Geodesic structure with force transmission along two perpendicular directions and method of construction.

Abstract

The invention relates to a geodesic structure (1) for transmitting forces along two perpendicular directions in a panel of a given configuration. The structure comprises stiffening elements (10) having faces (F1, F2, Fb) hinged on edges (A1, A2) extending parallel to one of the directions. Each element (10) has a triangular cross-section perpendicular to this direction, and the stiffening elements (10) are assembled together in at least one layer such that the faces (F1, F2, Fb) constitute, in cross-section, a geodesic surface. At least one skin (P1, P2) is bonded to each set of faces (Fb) and / or edges (A2) of end stiffening elements of the assembly, forming consolidation surfaces of the structure (1). Abstract figure: [Fig. 1]

Description

Title of the invention: Geodesic structure with transmission of forces in two perpendicular directions and method of production. Technical field

[0001] The invention relates to a geodesic type structure allowing the transmission of forces in two perpendicular directions in a panel of a given architecture. The term "geodesic structure" is conventionally understood to mean a surface defining a volume and consisting of elementary spacers made of metal alloy or other rigid material, arranged to form a rigid triangular lattice. This braiding gives great resistance to the assembly because each spacer supports the forces of other spacers, even those located at a distance. This type of structure has been used in the past to produce aircraft fuselages or wings.

[0002] The present invention applies to architectural panels having structures capable of transmitting forces in two directions. This need for a structure is found more particularly in the aeronautical field where it is necessary to transport shear forces in two perpendicular directions, in particular in the following architectures: - aircraft door made of beams and frame or landing gear door made of spars and frame, with one direction in the direction of the fuselage and one direction in the direction of the frames perpendicular to the first direction; - wing box made up of spars and ribs or moving elements of this wing (aileron, flap, blade, rudder) with one direction in the span direction and the other in the rib direction; - pressurized cabin floor, made of rails and frame.

[0003] This need can also be found in other areas of construction: building, railways, navigation, etc.

[0004] Conventionally, double stiffening in two perpendicular directions is implemented. For example, in an aircraft door structure, this double stiffening is achieved on the one hand by beams arranged along the axial direction of the fuselage and on the other hand by frames arranged parallel along the direction transverse to the axis of the fuselage, with the use of at least three frames. This structure leads to numerous crossings where only one of the two stiffening elements, beams or spars, can remain continuous. It therefore involves the use of numerous connecting parts on the stiffening element cut into sections, which also weakens the assembly, and adds weight as well as production time. STATE OF THE ART

[0005] In order to overcome this fundamental problem, it is proposed to use sandwich structures comprising two sheet metal skins separated by a material or a light structure forming the core allowing the shear force to pass in two perpendicular directions. In practice, two types of core have been developed: honeycomb structures and polymer foam structures (for example based on polyurethane, polypropylene or polyester).

[0006] This type of sandwich structure is easy to implement on simple geometries, in particular with constant thickness. As soon as the geometry becomes more complex, with variations in thickness such as for a wing trailing edge, bonding the cores to the skins can become problematic. In addition, these structures absorb a large quantity of water and are difficult to drain. It is therefore necessary to guarantee the watertightness of the skins surrounding the core. Statement of the invention

[0007] To overcome the drawbacks of the state of the art set out above, the main objective of the invention is to provide an architecture allowing continuous transmission of forces in two perpendicular directions and capable of adapting to variable thickness geometries.

[0008] To do this, the invention provides for forming the structure from identical stiffening elements extending in a single direction, either in the lengthwise direction or in the transverse direction of the architecture, thus forming either side members or beams. These basic stiffening elements, of triangular section, are joined together to be assembled in layers.

[0009] More specifically, the present invention relates to a geodesic structure of a panel of a given configuration architecture, in particular of an aircraft door, this structure comprising stiffening elements having a triangular section perpendicular to one of the directions, each stiffening element having two lateral faces articulated on an inner edge extending parallel to this direction, and a base face opposite the inner edge. This base face forms lateral edges with the lateral faces, each element having a triangular section perpendicular to this direction. The base faces of the stiffening elements are assembled together by their lateral edges in at least one layer having a surface continuity so as to constitute, in section, a geodesic surface.At least one skin is secured to each set of assembled faces and / or inner edges of end stiffening elements of the assembly forming a surface continuity for consolidating the structure. When it is unique, the skin is an inner skin secured to the inner edges.

[0010] Thus, the transmission of forces uses the faces of the components to be exerted in two directions capable of carrying the shear forces: that in which the stiffening elements extend and that perpendicular to this direction of extension of these elements, as in a geodesic structure. In addition, the stiffening elements have lengths and a triangular section of dimensions adapted to the configuration of the architecture.

