Abstract:
Maintenance requirements on a variable pitch propeller control system are minimized by eliminating the need for mechanical elements, such as a flyweight, mechanism in a secondary control for the variable pitch propeller. The system includes a secondary control ( 40 ) operating a pitch change mechanism ( 24 ) in response to the existence of at least one undesirable propeller condition and includes a target ( 34 ), ( 38 ) along with a target pickup ( 32 ), ( 36 ) for sensing the target ( 34 ), ( 38 ) and providing information to a function generator ( 76 ) which in turn provides information to a logic device ( 78 ), ( 80 ), ( 82 ), ( 84 ), ( 86 ), ( 88 ), ( 94 ) which in turn is connected to the pitch change mechanism ( 24 ) for operating the same.

Description:
FIELD OF THE INVENTION 
     This invention relates to a variable pitch propeller control system, and more particularly, to a variable pitch propeller and control system including a main pitch control for normally operating the pitch change mechanism of a variable pitch propeller along with a secondary control for operating the pitch change mechanism as a back up to the main control and which does not rely upon flyweight mechanisms or the like as part of the second control. 
     BACKGROUND OF THE INVENTION 
     Conventional variable pitch control systems for propellers employed in aircraft typically include a main control having two channels for redundancy which normally operate a pitch control mechanism in the propeller and a flyweight mechanism that acts as a back up or second control for the main control system. In systems of this sort, the flyweight mechanism is rotated with the propeller and as propeller angular velocity changes, the flyweights change their position in proportion to the angular velocity of the propeller. In the usual case, a spring or the like biases the flyweights toward one position and as the propeller angular velocity increases, the flyweights move against the bias of the spring. This movement is conventionally conveyed to a hydraulic valve which controls the flow of hydraulic fluid to a pitch change mechanism incorporated in the propeller itself. Should an undesirable propeller condition occur, such as an overspeed condition or movement of the propeller blades to a finer pitch than a so-called “flight idle” position, whereat the pitch of the blades is finer than a coarse position necessary to sustain flight, the valve controlled by the flyweight mechanism will direct hydraulic fluid at a greater pressure to the pitch change mechanism to move the blades toward a more coarse position, i.e., move the blades toward a feathered position, to alleviate the undesirable condition. 
     Such systems, like any other, require periodic maintenance to assure that they are operating properly, which is to say, to assure that they become operational at the proper threshold of propeller speed and/or flight idle pitch, amongst other things. At the same time, they must be adjusted so they do not interfere with the operational characteristics dictated by operation of the main control. 
     Experience with such systems in the field has tended to show that maintenance personnel working on such systems were spending more time adjusting and “tweaking” the mechanical back-up system, that is, the flyweight control, than on maintaining the main controls. This is not to say that maintenance of the main control is neglected, but rather, only that the costs of maintenance are undesirably large as a result of the effort to continually and properly adjust the secondary control system including a flyweight mechanism. 
     The present invention is directed to overcoming one or more of the above problems. 
     SUMMARY OF THE INVENTION 
     It is the principal object of the invention to provide a new and improved variable pitch propeller control system. More specifically, it is an object of the invention to provide a variable pitch propeller control system with a secondary control that eliminates mechanical components, such as a flyweight mechanism, that require an inordinate amount of time being maintained or adjusted to achieve proper balance in the system and compatibility with the main control. 
     An exemplary embodiment of the invention achieves the foregoing objects in a construction that includes a propeller having a hub rotatable about a rotational axis, and at least two propeller blades journal ed in the hub for rotation about axes crossing the rotational axis for blade pitch control as well as rotation about the rotational axis with the hub. The system includes a pitch change mechanism including a motor for rotating the blades in unison about the crossing axes between at least a coarse pitch position and a fine pitch position. A main pitch control is provided for normally operating the pitch change mechanism. There is further included a secondary control for operating the pitch change mechanism in response to the existence of at least one undesirable propeller condition and includes a target carried by the propeller along with a target pickup for sensing the target and generating a target signal. A function generator receives the target signal and generates at least one propeller condition signal indicating when at least one undesirable propeller condition occurs. A logic device is responsive to the propeller condition signal and is connected to the pitch change If mechanism and causes the pitch change mechanism to change the pitch of the blades to a pitch whereat the undesirable propeller condition no longer exists. 
