Patent Publication Number: US-2023159155-A1

Title: Propeller blade pitch change actuation system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. 21290076.5 filed Nov. 22, 2021, the entire contents of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a pitch change actuation system for varying a propeller blade pitch. 
     BACKGROUND OF THE INVENTION 
     Propellers, for example aircraft propellers, typically comprise a plurality of blades fixed to a rotating propeller hub. Variable-pitch propellers are provided with a pitch change system that enables the blade pitch of each blade to be controlled. Variable-pitch propeller blades allow the angle of attack of the blade relative to the oncoming airflow to be varied. For example, the blade pitch can be adjusted from a feather position (with the blades parallel to the oncoming airflow—minimum thrust/drag) to a reverse position that may provide reverse thrust. 
     Propeller pitch change systems commonly use hydraulic actuation systems to control the pitch of the propeller blades. A typical hydraulic actuator comprises an hydraulic piston that is located axially forward of the propeller blade plane and housed within a piston sleeve. A cover is usually provided at a forward-most end such that a first chamber is formed between the cover, sleeve and piston and a second chamber is formed between the sleeve and piston, the piston separating the chambers. A transfer tube usually extends from the piston chambers axially aftward to a hydraulic fluid source such that hydraulic fluid can be supplied to the first and second chambers. 
     A controller controls the hydraulic fluid flow to selectively supply hydraulic fluid to the chambers depending on the desired blade pitch angle. The difference in hydraulic pressure between the two hydraulic chambers causes the piston to axially translate. Coupled to the piston is usually a piston shaft that extends aftwards towards the propeller blade plane and proximate to the propeller blades a pitch change kinematic couples the shaft to the propeller blades. This allows the pitch of the propeller blades to be varied by varying hydraulic flow and pressure within the two chambers. 
     Existing propeller pitch change actuation systems can be time consuming to maintain and are at risk of pollutants being introduced into the piston chamber if the cover is removed to perform maintenance. Furthermore, existing pitch change actuation systems can be heavy and complex. 
     There therefore remains a need for an improved design of a pitch change actuation system for controlling propeller blade pitch. 
     SUMMARY 
     According to a first aspect, there is provided a pitch change actuation system for varying a propeller blade pitch, the pitch change actuation system comprising: an actuator body defining an interior volume; a chamber separator located within the interior volume of the actuator body and dividing the interior volume into a first chamber and a second chamber, the two chambers being fluidly separated by the chamber separator; wherein the first and second chambers are configured to receive hydraulic fluid; wherein the actuator body is configured to translate relative to the chamber separator in response to a difference in hydraulic pressure between the first chamber and second chamber; and wherein the translation of the actuator body is configured to effect a change in propeller blade pitch. 
     The translation of the actuator body may be a linear movement along a longitudinal axis of the pitch change actuation system. Therefore, where “axially” is referred to, this will be understood as relating to this longitudinal axis. It will be well understood by the skilled person that the longitudinal axis of the pitch change actuation system extends in a direction from forward to aft. That is, the actuator body may move forward and aft along a longitudinal axis. 
     It will be appreciated that relative positional terms such as “forward” and “aft” are with reference to the normal operational attitude of the pitch change actuation system. The pitch change actuation system may be installable in a propeller assembly of an aircraft, the propeller assembly being driven by an engine shaft. 
     It can be understood that an aft end of the pitch change actuation system may be towards the engine shaft and the forward end may be away from said engine shaft. By way of further example, an aircraft will have a forward (i.e. front) end, and an aft (i.e. rear) end. In the case of an aircraft having a tractor propeller configuration (i.e. the propeller pulling the aircraft) the forward end of the pitch change actuation system is towards the front of the aircraft when installed thereon. In this case, the forward and aft end of the pitch change actuation system is in correlation with the forward and aft ends of an aircraft in which it is installable. 
     Conversely, in the case of an aircraft having a pusher propeller configuration (i.e. the propeller pushing the aircraft) the forward end of the pitch change actuation system is towards the rear of an aircraft when installed thereon. In either a tractor or pusher configuration of the propeller the aft end of the pitch change actuation system is towards the engine shaft, i.e. the shaft that drives the propeller. 
     The actuator body, responsive to the difference in hydraulic pressure between the first chamber and second chamber, may be configured to translate between a first position and a second position. In the first position the second chamber may have a minimum volume and the first chamber may have a maximum volume. In the second position the first chamber may have a minimum volume and the second chamber may have a maximum volume. One of the first and second actuator body positions may correspond to the provision of a reverse propeller blade pitch, with the other of the actuator body positions corresponding to the provision of a feathering propeller blade pitch, and intermediary actuator body positions corresponding to the provision of intermediary propeller blade pitches. 
     The first chamber may be known as the decrease pitch chamber, wherein an increase in volume of the first chamber corresponds to a translation of the actuator body that causes the pitch of the blades to decrease, i.e. to cause the pitch to become finer. Conversely, the second chamber may be known as the increase pitch chamber, wherein an increase in volume of the second chamber corresponds to a translation of the actuator body that causes the pitch of the blades to increase, i.e. to cause the pitch to become coarser and move towards the feather position. Therefore, the first actuator body position (i.e. in which the first chamber has a maximum volume) may correspond to a reverse propeller blade pitch. The second actuator body position (i.e. in which the second chamber has a maximum volume) may correspond to a feathering propeller blade pitch. Intermediary actuator body positions between the first and second actuator body position may correspond to intermediary propeller blade pitches. 
