraft fuselage skin is connected to the frames by means of elements known as castellation's, shear webs, clips (short shear webs) and frame stabilizing elements (cleats or stabilizers). In the case of ribs for wing and tail planes, reference is made to rib stabilizing elements instead of frame stabilizing elements. Rib stabilizing elements are used in combination with shear webs or clips to fix the aircraft wing and tail plane skin to the ribs. A rib stabilizing element therefore is a cleat that is fastened to a rib and to the wing or tail plane skin. This can be performed directly to and only to these skins or to the stringers as well as to skin and stringers. The connection between the aforementioned elements is usually achieved by a riveted connection. However, bolts, for example those known as Hi-Loks, Hi-Lites and lockbolts may also be used in the context of the present invention. A bonded connection can also be applied, as well as combinations of mechanical fastening (bolts and/or rivets) and bonding. The castellations, shear webs, clips, frame and rib stabilizing elements used nowadays are based on a fiber reinforced composition, typically comprising carbon fibres. Clips and cleats are typically made by a production process, in which an assembled, multi-layered raw material is used. A sheet of the desired size and shape is pressed out of the multi- layered raw material, and subsequently heated in sheet form and adapted (folded) in a substantially solid state to the contours of a mould in the shape of the final component. The edges are then trimmed and/or removed, for instance by sawing or grinding, in order to obtain the exact form of the component. An edge sealing is usually required to prevent dry carbon fibres from being exposed at the borders. Exposed carbon fibers are prone to galvanic corrosion, which should be prevented, at least at locations with materials and structures prone to galvanic action with carbon fibres. The above described production method starts from a relatively flat multi-layered sheet and there is a reliance on the given form of the mould with regard to the radii that are created on the component produced. Trimming and edge sealing furthermore makes the known process very labour- and cost-intensive, since it requires a number of intermediate steps. In particular in the case of shear webs, and frame and rib stabilizing elements, which are applied in numbers of 3000 to over 10 000 per aircraft, the known process entails an enormous amount of work for the production of an aircraft.

SUMMARY OF THE INVENTION The present invention aims to provide an improved primary structure of an aircraft, in particular with regard to the amount of work involved and the costs as well as improved structural performances. The invention in particular aims to provide a connecting element for fastening an aircraft skin to a primary aircraft reinforcing element which offers the desired primary load capacity and yet can be produced in a reliable and cost-effective manner.

The object is achieved by a connecting element in accordance with claim 1. In particular, a connecting element is provided comprising a shear web and, integrally formed therewith, a frame or rib stabilizing element, wherein the connecting element comprises a fibre - reinforced composition of discontinuous fibres in a thermoplastic or thermoset matrix material, the connecting element is configured to transfer primary loads between the aircraft skin and the primary aircraft frame or rib, and at least part of the discontinuous fibres are intentionally and controllably oriented along paths of primary load transfer. Although not limiting to the invention, a particularly useful method for producing a connecting element in accordance with the invention concerns injection moulding.

Injection moulding of discontinuous fibers embedded in a thermoplastic or thermoset matrix material is known per se but has not been used for aircraft primary structural parts before, due to the general believe that the stiffness and strength offered by injection moulded components would be too low for application in aircraft primary structures. This prejudice has been overcome by configuring the connecting element in accordance with the invention, as will be elucidated in more detail below. The injection molding method in accordance with the invention comprises the steps of: providing a fibre-reinforced thermoset or thermoplastic composition,

optionally melting the fibre-reinforced composition,

injecting the fibre-reinforced composition through one or more gates into an injection moulding press with an injection mould provided with the one or more gates for producing the primary structure connecting element,

curing the thermoset composition or cooling the thermoplastic composition, and removing the primary structure connecting element produced from the injection- moulding device,

whereby the connecting element is configured to transfer primary loads between the aircraft skin and the primary aircraft frame or rib, and the position of the one or more inlets is selected such that at least a part of the discontinuous fibres are intentionally and controllably oriented along paths of primary load transfer.

In the context of the invention, the term "primary structural connecting element" should be seen as covering in particular a castellation, shear web, a frame stabilizing element, a clip or a rib stabilizing element as exemplary embodiments. The "primary structural element" can also be an integrally part of the substructure, i.e. frame and/or rib or a section thereof. In that case the frame/rib or frame/rib-section is also injection moulded.

At the same time, in the context of the invention, the term "shear web" should be understood as a means of introducing shear in a frame. This corresponds to the usual understanding of a person skilled in the art. It is understood as meaning a device for fixing a frame or a rib to the aircraft structure. A shear web thereby connects a frame or a rib to apart of the skin of the aircraft and extends over a number of stringers provided in between the frames or ribs.

Furthermore, in the context of the invention, the term "clip" should be understood as meaning a kind of short shear web in the sense that a clip only extends from a first stringer foot to a second stringer foot. For example, Figure 6 shows a shear web 605 and Figure 4 shows a clip 401, which in its installed position is only arranged between two stringers.

