WO2019064032A1

WO2019064032A1 - Support member for a load-bearing structure

Info

Publication number: WO2019064032A1
Application number: PCT/GB2018/052806
Authority: WO
Inventors: Daniel SOLA
Original assignee: Archangel Lightworks Ltd
Priority date: 2017-09-29
Filing date: 2018-10-01
Publication date: 2019-04-04
Also published as: GB201715884D0

Abstract

A load-bearing structure, such as an aircraft wing, is proposed including a light redirection mechanism, such that light incident at any point of a light collection region of the structure, is redirected along a guide path, which may be along the surface (i.e. substantially parallel to the surface), to at least one photoelectric element. Thus, the light is collected throughout the collection region and concentrated into a photoelectric element which has a size which does not scale with the area of the collection area. The light redirection mechanism is part of a support member which defines the shape of at least a portion of the structure by the rigidity of the support member. The support member typically includes a load-bearing portion which maintains the profile of the structure, and the guide path is inward (i.e. towards the centre of the wing) relative to the surface of the structure defined by the support member.

Description

Support member for a load-bearing structure

Field of the invention

The present invention relates to a support member for use in a load-bearing structure such as an aircraft wing, and particularly one which is able to generate electricity. The support member may be a panel defining the exterior profile of the aircraft (i.e. the three-dimensional shape of the aircraft "skin"), such as a panel of the aircraft wing.

Background of the invention

It is known to mount solar panels on the wing of an aircraft. An example is the Airbus® Zephyr, a lightweight solar-powered unmanned aerial vehicle (UAV) in which solar cells spread across the wings of the aircraft generate electricity during the day to charge batteries and drive two propellers, and at night the energy stored in the batteries is sufficient to drive the propellers to maintain altitude. Such aircraft have completed flights of many months' duration. The feasibility of such aircraft is dominated by the weight per unit area of the wings.

Summary of the invention The present invention aims to provide new and useful load-bearing structures. The load- bearing structures are at least self-supporting, and more are additionally able to withstand an external load (that is, an externally applied load).

In particular, the present invention aims to provide new and useful aircraft structures, such as wings for aircraft, as well as new and useful methods of assembling aircraft. In general terms, the present invention proposes that light incident in any point of a light collection region of a support member of a load-bearing structure, is redirected along a guide path to at least one photoelectric element. The guide path is at least partly on the other side of a load-bearing portion of the support member from the light collection region. That is, light has been transmitted through the load-bearing portion of the support member to the guide path.

In one example, the load-bearing structure may be at least a part of an aircraft. The term aircraft is used here to refer to any manned or unmanned aircraft, such as a balloon (an unpowered aerostat), an airship (a floating aircraft with an engine) or an aircraft having wings.

In the case of an aircraft having wings, the load-bearing structure may be a wing. In this case, the guide path may be along the wing surface (i.e. substantially parallel to the wing surface), to the at least one photoelectric element. More generally, the support member may define any part of the external shape of the aircraft, i.e. the external shape of the part of the wing of the aircraft, or of part of the body (fuselage) or empennage of the aircraft.

Thus, in contrast to a conventional solar-powered aircraft in which all points on the solar cells are capable independently of generating electricity, the present invention makes possible a system in which light is collected throughout the light collection region and concentrated into a photoelectric element which has a size which does not scale with the area of the collection area. Thus, the weight of the photoelectric element does not increase in proportion to the area of the light collection region, which results in a weight saving. In the case of an aircraft wing, the light incident in the light collection region of the aircraft wing is redirected to the photoelectric element by a light redirection mechanism which is part of a support member which defines the shape of at least a portion of the wing by the rigidity of the support member. The support member typically includes one or more load-bearing portions which maintain the profile of the wing, and the guide path is inward (i.e. towards the centre of the wing) relative to at least one of the load-bearing portions, and inward relative to the surface of the wing defined by the support member. For example, the guide path may be along a waveguide located between two load-bearing portions of the wing (as measured in the direction normal to the wing surface).

The primary function of the support member is to define the shape of a portion of the wing surface, but its secondary function is to redirect the light into the light guide path along the wing surface. Since it has these two functions, its weight may be minimized compared to providing the light collecting function by many independent electricity-generating elements mounted on a conventional wing.

