Patent Publication Number: US-2020298959-A1

Title: Combined overspeed, feathering, and reverse enabler control valve for a propeller assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of priority to Italian patent application No. 102019000003999, filed Mar. 19, 2019. Italian patent application No. 102019000003999 is hereby incorporated by reference in its entirety. 
     FIELD 
     The subject matter of the present disclosure is related generally to propeller control units. 
     BACKGROUND 
     Variable pitch propeller assemblies for aircraft are operatively configured to adjust propeller blades of the propeller assembly through a plurality of blade angles. In this manner, the propeller blades can be adjusted to a propeller blade angle that optimizes engine performance for given flight conditions or for ground operations. To adjust the propeller blade angle of the propeller blades, variable pitch propeller assemblies typically include a pitch control unit. Certain pitch control units can include a primary pitch control valve or governor. Based on one or more input signals, the primary control valve selectively allows an amount of hydraulic fluid to flow to or drain from a pitch actuation assembly positioned within the propeller assembly. By altering the amount of hydraulic fluid in the pitch actuation assembly, the blade angle of the propeller blades can be set to the desired pitch. 
     For constant-speed variable pitch propeller assemblies, the pitch control unit is configured to maintain constant engine speed by adjusting the propeller blade angle to vary the load on the propeller in response to changing flight conditions. In particular, the primary control valve modulates the pitch of the propeller blades to keep the reference speed. In some instances, the propeller assembly can experience an overspeed condition, which occurs when propeller RPM increases above the reference speed, and in some instances, the propeller assembly can experience an underspeed condition, which occurs when propeller RPM decreases below the reference speed. When an overspeed or underspeed condition is experienced, the primary control valve controls the flow of hydraulic fluid through the system such that the propeller assembly returns to an onspeed condition, or a condition in which the actual RPM of the engine is the same as the reference speed. 
     In some cases, however, the primary control valve can fail or can be unresponsive when the propeller assembly experiences an overspeed condition. To prevent the propeller assembly from reaching a destructive overspeed condition, pitch control units typically include an overspeed governor. Overspeed governors intervene when the propeller speed reaches an overspeed reference value typically higher than the reference speed and adjust pitch in a manner that overrides the primary control valve in a coarsening direction and governs to the overspeed reference value. In the past, overspeed governors have typically been mechanical devices (e.g., flyweight governors). However, such conventional mechanical overspeed governors include many parts, increase the weight of the engine, and typically include overspeed testing components (e.g., solenoid test valves) for ensuring proper operation of the overspeed governor. The weight of the overspeed governor and overspeed testing components are penalties on the efficiency of the engine. 
     Moreover, some variable pitch propeller assemblies are configured as feathering propeller assemblies. Such feathering propeller assemblies typically include a solenoid-operated feather valve. The solenoid-operated feather valve is operatively configured to switch the propeller assembly into a feather mode. The feathering mode can be commanded by a pilot by a dedicated cockpit switch, can be commanded by an engine controller after a normal shutdown, or can be commanded automatically by the engine controller (i.e., autofeather) when an engine flames out or an unexpected sudden reduction of power is detected. Such conventional solenoid-operated feather valves and accompanying sensing components can increase the weight of the engine, which is a penalty on the efficiency of the engine. 
     In addition, some variable pitch propeller assemblies include ground beta or reverse mode functionality. For instance, some propeller assemblies include a ground beta enable solenoid and a ground beta enable valve that effectively enable the propeller blades to move to a fine pitch position, e.g., for taxiing on the ground, or a reverse angle, e.g., for reverse and braking. These conventional solenoids and valves can increase the weight of the engine, which is a penalty on the efficiency of the engine. 
     Therefore, there is a need for improved propeller assemblies and/or methods therefore that address one or more of these challenges. 
     BRIEF DESCRIPTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In one aspect, a variable pitch propeller assembly for an engine defining an axial direction, a radial direction, and a circumferential direction is provided. The variable pitch propeller assembly includes a plurality of propeller blades rotatable about the axial direction and spaced apart along the circumferential direction. Each propeller blade is rotatable through a plurality of blade angles about respective pitch axes each extending in the radial direction. Further, the variable pitch propeller assembly includes a pitch actuation assembly for adjusting the plurality of propeller blades through the plurality of blade angles. The variable pitch propeller assembly also includes a pitch control unit. The pitch control unit includes a primary control valve operable to selectively allow a flow of hydraulic fluid to or from the pitch actuation assembly. The pitch control unit also includes a secondary control valve adjustable between a constant speed mode, a feather mode, and a reverse mode and operable to selectively allow a flow of hydraulic fluid to or from the pitch actuation assembly based at least in part on the mode of the secondary control valve. 
     In some embodiments, the secondary control valve has a valve body defining a chamber and a spool movable within the chamber, and wherein the spool is movable between a plurality of constant speed positions in the constant speed mode, one or more feather positions in the feather mode, and one or more reverse positions in the reverse mode to enable the plurality of propeller blades to rotate to a negative blade angle. 
     In some embodiments, the secondary control valve is an electrohydraulic servovalve (EHSV). 
     In some embodiments, the pitch actuation assembly includes a cylinder defining a chamber and a control piston translatable within the cylinder. Further, the pitch actuation assembly includes a piston rod connected to the control piston and extending into a propeller gear box of the engine, the piston rod translatable in unison with the control piston. Further, the pitch actuation assembly includes an oil transfer bearing surrounding the piston rod within the propeller gear box of the engine and defining a flight gallery fluidly connected with the secondary control valve and a ground gallery fluidly connected with the secondary control valve. Moreover, the pitch actuation assembly includes a beta tube enclosed within the piston rod and fluidly connecting the flight gallery with the chamber of the cylinder. 
     In some embodiments, when the secondary control valve is adjusted to the feather mode, the secondary control valve selectively allows the flow of hydraulic fluid to flow from the chamber of the cylinder to the secondary control valve. 
     In some embodiments, when the secondary control valve is adjusted to the reverse mode, the secondary control valve selectively allows the flow of hydraulic fluid to flow from the secondary control valve to the chamber and from the secondary control valve to the ground gallery. 
     In some embodiments, when the secondary control valve is adjusted to the constant speed mode, the secondary control valve selectively allows the flow of hydraulic fluid to flow between the chamber and the secondary control valve to maintain an onspeed condition. 
     In some embodiments, a flight gallery conduit fluidly connects the secondary control valve with the flight gallery and a ground gallery conduit fluidly connects the secondary control valve with the ground gallery. 
     In some embodiments, the primary control valve is an electrohydraulic servovalve (EHSV). 
     In another aspect, a method for controlling a variable pitch propeller assembly driven by a powerplant using a propeller control system is provided. The powerplant defines an axial direction and a radial direction and includes a controller. The variable pitch propeller assembly has a plurality of propeller blades rotatable about the axial direction and adjustable about respective pitch axes each extending along the radial direction. Further, the propeller control system includes a pitch actuation assembly for actuating the propeller blades about their respective pitch axes and a pitch control unit for driving the pitch actuation assembly. The propeller control system also includes a primary control valve and a secondary control valve both communicatively coupled with the controller. The primary control valve and the secondary control valve are each configured to selectively control a flow of hydraulic fluid to or from the pitch actuation assembly. The method includes operating the powerplant; receiving, by the controller, one or more operational parameters relating to operation of the powerplant; determining, by the controller, a condition of the powerplant based at least in part on the one or more operational parameters; and controlling, by the controller, the secondary control valve adjustable between a constant speed mode, a feather mode, and a reverse mode to selectively allow a controlled amount of hydraulic fluid to or from the pitch actuation assembly based at least in part on the condition determined. 
     In some implementations, the condition is an overspeed condition. 
     In some implementations, the condition is a reverse thrust condition. 
     In some implementations, the condition is an engine failure condition. 
     In some implementations, the one or more operational parameters relating to operation of the powerplant are indicative of a power setting of the powerplant and are indicative of a torque output of the powerplant, and wherein determining, by the controller, the condition of the powerplant based at least in part on the one or more operational parameters includes comparing the power setting with the torque output of the powerplant, and wherein if the torque is below a predetermined threshold, in determining, by the controller, the condition of the powerplant, the secondary control valve is controlled by the controller to selectively allow the controlled amount of hydraulic fluid to flow to or from the pitch actuation assembly such that the propeller blades are rotated to a feathered position. 