[0011] In addition, the structure thus produced has high torsional resistance with the advantages of sandwich structures without their disadvantages, in particular without the risk of the skins detaching.

[0012] According to preferred embodiments: - the layer of stiffening elements is compact by arranging a head-to-tail configuration of adjacent stiffening elements; - a reinforcement head (“noddle” in English terminology) is arranged between two edges belonging to two adjacent stiffening elements; - the stiffening elements are, for at least some of them, tubes empty of material, which allows the circulation of cables or any type of material of sufficient length and flexibility to pass through them; - the stiffening elements are adapted in length and cut to a given architectural configuration; - the stiffening elements and the skins are made of a material chosen from a composite material, a metal alloy and a plastic material.

[0013] Advantageously, the composite material is made of layers of carbon fibers (or glass, aramid such as Kevlar®, or equivalent) reinforced by an epoxy resin, the metal alloy is an aluminum alloy and the plastic material is a high-performance thermoplastic, in particular PEEK (polyetheretherketone), PEI (polyetherimide), PPS (polyphenylene sulfide) or equivalent.

[0014] The invention also relates to a method for producing such a stiffened structure. The method comprises the following steps for preparing a structure made of composite material: - rotation of mandrels of predetermined triangular sections; - draping of each mandrel by winding a pre-impregnated fiber of thermosetting resin over a predetermined length of each mandrel; - assembly of stiffening elements in geodesic structure layer; - co-firing of the assembly with the end skins attached to the end surfaces of said assembly. - demolding of the mandrels.

[0015] According to advantageous forms of implementation: - in order to facilitate their disassembly, the chucks are so-called key chucks having portions of walls blocked by a central core, the removal of the central core causing the separation of the portions of walls and therefore the unlocking of the chuck; - the mandrels are made of thermo-expandable silicone with a metal core, these mandrels contract when cooling, which facilitates the recovery of the stiffening elements; - alternatively, the mandrels being formed from a bag filled with microbeads under vacuum and arranged around a metal core, the elimination of the vacuum allows the beads to be evacuated and thus the stiffening elements to be released from their mandrels; - alternatively the mandrels being constituted by a metal core molded in a material soluble in a solvent in a given temperature range, after draping the assembly is immersed in this solvent at a temperature of said range, which makes it possible to dissolve the material and to remove the stiffening elements from the core; - reinforcement heads are inserted at the edges of two adjacent stiffening elements when assembling the stiffening elements in a geodesic structure layer. PRESENTATION OF FIGURES

[0016] Other characteristics and advantages of the present invention will emerge from the following reading of a detailed example of embodiment without limiting its scope, with reference to the appended figures which represent, respectively:

[0017] - [Fig. 1], a partial perspective view of an example of a geodesic structure elementary according to the invention;

[0018] - [Fig.2], a sectional view of an example of a layered geodesic structure compact according to the invention;

[0019] - [Fig.3], a perspective view of a rotating mandrel on which a fiber of carbon is wound at the start of draping in the manufacturing process of a stiffening element according to the invention;

[0020] - [Fig.4a], [Fig.4b] and [Fig.4c], sectional views of examples of mandrels which can be used when implementing said method;

[0021] - [Fig.5a] and [Fig.5b], a schematic front and perspective view of a door aircraft equipped with a geodesic structure of stiffening element panels extending in a beam according to the invention;

[0022] - [Fig.6], a schematic perspective view of a landing gear door aircraft equipped with a geodesic structure panel according to the invention, and

[0023] - [Fig.7], a schematic perspective view of an aircraft wing trailing edge equipped with a geodesic structure panel according to the invention. DETAILED DESCRIPTION

[0024] In the figures, identical reference signs refer to the same element as well as to the corresponding passages of the description. “Stiffener”, “stiffening element” and “element” designate the same object.

[0025] [Fig.l] illustrates a partial perspective view of a first example of geodesic structure 1 according to the invention. This structure is called “elementary” insofar as it is made up of identical stiffening elements 10 successively joined together along the lateral edges A1 to form a layer of stiffeners C1.

[0026] Each stiffening element 10 has a prismatic geometric shape of triangular section SI which extends parallel to a direction D1. The stiffening elements 10 are secured along the lateral edges Al so that the base faces Fb between the lateral edges Al form a base surface Sb which is flat in the direction D2 perpendicular to D1, or generally slightly curved as in the example illustrated. To do this, two base faces Fb of two consecutive stiffeners form an angle of small amplitude, a few degrees, along their common lateral edge AL. The curvature thus obtained makes it possible to globally adapt the structure to a targeted curved architecture.