In a preferred form the light redirection mechanism is unitary with the rest of the support member (i.e. the two form a one-piece, integral unit). For example, the light redirection mechanism may be formed as a layer attached to (e.g. laminated to) layers constituting the load-bearing portions of the support member.

In one possibility, a single support member of this kind may be sufficient to define all the outer profile of the wing. Alternatively, the wing can be produced by mechanically connecting support member(s) of the type described above, and optionally other support member(s) each defining by their rigidity a portion of the shape of the wing, to form the overall profile of the wing. Note that although, as mentioned above, the support member of the invention maintains the shape of the wing due to its rigidity, it may not be solely responsible for the shape of the wing. For example, the support member may be attached to an internal support structure of the wing (e.g. composed of rib(s) and/or spar(s)) which assists the rigidity of the support member in maintaining its shape and/or fixes the shape of a portion of the support member such that the rigidity of the support member determines the shape of portions of the support member which are not in contact with, and are spaced from, the support structure, and thereby maintains the profile of the wing.

The support member is generally sheet-like (i.e. laminar but not necessarily flat), with a thickness at least 5 times less in a thickness direction than its extent in directions transverse to the thickness direction. It is optionally curved. Preferably this is in a single plane. That is, the outer profile of at least part of the support member may have an axis of longitudinal symmetry (e.g. transverse to the thickness direction). Viewed along the axis of longitudinal symmetry, that part of the support member may be arcuate. One specific expression of the invention is a support structure, such as a wing for an aircraft, comprising: one or more support members having respective surfaces, the surfaces collectively defining the profile of the support structure, the profile of the support structure being at least partly maintained by rigidity of respective load-bearing portions of the support members; and at least one photoelectric element which converts light incident on the photoelectric element into an electric signal; at least one of the support members comprising a light redirection mechanism which deflects light incident in a light collection region of the corresponding surface to a light guide path to the photoelectric element, at least part of the light guide path being further from the surface than the closest load-bearing portion of the support member.

The photoelectric element may be a portion of one of the support members or may be a separate element.

The support member may be suitable (e.g. shaped, sized and of suitable weight) to define a portion of the surface of an aircraft, such as a portion of the wing of an aircraft. In the latter case, the surface of the support member is a wing surface.

The invention may be expressed as a portion of a load-bearing structure such as an aircraft (e.g. a wing), as a structural element for use in forming a load-bearing structure, as a load- bearing structure (e.g. an aircraft) including the structural element, or as a method for constructing a load-bearing structure (e.g. an aircraft, or the wing of an aircraft) by connecting structural elements.

Although an aircraft is an important example of the load-bearing structure, the load-bearing structure may alternatively be a building (i.e. a ground structure intended to be fixed to a piece of ground and defining one or more rooms within it, such as for human habitation), or a part (e.g. a chassis) of another type of vehicle, e.g. specifically a ground vehicle such as an automobile or bus, or even the sail of a fixed-sail boat, or even a spacecraft. Preferably, the load-bearing structure defines at least one chamber. Generally, at least part of the light collection region, and at least part of the guide path, are on opposite respective sides of the portion of the support member which, in use in the load-bearing structure, experiences maximum load-bearing stress. Light passes through this portion of the support member from the light collection region to the guide path.

Brief description of the figures Embodiments of the invention will now be described for the sake of example only with reference to the following figures, in which:

Fig. 1 is composed of Fig. 1 (a) which is a perspective view of a wing of known construction, and Fig. 1 (b) which shows a support member of the wing;

Fig. 2 is composed of Fig. 2(a) which is a perspective view of a support member which is an embodiment of the invention, and Fig. 2(b) which shows a portion of the support member of Fig. 2(a) in cross-section;

Fig. 3 is composed of Fig. 3(a) which is a perspective view of a second support member which is an embodiment of the invention, and Fig. 3(b) which shows a portion of the support member of Fig. 3(a) in cross-section; Fig. 4 is a cross-sectional view of a portion of a wing which is a third embodiment of the invention;

Fig. 5 is composed of Fig. 5(a) which is a perspective view of a fourth support member which is an embodiment of the invention, and Fig. 5(b) which shows a portion of the support member of Fig. 5(a) in cross-section. Detailed description of the embodiments Referring firstly to Fig. 1 (a), a wing of known construction is illustrated. The wing comprises an elongate spar 1 having a length direction A. The spar 1 supports ribs 2, 3, 4. The spar 1 and ribs 2, 3, 4 form an internal support structure of the wing. A number of sheet-like support members 6 are attached to the ribs 2, 3, 4, collectively defining the "skin" 5 of the wing, i.e. the profile of the wing. That is the support member 6 is a panel of the wing. In variants, there may be any number of rib(s) 2, 3. 4.

