Patent Publication Number: US-10773816-B2

Title: Single lever turboprop control systems and methods utilizing torque-based and power-based scheduling

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of co-pending U.S. application Ser. No. 14/861,712, filed with the United States Patent and Trademark Office on Sep. 22, 2015. 
    
    
     TECHNICAL FIELD 
     The following disclosure relates generally to gas turbine engines and, more particularly, to single lever turboprop control systems and methods utilizing torque-based and/or power-based scheduling to achieve a desired (e.g., substantially proportional) relationship between control lever position and the power output of a turboprop engine. 
     BACKGROUND 
     Fixed wing aircraft are commonly equipped with one of three types of propulsive gas turbine engines: turboprop, turbofan, or turbojet engines. Turbofan and turbojet engines are typically operated utilizing a single lever control system, which includes a cockpit lever movable through a range of angular positions to schedule engine thrust. Turboprop engines, by comparison, are typically operated utilizing a dual lever control system, which includes a first cockpit lever for controlling propeller blade angle and a second cockpit lever for controlling engine rotational speed. Thus, relative to single lever turbofan and turbojet control systems, dual lever turboprop control systems differ fundamentally in the design of the pilot interface and the manner in which the engine is controlled. In further contrast to single lever turbofan and turbojet control systems, dual lever turboprop control systems typically do not provide pilot controls for adjusting the thrust or power output of the turboprop engine in a direct manner. Such disparities in the control systems of turboprop, turbofan, and turbojet engines can increase operational complexity, necessitate additional pilot training, and result in a general lack of familiarity on behalf of the pilot when transitioning between aircraft equipped with different types of propulsive gas turbine engines. 
     BRIEF SUMMARY 
     Embodiments of a single lever turboprop control method are provided, which utilize torque-based and/or power-based scheduling to achieve a desired (e.g., substantially proportional) relationship between control lever position and the power output of a turboprop engine. In one embodiment, the method includes the step or process of monitoring, at an Engine Control Unit (ECU), for receipt of a Power Lever Angle (PLA) signal from a single lever control device. When a PLA control signal received at the ECU, a target torque or power output is established as a function of at least the PLA control signal. A first engine setpoint, such as a blade angle setpoint or an engine rotational speed setpoint, is selectively determined utilizing the target torque output. An operational parameter of the turboprop engine, such as engine rotational speed and/or propeller blade angle, is then adjusted in accordance with the first engine setpoint. 
     In a further embodiment, the single lever turboprop control method includes monitoring, at an ECU, for receipt of a PLA control signal from a single lever control device. When a PLA control signal is received at the ECU, a target power output for the turboprop engine is established as a function of at least the PLA control signal. A first engine setpoint, such as a blade angle setpoint or an engine rotational speed setpoint, can then be selectively determined utilizing the target power output. Finally, an operational parameter of the turboprop engine, such as engine rotational speed and/or propeller blade angle, is adjusted in accordance with the first engine setpoint. 
     Embodiments of a single lever turboprop control system are further provided. In one embodiment, the single lever turboprop control system includes a single lever control device to which an ECU is coupled. The ECU monitors for receipt of a PLA control signal from the single lever control device. When a PLA control signal received at the ECU, the ECU establishes a target engine output for the turboprop engine as a function of at least the PLA control signal. The target engine output is selected from the group consisting of a target torque output and a target power output. The ECU further determines a first engine setpoint utilizing the target engine output and then adjusts an operational parameter of the turboprop engine, such as engine rotational speed and/or propeller blade angle, in accordance with the first engine setpoint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       At least one example of the present invention will hereinafter be described in conjunction with the following figures, wherein like numerals denote like elements, and: 
         FIG. 1  is a schematic of a single lever turboprop control system and a turboprop engine, as illustrated in accordance with an exemplary embodiment of the present invention; 
         FIG. 2  is a schematic illustrating a power/torque-based scheduling process, which can be carried-out by the single lever turboprop control system of  FIG. 1  and which is illustrated in accordance with a further exemplary embodiment of the present invention; and 
         FIG. 3  is a schematic illustrating one manner in which a power/torque-based scheduling mode can be implemented by the control system of  FIG. 1 , as illustrated in accordance with a still further exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background or the following Detailed Description. 
