CASING ASSEMBLY FOR A GAS TURBINE ENGINE

Abstract

There is described a casing assembly for a gas turbine engine, the casing assembly comprising: an annular casing having a longitudinal axis; a unison ring assembly configured for rotation about the longitudinal axis; a plurality of vanes rotatably coupled to the annular casing; and a plurality of linkages, wherein each of the plurality of vanes is coupled to the unison ring assembly by a respective one of the plurality of linkages. Each linkage comprises: a first link coupled to the respective vane; and a second link rotatably coupled to the first link and rotatably coupled to the unison ring assembly, such that rotation of the unison ring assembly about the longitudinal axis effects rotation of the respective vane.

Claims

1. A casing assembly for a gas turbine engine, the casing assembly comprising: an annular casing having a longitudinal axis; a unison ring assembly configured for rotation about the longitudinal axis; a plurality of vanes rotatably coupled to the annular casing; and a plurality of linkages, wherein each of the plurality of vanes is coupled to the unison ring assembly by a respective one of the plurality of linkages, wherein each linkage comprises: a first link coupled to the respective vane; and a second link rotatably coupled to the first link and rotatably coupled to the unison ring assembly, such that rotation of the unison ring assembly about the longitudinal axis effects rotation of the respective vane.

2. The casing assembly as claimed in claim 1, wherein the unison ring assembly is configured for movement on a single plane.

3. The casing assembly as claimed in claim 1, wherein the second link of each of the linkages is rotatably coupled to its respective first link about a first axis of rotation that is substantially perpendicular to the longitudinal axis.

4. The casing assembly as claimed in claim 1, wherein the second link of each of the linkages is rotatably coupled to the unison ring assembly about a second axis of rotation that is substantially perpendicular to the longitudinal axis.

5. The casing assembly as claimed in claim 1, wherein the first link of each of the linkages is fixedly coupled to its respective vane.

6. The casing assembly as claimed in claim 1, wherein the second link of each of the linkages is rotatably coupled to its respective first link and/or the unison ring assembly by a hinge joint.

7. The casing assembly as claimed in claim 1, wherein the second link of each of the linkages is rotatably coupled to its respective first link and/or the unison ring assembly by a universal joint.

8. The casing assembly as claimed in claim 1, wherein the second link of each of the linkages is rotatably coupled to its respective first link and/or the unison ring assembly by a spherical joint.

9. The casing assembly as claimed in claim 1, wherein two or more of the first links or two or more of the second links have two or more different lengths.

10. The casing assembly as claimed in claim 1, wherein the unison ring assembly comprises two or more unison rings spaced from each other, wherein the second links of each of the linkages are rotatably coupled to the two or more unison rings.

11. The casing assembly as claimed in claim 10, wherein the two or more unison rings are connected by one or more connecting rods.

12. The casing assembly as claimed in claim 10, wherein each of the two or more unison rings comprise a plurality of clevises, wherein the second links of each of the linkages are rotatably coupled to the unison ring assembly via a respective one of the plurality of clevises.

13. The casing assembly as claimed in claim 10, wherein each of the two or more unison rings comprise a plurality of projections extending towards the longitudinal axis, wherein the annular casing comprises a plurality of grooves or slots for receiving the plurality of projections and locating the two or more unison rings relative to the annular casing.

14. The casing assembly as claimed in claim 10, wherein the two or more unison rings are substantially planar.

15. The casing assembly as claimed in claim 1, wherein the plurality of vanes are a plurality of stator vanes and/or a plurality of inlet guide vanes.

16. A gas turbine engine comprising a casing assembly as claimed in claim 1.

Description

[0049] Embodiments will now be described by way of example only, with reference to the Figures, in which:

[0050] FIG. 1 is a sectional side view of a gas turbine engine;

[0051] FIG. 2 is a close up sectional side view of an upstream portion of a gas turbine engine;

[0052] FIG. 3 is a partially cut-away view of a gearbox for a gas turbine engine;

[0053] FIG. 4 is a perspective view of a casing assembly of a high pressure compressor of the gas turbine engine;

[0054] FIG. 5 is a perspective view of a unison ring assembly of the casing assembly;

[0055] FIG. 6 is a close-up perspective view of a plurality of linkages of the casing assembly;

[0056] FIG. 7 is a close-up perspective view of one of the linkages of the casing assembly;

[0057] FIG. 8 is a cross-sectional view of the connection between a unison ring of the unison ring assembly and a casing;

[0058] FIG. 9 is a plan view of a plurality of linkages with the unison ring located at a first position;

[0059] FIG. 10 is a plan view of a plurality of linkages with the unison ring located at a second position;

[0060] FIG. 11 is a cross-sectional view of an alternative connection between an alternative unison ring and an alternative casing; and

[0061] FIG. 12 is a further cross-sectional view of the alternative connection between the alternative unison ring and the alternative casing taken in a direction perpendicular to the cross-sectional view of FIG. 11.

