Abstract:
A method for controlling a propeller of an aircraft, comprises receiving, with a processor, one or more signals indicative of commanded collective pitch of the propeller; receiving, with the processor, one or more sensed signals indicative of propeller axial speed, propeller rotational speed, and air density; estimating, with the processor, a propeller torque and propeller thrust from one or more of the propeller axial speed, the propeller rotational speed, and the air density; determining, with the processor, information indicative of an error value between a desired torque and a measured torque in the propeller; determining, with the processor, information indicative of a corrected pitch command in response to the determining of the error value; combining, with the processor, the corrected pitch command with the propeller rotational speed into an adjustment solution; providing, with the processor, the propeller with the adjustment solution.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates generally to the field of rotating blades and, more particularly, to a method of controlling a propeller in an aircraft having a controller that limits a commanded pitch of a propeller within torque and thrust limitations or controls the pitch in order to maintain a constant torque of the propeller. 
       DESCRIPTION OF RELATED ART 
       [0002]    Traditional aircraft with fixed-pitch propellers are optimized for either takeoff or climb as the efficiency of the engine is non-linear and a function of propeller speed. Aircraft equipped with constant speed propellers adjust the blade pitch to maintain the desired propeller speed thereby maximizing engine performance and efficiency over varying flight conditions. However, these constant speed propellers require a cluster of mechanical parts that add weight and complexity to the propeller design as well as reduce their reliability. Further, variable-pitch propellers can exceed structural limits of the airframe when operated in off-design conditions. An electronic controller for a variable pitch propeller that limits over-thrust or over-torque, or one which maintains a constant torque would be well received in the art. 
       BRIEF SUMMARY 
       [0003]    According to one aspect of the invention, a method for controlling a propeller of an aircraft, comprises receiving, with a processor, one or more signals indicative of commanded collective pitch of the propeller; receiving, with the processor, one or more sensed signals indicative of propeller axial speed, propeller rotational speed, and air density; estimating, with the processor, a propeller torque and propeller thrust from one or more of the propeller axial speed, the propeller rotational speed, and the air density; determining, with the processor, information indicative of an error value between a desired torque and a measured torque in the propeller; determining, with the processor, information indicative of a corrected pitch command in response to the determining of the error value; combining, with the processor, the corrected pitch command with the propeller rotational speed into an adjustment solution, the propeller rotational speed being governed by a full authority engine controller; providing, with the processor, the propeller with the adjustment solution; and receiving, with the processor, a subsequent error value between the desired torque and the measured torque in response to the providing of the adjustment solution. 
         [0004]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include determining an estimated upper limit of a collective pitch as a function of maximum torque and maximum thrust from a predefined schedule. 
         [0005]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include determining an estimated lower limit for the collective pitch as a function of minimum torque and minimum thrust from a predefined schedule. 
         [0006]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include determining each of an upper limit collective pitch command and a lower limit collective pitch command from the estimated upper and lower limits. 
         [0007]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include adjusting the corrected pitch command as a function of at least one of the commanded collective pitch, the propeller axial speed, and the propeller rotational speed. 
         [0008]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include determining a pitch rate from the commanded collective pitch, the estimated torque, and the estimated thrust. 
         [0009]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include applying each of a lower limit constant value and pitch rate limiting value to the commanded collective pitch to drive down the commanded collective pitch to the lower limit constant value, the lower limit constant value representing a safe lower limit of a collective pitch. 
         [0010]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include applying the lower limit constant value if at least one of the propeller thrust or the propeller torque is greater than its respective upper limit value. 
         [0011]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include applying each of an upper limit constant value and pitch rate limiting value to the commanded collective pitch to drive up the commanded collective pitch to the upper limit constant value, the upper limit constant value representing a safe upper limit of a collective pitch. 
         [0012]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include applying the upper limit constant value if at least one of the propeller thrust or the propeller torque is lesser than its respective lower limit value. 