     As briefly described above, turboprop engines are traditionally operated utilizing a dual lever control system, which includes a first lever for adjusting propeller blade angle (β) and a second, independent lever for adjusting engine rotational speed (N). Recently, certain turboprop control systems have been proposed that enable a pilot to adjust both propeller blade angle (β) and engine rotational speed (N) utilizing a single control lever located in the aircraft cockpit. During operation, such a single lever turboprop control system may convert or “schedule” the angular position of the control lever to a corresponding propeller blade angle setpoint (β set ) and a corresponding rotational speed setpoint (N set ). The blade angle and rotational speed setpoints (β set  and N set ) are then applied to the turboprop engine by commanding one or more actuation systems to adjust the blade angle and rotational speed of the turboprop engine in accordance with the newly-established setpoints. In this manner, the turboprop engine can be operated utilizing a single lever control system mimicking or emulating the single lever control systems utilized in the operation of turbofan and turbojet engines. Advantageously, such a single lever turboprop control system helps simplify operating procedures and improves commonality between the pilot interfaces utilized to operate turboprop, turbofan, and turbojet engines. 
     While providing the above-noted benefits, conventionally-proposed single lever turboprop control systems remain limited in a number of respects. For example, when implemented as described above, a single lever turboprop control system may not provide a substantially proportional relationship between PLA position and turboprop engine power (also referred to herein as “shaft horsepower” and identified by the abbreviation “SHP”). Significantly, engine power (SHP) is the controlled parameter of a turboprop engine most closely corresponding with the thrust output of turbofan and turbojet engines, which is typically scheduled in a direct proportional relationship with PLA position. Turboprop engine power (SHP) is the product of engine torque (Q) multiplied by engine rotational speed (N), as expressed by the following equation:
 
 Q×N =SHP  EQ. 1
 
     Engine torque (Q), engine power (SHP), and the thrust generated by the turboprop engine can vary for a given pair of blade angle (β) and rotational speed (N) setpoints depending upon a number of non-controlled variables, such as the current altitude at which the aircraft (A/C) is traveling, aircraft speed, and other conditions. In the case of a single lever turboprop controls system wherein the blade angle (β) and rotational speed (N) of a turboprop engine are derived directly from PLA position in the manner previously described, the turboprop engine may provide a different thrust output each time the pilot moves the control lever to a particular angular position. Consequently, there may continue to exist an undesirable disparity in the behavior of a turboprop engine controlled utilizing a single lever control system of the type described above as compared to a turbofan or turbojet engine operated utilizing a similar single lever control system. 
     The following describes embodiments of systems and methods enabling single lever control of a turboprop engine wherein blade angle (β) and/or rotational speed (N) are adjusted in a manner providing a desired (e.g., substantially proportional) relationship between PLA position and the power output (SHP) of the turboprop engine. Embodiments of the single lever turboprop control system provide such a desired PLA-to-SHP relationship by first converting a PLA input signal to target torque output (Q tar ), to a target power output (SHP tar ), or to a combination thereof. The target power output (SHP tar ) can be expressed as either a discrete power level (SHP) or a percentage (% SHP) of a reference (e.g., maximum) power level. In arriving at the target power output (SHP tar ) and/or the target torque output (Q tar ), other input data may also be considered in addition to the PLA input signal. Such other input data can include sensor data describing current environmental, flight, and operating conditions of the A/C. At least one engine setpoint is then established as a function of the target torque output (Q tar ) and/or the target power output (SHP tar ). The engine setpoint or setpoints can be one or both of a blade angle setpoint (β set ) and a rotational speed setpoint (N set ). The newly-established engine setpoints are then applied to the turboprop engine by commanding the appropriate actuation systems to make the desired adjustments in blade angle and/or engine speed. In this manner, embodiments of the turboprop control system not only provide a single lever pilot interface similar to that of a turbofan or turbojet control system, but further achieve a substantially proportional relationship between PLA position and engine thrust similar more closely resembling the PLA position-to-thrust relationships provided by turbofan and turbojet control systems. This is highly desirable. 
     Embodiments of the single lever turboprop control systems and methods described herein can provide still further benefits and functionalities. For example, in certain embodiments, the single lever turboprop control system may be operable in multiple different scheduling modes, which can be selected in response to pilot input, changes in flight phase, or other such factors to help optimize engine performance across the entire flight cycle of the A/C. Furthermore, in certain embodiments, the single lever turboprop control system can include an active feedback component (e.g., a closed feedback loop), which actively adjusts the blade angle (β) and/or rotational speed (N) of the turboprop engine to reduce any measured discrepancies between the target torque output (Q tar ) and the current torque output (Q current ) and/or the target power output (SHP tar ) and the current engine power (SHP current ). Through the inclusion of such an active feedback component, the desired (e.g., proportional) relationship between PLA position and turboprop power output can be better maintained, while further enabling the turboprop control system to automatically adapt to changes in environmental conditions. Finally, embodiments of the single lever turboprop control system can further include one or more limiting functions enabling the power/torque-based scheduling logic to be overridden when appropriate to prevent operating parameters of the turboprop engine (e.g., bleed air temperatures, exhaust gas temperatures, mechanical stress levels, flow rates, etc.) from exceeding predefined thresholds to help reduce component wear, avoid component damage, and generally prolong the serviceable lifespan of the turboprop engine. 