     Alternatively, the first chamber may be an increase pitch chamber, wherein an increase in volume of the first chamber corresponds to a translation of the actuator body that causes the pitch of the blades to increase, i.e. to cause the pitch to become coarser and move towards the feather position. Conversely, the second chamber may be a decrease pitch chamber, wherein an increase in volume of the second chamber corresponds to a translation of the actuator body that causes the pitch of the blades to decrease, i.e. to cause the pitch to become finer. Therefore, the first actuator body position (i.e. in which the first chamber has a maximum volume) may correspond to a feathering propeller blade pitch. The second actuator body position (i.e. in which the second chamber has a maximum volume) may correspond to a reverse propeller blade pitch. Intermediary actuator body positions between the first and second actuator body position may correspond to intermediary propeller blade pitches. 
     The first position of the actuator body may be an axially forward most position and the second position of the actuator body may be an axially aftward most position. The first chamber may be an axially forward most chamber and the second chamber may be an axially aftmost chamber. 
     The chamber separator may not translate in response to a difference in hydraulic pressure between the chambers. In other words it may be configured to be axially static with respect to a propeller hub in which the pitch change actuation system may be installed. The chamber separator may be in the form of a piston. This may be termed a fixed piston, e.g. because it does not translate in response to a difference in hydraulic pressure between the chambers, it is configured to be axially static with respect to a propeller hub in which the pitch change actuation system may be installed. The piston may take any typical known form, such as for example a substantially cylindrical segment. The actuator body may be considered as a piston sleeve. The chamber separator may be a yoke. 
     The actuator body may be configured to intersect a plane of the propeller blade(s) of a propeller assembly in which it is installable. In other words, the actuator body may be configured to intersect a plane of the propeller blade(s) when assembled in a propeller blade assembly. In other words, the pitch change actuation system may be configured, for example positionable in a propeller hub, such that the actuator body intersects a plane of blade(s) the pitch of which the pitch change actuation system varies. The pitch change actuation system may be configured such that the actuator body intersects a plane of the propeller blade at all times regardless of translation in response to a difference in hydraulic pressure. 
     Put another way, the actuator body may be configured to be located at least partially within a propeller blade plane. In other words, the actuator body may be configured to be located at least partially within a plane of blade(s) the pitch of which the pitch change actuation system varies. 
     The pitch change actuation system may comprise a shaft, for example a rod. This may extend axially from the chamber separator. It may extend axially in both forward and aft directions. The actuator body may be configured to translate with respect to the shaft. The actuator body may be configured to translate in a longitudinal direction of the shaft. The actuator body may be configured to translate along the shaft. The actuator body may be slidably engaged with the shaft. The actuator body may be slidably mounted on the shaft. The actuator body may slidably translate along the shaft with respect to the chamber separator. The shaft may extend axially from the chamber separator in a forward direction, an aft direction, or both. 
     The chamber separator and shaft may not be moveable with respect to each other, e.g. are in a fixed positional relationship. The chamber separator may be fastened to the shaft. The chamber separator and shaft may be integral with one another. The chamber separator may comprise a seal about its circumference to prevent hydraulic leakage between the first and second chambers. The seal may be a glide seal, slide ring, O-ring seal, an O-ring in combination with a back-up ring. The seal may be a dynamic seal. The seal may comprise a plurality of seals. 
     The shaft may extend through the first and/or second chambers. The chamber separator may be located at an axial mid-portion of the shaft. 
     The actuator body may comprise an aperture at an end of the actuator body and the shaft may extend through the aperture. The aperture may be a forward aperture at an axially forward end of the actuator body. The aperture may comprise a seal configured to prevent hydraulic leakage from within the actuator body. 
     The shaft may be configured to abut a propeller blade hub cover or other component of a propeller hub at an axially forward end of the shaft. The shaft may be configured to engage with a hub cover. 
     The shaft may be configured to be held axially static e.g. with respect to a propeller blade hub cover or other component of the propeller hub. The shaft may be configured to be fastened to a propeller hub cover. The shaft may be configured to receive a nut to fasten the shaft to the propeller hub cover. 
     The pitch change actuation system may comprise a transfer tube. The transfer tube may be coupled to the actuator body. A portion of the transfer tube may be received within the actuator body. The transfer tube may be configured to provide hydraulic fluid to the first and second chambers. The pitch change actuation system may comprise one or more fluid passageways to supply hydraulic fluid from the transfer tube to the first and/or second chambers. 
     The actuator body may comprise a coupling for coupling the transfer tube to the actuator body. The coupling may be configured to prevent relative axial displacement between the actuator body and transfer tube and prevent relative rotation between the actuator body and the transfer tube about a longitudinal axis of the pitch change actuation system. The coupling may be configured to allow the transfer tube to rotate about axes perpendicular to the longitudinal axis. 
