Structure comprising a heater layer and an erosion shield layer

Abstract

A de-icing structure 300 comprising a heater layer 310, with a plurality of heater regions 202-214 and a heatable parting region 220 separating adjacent heater regions, and an erosion shield layer 305

Description

FIELD OF THE INVENTION The present invention relates to a structure, for example an aerofoil, aircraft wing, or turbine blade, comprising a heater layer and an erosion shield layer. BACKGROUND Ice protection systems protect against the build-up of ice on structures. One common application of ice protection systems is on aircraft. During flight, the surfaces of an aircraft can be exposed to water at low temperatures and, if no preventative action is taken, ice can quickly form on the wings, on control surfaces, and on other parts of the aircraft in such a way as to alter the aerodynamic performance of the aircraft (for example by altering the airflow around the aircraft and by adding additional weight to it) with potentially catastrophic consequences. Example ice protection systems are discussed in the following patents and applications in the name of Ultra Electronics Limited (RTM) (the contents of which are hereby incorporated in their entirety by reference): US7580777, WO2008 / 145985, US20090149997, US20090230239 and US2010 / 0243811. Electrothermal ice protection systems comprise a number of heater devices (such as heater mats), which can be used as anti-icing zones in which a sufficient temperature is maintained at the surface of the wing in order to prevent the formation of ice on and behind the protected zone. These heater devices can also be used as de-icing zones to shed ice that has been allowed to accrete on the protected region. The de-icing mats are cyclically energised in order to melt the interface between the wing and the accreted ice, causing the ice to be shed. Most ice protection systems additionally comprise a parting region, or parting strip, separating adjacent heater regions of the wing. The parting region is permanently heated to prevent the formation of ice between interconnecting heater regions, and / or to prevent ice that has become detached from one heated region from becoming reattached to an adjacent interconnecting region. Furthermore, aircraft wings are often covered by an erosion shield, which helps to protect the wing from damage caused by high-speed debris such as sand. The erosion shield may also act as an electrically conductive shield to protect the heaters or other electrical systems from lightning. However, the erosion shield is typically also thermally conductive, which can cause a significant amount of the heat supplied to the parting region to be lost to the surrounding regions of the erosion shield. SUMMARY OF THE INVENTION The present invention is defined by independent claim 1, appended hereto. Further advantageous embodiments are also defined by the dependent claims, also appended hereto. We describe a structure comprising: a heater layer, the heater layer comprising a plurality of heater regions and a heatable parting region separating adjacent heater regions; and an erosion shield layer disposed over the heater layer; wherein at least one region of the erosion shield layer disposed over at least one heater region is substantially thermally insulated from the parting region. The skilled reader will appreciate that “substantially thermally insulated” means a relatively low thermal conductivity between the parting region and the region of the erosion shield layer disposed over the heater region in comparison to a thermal conductivity between the parting region and a region of the erosion shield layer disposed over the parting region, and / or between the heater region and the region of the erosion shield layer disposed over the heater region. Advantageously, substantially thermally insulating the region of the erosion shield layer disposed over the heater region from the parting region reduces the amount of power required to maintain a parting region temperature sufficient to prevent the formation or attachment of ice in the region(s) interconnecting the heater regions. The region of the erosion shield layer disposed over the heater region may be separated from a region of the erosion shield layer disposed over the parting region by a channel in the erosion shield layer. Advantageously, a thickness of the erosion shield layer is reduced in the region of the channel, and the thermal conductivity of the erosion shield layer is therefore reduced in this region. The channel may be filled with a thermally insulating filler material. The region of the erosion shield layer disposed over the heater region may be substantially thermally insulated from the parting region by a gap in the erosion shield layer. In some examples, the gap is situated directly over the parting region. In some examples, the gap separates the region of the erosion shield layer disposed over the heater region from a region of the erosion shield layer disposed over the parting region. The gap may be an air gap. The gap may be filled with a thermally insulating filler material. The gap may be bridged by a thermally insulating bridging material The erosion shield layer may comprise a first piece disposed over the heater region, and a second piece disposed over the parting region. The first piece may overlap the second piece. Alternatively, the first piece may be shaped to receive the second piece. The first piece may be attached to the second piece by a thermally insulating adhesive. The parting region may be disposed on a thermally insulating strip. The thermally insulating strip may advantageously prevent the parting region from heating any region of the erosion shield other than the region of the erosion shield disposed over