Method of fabricating an aircraft structural component

Abstract

A method of fabricating a component, e.g. a stringer, comprises a precursor 300 comprising a web 302 and a flange 308, the web has a length, a height which is shorter than the length, a thickness whic

Description

24 02 25 TECHNICAL FIELD

[0001] The present invention relates to methods of fabricating structural components for aircraft. BACKGROUND

[0002] In an aircraft, a curved shape is desirable for aerodynamic and weight saving purposes in some parts such as the wings or the fuselage. These parts normally comprise a skin at the surface. The skin is curved to accommodate or provide the required shape. The curved skin requires structural supports to be attached to it in order to have the structural rigidity suitable to withstand the severe demands of the operating conditions of the aircraft, such as the pressure in the cabin or the lift forces on the wing. The structural supports are thus subject to strict quality requirements.

[0003] It is desirable to provide aircraft parts with tight curvature. Doing so will reduce the design constrains and permit, for example, aircraft design closely inspired by natural shapes. Such aircraft parts in turn require structural supports with the desired curvature while complying with the property criteria for application in aircraft. SUMMARY

[0004] A first aspect of the present invention provides a method of fabricating an aircraft structural component for an aircraft, the method comprising: providing a component precursor comprising a web and a flange that extends from the web, wherein the web has a length, a height which is shorter than the length, a thickness which is shorter than the height, a first end proximal to the flange and a second end distal from the flange, applying tension to the component precursor in a direction parallel to the length of the web and with a magnitude that varies along the height of the web so as to increase the length of 24 02 25 the web to different extents between the first end of the web and the second end of the web, sectioning the component precursor, and releasing the tension to allow shaping of the component precursor into a form required for the aircraft structural component.

[0005] Increasing the length of the web by different extents between the first end of the web and the second end of the web elastically deforms the web to different extents between the first end of the web and the second end of the web. The sectioning the web and releasing the tension would then permit the web to spring back and reverse the elastic deformation. As parts of the web have been deformed to different extents and following the sectioning have different relaxed lengths, the springing back will result in the web having a shape that differs from a shape it had before the tension was applied. The shaping may then be performed in such a way that the resultant aircraft structural component need not undergo a (significant) compressive force in the length direction and thus is free from wrinkles. In contrast, had the shaping been performed by other methods without implementation of the present invention, then wrinkles could be formed in portions of the web that have to be compressed in the length direction to obtain the desired shape of the aircraft structural component.

[0006] The tension may be applied to the component precursor by pulling on the web in a direction exactly parallel to the length of the web or by pulling on the web in a direction that has a component that is parallel to the length of the web. Either way, the web experiences tension in the length direction, and thus becomes elongated over at least part of its height.

[0007] Optionally, the component precursor comprises a composite material.

[0008] Composite materials are used in aircraft due to their advantageous mechanical properties and low density. Low density permits a reduction in weight of the aircraft, for fuel efficiency purposes or to enable the aircraft to carry a greater load for a given engine rating, while the mechanical properties make composite materials suitable for the strict requirements for use in aircraft.

[0009] Optionally, the composite material is a fibre composite material. 24 02 25

[0010] Fibre composites comprise fibres as reinforcement of a light material, normally a resin or another polymer. This allows the advantageous properties of the fibres, such as their tensile strength, and of the polymer, such as its low density, to be harnessed.

[0011] Optionally, the fibre composite material comprises pre-impregnated fibres, curable into a rigid fibre composite material.

[0012] Pre-impregnated fibres may be cured into a rigid composite material, they are shapeable in their uncured form, and curable in a relatively simple process following or during shaping.

[0013] Optionally, the component precursor comprises dry fibres that are permeable by a matrix material that is curable to form a rigid fibre composite material with the fibres.

[0014] Dry fibre processes add additional complexity in term of additional steps to obtain a rigid composite material, but the composite material made this way is generally lighter and stronger compared to other methods of fabrication of the fibre composite material.