     The coupling may be a soft coupling for receiving the transfer tube. The soft coupling may be configured to allow a limited number of degrees of freedom between the transfer tube and actuator body. The soft coupling may prevent relative axial displacement between the actuator body and the transfer tube and prevent relative rotation between the actuator body and the transfer tube about a longitudinal axis of the pitch change actuation system; but allow other degrees of freedom for example rotational movement about axes perpendicular to the longitudinal axis. Deflection may also be accommodated, e.g. such that instances where an axis of the actuator body and an axis of the transfer tube are not parallel can be accommodated. The soft coupling may be in the form of a ball joint. 
     The transfer tube may translate with the actuator body. 
     The shaft may comprise an interior fluid passageway configured to fluidly couple at least one of the first and second chambers to the transfer tube. The shaft may comprise fluid outlets to provide fluid from the interior fluid passageway to at least one of the first and second chambers. 
     The shaft may comprise one or more fluid outlets at a position axially forward of the chamber separator to fluidly couple the first chamber to the interior passageway. The shaft may comprise fluid outlets at a position axially aft of the chamber separator to fluidly couple the second chamber to the interior passageway. 
     The actuator body may comprise a fluid passageway that extends from the transfer tube to the first or second chamber such that the transfer tube and the first or second chamber are in fluid communication, in other words are fluidly coupled. 
     The interior passageway of the shaft may fluidly couple the first chamber to the transfer tube and the actuator body fluid passageway may fluidly couple the second chamber to the transfer tube. Alternatively, the interior passageway of the shaft may fluidly couple the second chamber to the transfer tube and the actuator body fluid passageway may fluidly couple the first chamber to the transfer tube. 
     The transfer tube may comprise one or more inlets configured to fluidly couple with one or more outlets of a transfer tube receiving component. The transfer tube may comprise two hydraulic fluid flow paths. The hydraulic fluid flow paths may be in communication with the one or more inlets. The first hydraulic fluid flow path of the transfer tube may be in fluid communication with the interior passageway of the shaft. The second hydraulic flow path of the transfer tube may be in fluid communication with the actuator body fluid passageway. 
     The pitch change actuation system may be configured to be installed and/or uninstalled at least partially within a propeller hub of a propeller assembly as a single module. In other words, the pitch change actuation system may be configured to be replaceable as a single unit. That is, the components forming the pitch change actuation system, for example the actuator body, chamber separator and transfer tube, may be configured to be inserted or removed as one assembly into or out of a propeller assembly. By way of further example, the transfer tube is not uncoupled from the actuator body to remove the pitch change actuation system from a propeller assembly. 
     In an embodiment, the pitch change actuation system may be a line replaceable unit (LRU). If the pitch change actuation system is an LRU then no further rigging is required between the actuator body and the transfer tube. 
     Translation of the actuator body is configured to effect a change in propeller blade pitch. This may be by the actuator body being configured to engage with a propeller blade pitch kinematic. For example, the actuator body may comprise a flange extending radially outwards of an outer surface of the actuator body. The flange may be for mechanically coupling the actuator body to a propeller blade pitch kinematic. In other words, the flange may extend radially outwards of the surface not defining the interior volume. The flange may be configured to receive a fastener to mechanically couple the actuator body to a propeller blade pitch kinematic. The flange may be configured to be received within a slot of the propeller blade pitch kinematic. The flange may comprise two flange portions which are configured to receive the propeller blade pitch kinematic therebetween. 
     The flange may be a circumferential flange, e.g. extending annularly around the actuator body. The flange may be configured to receive a fastener such as a threaded bolt. The flange may comprise an aperture to receive a fastener. The flange may comprise a plurality of apertures to receive a plurality of fasteners. The pitch change actuation system may comprise a fastener installed in the flange, the fastener and flange may be configured to mechanically couple a propeller blade pitch kinematic to the actuator body. The pitch change actuation system may comprise a plurality of fasteners installed in the flange, the fasteners and flange may be configured to mechanically couple a plurality of propeller blade pitch kinematics to the actuator body. 
     The propeller blade pitch kinematic may be an eccentric roller, a crank shaft and cam, roller and plate/bearing or other suitable mechanisms for converting linear motion into rotational motion as is known in the art. In other words, the propeller blade pitch kinematic may be any suitable mechanism for converting the translational motion of the actuator body to a rotational motion at the propeller blade root. Accordingly, axial translation of the actuator body directly corresponds to a change in pitch of the propeller blade. 
     The chamber separator may be located at an axial position between an axial position of the radially extending flange when the actuator body is in the first actuator body position and an axial position of the radially extending flange when the actuator body is in the second actuator body position. The chamber separator may be located at an axial position between an axially forwardmost position of the radially extending flange and an axially aftmost position of the radially extending flange. 
     The actuator body may comprise a first actuator body portion and a second actuator body portion. The first and second actuator body portions may be coupled together to define the interior volume. The first and second actuator body portions may each comprise a flange. The first actuator body portion and the second actuator body portion may be fastened to one another via at least one fastener extending through the flanges of the first and second actuator body portions. 
     The first actuator body portion may be a forward actuator body portion and the second actuator body portion may be an aft actuator body portion. The forward actuator body portion flange may be at a location proximate the aft end of the forward actuator body portion. The aft actuator body portion flange may be at a location proximate the forward end of the aft actuator body portion. The forward actuator body portion and the aft actuator body portion may be fastened to one another via at least one fastener extending through the forward actuator body portion flange and the aft actuator body portion flange. 