the parting region. The structure described herein may be an aerofoil. We also describe an aircraft comprising the structure described herein. We further describe a turbine (e.g. a wind turbine) comprising the structure described herein. LIST OF FIGURES The invention will now be described, by way of example only with reference to the accompanying figures, in which: Figure 1 is an illustration of a portion of an aircraft; Figure 2 is an illustration of a portion of the leading edge of an aircraft wing comprising a plurality of heated regions; Figure 3 is a cross-section of an aircraft wing section; Figure 4 is a cross-section of an example structure comprising a channel in the erosion shield layer; Figure 5 is a cross-section of an example structure comprising a gap in the erosion shield layer; Figure 6 is a cross-section of a further example structure comprising a gap in the erosion shield layer; Figures 7A and 7B illustrate cross-sections of example structures comprising an erosion shield layer comprising first and second pieces; Figure 8 is a cross-section of an example structure in which a parting region is disposed on a thermally insulating strip; Figure 9 is an illustration of an aerofoil comprising a structure according to the present disclosure; and Figure 10 is an illustration of a wind turbine comprising a structure according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In brief, we describe a structure comprising a heater layer comprising a plurality of heater regions separated by a heatable parting region (also referred to as a parting strip), and an erosion shield layer disposed over the heater layer, wherein a region of the erosion shield layer that is disposed over a heater region is substantially thermally insulated from the parting region. Advantageously, thermally insulating the regions of the erosion shield layer that are disposed over the heater regions (i.e. not over the parting region) from the parting region reduces the amount of heat that is lost from the parting region to the surrounding areas of the erosion shield layer, thereby reducing the amount of power that is required to maintain the parting region at a temperature suitable to prevent the formation of ice in the vicinity of the parting region. As mentioned above, electrothermal ice protection systems comprise a number of heater devices (such as heater mats), which can be used as anti-icing zones in which a sufficient temperature is maintained at the surface of the wing in order to prevent the formation of ice on and behind the protected zone. These heater devices can also be used as de-icing zones to shed ice that has been allowed to accrete on the protected region. The de-icing mats are cyclically energised in order to melt the interface between the wing and the accreted ice, causing the ice to be shed. In such an ice protection system it is important to avoid overheating of the heater devices (heating mats) in order to avoid a failure either of the devices or in the structure to which the devices are attached. Many modern aircraft (and other structures) use composite materials, which can suffer damage (delamination of the material, for example) at a relatively low temperature. Temperature ‘overshoot’ of the heater devices must therefore be controlled whilst maintaining rapid heating of the protected surface(s). Aircraft are normally subject to a range of different icing conditions during flight, such as different air temperatures, air velocities, relative humidity, and so on, which can depend for example on the location, altitude, orientation, air speed or pitch of the aircraft, the prevailing meteorological conditions, and so on. Different icing conditions can determine not only the temperatures and velocities (and so on) at which ice will form on different parts of the aircraft structure, but also the heat loss from the aircraft structure. It will be appreciated that the term “heater region” as used herein may refer to a region of a structure in which a heater is located. Figure 1 is an illustration of a portion of an aircraft, showing the placement of heater mats of an ice protection system of an aircraft. The aircraft 100 includes a fuselage portion 102 and a wing portion 104. On the leading edge 106 of the wing 104 are provided a plurality of heating mats. Each heater mat may be divided into a number of heater zones. The number and size of the heater zones are chosen to suit a particular safety model, for example such that up to two heater zones can fail without causing a hazardous or catastrophic failure of the aircraft. In one aircraft design, safety requirements require each heater mat to be divided into six separate heater zones. However, ice protection systems are known for being power-hungry systems, which can be a burden to the power generation and distribution systems in aircraft. Figure 2 is an example illustration of a portion of a leading edge of an aircraft wing 104 comprising a plurality of heaters forming heater regions 202, 204, 206, 208, 210, 212, 214 (shown separated by dashed lines in Figure 2). The heaters are located under the surface of the wing, but are in thermal contact with the surface of the wing. Seven heaters (not to scale) are shown in this figure, although it would be clear to the skilled reader that there could be more or fewer regions than indicated, and also located along other edges of the aircraft or across greater areas. The heater regions 202 to 214 are arranged such that a heater region 202 is located on the leading edge of the structure 104, with heater regions 204, 206, 208, 210 running aft of the heater region 202 on an upper surface of the structure 104 in a direction of the airflow that would impinge on the surface. Other heater regions 212 and 214 are arranged aft of and beneath the