One of the support members 6, from the upper surface of the wing, is shown in perspective view in Fig. 1 (b). The support member 6 is typically a sheet of uniform material, which in use is substantially longitudinally symmetric in the direction A and attached to at least one of the ribs 2, 3, 4, for example on a side of the wing which is uppermost when the aircraft is in flight. The support member 6 presents a wing surface 7 which forms a portion of the outer profile of the wing.

Viewed in the direction A, the support member is may appear, for example, as arcuate, e.g. a portion of an ellipse. The support member 6 may have enough rigidity to be fully self- supporting in this configuration. Alternatively it may be maintained in this configuration by the ribs 2, 3, 4 which define the shape of the portion of the support member 6 at the portions of the support member 6 which are in contact with the ribs 2, 3, 4. In either case, other portions of the support member 6 which are not in contact with the ribs 2, 3, 4 are rigid enough for the support member 6 to maintain its configuration, resisting aerodynamic load in flight. The aerodynamic load may be a force applied to a major surface of the support member 6 by air. This force is opposed by the ribs 2, 3, 4, and the interaction of these forces tends to deform the support member (e.g. flex it around the ribs 2, 3, 4).

Fig. 2 shows a support member 10 which is a first embodiment of the invention, illustrated in perspective view in Fig. 2(a), and a portion of the support member 10 is shown in cross- section in Fig. 2(b). These figures are schematic, and dimensions will typically be different from those shown.

The support member 10 has the overall shape of support member 6 of Fig. 1 (b), so it can be used to replace the support member 6 in the wing of Fig. 1 (a). Although the wing of an aircraft is illustrated in Fig. 1 (a), the support member 10 may be used to define the external profile of any other part of an aircraft, e.g. a portion of the fuselage. When the support member 10 is incorporated in the wing, it is supported by a support structure composed of ribs 2, 3, 4 and a strut 1 . Like the support member 6, the support member 10 is substantially longitudinally symmetric in the direction A. It is rigid enough to maintain its configuration, at least in portions of the support member 10 which do not contact the support structure 1 , 2, 3, 4, The support member 10 has a sheet-like (i.e. laminar but not necessarily flat) first load- bearing layer 12. This is at least partially transparent (that is, it may permit the passage through it of at least one frequency component of electromagnetic radiation, such as a component in the UV or visible spectrum with a minimal attenuation, e.g. under 1 %), and may be fully transparent (i.e. cause a minimal attenuation (e.g. under 1 %) for all frequencies in a range such as the visible range and/or the UV range). The outer surface of the first load- bearing layer 12 constitutes the wing surface 1 1 , which in use forms an portion of the upper surface of the wing. The wing surface 1 1 has the same overall shape as the wing surface 7 of the support member 6). Under the load-bearing layer 12 is a waveguide layer 13, comprising a transparent core layer 14 and on respective sides of it a cladding layer 15 and mirror layer 16. The waveguide layer 13 defines a light guide path extending in a curve to an elongate photoelectric element 19 which extends in the direction A.

The mirror layer 16 lies over a second load-bearing layer 17. In use, the first load-bearing layer 12 may be under tension, and the second load-bearing layer 17 may be under compression. Together the load-bearing layers 12, 17 maintain the profile of the panel 10. The waveguide layer 13 substantially does not have this function. That is, in use, the waveguide layer 13 is under less load-bearing stress than the load-bearing layers 12, 17. In other words, it may be protected from stress by the layers 12, 17. It is less rigid than the load-bearing layers 12, 17. Thus, when a force is applied to the panel 6 which tends to deform the panel, the load-bearing layer 12, 17 may provide substantially all the resistance to the deformation. The load-bearing layers 12, 17 thus resist deformation of the waveguide layer 13, and may substantially prevent such deformation for a specified range of loads, such as the range of loads typically encountered by the wing of the aircraft. The layers 12, 13, 15, 16 and 17 are mutually attached together, to form an integral (one piece) unit.