Furthermore, in the context of the invention, the term "frame stabilizing element" should be understood as meaning a cleat or stabilizer. To a person skilled in the art, and according to the definitions and terminology used here, a "rib stabilizing element" is a stabilizing element similar to a frame stabilizing element that is used on a rib, that is to say for example in the area of a wing, a vertical tail surface or a horizontal tail surface. The rib stabilizing element supports and stabilizes a rib of the aircraft.

Castellation's are elements similar to shear webs, but with the objective to have some flexibility between frame and aircraft fuselage skin. In general these castellation's will be applied to small aircraft up to medium sized transport aircraft.

It should be noted that, in the description of the invention that follows, with regard to the primary structure connecting element reference is made variously to castellation's, shear webs, clips, frame stabilizing elements and/or rib stabilizing elements. At the same time, the individual description also respectively applies to the other exemplary embodiments of the generic feature of the primary structure connecting element.

According to an exemplary embodiment of the invention, a connecting element is configured such that it is provided with a shear web comprising flanges for establishing a connection to the aircraft skin or to a stringer, whereby the flanges extend at an angle with a planar web section of the shear web and at both sides of the planar web section of the shear web. The angle differs from zero and preferably ranges between 60 and 120 degrees, more preferably between 80 and 100 degrees, and most preferably extends about perpendicular to the planar web section of the shear web. This embodiment provides an efficient load transfer between aircraft skin and frame and therefore provides sufficient load bearing capacity to the connection element in order to be able to reinforce it with discontinuous fibers only. The use of discontinuous fibers allows a production method such as injection moulding. Another advantage of being able to use discontinuous fibers in the connecting element is that this provides much more freedom in designing the connecting element. Indeed, the connecting element may be configured to comprise (multiple) flanges, stiffening ribs and webs, in addition to the web sections provided by the folded planar web sections of the known relatively flat multi-layered sheet. In yet another embodiment of the invention, a connecting element is configured such that the frame stabilizing element comprises a planar web section and flanges for establishing a connection to the primary aircraft structure, whereby the flanges extend at an angle with a plane of the planar web section and at both sides of the plane of the planar web section. A further embodiment of the invention provides a connecting element configured such that a planar web section of the frame stabilizing element and the shear web connect over a distance of at least half the height of the shear web. In yet another embodiment, a connecting element is provided that is configured such that the planar web section of the frame stabilizing element and the shear web connect over a distance substantially equal to the height of the shear web.

Another useful embodiment of the invention provides a connecting element that is configured such that a planar web section of the frame stabilizing element comprises a stiffening element along an edge of the planar web section. The stiffening element may be formed integrally with the frame stabilizing element, for instance by injection moulding. It is also possible to make the stiffening element by another method, for instance by pultrusion, and subsequently attach it to the edge of the frame stabilizing element, preferably by bonding.

Yet another embodiment relates to a connecting element that is configured such that the wall thickness of the frame stabilizing element and/or of the shear web varies across the element and/or web. The injection moulding method conveniently provides such an embodiment.

Another embodiment of the invention provides a connecting element configured such that a web section of the frame stabilizing element and/or of the shear web comprises holes. Holes are conveniently provided in a web section of an element by providing an injection mould with inserts. Reinforcing element may be provided as well, and are preferably selected from the group consisting of a pultrusion element, an extrusion element, a metal insert for receiving a rivet or bolt, a plastic insert for receiving a rivet or bolt and any combination of these.

According to an exemplary embodiment of the invention, an injection-moulding process for producing a primary structure connecting element is specified. The process has the steps of providing a fibre-reinforced, thermoplastic composition, melting the fibre- reinforced, thermoplastic composition, and injecting the fibre-reinforced, thermoplastic composition into an injection-moulding device. In this case, the injection-moulding device has an injection mould for producing the primary structure connecting element.

Furthermore, the step of removing the primary structure connecting element produced from the injection-moulding device is included in the process.

In this way, a lightweight primary structure connecting element can be produced quickly and in one production process with low costs. In particular, a fibre -reinforced, thermoplastic composition with a high modulus of elasticity is suitable for this. Further details and aspects with respect to the modulus of elasticity of the fibres used and of the castellation, shear web, clip, frame stabilizing element produced and/or the rib stabilizing element produced are explained below.

In particular, the fibre-reinforced, preferably thermoplastic, material may be provided in pellet form, and be transported by a transporting device into a heating element. The composition can be melted and compressed, so that it is subsequently injected at the necessary pressure into the cavity, which is formed by the injection mould. For example, 1000 bar may be used as the injection pressure. However, other values above and below 1000 bar, for example 2000 bar, may also similarly be used. The injection mould may be prewarmed or preheated, in order to provide an improved or optimum temperature for a rapid crystallization process. As soon as the matrix has gone over into the solid state, the primary structure connecting element produced can be removed from the injection- moulding device. Since the component produced, i.e. the primary structure connecting element, in its final form is covered with the thermoplastic resin, no working, trimming or sealing of the edges of the component produced is necessary. After the completion of the process, resin is respectively present on the surfaces of the moulded primary structure connecting element, so that no fibre lies dry.

In this and in every other exemplary embodiment, the fibre-reinforced matrix composition may be provided by providing chopped reinforcing fibers, preferably carbon, graphite and/or glass fibers, in a thermoplastic or thermoset matrix. Various degrees of mixing are possible.