       FIG. 1  is a schematic of a single lever turboprop control system  10  and single shaft turboprop engine  12 , as illustrated in accordance with an exemplary embodiment of the present invention. While turboprop control system  10  is illustrated in conjunction with a particular type of turboprop engine  12  in  FIG. 1 , it will be appreciated that single lever turboprop control system  10  can be utilized in conjunction with various other types of turboprop engines, including both fixed shaft (e.g., single shaft) and free turbine-type turboprop engines. Single lever turboprop control system  10  is described below in detail in conjunction with  FIGS. 1-3 , as are several power/torque-based scheduling processes that can be carried-out by turboprop control system  10  during operation thereof. First, however, a description of turboprop engine  12  is provided to establish an exemplary and non-limiting context in which embodiments of single lever turboprop control system  10  can be better understood. 
     In the exemplary embodiment illustrated in  FIG. 1 , turboprop engine  12  includes a propeller  14  having a propeller dome  16  from which a number of blades  18  project in a radially-outward direction. A Propeller Pitch Control (PPC) actuator  20  is at least partially housed within propeller dome  16  and can adjust the angle or pitch of propeller blades  18  in accordance with commands received from turboprop control system  10 . PPC actuator  20  can include or assume the form of a mechanical actuation system, a hydraulic actuation system, a hydromechanical actuation system, or another type of actuation system suitable for adjusting the angle or pitch of propeller blades  18  in response to commands received from turboprop control system  10 . 
     A shaft  22  projects from propeller  14  in an aftward direction to mechanically couple propeller  14  to a gearbox  24  containing a gear reduction (referred to hereafter as “reduction gearbox  24 ”). As schematically indicated in  FIG. 1 , reduction gearbox  24  can contain a first sensor  26  for measuring the current rotational speed of turboprop engine  12  (N current ) and a second sensor  28  for measuring the current torque output of engine  12  (Q current ). A single shaft  30  links the mechanical input of gearbox  24  to a gas turbine engine core  32 . Engine core  32  includes a compressor section  34 , a combustion section  36 , a turbine section  38 , and an exhaust section  40  coupled in flow series. Compressor section  34  can contain any type of compressors (e.g., axial and/or centrifugal compressors commonly referred to as “impellers”), any number of compressors, and any number of compressor stages separated by non-rotating vanes. Similarly, turbine sections  38  can contain any type of turbines (e.g., axial and/or radial turbines), any number of turbines, and any number of turbine stages separated by non-rotating vanes. While shown as including a single shaft in the illustrated example, turboprop engine  12  can include two or more co-axial shafts and varying numbers of compressor and turbines in further embodiments. Furthermore, while turboprop engine  12  is depicted as a single or fixed-shaft engine in  FIG. 1 , it is again emphasized that that turboprop control system  10  can also be utilized in conjunction with free turbine turboprop engines. In free turbine platforms, rotation of the propeller is driven by a separate turbine, which is fluidly (rather than mechanically) coupled to and driven by the combustive gasses generated by the core gas turbine engine. 
     During operation of turboprop engine  12 , the compressor(s) within compressor section  34  rotate to compress airflow ingested by turboprop engine  12  through a non-illustrated intake section. The compressed airflow is then directed into one or more combustion chambers located within combustion section  36 , mixed with fuel, and ignited. The combustive gasses heat rapidly, expand, and flow from combustion section  36  into turbine section  38  to drive rotation of turbine or turbines contained therein. Rotation of the turbine(s) within section  38  drives rotation of shaft  30 , which, in turn, drives rotation of propeller  14  through reduction gearbox  24 . The desired thrust output of turboprop engine  12  is largely provided by the rotation of propeller  14 . However, a relatively small amount of additional thrust may also be provided the combustive gases discharged from turboprop engine  12  through exhaust section  40 . 