[0027] Alternatively, the overall curvature of the structure can also be obtained from triangular sections with a curvilinear base, the stiffening elements being formed from flat lateral faces and curved base faces compatible with a continuous overall curvature.

[0028] The stiffening elements 10 are tubular (empty of material) and made, in the example, of a composite material based on carbon fibers and epoxy resin. A consolidation skin called the inner skin PI is secured to the inner edges A2 formed by the lateral faces F1, F2 of the stiffeners 10. The inner skin PI ensures the continuity of the structure on the inner side thereof while closing the intercalary spaces between two stiffening elements 10.

[0029] Advantageously, for architectures which require superior reinforcement, a second skin P2, called the outer skin, is secured to the base faces Fb, which provides structural continuity which is thus generally reinforced.

[0030] The triangular section SI is an isosceles triangle in the example illustrated in order to distribute the forces equally on the two lateral faces F1, F2 of each stiffener 10. More generally, the stiffening elements can have a section formed from any triangular shape provided that this triangle is compatible with the mechanical constraints of the structure.

[0031] Under these conditions, the forces are evacuated along the stiffeners 10, parallel to the direction D1, or parallel to the direction D2 by using a geodesic structure formed perpendicular to the direction D1.

[0032] With reference to the sectional view of [Fig.2], a variant of the previous single-layer structure is shown using the same stiffeners: in this structure 2, the stiffeners 10 are mounted head to tail to form a compact layer C2 of stiffening elements 10.

[0033] In addition, reinforcing heads 30 are inserted between two edges A1, A2 belonging to two adjacent stiffening elements 10. Reinforcing heads can also be inserted between two adjacent stiffening elements of the elementary structure 1 described with reference to [Fig.l]. The structure 2 is completed by two end skins P1, P2, respectively inner and outer. The reinforcing heads are made of material of the same type as the stiffeners 10, namely metal alloy, in particular aluminum, composite material or plastic material, in particular thermoplastic.

[0034] In order to obtain particularly strong structures, it is possible to produce a multi-layer structure by stacking compact layers C2 of stiffening elements 10 interposed between skins of the same type as the skins PI or P2 at each layer or on the end surfaces of the stack of layers. In order to maintain a reasonable structural thickness while maintaining high strength, it may be advantageous to seek a compromise between the size of the stiffeners and the number of layers.

[0035] The perspective view of [Fig. 3] illustrates the first steps of an example of a method for producing a structural stiffener according to the invention. In this method, a mandrel 4 is rotated (arrow RI) via a shaft 40 mounted on a drive motor (not shown). A carbon fiber 3, pre-impregnated with epoxy resin and stored on a roller 8, is wound onto the mandrel 4 of prismatic shape with triangular section corresponding to the geometry of the stiffening elements.

[0036] The carbon fiber 3 then winds along the faces 41, 42, 43 of the mandrel 4 to form the faces of a stiffening element, such as the faces F1, F2, Fb of the stiffening element 10 (see figures 1 or 2). The draping of the mandrel 4 continues over a predetermined length beyond the desired stiffener length.

[0037] In addition, the angle of the fiber 3 on the mandrel 4 sets the desired stiffener strength. This angle is obtained by adjusting the ratio between the rotation speed of the mandrel 4 and the fiber 3 feed.

[0038] Once the rotation has stopped, the stiffening element is then assembled with the other stiffening elements to form one or more layers according to the desired geodesic configuration.

[0039] Reinforcing heads 30 are advantageously arranged between the stiffening elements 10 and the assembly is arranged on the outer skin P2 before placing the inner skin PI (or conversely after placing the inner skin PI), the end skins PI and P2 being attached to the end surfaces in accordance with the configuration of [Fig.2],

[0040] A co-cooking of the assembly thus arranged is then carried out in an autoclave.

[0041] Alternatively to the use of a resin prepreg, it is possible to use a dry fiber and subsequently impregnate the drape. This impregnation can be carried out using various known methods.

[0042] The mandrel 4 thus draped is pre-baked in an oven (not shown), then the stiffening element 10 is demolded and recovered to be cut to the desired length and end shape to constitute a panel according to the intended architecture (see figures 5 to 7).

[0043] It is also possible to impregnate the fiber when it is wound onto the mandrel by passing it through a resin bath.

[0044] In order to recover the stiffening element 10 (see figures 1 or 2) after co-firing the mandrel 4, it is advantageous to use mandrels 4a to 4c respectively conforming to the sectional views of figures 4a to 4c.

