Patent Publication Number: US-8991753-B2

Title: Method of positioning landing gear

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional application of U.S. patent application Ser. No. 12/730,598, filed Mar. 24, 2010, the contents of which are incorporated herein in their entirety. 
    
    
     TECHNOLOGICAL FIELD 
     Configurations of the present disclosure relate generally to landing gear and, more particularly, to a semi-levered landing gear and an associated method of positioning the truck beam of the landing gear. 
     BACKGROUND 
     An airplane includes landing gear to facilitate takeoff, landing and taxi. The landing gear of some aircraft includes a shock strut that is pivotally attached to a truck beam at a distal or lower end thereof. The truck beam includes two or more axles upon which tires are mounted. In this regard, the truck beam may include a forward axle positioned forward of the shock strut and an aft axle positioned aft of the shock strut. Upon takeoff, an airplane having a conventional landing gear with forward and aft axles will pivot about the pin that attaches the truck beam to the shock strut such that all of the landing gear tires have an equal load distribution. 
     In order to provide additional ground clearance for rotation of the aircraft during takeoff, semi-levered landing gear mechanisms have been developed. A semi-levered landing gear fixedly positions the shock strut and the forward end of the truck beam during takeoff such that the forward axle is in a raised position relative to the aft axle when the airplane has left the ground. As such, the aircraft pivots about the aft axle, rather than the pin that pivotally connects the truck beam to the shock strut provided that the extend pressure of the shock strut has been increased sufficiently. By rotating about the aft axle, the landing gear height is effectively increased so as to provide additional ground clearance for rotation of the aircraft during takeoff. As a result, the takeoff field length (TOFL) of the aircraft may be reduced, the thrust required of the engines may be reduced or the weight carried by the aircraft may be increased while maintaining the same takeoff field length. 
     In order to provide for rotation of the aircraft about the aft axle during takeoff, a semi-levered landing gear locks the truck beam in a “toes-up” attitude such that the tires mounted upon the aft axle support the aircraft, while the tires mounted upon the forward axle are raised above the surface of the runway. Following takeoff, the landing gear is generally stowed in a wheel well or the like. In order to fit within a conventional wheel well, the landing gear must typically be unlocked and the truck beam repositioned in a “stowed” attitude prior to retracting the landing gear into the wheel well. Thereafter, during landing, the landing gear is lowered and the truck beam is repositioned such that all of the wheels, including both those on the forward axle and the aft axle, equally bear the weight of the aircraft. Typically, the locking and unlocking of a semi-levered gear system and the resulting repositioning of the truck beam relative to the shock strut occurs without input from the pilot or the flight control system. 
     One type of semi-levered landing gear includes a locking hydraulic strut to lock the truck beam in the desired orientation for takeoff. The locking hydraulic strut is essentially a locking actuator, but has a number of additional chambers and an internal floating piston. See, for example, U.S. Pat. No. 6,345,564. While a semi-levered landing gear having a locking hydraulic strut is suitable for some aircraft, the landing gear of other aircraft may not have sufficient clearance or room for the hydraulic strut to be positioned between the shock strut and the truck beam in an efficient manner. In addition, the hydraulic strut disadvantageously adds to the cost and complexity of the landing gear. 
     Another semi-levered landing gear utilizes a mechanical linkage to lock the truck beam during takeoff, but requires a separate mechanical linkage, termed a shrink-link, to reposition the shock strut for retraction into the wheel well. The requirement for a shrink-link disadvantageously increases the complexity, expense and weight of the resulting semi-levered landing gear. 
     Accordingly, it would be desirable to provide an improved semi-levered landing gear that reliably positions the truck beam in a toes-up attitude during takeoff and then repositions the truck beam in a “stowed” attitude following takeoff for stowage in the wheel well. In particular, it would be desirable to provide a semi-levered landing gear that is both weight and cost efficient and that is not overly complex, while still satisfying the various operational requirements of the semi-levered landing gear. 