     Turboprop control system  10  enables a pilot to adjust a number of operational parameters of turboprop engine  12  including the blade angle (β) of propeller blades  18  and the rotational speed (N) of engine core  32 . As noted above, adjustments in propeller blade angle (β) can be implemented by applying the appropriate commands to PPC actuator  20 . Turboprop control system  10  can also modulate the engine rotational speed (N) of turboprop engine  12  by sending appropriate command signals to a non-illustrated fuel metering system. As indicated in  FIG. 1  by an arrow connecting turboprop control system  10  to combustion section  36 , the fuel metering system may then increase or decrease the amount of burn fuel supplied to combustion section  36  to effectuate the desired engine speed adjustment. In certain embodiments, turboprop control system  10  may also regulate the engine rotational speed (N) utilizing various other non-illustrated features or devices included within of turboprop engine  12  and well-known within the avionics industry. Such other features can include, but are not limited to, variable inlet guide vanes, variable stator vanes, and bypass bleed valves. 
     With continued reference to the exemplary embodiment shown in  FIG. 1 , single lever turboprop control system  10  is schematically illustrated as including the following components, each of which may be comprised of multiple devices, systems, or elements: (i) an engine control unit or “ECU”  42 , (ii) a pilot interface  44 , (iii) a number of onboard sensors  46 , and (iv) a memory  48 . As schematically indicated in  FIG. 1 , pilot interface  44 , onboard sensors  46 , and memory  48  can be coupled to various inputs and/or outputs of ECU  42 , as appropriate, to carry-out the processing and control functions described herein. In this regard, the components of turboprop control system  10  can be interconnected utilizing any suitable aircraft interconnection architecture, which may include any combination of wired and wireless data connections. In many cases, the components of turboprop control system  10  will communicate over an avionics bus, which permits bidirectional signal communication with ECU  42 . The individual elements and components of turboprop control system  10  can be implemented in a distributed manner using any number of physically-distinct and operatively-interconnected pieces of hardware or equipment. Furthermore, alternative embodiments of turboprop control system  10  can include other components in addition to or in lieu of those listed above. For example, in further embodiments, turboprop control system  10  can include a second ECU, which functions in parallel with ECU  42  and also performs the below-described processes for purposes of redundancy. 
     ECU  42  can include or assume the form of any electronic device, system, or combination of devices suitable for performing the processing and control functions described herein. More specifically, ECU  42  can be implemented utilizing any suitable number of individual microprocessors, automated flight control equipment, memories, power supplies, storage devices, interface cards, and other standard components known in the art. Additionally, the ECU  42  may include or cooperate with any number of software programs (e.g., automated flight control logic programs) or instructions designed to carry-out various methods, process tasks, calculations, and control functions described herein. In one embodiment, and by way of non-limiting example only, ECU  42  is a digital engine controller, such as a Full Authority Digital Engine Controller or “FADEC.” 
     Memory  48  can include any number of volatile and/or non-volatile memory elements. In many embodiments, memory  48  will include a central processing unit register, a number of temporary storage areas, and a number of permanent storage areas. Memory  48  can also include one or more mass storage devices, such as magnetic hard disk drives, optical hard disk drives, flash memory drives, and the like. Memory  48  can store various programs and applications, which are executed by ECU  42  to perform the below-described control functions. In certain embodiments, memory  48  will store multiple formulae, multi-dimensional lookup tables, and/or the like suitable for converting PLA position and other inputs into various combinations of target torque outputs (Q tar ), target power outputs (SHP tar ), blade angle setpoints (β set ), and/or rotational speed setpoints (N set ) as described in detail below. Memory  48  may also store predetermined operational thresholds, such as maximum temperatures and flow rates, below which the operational parameters of turboprop engine  12  are desirably maintained. Although illustrated as a distinct block in  FIG. 1 , memory  48  can be incorporated into ECU  42  in further embodiments of turboprop control system  10 . 
     Onboard sensors  46  generate, measure, and/or provide different types of data related to the operational status of the A/C, the operational environment in which A/C operates, current flight parameters, and the like. Onboard sensors  46  can include or cooperate any number of distinct avionic systems including, but not limited to, a Flight Management Systems (FMSs), Inertial Reference Systems (IRSs), and/or Attitude Heading Reference Systems (AHRSs). Data provided by onboard sensors  46  may include, without limitation: airspeed data; groundspeed data; altitude data; attitude data including pitch data and roll data; yaw data; geographic position data, such as Global Positioning System (GPS) data; time/date information; heading information; weather information; flight path data; track data; radar altitude; geometric altitude data; wind speed data; wind direction data; fuel consumption; etc. Although schematically illustrated as separate symbols for illustrative clarity in  FIG. 1 , onboard sensors  46  can also include various sensors deployed within turboprop engine  12  including, for example, sensors  26  and  28  contained in reduction gearbox  24 . ECU  42  is configured to process data obtained from onboard sensors  46  to perform the turboprop control functions described more fully below in conjunction with  FIGS. 2 and 3 . 