[0015] Optionally, the magnitude that varies along the height of the web varies from a maximum at one of the first and second ends of the web to a minimum at the other of the first and second ends of the web. The minimum may be zero or non-zero.

[0016] As a result, there is an increase in the length of the web which varies along the height of the web in such a way that there is a minimum (such as zero) increase in length and a smallest magnitude of elastic strain at one of the first and second ends of the web and a maximum increase in length and thus also a maximum elastic strain at the other of the first and second ends of the web. This difference in strain results in a corresponding variation in the springing back upon the release of the tension. 24 02 25

[0017] Optionally, the magnitude that varies along the height of the web varies linearly from the maximum to the minimum.

[0018] A linear variation from the maximum to the minimum is a simple way to introduce the variation along the height of the web, and can result in an aircraft structural component that has linear ends at the longitudinal ends of the web.

[0019] Optionally, the maximum is at the first end of the web and the minimum is at the second end of the web.

[0020] Accordingly, during subsequent shaping of the component precursor into the form required for the aircraft structural component, the web can take a curved form with the second end of the web having a greater radius than the first end of the web, which can be particularly beneficial in some circumstances such as those where the flange is to be attached to a convex surface in subsequent use of the aircraft structural component.

[0021] Optionally, the applying tension to the component precursor comprises applying tension to the flange.

[0022] This permits subsequent shaping of the flange with the web in a controlled manner that can avoid the formation of wrinkles in the flange.

[0023] Optionally, the applying tension to the component precursor comprises applying tension to the flange with a magnitude that is equal to the tension applied to the first end of the web.

[0024] This effectively means that the flange is treated in the same way as the first end of the web, from which it extends, which reduces any shear forces formed between the flange and the first end of the web during performance of the method.

[0025] Optionally, the method comprises the shaping the component precursor into the form required for the aircraft structural component after the releasing the tension. 24 02 25

[0026] Optionally, the shaping comprises curving the component precursor into a curved shape, wherein the curved shape has a first radius of curvature at the first end of the web and a second radius of curvature at the second end of the web, wherein the second radius of curvature is greater than the first radius of curvature.

[0027] Upon the releasing the tension, and because the web is sectioned and there is a difference in elastic strain between the first end of the web and the second end of the web, the component precursor will curve towards the end with the maximum elastic strain as the resulting relaxed length will be shorter than that at the other end of the web (where the releasing the tension results in a longer relaxed length). To produce an aircraft structural component wherein the curvature is towards the flange, the maximum strain needs to be produced at the first end of the web at which the flange extends from the web. This produces the curved shape which has a first radius of curvature at the first end of the web and a second radius of curvature at the second end of the web, wherein the second radius of curvature is greater than the first radius of curvature.

[0028] Optionally, the method comprises curing the component precursor during or after the shaping.

[0029] Curing the component precursor allows the component shape to be set to produce a rigid component.

[0030] Optionally, the flange extends from the web substantially perpendicularly.

[0031] A perpendicular orientation of the flange with respect to the web tends to provide an aircraft structural component that is more rigid in two dimensions. It can also facilitate use of the aircraft structural component in a typical aircraft architecture where holes for the passage of such components, such as in a rib in a wing, are formed with an expectation that the flange will be perpendicular to the web. Such holes are also more easily formed than holes to accept components in which the flange is at a non-perpendicular angle to the web.

[0032] Optionally, the sectioning the web causes the releasing the tension. 24 02 25

[0033] Releasing the tension by sectioning away any portions of the web to which the means of applying the tension is attached to the web is a simple way to release the tension. The sectioned portion of the web is then free to be shaped without needing to wait for means of applying the tension to be released. The releasing the tension by the sectioning also permits the sectioning and releasing to be performed in a single process, which is time saving and more precise than two separate operations which require precision timing.

[0034] Optionally, the applying tension continues during the sectioning the web.

[0035] Continuing the applying tension during the sectioning the web prevents premature release of the tension.