In other words, with this process a thermoplastic composition containing fibre material is provided, melted and injected into an injection-moulding device. This allows a primary structure connecting element (castellation, shear web, clip and/or a frame stabilizing element and/or a rib stabilizing element) to be produced not only at low cost and quickly but also with the required mechanical properties, in particular with regard to its modulus of elasticity.

In an embodiment, a primary structure connecting element that has been produced by an injection-moulding process and configured in accordance with the invention may comprise a compound having a modulus of elasticity E > 20 GPa , preferably E > 30 GPa, more preferably E > 35 GPa, and most preferably E > 40 GPa. However, values other than this are also possible, if desired. In another embodiment, the fibre -reinforced composition comprises discontinuous chopped carbon fibres with a modulus of elasticity Ef > 250 GPa, more preferably Ef > 300 GPa and most preferably Ef > 350 GPa. Suitable matrix materials include thermoset materials like epoxide resins for instance, as well as thermoplastics, like polypolefines and polyamides, high stiffness thermoplastics like polyphenylene sulphide (PPS),

polyetherether ketone (PEEK) and/or polyether imide (PEI) being preferred.

Depending on the area of use of the primary structure connecting element in the aircraft, a preferred connecting element is configured such that it has a (bending) stiffness of between 30 GPa and 40 GPa, 40 GPa and 50 GPa or more preferably greater than 50 GPa. This requirement can be satisfied by configuring the connecting element in accordance with the invention, as exemplified by the embodiments of the invention. According to exemplary embodiments of the invention, the fibre-reinforced, thermoplastic composition comprises graphite, carbon fibres and/or glass fibres. The use of glass fibers may in certain efficient embodiments of the connecting element be sufficient. Indeed, the configuration of the connecting element may be optimized such that relatively low modulus glass fibers may be used. The injection-moulding process for producing a primary structure connecting element offers the possibility and potential for optimizations with regard to the design and the form of these components and allows an integration of additional stabilizing elements. For example, metal inserts, plastic inserts, extrusion and/or pultrusion elements may be inserted into the injection mould in order to connect it in one working step to the component to be produced. Additional flanges to connect the primary structure connection to the skin may also be included.

In the aforementioned examples, it is advantageously possible to provide the fibre- reinforced composition in pellet form, which allows an injection-moulding process to be carried out in practice. According to a further exemplary embodiment of the invention, the composition is therefore provided in a pellet form and the process also has the step of transporting fibre -reinforced, thermoplastic pellets into a melting device for carrying out the melting.

According to a further exemplary embodiment of the invention, the ratio of the average length L of the fibres to the average diameter d of the fibres in the molded component is L/d > 10, more preferably L/d > 50, even more preferably L/d > 75 and most preferably L/d > 100. According to a further exemplary embodiment, the fibres have on average a length of between 0.1 mm and 5 mm. For example, 0.7 mm may be preferred. This may be a mean value about which the lengths of the individual fibres in the composition are randomly distributed. According to a further exemplary embodiment, a mean value of the fibres does not exceed the maximum value of 1 mm, 2 mm, 3 mm, 4 mm or 5 mm.

According to a further exemplary embodiment of the invention, the composition has carbon fibres with a modulus of elasticity E that has a value in a range of E > 250 GPa, E > 300 GPa or E > 350 GPa, and/or the composition has glass fibres with a modulus of elasticity E that has a value of E > 70 GPa, E > 85 GPa or E > 95 GPa.

Carbon fibres with such a high modulus of elasticity can be used to injection-mould in particular stiff primary structure connecting elements which absorb the usual loads that occur in the primary structure in an aircraft. Glass fibres, on the other hand, are cheaper and, depending on requirements, can be used as a single fibre constituent of the composition or be admixed with the carbon fibres. It should be mentioned that these exemplary embodiments can be combined readily and in particular with the exemplary embodiments described above and below with respect to the length of the fibres.

The choice of the injection-moulding technique for producing the aircraft fuselage fastening means, i.e. the primary structure connecting element, makes it possible to realize a particular reinforcement in certain regions of the component produced, whereby other regions of the component can for example be made thinner and/or shorter. In this case, a combination of an injection-moulding production of the aircraft fuselage fastening means or wing fastening means or fastening means for the tailplane, vertical tail surfaces and horizontal tail surfaces and a reinforcement by means of a pultrusion element or an extrusion element for example is therefore realized. In this case, the primary structure connecting element, for example the shear web and/or the frame stabilizing element may be combined together with a reinforcing element in a single component by the injection- moulding technique. The injection moulded process also allows to produce primary structure connecting element with flanges in two directions (so-called T-flange) and thereby reinforcing the connection to skin and rib or frame. This configuration is not possible with state of the art technology.

According to a further exemplary embodiment of the invention, a metal insert which has an opening that extends through the primary structure connecting element when the injection-moulding operation has been completed is inserted into the injection-moulding device before the moulding process. The metal insert surrounds this opening of the primary structure connecting element. This opening may be designed for receiving a rivet/bolt for fastening the aircraft skin to the airframe. One or more plastic inserts may also be used alternatively or in combination. A metal or plastic strip may also be provided as reinforcing element allowing support for one or more rivets/bolts.