     With continued reference to  FIG. 1 , pilot interface  44  can include any number of input devices (e.g., switches, dials, buttons, keyboards, cursor devices, cameras, microphones, etc.) suitable for receiving pilot input data useful in operating turboprop engine  12  via turboprop control system  10 . As a primary feature, pilot interface  44  includes a single lever control device  50  located in the A/C cockpit and coupled to an input of ECU  42 . Single lever control device  50  includes a control lever, which can be moved through a range of angular positions by a pilot when operating the A/C carrying turboprop control system  10  and turboprop engine  12 . When moved to a particular angular position, single lever control device  50  supplies a Power Lever Angle (“PLA”) position signal to ECU  42 . In response, ECU  42  converts the PLA control signal into a corresponding blade angle setpoint (β set ) and/or a corresponding rotational speed setpoint (N set ), which are then applied to turboprop engine  12  utilizing the appropriate actuation systems. For example, turboprop control system  10  may command PPC actuator  20  to implement any desired changes in blade angle (β) and/or command the non-illustrated fuel metering system to modulate the amount of burn fuel supplied to combustion section  36  to effectuate desired changes in engine rotational speed (N). In accordance with embodiments of the present invention, ECU  42  converts the PLA control signal to one or more setpoints (e.g., blade angle and/or rotational speed setpoints) utilizing a power/torque-based scheduling process. An exemplary embodiment of such power/torque-based scheduling process will now be described in conjunction with  FIG. 2 . 
       FIG. 2  is a schematic illustrating a power/torque-based scheduling process  58 , which can be implemented utilizing any suitable combination of software, hardware, and firmware and selectively carried-out by turboprop control system  10  ( FIG. 1 ) during operation thereof. Scheduling process  58  commences with receipt of a PLA control signal  60  at the input of a scheduling module  64 . PLA control signal  60  can be continually or periodically provided to scheduling module  64 ; or, instead, only supplied to scheduling module  64  when single lever control device  50  ( FIG. 1 ) is moved into a new position. Other inputs may also be provided to scheduling module  64  in conjunction with PLA control signal  60 . Such other inputs can include A/C sensor data  62  obtained from onboard sensors  46  ( FIG. 1 ) and describing environmental conditions, flight conditions, and/or operating characteristics of turboprop engine  12 . Such sensor data will often include the current rotational speed of turboprop engine  12  (N current ), as monitored by sensor  26  within gearbox  24  ( FIG. 1 ); and the current torque output of engine  12  (Q current ), as monitored by sensor  28  within gearbox  24 . In certain embodiments, A/C sensor data  62  may also include information from which the current flight phase can be derived and subsequently utilized in selecting amongst a number of scheduling modes, such as SCHEDULING MODES  1 - 4  shown in  FIG. 2  and described below. Onboard sensor data  62  may still further include other types of information utilized in determining the target torque output (Q tar ) and/or target power output (SHP tar ) values. 
     Scheduling module  64  may be operable in a single mode. Alternatively, scheduling module  64  may be selectively operable in multiple scheduling modes, which may be selectively implemented under varying conditions. In the illustrated example, scheduling module  64  is operable four different operational modes, which are identified in  FIG. 2  as “SCHEDULING MODES  1 - 4 .” When executed by ECU  42  ( FIG. 1 ), scheduling module  64  selects the appropriate scheduling mode for current usage. This selection may be made based upon pilot input received via pilot interface  44  ( FIG. 1 ) or in response to changes in any combination of environmental factors, flight conditions, or A/C system characteristics detected by onboard sensors  46 . Scheduling module  64  may, for example, select amongst SCHEDULING MODES  1 - 4  based upon the current flight phase of the A/C and the current operational mode of turboprop engine  12 . In one implementation, and by way of non-limiting example only, SCHEDULING MODES  1 ,  2 ,  3 , and  4  may be selectively activated depending upon whether turboprop engine  12  is operating in ground, flight idle, takeoff, climb, cruise, or descent modes. In further embodiments, scheduling module  64  and, more generally, scheduling process  58  executed by ECU  42  can include fewer or a greater number of scheduling modes. 
