Abstract:
A backup governing system for a variable pitch propeller, carried by the propeller hub which is hydraulically interposed between a main control for the propeller and a propeller blade moving, double acting piston, the backup governing system comprising a spool valve including a spool movable between a plurality of metering positions along with first and second opposing springs applying oppositely directed biasing forces to the valve member. A flyweight assembly is connected to the spool and applies a speed dependent valve positioning force to the spool in opposition to a first of the springs and in addition to the second of the springs. A piston valve is operatively associated with the second spring and operable to change the bias applied by the second spring to the spool.

Description:
FIELD OF THE INVENTION 
     This invention relates to variable pitch propellers used on aircraft, and more specifically, to an improved backup governing system for such propellers. 
     BACKGROUND OF THE INVENTION 
     Variable pitch propellers are employed on many differing types of aircraft having power plants ranging from piston engines to gas turbines. Conventionally, such systems include redundant main control systems with one of the control systems being operable to take over the pitch control function if the other main system malfunctions. Moreover, it is common practice to provide a backup governing system for the redundant main control systems, particularly where the main control systems are electronic pitch control (EPC) systems. The backup governing system should be capable of determining when a propeller overspeed condition exists and causing propeller blades to move toward a coarse pitch condition which is sufficient to slow the rate of rotation of the propeller to a maximum allowable speed. 
     In addition, the backup control system should also be capable of determining when a low pitch condition (also referred to as a “low pitch stop”) exists wherein the pitch of the propeller becomes less than that defined as a minimum, in-flight allowable pitch (often called “flight idle” pitch) and cause the propeller blades to return to a coarser pitch that is at least equal to or greater than the flight idle pitch. 
     Still further, if the variable pitch propeller system is one where the pitch of the blades can be changed to cause a reverse thrust condition, as, for example, employed at slowing an aircraft on a runway just after having landed, the backup governing system must include provision for manually disabling the backup functions providing overspeed protection and low pitch stop. 
     As propeller pitch control systems, including EPC systems conventionally employ hydraulic fluid under pressure as a means for controlling the pitch of the propeller blades, it is highly desirable to provide a backup control system which is operable notwithstanding flight conditions such as a momentary loss of hydraulic power. It is also desired to eliminate mechanical gear trains or other connections between the rotating and stationary parts of the propeller. 
     Furthermore, an additional constraint is the requirement that the backup governing system interfaced with three existing pressure signals which are available to the propeller from its control. This constraint minimizes cost and enhances the ability to retrofit a system on existing propeller control systems. 
     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 backup governing system for a variable pitch propeller. More specifically, it is an object of the invention to provide such a system wherein the components are carried by the propeller, eliminating a mechanical interface for the backup control between rotating and stationary parts of the propeller system and which may interface with existing pressure signals in a conventional system. 
     An exemplary embodiment of the invention achieves the foregoing object in a variable pitch propeller system that includes a rotatable propeller hub that is adapted to be driven by a prime mover. A double acting piston is carried by the hub and propeller blades having shanks journaled in the hub are provided. A linkage connects the piston to the shanks so that movement of the piston will cause rotation of the shanks within the hub. A transfer bearing is provided for providing at least first and second streams of hydraulic fluid under pressure. The first stream is adapted to be applied to one side of the piston and the second stream is adapted to be applied to the opposite side of the piston. A main control is provided for regulating the pressures of the first and second streams to set the pitch of the blades and the hub. According to the invention, there is provided a backup governing system carried by the hub and hydraulically interposed between the main control and the piston and which includes a metering valve having a metering valve member movable between a plurality of metering positions, first and second, opposing biasing elements applying oppositely directed forces to the valve member, a flyweight assembly connected to the valve member applying a speed dependent valve positioning force to the valve member in opposition to the first biasing element and in addition to the second biasing element. Also included is an actuator that is operatively associated with the second biasing element and which is operable to change the bias applied by the second biasing element to the valve member. A first stream control valve is connected to the actuator and is interposed between the transfer bearing and the piston and is operable to control the flow of the first stream to the one side of the piston. 
     In a preferred embodiment, the first stream is a stream that moves the propeller pitch towards a fine pitch condition and the second stream is a stream that moves the propeller towards a coarse pitch position. 
