GB2595667A

GB2595667A - An actuator and a method of operating the actuator

Info

Publication number: GB2595667A
Application number: GB2008226.9A
Authority: GB
Inventors: J Bickley Daniel
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2020-06-02
Filing date: 2020-06-02
Publication date: 2021-12-08
Also published as: GB202008226D0

Abstract

An actuator 26 and corresponding method of operating the actuator are provided. The actuator 26 comprises: a housing 28 defining a chamber 34; a piston 30 slidably received within the chamber 34 and able to move along an axis 44; and a spool 32 rotationally received within the piston 30 and configured for movement about the axis 44. The piston 30 includes: a seal 46 separating a first portion 48 of the chamber 34 from a second portion 50 of the chamber 34; a first opening 56 that opens into the first portion 48 of the chamber 34; and a second opening 58 that opens into the second portion 50 of the chamber 34. The spool 32 comprises: a first outlet slot 60 for supplying fluid to the first portion 48 of the chamber 34 via the first opening 56; a second outlet slot 62 for supplying fluid to the to the second portion 50 of the chamber 34 via the second opening 58; and an inlet slot 64 disposed between the first outlet slot 60 and the second outlet slot 62 for receiving fluid from the first portion 48 of the chamber 34 via the first opening 56 and the second portion 50 of the chamber 34 via the second opening 58. The first outlet lot 60, the second outlet slot 62 and the inlet slot 64 each extend both axially parallel to the axis 44 and circumferentially about the axis 44 such that rotation of the spool 32 about the axis 44 effects linear movement of the piston 30 along the axis 44. A method of operating the actuator 26 is also disclosed.

Description

AN ACTUATOR AND A METHOD OF OPERATING THE ACTUATOR

Field of the disclosure

The disclosure relates to an actuator and a method of operating the actuator.

Background

Hydraulic servos comprising actuators are commonly used to vary the rotary angles of compressor vanes in gas turbine engines in order to optimise the performance of the gas turbine engines across their operating ranges. However, such hydraulic servos can be large, complex and heavy. It is therefore desirable to provide an actuator and method of operating the actuator that overcomes these issues.

Summary of the disclosure

According to a first aspect there is provided an actuator, the actuator comprising: a housing defining a chamber; a piston slidably received within the chamber and configured for movement along an axis; and a spool rotationally received within the piston and configured for movement about the axis. The piston comprises a seal separating a first portion of the chamber from a second portion of the chamber, a first opening that opens into the first portion of the chamber and a second opening that opens into the second portion of the chamber. The spool comprises a first outlet slot for supplying fluid to the first portion of the chamber via the first opening, a second outlet slot for supplying fluid to the to the second portion of the chamber via the second opening and an inlet slot disposed between the first outlet slot and the second outlet slot for receiving fluid from the first portion of the chamber via the first opening and the second portion of the chamber via the second opening. The first outlet slot, the second outlet slot and the inlet slot each extend both axially parallel to the axis and circumferentially about the axis such that rotation of the spool about the axis effects linear movement of the piston along the axis.

The first outlet slot, the second outlet slot and the inlet slot may be parallel.

The first outlet slot, the second outlet slot and the inlet slot may be helical.

The width of the inlet slot in an axial direction parallel to the axis may be less than or equal to the axial distance parallel to the axis between the first opening and the second opening.

The axial width of the first opening in an axial direction parallel to the axis may be less than or equal to the axial distance parallel to the axis between the inlet slot and the first outlet slot.

The axial width of the second opening in an axial direction parallel to the axis may be less than or equal to the axial distance parallel to the axis between the inlet slot and the second outlet slot.

The axial distance between the first outlet slot and the second outlet slot in an axial direction parallel to the axis may be greater than or equal to the maximum axial distance parallel to the axis between the first opening and the second opening.

The first outlet slot, the second outlet slot and the inlet slot may extend in an axial direction parallel to the axis only along a central portion of the spool.

The spool may define at least part of an inlet channel. The first outlet slot and the second outlet slot may be fluidically connected to the inlet channel by one or more inlet passageways extending through the spool.

The spool may define at least part of an outlet channel. The inlet slot may be fluidically connected to the inlet channel by one or more outlet passageways extending through the spool.