         [0013]    According to another aspect of the invention, a system for controlling a plurality of propeller blades of an aircraft, comprise a propeller comprising the plurality of blades, wherein the propeller is associated with a sensor; a processor; and memory having instructions stored thereon that, when executed by the processor, cause the system to: receiving one or more signals indicative of commanded collective pitch of the propeller; receive one or more sensed signals indicative of propeller axial speed, propeller rotational speed, and air density; determine information indicative of an error value between a desired torque and a measured torque in the propeller; determine information indicative of a corrected pitch command in response to the determining of the error value; combine the corrected pitch command with the propeller rotational speed into an adjustment solution, the propeller rotational speed being governed by a full authority engine controller; provide the propeller with the adjustment solution; and receive a subsequent error value between the desired torque and the measured torque in response to the providing of the adjustment solution. 
         [0014]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor is configured to determine an estimated upper limit of a collective pitch as a function of maximum torque and maximum thrust from a predefined schedule. 
         [0015]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor configured to determine an estimated lower limit for the collective pitch as a function of minimum torque and minimum thrust from a predefined schedule. 
         [0016]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor configured to determine each of an upper limit collective pitch command and a lower limit collective pitch command from the estimated upper and lower limits. 
         [0017]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor configured to adjust the corrected pitch command as a function of at least one of the commanded collective pitch, the propeller axial speed, and the propeller rotational speed. 
         [0018]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor is configured to estimate a torque and thrust from one or more of the propeller axial speed, the propeller rotational speed, and the air density. 
         [0019]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor configured to apply each of a lower limit constant value and a pitch rate limiting value to the commanded collective pitch to drive down the commanded collective pitch to the lower limit constant value, the lower limit constant value representing a safe lower limit of a collective pitch. 
         [0020]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor is configured to apply the lower limit constant value if at least one of the propeller thrust or the propeller torque is greater than its respective upper limit value. 
         [0021]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor configured to apply each of an upper limit constant value and pitch rate limiting value to the commanded collective pitch to drive up the commanded collective pitch to the upper limit constant value, the upper limit constant value representing a safe upper limit of a collective pitch. 
         [0022]    In addition to one or more of the features described above or below, or as an alternative, further embodiments could include the processor configured to apply the upper limit constant value if at least one of the propeller thrust or the propeller torque is lesser than its respective lower limit value. 
         [0023]    Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
         [0024]    Technical effects of embodiments include the capability of electronically maintaining a constant propeller rotational speed to improve efficiency regardless of flight conditions. Ease of maneuvering is provided by control laws that automatically adjust propeller blade pitch to maintain a desired rotor speed and torque setting, eliminating the risk of over-torquing the propeller gearbox. Weight savings and reduced complexity to the mechanical propeller design are other benefits due to the elimination of electrically or hydraulically driven blade pitch changing mechanism. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0025]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which like elements are numbered alike in the several FIGURES: 
           [0026]      FIG. 1  is a perspective view of an exemplary rotary wing aircraft for use with embodiments of the invention; 
           [0027]      FIG. 2  is a schematic block diagram of an embodiment of a control system for a rotary wing aircraft; 
           [0028]      FIG. 3  is a schematic block diagram of a torque control law of the control algorithm of  FIG. 2  according to an embodiment of the invention; 
           [0029]      FIG. 4  is a schematic block diagram for implementing the control algorithm of  FIG. 2  according to an embodiment of the invention; and 
           [0030]      FIG. 5  is a schematic block diagram for implementing the control algorithm of  FIG. 2  according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Referring now to the drawings,  FIG. 1  illustrates a vertical takeoff and landing (VTOL) rotary-wing aircraft  10  having a dual, counter-rotating, coaxial rotor system  12  and a translational thrust system  16 . The aircraft  10  includes an airframe  14  which supports the dual, counter rotating, coaxial rotor system  12 , which rotates about a rotor axis of rotation A, and the translational thrust system  16 , which provides translational thrust generally parallel to an aircraft longitudinal axis, L. The coaxial rotor system  12  includes an upper rotor system  13  and a lower rotor system  15  and a plurality of rotor blades  18  connected thereto for rotation about the rotor axis of rotation A. Any number of blades  18  may be used with the rotor system  12 . The translational thrust system  16  includes a pusher propeller  20  mounted at an aerodynamic tail fairing  22  and also includes a plurality of propeller blades  21 . The translational thrust system  16  may be mounted to the rear of the airframe  14  to provide thrust for high-speed flight. A main gearbox  24  (illustrated schematically) may be located above the aircraft cabin  26  and drives the rotor system  12 . The translational thrust system  16  may be driven by the same main gearbox  24  that drives the coaxial rotor system  12 . The main gearbox  24  is driven by one or more engines (illustrated schematically at  28 ). Although a tail mounted translational thrust system  16  is disclosed in this embodiment, it should be understood that other configurations and/or machines, such as high speed compound rotary wing aircraft with supplemental translational thrust systems, dual contra-rotating coaxial rotor system aircraft, tilt-rotors, tilt-wing aircraft and fixed wing aircraft will also benefit from embodiments of the invention. 