     In one embodiment of the invention, the actuator includes an actuator piston having a side hydraulically connected to the metering valve. The metering valve is operable to direct the first or fine pitch stream to the actuator piston side when the metering valve member is moved by the flyweight assembly to a predetermined position indicative of an undesirable occurrence in the operation of the system as, for example, an overspeed condition or a low pitch condition. 
     In one embodiment, a linkage path, from the double acting piston to the flyweight assembly changes the spool of the flyweight assembly from a force control valve to a motion control valve whenever a low pitch stop is required. When this occurs the flyweight assembly simply becomes another link in the path. The link that follows the cam has enough mass so that it always follows the cam, thus, movement of the double acting piston, during a low pitch condition will move the spool valve directly without any spring or flyweight force influencing the motion of the valve. 
     One embodiment of the invention contemplates that the actuator and the first or fine pitch stream control valve include a piston valve having a piston surface hydraulically connected to the metering valve to receive the first or fine pitch stream when the metering valve is moved to a predetermined position by the flyweight assembly. Also included is a valve surface for halting flow of the first or fine pitch stream to the double acting piston when the metering valve is moved to the predetermined position by the flyweight assembly. 
     Preferably, the metering valve is a spool valve and the metering valve member is a spool having opposite ends. The first biasing element and the flyweight assembly are connected to one of the spool ends and the second biasing element is connected to the other of the spool ends. 
     Preferably, the biasing elements are springs. 
     The invention also contemplates the provision of a reverse enabling valve hydraulically interposed between the actuator piston and the metering valve and operable to prevent the first or fine pitch stream from being applied to the actuator piston side. In this embodiment, the reverse enabling valve may be a hydraulically operated valve responsive to a hydraulic signal in the form of a third stream of hydraulic fluid passing through the transfer bearing. 
     The invention also contemplates the provision of a hydraulic discharge path in fluid communication with the double acting piston fine pitch side, a flow limiter in the discharge path, and a valve operated bypass about the flow limiter. 
     The flow limiter may be an orifice and the discharge path operates as a hydraulically operated pitch delay valve. 
     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 sectional view of a variable pitch propeller embodying invention; and 
     FIG. 2 is an enlarged, partial schematic, partial mechanical view of a backup governing system for the variable pitch propeller and made according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary embodiment of a variable pitch propeller system made according to the invention is illustrated in the drawings and with reference to FIG. 1 is seen to include a rotatable hub, generally designated  10 , constituting the rotatable side of the variable pitch propeller system, and a stationary side, generally designated  12 , of conventional construction. The hub  10  is rotatable about an axis  14  and is driven by any suitable power plant, most often a gas turbine engine (not shown). A shaft  16  is bolted as by bolts  18  to the hub  10  and is journaled for rotation about the axis  14  by bearings including a transfer bearing, generally designated  20  of conventional construction. The transfer bearing  20 , in addition to journaling function, serves as an interface between the hub  10  and the stationary part  12  of the propeller system by serving to transmit, as is conventional, three streams of hydraulic fluid. One such stream commands the propeller system towards a coarse pitch and is designated P c . Another stream is operative to bias the propeller towards a fine pitch condition and is designated P f . The third stream is a governor disable signal and is designated P gds . The stream&#39;s P f  and P c  will be at selected variable, elevated pressures controlled by an EPC (not shown) or other conventional control while the stream P gds  will typically be at one or the other of two different pressure values. 
     The shaft  16  includes an interior cavity  22  in which a valving system, generally designated  24 , and shown in FIG. 2 is contained. The valving system  24  communicates in a manner to be seen with a shaft/transfer tube  26  and a concentric transfer tube  27  having a central flow path or conduit  28  and a concentric flow path  30 , formed by the shaft/transfer tube  26  and the transfer tube  27 , which respectively receive the streams P f  and P c . The shaft/transfer tube  26  extends into a double acting hydraulic cylinder, generally designated  32  having a double acting piston  34  therein. The piston  34  is connected to a piston rod  36  which extends out of the cylinder  32  and which is reciprocally mounted on the shaft/transfer tube  26  for movement along the axis  14 . The central conduit  28  in the shaft  26  opens through a radial port  37  to a first or fine pitch side  38  of the piston  34 . At the same time, the concentric conduit  30  opens via a port  40  to the opposite or coarse pitch side  42  of the piston  34 . 