A further chamber is defined by an end of the spool and an end of an interior of the piston within which the spool is rotationally received. The further chamber may be fluidically connected to the one or more outlet passageways.

The piston may comprise a housing portion within which the spool is rotationally received and a flange portion extending radially outwards from the housing portion. The flange portion may define the seal.

The actuator may further comprise a stepper motor configured to effect movement of the spool about the axis.

A hydraulic servo may comprise the actuator of any preceding statement.

A gas turbine engine may comprise a hydraulic servo comprising the actuator of any preceding statement.

According to a second aspect there is provided a method of operating the actuator of any preceding statement, the method comprising: supplying fluid to the first outlet slot and the second outlet slot; rotating the spool in a first direction about the axis such that the first outlet slot is in direct fluid communication with the first portion of the chamber via the first opening, the inlet slot is in direct fluid communication with the second portion of the chamber via the second opening and the second outlet slot is not in direct fluid communication with the second portion of the chamber such that fluid is supplied to the first portion of the chamber via the first opening, such that fluid is released from the second portion of the chamber via the second opening and such that the piston moves along the axis in a first direction away from the first portion of the chamber and towards the second portion of the chamber; and rotating the spool in a second direction about the axis such that the second outlet slot is in direct fluid communication with the second portion of the chamber via the second opening, the inlet slot is in direct fluid communication with the first portion of the chamber via the first opening and the first outlet slot is not in direct fluid communication with the first portion of the chamber such that fluid is supplied to the second portion of the chamber via the second opening, such that fluid is released from the first portion of the chamber via the first opening and such that the piston moves along the axis in a second direction away from the second portion of the chamber and towards the first portion of the chamber.

Rotation of the spool in the first and second directions about the axis may be effected by a stepper motor.

Brief description of the drawings

Arrangements will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a gas turbine engine; Figure 2 is a cross-sectional view of an actuator in a first position; Figure 3 is a perspective view of a spool; Figure 4 is a close-up cross-sectional view of the actuator in the first position; Figure 5 is a cross-sectional view of the actuator in a second position; Figure 6 is a cross-sectional view of the actuator in a third position; Figure 7 is a cross-sectional view of the actuator in a fourth position; and Figure 8 is a flowchart of a method of operating the actuator.

Detailed description

Figure 1 shows a ducted fan gas turbine engine 10 having a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted.

The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

The intermediate and high pressure compressors 13, 14 each comprise a plurality of vanes. The gas turbine engine 10 comprises a hydraulic servo comprising an actuator 26 that may be used to vary the rotary angle of the plurality of vanes in order to optimise the performance of the gas turbine engine 10 across its operating range.

Figure 2 is a cross-sectional view of the actuator 26. The actuator 26 comprises a housing 28, a piston 30 and a spool 32. The housing 28 comprises a main body 35 defining a cylindrical chamber 34, a first end 36 and a second end 38. The first end 36 is provided with a recess 40 for rotationally receiving a first end of the spool 32. In the arrangement shown in Figure 2, the first end 36 is integrally formed with the main body 35. However in alternative arrangements, the first end 36 may be distinct from the main body 35. The second end 38 is provided with a through-hole defined by a flange 42 for slidably receiving the piston 30 such that the piston 30 is slidably received within the chamber 34. An interior of the flange 42 is provided with a first annular dynamic seal 43 (e.g. an 0-ring or cap seal) and a second annular dynamic seal 45. The first and second dynamic seals 43, 45 are spaced apart along an axis 44. An annular drain channel 47 is disposed between the first and second dynamic seals 43, 45. The drain channel 47 is connected to a drain outlet 49.

The piston 30 is able to move along the axis 44. The piston 30 is however not able to move about the axis 44 (i.e. rotate about the axis 44). The piston 30 is provided with a through-hole 31 at its distal end. The piston 30 can be connected to the gas turbine engine 10 via the through-hole 31, which prevents rotation of the piston 30 about the axis 44. In alternative arrangements, rotational movement may be prevented through the use of a protrusion on the piston 30 that is configured to slide within an axially extending slot in the flange 42, or vice versa. The piston 30 comprises a housing portion 52 and a flange portion 54 extending radially outwards from the housing portion 52. The flange portion 54 defines a seal 46 that separates a first portion 48 of the chamber 34 from a second portion 50 of the chamber 34. The outer perimeter of the flange portion 54 is provided with an annular dynamic seal 55.