     According to a second aspect, a propeller assembly is provided comprising: a propeller hub; a propeller blade mounted to the propeller hub; and a pitch change actuation system as described above. 
     The pitch change actuation system may be configured to receive hydraulic fluid from a hydro-mechanical power and control assembly. 
     The actuator body may intersect a plane of the propeller blade. Put another way, the actuator body may be located at least partially within a plane of the propeller blade. The actuator body may intersect a plane of the propeller blade at all times regardless of translation in response to a difference in hydraulic pressure. 
     A portion of the actuator body may be configured to translate through a plane of the propeller blade in response to the difference in hydraulic pressure. 
     The actuator body, responsive to the difference in hydraulic pressure between the first chamber and second chamber, may be configured to translate between a first position and a second position. In the first position the second chamber may have a minimum volume and the first chamber may have a maximum volume. In the second position the first chamber may have a minimum volume and the second chamber may have a maximum volume. The pitch change actuation system may be positioned in the propeller hub such that during translation between the first and second actuator body positions, there is a position of the actuator body in which at least one of the chambers has a portion intersecting the blade plane. 
     The plane of the propeller blade (or the “propeller blade plane”) may be defined as a plane that extends perpendicular to the longitudinal axis of the pitch change actuation system at an axial position corresponding to the axial position of the centreline of propeller blade retention, i.e. the axis of blade pitch rotation. 
     The pitch change actuation system may be at least partially installed within the propeller hub. A hub cover may be provided to cover an axially forward end of the hub. The hub cover may therefore seal the hub. The hub cover may be integral with the propeller hub. The hub cover may be screwed to the hub. The hub cover may be a separate component to the pitch change actuation system. The hub cover may therefore not form part of the actuator body. The shaft may abut or otherwise engage with the propeller hub cover. The hub cover may be coupled to the shaft. 
     The actuator body is configured to translate relative to the chamber separator. The chamber separator may be considered to be static with respect to the propeller hub in which the pitch change actuation system is installed. Therefore the actuator body may be considered to translate with respect to the propeller hub. 
     The propeller assembly may comprise a longitudinal axis about which propeller blade(s) are mounted, i.e. an axis of propeller rotation. This may be termed the centreline of the propeller assembly. The actuator body may translate along this longitudinal axis of the propeller assembly. The actuator body may translate perpendicular to the propeller blade(s). 
     The propeller assembly may comprise a propeller blade pitch kinematic, for example as described previously above. The actuator body may be engaged with the propeller blade pitch kinematic. The actuator body may comprise a flange extending radially outwards of an outer surface of the actuator body. The flange may be coupled, e.g. fastened, to the propeller blade pitch kinematic. The propeller blade pitch kinematic may be coupled to a root of the propeller blade, optionally via a blade root pin. Therefore translation of the actuator body will cause translation of the kinematic, which converts the translational motion into rotation of the blade root coupled thereto. 
     It will be appreciated that although the propeller assembly has been described in relation to one propeller blade, typically the propeller assembly will comprise multiple propeller blades and the pitch change actuation system varies the pitch of the multiple propeller blades. Each propeller blade may have an associated propeller blade pitch kinematic for example as described above. The actuator body may be engaged with each propeller blade pitch kinematic, optionally via a flange of the actuator body that is coupled to each of the propeller blade pitch kinematics. 
     The propeller assembly may form part of a propeller system. Thus there is also provided a propeller system comprising the propeller assembly and a hydro-mechanical power and control assembly. 
     The hydro-mechanical power and control assembly may be mounted on a static part of the propeller system, for example the part comprising an engine. The hydro-mechanical power and control assembly may comprise multiple components located at different parts of the static part of the propeller system or aircraft to which it is mounted. The multiple components may be interconnected. The hydro-mechanical power and control assembly may comprise a hydraulic fluid reservoir or may be configured to receive hydraulic fluid from the engine side of the aircraft. 
     Where “static” is used herein, it will be appreciated that this is intended to differentiate from a rotary part of the propeller system. Of course, a vehicle having the propeller system may be moving and thus the “static” part of the propeller assembly will be moving with the vehicle and not truly static (stationary). 
     The propeller system may include a transfer tube receiving component configured to receive an end of the transfer tube. The end received in the transfer tube receiving component may be the opposite end to that coupled to the actuator body. The transfer tube receiving component may be configured to receive the transfer tube via an aperture. The transfer tube receiving component may be configured to receive the transfer tube without requiring fasteners. The transfer tube receiving component may be configured to receive hydraulic fluid from the hydro-mechanical power and control assembly and supply it to the transfer tube. 
     The transfer tube receiving component may be a component of the hydro-mechanical power and control assembly. The transfer tube receiving component may be configured to supply the hydraulic fluid to the first hydraulic fluid flow path and the second hydraulic fluid flow path of the transfer tube to create a desired pressure differential between the first and second chamber of the interior volume of the actuator body. The transfer tube receiving component may be configured to supply the hydraulic fluid to the first hydraulic fluid flow path and the second hydraulic fluid flow path responsive to a control input from a propeller electronic control unit which may be part of the hydro-mechanical power and control assembly. The propeller assembly may comprise such a propeller electronic control unit located on a static part of the propeller assembly. The transfer tube receiving component may comprise one or more outlets that fluidly couple with one or more inlets of the transfer tube. 