structure in a direction of the airflow that would impinge on the surface. As further illustrated in Figure 2, adjacent heater regions 202, 212 are separated by a parting strip or parting region 220 (shown as a solid line in Figure 2). In the example illustrated in Figure 2, the parting region 220 forms a continuous H-shaped region with strips running aft of the leading edge on the top and bottom of the structure 104, which strips bound the heater regions 202, 204, 206, 208, 210, 212, 214 perpendicular to the leading edge as well. The skilled reader will appreciate that, in some examples, each heater region may be surrounded by a parting strip or parting region 220, where the parting region 220 forms a continuous region that runs between multiple pairs of heater regions. In other examples, any number of pairs of adjacent heater regions may be separated by a parting region 220. In some examples, multiple separate parting regions 220 may be used. In operation, the parting region 220 is preferably continuously heated (or at least heated more often than the heater regions) to prevent the formation of ice between heater regions, and / or to prevent ice that has been melted, or detached, from one heater region from reforming, or becoming reattached, at the interconnecting region between heater regions. Figure 3 is a cross-section of an example structure 300. In the illustrated example, the structure 300 is that of an aircraft wing section. It will be appreciated that a similar arrangement may be provided on other exposed parts of the aircraft structure (such as on propeller leading edges or on engine inlets, for example). The figure shows the leading edge 302 of the wing in cross-section and an approximation of the airflow 304 over the wing whilst in flight. The structure 300 includes an erosion shield layer 306, typically a stiff, erosion-resistant aluminium shield (although this may be made of other materials including, but not limited to, Titanium or Stainless Steel), and a heater layer 310 comprising the heater regions 202 to 214 and the parting region 220. The wing may further include dielectric (electrical insulator) layers 308, 312, and a temperature sensor 314. Only one temperature sensor is shown for clarity, although each heater 202 to 214 may be in thermal contact with its own temperature sensor. The layers 306, 308, 310, 312 are much thinner than as shown, forming a thin sandwich at the edge of the wing section. While the parting region 220 in Figure 2 is located between the heater regions 202, 212 situated at the leading edge 302 of the structure, it will be appreciated that one or more parting regions 220 may be located between any pair of heater regions 202 to 214. The skilled reader will appreciate that, where the structure 300 is a section of a wing of an aircraft, the erosion shield layer 306 is preferably electrically grounded to an airframe of the aircraft. The main wing section 302 is formed from any appropriate material, such as composite materials that comprise a plurality of layers of stiff material bound together with glue. Composite materials have a good ratio of strength to weight, but are susceptible to failure by delamination (when the glue melts) at a relatively low temperature. Therefore, care needs to be taken to avoid ‘overshoot’ (overheating) of the heater. As further illustrated in Figure 3, a region 306A of the erosion shield layer 306 that is disposed over a heater region 208 can be defined. Similarly, a region 306B of the erosion shield layer 306 that is disposed over the parting region 220 can be defined. While for clarity only one of each region 306A, 306B is highlighted in Figure 3, the skilled reader will appreciate that any region of the erosion shield layer 306 that is disposed over a heater region 202, 204, 206, 208, 210, 212, 214, or over the parting region 220, can be defined as such. As described herein, substantially thermally insulating the region(s) of the erosion shield layer 306 disposed over the heater region(s) 202, 204, 206, 208, 210, 212, 214 from the parting region 220 leads to a reduction in power required to maintain the parting region 220 at a temperature suitable to prevent the formation of ice between the heater regions 202, 204, 206, 208, 210, 212, 214. As an example, in a simulated de-icing process for an aircraft having a wing structure comprising a heater layer and an aluminium erosion shield with no thermal isolation of the parting strip, an aircraft power consumption of 48 kW was determined. A similar structure with an insulated parting strip gave a power consumption of 32 kW for the same de-icing performance. Examples of structures that may provide the above-described substantial thermal insulation according to the present disclosure will now be described. Figure 4 illustrates an example structure 400 according to the present disclosure. The example structure 400 illustrated in Figure 4 is shown in cross-section and is substantially the same as the example structure 300 illustrated in Figure 3. In order to illustrate clearly the features of the example, Figure 4 shows a detail of a portion of the structure 400. A portion of a heater layer 310 is shown comprising two heater regions 202, 204 separated by a parting region 220, with an erosion shield layer 306 disposed thereon. The regions of the erosion shield layer 306 disposed over the heater regions 306A and over the parting region 306B are highlighted. Other (optional) layers of the structure 400 (such as dielectric layers) have been omitted for clarity. In the example structure 400, the region 306A of the erosion shield layer 306 disposed over the heater region 202, 204 is separated from the region 306B of the erosion shield layer 306 disposed over the parting region 220 by a channel 422 in the erosion shield