The housing portion 52 of the piston 30 defines a cylindrical chamber 53 and the spool 32 is rotationally received by the piston 30 within the cylindrical chamber 53. The spool 32 is able to move (i.e. rotate) about the axis 44. Movement of the spool 32 about the axis 44 is effected by an electrical effecter (i.e. motor) in the form of a stepper motor 41 having a detent that allows the actuation system to hold position when electrical power is not available. An electrical effector other than a stepper motor 41 could alternatively be used. The piston 30 comprises a first opening 56 that opens into the first portion 48 of the chamber 34. The piston 30 further comprises a second opening 58 that opens into the second portion 50 of the chamber 34. The flange portion 54 is thus disposed between the first opening 56 and the second opening 58.

Figure 3 is a perspective view of the spool 32. As shown, the spool 32 is cylindrical and comprises a first outlet slot 60, a second outlet slot 62 and an inlet slot 64. The first outlet slot 60, the second outlet slot 62 and the inlet slot 64 extend in an axial direction (i.e. a direction having a component parallel to the axis 44) and a circumferential direction (i.e. a direction having a component about the axis 44). The first outlet slot 60, the second outlet slot 62 and the inlet slot 64 are helical. The first outlet slot 60, the second outlet slot 62 and the inlet slot 64 have a constant rate of curvature about the spool 32 and are parallel (i.e. they are spaced by fixed distance along their lengths, and so never cross or come into contact with each other). The inlet slot 64 is disposed between the first outlet slot 60 and the second outlet slot 62. The first outlet slot 60, the inlet slot 64 and the second outlet slot 62 are therefore arranged in series.

The spool 32 comprises a reduced-diameter portion that defines part of an annular inlet channel 66. The inlet channel 66 is fluidically connected to an exterior of the actuator 26 by a servo supply line 67 (see Fig. 2). A first annular dynamic seal 68 and a second annular dynamic seal 70 are disposed either side of the inlet channel 66. The first outlet slot 60, the second outlet slot 62 and the inlet slot 64 extend only part way along the length of the spool 32 (i.e. only extend along part of the axial length of the spool).

In particular, the first outlet slot 60, the second outlet slot 62 and the inlet slot 64 do not extend to an upper surface 72 of the spool 32 (as shown in Figure 3). Further, the first outlet slot 60, the second outlet slot 62 and the inlet slot 64 do not extend to the first annular dynamic seal 68 or the inlet channel 66.

Returning to Figure 2, as shown, the inlet channel 66 is fluidically connected to the first outlet slot 60 and the second outlet slot 62 by an inlet passageway 74, which is internal to the spool 32. The inlet passageway 74 comprises a single channel that branches into two separate channels that connect to the first outlet slot 60 and the second outlet slot 62. It will be appreciated that the inlet passageway 74 could take other forms from that shown in Figure 2. For example, the inlet passageway 74 could comprise a single channel that branches into three or more separate channels that connect to the first outlet slot 60 and the second outlet slot 62. Alternatively, the inlet passageway 74 does not branch, and, instead, multiple individual inlet passageways 74 are provided.

A first end face of the spool 32 and the end of the recess 40 of the housing 28 define an annular outlet channel 76. The outlet channel 76 is fluidically connected to an exterior of the actuator 26 by a servo return line 77.

The axial length of the interior of the piston 30 (i.e. the cylindrical chamber 53) is greater than an axial length of the portion of the spool 32 that extends from the first end 36 of the housing 28. A second end face of the spool 32 and the end of the cylindrical chamber 53 defined by the housing portion 52 of the piston 30 define a further chamber 80. The outlet channel 76 is fluidically connected to the inlet slot 64 and the further chamber 80 by an outlet passageway 78, which is internal to the spool 32 and separate to the inlet passageway 74. The outlet passageway 78 comprises a single channel that branches into two separate channels that connect to the outlet channel 76, the inlet slot 64 and the further chamber 80. It will be appreciated that the outlet passageway 78 could take other forms from that shown in Figure 2. For example, the outlet passageway 78 could comprise a single channel that branches into three or more separate channels that connect to the outlet channel 76, the inlet slot 64 and the further chamber 80. Alternatively, the outlet passageway 78 does not branch, and, instead, multiple individual outlet passageways 78 are provided.