     The propeller system may include a sensor configured to detect the axial position of the transfer tube to determine the actuator body position and hence the propeller blade pitch. A plurality of sensors may be provided. The sensor(s) may include at least one of a Rotary Variable Differential Transformer (RVDT) sensor, a Linear Variable Differential Transformer (LVDT) sensor, a hall effect sensor, or a proximity sensor. 
     The sensor may be mounted to a static portion of the propeller system. The sensor may be installed proximate an aft end of the transfer tube. The sensor may be configured to measure the position of the transfer tube relative to the static portion of the propeller system. 
     The sensor may be configured to send data indicating the transfer tube position to the hydro-mechanical power and control assembly. The hydro-mechanical power and control assembly may be configured to determine the current propeller blade pitch angle based on the axial position of the transfer tube. The hydro-mechanical power and control assembly may be configured to compare the current propeller blade pitch angle to a desired propeller blade pitch angle and adjust the flow and pressures of the hydraulic fluid provided to the first and second chambers accordingly. 
     The propeller assembly may be mounted to an aircraft. The propeller system may be mounted to an aircraft. Thus there is also provided an aircraft comprising the propeller assembly as described above. An aircraft is also provided comprising the propeller assembly as described above and a hydro-mechanical power and control assembly, or the aircraft may comprise the propeller system as described above. 
     According to a third aspect, a method of installing a pitch change actuation system as described above in a propeller assembly is provided, the method comprising inserting the pitch change actuation system into the propeller hub. 
     The method may comprise inserting, optionally sliding, the pitch change actuation system into the propeller hub as a single integrated module. 
     The method may comprise removing a propeller hub cover of a propeller assembly prior to inserting the pitch change actuation system into the propeller hub. The pitch change actuation system may be inserted from a forward end. The propeller hub cover may be subsequently reinstalled. 
     The method may further comprise fastening a nut to the shaft of the pitch change actuation system to secure the shaft to the propeller hub cover. 
     The method may comprise inserting the pitch change actuation system into the propeller hub from an aft end of the propeller hub, for example, when a propeller hub cover is an integral component of the propeller hub. 
     The method may comprise coupling a flange of the actuator body to a propeller blade pitch kinematic. Optionally the coupling may comprise fastening with at least one fastener. 
     The method may further comprise mounting a propeller blade hub cover over the pitch change actuation system. Preferably the hub cover is screwed to the hub. The hub cover may be coupled to the shaft. The method may comprise fastening a nut to the shaft of the pitch change actuation system such that the shaft is fastened to the hub cover. The hub cover may seal the hub. 
     Installing a pitch change actuation system in a propeller assembly may include inserting an end of the transfer tube into a transfer tube receiving component of the propeller assembly. The transfer tube may be slid into the transfer tube receiving component. The transfer tube may be rotatable within the transfer tube receiving component. The transfer tube receiving component may be configured to supply hydraulic fluid into the transfer tube when the transfer tube is received. Inserting the transfer tube into the transfer tube receiving component may comprise axially inserting the transfer tube into an aperture of the transfer tube receiving component and may not require any fasteners. 
     Installing a pitch change actuation system in a propeller assembly may include locating the actuator body at least partially within a plane of the blade(s) of the propeller assembly. Installing a pitch change actuation system in a propeller assembly may include locating the actuator body to intersect a plane of the propeller blade(s). 
     Installing a pitch change actuation system in a propeller assembly may include aligning the flange of the actuator body with the propeller blade pitch kinematic. 
     Installing a pitch change actuation system in a propeller assembly may include aligning the shaft of the pitch change actuation system such that a nut can be fastened thereto to secure the shaft to the propeller blade hub cover. 
     Also disclosed is a method of controlling the blade pitch of a variable pitch propeller utilising the pitch change actuation system as described above. 
     According to a fourth aspect, a method of controlling the blade pitch of a variable pitch propeller is provided, the method comprising: varying the difference in hydraulic pressure acting on either side of a chamber separator such that an actuator body enclosing the chamber separator translates relative to the chamber separator, wherein the translation of the actuator body effects a change in propeller blade pitch. 
     The method may include providing hydraulic fluid to either side of the chamber separator via a transfer tube. 
     The actuator body may translate along a shaft that extends axially from the chamber separator. The actuator body may slidably translate along the shaft. 
     Optionally, at least a portion of the actuator body may translate through a plane of the propeller blade(s). A chamber may be provided on each side of the chamber separator. One of the chambers may be configured to translate through the blade plane in response to the difference in hydraulic pressure. 
     The method may include retaining the chamber separator in an axially fixed position relative to a propeller assembly having the variable pitch propeller. 
     The method may include providing hydraulic fluid from the transfer tube to a first chamber bounded by the actuator body and chamber separator via an interior passageway of a shaft that extends from the chamber separator. The method may include providing hydraulic fluid from the transfer tube to a second chamber bounded by the actuator body and chamber separator via an interior passageway of the actuator body. 
     The method may include translating the actuator body and transfer tube together. 
     The method may include sensing the translation of the actuator body. The method may include sensing translation of the transfer tube. Based on the translation of the actuator body or the transfer tube the method may comprise calculating the corresponding propeller blade pitch. 
