Abstract:
A turbine having a cam follower operable to control turbine blade pitch in association with a position thereof is provided and includes an axially movable plate, a rotational and axially movable flyweight and a system operably coupled to the plate and the flyweight whereby, at low RPMs, the system prevents flyweight rotation such that the plate and the flyweight position the cam follower at a first position, at medium RPMs, the system permits flyweight rotation such that the plate and the flyweight position the cam follower at a second position, and, at high RPMs, the system prevents further flyweight rotation and permits initial axial movement of the plate and the flyweight such that the plate and the flyweight position the cam follower at a third position.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to turbine yokeplate flyweights to improve RAT startup. 
     A Ram Air Turbine (RAT) is controlled by a governing mechanism to maintain a narrow operating speed range. The governor spring and blade aerodynamic forces rotate the blades toward fine pitch orientations at which substantially rapid rotation occurs with blade faces oriented substantially perpendicularly with respect to the airstream. Blade counterweights rotate the blade toward coarse pitch to prevent over speed conditions. The various forces of the governor spring, the blade aerodynamics and the blade counterweights balance each other to maintain the proper speed range but the counterweights require centrifugal forces to develop their restraining forces. 
     During startup, centrifugal forces are low, so the RAT governor is typically controlled by the governor springs with the blades in the fine pitch position. The airfoil shape is optimized to give power over the operating speed range, so it is somewhat inefficient at low RPM and fine pitch and there is very little torque available to overcome the large blade inertia and the RAT tare losses at low RPMs. Indeed, even if the RAT starts to turn, it takes a long time for a fine pitch RAT to get up to operating speed. Moreover, RATs are sometimes placed in turbulent zones under the aircraft where the dynamic pressure loss is high. This further reduces the available torque for start up. 
     Both fine pitch and coarse pitch start up RATs are currently in production. The fine pitch architecture has proven superior for reliability, robustness, weight, cost and packaging. Coarse pitch RATs contain more heavy, complicated parts and more failure points, but they start up more efficiently. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of the invention, a turbine having a cam follower operable to control turbine blade pitch in association with a position thereof is provided and includes an axially movable plate, a rotational and axially movable flyweight and a system operably coupled to the plate and the flyweight whereby, at low RPMs, the system prevents flyweight rotation such that the plate and the flyweight position the cam follower at a first position, at medium RPMs, the system permits flyweight rotation such that the plate and the flyweight position the cam follower at a second position, and, at high RPMs, the system prevents further flyweight rotation and permits initial axial movement of the plate and the flyweight such that the plate and the flyweight position the cam follower at a third position. 
     According to another aspect of the invention, a turbine having a cam follower operable to control turbine blade pitch in association with a position thereof is provided and includes a plate movable from an axial position, a flyweight rotatable between first and second rotational positions and movable from an axial position and a system operably coupled to the plate and the flyweight whereby: at low RPMs, the system prevents flyweight rotation from the first rotational position such that the plate and the flyweight position the cam follower at a first position, at medium RPMs, the system permits flyweight rotation to the second rotational position such that the plate and the flyweight position the cam follower at a second position, and, at high RPMs, the system prevents further flyweight rotation and permits axial movement of the plate and the flyweight from the respective axial positions such that the plate and the flyweight position the cam follower at a third position. 
     According to yet another aspect of the invention, a turbine having a cam follower operable to control turbine blade pitch in association with a position thereof is provided and includes a plate movable from an initial axial position, a flyweight rotatable between first and second rotational positions and movable from an initial axial position and a system operably coupled to the plate and the flyweight whereby: at low RPMs, the system prevents flyweight rotation from the first rotational position such that the plate and the flyweight position the cam follower at a first turbine blade pitch control position, at medium RPMs, the system permits flyweight rotation to the second rotational position such that the plate and the flyweight position the cam follower at a second turbine blade pitch control position, and, at high RPMs, the system prevents further flyweight rotation and permits axial movement of the plate and the flyweight from the respective initial axial positions such that the plate and the flyweight position the cam follower at a third turbine blade pitch control position. 
     According to yet another aspect of the invention, a turbine having a cam follower operable to control turbine blade pitch in association with a position thereof is provided and includes a plate movable in an axial direction within a turbine hub of the turbine, a flyweight rotatable about pivot pins, which are axially fixed within the turbine hub and a system operably coupled to the plate and the flyweight whereby, at low to medium RPMs, the plate is positioned neutrally with bias applied thereto and bias applied to the flyweight being substantially similar such that the plate and the flyweight position the cam follower at a first turbine blade pitch control position, at medium RPMs, greater net load is applied to the plate and the plate is axially moved toward a stop such that the plate and the flyweight position the cam follower at a second turbine blade pitch control position, and, at medium to high RPMs, axial movement of the plate is prevented and bias applied to the flyweight increases such that the plate and the flyweight position the cam follower at a third turbine blade pitch control position. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       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: 
         FIG. 1  is a side sectional view of an RAT at a low RPM start position; 
         FIG. 2  is a side sectional view of the RAT of  FIG. 1  with a fine pitch governing position; 
         FIG. 3  illustrate a typical governing position; 
         FIG. 4  illustrates an alternate embodiment; 
         FIGS. 5 and 6  show a side sectional view of an Rat in accordance further embodiments of the invention; and 
         FIG. 7  is a graphical display of blade angles vs. RAT RPMs for the RATs of  FIGS. 5 and 6 . 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with aspects of the invention, a fine pitch ram air turbine (RAT) construction is provided with enhancements needed to make an RAT startup process more efficient even under adverse conditions. These enhancements include a coarse pitch start mechanism that rotates the blades to a mid-range pitch only during startup to thereby provide the additional torque needed to aid startup. 