     In yet another aspect, a variable pitch propeller assembly for an engine defining an axial direction, a radial direction, and a circumferential direction is provided. The variable pitch propeller assembly includes a plurality of propeller blades rotatable about the axial direction and spaced apart along the circumferential direction, each propeller blade rotatable through a plurality of blade angles about respective pitch axes each extending in the radial direction. The variable pitch propeller assembly also includes a pitch actuation assembly for adjusting the plurality of propeller blades through the plurality of blade angles. Further, the variable pitch propeller assembly includes a pitch control unit. The pitch control unit includes a primary control valve operable to selectively allow a flow of hydraulic fluid to or from the pitch actuation assembly. The pitch control unit also includes a secondary EHSV control valve having a valve body defining a chamber and a spool movable within the chamber, the spool is movable between a plurality of constant speed positions to operate the variable pitch propeller assembly in a constant speed mode, one or more feather positions to operate the variable pitch propeller assembly in a feather mode, and one or more reverse positions to operate the variable pitch propeller assembly in a reverse mode. 
     In some embodiments, the spool defines a first groove and a second groove, and wherein the primary control valve is fluidly connected with the first groove when the spool is in one of the plurality of constant speed positions or in one of the one or more reverse positions. 
     In some embodiments, the primary control valve is not fluidly connected with the first groove of the spool when the spool is in one of the one or more feather positions. 
     In some embodiments, the pitch actuation assembly includes a cylinder defining a chamber, a control piston translatable within the cylinder, and a piston rod connected to the control piston and extending into a propeller gear box of the engine, the piston rod translatable in unison with the control piston. In such embodiments, the pitch actuation assembly also includes an oil transfer bearing surrounding the piston rod within the propeller gear box of the engine and defining a flight gallery fluidly connected with the secondary EHSV control valve and a ground gallery fluidly connected with the secondary EHSV control valve. Further, the pitch actuation assembly includes a beta tube enclosed within the piston rod and fluidly connecting the flight gallery with the chamber of the cylinder. 
     In some embodiments, the spool defines a first groove and a second groove, and wherein the first groove is fluidly connected with the flight gallery and the second groove is fluidly connected with the ground gallery when the spool is in one of the one or more reverse positions, and wherein the second groove is not fluidly connected with the ground gallery when the spool is in one of the one or more feather positions or when the spool is in one of the plurality of constant speed positions. 
     In some embodiments, the secondary EHSV control valve is fluidly connected with a drain, and wherein the spool defines a first groove, a second groove, and a third groove, and wherein when the spool is in one of the one or more reverse positions, the third groove does not fluidly connect the ground gallery with the drain, and wherein when the spool is in one of the one or more feather positions or one of the plurality of constant speed positions, the third groove fluidly connects the ground gallery with the drain. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  provides a side view of an example gas turbine engine according to an example embodiment of the present disclosure; 
         FIG. 2  provides a perspective, cutaway view of the gas turbine engine of  FIG. 1 ; 
         FIG. 3  provides a schematic view of an example propeller control system of the gas turbine engine of  FIG. 1 ; 
         FIG. 4  provides a schematic view of a propeller control unit of the propeller control system of  FIG. 3  depicting a secondary control valve in a constant speed mode; 
         FIG. 5  provides a schematic view of the propeller control unit of  FIG. 4  depicting the secondary control valve in a feather mode; 
         FIG. 6  provides a schematic view of the propeller control unit of  FIG. 4  depicting the secondary control valve in a reverse mode; 
         FIG. 7  provides an example controller of the gas turbine engine of  FIG. 1 ; and 
         FIG. 8  provides an example flow diagram according to an example embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “upstream” and “downstream” refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows. 
     The subject matter of the present disclosure is directed generally to variable pitch propeller assemblies and methods therefore for controlling the pitch of a plurality of propeller blades of a variable pitch propeller assembly. In one example aspect, the variable pitch propeller assembly includes features for combining overspeed, feathering, and reverse enabling functionality in a single electrohydraulic servovalve (EHSV). In particular, in one example aspect, a variable pitch propeller assembly includes a secondary EHSV control valve operatively configured to protect the propeller assembly and engine from an overspeed condition, and more generally for maintaining the propeller assembly and engine in an onspeed condition during flight, as well as providing feathering functionality in the event a primary pitch control valve fails or is otherwise unresponsive or operating conditions require it. Further, the secondary control valve is operatively configured to enable reverse functionality. That is, the secondary control valve is configured to enable the propeller blades to be actuated to a reverse pitch, e.g., to produce a reverse thrust. The secondary control valve is operable to selectively allow a controlled amount of hydraulic fluid to flow to or from a pitch actuation assembly such that the pitch of the propeller blades can be adjusted to operate the variable pitch propeller assembly in one of a constant speed mode, a feather mode, and a reverse mode. 
     By combining the overspeed (or more generally constant speed), feathering, and reverse enabling functionality into an electronically controlled secondary control valve, conventional fly-ball overspeed governors and their accompanying overspeed testing components, conventional separate solenoid-operated feather valves, and conventional reverse enabling solenoid valves can be eliminated. Thus, the weight of the engine or power plant can be reduced. Moreover, in some embodiments, the controller can control the protective overspeed, feathering, and reverse enabling functions, and thus, the electronically controlled secondary control valve offers more system flexibility. For example, thresholds and settings relating to when an engine is operating in an onspeed or overspeed can be adjusted, or these thresholds and settings can be adjusted when an engine failure condition has actually occurred. Conventional mechanical overspeed governors and binary feathering valves offered no such flexibility. 
       FIGS. 1 and 2  provide various views of an example engine  100  according to example embodiments of the present disclosure. Particularly,  FIG. 1  provides a side view of the engine  100  and  FIG. 2  provides a perspective, cutaway view of the engine  100  of  FIG. 1 . As shown in  FIG. 1 , for this embodiment, the engine  100  is a gas turbine engine, and more specifically, a turboprop engine. The gas turbine engine  100  defines an axial direction A, a radial direction R, and a circumferential direction C ( FIG. 2 ) extending three hundred sixty degrees (360°) around the axial direction A. The gas turbine engine  100  also defines a longitudinal or axial centerline  102  extending along the axial direction A. The gas turbine engine  100  extends generally along the axial direction A between a first end  103  and a second end  105 , which for this embodiment is the forward and aft end, respectively. Generally, the gas turbine engine  100  includes a gas generator or core turbine engine  104  and a propeller assembly  106  rotatable about the axial centerline  102 , or more generally, the axial direction A. 
     As shown best in  FIG. 2 , the core turbine engine  104  generally includes, in serial flow arrangement, a compressor section  110 , a combustion section  112 , a turbine section  114 , and an exhaust section  116 . A core air flowpath  118  extends from an annular inlet  120  to one or more exhaust outlets  122  of the exhaust section  116  such that the compressor section  110 , combustion section  112 , turbine section  114 , and exhaust section  116  are in fluid communication. 
     The compressor section  110  can include one or more compressors, such as a high pressure compressor (HPC) and a low pressure compressor (LPC). For this embodiment, the compressor section  110  includes a four-stage axial, single centrifugal compressor. In particular, the compressor includes sequential stages of compressor stator vanes and rotor blades (not labeled), as well as an impeller (not labeled) positioned downstream of the axial stages of stator vanes and rotor blades. The combustion section  112  includes a reverse-flow combustor (not labeled) and one or more fuel nozzles (not shown). The turbine section  114  can define one or more turbines, such as a high pressure turbine (HPT) and a low pressure turbine (LPT). For this embodiment, the turbine section  114  includes a two-stage HPT  126  for driving the compressor of the compressor section  110 . The HPT  126  includes two sequential stages of stator vanes and turbine blades (not labeled). The turbine section  114  also includes a three-stage free or power turbine  128  that drives a propeller gearbox  134 , which in turn drives the propeller assembly  106  ( FIG. 1 ). The exhaust section  116  includes one or more exhaust outlets  122  for routing the combustion products to the ambient air. 
     Referring still to  FIG. 2 , the core turbine engine  104  can include one or more shafts. For this embodiment, the gas turbine engine  100  includes a compressor shaft  130  and a free or power shaft  132 . The compressor shaft  130  drivingly couples the turbine section  114  with the compressor section  110  to drive the rotational components of the compressor. The power shaft  132  drivingly couples the power turbine  128  to drive a gear train  140  of the propeller gearbox  134 , which in turn operatively supplies power and torque to the propeller assembly  106  ( FIG. 1 ) via a torque output or propeller shaft  136  at a reduced RPM. The forward end of the propeller shaft  136  includes a flange  137  that provides a mounting interface for the propeller assembly  106  to be attached to the core turbine engine  104 . 
     The propeller gearbox  134  is enclosed within a gearbox housing  138 . For this embodiment, the housing  138  encloses the epicyclical gear train  140  that includes a star gear  142  and a plurality of planet gears  144  disposed about the star gear  142 . The planetary gears  144  are configured to revolve around the star gear  142 . An annular gear  146  is positioned axially forward of the star and planetary gears  142 ,  144 . As the planetary gears  144  rotate about the star gear  142 , torque and power are transmitted to the annular gear  146 . As shown, the annular gear  146  is operatively coupled to or otherwise integral with the propeller shaft  136 . In some embodiments, the gear train  140  may further include additional planetary gears disposed radially between the plurality of planet gears  144  and the star gear  142  or between the plurality of planet gears  144  and the annular gear  146 . In addition, the gear train  140  may further include additional annular gears. 