The first load-bearing layer 12 may be the portion of support member 10 which is under maximum load-bearing stress. The layers 12, 13, 14, 15, 16 and 17 may each have substantially constant thickness in different portions of the support member 10. In use, light 18 is incident on the wing surface 1 1 . The light (or at least one frequency component of it) is transmitted through the load-bearing layer 12 and the cladding layer 15 to the transparent core layer 14, where it propagates along the waveguide layer 13 to the photoelectric element 19 in a guidance direction B transverse to the direction A. The light is maintained within the transparent core layer 14 by reflections form the layers 15, 16. The guidance direction B is parallel to the closest parts of the two surfaces of the core layer 14, and has no component in the direction A. Thus, as the core layer 14 is curved, the guidance direction B is slightly different at respective portions of the waveguide layer 13 which are at different respective distances from the photoelectric element 19. However, at all portions of waveguide layer 13 at different distances from the photoelectric element 19, the guidance direction B is orthogonal to a normal direction to the closest part of the outer (upper) surface 1 1 of the first load-bearing layer 12. Substantially all the outer surface of the load-bearing layer 12 constitutes a light collection region.

Ideally, the mirror layer 16 is fully reflective. The cladding layer 15 is not fully reflective (since it has to allow light to pass through it from the first load-bearing layer 12), and so it allows some of the light to escape from the waveguide layer 13, but still a large fraction of the light incident on the wing surface 1 1 reaches the photoelectric element 19. The mirror layer 16 and cladding layer 15 may optionally be formed from different respective materials, and/or different respective thicknesses of the same material. They may be even be identical, in which case the mirror layer 16 also allows some light to pass through it, at least for light which is incident on the mirror layer 16 from a certain range of directions.

The photoelectric element 19 is elongate in the direction A, and extends along substantially the whole length of the edge of the support member 10 transverse to the wing surface 1 1 . Thus, its mass is proportional to the length of the edge, rather than to the area of the wing surface 1 1 . This means that it can be substantially lighter than a photoelectric element extending over the whole of the wing surface 1 1 .

The photoelectric element 19 may be adapted to produce an electric signal which can be used to power the aircraft, or to charge a battery.

Alternatively or additionally, if the light which is collected by the support member 10 is modulated to carry data, the electric signal generated by the photoelectric element 19 may be used to extract the data. For this purpose the aircraft may have a data extraction circuit which receives the electric signal generated by the photoelectric element 19, and extracts the data from it. The light may originally have been produced by a light source, such as a laser, on the ground (in which case the support member 10 may be on the lower surface of the wing) or a satellite orbiting the Earth. In this case the photoelectric element 19 may not be configured to generate useful electrical power, but instead may be configured to consume power provided by a battery. For example, it may be a light-sensitive diode or transistor. Although only a single photoelectric element 19 is shown in Fig. 2 at an edge of the support member 10, in fact plural photoelectric elements may be provided at two or more edges of the support member 10, e.g. at all four edges of the support member 10.

Although the photoelectric element 19 is shown as a portion of the support element 10, it may alternatively be provided as a separate element. Alternatively, two support elements may be provided in which one is the support element 10 and the second support element is identical to it, except that it omits the photoelectric element 19. The second support element may be arranged such that its waveguide is arranged to transmit light into the photoelectric element 19 of the support member 10. Optionally, the core layer 14 may have non-uniform refractive index, such as with maximal refractive index at a central portion (e.g. equidistant from the load-bearing layers 12, 17). This tends to concentrate light in the central portion of the core layer 14, and minimise the number of collisions between the light and the layers 15, 16. This improves the proportion of the light which reaches the photoelectric element 19. There are many other patterns of distribution of the levels of refractive index which give advantageous light transmission properties. For example, the core layer 14 may have a graduated refractive index.

Specifically, the core layer 14 may include at least three respective portions having different respective refractive indices and arranged in an order of increasing refractive index along a specific direction, which may be transverse to the guidance direction. The specific direction may be a thickness direction of the core layer.