[0036] Optionally, the sectioning the web comprises sectioning the web perpendicular to the length of the web.

[0037] This is a simple geometrical arrangement for the sectioning, which results in a difference in length of the first end of the web and the second end of the web, following the sectioning and the releasing the tension, that is proportional only to the magnitude of tension that is applied at the first end of the web and the second end of the web, respectively.

[0038] Optionally, the sectioning the web comprises performing two cuts through the web at respective locations that are spaced apart in the length direction of the web.

[0039] This permits the shape of the cut web to be more predictable, since it is dictated by the applied tension and the positioning of the cuts, and also permits the original longitudinal ends of the web not to need to be particularly neat.

[0040] Optionally, the two cuts are parallel to each other. 24 02 25

[0041] Two parallel cuts provide a geometrically and mechanically simple process, so that the final length of the first and second ends of the web may be predicted and controlled.

[0042] Optionally, the aircraft structural component is a stringer.

[0043] A skin section of a wing or a fuselage may be attached to a stringer for structural support. Stringers are conventionally curved through large radii where wrinkles in the material of the web of the stringer are avoided due to the inherent small degree of “play” in the material, allowing the material to stretch or resize sufficiently to avoid wrinkling. If stringers with tighter curvature are desired, embodiments of the present invention enable them to be provided to a satisfactory standard.

[0044] A second aspect of the present invention provides an aircraft structural component fabrication apparatus as defined in claim 21.

[0045] Optionally, the tensioner comprises a plurality of clamps removably attachable to portions of a longitudinal end of the web at respective points along the height of the web, wherein the clamps are operable independently of each other to tension the web with the magnitude that varies along the height of the web so as to increase the length of the web to different extents between the first end of the web and the second end of the web.

[0046] Such a set of clamps provides a relatively simple and versatile means to apply tension with a magnitude that varies along the height of the web.

[0047] Optionally, a second such plurality of such clamps is provided and is similarly removably attachable to portions of the other longitudinal end of the web at respective points along the height of the web.

[0048] Providing such a second plurality of clamps allows the tension applied to respective points along the height of the web to be accurately and independently applied and controlled. 24 02 25

[0049] Optionally, each of the clamps has a resilient contact surface and the clamps are configured to grip the web via the respective resilient contact surfaces.

[0050] This helps to protect the web from damage by the clamps.

[0051] Also disclosed herein is an aircraft structural assembly, comprising an aircraft structural component fabricated by the method of the first aspect of the present invention, and an aircraft skin section affixed to the flange of the aircraft structural component.

[0052] The affixing may be done by mechanical fasteners, such as bolts or rivets, or using a welding process (if the flange is of a weldable material) or an adhesive process or any other suitable method. The assembly may, for example, be a wing and / or a fuselage of the aircraft. Wings and fuselages are examples of aircraft structural assemblies with curvature where structural support is required. The assembly provides particular advantages, at least in enabling desired curvature to be achieved.

[0053] Also disclosed herein is an aircraft comprising the aircraft structural assembly.

[0054] An aircraft with curved portions unachievable by known methods may be fabricated through implementation of the present invention, as the shape of the assembly, for example the wing or the fuselage, is not so limited by the curvature restrictions previously imposed. This may result in more optimal tailoring of the shape of the assembly for aerodynamic or lift properties, structural resilience, weight savings or other benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0056] Figure 1 shows a schematic view of an aircraft.

[0057] Figure 2 shows an aircraft structural assembly. 24 02 25

[0058] Figures 3A to 3C show, respectively, a side view of an aircraft structural component precursor according to an embodiment of the present invention, a cross-sectional view and a top-down view of the same.

[0059] Figures 3D and 3E show an alternative example embodiment of an aircraft structural component precursor according to an embodiment of the present invention in a cross-sectional view and a top-down view, respectively.

[0060] Figures 3F and 3G show an alternative arrangement of the aircraft structural component precursor of Figures 3D and 3E, in a cross-sectional view and a top-down view, respectively.