     As noted above, the core turbine engine  104  transmits power and torque to the propeller gearbox  134  via the power shaft  132 . The power shaft  132  drives the star gear  142 , which in turn drives the planetary gears  144  about the star gear  142 . The planetary gears  144  in turn drive the annular gear  146 , which is operatively coupled with the propeller shaft  136 . In this way, the energy extracted from the power turbine  128  supports operation of the propeller shaft  136 , and through the power gear train  140 , the relatively high RPM of the power shaft  132  is reduced to a more suitable RPM for the propeller assembly  106 . 
     In addition, the gas turbine engine  100  includes one or more controllers  280  that control the core turbine engine  104  and the propeller assembly  106 . For this embodiment, the controller  280  is a single unit control device for a Full Authority Digital Engine (FADEC) system operable to provide full digital control of the core turbine engine  104 , and in some embodiments, the propeller assembly  106 . The controller  280  depicted in the illustrated embodiment of  FIGS. 1 and 2  can control various aspects of the core turbine engine  104  and the propeller assembly  106 . For example, the controller  280  can receive one or more signals from sensory or data collection devices and can determine the blade angle of a plurality of propeller blades  150  about their respective pitch axes, as well as their rotational speed about the axial direction A based at least in part on the received signals. The controller  280  can in turn control one or more components of the gas turbine engine  100  based on such signals. For example, based at least in part on one or more speed or blade pitch signals (or both), the controller  280  can be operatively configured to control one or more valves such that an amount of hydraulic fluid can be delivered or returned from a pitch actuation assembly of the gas turbine engine  100  as will be described in greater detail herein. The internal components of the controller  280  will likewise be described in detail herein. 
     With reference to  FIG. 1 , during operation of the gas turbine engine  100 , a volume of air indicated by arrow  148  passes across the plurality of propeller blades  150  circumferentially spaced apart from one another along the circumferential direction C and disposed about the axial direction A, and more particularly for this embodiment, the axial centerline  102 . The propeller assembly  106  includes a spinner  163  aerodynamically contoured to facilitate an airflow through the plurality of propeller blades  150 . The spinner  163  is rotatable with the propeller blades  150  about the axial direction A and encloses various components of the propeller assembly  106 , such as e.g., the hub, propeller pitch actuator, piston/cylinder actuation mechanisms, etc. A first portion of air indicated by arrow  152  is directed or routed outside of the core turbine engine  104  to provide propulsion. A second portion of air indicated by arrow  154  is directed or routed through the annular inlet  120  of the gas turbine engine  100 . 
     As shown in  FIG. 2 , the second portion of air  154  enters through the annular inlet  120  and flows downstream to the compressor section  110 , which is a forward direction along the axial direction A in this embodiment. The second portion of air  154  is progressively compressed as it flows through the compressor section  110  downstream toward the combustion section  112 . 
     The compressed air indicated by arrow  156  flows into the combustion section  112  where fuel is introduced, mixed with at least a portion of the compressed air  156 , and ignited to form combustion gases  158 . The combustion gases  158  flow downstream into the turbine section  114 , causing rotary members of the turbine section  114  to rotate, which in turn supports operation of respectively coupled rotary members in the compressor section  110  and propeller assembly  106 . In particular, the HPT  126  extracts energy from the combustion gases  158 , causing the turbine blades to rotate. The rotation of the turbine blades of the HPT  126  causes the compressor shaft  130  to rotate, and as a result, the rotary components of the compressor are rotated about the axial direction A. In a similar fashion, the power turbine  128  extracts energy from the combustion gases  158 , causing the blades of the power turbine  128  to rotate about the axial direction A. The rotation of the turbine blades of the power turbine  128  causes the power shaft  132  to rotate, which in turn drives the power gear train  140  of the propeller gearbox  134 . 
     The propeller gearbox  134  in turn transmits the power provided by the power shaft  132  to the propeller shaft  136  at a reduced RPM and desired amount of torque. The propeller shaft  136  in turn drives the propeller assembly  106  such that the propeller blades  150  rotate about the axial direction A, and more particularly for this embodiment, the axial centerline  102  of the gas turbine engine  100 . The exhaust gases, denoted by  160 , exit the core turbine engine  104  through the exhaust outlets  122  to the ambient air. 
     It should be appreciated that the example gas turbine engine  100  described herein is provided by way of example only. For example, in other example embodiments, the engine may include any suitable number or types of compressors (such as e.g., reverse flow and/or axial compressors), turbines, shafts, stages, etc. Additionally, in some example embodiments, the gas turbine engine may include any suitable type of combustor, and may not include the example reverse-flow combustor depicted. It will further be appreciated that the engine can be configured as any suitable type of gas turbine engine, including, for example, turboshaft, turbojets, etc. Moreover, in yet other embodiments, the engine can be configured as a reciprocating or piston engine. In addition, it will be appreciated that the present subject matter can be applied to or employed with any suitable type of propeller or fan configuration, including, for example, tractor and pusher configurations. 
     Furthermore, although the gas turbine engine  100  described above is an aeronautical gas turbine engine for propulsion of a fixed-wing aircraft, the gas turbine engine may be configured as any suitable type of gas turbine engine for use in any number of applications, such as marine applications. Furthermore, the invention could be used on other devices with variable pitch blades such as windmills. The propeller assembly  106  may rotate due to passing of a fluid, such as air or water, across the plurality of blades  150  of the propeller assembly  106 . 
       FIG. 3  provides a schematic view of an example propeller control system  200  for controlling the propeller assembly  106  of the gas turbine engine  100  of  FIGS. 1 and 2  according to an example embodiment of the present disclosure. As depicted in  FIG. 3 , the propeller assembly  106  is driven by the core turbine engine  104  ( FIG. 2 ) by the propeller shaft  136 . The propeller shaft  136  in turn drives a hub  162  from which the plurality of propeller blades  150  extend outwardly from in the radial direction R. As the propeller shaft  136  rotates about the axial direction A, the hub  162  in turn rotates the propeller blades  150  about the axial direction A. The propeller control system  200  includes features for controlling the rotational speed of the propeller blades  150  about the axial direction A and the pitch of the propeller blades  150 , as well as features for protecting the components of the propeller assembly  106 . As shown in  FIG. 3 , for this embodiment, generally, the propeller control system  200  includes a pitch actuation assembly  202 , a pitch control unit  204 , a power lever  206 , and controller  280 . 
     Generally, the pitch actuation assembly  202  is operatively configured to adjust the plurality of propeller blades  150  through a plurality of blade angles. Stated differently, the pitch actuation assembly  202  is operatively configured to rotate each propeller blade  150  about respective pitch axes P extending in the radial direction R (each pitch axis P is relative to a corresponding propeller blade  150 ). For the example embodiment of  FIG. 3 , the pitch actuation assembly  202  is operatively configured to rotate the plurality of propeller blades  150  between high or coarse pitch blade angles, including a fully feathered blade angle to low or fine pitch blade angles. Moreover, for this example embodiment, the pitch actuation assembly  202  is additionally operatively configured to rotate the plurality of propeller blades  150  through reverse pitch angles, which can be useful for ground or taxiing operations, particularly where an aircraft includes multiple engines. In this regard, the example propeller assembly  106  depicted in  FIG. 3  is a variable pitch, full feathering, and reverse enabled propeller assembly, and more particularly still, the propeller assembly is configured as a variable pitch constant-speed, full feathering, reverse enabled propeller assembly. A pilot or aircrew member can operate the propeller assembly  106  in one of the modes noted above utilizing one or more levers. For instance, as shown in  FIG. 3 , the aircraft to which the gas turbine engine  100  is operatively coupled includes control levers. In particular, for this embodiment, the aircraft includes power lever  206 . The power lever  206  can be set within a ground range GR (e.g., for taxiing), within a flight range FR, or within a feathering range FT (e.g., in the event of engine failure). In some embodiments, the aircraft to which the gas turbine engine  100  is operatively coupled includes other control levers, such as e.g., a condition lever, propeller speed levers, mixture levers, etc. 