     In a preferred embodiment, the main pitch control includes two control channels and the logic device causes the pitch change mechanism to change the pitch of the blades to a more coarse position. 
     In one embodiment of the invention, the undesirable propeller condition is a propeller overspeed condition and the function generator generates the propeller condition signal when the angular velocity of the propeller about the rotational axis exceeds a predetermined value. 
     According to one embodiment of the invention, the undesirable propeller condition is a propeller blade pitch less than a flight idle pitch and the function generator generates the propeller condition signal when the pitch of the blades about the crossing axes is a fine pitch finer than a predetermined flight idle pitch. 
     In a highly preferred embodiment, there are two undesirable propeller conditions including a propeller overspeed condition and a propeller blade pitch less than a flight idle pitch and the function generator generates the propeller condition signal when either or both 1) the angular velocity of the propeller about the rotational axis exceeds a predetermined value and/or 2) the pitch of the blades about the crossing axes is a fine pitch finer than a predetermined fine idle pitch. 
     In a preferred embodiment, the logic device includes a series of logic gates. 
     In a preferred embodiment, there is further included a device for selectively disabling the secondary control as, for example, during ground operations. 
     In a preferred embodiment, the secondary control includes two separate, generally identical control channels, each including one of the function generators and one of the logic devices. 
     Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a variable pitch propeller control system made according to the invention; 
     FIG. 2 is an enlarged, fragmentary somewhat schematic view of one target and associated pickup sensor illustrating their association with a propeller blade; and 
     FIG. 3 is a logic diagram of a logic device employed as part of a secondary control system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary embodiment of the invention is illustrated in FIG. 1 in schematic form and is seen to include a variable pitch propeller, generally designated  10 . The propeller  10  includes a hub  12  which is rotatable about an axis  14 . The propeller  10  is driven through a gear box  16  connected to a power plant  18  which will typically be a gas turbine engine but could, in some instances, be an internal combustion engine. The hub  12  mounts two or more propeller blades  20 . Specifically, the hub  12  journals the propeller blades  20  for rotation about axes  22  which cross the rotational axis  14 . Shown schematically within the hub  12  in dotted lines is a pitch control mechanism  24  which may be of conventional construction and which is ultimately driven by hydraulic fluid provided to it by one or more pumps  26  driven by the engine  18  and whose flow is modulated by a conventional, two channel hydraulic pitch control  28  which is the main control for the system. One channel is designated “A” while the other is designated “B”. An input  30 , conventionally pilot operated, provides information to the main control  28  for its use in controlling the propeller  10  in a conventional fashion. 
     The system includes a fixed target pickup unit  32  in close proximity to the propeller  10  to sense the presence of a metallic target  34 . The fixed target pickup unit  32  is stationarily mounted while the target  34  rotates with the propeller  10 . Though not shown in FIG. 1, in the usual case, there will be two of the fixed target pickup units, one for each of the channels A and B. The target  34  and pickup unit  32  may be of the general form disclosed in commonly assigned U.S. Pat. No. 5,865,599, the entire disclosure of which is herein incorporated by reference. 
     Also included is a movable target pickup unit  36  which senses the presence of another target  38  mounted on the propeller  10 . The target  38  is, as inferred by the name of the movable target pickup unit  36 , movable toward and away from the movable target pickup unit  36 . In one position, it may be sensed by the movable target pickup unit and in another position, it cannot be sensed by the movable target pickup unit. Specifically, when the pitch of the blades  20  moves to a position finer than the fine pitch of flight idle, the target  38  is moved by means to be described hereinafter, into proximity with the movable target pickup unit  36  so as to be sensed thereby. When the pitch of the blades  20  is as coarse as or coarser than the flight idle pitch, the target  38  cannot be sensed by the movable target pickup unit. 
     Both the fixed target pickup unit  32  and the movable target pickup unit  36  provide signals to a secondary or back-up control, generally designated  40 . The fixed target pickup unit also provides signals to each of the control channels A and B of the main control  28 . 