     When operating in SCHEDULING MODE  1 , scheduling module  64  first converts PLA control signal  60  and onboard sensor data  62  to a target torque output (Q tar ) utilizing a conversion function  66  (hereafter “PLA-to-Q conversion function  66 ”). PLA-to-Q conversion function  66  can be a multi-dimensional lookup table, a formula, or any other logic tool suitable for generating a target torque output (Q tar ) as a function of the input data. The target torque output (Q tar ) is then applied to two additional conversion functions: (i) a Q-to-N conversion function  68 , which converts the target torque output (Q tar ) to a corresponding rotational engine speed setpoint (N set ); and (ii) a Q-to-3 conversion function  70 , which converts the target torque output (Q tar ) to a corresponding blade angle setpoint (β set ). Onboard sensor data  62  may or may not be applied to Q-to-N conversion function  68  and Q-to-3 function  70  for consideration in establishing the engine speed setpoint (N set ) and the blade angle setpoint (β set ), respectively. Turboprop control system  10  ( FIG. 1 ) then generates commands in accordance with the newly-established engine speed setpoint (N set ) and the blade angle setpoint (β set ), which are delivered to the appropriate actuation systems to adjust the engine speed and blade angle of turboprop engine  12 . The actuation systems, turboprop engine  12 , and the sensors monitoring the operational parameter of engine  12  are collectively represented by a box  92  in  FIG. 2  and labeled as “TURBOPROP SYSTEM.” 
     SCHEDULING MODES  2  and  3  are similar to SCHEDULING MODE  1  in that, when operating in either of these modes, scheduling module  64  converts PLA control signal  60  and sensor data  62  to a target torque output (Q tar ). However, in contrast to SCHEDULING MODE  1 , only a single type of engine setpoint is generated by scheduling module  64  when operating in either SCHEDULING MODE  2  or SCHEDULING MODE  3 . When operating in SCHEDULING MODE  2 , specifically, scheduling module  64  converts PLA control signal  60  and onboard sensor data  62  to a target torque output (Q tar ) utilizing a PLA-to-Q conversion function  72 . Concurrently or sequentially, scheduling module  64  further converts PLA control signal  60  and onboard sensor data  62  to a rotational speed setpoint (N set ) utilizing a PLA-to-N conversion function  74 . Both the target torque output (Q tar ) and the rotational speed setpoint (N set ) are then applied to a Q/N-to-3 conversion function  76 , which generates a blade angle setpoint (β set ) for application to turboprop system  92 . By comparison, when operating in SCHEDULING MODE  3 , scheduling module  64  likewise converts PLA control signal  60  and onboard sensor data  62  to a target torque output (Q tar ) utilizing a PLA-to-Q conversion function  78 . However, in contrast to SCHEDULING MODE  2 , scheduling module  64  further converts PLA control signal  60  and onboard sensor data  62  to a blade angle setpoint (β set ) utilizing a PLA-to-3 conversion function  80 . Both the target torque output (Q tar ) and the blade angle setpoint (β set ) are then applied to a Q/β-to-N conversion function  82 , which generates a rotational speed setpoint (N set ). The rotational speed setpoint (N set ) is then applied to turboprop system  92  for implementation utilizing the appropriate actuation systems associated with turboprop engine  12  ( FIG. 1 ), as previously described. 
     Addressing lastly SCHEDULING MODE  4 , this operational mode is similar to SCHEDULING MODES  1 - 3  in that a target torque output (Q tar ) is calculated or otherwise established by scheduling module  64 . However, in the case of SCHEDULING MODE  4 , the target torque output (Q tar ) is not directly derived from PLA control signal  60 , but is instead determined from a target power output (SHP tar ), which is itself determined as a function of PLA control signal  60 . As can be seen in  FIG. 2 , two functions are initially performed when executing SCHEDULING MODE  4 : (i) a PLA/N-to-SHP conversion function  84 , and (ii) a PLA-to-N conversion function  86 . As does PLA-to-N conversion function  74  described above in conjunction with SCHEDULING MODE  2 , PLA-to-N conversion function  86  converts PLA control signal  60  and onboard sensor data  62  to a rotational speed setpoint (N set ). The rotational speed setpoint (N set ) generated by conversion function  86  is applied to PLA/N-to-SHP conversion function  84 , which then determines a target power output (SHP tar ) as a function of the current N set  value, the current PLA control signal  60 , and the current onboard sensor data  62 . The target power output (SHP tar ) can be expressed as either a discrete power level (SHP) or a percentage (% SHP) of a reference power level. The target power output (SHP tar ) output from PLA/N-to-SHP conversion function  84  is then supplied to a conversion function  86  along with the current rotational speed setpoint (N set ). Conversion function  86  next determines the target torque output (Q tar ) from these data inputs by, for example, dividing-out the current N set  value from the target power output (SHP tar ). Finally, the target torque output (Q tar ) and the rotational speed setpoint (N set ) are applied to a Q/N-to-3 conversion function  90 , which generates a blade angle setpoint (β set ) for application to turboprop system  92 . 