[0062] FIG. 1 illustrates a gas turbine engine 10 having a principal rotational axis 9. The engine 10 comprises an air intake 12 and a propulsive fan 23 that generates two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 comprises a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow series, a low pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, a low pressure turbine 19 and a core exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow B flows through the bypass duct 22. The fan 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30.

[0063] In use, the core airflow A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air exhausted from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high pressure and low pressure turbines 17, 19 before being exhausted through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gearbox.

[0064] An exemplary arrangement for a geared fan gas turbine engine 10 is shown in FIG. 2. The low pressure turbine 19 (see FIG. 1) drives the shaft 26, which is coupled to a sun wheel, or sun gear, 28 of the epicyclic gear arrangement 30. Radially outwardly of the sun gear 28 and intermeshing therewith is a plurality of planet gears 32 that are coupled together by a planet carrier 34. The planet carrier 34 constrains the planet gears 32 to precess around the sun gear 28 in synchronicity whilst enabling each planet gear 32 to rotate about its own axis. The planet carrier 34 is coupled via linkages 36 to the fan 23 in order to drive its rotation about the engine axis 9. Radially outwardly of the planet gears 32 and intermeshing therewith is an annulus or ring gear 38 that is coupled, via linkages 40, to a stationary supporting structure 24.

[0065] Note that the terms low pressure turbine and low pressure compressor as used herein may be taken to mean the lowest pressure turbine stages and lowest pressure compressor stages (i.e. not including the fan 23) respectively and/or the turbine and compressor stages that are connected together by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e. not including the gearbox output shaft that drives the fan 23). In some literature, the low pressure turbine and low pressure compressor referred to herein may alternatively be known as the intermediate pressure turbine and intermediate pressure compressor. Where such alternative nomenclature is used, the fan 23 may be referred to as a first, or lowest pressure, compression stage.

[0066] The epicyclic gearbox 30 is shown by way of example in greater detail in FIG. 3. Each of the sun gear 28, planet gears 32 and ring gear 38 comprise teeth about their periphery to intermesh with the other gears. However, for clarity only exemplary portions of the teeth are illustrated in FIG. 3. There are four planet gears 32 illustrated, although it will be apparent to the skilled reader that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of a planetary epicyclic gearbox 30 generally comprise at least three planet gears 32.

[0067] The epicyclic gearbox 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. By way of further example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring (or annulus) gear 38 allowed to rotate. In such an arrangement the fan 23 is driven by the ring gear 38.

[0068] By way of further alternative example, the gearbox 30 may be a differential gearbox in which the ring gear 38 and the planet carrier 34 are both allowed to rotate.

[0069] It will be appreciated that the arrangement shown in FIGS. 2 and 3 is by way of example only, and various alternatives are within the scope of the present disclosure. Purely by way of example, any suitable arrangement may be used for locating the gearbox 30 in the engine 10 and/or for connecting the gearbox 30 to the engine 10. By way of further example, the connections (such as the linkages 36, 40 in the FIG. 2 example) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of stiffness or flexibility. By way of further example, any suitable arrangement of the bearings between rotating and stationary parts of the engine (for example between the input and output shafts from the gearbox and the fixed structures, such as the gearbox casing) may be used, and the disclosure is not limited to the exemplary arrangement of FIG. 2. For example, where the gearbox 30 has a star arrangement (described above), the skilled person would readily understand that the arrangement of output and support linkages and bearing locations would typically be different to that shown by way of example in FIG. 2.

[0070] Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox styles (for example star or planetary), support structures, input and output shaft arrangement, and bearing locations.

[0071] Optionally, the gearbox may drive additional and/or alternative components (e.g. the intermediate pressure compressor and/or a booster compressor).