     BRIEF SUMMARY 
     In accordance with configurations of the present disclosure, a semi-levered landing gear is provided that is configured to position a truck beam in a toes-up attitude during takeoff and then reposition the truck beam in a “stowed” attitude for stowage in a wheel well following takeoff. The semi-levered landing gear of configurations of the present disclosure may be constructed in a manner that is effective from both a cost and a weight standpoint. 
     In one configuration, a semi-levered landing gear is provided that includes a shock strut having inner and outer cylinders, a truck beam pivotally connected to the inner cylinder of the shock strut and a pair of torsion links connecting the inner and outer cylinders. The semi-levered landing gear of this configuration also includes a first link connected to the truck beam at a first pivot, a second link connected to the first link at a second pivot and a third link connected to the second link at a third pivot and connected to the outer cylinder at a fourth pivot. Further, the semi-levered landing gear of this configuration includes a truck pitch actuation system operatively connected to the third pivot and configured to position the third pivot in one of a first position and a second position. 
     The truck pitch actuation system of one configuration is configured to maintain the third pivot in the first position in which the third pivot has a fixed first relationship with respect to the outer cylinder, thereby facilitating raising of a forward end of the truck beam, relative to an aft end of the truck beam, during extension of the shock strut. The truck pitch actuation system may also be configured to maintain the third pivot in the second position in which the third pivot has a fixed second relationship with respect to the outer cylinder, thereby facilitating positioning of the truck beam in a stow orientation. 
     In a further configuration, a method of positioning a truck beam of a landing gear is provided. The method provides a semi-levered landing gear mechanism. The semi-levered landing gear mechanism includes a plurality of links operably connected between a shock strut and a truck beam and a truck pitch actuation system. The method of this configuration positions a forward end of the truck beam in a raised position relative to the aft end of a truck beam by causing the truck pitch actuation system to be in a take-off position while the airplane is in the air. The method of this configuration also positions a forward end of the truck beam in a lower position relative to the aft end of the truck beam by causing the truck pitch actuation system to be in a stow position while the airplane is in the air to facilitate retraction of the landing gear. 
     In another configuration, a method of positioning the landing gear of an airplane is provided in which the landing gear includes a shock strut, a truck beam operatively pivotally connected to the shock strut, first and second interconnected links operatively connected to the truck beam and a third link extending between the shock strut and the first and second links with the third link pivotally connected to the second link at a third pivot. In the method of this configuration, the landing gear is commanded to a raised or lowered position. In an instance in which the landing gear is commanded to a lowered position and the landing gear is operational, the third pivot positioned in a first position to support a taxi mode, a take-off mode and a landing mode. In an instance in which the landing gear is commanded to a lowered position and the landing gear is not fully operational, the third pivot is positioned in a second position to support an alternate landing mode. Further, in an instance in which the landing gear is commanded to a raised position the third pivot is positioned in the second position to support a stow mode. 