layer 306. The local thickness of the erosion shield layer 306 is reduced in the region of the channel 422, thus reducing the thermal conductivity of the erosion shield layer 306 in this region, and thereby thermally insulating the region 306A of the erosion shield layer 306 disposed over the heater region 202, 204 thermally insulated from the parting region 220. The channel may be formed by etching, for example by chemical etching. In some examples, the channel 422 is filled with a thermally insulating filler material 424, such as epoxy, resin, or polyamide, to provide further thermal insulation. In the example illustrated in Figure 4, the channel 422 is formed on a surface 410 of the erosion shield layer 306 facing the heater layer 310 (i.e. an inner surface, or inside mould line). By forming the channel 422 on the inner surface, the shape of the structure 400 is unchanged for the purposes of aerodynamics. However, a channel 422 may alternatively or additionally be formed on an outer surface (or outside mould line) (i.e. opposite to the surface 410 illustrated in Figure 4). It will be appreciated that, where the channel 422 is formed on the outer surface, it may be preferable to fill the channel 422 with a thermally insulating filler material 424, and mould the thermally insulating filler material 424 to return the structure 400 to its original shape (i.e. without a channel) for the purposes of aerodynamics. Figure 5 illustrates a further example structure 500 according to the present disclosure. The example structure 500 illustrated in Figure 5 is shown in cross-section and is substantially the same as the example structure 300 illustrated in Figure 3. In order to illustrate clearly the features of the example, Figure 5 shows a detail of a portion of the structure 500. A portion of a heater layer 310 is shown comprising two heater regions 202, 204 separated by a parting region 220, with an erosion shield layer 306 disposed thereon. The regions 306A of the erosion shield layer 306 disposed over the heater regions 202, 204 are highlighted. Other (optional) layers of the structure 500 (such as dielectric layers) have been omitted for clarity. In the example structure 500, the region 306A of the erosion shield layer 306 disposed over the heater region 202, 204 is substantially thermally insulated from the parting region 220 by a gap 522 in the erosion shield layer 306. In other words, the regions 306A of the erosion shield layer 306 disposed over the heater regions 202, 204 constitute separate pieces of the erosion shield material. In the example structure 500, only two pieces of erosion shield material are shown, such that there is no region of the erosion shield layer 306 disposed over the parting region 220. In such an example, the gap 522 may be filled with a thermally insulating filler material 524 having erosion shielding properties (such as epoxy, Kapton (RTM), or polyamide). The thermally insulating filler material 524 may be shaped, moulded, or contoured according to the desired aerodynamic shape of the overall structure 500. Alternatively, or in addition, to filling the gap 522 with a thermally insulating filler material 524, the gap 522 may be bridged by a thermally insulating bridging material 526. The thermally insulating bridging material 526 is preferably electrically conductive to enable the erosion shield layer 306 to perform its function as a lightning shield. An example of a suitable thermally insulating bridging material 526 is Astrostrike (RTM). The erosion shield layer 306 may comprise a recess 527 to receive the thermally insulating bridging material 526, to preserve the aerodynamic shape of the structure 500. Figure 6 illustrates an alternative example cross-section of a structure 600 in which the region 306A of the erosion shield layer 306 disposed over the heater region 202, 204 is substantially thermally insulated from the parting region 220 by a gap 622. In the example of Figure 6, one or more gap(s) 622 separate(s) one or more region(s) 306A of the erosion shield layer 306 disposed over the heater region 202, 204 from a region 306B of the erosion shield layer 306 disposed over the parting region 220. A gap 622 may be 500 microns wide or less. The gap 622 may be an air gap. Even a very small air gap 622 (e.g. approximately 50 microns) can lead to a significant reduction in heat transfer between the parting region 220 and the region 306A of the erosion shield layer 306 disposed over the heater region 202, 204. The gap 622 may be filled with a thermally insulating filler material 624 such as epoxy, Kapton (RTM), or polyamide. The thermally insulating filler material 624 may have erosion shielding properties. The thermally insulating material 624 may be shaped, moulded, or contoured according to the desired aerodynamic shape of the overall structure 600. In some examples, a filled gap 622 may between approximately 100 microns and 500 microns wide. Alternatively, or in addition, to filling the gap 622 with a thermally insulating filler material 624, the gap 622 may be bridged by a thermally insulating bridging material 626. The thermally insulating bridging material 626 is preferably electrically conductive to enable the erosion shield layer 306 to perform its function as a lightning shield. An example of a suitable thermally insulating bridging material 626 is Astrostrike (RTM). The erosion shield layer 306 may comprise a recess 627 to receive the thermally insulating bridging material 626, to preserve the aerodynamic shape of the structure 600. Figures 7A and 7B illustrate further example structures 700A, 700B according to the present disclosure. The example structures 700A, 700B illustrated in Figures 7A and 7B are shown in cross-section and is substantially the same as the example structure 