Optionally, the load-bearing layer 17 may also be (at least partially) transparent, and the mirror layer 16 may transmit a portion of light incident on it. In this case, light which is incident on the surface of the load-bearing layer 17 which is lowermost in Fig. 2(b) can also enter the waveguide layer 13. This may be valuable, for example, in a case in which the support member 10 provides the whole thickness of the wing, so that the outer surface of the load-bearing layer 17 provides an outer surface of the wing. This outer surface may be directed downwardly. Thus, substantially all of the major surfaces of the support element 10 constitute a light collection region. In some situations, e.g. when the aircraft is flying over cloud, the amplitude of light which is incident on the aircraft from below is approximately as great as the amplitude of light which is received from above (at least for certain

wavelengths). In this case, light incident on the panel 10 from both its outer profile faces can enter the waveguidle layer 13 and be propagated towards the photoelectric element 19.

Turning to Fig. 3, a support member 20 which is a second embodiment of the invention is illustrated in perspective view in Fig. 3(a), and a portion of the support member 20 is shown in cross-section in Fig. 3(b). These figures are schematic, and dimensions will typically be different from those shown.

Like the support member 10, the support member 20 has the overall shape of support member 6 of Fig. 1 (b), so it can be used to replace the support member 6 in the wing of Fig. 1 (a). When the support member 20 is incorporated in the wing, it is supported by a support structure composed of ribs 2, 3, 4 and a strut 1 . Like the support member 6, the support member 20 is substantially longitudinally symmetric in the direction A. It is rigid enough to maintain its configuration, at least in portions of the support member 20 which do not contact the support structure 1 2, 3, 4, The support member 20 has a sheet-like first load-bearing transparent (or at least partially transparent) layer 21 , which is covered by a plurality of elongate refractive elements 22, which are viewed in Fig. 3(b) looking along their length direction A. The refractive elements 22 present respective convex, contoured outer refractive surfaces. The outer surfaces of the elongate elements 22, together with any portions of the first load-bearing layer 21 which are not covered by the refractive elements 22, constitute the wing surface 29, which in use forms a portion of the upper surface of the wing. The wing surface 29 has the same overall shape as the first load-bearing layer 21 (and of the wing surface 7 of the support member 6), though its detailed surface structure is modified by the refractive elements 22. The refractive elements 22 provide a surface roughness which may actually be beneficial from the point of view of increasing turbulence in air flowing over the upper surface of the wing, and thereby increasing lift.

Under the load-bearing layer 21 is a waveguide layer 23, comprising a transparent core layer 24 and on its respective surfaces a cladding layer 25 and mirror layer 26. The waveguide layer 23 defines a light guide path extending in a curve to an elongate photoelectric element 30 which extends in the direction A. At each point at the centre of the waveguide layer 23, there is a guidance direction B which is generally towards the photoelectric element 30, and which is parallel to the closest portion of the interface between the core layer 24 and the cladding layer 25, and which has no component in the direction A. The guidance direction B, at any given point in the waveguide layer 23, is parallel to the closest part of the inner (lower in Fig. 2(b)) surface of the load-bearing layer 21 . Thus, the guidance direction is parallel to the overall shape of the wing surface 29 at that distance from the photoelectric element 30. The photoelectric element 30 has the same construction as the corresponding photoelectric element 19 of the support member 10.

The mirror layer 26 lies over a second load-bearing layer 27. In use, the first load-bearing layer 21 may be under tension, and the second load-bearing layer 27 may be under compression. The first load-bearing layer 21 may be the portion of support element 20 which is under maximum load-bearing stress. In contrast to the support member 10 of Fig. 2, the mirror layer 26 of the support member 20 is not of constant thickness, but instead includes a number of reflective surfaces 261 which are inclined to the guidance surface B, so as to direct light which falls on them into a direction which is more closely parallel to the guidance direction B. The reflective surfaces 261 are spaced by step-surfaces substantially transverse to the guidance direction B at that point in the waveguide layer 23.

The refractive elements 22 form a light redirection mechanism which redirects a light ray 31 , impacting on each refractive element 22 towards a corresponding one of the reflective surfaces 261 , which in turn reflects it towards the guidance direction B. Each refractive element 22 has a contoured surface with contours which extend in (i.e. which are elongate in, such as having an axis of longitudinal symmetry in) the direction A, transverse to the guidance direction B.