According to a further exemplary embodiment of the invention, the composition contains polyphenylene sulphide (PPS), polyetherether ketone (PEEK) and/or polyether imide (PEI).

The primary structure connecting elements according to the invention can be used in particular in helicopters, satellites or other transport or fighter aircraft. According to a further exemplary embodiment of the invention, such an aircraft is specified.

The primary structure connecting element may also be regarded as an aircraft fuselage fastening means, which is suitable and designed for fastening the aircraft skin to a frame/rib of the airframe or for ribs in the wing (for example at the "tankhole or manhole"). In particular, it is clear here to a person skilled in the art that typically occurring loads must and can be absorbed by such a primary structure connecting element.

According to a further exemplary embodiment of the invention, the primary structure connecting element has a substantially triangular or a triangular structural form. According to a further exemplary embodiment of the invention, the primary structure connecting element has a substantially trapezoidal or a trapezoidal structural form.

According to a further exemplary embodiment of the invention, the primary structure connecting element has at least a first flange for establishing a connection to the aircraft skin and has at least a second flange for establishing a connection to the aircraft skin.

In a particular embodiment, the two flanges lie (substantially) in one plane, such as for example the flanges 501 and 511 in Figure 5. The use of the injection-moulding process allows such a component to have two different flanges for fastening to the aircraft skin. This is not possible with previous production methods, particularly not for an integrated component. A specific exemplary embodiment of this is shown in Figure 5 and described. In the embodiment shown, flange 506 is also supplemented with another flange 512 on an opposite side of part 510, the flange 512 extending in the same plane as flange 506. According to the prior-art process, a primary structure connecting element with two flanges cannot be produced, since there the component is folded from a sheet. Differences in wall thickness also cannot be realized on the component. Since the present invention makes both possible, however, it is of particular value for primary structure connecting elements with two, three, four or more flanges. A primary structure connecting element with differences in wall thickness may also be provided alternatively or in combination by the present invention. According to a further exemplary embodiment of the invention, the primary structure connecting element comprises a compound with a modulus of elasticity E that has a value in a range of 20 GPa < E < 60 GPa. It needs to be mentioned that application of the additional stiffening elements allows for lower values of the compound elastic modulus, if desired.

According to a further exemplary embodiment of the invention, the primary structure connecting element has a reinforcing element that is selected from the group consisting of a pultrusion element, an extrusion element, a metal insert or an insert of plastic for receiving a rivet/bolt (or more rivets & bolts) and any combination of these.

According to a further exemplary embodiment of the invention, the primary structure connecting element has at least a first, a second, a third and a fourth flange.

According to a further exemplary embodiment of the invention, an aircraft that has a primary aircraft structure with at least one frame and at least one rib and an aircraft skin is specified. It also includes a castellation or a shear web or a clip, a frame stabilizing element and a rib stabilizing element. A first part of the aircraft skin is fastened to the frame of the primary aircraft structure and a second part of the aircraft skin is fastened to the rib of the primary aircraft structure. Furthermore, the first part of the aircraft skin is fastened to the frame of the primary aircraft structure by means of the shear web or the clip and the frame stabilizing element, and the second part of the aircraft skin is fastened to the rib of the primary aircraft structure by means of the rib stabilizing element. At least one element from the group consisting of the castellation or shear web, the clip, the frame stabilizing element and the rib stabilizing element is produced from a fibre-reinforced, thermoplastic composition by means of injection moulding. Further features, advantages and application possibilities of the invention emerge from the following description of the exemplary embodiments. The same or similar elements in the figures can be denoted by the same or similar reference signs. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a flow diagram of an injection-moulding process for producing a shear web, clip, frame stabilizing element and/or a rib stabilizing element according to an exemplary embodiment of the invention;

Figure 2 shows a detail from a primary aircraft structure with a plurality of primary structure connecting elements according to an exemplary embodiment of the invention;

Figure 3 shows a primary aircraft structure with a multiplicity of shear webs and frame stabilizing elements according to a further exemplary embodiment of the invention;

Figure 4 is a schematic representation of a primary structure connecting element which has as an integrated solution both a clip and a frame stabilizing element or a clip and a rib stabilizing element according to an exemplary embodiment of the invention;

Figure 5 is a schematic representation of a primary structure connecting element which additionally has as an integrated solution reinforcing elements according to an exemplary embodiment of the invention;

Figure 6 is a schematic representation of a typical installation location of a shear web and a frame stabilizing element according to a further exemplary embodiment of the invention; and