     As is well known, conventional hydraulic pitch main control systems such as that schematically shown at  28  include a so-called feather solenoid  42  which is controlled by airframe logic. This solenoid operates hydraulic valves which are opened when the airframe logic determines that the propeller should be feathered. When that occurs, the pressures of hydraulic fluids directed to the pitch change mechanism  24  are modified. This hydraulic fluid is applied to a motor within the pitch change mechanism  24 , typically in the form of a reciprocating piston, and the change in pressure differential across the piston, drives the piston toward a feathering or coarse pitch position for the blades  20 . This same solenoid  42  and a second feathering solenoid  44  may also receive signals for opening the associated valves from the secondary control  40 . Alternatively, for additional redundancy, two further feathering solenoids, one having a valve in parallel with the solenoid  42  and the other having a valve in parallel with the solenoid  44  may be operated by the secondary control  40 . 
     It is to be particularly noted that while the invention is being described in the context of a hydraulic pitch control system, the same may be used with equal advantage in an electrical pitch control system wherein a motor, such as a stepper motor, forms part of the pitch change mechanism  24  and is employed in altering the pitch of the blades  20  in lieu of a motor in the form of a piston. 
     One form of the target  38  is illustrated in FIG.  2 . An element  50  rotatable with the propeller  10  includes an aperture  52  through which the target  38  extends and which is aligned to pass the movable target pickup unit  36 . The target  38  has a head  54  on its end most remote from the pickup unit  36  and a compression coil spring  56  extends between the head  54  and the mounting element  50 . Any suitable retaining device  58  may be located on the pickup unit  36  near its end closest to the pickup unit  36  and a shoulder  60  is disposed on one of the blades  20  in a position to engage the head  54 . The shoulder  60  is located angularly about the crossing axis  22  for the blade  20  in question such that for all pitch conditions as coarse as or more coarse than flight idle, the target  38  will be in the solid line position illustrated in FIG.  2  and will consequently be sufficiently remote from the pickup  36  that it cannot be detected thereby. On the other hand, for pitches finer than flight idle, the shoulder  60  will cam the target  38  toward the pickup unit  36  to a position sufficiently close that it may be sensed thereby. 
     Turning now to FIG. 3, the secondary control  40  will be described. As noted previously, it preferably is a two channel secondary control with one channel being designated  70  and the other being designated  72 . As both the channels  70  and  72  are identical, one to the other, only the channel  70  will be described. The channel  70  is associated with the A channel of the main control  28  while the channel  72  is associated with the B channel of the main control  28 . To this end, each receives inputs as indicated by a block  74  from the fixed target pickup unit  32  and the movable target pickup unit  36  for the associated channel. These inputs are in the form of a string of pulses with each pulse being generated each time one of the targets  34 ,  38 , is sensed by the corresponding pickup unit  32 , 36 . In normal operation, pulse trains corresponding only to those sensed by the fixed target pickup unit will be present. This pulse train is provided to a function generator  76  of conventional construction. And, when present, a pulse train from the movable target pickup unit  36  will likewise be fed to the function generator  76 . 
     In the usual case, with only the fixed target pickup unit  32  generating pulses, the number of pulses sensed over a given period of time is indicative of the angular velocity of the propeller  10  and the function generator  76  will be quiescent, that is, will not provide an output. However, if an overspeed condition occurs, over the same time interval, a greater number of pulses will be sensed by the function generator  76 . Should this number of pulses be indicative of an angular velocity greater than 102% of the predetermined maximum velocity for the propeller  10 , an overspeed signal will be issued as indicated by a box  78 . Similarly, should the function generator  76  begin to receive pulses from the movable target pickup unit  36 , those pulses, in addition to those received from the fixed target pickup unit will generate a pulse input of approximately twice that for normal operation. As this can occur only when the propeller  20  has been moved to a pitch less than flight idle, the function generator  76  will respond to the vastly increased number of pulses to generate a pitch less than flight idle signal as indicated by a box  80 . These signals are provided as an input to an OR gate  82  which in turn provides an output on a line  84  indicative of the presence of either or both of the above-described undesirable propeller conditions. 
     The line  84  is connected as an input to an AND gate  86  which in turn receives a second input from an inverter or NOT gate  88 . The inverter  88  receives an input signal indicating that the associated channel of the secondary system  40  should be disabled as indicated by a box  90 . This disabling signal will typically be generated by the pilot through an appropriate control during certain situations, most notably, ground operations when propeller pitch less than flight idle may be desired or, if the propeller is constructed to provide the reverse thrust, when reverse thrust is applied. As a consequence of this construction, the AND gate  86  can only issue a signal for ultimately causing operation of the secondary control system  40  when no disabling signal is present at the box  90 . Needless to say, associated interlocks will be provided so that the disabling signal cannot be inadvertently issued when the aircraft is airborne as is well known in the art. 