Like the support element 6 of Fig. 1 (b), the support element 20 may be curved transverse to the direction A. However, the respective portions of the layers 21 , 25, 27 at the same distance from the photoelectric element 30 are substantially parallel to each other. In this case, the guidance direction B is slightly different at portions of the waveguide 25 which are at different respective distances from the photoelectric element 30. However, at all portions of waveguide layer 25 at different distances from the photoelectric element 30, the guidance direction B is orthogonal to a normal direction 28 to the closest part of the outer (upper) surface of the first load-bearing layer 21 .

As illustrated in Fig. 3(b), a light ray 31 which is incident at an angle to the normal direction 28 to closest part of the load-directing layer 21 , is redirected be at a higher angle β. In other words it is closer to a guidance direction B, which is orthogonal to the direction A and parallel to the centre of the waveguide layer 24. Following reflection from one of the reflective surfaces 261 , the light ray 31 propagates in a direction even closer to the guidance direction B. The light ray has a positive component in the guide direction B, and thus is successively reflected along the waveguide to the photoelectric element 30 after multiple reflections from the layers 25, 26. The refractive elements 22 have the effect of increasing the proportion of the light which impacts one of the reflective surfaces 261 , by directing light away from the step-like surfaces which separate neighbouring ones of the reflective surfaces 261 . Thus, the refractive elements 22 provide greater control over the direction of light within the waveguide layer 23. They furthermore increase the range of incident light angles which result in light reaching the photoelectric element 30. As for the core layer 14 of the support member 10, a central portion of the transparent core layer 24 may have a different refractive index than outer portions of the transparent core layer 24, to increase the proportion of light which is transmitted in directions close to the guidance direction B.

Turning to Fig. 4, a cross-sectional view of a portion of a wing which is a second

embodiment of the method is shown. Elements of Fig. 4 corresponding to elements of Fig. 3 are given the same reference numerals. The wing of Fig. 4 is identical to the wing of Fig. 3, except that the support element 20 of the wing of Fig. 4 is covered with three successive layers 281 , 282, 283. The three layers 281 , 282, 283 have successively increasing refractive index (with layer 283 having the highest and layer 281 the lowest). The refractive index of layer 283 is slightly below that of the refractive elements 22. The refractive layers 283 increase the power of the light redirection mechanism, and hence increase the range of angles which produce a suitable angle β (where these angles are defined as in Fig. 3(b)).

It was explained above that the support member (panel) 10 of Fig. 2 can be used to collect light which is incident on both of its major surfaces. The same is true of the support members (panels) 20 of Figs.3 and 4. In this case, the load-bearing layer 27 is made at least partially transparent, and the mirror layer 26 is arranged to transmit light into the waveguide layer 23. Optionally refractive elements with a construction similar to the refractive elements 22 (and optionally covered by one or more cover layers such as the layers 281 , 282, 283) may be provided on the outer surface of the load-bearing layer 27 (i.e. presenting respective convex refractive surfaces directed downwardly). The cladding layer 25 may be replaced with a stepped surface having reflective surfaces similar to the surface 261 for reflecting light incident on them from the waveguide layer 24 towards the guidance direction B. The refractive elements on the outer surface of the load-bearing layer 27 may be arranged to direct light towards corresponding ones of these reflective surfaces.