         [0032]      FIG. 2  illustrates a control system  30  that provides, in an embodiment, a torque feedback loop closure in order to keep the propellers  20  ( FIG. 1 ) at a constant torque. Control system  30  also provides a blade pitch command to propellers  20  ( FIG. 1 ) that limits a pilot&#39;s command to change the propeller pitch such that the propeller thrust and propeller torque do not exceed structural limitations. The control system  30  implements a torque control algorithm  42  to provide control to the propellers  20  ( FIG. 1 ) including implementing a torque control strategy and control circuitry that is described in the various embodiments herein. A schematic of a control system  30  to accomplish this is illustrated. Pilot commands/inputs  34  from pilot inceptors are received by a flight controller  32  as a commanded change to the propeller pitch. A number of sensors  36  are provided on aircraft  10  in order to sense flight conditions of aircraft  10  such as, in some non-limiting examples, propeller axial speed, propeller rotational speed, airspeed, measured thrust, measured torque, or the like. Data from sensors  36  is directed to flight controller  32  operably connected to sensors  36  where they are compared to control laws  38 . Control laws  38  define flight control actuator commands  40  which result in the desired response e.g., actual torque and thrust for aircraft  10 . In embodiments, the flight control commands  40  may be estimated from aircraft parameters or determined according to a schedule of propeller pitch commands as a function of propeller axial airspeed, propeller rotational speed, air density, thrust coefficients, and torque coefficients that are stored in memory  46  in one or more lookup tables. Control system  30  includes a Full Authority Digital Engine Controller (FADEC)  35  for each engine  28  ( FIG. 1 ) to control engine speed and torque. In an embodiment, control system  30  uses FADEC  35  and data from pilot commands  34  and sensors  36  to control blade pitch and torque commands to the propeller  20  and keep propeller  20  rotating at a constant speed over varying flight conditions. 
         [0033]    In an embodiment, controller  32  includes a memory  46 . The memory  46  stores torque control algorithm  42  as executable instructions that is executed by processor  44 . The instructions may be stored or organized in any manner and at any level of abstraction, such as in connection with the execution of the torque control algorithm  42 . Processor  44  may be any type of processor (CPU or GPU), including a general purpose processor, a digital signal processor, a microcontroller, an application specific integrated circuit, a field programmable gate array or the like. Also, in embodiments, memory  46  may include random access memory (RAM), read only memory (ROM), or other electronic, optical, magnetic or any other computer readable medium onto which is stored torque control algorithm  42  described below. 
         [0034]      FIG. 3  illustrates a schematic view of a torque control strategy  50  as part of torque control algorithm  42  that is implemented by Flight Controller  32  for providing blade pitch commands to the propeller  20  according to an embodiment of the invention. Initially, a signal for a torque error value  58  is determined in Summation block  54 . Torque error value  58  is the difference between a reference signal for a desired torque  52  and a signal representing measured/actual torque  56  received from torque sensor block  62 . The torque sensor  62  determines the actual torque  70  (sensed torque) at the propeller  20  ( FIG. 1 ). The error value  58  is fed as a signal to a torque controller block  60 . Torque controller block  60  determines data that represents a corrected pitch command  66  for collective pitch control of propeller  20  ( FIG. 1 ). The torque controller  60  processes the error value  58  through iterative cycles and multiplies it by a gain(s) to arrive at a signal that represents a calculated blade pitch command  66 . The blade pitch command  66  represents a degree of change in attitude of propeller blade  21  for propeller  20  ( FIG. 1 ). Also, the FADEC  35  ( FIG. 2 ) provides a rotor/propeller rotational speed loop closure  64  to keep propeller  20  ( FIG. 1 ) rotating at a constant speed. As illustrated, torque control strategy  50  is an iterative process for determining the propeller blade pitch commands  66  to be applied to the propeller  20  ( FIG. 1 ) in order to achieve a desired torque setting  52  of propeller  20  ( FIG. 1 ) and maintain a constant rotational speed thereby maximizing flight efficiency in varying flight conditions and provide the pilots with care-free maneuvering capability. 