Figure 7 is a schematic representation of an aircraft according to a further exemplary embodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figure 1 represents an injection-moulding process according to an exemplary embodiment of the invention by means of a flow diagram in which the steps SI, S2, S3 and S4 are shown. The injection-moulding process is used for producing a primary structure connecting element (a shear web, clip and/or a rib or frame stabilizing element) and uses a fibre -reinforced thermoplastic composition as the injection-moulding medium. This composition is provided in step SI. In step SI, the fibre-reinforced, thermoplastic composition lati larton k/40 hm, or lati larpeek 1 Ok/40 hm may be used for example. However, these are only exemplary embodiments and the use of other materials is also additionally or alternatively possible. Furthermore, this composition is melted in step S2, so that in step S3 the molten composition can be injected into an injection-moulding press with an injection mould for producing the primary structure connecting element. To be able to control the orientation of at least a part of the discontinuous fibres, one or more gates are provided in the injection mould at preselected positions. These positions depend for instance on the particular shape of the element to be injection moulded, on the primary loads to be transferred, the specific injection moulding process to be applied and other factors, but one skilled in the art of injection moulding will be able to select the positions such that at least part of the discontinuous fibres are oriented along paths of primary load transfer. Subsequently, in step S4, the primary structure connecting element produced can be removed or ejected, so that a further cycle of the injection-moulding process can be carried out. It should be noted in this respect that, when the injection-moulding process is carried out according to Figure 1 , either a castellation or a shear web or clip or a frame stabilizing element or a rib stabilizing element can be produced. The corresponding injection mould should be used for this. However, if desired, it is also possible to injection-mould an integrated variant that includes both a shear web and a frame stabilizing element. This one-piece component should be regarded as a special embodiment of the invention, two exemplary embodiments being shown in Figures 4 and 5. The injection-moulding process shown in Figure 1 may be supplemented by additional steps in exemplary embodiments that are taken further.

The following steps and parameters of the moulding process may be combined with the exemplary embodiments mentioned above and below. Pellets of plastic, i.e. the fibre- reinforced, thermoplastic composition, are loaded into a container by means of a screw pump. The pellets of plastic are melted by means of heating elements. Furthermore, the molten pellets of plastic are injected into the moulding device by means of the screw. An increase in pressure of up to 2000 bar may be used for this. The cold mould causes the molten pellets of plastic to solidify and go over into the solid state. After the opening of the mould, the moulded primary structure connecting element can be removed. The temperature is dependent on the mould that is used. It may be 250°C to 400°C or else less than 250°C or greater than 400°C.

In particular, the steps described above of inserting a reinforcing element into the injection-moulding device and connecting the reinforcing element to the injected composition in the injection-moulding device may be included in a further embodiment. Details of various reinforcing elements are further elucidated below. Figure 2 is a schematic representation of part of an primary aircraft structure 200 in the main fuselage area of an aircraft, in which multiple frames 205 and 206 are shown. Such an aircraft is represented for example in Figure 7. Similarly shown are multiple stringers 207 and 208, which are fixedly connected to the two frames shown. Similarly shown, as a primary structure connecting element, are a multiplicity of castellation's, shear webs and clips 201 and 202, which are fastened to the respective frame by rivets and/or bolts. In this case, the castellation's or shear webs 201 and 202 have been produced from a fibre- reinforced, thermoplastic composition by means of injection moulding. Similarly shown in Figure 2 are the frame stabilizing elements (cleats) 203, 204, which similarly consist of a fibre -reinforced, thermoplastic material and have been produced by means of injection moulding, as described herein. These primary structure connecting elements 201-204 therefore do not need any subsequent sealing' s that could disadvantageously peel off and entail additional weight. Similarly, the frame stabilizing elements 203 and 204 shown in Figure 2 have chopped glass and/or carbon fibres, which have an average length of between 0.1 mm and 5 mm. For other applications, shorter and/or longer fibres may also be used without departing from the scope of the invention. This clearly distinguishes the frame stabilizing elements 203 and 204 from conventionally produced frame stabilizing elements. The same applies to the castellation's or shear webs and clips 201, 202. The longitudinally extending stringers 208 are connected to the frames 205 and 206 both by way of the castellation's or shear webs and clips 201, 202 and moreover by way of the frame stabilizing elements 203 and 204. Similarly, the aircraft skin of the associated aircraft is connected to the primary structure 200 by way of the elements 201, 202, 203 and 204. At the same time, the aircraft skin is not shown here. Riveted connections and/or bolts are used in particular for this.

According to a further exemplary embodiment of the invention, a plurality of primary structure connecting elements that are arranged in a primary aircraft structure 300 are shown in Figure 3. In this case, two frames 307 and 308 that extend in the form of an arc of a circle are represented in Figure 3. Multiple longitudinally extending stringers 309 can be seen in Figure 3. In this case, multiple shear webs and clips 301, 302 and 303 and also multiple frame stabilizing elements 304, 305 and 306 are shown. Each frame stabilizing element 304, 305 and 306 extends substantially perpendicularly away from the frame 307, to be precise perpendicularly from the direction running in the form of an arc of a circle. On the other hand, the shear webs 301, 302 and 303 extend parallel to the face 310 of the frame 307 that extends perpendicularly to the longitudinal direction of the aircraft. The longitudinal direction of the aircraft is represented in Figure 3 by the arrow 311. In this case, the clips and cleats may be connected to the aircraft skin by riveting with riveted connections or by means of bolts (Hi-Loks, Hi-Lites or lockbolts).