Many other variations of the light redirection mechanism are possible. Some such mechanisms are described for example in the reference US9274226. For example, the refractive elements 22 may not be elongate, but may appear as globules of reflective material on the surface of the first load-bearing layer 21 . That is, the refractive elements may not have a longitudinal symmetry in the direction A, but may instead be, for example, round as viewed from a direction transverse to the first load-bearing layer 21 . This permits redirection of the light along each of two non-parallel axes parallel to the surface of the load- bearing layer 21 . The refractive elements may be arranged in a two-dimensional array (such as a regular two-dimensional array having periodicity in the two axis directions) in a plane parallel to the surface of the load-bearing layer 21 . Fig. 5 illustrates yet a further possible support member 40 which is a fourth embodiment of the invention. Elements having the same meaning as in Fig. 3 are given reference numerals 20 higher. In this embodiment, the cladding layer 25 of the support member 20 is replaced with a mirror layer 45 which has a plurality of small openings ("gaps") 451. Refractive elements 42 (which may be elongate like the refractive elements 22 of the support member 20, or globules) have the effect of focusing light 51 incident on them from various directions onto respective ones of the gaps 461 . For example, the gaps 461 may be in a two- dimensional array in the plane of the cladding layer 25, and the refractive elements may be provided as a parallel two-dimensional array (globules of refractive material), comprising respective lenses in the proximity of each of the gaps 461 , e.g. with a centre of the lens spaced from the respective gap in a direction transverse to the surface of the load-bearing layer 21 . Provided the gaps 461 are small (i.e. have low total area compared to the area of the mirror layer 46), almost all the light which enters the transparent core layer 44 through the gaps 461 eventually reaches the photoelectric element 50, irrespective of the direction in which it passes into the transparent core layer 44 through one of the gaps 461 . Optionally, the refractive elements 42 may be provided with cover layers (not shown) resembling the layers 281 , 282, 283 of the support member 20 of Fig. 4.

As in the support members 10, 20 of Figs. 2-3, in the support member 40 the load-bearing layer 47 also may be (at least partially) transparent, so that light incident on the outer surface of the load-bearing layer 47 can also enter the waveguide layer 43. Thus, the support member 40 can collect light which is incident on either of its major surfaces. Optionally, the mirror layer 46 may also be formed with a plurality of gaps (not shown in Fig. 5), and refractive elements (not shown) may be provided on the outer surface of the load-bearing layer 47 so as to focus light onto corresponding ones of the gaps in the mirror layer 46. Although only a small number of embodiments of the invention have been described, many variations are possible within the scope of the claims. For example, in some wings according to the invention, the strut 1 and/or the ribs 2, 3, 4 may be omitted, if the structural members are rigid and strong enough not to require them.

In other variations, instead of the support members being arranged just to redirect light generally towards their edges transverse to their major surface (e.g. without modifying a component of the light propagation direction which is parallel to the major surface of the support member and orthogonal the guidance direction), they may be arranged to concentrate it at one more specific locations, which may be at one or more of the edges of the support member, or elsewhere. The photoelectric elements may be provided only at those location(s). In this case, the size (and weight) of the photoelectric layers may not vary linearly with the length dimensions of the support element transverse to the wing surface.

In further variations, the convex refractive surfaces of the refractive elements 22, 42 may be concave rather than convex, or even alternating convex and concave elements to allow redirection of light in a wide range of incident angles, albeit in some embodiments with reduced overall efficiency for certain specific incident angle(s).

Claims

1 . A load-bearing structure for bearing an external load, comprising: one or more support members having respective surfaces, the surfaces collectively defining the profile of the structure; and at least one photoelectric element which converts light incident on the photoelectric element into an electric signal; at least one of the support members comprising:

(i) one or more load-bearing portions, the profile of the structure being at least partly maintained by rigidity of the load-bearing portions of the one or more support members; and (ii) a light redirection mechanism which deflects light incident in a light collection region of the corresponding surface into a light guide path transverse to the surface of the support member to the photoelectric element, at least part of the light guide path being further from the surface than one of the load-bearing portions of the support member, whereby light incident in the light collection region is transmitted through at least one of the load-bearing portions, and transmitted along the light guide path to the photoelectric element; wherein in use, when the load-bearing structure subject to an external load, the load- bearing portions of each support member maintain the shape of the support member.

2. A structure according to claim 1 in which, when in use the load-bearing structure is under load-bearing stress, said at least one of the load-bearing portions through which light incident in the light collection region passes to the light redirection mechanism, is the part of the support member which experiences the greatest load-bearing stress.

3. A structure according to claim 1 or 2 in which at least part of the light guide path is defined by a waveguide for transmitting the light to the photoelectric element.

4. A structure according to claim 3 in which the waveguide is located between multiple load-bearing portions of the support member.

5. A structure according to claim 4 in which, when the support member is subject to an external load, the at least one of the load-bearing portions through which light incident in the light collection region passes to the waveguide is under tension.

6. A structure according to claim 5 in which the at least one of the load-bearing portions through which light incident in the light collection region passes to the waveguide, is on a first side of the waveguide, and when in use the load-bearing structure is under load-bearing stress another of the load bearing portions, on another other side of the light waveguide, is under compression.