     As further shown in  FIG. 3 , for this embodiment, the pitch actuation assembly  202  includes a single-acting system for controlling or adjusting the pitch of the propeller blades  150 . It will be appreciated, however, that the pitch actuation assembly  202  can be a double-acting system in other example embodiments. The single-acting system pitch actuation assembly  202  of  FIG. 3  includes a housing or cylinder  166  that defines a chamber and encloses a control piston  168  that is translatable along the axial direction A within the chamber of the cylinder  166 . In particular, as shown, the cylinder  166  and the outboard side  169  of the control piston  168  define a first side  173  of the chamber and the cylinder  166  and the inboard side  167  of the control piston  168  define a second side  174  of the chamber. The control piston  168  separates the first side  173  from the second side  174  of the chamber along the axial direction A. The control piston  168  is biased on its outboard side  169  by a feather spring  172  positioned within the first side  173  of the chamber, as well as by one or more counterweights  182  operatively coupled with one or more propeller blades  150 . 
     The control piston  168  is operatively coupled with a piston rod  184  that extends along the axial direction A. In particular, the piston rod  184  extends from the propeller assembly  106  (where the piston rod  184  is connected to the control piston  168 ) to the propeller gearbox  134  along the axial direction A. The piston rod  184  and the control piston  168  are translatable in unison. The piston rod  184  encloses an oil transfer or beta tube  170  that also extends along the axial direction A. When the propeller blades  150  are rotated about the axial direction A, the piston rod  184  and the beta tube  170  are likewise rotatable about the axial direction A. Like the piston rod  184 , the beta tube  170  extends at least partially into the propeller assembly  106  and at least partially into the propeller gearbox  134  positioned within the gearbox housing  138  ( FIG. 2 ). To control the blade angles of the propeller blades  150 , hydraulic fluid (e.g., oil) can be fed through the beta tube  170  and/or other fluid channels to the second side  174  of the chamber (or to the first side  173  of the chamber in a double-acting system) to translate the control piston  168  along the axial direction A. In some embodiments, the beta tube  170  can define one or more orifices  176  that permit hydraulic fluid to flow from the hollow beta tube  170  to the second side  174  of the chamber depending on the desired blade pitch. Hydraulic fluid can enter and exit the beta tube  170  through an oil transfer bearing  186  surrounding the piston rod  184  within the propeller gear box  134 . The oil transfer bearing  186  defines an annular flight gallery  221  and an annular ground gallery  222 . 
     With reference still to  FIG. 3 , during operation of the gas turbine engine  100 , for this example embodiment, the spring  172  and the counterweights  182  constantly urge the control piston  168  along the axial direction A (a direction to the right in  FIG. 3 ) such that the propeller blades  150  operatively coupled with the control piston  168  (e.g., by the piston rod and an actuation lever coupled thereto) are driven toward a coarse or high pitch position. 
     To actuate the propeller blades  150  toward a low or fine pitch position, an amount of hydraulic fluid is delivered to the second side  174  of the chamber such that a force sufficient to overcome the biasing force of the spring  172  and the counterweights  182  is applied to the inboard side  167  of the control piston  168 . The hydraulic force on the inboard side  167  of the control piston  168  actuates the control piston  168  along the axial direction A (a direction to the left in  FIG. 3 ). This in turn causes the piston rod  184  and enclosed beta tube  170  to translate forward along the axial direction A (or toward the left in  FIG. 3 ). When the control piston  168  is moved forward along the axial direction A, the propeller blades  150  are rotated to a more fine or low pitch position. When rotated to a more fine position, the propeller blades  150  take less “bite” out of the air when the propeller is operating in a forward mode. In a reverse mode, the propeller blades  150  take a greater “bite” out of the air when rotated to a more fine position. 
     When it is desired to adjust the angle of the propeller blades  150  back toward coarse or high pitch, an amount of hydraulic fluid within the second side  174  of the chamber is returned or scavenged back to the engine (e.g., via one of the drains  224 ) such that the spring  172  and the counterweights  182  can urge the control piston  168  rearward along the axial direction A (a direction to the right in  FIG. 3 ). The hydraulic fluid can drain through the beta tube  170  and to the oil transfer bearing  186  positioned within the propeller gearbox  134 . The hydraulic fluid can then be drained to a sump or other like structure. When rotated to a more coarse position, the propeller blades  150  take a greater “bite” out of the air when the propeller is operating in a forward mode. In a reverse mode, the propeller blades  150  take less “bite” out of the air when rotated to a more coarse position. 
     The translation of the control piston  168  along the axial direction A in turn causes the piston rod  184  to translate along the axial direction A as well. To move the propeller blades  150  about their respective pitch axes P, the propeller assembly  106  includes a pitch actuation or propeller pitch actuator  178  to pitch or actuate the propeller blades  150 . When the control piston  168  is translated along the axial direction A, the propeller pitch actuator  178 , which is operatively coupled to the piston rod  184  in this embodiment, rotates the propeller blades  150  about their respective pitch axes P. Accordingly, the axial position of the piston rod  184  and beta tube  170  corresponds with a particular blade angle or angular position of the propeller blades  150 . 
     As further shown in  FIG. 3 , the piston rod  184  encloses beta tube  170  as well as the propeller pitch actuator  178  operatively coupled thereto. The piston rod  184  is operatively coupled with the propeller pitch actuator  178 , which in this embodiment includes an actuation lever  180 . The actuation lever  180  is operatively coupled to the plurality of blades  150  such that movement of the actuation lever  180  along the axial direction A moves or rotates the plurality of blades  150  about their respective pitch axes P. Stated alternatively, as the piston rod  184  and enclosed beta tube  170  translate along the axial direction A (caused by the axial displacement of the control piston  168 ), the actuation lever  180  also translates along the axial direction A. This in turn causes the plurality of blades  150  to rotate about their respective pitch axes P, thereby adjusting the blade angles of the propeller blades  150  to the desired pitch. Thus, by controlling the quantity of hydraulic fluid within the second side  174  of the chamber, the propeller blades  150  can be controlled through a plurality of blade angles about their respective pitch axes P by the actuation lever  180 . 
     In some example embodiments, it will be appreciated that the propeller pitch actuator  178  may include additional or alternative structures that provide pitch actuation functionality. For example, such structures may include actuation linkages linking the control piston  168 , piston rod, or other axially displaceable components with the propeller blades  150 . Other structures may include a yoke and cam assembly operatively coupled with the beta tube  170  and/or piston rod  184  enclosing the beta tube  170 . Any suitable structure can be used to rotate the propeller blades  150  about their respective pitch axes P. Stated alternatively, any known assemblies or structures for converting the translatory motion of the piston rod  184  into rotational motion of the propeller blades  150  is contemplated. 
     As further depicted in  FIG. 3 , an example pitch control unit  204  of the propeller control system  200  is provided. Generally, the pitch control unit  204  is operatively configured to provide an amount of hydraulic fluid to the pitch actuation assembly  202  such that the pitch actuation assembly  202  can adjust the plurality of propeller blades  150  through a plurality of blade angles. More specifically, the pitch control unit  204  is operatively configured to deliver or return an amount of hydraulic fluid from the second side  174  of the chamber such that the control piston  168  is translated along the axial direction A, which in turn drives the piston rod  184  along the axial direction A, causing the propeller pitch actuator  178  to adjust the plurality of propeller blades  150  about their respective pitch axes P. 
     For this embodiment, the pitch control unit  204  includes a high pressure pump  210  positioned downstream of and in fluid communication with a lubrication supply  212 , such as e.g., hydraulic fluid from the engine. The lubrication supply  212  is configured to supply hydraulic fluid, such as e.g., oil, to the propeller control system  200 . The high pressure pump  210  is operatively configured to increase the pressure of the hydraulic fluid as it flows from the lubrication supply  212  downstream to the components of the propeller control system  200 . A lubrication supply conduit  214  provides fluid communication between the lubrication supply  212  and the high pressure pump  210 . 
     A pressure relief valve  216  is positioned downstream of the high pressure pump  210  and is in fluid communication with the high pressure pump  210 . For this example embodiment, the pressure relief valve  216  is in fluid communication with the high pressure pump  210  via a high pressure (HP) conduit  218 . The pressure relief valve  216  is operatively configured to regulate the pressure of the hydraulic fluid within the propeller control system  200 . In the event the pressure of the hydraulic fluid within the HP conduit  218  exceeds a predetermined threshold, the pressure relief valve  216  can drain an amount of hydraulic fluid from the HP conduit  218 . In particular, the pressure of the hydraulic fluid acting on the control piston of the pressure relief valve  216  overcomes a spring biasing force applied by a spring of the pressure relief valve  216 , allowing an amount of hydraulic fluid to drain from the system, as indicated by  224 . The hydraulic fluid can then be scavenged to the lubrication supply  212 , for example. 