[0072] Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and/or turbines and/or an alternative number of interconnecting shafts. By way of further example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 20, 22 meaning that the flow through the bypass duct 22 has its own nozzle that is separate to and radially outside the core engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also apply to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed flow nozzle. One or both nozzles (whether mixed or split flow) may have a fixed or variable area. Whilst the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as an open rotor (in which the fan stage is not surrounded by a nacelle) or turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not comprise a gearbox 30.

[0073] The geometry of the gas turbine engine 10, and components thereof, is defined by a conventional axis system, comprising an axial direction (which is aligned with the rotational axis 9), a radial direction (in the bottom-to-top direction in FIG. 1), and a circumferential direction (perpendicular to the page in the FIG. 1 view). The axial, radial and circumferential directions are mutually perpendicular.

[0074] FIG. 4 is a perspective view of a casing assembly 42 of a high pressure compressor, such as the high pressure compressor 15 described with reference to FIGS. 1 and 2. The casing assembly 42 generally comprises a casing 44, a unison ring assembly 46, a plurality of linkages 48 and a plurality of vanes 50. Although only a single vane 50 is shown in FIG. 4 for clarity, each of the linkages 48 is connected to a separate vane 50. The vanes 50 may be variable inlet guide vanes and/or variable stator vanes.

[0075] The casing 44 and the unison ring assembly 46 each have annular profiles. The longitudinal axes of the annular casing 44 and the annular unison ring assembly 46 (i.e. the axes around which the casing 44 and the unison ring assembly 46 extend) are aligned with a longitudinal axis 41 of the casing assembly 42 (i.e. the axis around which the casing assembly 42 extends). The longitudinal axis 41 of the casing assembly 42 is aligned with the principal rotational axis 9. Accordingly, the casing 44, the unison ring assembly 46 and the casing assembly 42 extend around the principal rotational axis 9. The unison ring assembly 46 is configured for rotation about the longitudinal axis 41. The unison ring assembly 46 is configured for movement on a single plane perpendicular to the longitudinal axis 41, and does not move in an axial direction along the longitudinal axis 41.

[0076] The casing 44 is disposed within the unison ring assembly 46, such that the unison ring assembly 46 extends around the casing 44. The plurality of vanes 50 are disposed within the casing 44, such that the casing 44 extends around the vanes 50. The plurality of vanes 50 are disposed circumferentially around the longitudinal axis 41. The plurality of vanes 50 are arranged in three distinct rows (i.e. on three distinct planes) disposed along the longitudinal axis 41. The plurality of vanes 50 are each configured for rotation about respective axes 51 oriented perpendicularly to the longitudinal axis 41.

[0077] FIG. 5 is a perspective view of the unison ring assembly 46 in isolation. The unison ring assembly 46 comprises a first unison ring 52, a second unison ring 54 and a third unison ring 56. Each unison ring 52, 54, 56 comprises an outer ring portion 58 and a series of alternating clevises 60 and projections or centralisers 62 extending radially inwards from the outer ring portion 58 towards the longitudinal axis 41 around the circumference of the unison rings 52, 54, 56. The projections 62 and the clevises 60 are integrally formed with the outer ring portion 58. Accordingly, each of the unison rings 52, 54, 56 may be cut out of a single plate of metal using laser cutting or water jet cutting processes.

[0078] The outer ring portions 58 are provided with a plurality of axially extending holes. The plurality of axially extending holes are disposed circumferentially around the outer ring portions 58 and are spaced at two distinct radial distances from the longitudinal axis 41. Accordingly, the axially extending holes are arranged in pairs of concentric circles. A plurality of identical threaded connecting rods 64 extend through corresponding axially extending holes of the first, second and third unison rings 52, 54, 56 and are fixed to each of the first, second and third unison rings 52, 54, 56 by bolts. Accordingly, the first, second and third unison rings 52, 54, 56 are secured together to form a squirrel-cage structure that provides good out-of-plane stiffness. The first, second and third unison rings 52, 54, 56 are substantially identical, with the exception of a tab 66 that extends outwards from the outer ring portion 58 of the second unison ring 54 and which contains a hole for direct connection to an actuator.