Figure 4 is a close-up cross-sectional view of the actuator 26. The actuator 26 is not shown to scale. As shown, the width Al of the inlet slot in the axial direction is less than the axial distance A2 between the first opening 56 and the second opening 58 (i.e. the minimum axial distance). The width Al of the inlet slot in the axial direction may alternatively be equal to the axial distance A2 between the first opening 56 and the second opening 58. The axial width A3 of the first opening 56 is equal to the axial distance A4 between the inlet slot 64 and the first outlet slot 60 (i.e. the minimum axial distance). The axial width A3 of the first opening 56 may alternatively be less than the axial distance A4 between the inlet slot 64 and the first outlet slot 60. The axial width A5 of the second opening 58 is slightly less than the axial distance A6 between the inlet slot 64 and the second outlet slot 62 (i.e. the minimum axial distance). The axial width AS of the second opening 58 may alternatively be equal to the axial distance AS between the inlet slot 64 and the second outlet slot 62. The axial distance A7 between the first and second outlet slots 60, 62 (i.e. the minimum axial distance) is equal to the maximum axial distance A8 between the first and second openings 56, 58. The axial distance A7 between the first and second outlet slots 60, 62 may alternatively be greater than the maximum axial distance A8 between the first and second openings 56, 58.

The abovementioned geometry is designed to prevent the first portion 48 of the chamber 34 and the second portion 50 of the chamber 34 having appreciable communication with the supply pressure (i.e. the pressure from the servo supply line 67) simultaneously, and also prevents the first portion 48 of the chamber 34 and the second portion 50 of the chamber 34 having appreciable communication with the return pressure (i.e. the pressure from the servo return line 77) simultaneously. This minimises parasitic leakage from the servo supply to the servo return and maximises the force gain of the actuator 26 (i.e. creates a high rate of change of actuator load with respect to movement of the spool 32 relative to the piston 30). The high gain aids accurate actuator 26 positioning in the presence of high levels of friction within the actuated mechanism.

Operation of the actuator 26 will now be described with reference to Figures 2 and 5 to 7.

Figure 2 shows the actuator 26 with the piston 30 in a retracted position. Fluid is supplied to the first and second outlet slots 60, 62 via the servo supply line 67, the inlet channel 66 and the inlet passageway 74. Since the first and second outlet slots 60, 62 are connected to a common source, the pressure of the fluid supplied to the first outlet slot 60 is equal to the pressure of the fluid supplied to the second outlet slot 62. Fluid is disposed within the first and second portions 48, 50 of the chamber 34. The first outlet slot 60 is occluded by a wall of the housing portion 52 of the piston 30 such that the first outlet slot 60 is not in direct fluid communication with the first portion 48 of the chamber 34 via the first opening 56. Likewise, the second outlet slot 62 is occluded by a wall of the housing portion 52 of the piston 30 such that the second outlet slot 62 is not in direct fluid communication with the second portion 50 of the chamber 34 via the second opening 58. Further, the inlet slot 64 is occluded by a wall of the housing portion 52 of the piston 30 such that the inlet slot 64 is not in direct fluid communication with either of the first or second portions 48, 50 of the chamber 34 via the first or second openings 56, 58. Accordingly, fluid within the first and second portions 48, 50 of the chamber 34 is such that no net force is exerted on the piston 30 by the fluid within the first and second portions 48, 50 of the chamber 34, the fluid volume 80 and the end of the piston 30 exposed to the atmosphere. Consequently, the piston is stationary.