300 illustrated in Figure 3. In order to illustrate clearly the features of the example, Figures 7A and 7B show details of a portion of the structure 700A, 700B. A portion of a heater layer 310 is shown comprising two heater regions 202, 204 separated by a parting region 220, with an erosion shield layer 306 disposed thereon. The regions 306A of the erosion shield layer 306 disposed over the heater regions 202, 204 are highlighted. Other (optional) layers of the structure 700A, 700B (such as dielectric layers) have been omitted for clarity. In the example structure 700A illustrated in Figure 7A, the erosion shield layer 306 comprises one or more first pieces 306X disposed over the heater region(s) 202, 204, and one or more second pieces 306Y disposed over the parting region(s) 220. The region 306A disposed over the heater region 202, 204 is therefore a region of the first piece 306X, and the region 306B disposed over the parting region is a region of the second piece 306Y. The first piece 306X overlaps the second piece 306Y. While in Figure 7A the second piece 306Y is illustrated as positioned over the first pieces 306X, in other examples a first piece 306X may be positioned over a second piece 306Y. The separation of the erosion shield layer 306 into pieces leads to a reduction in heat transfer between the first piece 306X and the second piece 306Y, and thus substantially thermally insulates the region 306A of the erosion shield layer 306 disposed over the heater region 202, 204 from the parting region 220. The first piece 306X may be attached to the second piece 306Y by an adhesive 725, preferably a thermally insulating adhesive, for example, an epoxy based adhesive with a plastic carrier or scrim such as 3M AF3109, or Solvay FM300. The first 306X and / or second 306Y erosion shield pieces of the structure 700A illustrated in Figure 7A may be shaped, moulded, or contoured according to the desired aerodynamic shape of the overall structure 700A. The example structure 700B illustrated in Figure 7B is similar to the example structure 700A illustrated in Figure 7A, but instead of overlapping first 306X and second 306Y pieces of the erosion shield layer 306, the first piece 306X is chamfered or otherwise shaped to receive the second piece 306Y. Shaping the first piece 306X to receive the second piece 306Y means that the aerodynamic shape of the overall structure 700B can be preserved across the first 306X and second 306Y pieces. The first piece 306X may be attached to the second piece 306Y by an adhesive 725, preferably a thermally insulating adhesive for example, an epoxy based adhesive with a plastic carrier or scrim such as 3M AF3109, or Solvay FM300. Figure 8 illustrates another example structure 800 according to the present disclosure. The example structure 800 illustrated in Figure 8 is shown in cross-section and is substantially the same as the example structure 300 illustrated in Figure 3. In order to illustrate clearly the features of the example, Figure 8 shows a detail of a portion of the structure 800. A portion of a heater layer 310 is shown comprising two heater regions 202, 204 separated by a parting region 220, with an erosion shield layer 306 disposed thereon. The regions 306A of the erosion shield layer 306 disposed over the heater regions 202, 204 are highlighted. Other (optional) layers of the structure 800 (such as dielectric layers) have been omitted for clarity. In the example of Figure 8, the heater layer 310 further comprises a thermally insulating strip 802 on which the parting region 220 is disposed. The thermally insulating strip 802 may be, for example, a thermally insulating tape (e.g. Astrosrike (RTM)). Preferably, the erosion shield layer 306 comprises first 306X and second 306Y pieces, similarly to the examples illustrated in Figures 7A and 7B, and / or a gap between the region(s) 306A of the erosion shield layer 306 disposed over the heater region(s) 202, 204 and the region 306B of the erosion shield layer 306 disposed over the parting region 220, similarly to the example illustrated in Figure 6. The first 306X and second 306Y pieces may be attached by a thermally insulating adhesive (not shown). The parting region 220 is thus thermally insulated from the region(s) 306A (i.e. first piece(s) 306X) of the erosion shield layer 306 disposed over the heater region(s) 202, 204 by the thermally insulating strip 802. In other words, the parting region 220 effectively only supplies heat to the region 306B (i.e. second piece 306Y) of the erosion shield layer 306 disposed over the parting region 220. The skilled reader will appreciate that a thermally insulating filler or bridging material, thermally insulating adhesive, or thermally insulating strip, means a material, adhesive, or strip having a relatively low thermal conductivity compared to the heater layer 310 and / or the erosion shield layer 306. While the examples described herein primarily relate to aeroplane wings, it will be appreciated that the structure according to the present disclosure may be implemented in any application including an ice protection system. As illustrated in Figure 9, a structure 900 according to the present disclosure may have an aerofoil shape. As illustrated in the example, a heater layer 906, and an erosion shield layer 910 disposed over the heater layer 906, may be positioned at a leading edge 902 of the structure 900. It will be appreciated that an aircraft of the kind illustrated in Figures 1 and 2 may comprise the any of the structures described herein. In some examples, a turbine 1000, such as a wind turbine as illustrated in Figure 10, may comprise any of the structures described herein. For example, the structure may be a blade 1001 of the turbine 1000. No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