     The method may include comparing the current propeller blade pitch to a desired propeller blade pitch and varying the flow and pressures of hydraulic fluid within the first and second chambers accordingly to achieve the desired propeller blade pitch. 
     The method of the fourth aspect may be performed by providing and/or using the pitch change actuation system of the first aspect, the propeller assembly of the second aspect, or the propeller system described above. The previously described features are applicable, as appropriate, to both the pitch change actuation system of the first aspect, the propeller assembly of the second aspect, the propeller system described above, and the method of controlling the pitch of a variable pitch propeller of the fourth aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some exemplary embodiments of the present disclosure will now be described by way of example only and with reference to the accompanying drawings, in which: 
         FIG.  1    shows a propeller system comprising a pitch change actuation system according to the prior art; and 
         FIG.  2    shows a pitch change actuation system according to an embodiment of the present disclosure with a propeller system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a propeller system  70  comprising a propeller assembly  20  comprising a pitch change actuation system  10  according to the prior art. 
     A centreline (longitudinal) axis A-A of the propeller assembly  20  is shown about which components of the propeller assembly are mounted. For example, propeller blades of the propeller assembly are mounted about the axis A-A. Reference to axially forward is understood to mean along axis A-A in the direction of the left hand side of the figure and aft is understood to mean along axis A-A in the direction of the right hand side of the figure. Reference to a radial direction is understood to refer to extending perpendicularly from axis A-A, for example away from axis A-A towards the top and bottom of the figure. 
     The propeller assembly  20  comprising a rotary hub  30  is generally configured to rotate propeller blades to propel a vehicle, such as an aircraft, to which the propeller assembly  20  is mounted. In this example the propeller assembly  20  comprises propeller blades with variable pitch, which allows control of the angle of attack of the blade relative to the oncoming airflow. The propeller blades each have a blade root pin  50  configured to interface with a pitch change actuation system such that the angle of the blade can be adjusted. For example, the blade pitch can be adjusted from a feather position (with the blades parallel to the oncoming airflow—minimum thrust/drag) to a reverse position that may provide reverse thrust. 
     To adjust the pitch of the propeller blades the pitch change actuation system  10  is provided within the rotary hub  30 . The pitch change actuation system  10  comprises a piston  40  located within a piston chamber  60 , an actuator shaft  80  coupled to and extending from the piston  40 , an actuator flange  100  extending from the actuator shaft  80 , and a transfer tube  120  extending from the piston chamber  60 . 
     The transfer tube  120  receives hydraulic fluid from a transfer tube receiving component  140  of the propeller system  70  and supplies hydraulic fluid to a first and a second side of piston  40 . To supply hydraulic fluid to either side of the piston  40  the transfer tube  120  comprises two concentric hydraulic fluid passageways. The first inner hydraulic fluid passageway  160  is configured to supply hydraulic fluid from the transfer tube receiving component  140  to the first side of the piston  40 . The second outer hydraulic fluid passageway  180  is configured to supply hydraulic fluid from the transfer tube receiving component  140  to the second side of the piston  40 . 
     To vary the propeller blade pitch, a hydro-mechanical power and control assembly of the propeller system  70  in combination with the transfer tube receiving component  140  cause a pressure difference across the two sides of the piston  40  by altering the hydraulic fluid flow and pressure supplied to the first and second side of the piston  40 . This causes the piston  40  to translate either axially forward or aft dependent on the pressure difference across the two sides of the piston  40 . Translation of the piston  40  causes the actuator shaft  80  and the actuator flange  100  to also translate as each of the piston  40 , actuator shaft  80  and actuator flange  100  are mechanically coupled to one another. The actuator flange  100  is configured to interface with a propeller blade pitch kinematic  200  which converts the translation of the actuator flange  100  into a rotational motion of a propeller blade, i.e. to change the pitch. Typically, multiple blades are provided, each with a propeller blade pitch kinematic, and the actuator flange interfaces with each propeller blade pitch kinematic. 
     In this example of an existing pitch change actuation system  10  the piston chamber  60  is bounded by portions of a propeller blade hub  220  and a chamber cover  240  and is located axially forward of the plane in which the propeller blades are mounted. This means that the actuator shaft  80  is required to extend from the piston  40  to the propeller blade plane (indicated by line P-P) such that translation of the piston  40  can be used to vary the propeller blade pitch at the root of the propeller blades via the actuator flange  100 . The actuator shaft  80  and actuator flange  100  add weight and complexity to the pitch change actuation system  10 . Furthermore, to remove the pitch change actuation system  10 , for instance during regular maintenance, it is necessary to remove chamber cover  240  such that the pitch change actuation system  10  can be extracted from the propeller blade assembly  20 . In this case the piston chamber is placed at risk of contaminants inadvertently being introduced which may damage or reduce the performance of the system. 
     The embodiment of the present disclosure shown in  FIG.  2    provides an improved pitch change actuation system  310  of a propeller assembly  320 , which is part of a propeller system  370 . 
     A centreline (longitudinal) axis A-A of the propeller assembly  320  is shown about which components of the propeller assembly are mounted. In other words, this is the axis of propeller rotation. For example, propeller blades (not visible in  FIG.  2   ) of the propeller assembly are mounted about the axis A-A. Reference to axially forward is understood to mean along axis A-A in the direction of the left hand side of the figure and aft is understood to mean along axis A-A in the direction of the right hand side of the figure. Reference to extending radially is understood to refer to extending perpendicularly from axis A-A, for example away from axis A-A towards the top and bottom of the figure. 
