Abstract:
A helicopter tail rotor pitch-range altering device is disclosed. The device is incorporated into the existing helicopter tail rotor pitch-change control system of a helicopter and automatically alters the amount of tail rotor pitch available from the helicopter&#39;s tail rotor pitch-change control system based on changes in density altitude. The device comprises an ambient air density sensing device and a movable member located within or located in proximity to the helicopter tail rotor pitch-change flight control system. In one embodiment, a sealed bellows is connected directly to a movable member within the helicopter tail rotor pitch-change control system. As density altitude changes, the length of the bellows changes accordingly and moves the movable member within the helicopter&#39;s tail rotor pitch-change control system providing a varying tail rotor pitch-range based on density altitude.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. Provisional Application No. 60/603,692, filed Aug. 23, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates to a device for altering the available pitch-range of a helicopter tail rotor based on changes in air density or changes in components of air density such as ambient air pressure, ambient air temperature or ambient air moisture content.  
         [0003]     Most common rotor wing aircraft (helicopters) have a rotor system that consists of a main rotor and a tail rotor. The main rotor provides lift and translational force. The tail rotor provides sideward thrust that counteracts the torque affects induced on the helicopter by the driving of the main rotor. The sideward thrust not only counteracts the main rotor torque, it also provides yaw control or directional control for the helicopter. The pilot can vary the amount of sideward thrust put out by the tail rotor through controls, which are typically in the form of pedal inputs. The controls change the amount of pitch on the tail rotor. By actuating the controls, the pilot can adjust the amount of thrust that is produced by the tail rotor by varying the pitch of the tail rotor blades. More tail rotor pitch produces more tail rotor thrust.  
         [0004]     Air density varies as a function of air pressure, air temperature, and the amount of moisture in the air. Air density decreases with decreasing air pressure, increasing air temperature and to a smaller extent increasing moisture content. Air density is sometimes expressed in terms of “density altitude”, which describes air density in terms of the equivalent altitude at which that same air density occurs in the Standard Atmosphere (a standardized mathematical model of the atmosphere). The higher the density altitude, the less dense the air is.  
         [0005]     A helicopter tail rotor works less efficiently at higher density altitudes. The aircraft therefore has less tail rotor authority (i.e., maximum thrust that can be produced by the tail rotor) and less yaw control at higher density altitudes. If, however, one or more of the factors affecting tail rotor authority is properly changed, the tail rotor authority can be increased. The problem, however, with simply changing tail rotor operating parameters to provide the tail rotor authority needed at higher density altitudes is that these changes often cause too much thrust to be produced at lower density altitudes. Too much thrust can overload the helicopter airframe and drive train components as well as the tail rotor flight controls.  
         [0006]     It is therefore desirable to vary the maximum level of thrust achievable by the tail rotor based on air density or density altitude. Numerous devices have been developed to accomplish this. U.S. Pat. No. 5,607,122 to Hicks et al. describes an apparatus including a microprocessor which calculates density altitude based on ambient air sensor inputs. The microprocessor produces an electronic control signal to an actuator that varies the geometry of a linkage member in the tail rotor control system, which in turn varies the pitch of the tail rotor blades. A similar system is described in a 1979 service manual for a Russian-manufactured helicopter.  
         [0007]     U.S. Pat. No. 6,371,408 B1 to Halwes describes an apparatus that uses a sealed bellows that extends and retracts based on air temperature and air pressure changes. In this manner, the movement of the bellows closely reflects changes in air density. The bellows moves a target that is sensed by proximity sensors. The proximity sensors send signals to a logic circuit that activates a drive motor, varying the geometry of a linkage member while simultaneously moving the proximity sensor mount to bring the proximity sensors and mount into alignment with the target. This logic circuit also must detect whether proximity sensors are on or off and provide the appropriate signal to the drive motor for each condition. The Halwes apparatus, like the others, will cease operation if power to the unit is lost for any reason. Because of the operation of the proximity sensors, the Halwes apparatus is not as responsive and accurate as desired.  
         [0008]     Accordingly, there is a need to provide a simple, dependable and accurate device for altering the available pitch-range of a helicopter tail rotor based on changes in air density or changes in components of air density such as ambient air pressure, ambient air temperature or ambient air moisture content.  