     Within hub  10 , and disposed between the double acting cylinder  32  and the valve assembly  24 , the piston rod  36  mounts a conventional reciprocating to rotary motion converting mechanism, generally designated  42 . This mechanism may be of any conventional form and as illustrated, includes a pair of bell-shaped plates  44 , 46  that are abutted near their center and at their point of connection to the piston rod  36 . They are separated to provide a bearing receiving space  48  at their peripheries. 
     A self-aligning spherical bearing  50  is located in the space  48  for each of a plurality of propeller blades  52  carried by the hub  10 . The blades  52  have shanks  54  which are journaled to the hub  10  and retained in the hub  10  by a retention bearing system, generally designated  56 , of conventional construction. The rotational axis of one blade is shown at  58  and it will be observed that the shank  54 , at its radially innermost end, includes an eccentrically located pin  60  on which the bearing  50  is mounted. As a consequence, when the piston  34  moves within the cylinder  32 , the reciprocating to rotary motion converting mechanism  42  reciprocates along the axis  14  and such motion, because of the eccentricity of the pin  60 , is converted to rotary motion of the blades  52  within the hub  10 . As viewed in FIG. 1, when the piston  34  is moved to the left, the blades  52  will be pivoted towards a coarse pitch position. Conversely, when the piston  34  is moved to the right as viewed in FIG. 1, the propeller blades  52  will be moved toward a fine pitch position and, in a case where reverse thrusting propellers are involved, ultimately toward a reverse thrust position. 
     Finally, with reference to FIG. 1, it is to be noted that a reciprocal push rod  62  optionally having a roller  64  thereon is positioned to be engaged by an end  66  of the piston rod  36  to move reciprocally in a path that is generally parallel to the rotational axis  14 . The purpose of this linkage will be described hereinafter. 
     Turning now to FIG. 2, the backup governing system contained within the valve assembly  24  will be described in greater detail. The system includes a spool valve, generally designated  68 , having a spool  70  reciprocally mounted therein. Adjacent one end  72  of the spool  70 , a chamber  74  is provided for housing a flyweight assembly, generally designated  76 . The flyweight assembly  76  includes a plurality of flyweights  78  that are generally L-shaped and which include arms  80  in operative relation with a radial flange  82  on the end  72  of the spool  70 . A bearing  84  is interposed between the ends of the arms  80  and the flange  82  and each of the flyweights  78  is mounted for pivoting movement about a pivot pin  86 . As a consequence of this, as the rotational speed of the hub  10  increases, an increasing amount of centrifugal force will be generated within the flyweight assembly  76  which in turn will be conveyed via the arms  80  and the bearings  84  to the flange  82  on the spool  70 . This speed dependent force will tend to drive the spool  70  to the right as viewed in FIG.  2 . 
     Also within the chamber  74  is a spring retainer  88  which retains a compression coil spring  90  against the side of the flange  82  opposite the bearings  84 . This spring  90  applies a biasing force against the spool  70  that is to the left as viewed in FIG.  2 . Suitable means (not shown) are provided for varying the position of the retainer  88  to pre-set the degree of bias applied by the spring  90 . 
     Also within the chamber  74  is a bell crank  92  mounted for pivotal movement by a pivot pin  94 . The bell crank  92 , at one end, includes a roller  96  that may be abutted against one of the flyweights  78  to move the same. Specifically, the bell crank  92  has sufficient mass to assure this movement of the flyweight  78 . The contact occurs on the radially inner side of the flyweight  78  and is such that the motion of the rod  62  in the decrease pitch direction will cause the flyweight  78 , either by the addition of mass or by physical displacement to move radially outward. The effect of such is to drive the spool  70  to the right as viewed in FIG.  2  and the resulting action in response to a low pitch condition is similar to that caused by an increase in rotational speed. The other end of the bell crank  92  includes a roller  98  engaged with a cam surface, generally designated  100 , on an end of the push rod  62 . The cam surface  100  includes a valley  101  between two lobes  103 . 