     The features, functions and advantages that have been discussed may be achieved independently in various configurations of the present disclosure and may be combined in yet other configurations, further details of which may be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Having thus described configurations of the present disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
         FIG. 1  is a schematic representation of a semi-levered landing gear in accordance with one configuration of the present disclosure; 
         FIG. 2  is a functional block diagram of a semi-levered landing gear in accordance with one configuration of the present disclosure; 
         FIG. 3  is a side view of a semi-levered landing gear in accordance with one configuration of the present disclosure; 
         FIG. 4  is another functional block diagram of a semi-levered landing gear in accordance with one configuration of the present disclosure; 
         FIG. 5  is a side view of the semi-levered landing gear of  FIG. 3  during taxi operations in a truck pitch down configuration; 
         FIG. 6  is a side view of the semi-levered landing gear of  FIG. 3  during taxi operations in a truck pitch up configuration; 
         FIG. 7  is a more detailed side view of a portion of a semi-levered landing gear of  FIG. 3  which illustrates the over-center, locked position of a pair of lock links in accordance with one configuration of the present disclosure; 
         FIG. 8  is a side view of a semi-levered landing gear in a takeoff position with a forward end of the truck beam being in a raised position relative to an aft end of the truck beam in accordance with one configuration of the present disclosure; 
         FIG. 9  is a side view of the semi-levered landing gear of  FIG. 8  during transition of the semi-levered landing gear from the takeoff position of  FIG. 7  to a stowed position in accordance with one configuration of the present disclosure; 
         FIG. 10  is a detailed side view of a portion of the semi-levered landing gear that illustrates the further movement of a pair of lock links as a result of the extension of a truck pitch actuator during transition of the semi-levered landing gear from the takeoff position of  FIG. 8  to a stowed position in accordance with one configuration of the present disclosure; 
         FIG. 11  is a side view of a semi-levered landing gear in a stowed position in which the forward end of the truck beam is in a lower position relative to the aft end of the truck beam in accordance with one configuration of the present disclosure; 
         FIG. 12  is a detailed side view of a portion of a semi-levered landing gear of  FIG. 11  which illustrates the over-center position of the pair of lock links in accordance with one configuration of the present disclosure; 
         FIG. 13  is a side view of the semi-levered landing gear of  FIG. 11  as the semi-levered landing gear linkage begins to fold upon contact with the ground during an alternate extension landing in accordance with one configuration of the present disclosure; 
         FIG. 14  is a detailed perspective view of a portion of the semi-levered landing gear of  FIG. 13  which illustrates the interaction of the stop features in accordance with one configuration of the present disclosure; 
         FIG. 15  is a side view of the semi-levered landing gear of  FIGS. 11 and 13  as the semi-levered landing gear linkage continues to fold during an alternate extension landing in accordance with one configuration of the present disclosure; 
         FIG. 16  is a side view of the semi-levered landing gear of  FIGS. 11 ,  13  and  15  which illustrates the truck pitch actuator being back-driven in accordance with one configuration of the present disclosure; 
         FIG. 17  is a side view of the semi-levered landing gear of  FIGS. 11 ,  13 ,  15  and  16  illustrating the nominal position of a semi-levered landing gear once the aircraft is on the ground after an alternate extension landing in accordance with one configuration of the present disclosure; and 
         FIG. 18  is a side view of a semi-levered landing gear in accordance with another configuration of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all configurations of the disclosures are shown. Indeed, these configurations may take many different forms and should not be construed as limited to that set forth herein; rather, these configurations are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout. 
     Referring now to  FIGS. 1 and 2 , a schematic representation and a functional block diagram of a semi-levered landing gear  10  in accordance with one configuration are respectively depicted. As shown, the semi-levered landing gear includes a shock strut  12  extending downwardly from the fuselage of an aircraft or other air vehicle. As described below, the shock strut generally includes an outer cylinder  14  and an inner cylinder  16 . The semi-levered landing gear may include torsion links  20  extending between the inner and outer cylinders for preventing relative rotation therebetween. The semi-levered landing gear also includes a truck beam  18  pivotally connected to the shock strut. In the illustrated configuration, the truck beam is pivotally attached to a distal end or lower end of the shock strut so as to vertically move in concert with the inner cylinder. The truck beam extends from a forward end  18   a  to an opposed aft end  18   b  with the forward end extending toward the forward end of the aircraft and the aft end extending toward the aft end of the aircraft. The truck beam of the illustrated configuration includes a pair of axles with one axle pivotally connected proximate the forward end of the truck beam and another axle pivotally connected proximate the aft end of the truck beam. As shown in  FIG. 1 , one or more tires  22   a ,  22   b  may be mounted upon each axle so as to support the aircraft during ground operations. 