     A partial cross-section of an RAT  10  is shown in  FIG. 1  and is similar to a typical fine pitch startup turbine but has been modified to start up at a blade angle that is coarser than normal to develop higher torque at low RPMs. In a typical fine pitch startup turbine, a yokeplate acts as a single body that shifts along a governor shaft as a cam follower changes position. In accordance with embodiments of the present invention, however, and with reference to  FIG. 1 , a flyweight  20  has been added to a yokeplate  30  and is configured to have an adjustable position based on the starting turbine speed. 
     To this end, the flyweight  20  includes a flyweight body  21  that is rotatable away from a plane of the yokeplate  30  about a flyweight pivot axle  22 . The flyweight body  21  is formed to define a detent pocket  23  as well as an elongate through-hole  24  and a recessed corner section  25  that reduce an overall weight of the flyweight  20 . The flyweight body  21  also includes a surface to which a boss  26  is integrally coupled. Movement of the boss  26  as the flyweight  20  rotates is guided by a rotational guide  27  having a guide slot defined therein. 
     The RAT  10  is generally formed of a turbine casing  11  having a governor shaft  12  proximate to a turbine rotational axis  34 . Along with the other components described herein, the yokeplate  30  is disposed within the turbine casing  11  and includes a yokeplate body  31  that is axially movable with respect to the turbine casing  11  along the governor shaft  12  as RPMs increase and decrease. At the low RPM start position shown in  FIG. 1 , the yokeplate  30  is in the normal fine pitch axial position whereby an end  32  of the yokeplate body  31  is proximate to turbine casing shoulder  33  but the flyweight  20  is rotationally positioned close to the turbine rotational axis  34 . 
     A flyweight spring  40  is anchored to a protrusion  41  of the turbine casing  11  and to a plate member  42 , which is axially movable with respect to the turbine casing  11 . The flyweight spring  40  thereby biases a cam follower  50  and turbine blades, which are operably coupled to the cam follower  50  for turbine blade pitch positional control, into a mid range startup position with the cam follower  50  abutting against a surface  43  of the flyweight  20 . The surface  43  approaches and recedes from the plate member  42  as the flyweight  20  rotates with the cam follower  50  disposed between the plate member  42  and the surface  43  such that the cam follower  50  is biased to move in accordance with the flyweight  20  rotation. 
     The RAT  10  further includes a governor spring  60  having a detent  61  at an end thereof. The detent pocket  23  defined by the flyweight body  21  registers with the detent  61  such that the flyweight  20  can be held in the start position of  FIG. 1  by the governor spring  60 . 
     With reference to  FIG. 2 , at low RPMs, the flyweight spring  40  and the cooperation between the detent  61  and the detent pocket  23  cooperatively supply enough load to hold the flyweight  20  and the cam follower  50  in the start position. When the turbine reaches a transition point, however, centrifugal forces on the flyweight  20  balance the flyweight spring  40 , blade, counterweight and detent forces. At slightly higher RPMs, the flyweight  20  centrifugal force increases faster than the opposing blade and counterweight forces increase, so the flyweight  20  moves outward toward the  FIG. 2  position. If one of the flyweights  20  moves out prior to the other flyweights  20 , the flyweight spring  40  load is removed from the lagging flyweights  20  to thereby allow them to also rotate outward. 
     Still referring to  FIG. 2  and with additional reference to  FIG. 3 , with the rotation of the flyweight  20  and the corresponding movement of the cam follower  50 , the blade(s) coupled to the cam follower(s)  50  now occupy a fine pitch position for the start of normal RAT  10  governing. A stop  70  at an end of the guide slot of the rotational guide  27  stops the boss  26  and the flyweight  20  from continuing to rotate and so prevents over rotation. As such, the yokeplate  30  and the flyweight  20  now act as a single body during normal governing and, at higher RPMs in the normal governing range, the yokeplate  30  and the flyweight  20  are caused to axially move or shift (i.e., the yokeplate  30  and the flyweight  20  move to the left in the image of  FIG. 2 ) against the governor spring  60 , as shown in  FIG. 3 . 
     As the yokeplate  30  moves in this manner (i.e., to the left), the cam follower  50  moves closer to the flyweight pivot axle  22 . A moment arm is thereby reduced to help prevent cam follower  50  forces from overcoming centrifugal forces exerted on the flyweight  20  and turbine blade counterweight forces increase at a mid range blade angle such that there is more cam follower  50  load at a shorter moment arm. The flyweight  20  mass is chosen to maintain the  FIG. 2  position against cam follower  50  loads throughout the governing range. 