     The propeller assembly  320  comprising a rotary hub  330  is generally configured to rotate propeller blades to propel a vehicle such as an aircraft to which the propeller assembly  320  is mounted. In this example the propeller assembly  320  comprises propeller blades with variable pitch, which allows control of the angle of attack of the blade relative to the oncoming airflow. The propeller blades each have a blade root pin  350  configured to interface with the pitch change actuation system such that the angle of the blade can be adjusted. For example, the blade pitch can be adjusted from a feather position (with the blades parallel to the oncoming airflow—minimum thrust/drag) to a reverse position that may provide reverse thrust. 
     To adjust the pitch of the propeller blades the pitch change actuation system  310  is provided within the rotary hub  330 . The pitch change actuation system comprises: an actuator body  340  that defines an interior volume  360 ; a chamber separator  380  in the form of a fixed piston located within said interior volume  360 , which divides the interior volume into a first chamber  360   a  and a second chamber  360   b ; and a transfer tube  420 . 
     In this embodiment the actuator body  340  is formed of two portions, a first actuator body portion  340   a  and a second actuator body portion  340   b . The two portions being coupled together at respective interfaces such that the first actuator body portion  340   a  is axially forward of the second actuator body portion  340   b.    
     A shaft  500  extends both axially forward and aft from the chamber separator  380 . In the axially forward direction the shaft  500  extends through an aperture  342  in the first actuator body portion  340   a . In the axially rearward direction the shaft  500  extends into the second actuator body portion  340   b . At the axially forward end of the shaft  500  the shaft  500  is mechanically coupled to a propeller hub cover  520  such that the shaft  500  and the chamber separator  380  are held axially fixed with respect to the propeller assembly  320 . The actuator body portions  340   a ,  340   b  are configured to axially translate, in this case slide, along the shaft  500 . In other words the actuator body  340  translates with respect to the shaft  500 . In this embodiment the shaft  500  and chamber separator  380  are integrally formed as one piece. 
     An axially forward end of the transfer tube  420  is received within the second actuator body portion  340   b  at an axially aft position thereof. Hence, the transfer tube  420  is linked to the actuator body  340  and translates therewith. The second actuator body portion  340   b  comprises a ball joint coupling (not shown) that connects the transfer tube  420  to the actuator body  340  and prevents at least relative axial displacement between the actuator body  340  and transfer tube  420 , and prevents at least relative rotation between the actuator body  340   b  and the transfer tube  420  about a longitudinal axis of the pitch change actuation system  310 , but allows rotation about the axes perpendicular to the longitudinal axis. 
     An axially aft end of the transfer tube  420  is received within a transfer tube receiving component  440  of the propeller system  370 . In this embodiment the transfer tube receiving component  440  is a component of a hydro-mechanical power and control assembly illustrated schematically by a box  450 . However it will be appreciated that the hydro-mechanical power and control assembly can comprise multiple interconnected components located in different parts of the static part of the propeller system or aircraft to which it is mounted. The transfer tube  420  receives hydraulic fluid from the transfer tube receiving component  440  and supplies hydraulic fluid to both a fluid passageway  540  within the shaft  500  and to a fluid passageway  560  within the second actuator body portion  340   b . The fluid passageway  540  within the shaft communicates the hydraulic fluid received from the transfer tube  420  to the first chamber  360   a  via apertures in the shaft  500  not illustrated in  FIG.  2   . The apertures in the shaft  500  may be axially forward of the chamber separator  380  such that the fluid passageway  540  and the first chamber  360   a  are in fluid communication. The fluid passageway  560  within the second actuator body portion  340   b  communicates the hydraulic fluid received from the transfer tube  420  to the second chamber  360   b.    
     To supply hydraulic fluid to the fluid passageway  540  within the shaft  500  and the fluid passageway  560  within the second actuator body portion  360   b  the transfer tube  420  comprises two concentric hydraulic fluid passageways. In other embodiments the two hydraulic fluid passageways may be non-concentric within the transfer tube. The first inner hydraulic fluid passageway  460  is configured to supply hydraulic fluid from the transfer tube receiving component  440  to the fluid passageway  540  within the shaft  500 . The second outer hydraulic fluid passageway  480  is configured to supply hydraulic fluid from the transfer tube receiving component  440  to the fluid passageway  560  within the second actuator body portion  360   b.    
     The actuator body  340  comprises a radially extending flange  346  which is connected to a propeller blade pitch kinematic  200 . The propeller blade pitch kinematic  200  is coupled to a blade tulip of the propeller blade via the blade root pin  350  and converts translation of the actuator body  340  into a rotational motion of the blade tulip. This rotational motion thereby alters the pitch of the blade. Whilst not illustrated the flange is mechanically coupled to the propeller blade pitch kinematic  200  via a fastener. Typically, multiple blades are provided, each with an associated propeller blade pitch kinematic. The flange  346  will be coupled to all the kinematics so as to effect pitch change of all blades. 
     The propeller system  370  comprises a sensor (not illustrated) mounted to a static portion of the propeller system  370  at an aft end of the transfer tube  420 . 