Figure 5 also shows the actuator 26 with the piston 30 in the retracted position. However, the stepper motor 41 has rotated the spool 32 about the axis 44 in a first rotational direction. Therefore, owing to the helical nature of the inlet 64 and outlet slots 60, 62, the positions of the slots relative to the first 56 and second 58 openings has changed. In the arrangement shown in Figure 5, the first outlet slot 60 is in direct fluid communication with the first portion 48 of the chamber 34 via the first opening 56. Fluid therefore passes from the first outlet slot 60 into the first portion 48 of the chamber 34 via the first opening 56. The second outlet slot 62 is occluded by a wall of the housing portion 52 of the piston 30 such that the second outlet slot 62 is not in direct fluid communication with the second portion 50 of the chamber 34 via the second opening 58. Fluid therefore does not pass from the second outlet slot 62 into the second portion 50 of the chamber 34 via the second opening 58. The inlet slot 64 is in direct fluid communication with the second portion 50 of the chamber 34 via the second opening 58. Fluid therefore passes from the second portion 50 of the chamber 34 into the inlet slot 64 via the second opening 58, passes into the outlet channel 76 via the outlet passageway 78 and passes out of the actuator 26 via the servo return line 77.

The fluid in the inlet slot 64, the outlet passageway 78, the outlet channel 76 and the servo return line 77 has a lower pressure than the fluid in the first and second outlet slots 60, 62, the inlet passageway 74, the inlet channel 66 and the servo supply line 67.

Accordingly, the fluid within the first and second portions 48, 50 of the chamber 34 is such that the net force exerted on the piston 30 by the fluid within the first and second portions 48, 50 of the chamber 34, the fluid volume 80 and the end of the piston 30 exposed to the atmosphere is in a direction from the first portion 48 of the chamber 34 to the second portion 50 of the chamber 34 and the piston 30 moves along the axis 44 in a first direction 82 away from the first portion 48 of the chamber 34 and towards the second portion 50 of the chamber 34.

Figure 6 shows the actuator 26 after the stepper motor 41 has rotated the spool 32 about the axis 44 in a first rotational direction to its maximum extent. The piston 30 has been forced to the extended position shown in Figure 6 by the pressure of the fluid within the first portion 48 of the chamber 34 such that the first outlet slot 60 is again occluded by the wall of the housing portion 52 of the piston 30 and such that the first outlet slot 60 is no longer in direct fluid communication with the first portion 48 of the chamber 34 via the first opening 56. The second outlet slot 62 remains occluded by the wall of the housing portion 52 of the piston 30 such that the second outlet slot 62 is not in direct fluid communication with the second portion 50 of the chamber 34 via the second opening 58. The inlet slot 64 is again occluded by the wall of the housing portion 52 of the piston 30 such that the inlet slot 64 is not in direct fluid communication with the first or second portions 48, 50 of the chamber 34 via the first or second openings 56, 58. Accordingly, fluid within the first and second portions 48, 50 of the chamber 34 is such that no net force is exerted on the piston 30 by the fluid within the first and second portions 48, 50 of the chamber 34, the fluid volume 80 and the end of the piston 30 exposed to the atmosphere. Consequently, the piston is stationary.

Figure 7 also shows the actuator 26 with the piston 30 in the extended position. However, the stepper motor 41 has rotated the spool 32 about the axis 44 in a second rotational direction. In the arrangement shown in Figure 7, the second outlet slot 62 is now in direct fluid communication with the second portion 50 of the chamber 34 via the second opening 58. Fluid therefore passes from the second outlet slot 62 into the second portion 50 of the chamber 34 via the second opening 58. The first outlet slot 60 is occluded by a wall of the housing portion 52 of the piston 30 such that the first outlet slot 60 is not in direct fluid communication with the first portion 48 of the chamber 34 via the first opening 56. Fluid therefore does not pass from the first outlet slot 60 into the first portion 48 of the chamber 34 via the first opening 56. The inlet slot 64 is in direct fluid communication with the first portion 48 of the chamber 34 via the first opening 56. Fluid therefore passes from the first portion 48 of the chamber 34 into the inlet slot 64 via the first opening 56, passes into the outlet channel 76 via the outlet passageway 78 and passes out of the actuator 26 via the servo return line 77.

Accordingly, the fluid within the first and second portions 48, 50 of the chamber 34 is such that the net force exerted on the piston 30 by the fluid within the first and second portions 48, 50 of the chamber 34, the fluid volume 80 and the end of the piston 30 exposed to the atmosphere is in a direction from the second portion 50 of the chamber 34 to the first portion 48 of the chamber 34 and the piston 30 moves along the axis 44 in a second direction 83 away from the second portion 50 of the chamber 34 and towards the first portion 48 of the chamber 34. This will return the actuator 26 towards the position shown in Figure 2.