Claims

:

1. A structure comprising:a heater layer, the heater layer comprising a plurality of heater regions 5 and a heatable parting region separating adjacent heater regions; andan erosion shield layer disposed over the heater layer;wherein at least one region of the erosion shield layer disposed over at least one heater region is substantially thermally insulated from the parting region by a channel or gap in the erosion shield layer.

102. A structure according to claim 1, wherein the channel or gap separates the region of the erosion shield layer disposed over the heater region from a region of the erosion shield layer disposed over the parting region.15 3. A structure according to claim 1 or 2, wherein the channel or gap is filled with aCM thermally insulating filler material.CMv— 4. A structure according to any one of the preceding claims, wherein the at leastone region of the erosion shield layer disposed over at least one heater region 20 is substantially thermally insulated from the parting region by a gap in theerosion shield layer, and wherein the gap is bridged by a thermally insulating bridging material.

5. A structure according to any one of the preceding claims, wherein the erosion 25 shield layer comprises a first piece disposed over the heater region, and asecond piece disposed over the parting region.

6. A structure according to claim 5, wherein the first piece is attached to the second piece by a thermally insulating adhesive.

307. A structure according to the any one of the preceding claims, wherein the parting region is disposed on a thermally insulating strip.

8. A structure according to any one of the preceding claims, wherein the structure 35 is an aerofoil.CM9. An aircraft comprising the structure of any one of the preceding claims.

10. A turbine comprising the structure of any one of claims 1 to 8.5

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