     With reference still to  FIG. 3 , the pitch control unit  204  includes a primary pitch control valve  230 . The primary control valve  230  is operatively configured to adjust the propeller pitch or blade angles of the propeller blades  150  during normal operation of the engine. For this embodiment, the primary control valve  230  is a spool-type directional EHSV. The primary control valve  230  is positioned downstream of and is in fluid communication with the high pressure pump  210 . In particular, the primary control valve  230  is in fluid communication with the high pressure pump  210  via the HP conduit  218 . A first portion of the high pressure hydraulic fluid from the high pressure pump  210  is delivered to a first stage  231  of the primary control valve  230 , which is a double nozzle-flapper valve that includes a toque motor, a flapper, two nozzles, and a feedback spring. A second portion of the high pressure hydraulic fluid from the high pressure pump  210  is delivered to a second stage  232  of the primary control valve  230 , which is a precision control spool valve. The second stage  232  of the primary pitch control valve  230  has a valve body  235  defining a chamber and a spool  233  movable within the chamber. The first portion of the high pressure hydraulic fluid delivered to first stage  231  can be used to actuate the second stage  232  precision control spool. In this way, the primary control valve  230  can selectively control or allow a flow of hydraulic fluid to or from the pitch actuation assembly  202 . For instance, the first stage  231  can control the spool  233  of the second stage  232  to actuate or remain in a null position depending on the condition in which the propeller is operating. At times, if there is excess hydraulic fluid within the primary control valve  230 , the fluid can be scavenged to the lubrication supply  212 , for example, as denoted by drain  224 . 
     Generally, the propeller assembly  106  operates in one of three conditions while the aircraft is in flight, including an onspeed condition, an overspeed condition, or an underspeed condition. An onspeed condition results when the engine is operating at the RPM set by the pilot. An overspeed condition results when the engine is operating above the RPM set by the pilot. As an example, if the aircraft begins to pitch downward into a descent maneuver, the airspeed increases across the propeller blades. When this occurs, the propeller blades are unable to fully absorb the engine power, and as a result, the engine RPM increases above the desired setting resulting in an overspeed condition. An underspeed condition results when the engine is operating below the RPM set by the pilot. As an example, if the aircraft begins to pitch upward into a climb maneuver, the airspeed decreases across the propeller blades. When this occurs, the RPM of the engine decreases below the desired setting. During normal operation, the primary pitch control valve  230  selectively controls a flow of hydraulic fluid to or from the pitch actuation assembly  202  to maintain the RPM of the engine as near as possible to the desired setting, or stated alternatively, to maintain an onspeed condition. 
     Moreover, for this embodiment, the primary control valve  230  is operatively configured to feather the propeller blades  150  to a feathered position but only upon the failure of a secondary control valve (described below) and upon the occurrence of a failure condition (e.g., an engine failure condition) or upon a user input. For example, if the torque sensor  268  operatively configured to sense the output torque of the propeller shaft  136  senses that the torque is below a predetermined threshold, for this example, the engine is determined to have experienced an engine failure condition. When it is determined that the engine has experienced an engine failure condition and the secondary control valve has failed, the primary control valve  230  is operatively configured to selectively allow a controlled amount of hydraulic fluid to the pitch actuation assembly  202  such that the propeller blades  150  are actuated to a feathered position. This prevents windmilling and cuts drag to a minimum. 
     Referring still to  FIG. 3 , the pitch control unit  204  also includes a secondary pitch control valve  240 . For this embodiment, the secondary pitch control valve  240  is operatively configured to take over overspeed protection functionality in the event the primary control valve  230  fails, becomes unresponsive, or erroneously drives the pitch of the propeller blades  150  toward a fine pitch position. In addition, for this example embodiment, the secondary pitch control valve  240  is also operatively configured to feather the propeller blades  150  to a full feather position when an engine failure condition has been determined, which can be determined, for example, by sensing an inadequate torque output of the engine. Moreover, for this embodiment, the secondary pitch control valve  240  is operatively configured to provide reverse enabling functionality (e.g., removal of the hydraulic lock for minimum pitch) in a way that, by design, avoids the intervention of the overspeed functionality of the secondary pitch control valve  240 . Accordingly, the secondary pitch control valve  240  of the present disclosure includes overspeed protection functionality, feathering functionality, and reverse enabling functionality. That is, overspeed, feathering, and reverse functionality is combined into and provided by the secondary pitch control valve  240 . 
     As shown in  FIG. 3 , for this embodiment, the secondary pitch control valve  240  is a spool-type directional EHSV. The secondary pitch control valve  240  has a first stage  241 , which is a double nozzle-flapper valve that includes a toque motor, a flapper, two nozzles, and a feedback spring. The secondary pitch control valve  240  also has a second stage  242 , which is a precision control spool valve. The second stage  242  of the secondary pitch control valve  240  has a valve body  245  defining a chamber and a spool  243  movable within the chamber. The secondary pitch control valve  240  is positioned downstream of and is in fluid communication with the high pressure pump  210  as well as the primary control valve  230 . In particular, the secondary pitch control valve  240  is in fluid communication with the high pressure pump  210  via HP conduit  218 . A portion of the high pressure hydraulic fluid from the high pressure pump  210  is delivered to the first stage  241  of the secondary pitch control valve  240  such that the high pressure hydraulic fluid can be used to actuate the spool  243  of the second stage  242 . Moreover, hydraulic fluid can flow from the primary control valve  230  to the secondary control valve  240  via a control conduit  270 . The control conduit  270  splits into a first control conduit  271  and a second control conduit  272  that feed different ports of the second stage  242  of the secondary control valve  240 . 
     Depending on how the first stage  241  is controlled to actuate the spool  243 , the secondary control valve  240  can selectively allow a flow of hydraulic fluid to and from the pitch actuation assembly  202 . The first stage  241  controls the spool  243  of the secondary pitch control valve  240  to allow the primary control valve  230  to be in fluid communication with the pitch actuation assembly  202  or to drain fluid from the pitch actuation assembly  202  through the drain  224  depending on the condition in which the propeller is operating or if the engine has experienced a failure condition. 
     The secondary control valve  240  is fluidly connected with the oil transfer bearing  186  as shown in  FIG. 3 . Specifically, a flight conduit  225  fluidly connects the secondary control valve  240  with the flight gallery  221  of the oil transfer bearing  186  and a ground conduit  226  fluidly connects the secondary control valve  240  with the ground gallery  222  of the oil transfer bearing  186 . The beta tube  170  fluidly connects the flight gallery  221  with the chamber of the cylinder  166 , and more particularly, the beta tube  170  fluidly connects the flight gallery  221  with the second side  174  of the chamber of the cylinder  166 . 
     In the event that the primary control valve  230  fails, becomes unresponsive, or otherwise becomes inoperable, the secondary control valve  240  is operatively configured to take over the functionality of the primary control valve  230 . That is, the secondary control valve  240  takes over constant speed functionality, e.g., maintaining an onspeed condition, feather functionality, and reverse enabling functionality. Accordingly, the secondary control valve  240  is adjustable between a constant speed mode, e.g., to maintain an onspeed a condition, a feather mode, and a reverse mode and is operable to selectively allow a flow of hydraulic fluid to or from the pitch actuation assembly  202  based at least in part on the mode of the secondary control valve  240 . Examples are provided below. 
       FIGS. 4, 5, and 6  provide schematic views of the propeller control unit  204  of  FIG. 3 . In particular,  FIG. 4  depicts the secondary control valve  240  in a constant speed mode,  FIG. 5  depicts the secondary control valve  240  in a feather mode, and  FIG. 6  depicts the secondary control valve  240  in a reverse mode. As noted above, the secondary control valve  240  has valve body  245  defining a chamber  247 . The spool  243  is movable within the chamber  247 . Particularly, the spool  243  is movable between a plurality of constant speed positions in the constant speed mode ( FIG. 4 ), one or more feather positions in the feather mode ( FIG. 5 ), and one or more reverse positions in the reverse mode ( FIG. 6 ) to enable the plurality of propeller blades to rotate to a negative blade angle. Stated differently, the spool  243  is movable between a plurality of constant speed positions ( FIG. 4 ) to operate the variable pitch propeller assembly  106  ( FIG. 2 ) in a constant speed mode, one or more feather positions ( FIG. 5 ) to operate the variable pitch propeller assembly  106  in a feather mode, and one or more reverse positions ( FIG. 6 ) to operate the variable pitch propeller assembly  106  in a reverse mode. 
     As shown in  FIG. 4 , the secondary control valve  240  is in a constant speed mode. In the constant speed mode, the secondary control valve  240  controls the flow of hydraulic fluid to the pitch actuation assembly  202  ( FIG. 3 ) to maintain an onspeed condition, e.g., by correcting overspeed and underspeed conditions. When the second control valve  240  is adjusted to the constant speed mode, the secondary control valve  240  selectively allows a flow of hydraulic fluid to flow between the chamber of the cylinder  166  (e.g., the second side  174  of the chamber) ( FIG. 3 ) and the secondary control valve  240  to maintain an onspeed condition. 