Figure 8 shows a flowchart of the abovementioned method of operating the actuator 26, divided into a first step Al, a second step A2 and a third step A3. The order and number of successive iterations of the second and third steps A2, A3 may be altered in order to provide the desired pattern of movement of the piston 30, and, thus, the engine variable vanes. The amount of rotation of the spool 32 may be varied in order to provide the desired amount of movement of the piston 30, and, thus, the vanes. The spool 32 therefore does not need to be rotated to its maximum and minimum rotational extent and can be rotated to positions between its maximum and minimum rotational extent in order to achieve piston positions between the maximum and minimum extents.

The supply and return pressures only communicate via leakage along a small radial gap formed between the spool 32 and the piston 30. Leakage between the volumes in the actuator 26 and out of the actuator 26 is limited by the dynamic seals 43, 45, 55, 68, 70. The drain channel 47 collects a portion of fluid that leaks from the second portion 50 of the chamber 34. Fluid within the drain channel 47 passes out of the actuator 26 via the drain outlet 49.

A position sensor (not shown) can be used to measure the position of the actuator 26 (e.g. the piston 30) or any other component in the actuated mechanism such that the position of the actuator 26 and engine vane angle is known. The position sensor may be a linear position sensor or a rotary position sensor. The position value produced by the position sensor can be used to ensure that the first outlet slot 60, the second outlet slot 62 and the inlet slot 64 are translated to their desired positions If electrical power is lost to the stepper motor 41, the piston 30 stays in a fixed position.

Any movement of the actuator 26 in this condition will open communication between the pressures within the slots 60, 62, 64 and the first and second portions 48, 50 of the chamber 34 such that the position of the actuator 26 is restored. Thus, the abovementioned arrangement maintains the actuator 26 in a fixed position when electrical power is lost.

The abovementioned arrangement results in a relatively compact, lightweight, simple, accurate and reliable actuator that provides high pressure gain. The high pressure gain results in high accuracy position control for an actuated mechanism with high levels of friction. Linear movement of the piston 30 is achieved without having translate the rotary motion of the stepper motor 41 into linear actuation of the spool 32. Rather, the rotary motion of the stepper motor can be directly imparted to rotate the spool, allowing the fluid pressures in the first and second portions 48, 50 to effect linear movement on the piston 30. Because components required to translate the rotary motion of the stepper motor to linear actuation of the spool 32 are not needed, the system is less complex and cheaper to construct, and more reliable.