     The semi-levered landing gear  10  of  FIGS. 1 and 2  may also include a plurality of links for angularly orienting the truck beam  18 . In this regard, the semi-levered landing gear may include a semi-levered landing gear linkage  30  including, for example, a first link  30   a  connected to the truck beam at a first pivot, such as a first pivot pin  32 , and a second link  30   b  connected to the first link at a second pivot, such as a second pivot pin  31 . Additionally, the semi-levered landing gear may include a third link  24  connected to the second link at a third pivot, such as a third pivot pin  28 , and connected to the outer cylinder  14  of the shock strut  12  at a fourth pivot, such as a fourth pivot pin  26 . Although the torsion links  20  are also shown to be connected to the outer cylinder at the fourth pivot pin, such coincidence is not necessary and, instead, the third link and the torsion links may be connected to the outer cylinder at different locations. Further, the semi-levered landing gear of  FIGS. 1 and 2  may include a truck pitch actuation system  40  configured to position at least one of the plurality of links so as to angularly orient the truck beam. Although one configuration of the truck pitch actuation system is described in detail below, the truck pitch actuation system may be configured in a number of different manners and, as such, may comprise a plurality of links, levers, linear or rotary actuators that can be powered hydraulically, electrically, pneumatically, etc., or the like. In one operational mode, such as during take-off, the truck pitch actuation system of one configuration is configured to maintain the third pivot in a fixed first relationship with respect to the outer cylinder, thereby allowing a forward end  18   a  of the truck beam  18  to be raised, relative to an aft end  18   b  of the truck beam, during extension of the shock strut in this operational mode. In another operational mode, such as during flight with the landing gear stowed, the truck pitch actuation system may also be configured to maintain the third pivot in a fixed second relationship with respect to the outer cylinder, thereby allowing the truck beam to be positioned by the aircraft in a stow orientation. 
     In one configuration, the first and second links  30   a ,  30   b  have a range of angular rotation with respect to one another that is limited to an off-center orientation. In this regard, the truck beam  18  may have a pitch at take-off that is limited by the off-center orientation between the first and second links, the extension of the shock strut  12  and the positional relationship of the third pivot to the outer cylinder  14 . As described below, the first and second links may include respective stops for limiting relative movement of the first and second links. 
     In accordance with one, more detailed configuration, a semi-levered landing gear  10  is depicted in  FIG. 3 . The semi-levered landing gear includes a shock strut  12  extending downwardly from the fuselage of an aircraft. As noted above, the shock strut generally includes an outer cylinder  14  and an inner cylinder  16 . In one configuration, the shock strut is maintained under a relative high pressure, such as a nitrogen pressure of about 2500 pounds per square inch (PSI), that attempts to extend the inner cylinder. While the aircraft is on the ground, however, the weight on the landing gear overcomes the pressure under which the strut is maintained such that the shock strut remains in a compressed position as shown in  FIG. 3 . As shown in  FIG. 3 , the semi-levered landing gear may include torsion links  20 . The torsion links may include a pair of interconnected links connected at opposed ends to respective lugs carried by the outer cylinder and the inner cylinder. The semi-levered landing gear of the illustrated configuration also includes a truck beam  18  pivotally connected to a distal end or lower end of the shock strut so as to vertically move in concert with the inner cylinder. The truck beam may be pivotally connected to the shock strut by means of a pivot pin extending through both the distal or lower end of the shock strut and an intermediate portion of the truck beam. 
     The semi-levered landing gear  10  also includes a semi-levered landing gear mechanism including at least three links configured to angularly orient the truck beam  18 . The at least three links include a pair of off-center links, such as the first and second links  30   a ,  30   b  discussed above and a third link  24 , such as a pivot link. The off-center links may be pivotally connected to the truck beam at a first pivot pin  32 , to one another at a second pivot pin  31  and to the third link at a third pivot pin  28 . In this regard, the truck beam may include a lug proximate the forward end  18   a  with the off-center links being connected to the lug of the truck beam by means of the first pivot pin. The third link may, in turn, extend between the third pivot pin at which the third link is pivotally connected to the pair of off-center links and a fourth pivot pin  26  at which the third link is pivotally connected to the shock strut, such as the outer cylinder  14  of the shock strut. As shown in  FIG. 3 , for example, the fourth pivot pin may also serve to pivotally attach one of the torsion links  20  to a lug carried by the outer cylinder of the shock strut. As noted above, however, the third link and the torsion links need not both be pivotally attached to the outer cylinder by the fourth pivot pin and, instead, the torsion links may be pivotally attached to the outer cylinder at a position offset from the third link. 