       SUMMARY OF THE INVENTION  
       [0009]     In one aspect, this invention is a helicopter tail rotor pitch-range control mechanism comprising:  
         [0010]     a. a helicopter tail rotor pitch-change control system having at least one linkage;  
         [0011]     b. a helicopter tail rotor having a tail rotor pitch that is variable in response to the helicopter tail rotor pitch-change control system; and  
         [0012]     c. an air density compensation device that includes (1) a mechanical air sensing device having a moving member that moves in response to changes in ambient air pressure, ambient air temperature, ambient air moisture content or a combination of two or more of these and (2) a movable mechanism attached to the moving member of the air sensing device and movable in response to movement of the moving member of the air sensing device, said movable mechanism adapted to engage with at least one linkage member of the helicopter tail rotor pitch-change control system such that, in response to movement of the moving member of the air sensing device, the movable mechanism alters the range of pitch through which the helicopter tail rotor can be varied by the helicopter tail rotor pitch-change control system.  
         [0013]     In a second aspect, this invention is an air density compensation device for a helicopter tail rotor pitch-change control system, the air density compensation device comprising (1) a mechanical air sensing device having a moving member that moves in response to changes in ambient air pressure, ambient air temperature, ambient air moisture content or a combination of two or more of these and (2) a movable mechanism attached to the moving member of the air sensing device and movable in response to movement of the moving member of the air sensing device, said movable mechanism adapted to engage with at least one linkage member of a helicopter tail rotor pitch-change control system such that the range of pitch through which the helicopter tail rotor can be varied by the helicopter tail rotor pitch-change control system is altered in response to movement of the moveable mechanism.  
         [0014]     The invention provides a simple, mechanical apparatus for controlling the range of available tail rotor pitch in response to changes in ambient air conditions that affect air density. Those ambient air conditions may include ambient air pressure, ambient air temperature and, to a lesser extent, ambient air moisture content, all of which affect air density.  
         [0015]     A “range” of tail rotor pitch is generally specified in terms of (1) the total included angle of tail rotor pitch through which the tail rotor can be adjusted (typically from a full left pedal to a full right pedal position) and (2) the location of that total included pitch-angle relative to zero tail rotor pitch. For example, for a helicopter where the tail rotor pitch can vary between minus five (−5) degrees (at full right pedal) and seventeen (17) degrees (at full left pedal) the tail rotor pitch range is defined by the total included pitch-angle of twenty-two (22) degrees with full right pedal at minus five (−5) degrees relative to zero tail rotor pitch. In this invention, a change or alteration in the range of tail rotor pitch may include a change in the total included pitch-angle, a change in the location of the included pitch-angle relative to zero tail rotor pitch, or changes to both the total included pitch-angle and its location. Thus, in the foregoing example, one way in which the pitch range can be altered is by increasing the included angle. For example, pitch range can be adjusted by changing the full left pedal setting to twenty (20) degrees, thereby increasing the included angle to twenty-five (25) degrees. Another way of changing the pitch range in the foregoing example is to move both full pedal positions by the same amount (and in same direction), to preserve the original included angle but change its location relative to zero tail rotor pitch. The full right pedal position may be changed to minus one (−1) degree and the full left pedal position to twenty-one (21) degrees, for example. This preserves the original twenty-two (22) degree included angle but changes its location relative to zero tail rotor pitch. It is often desirable to change both the included angle and location of the included angle relative to zero tail rotor pitch.  
         [0016]     In its usual configuration, the mechanism of the invention permits the maximum pitch to which the tail rotor can be adjusted to increase with decreasing air density. This allows the tail rotor to assume a greater pitch under lower air density operating conditions, thereby increasing the thrust that can be generated by the tail rotor under the lower air density conditions. At higher air density conditions, the maximum available pitch that can be imparted to the tail rotor is more limited by the mechanism of the invention. This has the effect of limiting the maximum thrust that can be generated by the tail rotor at the higher air density conditions and helps to prevent over-thrust at higher air density conditions. The ability to vary the range of allowable tail rotor pitch with air density improves control over the vehicle under low air density conditions without over-thrusting during higher air density conditions.  