     The mechanism described herein with reference to  FIGS. 1-3 , automatically resets from the  FIG. 3  and  FIG. 2  positions back to the  FIG. 1  position when the RAT  10  RPM is decreased. The detent pocket  23  registers with the detent  61  and the rotational guide  27  is ramped to ease detent  61 /detent pocket  23  re-registry when the RAT  10  slows to a rest. That is, as the flyweight  20  rotates back to the startup position of  FIG. 1 , the boss  26  slides through the guide slot of the rotational guide  27  until the detent  61  moves back into the detent pocket  23 . The flyweight spring  40  is sized large enough to overcome any friction forces between the flyweight(s)  20  and the detent(s)  61  during the resetting. 
     With reference to  FIG. 4 , an alternate embodiment is shown. Here, the detent  61  is removed and the flyweight  40  spring is designed to supply a higher load to prevent early transition to fine pitch position and the counterweight of the flyweight  20  needs to be more massive to overcome the heavier flyweight spring  40  in the governing range. The governor spring(s)  60  also need to be stronger to overcome the heavier flyweight spring  40 . All of these changes are feasible with only small adjustments to the envelope. The flyweight  20  mechanism operates in a similar manner to the version described above. 
     In accordance with further aspects of the invention and, with reference to  FIGS. 5-7 , an RAT  10  includes a strong flyweight spring  70 , which is disposed inside the turbine driveshaft  80 , and which opposes the governor springs  60  at start up. The governor shaft  12  and the yokeplate  30  are positioned at a neutral point where the load provided by the governor spring  60  is substantially equal to the load provided by the flyweight spring  70 . This positions the cam follower  50  and the turbine blades at a mid-range angle, which allows the blades to develop more starting torque. 
     As the RAT  10  begins to spin, the turbine blades and counterweight forces apply a load toward the governor spring  60  and the flyweights  20  provide a larger load in the opposite direction. At each speed, an equilibrium point is reached between all these forces to locate the blade angle. A representative blade angle vs. RPM plot is shown in  FIG. 7 . As the RAT  10  speeds up, the counterweight provides greater net load to the governor shaft  12  to gradually shift the blades to a finer angle. When normal governing speed is reached, the governor shaft  12  has shifted to the right until it rests on a stop. Higher RAT  10  speeds within the governing range increase the flyweight  20  load, but the governor shaft  12  no longer shifts any further. Meanwhile, the yokeplate  30  is free to move to the left in the normal governing mode. All normal governing operation occurs with the governor shaft  12  firmly seated on the stop  85  as shown in  FIG. 6 . 
     The angle required for a turbine blade to produce the maximum torque is near coarse pitch at very low RPMs. The angle becomes finer with increasing RPMs. The blade angles shown in  FIG. 7  allow the RAT  10  to start up at a favorable torque producing angle, and adjust that angle as the RAT  10  speeds up to maintain favorable torque capacity at all RPMs. In accordance with aspects of the invention, a RAT  10  may start up much quicker than the previous fine pitch start up turbines. It would be comparable to typical coarse pitch start up turbines up to about 1000 to 2000 RPM, where other coarse pitch turbines would quickly transition to fine pitch for less rapid acceleration. The gradual transition of this mechanism promotes a faster start up from about 1000 to 2000 RPM to the governing range. 
     When the RAT  10  slows down and comes to a rest after operation, the flyweight spring  70  automatically pushes the governor shaft  12  back to the left until an equilibrium position is reached with the governor spring  60 . This resets the blades in the mid-range startup position shown in  FIG. 1 . 
     The flyweight  20  rotates about pivot pins  100 , as shown in  FIGS. 1 and 2 . The pins  100  are held stationary by a yoke connected either to the turbine hub or the turbine driveshaft (not shown). Motion from the counterweights is transferred to the governor shaft  12  through a rack and pinion type of connection. Only 1 tooth of the rack and 1 tooth of the pinion is shown, but a second tooth could be added to both for smoother load transmission. Other types of load transfer devices could be used, such as a cam follower pushing against the governor shaft. 
     The flyweight spring  70  shown is a helical compression spring, which fits comfortably within the turbine driveshaft shown. A larger diameter turbine driveshaft  80  would allow a stack of disk springs to be substituted, if desired, for a modest weight savings. 
     In accordance with aspects of the present invention, additional weight increase from the fine pitch version is limited by the addition of moderately sized flyweights with a small increase in overall length. Also, automatic reset to the start position when the RAT  10  slows to a stationary position is possible with simple component parts that do not add much complexity to the traditional turbine design. Further, the flyweight(s)  20  are supported with a small amount of additional material and flyweight  20  centrifugal loads do not require a stronger hub to support them while compliance is added to the yokeplate  30 , which may reduce impact loading into the coarse pitch stop. Still further, whereas current design requires a spring pack to blunt impact loading, an existing governor spring  60  can be used as the detent  61  o minimize the size of the flyweights  20  and the flyweight spring  40 . 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while 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.