     Another input to the channel  70  is an air frame feather request input as indicated by a box  92 . This signal will typically be issued by the pilot when, for any reason, he desires the propeller to be moved to a feathered pitch condition. This signal is applied as an input to an OR gate  94  which also receives the output of the AND gate  86 . Thus, the output of the OR gate  94  will go true whenever there is an air frame feather request or a determination of overspeed or a propeller blade pitch of less than flight idle has been made with the system not disabled. 
     When either of the OR gates  94  in the channels  70  and  72  go true, they are integrated with signals generated elsewhere on the aircraft as well as in the other channel. For example, if the main control channel A issues a feather request as indicated by a box  96 , or if a similar feather request is received from channel B as indicated by a box  98 , these signals are provided to various gates to be described. Ultimately, a true output from an OR gate  100  will cause operation of the feather solenoid for the A channel  42  while a true output from an OR gate  102  will result in a similar signal to the feather solenoid  44  for the B channel of the main control. 
     Viewing first the inputs to the OR gate  100 , one input can be received on a line  104  from the output of an AND gate  106 . The AND gate  106  is connected to receive the output of the OR gate  94  for the channel  70  as well as a feather request from the A channel of the main control  28 . Consequently, the solenoid  42  will be energized whenever the output  94  for the channel  70  goes true and a channel A feather request as shown at block  96  is present. 
     A similar AND gate  108  operates in the same fashion when the OR gate  94  for the channel  72  shows true and a channel B feather request is received from the main control  28 . 
     In addition, an AND gate  110  is connected to the output of the OR gates  94  for both of the channels  70 ,  72  and has its output connected as an input to both of the OR gates  100  and  102 . Consequently, if both the channels  70  and  72  detect one or the other or both of the undesirable propeller conditions, both of the solenoids  42  and  44  will be energized to cause the pitch change mechanism  24  (FIG. 1) to move the blades  20  towards a feathered or coarse position. 
     Finally, an AND gate  112  is connected to receive both of the main control feather requests shown at blocks  96  and  98 . When both are present, its output will go true and is fed to both the OR gates  100 ,  102  to energize both of the solenoids  42  and  44 . 
     Thus, it will be appreciated that either one of the channels  70 ,  72  of the secondary control  40  may be disabled when a disable signal is present at the corresponding input block  90 . This signal effectively prevents the corresponding AND gate  86  from going true and providing a true input to the corresponding OR gate  94 . At the same time, if an air frame feather request, as for example, generated by the pilot, and present at corresponding block  92  is present, the output of the corresponding OR gate  94  may go true. 
     Assuming the absence of a disabling signal, the outputs of the AND gates  86  may go true to the corresponding OR gates  94  whenever an overspeed condition or a pitch less than flight idle pitch is determined to exist. The secondary channel  70  associated with the A channel can have its associated solenoid  42  energized to cause the pitch change mechanism to move the blades  20  towards a coarser position only when one of the AND gates  106 ,  110  or  112  goes true. The AND gate  106  will go true only when there is an undesirable propeller condition determined by the channel  70  or when the same receives an air frame feather request and a feather request is received from the main channel at block  96 . The output of the AND gate  110  may only go true when both the channels  70  and  72  have their OR gates  94  with true outputs which, in turn, can only occur during the presence of an undesirable flight condition or an air frame feather request. The AND gate  112  can only go true when feather requests are received from both channels of the main control. 
     The AND gate  108 , which may cause the solenoid  44  to be energized, can only go true for the same conditions as the AND gate  106  except for the fact that the channel  72  and the B channel of the main control are involved. As a consequence, in a system where the main control has two channels and the secondary control  40  has two channels, it will be appreciated that at least two of the channels must be directing a change in blade pitch towards the feathering condition before the system will respond. Consequently, failure in any one of the four channels will not be recognized as the presence of a command from the pilot or as the existence of an undesirable propeller condition. Reliability, is thus assured. 
     It will especially be appreciated that the system completely eliminates the need for mechanisms such as fly weight mechanisms as part of the secondary control function. Consequently, adjustments are virtually eliminated from the system thereby eliminating the maintenance cost associated therewith.