         [0017]     As discussed in more detail below, the moveable mechanism may engage with a helicopter tail rotor pitch-change control system in various ways to control the range of available tail rotor pitch. One general type of design uses a moveable stop mechanism that engages with a linkage member of the helicopter tail rotor pitch-change control system and limits its range of movement. Movements of the stop mechanism alter the range of movement available to the helicopter tail rotor pitch-change control system, increasing or decreasing the available ranges of pitch. Another type of design includes a variable geometry link in the helicopter tail rotor pitch-change control system. Changes in the geometry of the variable geometry link (for example, a change in length of a member or a component thereof as described more below) increase or decrease the range of pitch through which the helicopter tail rotor pitch-change control system can move the tail rotor.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Having described the invention generally, specific embodiments are now described in more detail with respect to the Figures.  
         [0019]      FIG. 1  is a perspective view of an embodiment of the tail rotor pitch-range control mechanism of the invention.  
         [0020]      FIG. 2  is a side view, partially in section, of an air density compensation device of the invention.  
         [0021]      FIG. 2A  is a side view of an alternative embodiment of a moveable stop for use in the invention.  
         [0022]      FIG. 3  is a side view of another embodiment of the air density compensation device of the invention.  
         [0023]      FIG. 3A  is a top view of an alternative embodiment of a moveable stop for use in the invention.  
         [0024]      FIG. 4  is a side view of another embodiment of the air density compensation device of the invention.  
         [0025]      FIG. 5  is an isometric view of an embodiment of an air sensing device for use in the invention.  
         [0026]      FIG. 6  is a perspective view of a second embodiment of the tail rotor pitch-range control mechanism of the invention.  
         [0027]      FIG. 7  is a side view of a variable geometry link for use in the invention.  
         [0028]      FIG. 8  is a side view of another type of variable geometry link for use in the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     In  FIG. 1 , helicopter tail rotor pitch-range control mechanism  1  contains a helicopter tail rotor pitch-change control system that includes control pedals  10 A and  10 B and a series of linkage members that communicate with rotors  20 A and  20 B. Pedals  10 A and  10 B are mounted on cranks  11 A and  11 B, respectively. When pedals  10 A and  10 B are depressed, cranks  11 A and  11 B move about axis  12 , actuating a series of linkage members that move lever  21  inwardly or outwardly in the direction shown by double-headed arrow  22 . Lever  21  connects to rotor hubs  25 A and  25 B or directly to rotors  20 A and  20 B. The inward and outward movement of lever  21  causes rotor hubs  25 A and  25 B to rotate about axis  24 , thereby changing the pitch of rotors  20 A and  20 B. As shown, the linkage members within the helicopter tail rotor pitch-change control system also include components such as linkage members  30 A-D and  60 , linkage member  50 , and push/pull tubes  40 A-H. However, the type and arrangement of the various components of the helicopter tail rotor pitch-change control system may vary significantly depending on the design of the particular aircraft. Linkage members and push/pull tubes may assume many alternative configurations in addition to the specific types shown in  FIG. 1 . In addition, cables, chains, and other mechanical devices and means (not shown) can be used as alternative or additional linkage members to create a suitable helicopter tail rotor pitch-change control system for a particular aircraft.  
         [0030]     The embodiment shown in  FIG. 1  includes an air density compensation device  100  with a moveable stop mechanism, which engages with linkage member  50 . When either pedal  10 A or  10 B is actuated, linkage member  30 A rotates about pivot point  31 , moving push/pull tube  40 C in the direction indicated by double-headed arrow  32 . Push/pull tube  40 C engages with linkage member  50 , causing it to rotate about pivot point  35 . The rotation of linkage member  50  causes arms  51  and  52  to rotate in the directions indicated by double-headed arrow  34 . Thus, for example, when pedal  10 A is depressed, arms  51  and  52  rotate to the right (i.e., toward arrowhead a′), until arm  51  contacts the moveable-stop mechanism of air density compensation device  100  and further movement of linkage member  50  is prevented. Conversely, when pedal  10 B is depressed, arms  51  and  52  rotate to the left (i.e., toward arrowhead a) until arm  52  contacts the moveable stop mechanism of air density compensation device  100  and further movement of linkage member  50  is prevented. Thus, the pitch of tail rotors  20 A and  20 B can be increased or decreased by actuating pedals  10 A and/or  10 B, with the movement of the helicopter tail rotor pitch-change control system and the range of allowable pitch being limited by the engagement of either of arms  51  or  52  with the moveable stop mechanism of air density compensation device  100 .  