7. A structure according to any preceding claim in which the waveguide comprises a core having a non-uniform refractive index.

8. A structure according to claim 8 in which the waveguide core has a graduated refractive index.

9. A structure according to any preceding claim in which the redirection mechanism comprises one or more refractive elements defining respective refractive surfaces.

10. A structure according to claim 9 in which the refractive surface includes contours extending transverse to a direction towards the photoelectric element

1 1 . A structure according to claim 9 or claim 10 in which the refractive surface of at least one of the refractive elements is a convex surface.

12. A structure according to any of claims 9 to 1 1 in which the refractive elements are arranged to direct light towards corresponding reflective surfaces which are inclined to a guidance direction towards the photoelectric element, and which are for reflecting light incident upon them to propagate in a direction closer to the guidance direction.

13. A structure according to any of claims 9 to 1 1 in which the refractive elements are arranged to direct light through respective gaps in a reflective layer which defines a side of the light guide path.

14. A structure according to any preceding claim comprising a cover portion extending over the surface of at least one of the support members, and having successively increasing refractive index in at least three locations spaced apart parallel to the normal direction of the surface of the support member.

15. A structure according to claim 14 in which the cover portion comprises at least three layers, the three locations being within the respective layers.

16. A structure according to any preceding claim which is a portion of an aircraft, said external force being aerodynamic load.

17. A structure according to claim 16 which is a wing of an aircraft.

18. A structure according to claim 17 which is a static wing.

19. A structure according to any of claims 1 to 15 which is a ground structure.

20. A structure according to any of claims 1 to 15 which is a portion of a ground-based vehicle.

21 . A structure according to any of claims 1 to 15 which is a portion of a spacecraft.

22. An aircraft having at least one portion which is a load-bearing structure according to any preceding claim.

23. A support member for mechanical coupling to other support members to form a load- bearing structure, the support member having a surface for forming a portion of the profile of the structure, whereby the surfaces of the support members collectively define the profile of the structure, the profile of the surface being maintained by the rigidity of one or more load- bearing portions of the support member, the support member comprising a light redirection mechanism which deflects light incident on the surface and transmitted through at least one of the load-bearing portions of the support member, into a light guide path at the other side of the surface, the light guide path extending transverse to the surface of the support member.

24. A support member according to claim 23 which is an aircraft surface panel.

25. A support member according to claim 23 or claim 24 further including a waveguide element for receiving light redirected by the light redirection mechanism.

26. A support member according to claim 25 in which the waveguide is located between two load-bearing portions of the support member.

27. A support member according claim 26 in which both load-bearing portions are at least partially transparent.

28. A support member according to any of claims 23 to 27 in which the redirection mechanism comprises one or more refractive elements defining a respective refractive surface.

29. A support member according to claim 27 in which the refractive elements include contours extending transverse to a guidance direction, and are arranged to direct light towards corresponding reflective surfaces which are inclined to a guidance direction, and which are for reflecting light incident upon them to propagate in a direction closer to the guidance direction.

30. A support member according to claim 27 in which the refractive elements are arranged to direct light through respective gaps in a reflective layer which defines a side of the light guide path.

31 . A support member according to any of claims 27 to 30 in which the refractive surface of at least one of the refractive elements is a convex surface.

32. A support member according to any of claims 23 to 31 comprising a cover portion extending over the surface and having successively increasing refractive index in at least three locations spaced apart parallel to the normal direction of the surface.

33. A support member according to claim 32 in which the cover portion comprises at least three layers, the three locations being within the respective layers.

34. A method for constructing a load-bearing structure, the method comprising connecting multiple support members, including at least one support member according to any of claims 22 to 33, together, respective surfaces of the support members collectively defining the profile of the structure.

35. A method according to claim 34 in which the structure is a portion of an aircraft.

36. A method according to claim 35 in which the structure is a wing of an aircraft.

37. A method according to claim 36 in which the structure is a static wing.

38. A method according to claim 34 in which the structure is a ground structure.

39. A method according to claim 34 in which is structure is a ground-based vehicle.

40. A method according to claim 34 in which the structure is a portion of a spacecraft.