Claims

CLAIMS: 1. An actuator (26), the actuator (26) comprising: a housing (28) defining a chamber (34); a piston (30) slidably received within the chamber (34) and configured for movement along an axis (44); and a spool (32) rotationally received within the piston (30) and configured for movement about the axis (44), wherein the piston (30) comprises a seal (46) separating a first portion (48) of the chamber (34) from a second portion (50) of the chamber (34), a first opening (56) that opens into the first portion (48) of the chamber (34) and a second opening (58) that opens into the second portion (50) of the chamber (34), wherein the spool (32) comprises a first outlet slot (60) for supplying fluid to the first portion (48) of the chamber (34) via the first opening (56), a second outlet slot (62) for supplying fluid to the to the second portion (50) of the chamber (34) via the second opening (58) and an inlet slot (64) disposed between the first outlet slot (60) and the second outlet slot (62) for receiving fluid from the first portion (48) of the chamber (34) via the first opening (56) and the second portion (50) of the chamber (34) via the second opening (58), wherein the first outlet slot (60), the second outlet slot (62) and the inlet slot (64) each extend both axially parallel to the axis (44) and circumferentially about the axis (44) such that rotation of the spool (32) about the axis (44) effects linear movement of the piston (30) along the axis (44).
2. The actuator (26) of claim 1, wherein the first outlet slot (60), the second outlet slot (62) and the inlet slot (64) are parallel.
3. The actuator (26) of claims 1 or 2, wherein the first outlet slot (60), the second outlet slot (62) and the inlet slot (64) are helical.
4. The actuator (26) of any preceding claim, wherein the width (Al) of the inlet slot (64) in an axial direction parallel to the axis (44) is less than or equal to the axial distance (A2) parallel to the axis (44) between the first opening (56) and the second opening (58).
5. The actuator (26) of any preceding claim, wherein the axial width (A3) of the first opening (56) in an axial direction parallel to the axis (44) is less than or equal to the axial distance (A4) parallel to the axis (44) between the inlet slot (64) and the first outlet slot (60).
6. The actuator (26) of any preceding claim, wherein the axial width (A5) of the second opening (58) in an axial direction parallel to the axis (44) is less than or equal to the axial distance (A6) parallel to the axis (44) between the inlet slot (64) and the second outlet slot (62).
7. The actuator (26) of any preceding claim, wherein the axial distance (A7) between the first outlet slot (60) and the second outlet slot (62) in an axial direction parallel to the axis (44) is greater than or equal to the maximum axial distance (A8) parallel to the axis (44) between the first opening (56) and the second opening (58).
8. The actuator (26) of any preceding claim, wherein the first outlet slot (60), the second outlet slot (62) and the inlet slot (64) extend in an axial direction parallel to the axis (44) only along a central portion of the spool (32).
9. The actuator (26) of any preceding claim, wherein the spool (32) defines at least part of an inlet channel (66), wherein the first outlet slot (60) and the second outlet slot (62) are fluidically connected to the inlet channel (66) by one or more inlet passageways (74) extending through the spool (32).
10. The actuator (26) of any preceding claim, wherein the spool (32) defines at least part of an outlet channel (76), wherein the inlet slot (64) is fluidically connected to the inlet channel (66) by one or more outlet passageways (78) extending through the spool (32).
11. The actuator (26) of claim 10, wherein a further chamber (80) is defined by an end of the spool (32) and an end of an interior of the piston (30) within which the spool (32) is rotationally received, wherein the further chamber (80) is fluidically connected to the one or more outlet passageways (78).
12. The actuator (26) of any preceding claim, wherein the piston (30) comprises a housing portion (52) within which the spool (32) is rotationally received and a flange portion (54) extending radially outwards from the housing portion (52), wherein the flange portion (54) defines the seal (46).
13. The actuator (26) of any preceding claim, further comprising a stepper motor (41) configured to effect movement of the spool (32) about the axis (44).
14. A hydraulic servo comprising the actuator (26) of any preceding claim.
15. A gas turbine engine (10) comprising the hydraulic servo of claim 14. 10
16. A method of operating the actuator (26) of any preceding claim, the method comprising: supplying fluid to the first outlet slot (60) and the second outlet slot (62); rotating the spool (32) in a first direction about the axis (44) such that the first outlet slot (60) is in direct fluid communication with the first portion (48) of the chamber (34) via the first opening (56), the inlet slot (64) is in direct fluid communication with the second portion (50) of the chamber (34) via the second opening (58) and the second outlet slot (62) is not in direct fluid communication with the second portion (50) of the chamber (34) such that fluid is supplied to the first portion (48) of the chamber (34) via the first opening (56), such that fluid is released from the second portion (50) of the chamber (34) via the second opening (58) and such that the piston (30) moves along the axis (44) in a first direction away from the first portion (48) of the chamber (34) and towards the second portion (50) of the chamber (34); and rotating the spool (32) in a second direction about the axis (44) such that the second outlet slot (62) is in direct fluid communication with the second portion (50) of the chamber (34) via the second opening (58), the inlet slot (64) is in direct fluid communication with the first portion (48) of the chamber (34) via the first opening (56) and the first outlet slot (60) is not in direct fluid communication with the first portion (48) of the chamber (34) such that fluid is supplied to the second portion (50) of the chamber (34) via the second opening (58), such that fluid is released from the first portion (48) of the chamber (34) via the first opening (56) and such that the piston (30) moves along the axis (44) in a second direction away from the second portion (50) of the chamber (34) and towards the first portion (48) of the chamber (34).
17. The method of claim 16, wherein rotation of the spool (32) in the first and second directions about the axis (44) is effected by a stepper motor (41).