         [0031]     In the embodiment shown in  FIG. 1 , a common arrangement of pedals is shown, in which operation of the left pedal  10 A increases tail rotor pitch (and correspondingly, thrust), and right pedal  10 B decreases tail rotor pitch (correspondingly reducing thrust).  
         [0032]     The moveable stop mechanism and air density compensation device  100  shown in  FIG. 1  are shown in greater detail in  FIG. 2 . In  FIG. 2 , air density compensation device  100  includes sealed bellows  120 , which is a preferred type of mechanical air sensing device. Moving member  122  is attached to bellows  120  and moves in response to changes in ambient air pressure, ambient air temperature, ambient air moisture content, or two or more of these. Bellows  120  is charged with a gas and sealed from the ambient atmosphere. Changes in ambient air pressure, ambient air temperature and/or ambient air moisture content cause the gas sealed within bellows  120  to expand or contract, causing bellows  120  to correspondingly change in volume. Those volume changes cause moving member  122  to extend and retract relative to bellows frame  121  (and relative to a fixed position on the aircraft), in the direction indicated by double-headed arrow  123  in  FIG. 2 . Frame  121  is generally a rigid member having openings allowing bellows  120  to be open to the atmosphere.  
         [0033]     Stop member  151  is affixed to moving member  122  and moves with it. In the embodiment shown in  FIG. 2 , stop member  151  has a varying geometry along its length, decreasing in width from top to bottom (in the orientation shown). Stop member  151  is positioned between arms  51  and  52  of linkage member  50 . Upward or downward movements of moving member  122  change the position of stop member  151  relative to arms  51  and  52 , so that the width of stop member  151  at the level of arms  51  and  52  in turn changes. When moving member  122  moves downwardly (in the orientation shown), a wider width portion of stop member  151  is positioned between arms  51  and  52 . Conversely, when moving member  122  moves upwardly (in the orientation shown), a narrower portion of stop member  151  becomes positioned between arms  51  and  52 .  
         [0034]     Higher air density conditions cause the ambient atmospheric conditions to compress bellows  120 . This causes moving member  122  to retract and, in the orientation shown, move downward. The downward movement of moving member  122  presents a wider width portion of stop member  151  to arms  51  and  52  of linkage member  50 . This wider width reduces the range of movement that is available to linkage member  50  before one of arms  51  and  52  contacts stop member  151 . This in turn reduces the range of pitches to which rotors  20 A and  20 B can be adjusted. As illustrated in  FIG. 2 , the change in width of stop member  151  affects the stop position of both of arms  51  and  52 . However, the geometry of stop member  151  may be designed such that changes in the position of stop member  151  alter the allowable range of movement of only one of arms  51  and  52 . It is particularly desirable that the movement of arm  51  before it contacts stop member  151  becomes more limited with increasing air density conditions as this reduces the maximum pitch angle that can be imparted to rotors  20 A and  20 B at higher air density conditions, and thus helps to avoid over-thrust conditions. It may or may not be necessary or desirable to change the movement of arm  52  before it contacts stop member  151 .  
         [0035]     In lower air density conditions, the ambient atmospheric conditions allow the bellows  120  to expand, causing moving member  122  to extend and (in the orientation shown) move upwardly. This presents a narrower portion of stop member  151  to linkage member  50 , allowing it a greater range of motion before arms  51  or  52  contact stop member  151 . This in turn allows rotors  20 A and  20 B to be moved through a greater range of pitch. In the lower air density conditions, it is generally desired to increase the permitted movement of arm  51  before it contacts stop member  151 , as this allows the maximum pitch angle that can be imparted to rotors  20 A and  20 B to be increased under those conditions, increasing the amount of thrust that is available. As before, it may or may not be necessary or desirable to change the movement of arm  52  before it contacts stop member  151 .  