     It will be observed from FIG. 2 that when the push rod  62  is in the position illustrated, the bell crank  92  will be rotated to a counterclockwise most position with the result that the roller  96  will be at its radially inward most position and out of contact with the flyweight  78 . It should be noted that bell crank  92  has enough mass so that it will over power all spring forces in the flyweight system, insuring that it will always be in contact with the cam surface  100 . On the other hand, when allowed to contact the flyweight  78 , it will physically position the flyweight  78 . Thus, when the push rod  62  is moved to the right as viewed in FIG. 2, the roller  98  will follow the cam surface  100  into the cam surface valley  101 , thereby allowing the bell crank  92  to pivot in a clockwise direction with the result that the roller  96 , in contact with the radially inner side of a flyweight  78 , will move the flyweight  78  in the counterclockwise direction. Consequently, in the illustrated embodiment, the bell crank  92  serves to position the flyweight assembly  76 . Specifically, when the bell crank  92  is introduced into the flyweight assembly  76 , as will occur when a low pitch condition is sensed as will be explained in greater detail hereinafter, the same urges the upper flyweight  78  in a counterclockwise direction about its pivot  86  which allows the lower flyweight  78  to rotate in the clockwise direction, thereby moving the flyweight assembly  76  against the flange  82  of the spool  70 . Thus, movement of the spool  70  to the right will occur as a result. The actuator  34 , in turn, will reposition the spool to a position where the balance of forces on the actuator will cause equilibrium of the system. In short, when the actuator  34  is positioned in response to a low pitch condition, it will always position the spool  70  accordingly, thereby guaranteeing direct control of the low pitch stop position and the flyweights  78  have no effect at this time. 
     Still a further biasing force is applied to the spool  70  by a compression coil spring  102  abutted against the end  104  of the spool  70 , opposite the end  72 . The spring  102  is interposed between the spool end  104  and an end  106  of a piston valve  108 . The piston valve  108  has a seal  110  at the end  106  and an enlarged end  112  also bearing a seal  114 . The same is disposed in a stepped bore  116  communicating with the bore in which the spool  70  is received. The step is shown at  118  and acts as a valve seat when the piston valve  108  is shifted to the right from the position illustrated in FIG.  2 . 
     Returning to the spool valve  68 , the valve body includes two spaced annuluses  120  and  122  while the spool  70 , for purposes of the present invention, includes three lands  124 , 125  and  126  separated by grooves  127  and  128 . A conduit  129  opens the groove  127  to the conduit  134 , which eventually communicates with the sump pressure. An internal conduit  130  is connected to the transfer bearing  20  (FIG. 1) to receive the P f  stream of hydraulic fluid under pressure. The conduit  130  is connected to a first port  132  within the piston valve  108  and located to the side thereof closest the spring  102 . The conduit  130  has a second port  134  which opens to the spool  70  between the annuluses  120  and  122  in the body of the spool valve  68 , depending upon the position of spool  70 . A conduit  136  is connected to the annulus  120  and extends to a pitch delay valve, generally designated  138 . A further conduit  140  extends to the stepped bore  116  on the large side of the step  118  while a further conduit  142  extends from the same location to the central conduit  28  in the shaft/transfer tube  26  and the transfer tube  27 . It is to be noted that an orifice  144  interconnects the conduits  136  and  140  in bypass relation to the pitch delay valve  138 . 
     A conduit  150  is connected to the annulus  122  and extends to an annulus  152  in a reverse enable valve, generally designated  154 . The reverse enable valve  154  includes a second annulus  156  that is connected to the sump. A biasing spring  158  biases a valve spool  160  within the reverse enable valve  154  toward the right as viewed in FIG.  2  and includes a groove  162  sized to allow fluid communication between annulus  152  and groove  162  when the valve spool  160  is moved to the left. It should be noted that a conduit  163  communicates through the groove  162 , with either the annulus  152  or the annulus  156 , but not both for any position of the spool  160 . 