[0079] FIG. 6 is a close-up perspective view of the casing assembly 42 showing one of the vanes 50 and its respective linkage 48 in detail. The casing 44 is provided with a plurality of through holes 68 directed towards the longitudinal axis 41. The vane 50 extends through the through hole 68 such that a radially outer end thereof is disposed outside of the casing 44 and such that an aerofoil portion 70 thereof is disposed within the casing 44. The longitudinal axis of the vane 50 is aligned with the axis 51. The vane 50 is able to rotate within the through hole 68 about the axis 51. The linkage 48 comprises a first link or lever 72 and a second link 74. A first end of the first link 72 is fixedly coupled to the radially outer end of the vane 50. A second end of the first link 72 is pivotally coupled to a first end of the second link 74. A second end of the second link 74 is pivotally coupled to the clevis 60. The first and second links 72, 74 are manufactured using a metal injection moulding (MIM) process.

[0080] FIG. 7 is a further close-up perspective view showing one of the linkages 48. As shown, the radially outer end of the vane 50 is provided with a shoulder 75 against which the first end of the first link 72 is secured by a bolt 76. Accordingly, the first link 72 and the vane 50 are unable to move relative to each other. The second end of the first link 72 comprises a clevis 78 formed by two arms separated by a gap. Each of the arms comprises a through hole, through which a first pin 80 extends. The first pin 80 is rotatable within the through holes about an axis 82 that is substantially oriented towards and perpendicular to the longitudinal axis 41. The first end of the second link 74 comprises a notch within which the first pin 80 is located. A second pin 84 extends through the second link 74 and the first pin 80 so as to secure the second link 74 to the first pin 80. The second link 74 is therefore able to pivot relative to the first link 72 about the axis 82. The connection between the second link 74 and the first link 72 is a hinge joint that allows motion on a single plane. However, as outlined below, it may alternatively be a different type of joint such as a universal joint or a spherical joint formed by a spherical bearing.

[0081] Wth reference to both FIGS. 6 and 7, the second end of the second link 74 comprises a clevis 86 formed by two arms separated by a gap. Each of the arms comprises a through hole, through which a third pin 90 extends. The third pin 90 is rotatable within the through holes about an axis 92 that is substantially oriented towards and perpendicular to the longitudinal axis 41. The clevis 60 of the unison ring 52 is likewise formed by two arms separated by a gap. Each of the arms comprises a through hole through which a fourth pin 94 extends. The fourth pin 94 is rotatable within the through holes about an axis 96 extending circumferentially with respect to the unison ring 52. The fourth pin 94 is fixedly connected to the third pin 90. The second link 74 is therefore rotatable about the axes 92 and 96. The connection between the second link 74 and the unison ring 52 is a universal joint allowing relative movement of the two components about multiple axes.

[0082] FIG. 8 is a cross-sectional view of the connection between the unison ring 52 and the casing 44. The casing 44 comprises a first flange 98 and a second flange 100. The first and second flanges 98, 100 are spaced apart from each other so as to define a groove or slot 102. A low-friction pad or coating 103 is disposed at the base of the groove 102. The first and second flanges 98, 100, the groove 102 and the low-friction pad 103 extend around the circumference of the casing 44 (see FIGS. 4 and 6). The projection 62 is located within the groove 102, is supported by the low-friction pad 103 and is able to slide along the groove 102 in a circumferential direction. Accordingly, the unison ring assembly 46 can be rotated about the longitudinal axis 41 (i.e. in a circumferential direction) via actuation of the actuator. The first and second flanges 98, 100 prevent the projection 62 and thus the unison ring 52 and the unison ring assembly 46 as a whole moving in an axial direction, along the longitudinal axis 41 and along the casing 44. Further, since a plurality of projections 62 are provided that are disposed around the circumference of the unison rings 52, 54, 56 (i.e. around the principal longitudinal axis 41), the unison ring assembly 46 is centred with respect to the casing 44. Accordingly, deformation of the unison rings 52, 54, 56 (e.g. from a circular shape to an oval shape) is prevented. Since the unison ring assembly 46 has good out-of-plane stiffness, the number of projections 62 can be minimised.