         [0036]     In  FIG. 2 , stop member  151  has a width that decreases continuously along its length. The continuous variation in width allows for a potentially infinite number of potential stop positions. In an alternative embodiment, as shown in  FIG. 2A , the width of stop member  151 A changes in a step-wise manner along its length. This design allows stop member  151 A to make a series of discrete changes in the stop position of the helicopter tail rotor pitch-change control system as stop member  151 A moves in response to changes in ambient air conditions. Each discrete width is designed to allow an appropriate range of tail rotor pitch positions for a discrete range of ambient air conditions. Depending on the design of the aircraft and the anticipated usage conditions (typically, altitude ranges for which the aircraft is designed), as few as two or as many as 20 or more (such as from 3-15 or from 3-10) discrete stop positions can be provided by the variable width stop member. Typically, each discrete stop position on the variable width stop member would correspond to a specific range of density altitude conditions under which that particular stop position would be engaged. For example, a first (widest) stop position may correspond to a density altitude of sea level to 500-2000 feet. The next widest stop position might then correspond to a density altitude of, for example, from 2000 to 3000 feet. Thus, each succeeding smaller width portion of stop member  151 A would represent a setting that is implemented at a correspondingly higher density altitude range. The particular minimum and maximum density altitudes that correspond to each stop position are a matter of design choice. The range of density altitudes for each stop position (i.e., the difference between the maximum and minimum density altitudes for each particular stop position) is also a matter of design choice. It is contemplated that each stop position will correspond to a density altitude range of from 500 to 3000 feet, especially about 500 to 2000 feet or about 500-1200 feet, from the minimum to maximum density altitude at which it will be engaged.  
         [0037]     Air density is related to air pressure, air temperature, and air moisture content according to the relationship 
 
 D=P   a   /R   a   T+P   w   /R   w   T  
 
 where D represents air density, P a  is the dry air pressure, R a  is the gas constant for dry air, P w  is the water vapor pressure, R w  is the gas constant for water vapor, and T is the absolute temperature. Thus, at constant temperature, decreases in air pressure reduce air density, whereas at constant air pressure, temperature increases cause air density to decrease. The gas sealed in bellows  120  expands and contracts in response to changes in ambient air pressure and in response to changes in ambient air temperature, and therefore can react to air density increases or decreases that arise due to a change in either air pressure or air temperature. The bellows typically cannot respond to changes in air moisture content. However, as changes in air moisture content tend to have lesser affects on air density than do changes in pressure and temperature, the bellows nonetheless will provide responses that closely approximate changes in air density. 
 
         [0038]     Various alternative stop member designs can be substituted for those shown in  FIGS. 1, 2  and  2 A. In  FIGS. 3 and 3 A, stop member  156  is a cam mounted asymmetrically about moving member  122 . Cam  156  is mounted on moving member  122  such that as moving member  122  extends and retracts with changes in volume in bellows  120 , cam  156  is rotated about axis  157 . This can be accomplished using a moving member  122  with a helical exterior, and a cam having a correspondingly threaded bore. Up and down movement of moving member  122  will result in a rotation of cam  156  if the vertical (as shown) position of cam  156  is held constant. The rotation of cam  156  about axis  157  changes the effective cross-section (or width) of cam  156  that is presented to arms  51  and  52  of linkage member  50  as air density changes. An example of the type of rotation that can be produced is illustrated in  FIG. 3A , in which a low air density position of cam  156  is shown in phantom, and the direction of movement from high air density to low air density is indicated by arrow  158 . This in turn alters stop positions with changes in air density (or component thereof), affecting the available rotor pitch range of motion as before. Note that in this embodiment, the allowable movement of arm  51  increases with decreasing air density, whereas the allowable movement of arm  52  in the opposite direction may be increased, unchanged or also decreased, depending on the particular geometry of stop member  156 . In the embodiment shown in  FIG. 3A , rotation of cam  156  changes the stop position of arm  51  of linkage member  150 , but affects little change in the stop position of arm  52 .  