     Between the annuluses  152  and  156 , the conduit  163  is in fluid communication with the interior of the valve  154  and extends to the pitch delay valve  138 . The pitch delay valve includes an internal spool  164  which is biased to the left as viewed in FIG. 2 by a spring  166 . An end  168  of the pitch delay valve spool  164  is subjected to the hydraulic stream P c  by a conduit  170 , which also includes a branch  172  extending to and in fluid communication with the conduit  30  between the shaft/transfer tube  26  and the transfer tube  27 . 
     The spool  164  includes a pair of grooves  174  and  176  separated by a land  178 . The groove  176  is sized to allow fluid communication between the conduits  136 ,  140  when the valve  164  is in the position illustrated in FIG. 2 while the groove  174  is sized to allow fluid communication between the conduit  163  and a conduit  180  that extends to the large side of the stepped bore  116  and is in fluid communication with the side of the piston valve  108  opposite the spring  102 . The land  178  is sized so that when the valve  164  moves to the left from the position illustrated in FIG. 2, communication between the conduits  163 ,  180  is cut off and communication between the conduits  180  and  140  is established, while communication between the conduits  136 ,  140  is also cut off, with the exception of flow through orifice  144 . 
     Operation is generally as follows: 
     In normal operation, the components are generally in the position illustrated in FIG.  2 . The spool  70  will be essentially ineffective with flow to the conduit  150  blocked by the land  126 , with the conduit  150  ported to sump pressure via the groove  127  and the conduit  129 . At the same time, the P f  stream will be directed to the fine pitch side  38  (FIG. 1) of the piston  34  via the conduit  130 , the port  132 , past the valve seat  118 , to the conduit  142  and then to the central conduit  28  within the shaft/transfer tube  26  and the transfer tube  27 . Similarly, the P c  stream will be directed via the conduit  170 , the branch  172  and the concentric conduit  30  to the coarse pitch side  42  of the piston  34 . Control of the pitch of the propeller will then be effected by the relative pressures P f  and P c  in a conventional fashion, i.e., controlled by the electrohydraulic servo valve, or a hydro-mechanical control valve, in the stationary part of the propeller. 
     In the case of an overspeed condition coming into existence, the flyweight  78  (FIG. 2) will exert an increasing bias against the spool  70  tending to move the same against the spring  90 . As that occurs, the groove  128  on the spool  70  begins to meter the P f  stream entering at the port  134  into the annulus  122  from which it enters the conduit  150 , passes through the reverse enable valve to the conduit  163 , passes through the pitch delay valve  138  to the conduit  180  to be applied to the piston valve  108  on the side there of opposite the spring  102 . As a consequence, the piston valve  108  shifts to the right and will close against the seat  118  cutting off the flow of the P f  from the port  132  to the conduit  142 . The shifting of the piston valve  108  increases the biasing force applied by the spring  102  to the spool  70  as well as the counteracting force applied to the spool  70  by the spring  90 . 
     The spring constant of the springs  90  and  102  as well as the force supplied by the flyweight  78  is chosen so that the balance of forces positions the spool  70  so that as propeller speed reaches 101.5% of maximum speed, the land  126  begins to open the annulus  122  to the port  134 . The resulting movement of the piston valve  108  changes the set point of the system to 103% of maximum speed. It is to be particularly noted that as the spring  102  is further compressed, it tends to cause a greater opening to the annulus  122  at the land  126 , thus providing positive feedback, which establishes a new set point at 103% of maximum speed. 
     As mentioned above, the piston valve  108  will have shifted to the right as viewed in FIG. 2 to close against the seat  118 . As a consequence, flow from the conduit  130  to the conduit  142  about the seat  118  is terminated, and the resetting of the set speed to 103% allows speed to increase to 103% before the governor can control the overspeed. At this speed and time, the shifting of the spool  70  to the right allows a groove  182  in the spool to come into fluid communication with the annulus  120 . The groove  182  is in fluid communication with a conduit  184  extending to the sump. Thus, the conduit  136  is gradually connected to the sump via the groove  182 . The conduit  136  remains connected to the central conduit  28  in the shaft/transfer tube  26  leading to the fine pitch side  38  (FIG. 1) of the double acting piston  34 . Hydraulic fluid on that side of the piston is then permitted to flow to the sump out of the center conduit  28 , through the conduit  142  to the conduit  140  and either through the orifice  144  or the groove  176  in the pitch delay valve  138  to the conduit  136 . Thus, pressure is relieved in the double acting cylinder  32  allowing the rotational and aerodynamic force existing in the propeller assembly and the P c  pressure signal to urge the piston  34  to the left as viewed in FIG. 1 thereby increasing the propeller pitch in the coarse direction. 