According to a further exemplary embodiment of the invention, a primary structure that uses ribs, that is to say for example in wings, vertical tail surfaces or horizontal tail surfaces, is specified by analogy with Figures 2 and 3. The invention also relates to the rear fuselage area, in which integral frames are used. Here, castellation's or shear webs 201, 202 and rib stabilizing elements 203 and 204, which are produced by means of injection moulding and from a fibre -reinforced, thermoplastic composition, are then used on the ribs of the aircraft. Since such ribs are known to a person skilled in the art, a further description is not necessary.

According to an embodiment of the invention, Figure 4 shows a primary structure connecting element 400, which has as an integrated solution both a clip and a frame stabilizing element or a clip and a rib stabilizing element. Element 400 has both a first component region 401, which is designed as a clip and can be used as such, and a second component region 402, which is designed as a frame stabilizing element and can be used as such. Between these two regions 401 and 402 there is a planar transitional region 403, so that a common flange is formed for the clip 401 and the frame stabilizing element 402. This integral solution, which combines a shear web and a frame stabilizing element in one component, is produced by the injection-moulding process according to the invention. The primary structure connecting element shown in Figure 4 consists of a fibre-reinforced, thermoplastic material and has a stiffness, i.e. a modulus of elasticity, that is greater than 40 GPa. In this example, carbon fibres in a chopped form have been used together with PPS as the thermoplastic composition. In particular, the following materials have proven to be suitable for the injection-moulding process according to the invention for producing aircraft shear webs and aircraft frame stabilizing elements. Fibres in a chopped form, for example with a length/maximum length of 0.1 mm to 5 mm, are suitable. Lenghts other than these of the fibres are also possible. In particular, such long carbon fibres may be used with polyphenylene sulphide (PPS), polyetherether ketone (PEEK) and/or polyether imide (PEI) as the fibre-reinforced, thermoplastic composition. Lati larton k/40 hm, lati larpeek 10k/40 hm may be mentioned by way of example. However, the use of other materials is also possible.

The primary structure connecting element 400 has a first two-dimensional region 404, which is designed to be fastened parallel to the main direction of extent of an aircraft frame to said frame. Such fastening can be seen for example from Figure 6. A two- dimensional region 405 extending substantially perpendicularly thereto is designed as a first flange for the purpose of being fastened in its installed position of the primary aircraft structure to a stringer, such as for example the stringers 606 in Figure 6. Furthermore, the component region 402 has a substantially triangular region 407, which serves as a frame stabilizing element. Substantially perpendicular to this triangular region 407, a first upper region 408 extends in a perpendicular direction in relation to the plane of the region 407. The first upper region may be regarded as a flange for fixing to an aircraft frame/to an aircraft rib. As can be seen for example from Figure 6, the component 400 can thereby also be fastened in the region 408 to a frame 601. Furthermore, this region 402 extends in the lower part almost perpendicularly into a two-dimensional region 409, by means of which the frame stabilizing element 402 can be fastened to the stringer foot, for example stringer 606 in Figure 6. The clip-cleat combination 400 is also referred to by a person skilled in the art as simply a clip. According to a particularly preferred embodiment of the invention, Figure 5 shows a primary structure connecting element 500, which by analogy with Figure 4 has both a region that can be used as a clip and a region that can be used as a frame stabilizing element/rib stabilizing element. With regard to the disclosure in this respect, reference is made to Figure 4. In addition to Figure 4, the exemplary embodiment of Figure 5 has multiple reinforcing elements 501, 502, 503, 504, 505, 507 and 512. It should be noted in this respect that the reinforcing elements described below can also be used individually and in any desired combination, if desired, on a clip or shear web or on a frame stabilizing element. A person skilled in the art infers from these variants that the primary structure connecting element that can be produced by the injection-moulding process can be specifically optimized by means of different reinforcing elements and/or different wall thicknesses.