     More particularly, to maintain an onspeed condition, the controller  280  first determines (e.g., automatically or via pilot input) whether an overspeed or underspeed condition is present. The controller  280  causes one or more electrical signals to be routed to a torque motor  244  of the first stage  241  of the secondary control valve  240 . The torque motor  244  can include a first coil and a second coil spaced from the first coil. The first and second coils can be in electrical communication with the controller  280 , and in some embodiments, a dedicated power supply (e.g., a voltage or current source). In some embodiments, the controller  280  can provide the required electrical power. When the electrical signals are provided to one or both of the coils, an electromagnetic torque is applied to an armature of the torque motor  244  that in turn causes a flapper  246  to deflect or move between a pair of opposing nozzles  248  from its resting or neutral position. Particularly, the flapper  246  moves closer to one nozzle and away from the other, causing a pressure differential over the spool  243 . The pressure differential drives the spool  243  to slide or move within the chamber of the valve body  245 . The displacement of the spool  243  is fed back to the flapper  246  via a feedback spring  250 . The spool  243  continues to slide or move until the flow forces reach equilibrium. The secondary control valve  240  can deliver an output flow proportional to the input electrical power. 
     The spool  243  defines a first groove  251 , a second groove  252 , and a third groove  253  spaced between lands of the spool  243 . When the spool  243  is in the constant speed mode, the primary control valve  230  is fluidly connected with the first groove  251  via the first control conduit  271  (as well as main control conduit  270 ); thus, hydraulic fluid can flow from the primary control valve  230  into the first groove  251  of the spool  243  when the spool  243  is in constant speed mode. The first groove  251  is also fluidly connected with the flight conduit  225  in the constant speed mode. Accordingly, hydraulic fluid can flow to the flight gallery  221  ( FIG. 3 ) from the first groove  251  of the secondary control valve  240  (e.g., to move the control piston  168  to the left in  FIG. 3  so that propeller blades  150  are moved to a more fine pitch position), or in some instances, hydraulic fluid can flow from the flight gallery  221  ( FIG. 3 ) to the first groove  251  of the secondary control valve  240  (e.g., to move the control piston  168  to the right in  FIG. 3  so that propeller blades  150  are moved to a more coarse pitch position). When the spool  243  is in one of the plurality of constant speed positions, the second groove  252  is not fluidly connected with the ground gallery  222  ( FIG. 3 ). In addition, in the constant speed mode, the ground gallery  222  is fluidly connected with a drain  274  thru the third groove  253 , which prevents the pitch of the blades from going below the minimum flight pitch during the flight. Drain  274  can be a common scavenge drain. Hydraulic fluid flowing along the drain  274  can be scavenged to the lubrication supply  212 , for example. 
     By changing the electrical power input to the torque motor  244 , the spool  243  can be moved or controlled within the chamber  247  to increase to decrease the hydraulic flow to the pitch actuation assembly  202 . Stated more particularly, the amount of fluid within the second side  174  of the chamber of the cylinder  166  can be adjusted so that the control piston  168  can be actuated along the axial direction A, which as noted previously, ultimately adjusts the pitch of the propeller blades  150 , e.g., to a more fine or coarse pitch to maintain the onspeed condition. When the propeller blades  150  are moved to a coarsened or higher pitch position to compensate for an overspeed condition, the propeller blades  150  are able to better absorb the engine power, and as a result, the engine RPM decreases to the desired setting. Consequently, the engine can return to an onspeed condition. On the other hand, when the propeller blades  150  are moved to a finer or lower pitch position to compensate for an underspeed condition, the propeller blades  150  absorb less of the engine power, and as a result, the engine RPM increases to the desired setting. Consequently, the engine can return to an onspeed condition. 
     As shown in  FIG. 5 , the secondary control valve  240  is in feather mode. As noted, the secondary pitch control valve  240  is operatively configured to feather the propeller blades  150  to a full feather position when an engine failure condition has been determined or via a pilot input. As noted previously, when the secondary control valve  240  is in feather mode, the spool  243  is movable between one or more feather positions. For instance, as depicted in  FIG. 5 , the spool  243  is moved by the torque motor  244  in a similar manner as described above to a feather position. That is, for this embodiment, the spool  243  is moved in a direction slightly downward relative to the position of the spool  243  in the constant speed mode shown in  FIG. 4 . The deflection of the feedback spring  250  confirms the slight downward movement of the spool  243  in  FIG. 5 . 
     When the secondary control valve  240  is adjusted to the feather mode, the secondary control valve  240  selectively allows the flow of hydraulic fluid to flow from the second side  174  of the chamber of the cylinder  166  to the secondary control valve  240 . More particularly, when the secondary control valve  240  is in the feather mode and thus the spool  243  is moved into one of the one or more feather positions, the primary control valve  230  is not fluidly connected with the first groove  251  of the spool  243 . Particularly, the first control conduit  271  is not fluidly connected with the first groove  251 . Accordingly, no additional hydraulic fluid can flow from primary control valve  230  to secondary control valve  240  and ultimately to the second side  174  of the chamber of cylinder  166  ( FIG. 3 ). Further, as shown in  FIG. 5 , the second groove  252  is not fluidly connected with the ground gallery  222  when the spool  243  is in one of the one or more feather positions. More specifically, a land of the spool  243  that separates the second groove  252  from the third groove  253  prevents hydraulic fluid from flowing along the second control conduit  272  into the second groove  252  and into the ground conduit  226  to eventually flow to the ground gallery  222 . Accordingly, additional hydraulic fluid is completely cutoff from flowing to the second side  174  of the chamber of cylinder  166 . Hydraulic fluid can be drained from the second side  174  of the chamber such that the control piston  168  is biased by the spring  172  and the counterweights  182  toward a full feather position (i.e., the control piston  168  can translate along the axial direction A to a position furthest to the right in  FIG. 3  for this embodiment). In this manner, the propeller blades  150  can be adjusted to a full feather position. In feather mode, the propeller blades  150  can cease rotation about the axial direction A, for example. Further, as shown in  FIG. 5 , the third groove  253  of the spool  243  provides fluid communication between ground conduit  226  and drain  274  and the first groove  251  of the spool  243  provides fluid communication between flight conduit  225  and drain  274 . In this way, hydraulic fluid from the flight gallery  221  and ground gallery  222  can be scavenged, e.g., to lubrication supply  212 . 
     As shown in  FIG. 6 , the secondary control valve  240  is in reverse mode. As noted, the secondary pitch control valve  240  is operatively configured to reverse the pitch angle of the propeller blades  150 , e.g., to create reverse thrust. When the secondary control valve  240  is in reverse mode, the spool  243  is movable between one or more reverse positions. For instance, as depicted in  FIG. 6 , the spool  243  is moved by the torque motor  244  in a similar manner as described above to a reverse position. That is, for the depicted embodiment of  FIG. 6 , the spool  243  is moved in a direction slightly upward relative to the position of the spool  243  in the constant speed mode shown in  FIG. 4 . The deflection of the feedback spring  250  confirms the slight upward movement of the spool  243  in  FIG. 6 . 
     When the secondary control valve  240  is adjusted to the reverse mode, the secondary control valve  240  selectively allows the flow of hydraulic fluid to flow from the secondary control valve  240  to the second side  174  of the chamber ( FIG. 3 ) and from the secondary control valve  240  to the ground gallery  222 . More particularly, when the secondary control valve  240  is in the reverse mode and thus the spool  243  is in one of the one or more reverse positions, the primary control valve  230  is fluidly connected with the first groove  251  of the spool  243 . The first groove  251  is also fluidly connected with the flight gallery  221  via the flight conduit  225  when the spool  243  is in one of the one or more reverse positions as shown in  FIG. 6 . That is, in one of the reverse positions, the first groove  251  of the spool  243  fluidly connects the first control conduit  271  and the flight conduit  225 . Thus, hydraulic fluid can flow from the primary control valve  230  to the secondary control valve  240  and ultimately to the second side  174  of the chamber of the cylinder  166  ( FIG. 3 ). 
     Moreover, when the secondary control valve  240  is adjusted to the reverse mode, the second groove  252  of the spool  243  fluidly connects the primary control valve  230  with the secondary control valve  240 , e.g., via the second control conduit  272  (as well as main control conduit  270 ). The second groove  252  is also fluidly connected with the ground gallery  222  via the ground conduit  226  when the spool  243  is in one of the one or more reverse positions as shown in  FIG. 6 . Thus, hydraulic fluid can flow from the primary control valve  230  to the secondary control valve  240  and ultimately to the ground gallery  222 . The flow of hydraulic fluid into the ground gallery  222  can enable the reverse functionality of the propeller assembly  106  ( FIG. 3 ) and the flow of hydraulic fluid into the flight gallery  221  and ultimately to the second side  174  of the chamber can fill into and force the control piston  168  to engage a stop  188  ( FIG. 3 ) (i.e., the control piston  168  is moved to a far left position in  FIG. 3  by the hydraulic fluid). Moreover, in reverse mode, fluid can move from the flight gallery  221  and the ground gallery  222  to the primary control valve  230 . The primary control valve  230  can drain the fluid (e.g., oil) to the oil system as needed, e.g., to increase the pitch angle of the blades. 