[0083] FIG. 9 is a plan view of a pair of adjacent linkages 48, with the unison ring 52 located at a first position. In the configuration shown in FIG. 9, the unison ring 52 is positioned at a relatively downwards position as viewed from FIG. 9 and a relatively anti-clockwise position as viewed from the right-hand side of FIG. 4. In the configuration shown in FIG. 9, the distance between the coupling between the vane 50 and the first link 72 and the coupling between the second link 74 and the unison ring 52 (i.e. the distance between axes 51, 92) is relatively small. Accordingly, in order to accommodate the lengths of the first link 72 and the second link 74, it is necessary for the first link 72 to be pivoted to a relatively anti-clockwise position as viewed from FIG. 9, such that a relatively small angle .sub.1 is formed between the first link 72 and the second link 74 and a relatively large angle is formed between the first link 72 and the longitudinal axis 41.

[0084] FIG. 10 is a plan view of a pair of adjacent linkages 48, with the unison ring 52 located at a second position. In the configuration shown in FIG. 10, the unison ring 52 is positioned at a relatively upwards position as viewed from FIG. 10 (as shown by the difference between the positions of the connecting rods 64 in FIGS. 9 and 10) and a relatively clockwise position as viewed from the right-hand side of FIG. 4. In the configuration shown in FIG. 10, the distance between the coupling between the vane 50 and the first link 72 and the coupling between the second link 74 and the unison ring 52 (i.e. the distance between axes 51, 92) is relatively large. Accordingly, in order to accommodate the lengths of the first link 72 and the second link 74, it is necessary for the first link 72 to be pivoted to a relatively clockwise position as viewed from FIG. 10 such that a relatively large angle .sub.2 is formed between the first link 72 and the second link 74 and a relatively small angle is formed between the first link 72 and the longitudinal axis.

[0085] As mentioned previously, the first link 72 is secured to the vane 50 such that they are unable to move relative to each other. Accordingly, when the unison ring 52 is positioned as shown in FIG. 9, the vane 50 is rotated to a relatively anti-clockwise position as viewed from FIG. 9, thereby reducing the flow area through the compressor. Likewise, when the unison ring 52 is positioned as shown in FIG. 10, the vane 50 is rotated to a relatively clockwise position as shown in FIG. 10, thereby increasing the flow area through the compressor. Accordingly, by actuating the actuator and thus the position of the unison ring assembly 46, the angle of the vane 50 and the performance of the compressor can be changed.

[0086] The desired range of motion of the vane 50 may change before or after manufacture of the gas turbine engine 10. In such circumstances, the range of motion of the vane 50 may be changed simply by changing the length of the first and/or second link 72, 74. This has minimal effect on surrounding components within the casing assembly 42 and gas turbine engine 10 as a whole. Accordingly, the need for significant redesign of the casing assembly 42 or the gas turbine engine 10 as a whole is prevented. In some circumstances, it may be preferable to change the length of the second link 74 whilst keeping the length of the first link 72 the same. In other circumstances, it may be preferable to change the length of the first link 72 whilst keeping the length of the second link 74 the same. The range of motion of the vane 50 may alternatively or additionally be modified by changing the distance between the unison ring 52 and the axis around which the vane 50 rotates (i.e. the axial distance between the axes 51 and 92). The lengths of the first and second links 72, 74 and the distance between the unison ring 52 and the axis around which the vanes 50 rotate may be different for different row of vanes 50 so as to effect different amounts of rotation of the vanes 50 within different rows.

[0087] By way of example, in a casing assembly 42 having a first sizing, the length of the first link 72 is 50 millimetres, the length of the second link 74 is 35 millimetres and the distance between the axis along which the unison ring 52 rotates and the axis about which the vane 50 rotates is 45 millimetres. This results in a total range of unison ring 52 travel of approximately 37 millimetres and a range of motion of the vane 50 of approximately 48 degrees. In a casing assembly 42 having a second sizing, the length of the first link 72 is 50 millimetres, the length of the second link 74 is 70 millimetres and the distance between the axis along which the unison ring 52 rotates and the axis about which the vane 50 rotates is 80 millimetres. This results in a total range of unison ring 52 travel of approximately 37 millimetres and a range of motion of the vane 50 of approximately 37 degrees.

[0088] The linkages 48 act as straight line linkages or straight line mechanisms. The linkages 48 allow the unison ring assembly 46 to be rotated in a circumferential direction (i.e. around the longitudinal axis 41) without moving axially (i.e. along the longitudinal axis 41). Since each of the unison rings 52, 54, 56 do not move axially, they can be connected to form a single unison ring assembly 46 that can be controlled directly by a single actuator. Accordingly, there is no need for the unison rings 52, 54, 56 to be controlled individually, such as via a plurality of control rods connected to a crankshaft, as is typically done in existing casing assemblies. The need for a plurality of control rods and crankshaft is therefore avoided. As mentioned above, the lengths of the first and second links 72, 74 and the distance between the unison ring 52 and the axis around which the vanes 50 rotate may be different for different rows of vanes 50, such that rotation of the unison ring assembly 46 effects different amounts of rotation in different vanes 50.