         [0039]     Another alternative stop member design is shown in  FIG. 4 . In  FIG. 4 , cam  176  is pivotably and asymmetrically mounted about pivot point  177 . Cam  176  is pivotably mounted to moving member  122  at pivot point  159 . In this embodiment, decreasing air density causes moving member  122  to move upwardly (in the orientation shown) as before, causing cam  176  to rotate in the direction indicated by arrow  171 . This reduces the effective cross-section (or width) of cam  176  that is presented to arm  51 , increasing the allowable range of motion of arm  51  before it contacts cam  176 , and increasing the range of allowable pitch motion as before. Note that in this embodiment as well, the allowable movement of arm  51  increases with decreasing air density, whereas the allowable movement of arm  52  in the opposite direction may be increased, unchanged or also decreased, depending on the particular geometry of stop member  176 . In the embodiment shown in  FIG. 4 , rotation of cam  176  results in a significant change in the stop position of arm  51  but little or no change in the stop position of arm  52 .  
         [0040]     Other moveable stop designs can of course be substituted for the particular types described above. For example, the cam designs shown in FIGS.  3  and/or  4  may be replaced with a cam having a serrated outer surface. This allows for step-wise changes in the allowable position of arms  51  and/or  52 , as described with respect to the moveable stop design illustrated in  FIG. 2A . In addition, any design that changes the available pitch range in the desired manner can be used. As before, preferred designs will permit the maximum allowable tail rotor pitch to decrease with increasing air density (or component thereof) and permit it to increase with decreasing air density (or component thereof).  
         [0041]     Design modifications can be made to the bellows and to the linkage system that connects the moving member of the air sensing device with the moveable stop. In general, the air sensing device can be of any mechanical design, provided that it includes a movable member that moves in a predictable way in response to changes in ambient air pressure, ambient air temperature and/or ambient air moisture content.  
         [0042]     For example, an alternative mechanical air sensing device design includes a housing member and a piston. The housing and piston together define a gas-filled chamber which is sealed from the atmosphere, such as through O-rings or similar seals. The piston is slidably mounted within the housing. The piston extends and contracts with changes in ambient air density (or component thereof such as air pressure and/or temperature) in much the same manner as moveable member  122  of  FIG. 2 , moving a moveable stop member as before. Another alternative air sensing device replaces bellows  120  of  FIG. 2  with a flexible, gas-filled bladder. Like the bellows described above, the gas-filled bladder will respond to changes in ambient air pressure and ambient air temperature, but not changes in ambient air moisture content. As before, this provides a good approximation of air density changes.  
         [0043]     A bellows containing a vacuum can be substituted for the bellows, housing-and-piston or gas-filled bladder described above. The vacuum-filled bellows is made of a flexible material or otherwise constructed such that the volume enclosed by the bellows changes with air pressure only. A moving member is affixed to the bellows as before, with the moving member extending with reduced ambient air pressure and retracting with increasing ambient air pressure. This type of bellows does not react to variations in ambient air temperature, and thus its movement is sometimes a poorer approximation of air density than the bellows designs described before. However, a vacuum-filled bellows can if desired be used in conjunction with another device that produces a mechanical motion in response to changes in ambient air temperature, so that the devices together produce a movement that more closely correlates with changes in air density.  
         [0044]     The linkage system connecting the air sensing device to the moveable stop that is illustrated in  FIG. 2  is a particularly simple design, in which the moveable stop is connected directly to the moving member and moves with it. However, the particular design of the linkage system is not considered to be critical. Various kinds of linkage systems can be used for this purpose, including for example, various levers, bell cranks, push-pull rods or tubes, cables, sleeve-and-cable systems, hydraulic systems, and the like. All that is required is that the moveable stop be mechanically connected to the moving member so the moveable stop moves in a known way in response to movements of the moving member.  
         [0045]     As shown in  FIG. 5 , multiple air sensing devices can be used together. In  FIG. 5 , air sensing devices  110 A,  110 B and  110 C have moving members that are joined together such that they produce a common moving end  132 . The moveable stop (not shown) is connected to common moving end  132  in a manner as described before. The movement of moving end  132  is a summation of the output forces of the individual air sensing devices  110 A,  110 B and  110 C. Multiple air sensing devices may be used for purposes of providing redundancy, thus providing an additional margin of safety. Air sensing devices of different types may be used together. For example, an air sensing device that responds only to ambient air pressure changes may be coupled to another air sensing device that responds to ambient air temperature changes and/or ambient air moisture content changes. Another reason to use multiple air sensing devices is simply to multiply the force that is available to move the moveable stop or to resist loads that may be applied by the tail rotor pitch-control system.  