         [0035]      FIG. 4  illustrates an exemplary schematic block diagram  80  for implementing torque control algorithm  42  for implementation by flight controller  32  ( FIG. 2 ) in order to determine blade pitch commands  116  for controlling actuators of propeller blades  21  to stay within predefined thrust and torque limits. As such,  FIG. 2  is also being referenced in the description of  FIG. 4 . Blade pitch commands  116  utilize a schedule of predefined thrust and torque limits for commanded propeller pitch. The predefined limits are accessed by torque control algorithm  42  from a plurality of lookup tables  88 ,  90 ,  92 , and  94  that are stored in memory  46 . 
         [0036]    In an embodiment, implementation of torque control algorithm  42  begins when flight controller  32  receives and stores signal inputs  82  for a commanded rate of change in propeller pitch received from pilot inceptors  34 . Constants for each of a pitch kinematics upper limit and a pitch kinematics lower limit are applied to limited integrator  86  such that the output of limited integrator  86  is a commanded propeller collective pitch reference signal  112  that is bounded within these limits. 
         [0037]    Signal inputs  84  for sensed flight conditions from sensors  36  are also received by flight controller  32 . Signal inputs  84  can include Signal inputs  84  for sensed flight conditions from sensors  36  are also received by flight controller  32 . Signal inputs  84  can include propeller axial speed (i.e., vehicle speed), propeller rotational speed, air temperature, and air density. 
         [0038]    Lookup tables  88 - 94  include estimated values for upper and lower pitch command limits based on thrust and torque values that are predefined for rotorcraft  10  ( FIG. 1 ). These estimated values may be predetermined or derived from, in some non-limiting examples, simulated data, or flight test data. For example, lookup table  88  includes upper limits on propeller pitch command based on a maximum torque, lookup table  90  includes predefined values for upper limits on propeller pitch command based on a maximum thrust, lookup table  92  includes lower limits on propeller pitch command based on a minimum torque, and lookup table  94  includes lower limits on propeller pitch command based on a minimum thrust. Lookup table  88  provides a signal  96  that is an estimate for an upper limit of propeller pitch command for a maximum torque value. Lookup table  90  provides a signal  98  that is an estimate of an upper limit for a propeller pitch command for a maximum thrust value. These signal values  96 ,  98  are compared in minimum block  104 , and the minimum of the two values is passed as a signal  108  on to asymmetric limiter  114  as its upper limit. Also, lookup table  92  provides a signal  100  that is an estimate for a lower limit of a propeller pitch command for a maximum torque value and lookup table  94  provides a signal  102  that is an estimate for a lower limit for a propeller pitch command for a minimum thrust value. These values  100 ,  102  are compared in maximum block  106  and the maximum of the two values is passed as signal  110  on to asymmetric limiter  114  as its lower limit. The asymmetric limiter  114  determines if the commanded propeller collective pitch reference signal  112  from a pilot may subject the propeller or its supporting structure to excess stresses and imposes the respective upper and lower pitch command limits  108 ,  110  on the commanded collective pitch  112 . Output signal  116  represents blade pitch commands that are implemented on actuators for controlling blade pitch of propellers blades  21 . 