     In the illustrated configuration, the pair of off-center links include the first and second links  30   a ,  30   b  that form a semi-levered landing gear linkage  30  that may be connected at the third pivot pin  28  to the third link  24  and at the first pivot pin  32  to the truck beam  18 . The second link of this configuration may be connected at the third pivot pin to the third link and at the second pivot pin  31  to the first link. The first link is, in turn, connected at the second pivot pin to the second link and at the first pivot pin to the truck beam, such as the lug carried by the forward end  18   a  of the truck beam. As described below, the semi-levered landing gear linkage may also include a biasing device, such as a spring  34 . Although the spring may be differently positioned in other configurations, the spring of the configuration illustrated in  FIG. 3  is connected to and extends between the first and second links. The spring may be a tension spring such that in instances in which the first and second links are on-center or otherwise aligned, the spring attempts to maintain the first and second links in the on-center or aligned relationship. Other types of biasing devices may be employed in other configurations. 
     The semi-levered landing gear mechanism may also include a pair of lock links. The pair of lock links are connected at the third pivot pin  28  to both the third link  24  and to the semi-levered landing gear linkage  30  and at a sixth pivot pin  38  to the shock strut  12 . In this regard, the shock strut may include a lug, such as a lug carried by the outer cylinder  14  of the shock strut, with the sixth pivot pin extending through both the lug and a respective lock link. In the illustrated configuration, the pair of lock links includes first and second lock links  36   a  and  36   b . The first lock link may be connected at the third pivot pin to both the third link and to the semi-levered landing gear linkage and at a fifth pivot pin  37  to the second lock link. The second lock link may, in turn, be connected at the fifth pivot pin to the first lock link and at the sixth pivot pin to the shock strut. 
     The semi-levered landing gear mechanism may also include a truck pitch actuation system  40 . In the illustrated configuration, for example, the truck pitch actuation system includes a truck pitch actuator that extends between the shock strut and a respective lock link. In this regard, the truck pitch actuator may be connected at one end, such as via a pin, to a lug carried by the shock strut, such as the outer cylinder  14  of the shock strut, and at the other end to a lug carried by a respective one of the lock links, such as the first lock link  36   a . In this regard, the truck pitch actuator may be connected to an intermediate portion of the first lock link between the third pivot pin  28  and the fifth pivot pin  37 . As described below, the truck pitch actuator may be actuated hydraulically, pneumatically or otherwise such that an inner cylinder  44  may be controllably extended, retracted or otherwise positioned relative to an outer cylinder  42  in order to at least partially position the truck beam  18  relative to the shock strut when the airplane is in the air. As noted above, the truck pitch actuation system may be differently configured in other configurations of the present disclosure. 