[0061] Figures 4A to 4E illustrate the performance of a method according to an embodiment of the present invention.

[0062] Figure 5 shows a schematic depiction of an apparatus according to an embodiment of the present invention, to perform the method of Figures 4A to 4E. DETAILED DESCRIPTION

[0063] It is known in comparative examples for aircraft structural components, such as stringers, to suffer from unacceptable levels of wrinkling, when fabricated with smallradii curvatures. As previously outlined, small-radius curvature in structural components may be desirable to support the skin in small-radius curved aircraft parts, which seek to improve the overall performance of the aircraft. In comparative manufacturing techniques for a curved stringer comprising a flange and a web extending from the flange, for example, wrinkling can occur in the web or flange during bending of the stringer following laying up in a flat rectangular state. This wrinkling is due to a difference in 24 02 25 length of the two respective ends of the web (one of which is proximal the flange and the other of which is distal from the flange) after bending, which results in an excess of material of the bent component at the inner, or smaller, radius end of the web. In the example embodiment described below, were the present invention not implemented, the excess would be formed at an end of the web proximal to the flange and the flange would also wrinkle. Wrinkling would be detrimental to the performance of the structural component, and so avoiding or minimizing such wrinkles has until now been a limitation on the achievable curvature in aircraft components. A wrinkle or wrinkling can be considered a deviation, over a short distance of, for example, less than 10cm, or less than 5 cm, or even less than 10 mm in the direction of the length of the component, of the web or the flange from its remaining general profile or path. Some such wrinkles or wrinkling may comprise folds in the web or the flange.

[0064] Embodiments of the present invention seek to overcome the issue of wrinkling in such components by preventing or limiting the generation of excess material when forming a bent component. This is done by modifying the length of a component precursor by a varying magnitude between what will become an inner and an outer radius of curvature in the finished product. Such wrinkle mitigation reduces the likelihood of harmful points of stress concentrations, which could otherwise present an unacceptable level of failure risk in the finished product.

[0065] Figure 1 shows an example of an aircraft 1. The aircraft has multiple structural assemblies that may benefit from design unrestricted by curvature limits. Such assemblies are included in the fuselage 11, the wings 12, and the tail 13.

[0066] A cross section through one of the wings 12 of the aircraft 1 is shown in Figure 2. The wing 12 is an aircraft structural assembly and comprises a skin 21 defining its outer surface, spars 23 (shown in dashed lines in Figure 2) and ribs 24 (only one of which is visible in Figure 2) supporting the skin 21, and a series of stringers 22. The rib 24 has openings 241 to accommodate the stringers 22 passing through the opening. The stringers 22 are attached to the inner surface 211 of skin section 21 at their respective flanges 221. This is achievable by mechanical joining, for example by mechanical fasteners such as 24 02 25 rivets or bolts (not shown). Alternatively, other joining methods such as adhesives or welding can be envisaged. A stringer is an example of an aircraft structural component which can be made by the method disclosed therein.

[0067] A component precursor to an aircraft structural component according to an embodiment of the present invention is shown in Figure 3 A. In this example embodiment, the component precursor 300 is a stringer precursor. In other words, it is a forerunner to a stringer, or the part from which the stringer will derive. The component precursor 300 is made of a carbon fibre composite material, in this example embodiment. Other alternative materials include, for example, other fire composite materials such as glass fibre, or other composite or non-composite materials such as metallic materials. Composite materials are employed for aircraft components as they are lightweight and have favourable mechanical properties such as strength and stiffness.