         [0039]      FIG. 5  illustrates a schematic block diagram of circuitry  150  for implementing torque control algorithm  42  by Flight Controller  32  ( FIG. 2 ) for providing limited propeller pitch command signal  206  to propeller  20  according to an embodiment of the invention. As such  FIG. 2  is also referenced in the description of  FIG. 5 . Blade pitch commands  206  control blade pitch of propeller blades  21  through propeller pitch commanded inputs and measured torque values. In an embodiment, implementation of torque control algorithm  42  begins when Flight Controller  32  receives and stores signal inputs such as, for example, pilot commanded rate of change in propeller pitch. As an alternative, estimated values for thrust  174  and torque  176  can be used in lieu of measured values of thrust  174  and torque  176  respectively. These estimated values may be predetermined or derived from, in some non-limiting examples, simulated data, or flight test data. Thrust  174 , signal  180 , and signal  182  are applied to an asymmetric thrust limiter block  178 . Also, torque  176 , signal  184 , and signal  186  are applied to an asymmetric torque limiter block  181 . Signal  180  is a predetermined or defined constant value for an upper structural limit of propeller thrust while signal  182  is a predetermined or define constant value for a lower structural limit of propeller thrust. Further, signal  184  is a predetermined or defined constant value for an upper structural limit of propeller torque while signal  186  is a predetermined or defined constant value for a lower structural limit of propeller torque. 
         [0040]    Also depicted in  FIG. 5 , a beep pitch down command  152 , a beep pitch up command  154 , beep down rate  156 , and beep up rate  158  are applied to a logic block  160 . Also, Boolean value signal  190  (output from OR-gate  191 ) and Boolean value signal  192  (output from OR-gate  193 ) are applied to logic block  160 . Beep pitch commands  152  and  154  represent a commanded rate of change in collective pitch that is received through pilot inceptors. In an embodiment, logic block  160  compares the commanded rate of change in propeller pitch to the status of the limiting algorithm as a means of rapidly resetting a reference signal  172  representing a pilot commanded value of propeller pitch (also called a reference signal). Output signal  164  represents a defined magnitude of a beeper rate to be applied to a limited integrator block  170 . In an alternative embodiment, output signal  164  can include a variable magnitude for the beeper rate. Output signal  164  is provided to a limited integrator block  170  together with signal  163 , pitch kinematic upper limit signal  166 , and pitch kinematic lower limit signal  168 . Output signal  172  from limited integrator block  170 , signal  194  representing a predetermined or defined safe value for a lower limit of pitch command, signal  192 , signal  163 , and signal  196  representing a predetermined or defined prescribed rate for pitch limiting are applied to an increase pitch switch block  198  for comparison. Output signal  201  representing commanded value of propeller pitch, signal  200  representing a predetermined or defined safe value for an upper limit of pitch command, signal  192 , signal  196 , and signal  163  are applied to a decrease pitch switch block  202  for comparison. In an embodiment of the invention, blocks  198  and  202  can include rates, in lieu of signal  196  for prescribed rate, which are proportional to the difference between the thrust signal  174  and its limits  180 ,  182  and/or proportional to the difference between the torque signal  176  and its limits  184 ,  186 . Output signal  203  from decrease pitch switch block  202 , feedback signal for limited propeller pitch command signal  206 , and signal  163  are filtered in a smoothing filter block  204  to provide a limited propeller pitch command signal  206 . 
         [0041]    In operation, if either the thrust signal  174  (measured or estimated) or the torque signal  176  (measured or estimated) is greater than their respective upper limit signals  180  and  184 , then signal  190  is TRUE and the pilot&#39;s commanded value of propeller pitch  172  (or reference signal) is driven down towards a safe value signal  194  at a prescribed rate signal  196  resulting in output signal  201 . Similarly, if either the thrust signal  174  (measured or estimated) or the torque signal  176  (measured or estimated) is lesser than their respective lower structural limit signals  182  and  186 , then signal  192  (output from OR-gate  193 ) is TRUE and signal  201  representing a commanded value of propeller pitch  201  is driven up towards a safe value signal  200  at a prescribed rate through signal  196  resulting in output signal  203 . Signal  203  is passed through a smoothing filter block  204  to obtain a limited propeller pitch command signal  206 . In addition, if the pilot&#39;s input beep pitch command signals  152 ,  154  are also persistently in a direction consistent with the automatic limiting as determined by comparison to signals  190  and  192 , then signal  163  initiates a reset of the integrator block  170  and the smoothing filter block  204  in order to align the propeller pitch reference signal  172  with the limited propeller pitch command  206 . 
         [0042]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. For instance, aspects of the invention are not limited to rotorcraft, and can be used in wind turbines, engine turbines, and other systems with rotary elements. Many modifications, variations, alterations, substitutions or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while the various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.