Element 501 represents an additional, second flange, which can be realized on the primary structure connecting element 500 by means of the injection-moulding process of the present invention in a simple and low-cost way in one moulding step. Furthermore, it is possible to carry out the injection-moulding process in such a way that a rib 502 is arranged on the element 500. The possibility of being able to carry out the injection- moulding process relatively quickly means that relatively complex supplemental mouldings of the primary structure connecting element can be integrated thereby. It is similarly possible, as an additional or supplemental measure, to insert a metal insert or plastic insert 503 into the injection-moulding device, so that there are correspondingly reinforced rivet positions in the component 500 thereby produced. Instead of separate inserts 503 a strip can be applied which contains the (rivet/bolt) holes. Similarly, a reinforcement 504 around a hole or a clearance 509 may be included, so that an overall weight saving is obtained for the region 510, but no sacrifices have to be made with regard to the necessary mechanical stiffness. Figure 5 shows furthermore that the stiffening element 505, which may be formed for example as a pultrusion element and/or as an extrusion element, has been arranged locally along an edge. Such pultrusion and/or extrusion elements may similarly be inserted into the moulding device at the desired location, in order subsequently to obtain a one-part component. Similarly, the injection- moulding technology used makes it possible that variations in thickness are possible locally, which is shown for example as an increase of the wall thickness in the regions 506, 507 and locally in the radius 508 in Figure 5. A reduction of the wall thickness can also be performed very easily for the user on account of the use of the injection-moulding technique. The gap between 404 and 408 in Figure 4 cannot be avoided at present by the prior-art process. On the other hand, the injection-moulding process according to the invention can close this gap, if desired, as is shown in Figure 5. Elements 506 and 507 are connected there. This significantly increases the stability of the clip-cleat combination, as shown in Figure 5. The same applies to the two flanges 501 and 502. Similar to the flanges 501 and 502, a double flange 512 is preferably applied at 506 on the opposite side of 510. Figure 6 shows part of a primary aircraft structure 600, on which a shear web 605 and a frame stabilizing element 602 are arranged. This is a shear web 605 because this component extends over a number of stringers and does not only extend from a first stringer to a second stringer like a clip. A frame 601 and also a stringer 606 are also represented in Figure 6. The frame stabilizing element, i.e. the cleat or stabilizer, is of a substantially triangular design here and has two fastening regions 607 and 608 that are perpendicular to this triangular plane. The cleat 602 is fastened to the stringer by means of rivets/bolts 603. Similarly, rivets/bolts are used to connect the cleat 602 fixedly to the frame 601 in the region 607. It can similarly be seen from Figure 6 that the shear web 605 (by means of introducing shear in a frame) has a first larger main area 609, which is fastened flat against the frame 601 by means of rivets/bolts 604. A second region 610, extending perpendicularly thereto, is similarly included in the shear web 605 as a flange, by means of which the clip can be fixed by riveting to the skin/stringer located under it.

In this case, at least two different fastenings of the aircraft skin to the primary aircraft structure are possible. The primary aircraft structure is represented by way of example in Figure 6 by the frame 601 and the stringer 606. As shown in Figure 6, the frame stabilizing element 602 can be riveted/connected by bolts directly onto the stringer, to which the aircraft skin is then in turn riveted or adhesively bonded. The rivets/bolts are fastened into the fastening region 608. The stringer and the skin in this case form an integral component. However, the frame stabilizing element 602 may also be arranged such that the fastening region 608 lies directly on the skin and the rivets/bolts reach through the stabilizing element and through the skin. This can also be applied to all of the exemplary embodiment of the invention, and other ways of attaching the aircraft skin to the primary structure are not ruled out either.

According to a further exemplary embodiment of the invention, Figure 7 shows an aircraft 700, which has a first part of the aircraft skin 701 in the main fuselage area. In this case, the aircraft skin 701 is fastened to a primary aircraft structure, such as that described above, by means of multiple shear webs and/or multiple frame stabilizing elements. In this case, the shear webs used and/or the frame stabilizing element have been produced from a fibre -reinforced, thermoplastic material by an injection-moulding technique, which leads to the aforementioned advantages for the user. The second part of the aircraft skin 703 is attached to the wing 702, ribs being arranged here in the primary structure, for which reason rib stabilizing elements are used here. A third part of the aircraft skin (704, 705) is attached to the rear fuselage section (706) with vertical and horizontal tail surfaces (tailplane), ribs being arranged here in the primary structure, for which reason rib stabilizing elements are used here.

In other words, Figure 7 shows according to a further exemplary embodiment of the invention an aircraft that has a primary aircraft structure with multiple frames and multiple ribs and an aircraft skin. Similarly, multiple castellation' s or shear webs, clips, frame stabilizing elements and rib stabilizing elements are included in the aircraft. The first part of the aircraft skin is fastened to the frames of the primary aircraft structure by means of the shear webs and the frame stabilizing elements and the second part of the aircraft skin is fastened to the respective ribs of the primary aircraft structure by means of shear webs and rib stabilizing elements. Furthermore, at least one castellation or shear web, a clip, a frame stabilizing element or a rib stabilizing element has been produced from a fibre -reinforced, thermoplastic composition by means of injection moulding, as described herein.

It should additionally be pointed out that "having" does not exclude other elements or steps and "a" or "one" does not exclude any greater number. Furthermore, it should be pointed out that the features that have been described with reference to one of the above exemplary embodiments can also be used in combination with other features of other exemplary embodiments described above.

Claims

1. Connecting element for fastening an aircraft skin (200) to an aircraft primary reinforcing element (205 and 206)), the connecting element comprising a castellation or shear web (201, 202) and, separately or integrally formed therewith, a frame stabilizing element (203, 204), clip or rib stabilizing element, wherein the connecting element comprises a fibre -reinforced composition of discontinuous fibres in a thermoplastic or thermoset matrix material, the connecting element is configured to transfer primary loads between the aircraft skin (200) and the primary aircraft frame or rib (205 and 206), and at least part of the discontinuous fibres are intentionally and controllably oriented along paths of primary load transfer.

2. Connecting element according to claim 1 , wherein the primary aircraft component comprises shape defining frame or rib members (205, 206) and skin supporting stringers (207, 208) provided between the frame members, and the shear web (201, 202) is adapted to be connected to a web section of a frame or rib member (205, 206) and to the aircraft skin (200) or stringer (207, 208).

3. Connecting element according to claim 1 or 2, wherein the shear web (201, 202) comprises flanges for establishing a connection to the aircraft skin or to a stringer, whereby the flanges extend at an angle with a planar web section of the shear web and at both sides of the planar web section of the shear web.