     As further shown in  FIG. 6 , when the spool  243  is in one of the one or more reverse positions, the drain  274  is not fluidly connected with the first control conduit  271 , the second control conduit  272 , the flight conduit  225 , or the ground conduit  226 . Thus, the third groove  253  of the spool  243  does not fluidly connect the drain  274  with the flight gallery  221 , the ground gallery  222 , or the primary control valve  230  when the spool  243  is in one of the one or more reverse positions. Accordingly, hydraulic fluid can flow from the primary control valve  230  through first groove  251  of the spool  243  and to the flight gallery  221  via flight conduit  225  without any of the hydraulic fluid draining via drain  274 . Moreover, hydraulic fluid can flow from the primary control valve  230  through second groove  252  of the spool  243  and to the ground gallery  222  via ground conduit  226  without any of the hydraulic fluid draining via drain  274 . In contrast, as shown in  FIGS. 4 and 5 , when the spool  243  is in one of the one or more feather positions ( FIG. 5 ) or one of the plurality of constant speed positions ( FIG. 4 ), the third groove  253  of the spool  243  fluidly connects the ground gallery  222  with the drain  274  via the ground conduit  226 . Thus, when the spool  243  is in either a feather or constant speed positions, at least some portion of the hydraulic fluid can drain from the ground gallery  222 . 
     Returning to  FIG. 3 , as noted above, the gas turbine engine  100  includes a controller  280 . The controller  280  is communicatively coupled with various components of the propeller control system  200 . More specifically, the controller  280  is communicatively coupled with a primary speed sensor  260 , a primary blade angle feedback sensor  262 , a secondary speed sensor  264 , a secondary blade angle feedback sensor  266 , the primary pitch control valve  230 , the secondary pitch control valve  240 , a torque sensor  268 , the power lever  206 , and other components of the propeller assembly  106 . The various components of the propeller control system  200  can be communicatively coupled with the controller  280  in any suitable manner, such as e.g., by wired or wireless communication lines (shown by dashed lines in  FIG. 3 ). The communication between the controller  280  and the various components of the propeller control system  200  will be described in turn. 
     As shown in  FIG. 3 , the controller  280  is communicatively coupled with the primary speed sensor  260  and the primary blade angle feedback sensor  262 . The primary speed sensor  260  is operatively configured to sense the rotational speed of the piston rod  184 , the beta tube  170 , or some other rotatory component of the propeller assembly  106  that rotates in unison about the axial direction A with the propeller blades  150 . During operation, the primary speed sensor  260  sends or otherwise transmits one or more signals indicative of the rotational speed of the propeller blades  150 . The controller  280  receives or otherwise obtains the one or more signals indicative of the rotational speed of the propeller blades  150  and can compare the actual rotational speed of the propeller blades  150  with the RPM set by controller  280 . In this manner, the controller  280  can determine whether the propeller assembly  106  is operating in an onspeed condition, an overspeed condition, or an underspeed condition. Based on the determined condition, the controller  280  can send one or more signals to the primary control valve  230  to control the spool  233  of the primary control valve  230  to selectively allow an amount of hydraulic fluid to flow to or from the pitch actuation assembly  202  so that the pitch of the propeller blades  150  can ultimately be adjusted. In this way, the propeller assembly  106  is maintained in or as close as possible to an onspeed condition. 
     The controller  280  is also communicatively coupled with the secondary speed sensor  264  as well as the secondary blade angle feedback sensor  266 . As noted above, in the event the primary control valve  230  fails, becomes unresponsive, or erroneously drives the pitch of the propeller blades  150  toward a fine pitch position, the secondary pitch control valve  240  takes over operation of governing overspeed conditions as well as feathering the propeller blades  150  to a full feather position. The controller  280  then utilizes the secondary speed sensor  264  and may use the secondary blade angle feedback sensor  266  in conjunction with the secondary pitch control valve  240  to control the propeller assembly  106 . 
     The secondary speed sensor  264  is operatively configured to sense the rotational speed of the piston rod  184 , the beta tube  170 , or some other rotational component of the propeller assembly  106  that rotates in unison about the axial direction A with the propeller blades  150 . The secondary speed sensor  264  can continuously sense the rotational speed of the propeller blades  150 . The secondary speed sensor  264  sends or otherwise transmits one or more signals indicative of the rotational speed of the propeller blades  150 . The controller  280  receives or otherwise obtains the one or more signals indicative of the rotational speed of the propeller blades  150  and can compare the actual rotational speed of the propeller blades  150  with the RPM set in the FADEC system for overspeed governing. In this manner, the controller  280  can determine whether the propeller assembly  106  is operating in an onspeed condition, an overspeed condition, or an underspeed condition. Based on the determined condition, the controller  280  can send one or more signals to the secondary pitch control valve  240  to control the spool  243  to selectively allow an amount of hydraulic fluid to flow to or from the pitch actuation assembly  202  so that the pitch of the propeller blades  150  can ultimately be adjusted. In this way, the propeller assembly  106  can be returned to an overspeed governing onspeed condition. 
     To improve the accuracy and overall efficiency of the engine  100  and the propeller assembly  106 , the controller  280  can receive or otherwise obtain one or more signals from the primary blade angle feedback sensor  262  and/or the secondary blade angle feedback sensor  266 . The primary and secondary blade angle feedback sensors  262 ,  266  are operatively configured to sense the blade angle or pitch of the propeller blades  150  by measuring or sensing the axial position of the piston rod  184 , the beta tube  170 , or some other rotary component that is translated along the axial direction A in unison with the control piston  168 . One or more signals indicative of the axial position of the piston rod  184  are sent or otherwise transmitted from the primary and/or secondary blade angle feedback sensors  262 ,  266  to the controller  280 . The controller  280  receives or otherwise obtains the one or more signals indicative of the axial position of the piston rod  184 , and based at least in part on the axial position of the piston rod  184 , the controller  280  can determine the blade angle of the propeller blades  150 . By knowing the pitch or blade angle of the propeller blades  150 , the controller  280  can ensure that the various components of the propeller control system  200  are functioning properly. Moreover, the controller  280  can use the sensed information to improve the timing and flows of the various valves of the system such that the propeller control system  200  can become more efficient and effective at adjusting the pitch of the propeller blades  150 . 
     For certain ground operations as well as inflight reverse thrust requirements, the primary blade angle feedback sensor  262  and/or the secondary blade angle feedback sensor  266  can sense the blade angle or pitch of the propeller blades  150  by measuring or sensing the axial position of the piston rod  184 , the beta tube  170 , or some other rotary component that is translated along the axial direction A in unison with the control piston  168  in the same or similar manner as noted above. One or more signals indicative of the axial position of the piston rod  184  can be sent or otherwise transmitted from the primary and/or secondary blade angle feedback sensors  262 ,  266  to the controller  280 . The controller  280  receives or otherwise obtains the one or more signals indicative of the axial position of the piston rod  184 , and based at least in part on the axial position of the piston rod  184 , the controller  280  can determine the negative blade angle of the propeller blades  150 . 
       FIG. 7  provides an example controller  280  of the gas turbine engine of  FIGS. 1 and 2  for controlling the propeller control system  200  in a manner as described above. The controller  280  includes various components for performing various operations and functions, such as e.g., receiving one or more signals from the sensors of the propeller control system  200  and the power lever  206 , determining the condition of the propeller assembly  106  and engine  100 , sending one or more signals to the first pitch control valve  230  to control the amount of hydraulic fluid to the pitch actuation assembly  202  if the propeller is determined to be in the overspeed condition or underspeed condition, and the secondary control pitch valve  240  to control the amount of hydraulic fluid to the pitch actuation assembly  202  if the propeller is in an engine failure condition, a feather condition based on a pilot or user input, etc. That is, the controller  280  controls the primary control valve  230  to supply/drain oil to/from the flight gallery  221  and controls the secondary control valve  240  to select the “working” mode in case of a failure of the primary control valve  230 . 
     As shown in  FIG. 7 , the controller  280  can include one or more processor(s)  281  and one or more memory device(s)  282 . The one or more processor(s)  281  can include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, logic device, and/or other suitable processing device. The one or more memory device(s)  282  can include one or more computer-readable media, including, but not limited to, non-transitory computer-readable media, RAM, ROM, hard drives, flash drives, and/or other memory devices. 