         [0046]      FIG. 6  illustrates an alternative embodiment of the invention, in which a variable geometry linkage is used to affect the range of available rotor pitches.  
         [0047]     In  FIG. 6 , helicopter tail rotor pitch-range control mechanism  61  contains a helicopter tail rotor pitch-change control system that includes control pedals  610 A and  610 B and a series of linkage members  630 A-D and  660  and push/pull tubes  640 A-H that communicate with rotors  620 A and  620 B. Pedals  610 A and  610 B are mounted on cranks  611 A and  611 B, respectively, as before, actuating a series of linkage members that move lever  621  inwardly or outwardly in the direction shown by double-headed arrow  622  to effect changes in the pitch of rotors  620 A and  620 B. In this embodiment, linkage members  630 A-D and push/pull tubes  640 A-H perform the same functions as described with respect to analogous features shown in  FIG. 1 . However, the embodiment shown in  FIG. 6  does not include an air density compensation device with a moveable stop as shown in  FIG. 1 .  
         [0048]     Instead, the helicopter tail rotor pitch-change control system shown in  FIG. 6  includes an air density compensation device with a variable geometry link  200 . A specific embodiment of variable geometry link  200  is illustrated in  FIG. 7 . In the embodiment shown in  FIG. 7 , the variable geometry link  200  includes lever  215  having a pivot point  201  through which link  200  is pivotably affixed to a fixed point. Near one end of lever  215  is attachment point  203  for receiving input from the pedal side of the helicopter tail rotor pitch-change control system. Near the opposite end of lever  215  is a second, moveable attachment point  202  for providing output to the tail rotor side of the helicopter tail rotor pitch-change control system. In the embodiment shown, the position of attachment point  202  relative to pivot point  201  is variable in response to movements of mechanical air sensing device  210 . Mechanical air sensing device  210  is as described before, and includes moving member  212  that extends and retracts in the direction indicated by double-headed arrow  218  with changes in ambient air density (or a component thereof such as air pressure, air temperature or air moisture content). In the embodiment shown in  FIG. 7 , air sensing device  210  is mounted onto variable geometry link  200 , being held in a fixed position against support  211 . Moving member  212  passes through optional guide  213  and connects to moveable attachment point  202  through tab  232 .  
         [0049]     As air sensing device  210  and moving member  212  extend due to a decrease in ambient air density (or component thereof), tab  232  and moveable attachment point  202  are pushed outwardly, away from pivot point  201 . The distance from pivot point  201  to attachment point  202  is thereby increased in response to reduced ambient air density. When the helicopter tail rotor pitch-change control system is actuated, push/pull tube  640 C moves attachment point  203  of lever  215 . The farther that attachment point  202  is pushed away from pivot point  201 , the greater the movement of attachment point  202  will become in response to a given movement of attachment point  203 . The greater range of movement of attachment point  202  creates a correspondingly greater range of movement of push/pull tube  640 D and increases the range of pitch (in particular, the included angle) that is imparted to tail rotors  620 A and  620 B. Similarly, increases in air density cause air sensing device  210  and moving member  212  to retract, reducing the distance from pivot point  201  to attachment point  202 , thereby reducing the amount of pitch change (i.e. reduces the included angle) that is translated to tail rotors  620 A and  620 B by a given movement of attachment point  203 .  
         [0050]     Equivalent results can be obtained by modifying variable geometry link  200  so that the position of pivot point  201  and/or attachment point  203  is changed in response to changes in air density. Note that the effect of changing the position of attachment point  203  is opposite that of moving attachment point  202 . Outward movement of attachment point  203  (i.e. to increase its distance from pivot point  201 ) will reduce the amount of pitch change that results from a given lateral movement of attachment point  203 . For that reason, the air sensing device in that case must be configured so that attachment point  203  is moved closer to pivot point  201  in response to decreasing air density. The effect of moving pivot point  201  in response to air density changes will vary according to the particular design of the variable geometry linkage member.  