[0089] By reducing the number of components relative to existing casing assemblies, the size of the casing assembly 42 can be reduced, thereby allowing the casing assembly 42 to fit within existing gas turbine engines 10, as well as gas turbine engines having less available space for the casing assembly 42. Further, by both reducing the number of components and providing multiple components that are identical or substantially identical to each other (e.g. the first, second and third unison rings 52, 54, 56 and the connecting rods 64), the casing assembly 42 is simple, quick and inexpensive to manufacture.

[0090] An additional benefit of providing a casing assembly 42 that does not move axially is that the connection between the unison ring assembly 46 and the casing 44 can be relatively simple in comparison to existing designs. One example of such a simple connection has already been described with reference to FIG. 8. Alternative simple connections between unison ring assemblies and casings may alternatively be provided, such as that shown in FIGS. 11 and 12.

[0091] FIGS. 11 and 12 are cross-sectional views of a connection between an alternative unison ring 104 and an alternative casing 106. The orientation of the cross-sectional plane of FIG. 11 corresponds that of FIG. 8. The cross-sectional plane of FIG. 12 is perpendicular to the cross-sectional plane of FIG. 11. The alternative unison ring 104 substantially corresponds to the unison ring 52. However, a first pin 108 and a second pin 110 extend from a distal end (i.e. a radially inner end) of the projection 62. The alternative casing 106 comprises an alternative first flange 112 and an alternative second flange 114 that are spaced apart from each other so as to define a groove 116. As best shown in FIG. 12, the alternative first and second flanges 112, 114 only extend part way around the circumference of the alternative casing 106. The alternative first and second flanges 112, 114 are provided with a first slot 118 and a second slot 120, respectively. The first and second pins 108, 110 extend through and are supported by the first and second slots 118, 120, respectively. In alternative arrangements one or more rollers could run along the first and second slots 118, 120, rather than the first and second pins 108, 110.

[0092] Although it has been described that the connection between the second link 74 and the first link 72 is a hinge joint and that the connection between the second link 74 and the unison ring 52 is a universal joint, in alternative arrangements both the connection between the second link 74 and the first link 72 and the connection between the second link 74 and the unison ring 52 may be universal joints. This may increase the amount by which the unison ring assembly 46 is able to rotate about the longitudinal axis 41 upon actuation by the actuator. In further alternative arrangements, the connection between the second link 74 and the first link 72 may be a universal joint and the connection between the second link 74 and the unison ring 52 may be a hinge joint. In yet further alternative arrangements, both of the connections may be hinge joints. In yet further alternative arrangements, one or both of the connections may be spherical bearings (i.e. ball joints).

[0093] Although it has been described that the casing assembly 42 is a casing assembly of a high pressure compressor, it may alternatively be a casing assembly of a low pressure compressor such as the low pressure compressor 14 described with reference to FIGS. 1 and 2.

[0094] Although it has been described that three unison rings are provided for three rows of vanes, any number of unison rings may be provided for any number of rows of vanes. For example, four unison rings may be provided for four rows of vanes. Such an arrangement may be used in a high pressure compressor 15 such as that described with reference to FIGS. 1 and 2, which typically comprise four rows of vanes. By way of further example, a unison ring assembly comprising a single unison ring may be provided for a single row of vanes, or a unison ring assembly comprising five unison rings may be provided for five rows of vanes.

[0095] Although it has been described that the first link 72 and the vane 50 are separate components, in alternative arrangements they may be formed integrally with each other. In such arrangements, as with the previously described arrangements, the first link comprises an elongate body extending away from the main body of the vane 50 (i.e. the aerofoil portion 70 of the vane 50) and the rotational axis 51 thereof.

[0096] Although the structure and operation of a single vane 50 and corresponding portions of the rest of the casing assembly 42 have been described, the remaining vanes and corresponding portions of the rest of the casing assembly 52 may be structured and operate in a similar manner.

[0097] It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.