[0045] The mandrel 4a of [Fig.4a] called a key mandrel is the mandrel illustrated in [Fig.3]. It comprises locking keys constituting wall portions 41a, 42b, 42c and 43d forming the faces 41, 42 and 43 and fitted into each other outside the draping zone. Each wall portion 41a and 43d, as well as the portions 42b and 42c taken together, extend in this example symmetrically with respect to the plane of symmetry Ps. This symmetry makes it possible to make the portions 42b and 42c interchangeable and to not distinguish sides or directions for the positioning of the portions 42a and 42d.

[0046] A central metal core 4N keeps the wall portions 41a, 42b, 42c and 43d locked. Removing this central core 4N unlocks and separates the wall portions 41a, 42b, 42c and 43d. To facilitate the detachment of the composite material from the stiffening element 10, the wall portions 41a, 42b, 42c and 43d have inclined draft angles. These portions are then removed and the stiffening element 10 recovered or demolded if this element is already included in the panel.

[0047] Alternatively, the locking means are constituted by ratchets between the walls. Once the ratchets are unlocked, the walls are extended, which allows the stiffener 10 to be released.

[0048] The mandrel 4b illustrated in [Fig.4b] has a layer of thermo-expandable silicone 44 arranged around a metal core 45. The layer of silicone contracts when it is immersed in a low temperature environment, which makes it possible to release and recover the stiffening element 10.

[0049] With reference to [Fig.4c], the mandrel 4c has, in place of the silicone layer 44 of the mandrel 4b, a bag 46 arranged around the metal core 45 and filled with the microbeads 47 under vacuum. Once the vacuum has been eliminated, the microbeads are evacuated and the stiffening element 10 is released from its mandrel 4b.

[0050] Alternatively, the mandrel is a metal core molded from a material soluble in a solvent over a temperature range, a water-soluble material in the example. After draping, the assembly is immersed in water at room temperature, which dissolves the material and removes the stiffening elements from the core.

[0051] The stiffening elements thus assembled are intended to constitute force passage panels in two perpendicular directions for given architectures in which these panels are integrated. The stiffeners extend along side members or beams depending on the architecture.

[0052] For example, for the aircraft door 5 as illustrated in front view and in perspective according to figures 5a and 5b, the stiffening elements 10 of stiffening structure of the type referenced 2 (cf. [Fig.2]), are assembled in a panel 50 in the form of spars 11 extending in the fuselage direction D.

[0053] The door 5 being equipped with a central support 51 comprising a control screen and other equipment (window, handles, controls, etc.), the side members 11a, 11b and 11c, 11d are previously cut to free up a space 5E ([Fig.5b]) which follows the contour of the support 51. Other panel assemblies are possible, for example two panels joined along the median MM'. By having an autoclave of suitable dimensions, a single panel provides greater overall rigidity than an assembly of several panels.

[0054] The landing gear hatch 6, seen in perspective according to [Fig. 6], takes up the structure 1 of [Fig. 1] according to a globally rectangular panel configuration in accordance with the configuration of the hatch architecture. Indeed, the constraints being of limited intensity, the structure 1, integrating a lower density of stiffening elements 10, can be advantageously used for this type of architecture.

[0055] Furthermore, a wing trailing edge 7 as shown in [Fig.7] by a perspective view, uses an assembly of stiffening elements 12 of prismatic geometry with adapted dimensions. Indeed, such a trailing edge architecture 7 here uses a stiffening structure panel according to the invention of type 1 (cf. [Fig.1]) or 2 (cf. [Fig.2]), with stiffening elements 12 according to the invention which regularly decrease in dimensions as one moves towards the end 7E of the trailing edge 7. In addition, the triangular section of these stiffeners 12 is not inscribed in an isosceles triangle but in a triangle adapted to the reduction in distance between the walls 7a, 7b of the trailing edge 7: the section decreasing linearly as a function of the span, the triangular sections are deduced by homothety.

[0056] The invention is not limited to the examples described and shown. Thus, the mandrels may be made of a sacrificial material (wax or equivalent) that is easily destructible in order to facilitate the recovery of the stiffening elements. The core mandrels may alternatively be made of a layer of foam, in particular a polymer foam, which remains chemically bonded to the stiffening element once the core has been separated or removed in order to reinforce the stiffener.

[0057] Furthermore, the base fiber may be a glass fiber, linen fiber, aramid fiber, in particular Kevlar®, a fiber made of thermoplastic material, in particular polyethylene, or an equivalent material.