[0068] The component precursor comprises a web 302, which has a length L and a height H. The height H is perpendicular to the length L. A flange 308 extends perpendicularly from a first end 304 of the web 302, and the web 302 also has a second end 306 distal from the flange 308. The component precursor 300 is depicted in a cross-sectional view in Figure 3B, wherein it is shown that the web 302 also has a thickness T. The thickness T is perpendicular to the length L and the height H. A further view of the component

Claims

30 05 251. A method of fabricating an aircraft structural component for an aircraft, the method comprising:providing a component precursor comprising a web and a flange that extends from the web, wherein the web has a length, a height which is shorter than the length, a thickness which is shorter than the height, a first end proximal to the flange and a second end distal from the flange,applying tension to the component precursor in a direction parallel to the length of the web and with a magnitude that varies along the height of the web so as to increase the length of the web to different extents between the first end of the web and the second end of the web,sectioning the component precursor, andreleasing the tension to allow shaping of the component precursor into a form required for the aircraft structural component.

2. The method according to claim 1, wherein the component precursor comprises a composite material.

3. The method according to claim 2, wherein the composite material is a fibre composite material.

4. The method according to claim 3, wherein the fibre composite material comprisespre-impregnated fibres, curable into a rigid fibre composite material.

5. The method according to claim 1, wherein the component precursor comprises dry fibres that are permeable by a matrix material that is curable to form a rigid fibre composite material with the fibres.

6. The method according to any preceding claim, wherein the magnitude that varies along the height of the web varies from a maximum at one of the first and second ends of the web to a minimum at the other of the first and second ends of the web.30 05 257. The method according to claim 6, wherein the magnitude that varies along the height of the web varies linearly from the maximum to the minimum.

8. The method according to claim 6 or claim 7, wherein the maximum is at the first end of the web and the minimum is at the second end of the web.

9. The method according to any preceding claim, wherein the applying tension to the component precursor comprises applying tension to the flange.

10. The method according to claim 8 and claim 9, wherein the applying tension to the component precursor comprises applying tension to the flange with a magnitude that is equal to the tension applied to the first end of the web.

11. The method according to any preceding claim, comprising the shaping the component precursor into the form required for the aircraft structural component after the releasing the tension.

12. The method according to claim 11, wherein the shaping comprises curving the component precursor into a curved shape, wherein the curved shape has a first radius of curvature at the first end of the web and a second radius of curvature at the second end of the web, wherein the second radius of curvature is greater than the first radius of curvature.

13. The method according to claim 11 or claim 12, comprising curing the component precursor during or after the shaping.

14. The method according to any preceding claim, wherein the flange extends from the web substantially perpendicularly.30 05 2515. The method according to any one of claims 1 to 14, wherein the sectioning the web causes the releasing the tension.

16. The method according to any one of claims 1 to 14, wherein the applying tension continues during the sectioning the web.

17. The method according to any preceding claim, wherein the sectioning the web comprises sectioning the web perpendicular to the length of the web.

18. The method according to any preceding claim, wherein the sectioning the web comprises performing two cuts through the web at respective locations that are spaced apart in the length direction of the web.

19. The method according to claim 18, wherein the two cuts are parallel to each other.

20. The method according to any preceding claim, wherein the aircraft structural component is a stringer.

21. An aircraft structural component fabrication apparatus comprising:a tensioner configured to apply tension to a component precursor, the component precursor comprising a web and a flange that extends from the web, wherein the web has a length, a height which is shorter than the length, a thickness which is shorter than the height, a first end proximal to the flange and a second end distal from the flange, wherein the tensioner is configured to apply the tension in a direction parallel to the length of the web and with a magnitude that varies along the height of the web so as to increase the length of the web to different extents between the first end of the web and the second end of the web,a cutter configured to section the web and release the tension, anda jig configured to support the component precursor and to guide a shaping of the component precursor into a form required for the aircraft structural component.

22. The aircraft structural component fabrication apparatus according to claim 21, wherein the tensioner comprises:a plurality of clamps removably attachable to portions of a longitudinal end of the web at respective points along the height of the web,wherein the clamps are operable independently of each other to tension the web with the magnitude that varies along the height of the web so as to increase the length of the web to different extents between the first end of the web and the second end of the web.

23. The aircraft structural component fabrication apparatus according to claim 22, wherein each of the clamps has a resilient contact surface and the clamps are configured to grip the web via the respective resilient contact surfaces.30 05 25

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