     By executing scheduling process  58  ( FIG. 2 ) in the above-described manner, ECU  42  ( FIG. 1 ): (i) establishes a target power output (SHP tar ) and/or the target torque output (Q tar ) as a function of at least the PLA control signal  60 , (ii) determines one or more engine setpoint adjustments to bring turboprop engine  12  into closer conformity with the target power output (SHP tar ) or the target torque output (Q tar ), and (iii) implements the setpoint adjustments by providing the appropriate commands to the turboprop actuation systems. ECU  42  can thus perform scheduling process  58  to provide a desired (e.g., substantially proportional relationship) between PLA position and the power output of turboprop engine  12  and, therefore, the thrust output of engine  12 . This notwithstanding, there may still exist unavoidable discrepancies between the actual thrust output of turboprop engine  12  and the desired thrust output of engine  12  after initial adjustment of the engine setpoints. Therefore, as further indicated in  FIG. 2 , it may be desirable to integrate feedback logic  94  into scheduling process  58 . Feedback logic  94  can receive data from sensors included within turboprop system  92  (e.g., sensors  26  and  28  illustrated in  FIG. 1 ) indicating the current measured torque output (Q current ) and/or the current measured rotational speed (N current ) of turboprop engine  12  ( FIG. 1 ). Feedback logic  94  may then cooperate with scheduling module  64  to determine further adjustments to the blade angle (β) and/or rotational speed (N) of the turboprop engine to reduce discrepancies between the target torque output (Q tar ) and the currently-measured torque output (Q current ) and/or the target power output (SHP tar ) and the currently-measured engine power (SHP current ). One exemplary manner in which feedback logic  94  and scheduling module  64  can perform this function when module  64  operates in a variation of SCHEDULING MODE  4  will now be described in conjunction with  FIG. 3 . 
       FIG. 3  is a schematic illustrating a power/torque-based scheduling sub-process  98  carried-out by scheduling module  64  and feedback logic  94  when operating in a variation of SCHEDULING MODE  4 , as illustrated in accordance with a further exemplary embodiment of the present invention. Certain features are shared between sub-process  98  shown in  FIG. 3  and the master scheduling process  58  shown in  FIG. 2 ; like reference numerals have thus been utilized to denote like logic or functional elements. Furthermore, as the manner in which the target torque output (Q tar ) and the rotational speed setpoint (N set ) are determined utilizing functions  84 ,  86 , and  88  has been previously described, this description will not be repeated at this juncture to avoid redundancy. In the variation of SCHEDULING MODE  4  shown in  FIG. 3 , and in contrast to SCHEDULING MODE  4  shown in  FIG. 2 , the blade angle setpoint (β set ) is determined based on a target torque input (rather than a target torque input and an engine speed input) and adjustments in engine speed are further generated from the current PLA position, as described below. 
     As can be been seen in  FIG. 3 , the target torque output (Q tar ) and the rotational speed setpoint (N set ) are further applied to comparators  100  and  102 , respectively, along with current torque output (Q current ) and the current engine rotational speed (N current ) detected by the engine sensors, such as sensors  26  and  28  shown in  FIG. 1 . The output signals of comparators  100  and  102  are then applied to control functions  104  and  106 , respectively, which condition the signals by noise filtering, gain amplification, the application of leads or lags, and/or the like. The output signal of torque control function  104  is subsequently provided to Q/N-to-3 conversion function  90 , which generates a blade angle setpoint (β set ) for application to turboprop system  92  in the manner previously described. In contrast to torque control function  104 , the output of engine speed control function  106  may be applied directly to turboprop system  92  for performing adjustment in engine rotational speed. A closed loop control system is thus provided, which functions to iteratively reduce any discrepancies that may exist between: (i) the target torque output (Q tar ) and the measured torque output (Q current ), as determined by comparator  100  and applied to function  90 ; and (ii) the rotational speed setpoint (N set ) and the measured engine rotational speed (N current ), as determined by comparator  102  and further applied to function applied  90 . Similar closed loop feedback schemes can also be utilized when scheduling module  64  operates in the other control modes illustrated in  FIG. 2 . 