[0058] In addition, the tooling used is preferably thermo-deformable in order to facilitate the implementation of the manufacturing steps.

[0059] The invention also applies to wing boxes or any other type of beam stiffened in two directions.

Claims

Claims

1. Geodesic structure for transmitting forces in two perpendicular directions (Dl, D2) in a panel of a given configuration architecture (6, 7), in particular of an aircraft door, characterized in that this structure (1, 2) comprises stiffening elements (10, 11, 12) having a triangular section perpendicular to one of the directions (Dl), each stiffening element having two lateral faces (Fl, F2) articulated on an inner edge (A2) extending parallel to this direction (Dl), and a base face (Fb) opposite the inner edge (A2) which forms lateral edges (A1) with the lateral faces (Fl, F2), in that the base faces (Fb) of the stiffening elements (10, 11, 12) are assembled together by their lateral edges (A1) in at least one layer having a surface continuity so that the faces (Fl, F2, Fb) constitute, in section, a geodesic surface, and in that at least one skin (P1,P2) is secured to each set of assembled faces (Fb) and / or internal edges (A2) of stiffening elements forming a continuity of consolidation surface of the structure (1,2), the skin being an internal skin (PI) secured to the internal edges (A2) when it is unique.,

2. A geodesic structure according to claim 1, wherein the layer of stiffening elements (10, 11, 12) is compact by arranging a head-to-tail configuration of adjacent stiffening elements (10, 11, 12).

3. Geodesic structure according to any one of Claim 1 to Claim 2, in which a reinforcing head (30) is arranged at the edges (A1; A1, A2) belonging to two adjacent stiffening elements (10, 11, 12).

4. Geodesic structure according to any one of Claim 1 to Claim 3, in which the stiffening elements (10, 11, 12) are, for at least some of them, tubes empty of material.

5. Geodesic structure according to any one of Claim 1 to Claim 4, in which the stiffening elements (10, 11, 12) are adapted in length and in cutting to a given architectural configuration (6, 7, 8).

6. Geodesic structure according to any one of Claim 1 to Claim 5, in which the stiffening elements (10, 11, 12) and the skins (PI, P2) are made of a material chosen from a composite material, a metal alloy and a plastic material.

7. Method for producing a geodesic structure according to any one of Claim 1 to Claim 6, characterized in that it comprises the following steps: - rotating mandrels (4) of predetermined triangular sections; - draping each mandrel (4) by winding a fiber (3) pre-impregnated with thermosetting resin over a predetermined length of each mandrel (4); - assembling the stiffening elements (10, 11, 12) into a layer of geodesic structure (1, 2); - co-firing the assembly with the end skins (PI, P2) attached to the end surfaces of said assembly; - demolding the mandrels (4).

8. Method for producing a geodesic structure according to claim 7, in which the mandrels (4) are so-called key mandrels, each key mandrel (4a) having wall portions (41a, 42b, 42c, 43d) blocked by a central core (4N), the removal of the central core causing the separation of the wall portions (41a, 42b, 42c, 43d) and the unlocking of the mandrel (4a).

9. A method of producing a geodesic structure according to claim 7, wherein the mandrels (4b) are made of thermo-expandable silicone with a metal core, these mandrels (4b) contracting upon cooling to facilitate extraction of the mandrels (4b).

10. A method of producing a geodesic structure according to claim 7, wherein the mandrels (4c) are formed from a bag filled with microbeads under vacuum and arranged around a metal core, the elimination of the vacuum evacuating the microbeads and releasing the stiffening elements from their mandrels (4c).

11. Method for producing a geodesic structure according to claim 7, in which the mandrels being constituted by a metal core molded in a material soluble in a solvent in a given temperature range, after draping the assembly is immersed in this solvent at a temperature of said range, causing the dissolution of the material and the extraction of the stiffening elements (10, 11, 12) from the core.

12. A method of producing a geodesic structure according to claim 7, wherein the mandrels are formed from a layer of foam arranged around a metal core, the foam layer remaining chemically bonded to the stiffening element (10, 11, 12).

13. A method of producing a geodesic structure according to claim 7, wherein the mandrels are made of sacrificial material.

14. A method of producing a geodesic structure according to claim 7 when this claim is attached to any one of claim 3 to claim 6, in which the reinforcing heads (30) are inserted at the edges (A1; A1, A2) of two adjacent stiffening elements when assembling the stiffening elements into a geodesic structure layer (1, 2).

15. Method for producing a geodesic structure according to any one of Claim 7 to Claim 14, in which the method uses thermo-deformable tooling.

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