     As a consequence, propeller speed will begin to diminish as the pitch increases resulting in the flyweight  78  applying a lesser biasing force to the spool  70  which tends to allow the spool  70  to shift to the left until the new equilibrium point is established by the movement of the piston valve  108  is reached. At this time, the land  124  will be modulating flow to or from the fine pitch side  38  of the piston  34  to the sump or from groove  134  at the annulus  120 . Essentially, the main control system has been locked out by shifting of the piston valve  108  until propeller speed decreases to 100% of maximum speed, at which time the flyweight  78  allows the spool  70  to return to its normal-operating position. If one or the other of the main controls is operating properly, propeller pitch to prevent overspeed is maintained by it. If not, as speed increases, the backup system again cycles into backup operation as described above. 
     In a low pitch condition, the same sort of action occurs. However, in this particular case, it is initiated by the push rod  62  being engaged by the end  66  of the piston rod  36  to cause the cam  100  to cause the bell crank  92  to physically position the flyweight assembly  76 . Consequently, the spool  70  now becomes a motion control valve rather than a force control valve and pitch is increased. 
     When it is desired to reverse pitch, a manual control is shifted to the conventional ground stop position. This in turn energizes a solenoid valve (not shown) which allows the stream P gds  signal to be applied to the right-hand side of the reverse enable valve  154 . The resulting shift of the spool  160  causes the groove  162  to establish fluid communication between the line  161  and the sump while cutting off flow from the annulus  152 . As a consequence, the piston  108 , if not already in the position illustrated in FIG. 2, will he shifted back to that position primarily by the balance of pressure forces on piston  108  and secondarily by the bias of the spring  102 . At the same time, the flow path to the conduit  180  is cut off within the reverse enable valve  154  to again prevent the piston valve  108  to be shifted to the right. Consequently, the backup governing system is disabled, allowing the propeller to be operated below flight idle or even in the reverse thrust position. 
     In some instances, during aircraft maneuvers that could result in so-called negative G&#39;s coming into effect, oil pressure may be temporarily lost. In such a situation, it is not desirable that a rapid pitch change in the propeller occur during normal operation. In such a situation, the pressures of streams P f  and P c  may momentarily drop. When P c  drops in pressure it allows the spring  166  to move the spool  164  of the pitch delay valve to the left as viewed in FIG.  2 . 
     This not only cuts off communication between the conduits  163 ,  180 , but it also connects conduit  180  and conduit  140 , and cuts off communication between the conduits  136  and  140  through the pitch delay valve  138  and allowing communication between those conduits only through the orifice  144 . It is to be noted that where the propeller is counterweighted, as is frequently the case, the blades will naturally tend toward coarse pitch under the influence of rotational and aerodynamic forces. This causes the cavity on the side  38  of the piston  34  to be pressurized as the piston  34  moves toward coarser pitch. 
     With the conduits  140  and  180  connected by the spool  164 , continued flow from the fine pitch side  38  of the piston  34  is directed against the piston  108 , shifting it to the right as viewed in FIG. 2 to seat against the valve seat  18 . Only at this time does the flow from the fine pitch side  38  pass through the orifice  44 , which now acts as a flow limiter, limiting the flow back to the conventional electro hydraulic servo valve (not shown) in the main control to a limited flow rate so that a rapid pitch change will not be effected. There will be, however, an initial flow rate greater than such limited flow rate until the piston  108  closes against the valve seat  118 . 
     From the foregoing, it will be appreciated that a backup governing system made according to the invention provides excellent control of the propeller during situations such as overspeed for low pitch and governs the propeller at 103%+/−3% of maximum speed. The same eliminates mechanical components at the interface between the fixed and rotating propeller system parts and yet is completely compatible with conventional systems to the point where it may be readily retrofitted therewith.