     Briefly referring again to  FIG. 2 , it may also be desirable to integrate a limiting function  96  into master scheduling process  58 . Limiting function  96  can serve to override the power/torque-based scheduling logic in instances when the operating parameters of turboprop engine  12  risk exceeding maximum limits or predefined thresholds should the engine setpoints be adjusted in accordance with the target torque output (Q tar ) or target power output (SHP tar ). Consider, for example, sub-process  98  ( FIG. 3 ) wherein such a limiting function is implemented, in part, through the provision of a SELECT LEAST function  108  between control function  104  and conversion function  90 . Here, engine sensor data, as measured by sensors associated with turboprop engine  12  ( FIG. 1 ), is applied to a comparator  110  and compared against one or more predetermined operational thresholds or preset limits, as represented in  FIG. 3  by arrow  112 . The preset limits may be recalled from memory  48  of turboprop control system  10  ( FIG. 1 ). The particular type and number of operating characteristics that are monitored by turboprop control system  10  ( FIG. 1 ) and compared to the operational limits at comparator  110  will vary amongst embodiments. However, a non-exhaustive list of such characteristics includes exhaust gas temperatures, bleed air temperatures, bleed air flow rates, mechanical stress levels, and engine rotational speeds. The discrepancies, if any, between the measured characteristic(s) and the preset limit(s) is outputted by comparator  110  and applied to SELECT LEAST function  108  along with the output of control function  104 . A control function  114  can further be provided between the output of comparator  110  and the input of SELECT LEAST function  108  to provide signal conditioning similar to control functions  104  and  106 , as previously described. Additionally, if desired, control function  114  can convert the signal provided by comparator  110  to a torque value to facilitate comparison by SELECT LEAST function  108  with the torque value output by comparator  100  and control function  104 , as further described below. 
     With continued reference to  FIG. 3 , SELECT LEAST function  108  selects the minimum of the two inputs applied thereto. For example, SELECT LEAST function  108  can select between the applied inputs in accordance with the following instruction: “Select Minimum of {[predetermined operational characteristic—current value of the operational characteristic], [target torque output—current torque value]}.” If the current operational characteristic or characteristics of turboprop engine  12  ( FIG. 1 ) exceed their preset limit(s), SELECT LEAST function  108  effectively overrides torque-based scheduling of the blade angle (β) of turboprop engine  12  ( FIG. 1 ). Instead, in such an instance, SELECT LEAST function  108  applies the output of comparator  110  to conversion function  90 , which then schedules the engine setpoint or setpoints (in this case, blade angle) in a manner to prevent or at least reduce the likelihood of the monitored engine characteristic exceeding its predetermined operational threshold or preset limit. If the current value of the measured operational characteristics of turboprop engine  12  should exceed its limit, while the current torque output of turboprop engine  12  (Q current ) likewise exceed its target (Q tar ), SELECT LEAST function  108  will select the parameter that exceeds its limit or target by the greatest amount for subsequent application to conversion function  90 . In this manner, if the disparity between the current torque output (Q current ) and the target torque (Q tar ) is more pronounced, blade angle (β) will be scheduled to reduce or eliminate the pronounced disparity in engine torque. Conversely, if there should exist a more pronounced disparity in the current value of the monitored operational characteristic of turboprop engine  12  relative to its preset limit, blade angle (β) will be scheduled to reduce or eliminate this disparity. Thus, by integrating SELECT LEAST function  108  or similar logic into power/torque-based scheduling sub-process  98 , critical turboprop system parameters (e.g., torque, exhaust gas temperature, and other characteristics measured by the engine sensors and applied to comparator  110 ) can be better maintained within acceptable limits during operation of turboprop engine  12  ( FIG. 1 ). 
     The foregoing has thus provided embodiments of systems and methods enabling single lever control of a turboprop engine. In the above-described exemplary embodiments, blade angle (β) and/or rotational speed (N) of a turboprop engine are adjusted in a manner providing a desired (e.g., substantially proportional) relationship between PLA position and the power output (SHP) of the engine. Such an approach enables the single lever control system to effectively schedule thrust for a turboprop engine in a manner similar to single lever turbofan and turbojet control systems. In this manner, embodiments of the single lever turboprop control system can provide improved continuity with the single lever control systems of turbojet and turbofan engines from both pilot interface and engine behavior standpoints. Still further benefits that can be provided by embodiments of the single lever turboprop control system may include the ability to operate multiple scheduling modes based on flight mode or otherwise selected to optimize engine performance; the ability to actively adjust the engine setpoint to reduce discrepancies between the target torque output (Q tar ) and/or the target power output (SHP tar ) and measured outputs of the turbofan engine through the inclusion of feedback control logic; and/or the inclusion of limiting functions enabling the power/torque-based scheduling logic to be overridden when appropriate to prevent or reduce the likelihood of operating parameters exceeding predetermined threshold or limits during operation of the turboprop engine. 
     While at least one exemplary embodiment has been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. Various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set-forth in the appended Claims.