     To vary the propeller blade pitch, the hydro-mechanical power and control assembly  450  in combination with the transfer tube receiving component  440  cause a pressure difference across the two sides of the chamber separator  380  by altering the flow and pressure of the hydraulic fluid supplied to the first and second chambers  360   a ,  360   b  via the transfer tube  420 . Because the chamber separator  380  is held axially fixed, e.g. via the shaft  500  being mechanically coupled to the propeller hub cover  520 , the pressure difference causes the actuator body  340  to translate axially either forward or aft along the shaft  500 . The actuator body can translate between a first position axially forwardmost, and a second position axially aftmost.  FIG.  2    illustrates the axially forwardmost position. The radially extending flange  346  of the actuator body  340  is configured to interface with a propeller blade pitch kinematic  200  which converts the translation of the actuator body  340  into a rotational motion of the propeller blade, i.e. pitch change. In other words, the blade tulip is connected to the moving actuator body  340  via the flange  346  of the actuator body and the kinematic  200 . 
     The hydro-mechanical power and control assembly  450  is configured to determine the current propeller blade angle based on the axial position of the transfer tube  420  sensed by the sensor. The hydro-mechanical power and control assembly  450  is further configured to compare the current propeller blade pitch angle to a desired propeller blade pitch angle and to adjust the flow and pressures of the hydraulic fluid provided to the first and second chambers  360   a ,  360   b  accordingly. The desired blade pitch may for example be selected by the pilot or be determined automatically for example based on a desired speed of the aircraft. 
     In the pitch change actuation system  310  of the present embodiment, the actuator body  340  intersects the plane P-P of the propeller blades of the propeller assembly  320 . 
     Since the actuator body  340  translates relative to the chamber separator  380 , i.e. the inverse of a typical piston and cylinder arrangement, the actuator body  340  can be located within, i.e. so as to intersect, the propeller blade plane indicated by line P-P and accordingly minimal linkages are required to mechanically couple the actuator body  340  to the propeller blade pitch change kinematic  200 . In other words, long and complicated shafts and flanges aren&#39;t required to extend from the component actuated by hydraulic fluid to transfer said actuation to the propeller blade pitch change kinematic. Instead, a comparably direct link can be provided between the actuator body  340  and the propeller blade pitch change kinematic  200 . In addition, since the pitch change actuation system  310  intersects the blade plane, the transfer tube  420  can be shorter than in the prior art systems where the transfer tube needs to extend to the front of the propeller hub. Again, this reduces cost and complexity. Furthermore, this means that the volume of actuating fluid in the system can be reduced which in turn reduces the weight of the system. 
     Advantageously, the actuator body  340  being located within the blade plane (e.g. by means of the disclosed arrangement in which the actuator body translates) both reduces the weight of the pitch change actuation system  310  as well as moves the centre of gravity of the pitch change actuation system  310  to a more desirable position when compared to existing pitch change actuation systems. In other words, embodiments of the present disclosure provide a system that does not require a pitch change actuator being located axially forward of the propeller blades and hence does not require the heavy and complex linkages seen in the existing art that couple the actuator to the propeller blade kinematic. Thus, the linkage between the actuator and the propeller blade pitch kinematic in embodiments of the present disclosure (e.g. the flange  346  directly connected to the blade kinematic  200 ) is more compact and therefore lighter than the prior art. This offers a weight saving which is clearly advantageous in the context of aircraft, thereby reducing fuel consumption and consequently cost. Furthermore, the simpler connection utilises less parts than the prior art linkage arrangement, again resulting in reduced cost. The more compact design is also stiffer and more robust, which improves the life of the dynamic seals. 
     A further advantage provided by embodiments of the present disclosure includes that the pitch change actuation system  310  can be installed or uninstalled as a single module. That is the actuator body  340 , chamber separator  380  and transfer tube  420  can be inserted or removed as a single module. No further rigging between the actuator body  340  and the transfer tube  420  is required. This provides for easier maintenance of a propeller assembly  320  as fewer components need to be disassembled or reassembled to install or uninstall the pitch change actuation system  310 . This reduces the time required for maintenance, and therefore reduces maintenance cost. In this embodiment the pitch change actuation system  310  is a line replaceable unit, however it will be appreciated that in other embodiments the pitch change actuation system  310  may not be a line replaceable unit, whilst still being a single module. 
     Furthermore, as the pitch change actuation system  310  can be installed or uninstalled as a single module the first and second chambers  360   a ,  360   b  remain enclosed by the actuator body  340  during installation or uninstallation of the pitch change actuation system  310 . That is, the first and second chambers  360   a ,  360   b  do not need to be exposed to the external environment during maintenance. Therefore, the risk of pollutants or contaminants entering the first and second chamber  360   a ,  360   b  from the external environment during maintenance is reduced. This has the advantage of reducing the wear to dynamic seals which can be caused by pollution in the chamber. Dynamic seals may be located between the shaft  500  and the actuator body  340  as well as between the chamber separator  380  and the actuator body  340 . Thus the wear of these seals may be reduced by the pitch change actuation system  310  provided. 
     It can therefore be seen that the pitch change actuation system of embodiments of the disclosure offers a simplified architecture providing significant advantages over the prior art. 
     The term “about” is intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof. 
     While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.