     The one or more memory device(s)  282  can store information accessible by the one or more processor(s)  281 , including computer-readable instructions  284  that can be executed by the one or more processor(s)  281 . The instructions  284  can be any set of instructions that when executed by the one or more processor(s)  281 , cause the one or more processor(s)  281  to perform operations. In some embodiments, the instructions  284  can be executed by the one or more processor(s)  281  to cause the one or more processor(s)  281  to perform operations, such as any of the operations and functions for which the controller  280  or controllers are configured, such as e.g., receiving one or more signals from sensors and determining an axial position of the beta tube  170  such that the blade angle of the propeller blades  150  can be determined. The instructions  284  can be software written in any suitable programming language or can be implemented in hardware. Additionally, and/or alternatively, the instructions  284  can be executed in logically and/or virtually separate threads on processor(s)  281 . 
     The memory device(s)  282  can further store data  283  that can be accessed by the one or more processor(s)  281 . The data  283  can also include various data sets, parameters, outputs, information, etc. shown and/or described herein. The controller  280  can also include a communication interface  285  used to communicate, for example, with other components of an aircraft in which the gas turbine engine  100  is mounted to, such as e.g., another controller configured to control another engine of the aircraft. The communication interface  285  can include any suitable components for interfacing with one or more network(s), including for example, transmitters, receivers, ports, controllers, antennas, and/or other suitable components. 
     As shown further in  FIG. 7 , the controller  280  includes primary logic  286  and secondary logic  287 . Although the primary logic  286  and the secondary logic  287  are shown as separate from the one or more processor(s)  281  and the one or more memory device(s)  282 , the primary and secondary logic  286 ,  287  can be embodied in the one or more processor(s)  281  and the one or more memory device(s)  282  described above. The primary logic  286  is operatively configured to control the primary control valve  230 . The secondary logic  287  is operatively configured to control the secondary pitch control valve  240 . In particular, the secondary logic  287  includes a constant speed logic module  288 , a feathering logic module  289 , and a reverse logic module  290 . The constant speed logic module  288  provides controller  280  with the logic to control the secondary pitch control valve  240  in actuating the propeller blades  150 , e.g., to a higher more coarse pitch to ultimately move propeller assembly  106  from an overspeed condition to governing to a selected speed condition. Likewise, the feathering logic module  289  provides controller  280  with the logic to control the secondary pitch control valve  240  in actuating the propeller blades  150  to a full feather position when an engine failure condition has been determined by the controller  280  or upon a user or pilot input. Further, the reverse logic module  290  provides controller  280  with the logic to control the secondary pitch control valve  240  in actuating the propeller blades  150  to a negative pitch position when a reverse condition has been determined by the controller  280  or upon a user or pilot input. 
       FIG. 8  provides a flow diagram of an example method ( 300 ) for controlling a variable pitch propeller assembly driven by a powerplant using a propeller control system. The powerplant, such as e.g., the engine  100  of  FIGS. 1 and 2 , defines an axial direction and a radial direction. The engine has a controller, such as e.g., controller  280  described herein. The variable pitch propeller assembly has a plurality of propeller blades rotatable about the axial direction and adjustable about respective pitch axes each extending along the radial direction. The propeller control system has a pitch actuation assembly for actuating the propeller blades about their respective pitch axes and a pitch control unit for driving the pitch actuation assembly. The pitch control unit has a primary control valve and a secondary control valve both communicatively coupled with the controller. The primary control valve and the secondary control valve are each configured to selectively control a flow of hydraulic fluid to or from the pitch actuation assembly. For instance, the primary control valve can control the flow of hydraulic fluid during normal operation. The secondary control valve can control the flow of hydraulic fluid to the pitch actuation assembly when the primary control valve fails or otherwise becomes unresponsive. In this way, the secondary control valve acts as a failsafe. Some or all of the method ( 300 ) can be implemented by one or more of the components described herein, such as e.g., the controller  280 , the sensors  260 ,  262 ,  264 ,  266 ,  268 , physical components, etc. 
     At ( 302 ), the method ( 300 ) includes operating the powerplant. For example, as noted above, the powerplant can be the gas turbine engine shown and described in  FIGS. 1 and 2 . 
     At ( 304 ), the method ( 300 ) includes receiving, by the controller, one or more operational parameters of the powerplant. For instance, in some example implementations, the one or more operational parameters can be indicative of a power setting of the powerplant. The one or more operational parameters indicative of the power setting of the powerplant can be obtained by the controller  280 . The power lever  206 , or an angular position sensor device, can send one or more signals indicative of the angle of the power lever  206 . Based on the angle of the power lever  206 , the controller  280  can determine the power setting selected by the pilot. As another example, the power setting selected by the pilot can be digitized, and thus, the power setting can be transmitted to the controller  280  digitally. 
     In some example implementations, the one or more operational parameters can be indicative of the rotational speed of the propeller blades  150  about the axial direction A. For instance, the rotational speed of the propeller blades  150  can be determined by the controller  280  based on one or more signals from the primary speed sensor  260  and/or the secondary speed sensor  264 . The primary or secondary speed sensors  260 ,  264  can sense or measure the rotational speed of a rotary component, such as, e.g. the piston rod  184 , the beta tube  170 , or some other rotary component that rotates about the axial direction A in unison with the propeller blades  150 . 
     In some example implementations, the one or more operational parameters can be indicative of a torque output of the powerplant. For instance, the torque sensor  268  positioned proximate the propeller shaft  136  ( FIG. 3 ) can sense the torque output of the core turbine engine  104  of the powerplant. One or more signals indicative of the torque output can be routed to the controller  280 . 
     In some example implementations, the one or more operational parameters can be indicative of an angular position of a condition lever or a selected condition of the powerplant. For instance, the cockpit of the aircraft or vehicle in which the turboprop and propeller assembly are mounted can include a condition lever. A pilot or crew member can selectively adjust the condition lever to select a condition of the propeller assembly. For instance, the angular position of the condition lever can be indicative of a reverse mode or a feather mode. 
     At ( 306 ), the method ( 300 ) includes determining, by the controller, a condition of the powerplant based at least in part on the one or more operational parameters. For example, the condition could be one of an overspeed condition, an underspeed condition, a feather condition or an engine or powerplant failure condition, a reverse thrust condition, etc. 
     For example, in implementations in which the one or more operational parameters are indicative of the rotational speed of the propeller blades  150  about the axial direction A, at ( 306 ) the method ( 300 ) can include determining the rotational speed of the propeller blades  150  and comparing the power setting with the rotational speed of the propeller blades. In this way, the controller  280  can determine whether the powerplant or engine is operating in an onspeed condition, an underspeed condition, or an overspeed condition. Once the condition of the powerplant or engine is known, the propeller control system  200  can make the necessary adjustments to the pitch of the propeller blades  150 , e.g., at ( 308 ) below. 
     As another example, in implementations in which the one or more operational parameters are indicative of a torque output of the powerplant, at ( 306 ) the method ( 300 ) can include comparing the power setting with the torque output of the powerplant. If the torque output of the powerplant is at or below a predetermined threshold for the given power setting, the controller  280  can determine that a powerplant or engine failure condition has occurred. When such a powerplant failure condition has been determined, the controller  280  can send one or more signals to the primary control valve  230  to actuate the primary control valve  230  such that the propeller blades  150  are actuated to a fully feathered position. If however, the primary control valve  230  fails or is otherwise unresponsive, the controller  280  can send one or more signals to the secondary control valve  240  to actuate the secondary control valve  240  such that the propeller blades  150  are actuated to a fully feathered position. 
     As a further example, in implementations in which the one or more operational parameters are indicative of an angular position of a condition lever or a selected condition of the powerplant, at ( 306 ) the method ( 300 ) can include determining the condition of the powerplant based at least in part on the angular position of the condition lever or the selected user input. 
     At ( 308 ), the method ( 300 ) includes controlling, by the controller, the secondary control valve adjustable between a constant speed mode, a feather mode, and a reverse mode to selectively allow a controlled amount of hydraulic fluid to or from the pitch actuation assembly based at least in part on the condition determined. For instance, the spool  243  of the secondary control valve  240  of  FIGS. 4, 5, and 6  can be moved to selectively allow a controlled amount of hydraulic fluid to or from the pitch actuation assembly based at least in part on the condition determined. For instance, if an overspeed condition or feather condition (e.g., an engine failure condition) is determined, hydraulic fluid can be drained from the pitch actuation assembly in a manner described herein. If, a reverse thrust condition is determined, the hydraulic fluid can be directed to the flight gallery  221  and the ground gallery  222  to enable reverse functionality and to actuate the control piston  168  to a more fine position such that the propeller blades are ultimately pitched to a reverse angle. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.