         [0051]     In the embodiment shown in  FIG. 7 , air sensing device  210  is mounted directly onto variable geometry link  200 . This is not required. Air sensing device  210  may be mounted separately from variable geometry link  200  and operatively attached to it using a variety of types of linkages. As before, various types of linkage members may be inserted between the moveable member of the air sensing device and the variable geometry link. Suitable such linkage members include those described above. The linkage design should be such that the linkage does not impede movement of lever  215  about pivot point  201 .  
         [0052]     Although the variable geometry link is illustrated in  FIG. 7  with a linear geometry, equivalently functioning variable geometry linkage members can be made having various non-linear geometries. For example, angled linkage members  630  may be adapted in analogous fashion to function as the variable geometry link.  
         [0053]     Another type of variable geometry link is illustrated in  FIG. 8 . In  FIG. 8 , a variable length push/pull tube mechanism  300  is positioned as a linkage replacing a conventional push/pull tube in the helicopter tail rotor pitch-change control system, such as any of tubes  40  or  640  in  FIGS. 1 and 6 . Thus, in  FIG. 8 , variable length push/pull tube  300  includes sections  314  and  315  that are slidably mounted together so that section  314  can move with respect to section  315  in the direction indicated by double-headed arrow  325 . Air sensing device  310 , which may be of any of the types described previously, is mounted onto section  315  of variable length push/pull tube  300  via support  311 . Moveable member  312  connects to tab  313  on moving section  314 . As moveable member  312  extends and retracts as a function of changes in air density (or component thereof) as before, section  314  moves correspondingly outwardly and inwardly, and the length of the variable length push/pull tube mechanism  300  is correspondingly altered. This change in the length of variable length push/pull tube mechanism  300  alters the range of available helicopter tail rotor pitch. This approach tends to bias the tail rotor pitch range towards one full pedal position or the other, i.e., tends to preserve the included angle of the pitch range while changing its location relative to zero tail rotor pitch. For example, if this approach were used to give a seven degree increase in tail rotor pitch at full left pedal, the tail rotor blade pitch angle will be increased by seven degrees at full left pedal and at full right pedal the tail rotor blade pitch will be biased towards the full left pedal position by about seven degrees as well. Therefore, in this example, if the full right pedal position were originally minus eight degrees, the new full right pedal position will be about minus one degree.  
         [0054]     It is also noted that in this embodiment, the push/pull tube is subjected to compressive and/or tensile forces when the pitch change control system is actuated. These forces can in some cases diminish, exaggerate or overcome the motion of moving member  312  in response to air density changes. For this reason, it is preferred that the moveable mechanism of the air density compensation device be designed and used in a manner such that compressive or tensile forces that are applied to it during operation of the helicopter tail rotor pitch-change control system are minimized or eliminated.  
         [0055]     Also as before, air sensor  310  can be separated from push/pull tube mechanism  300  and be operatively joined to it via a variety of types of linkages.  
         [0056]     The various types of moveable mechanisms as described above can be used in combination if desired or necessary to obtain the desired effect on tail rotor pitch range.  
         [0057]     In addition, the air density compensation device may be used in conjunction with or in addition to conventional types of air compensation devices, to provide, for example, redundant or back-up systems. The mechanical air sensing device may be supplemented with other types of air sensing devices, such as air temperature, air pressure or air moisture content sensors or detectors, to supplement the mechanical air sensing device. The air density compensation device may further include one or more display means for reporting one or more operating parameters or other information, such as the position of the moving member, the pitch range (or component thereof), the maximum allowable tail rotor pitch, the air density (or component thereof) represented by the position of the moving member of the air sensing device, and the like.  
         [0058]     Although several preferred embodiments of the present invention have been described in detail herein, the invention is not limited hereto. It will be appreciated by those having ordinary skill in the art that various modifications can be made without materially departing from the novel and advantageous teachings of the present invention. Accordingly, the embodiments disclosed herein are provided by way of example only. It is to be understood that the scope of the present invention is not to be limited thereby, but is to be determined by the claims which follow.