4. Connecting element according to claim 2 or 3, wherein the frame stabilizing element (203, 204) is adapted to be connected to a web section of a frame member and to the aircraft skin or stringer.

5. Connecting element according to any one of claims 1-4, wherein the frame stabilizing element (203, 204) comprises a planar web section and flanges for establishing a connection to the primary aircraft structure, whereby the flanges extend at an angle with a plane of the planar web section and at both sides of the plane of the planar web section.

6. Connecting element according to any one of the preceding claims, wherein a planar web section of the frame stabilizing element (203, 204) and the shear web (201, 202) connect over a distance of at least 25% of the height of the shear web, and more preferably to at least 50% of the height of the shear web.

7. Connecting element according to claim 6, wherein the planar web section of the frame stabilizing element (203, 204) and the shear web (201, 202) connect over a distance substantially equal to the height of the shear web.

8. Connecting element according to any one of the preceding claims, wherein a planar web section of the frame stabilizing element (203, 204) comprises a stiffening element along an edge of the planar web section.

9. Connecting element according to any one of the preceding claims, wherein the thickness of the frame stabilizing element (203, 204) and/or of the shear web (201, 202) varies across the element and/or web.

10. Connecting element according to any one of the preceding claims, wherein a web section and/or a flange of the frame stabilizing element (203, 204) and/or of the shear web (201, 202) comprises holes, preferably provided with a metal or plastic insert.

11. Connecting element according to any one of the preceding claims, wherein the composition comprises reinforcing fibres like carbon, aramid and/or glass fibres.

12. Connecting element according to any one of the preceding claims, wherein the ratio L/d of the mean length L of the fibres and the mean diameter d of the fibres is L/d > 10, more preferably L/d > 50, more preferably L/d > 75 and most preferably L/d > 100.

13. Connecting element according to any one of the preceding claims, the connecting element being manufactured by means of injection moulding.

14. Connecting element according to any one of the preceding claims, wherein the connecting element comprises a first flange (511) for establishing a connection to the aircraft skin, and a second flange (501) for establishing a connection to the aircraft skin.

15. Connecting element according to any one of the preceding claims, wherein the connecting element comprises a first flange (506) for establishing a connection to a frame or rib element, and a second flange (512) for establishing a connection to the frame or rib element.

16. Connecting element according to any one of the preceding claims, comprising a compound having a modulus of elasticity of E > 20 GPa, preferably E > 30 GPa, more preferably E > 35 GPa and most preferably E > 40 GPa.

17. Aircraft (700) comprising a primary aircraft structure with at least one frame member (601) and at least one rib; an aircraft skin (701, 703, 704, 705); a castellation or shear web (605) or a clip; a frame stabilizing element (602) and/or a rib stabilizing element:

a first part (701) of the aircraft skin being fastened to the frame of the primary aircraft structure, a second part (703) of the aircraft skin is fastened to the rib of the primary aircraft structure and a third part (704, 705) of the aircraft skin is fastened to another rib of the primary aircraft structure;

the first part (701) of the aircraft skin being fastened to the frame of the primary aircraft structure by means of the castellation or shear web or the clip and the frame stabilizing element, and

the second part of the aircraft skin being fastened to the rib of the primary aircraft wing structure by means of the rib stabilizing element, and

the third part of the aircraft skin being fastened to the rib of the primary aircraft tail plane structure by means of the rib stabilizing element

at least one element from the group consisting of the castellation or shear web, the clip, the frame stabilizing element and the rib stabilizing element being produced from a fibre-reinforced composition by means of injection moulding.

18. Method for producing a connecting element in accordance with any one of the preceding claims, the method comprising the steps of:

providing a fibre-reinforced thermoset or thermoplastic composition, optionally melting the fibre-reinforced composition,

injecting the fibre-reinforced composition through one or more gates into an injection-moulding press with an injection mould provided with the one or more gates for producing the primary structure connecting element,

curing the fibre -reinforced thermoset composition or cooling the thermoplastic composition, and

removing the primary structure connecting element produced from the injection- moulding device,

whereby the connecting element is configured to transfer primary loads between the aircraft skin (200) and the primary aircraft frame or rib (205 and 206), and the position of the one or more inlets is selected such that at least part of the discontinuous fibres are intentionally and controllably oriented along paths of primary load transfer.

19. Method according to claim 18, the composition having carbon fibres with a modulus of elasticity E > 250 GPa, E > 300 GPa or E > 350 GPa, and/or the composition having glass fibres with a modulus of elasticity E > 70 GPa, E > 85 GPa or E > 95 GPa.

20. Method according to claim 18 or 19, the method further comprising the steps of: inserting a reinforcing element into the injection-moulding device,

connecting the reinforcing element to the injected composition in the injection- moulding device, and

the reinforcing element being selected from the group consisting of a pultrusion element, an extrusion element, a metal insert for receiving a rivet or bolt, a plastic insert for receiving a rivet or bolt and any combination of these.

21. Method according to any one of claims 18-20, the composition comprising fibres with an average length of between 0.1 mm and 5 mm.