     As shown in  FIG. 4 , the semi-levered landing gear mechanism allows multiple operational modes depending upon whether the commanded position of the landing gear is up (raised) or down (lowered), whether the landing gear is functioning properly and is fully operational and whether the third pivot is in a first position or a second position as shown in blocks  100 - 108 . For example, during normal taxi operations as shown in block  110  of  FIG. 4 , both the shock strut  12  and the truck pitch actuation system  40  are in a taxi position with the shock strut being compressed as shown in block  112 . The semi-levered landing gear linkage  30  is configured during normal taxi operations so as to permit the truck beam  18  to pivot freely as indicted in block  114  so as to have a horizontal orientation, as shown in  FIG. 3 , a truck pitch down orientation, as shown in  FIG. 5 , or a truck pitch up orientation as shown in  FIG. 6 , in order to accommodate some variation in the angle between the shock strut and the ground over which the aircraft is taxiing. As shown by  FIGS. 4-6 , the third pivot remains in substantially the same first position in each of the orientations, e.g., the horizontal, truck pitch down and truck pitch up orientations, so as to control the range of pivotal motion permitted for the truck beam as a result of opening and closing of the first and second links  30   a ,  30   b . In this regard, the first and second links are generally opened or extended to define an obtuse angle therebetween that opens toward the truck beam as shown in  FIG. 5  in order to permit the forward end  18   a  of the truck beam to be lowered relative to the aft end  18   b  of the truck beam in a truck pitch down configuration. Conversely, the first and second links may be further folded so as to define a smaller acute angle therebetween, as shown in  FIG. 6  in order to raise the forward end of the truck beam relative to the aft end of the truck beam in the truck pitch up configuration. The first and second links may include respective lock stops  50 ,  52  for limiting relative movement of the semi-levered landing gear links. As shown in  FIG. 7 , for example, an end of the first link  30   a  proximate the second pivot pin  31  may include a lock stop  50 , while an intermediate portion of the second link  30   b  may include a corresponding lock stop  52 . As such, as the semi-levered landing gear linkage is opened as shown in  FIG. 5 , the lock stops will engage, such as by bringing the lock stop of the second link into contact with the corresponding lock stop of the first link in order to limit or prevent further opening of the semi-levered landing gear linkage and to correspondingly prevent further downward movement of the forward end of the truck beam relative to the aft end of the truck beam. 
     Similarly, the first and second lock links  36   a ,  36   b  may include respective lock stops  54 ,  56  for limiting relative movement of the first and second lock links. As also shown in  FIG. 7 , the second lock link  36   b  may include a lock stop  54  at the end of the second lock link proximate the fifth pivot pin  37 , while an intermediate portion of the first lock link  36   a  may include a corresponding lock stop. As the pair of lock links are opened, the corresponding lock stops of the pair of lock links will engage or physically contact one another and prevent further unfolding or opening of the pair of lock links. While the pair of lock links and the corresponding lock stops may be configured in various manners, the pair of lock links may be configured to be in-line in instances in which the lock stops have engaged one another or to be in an over-center configuration as shown in  FIG. 7  in instances in which the pair of lock links have been opened beyond an in-line position by a predetermined amount. As indicated by the solid lines in the illustrated configuration, for example, the pair of lock links define an interior angle of about 175 degrees in instances in which the respective lock stops have been engaged. For point of comparison, an in-line position is also depicted by  FIG. 7  in dashed lines. However, the pair of lock links and the respective lock stops may define other degrees of being over-center in other configurations. By configuring the pair of lock links to have an over-center position once the corresponding lock stops have been engaged, the pair of lock links will be effectively prevented from collapsing under a compressive load. 
     As noted above, the shock strut  12  is maintained under a relatively high pressure. In the take-off mode shown in block  116  of  FIG. 4 , as the aircraft accelerates down a runway during takeoff, lift is created by the wings and other aerodynamic surfaces. The lift removes at least some of the load from the landing gear  10  with a reduced load allowing the shock strut to extend in response to the relatively high pressure maintained thereby. See block  118  of  FIG. 4 . In order to provide additional ground clearance for rotation of the aircraft during takeoff, the semi-levered landing gear mechanism of one configuration is configured to position the forward end  18   a  of the truck beam  18  in a raised position relative to the aft end  18   b  of the truck beam as shown in the toes-up configuration of  FIG. 8 . See also block  120  of  FIG. 4 . In this regard, the semi-levered landing gear mechanism is configured to cooperate with an extension of the shock strut  12 , such as an extension of the inner cylinder  16  relative to the outer cylinder  14  of the shock strut in response to the lift that is created and the corresponding reduction in the load on the landing gear. Since the truck pitch actuation system  40  remains in a retracted position and the third pivot remains in the first position in the take-off mode, the third link  24  and the first and second lock links  36   a ,  36   b  remain in the same fixed position relative to the outer cylinder  14  of the shock strut  12  as the inner cylinder  16  of the shock strut is being extended. However, the semi-levered landing gear linkage  30  opens to the extent permitted by the respective lock stops  50 ,  52  (see  FIG. 7 ) of the first and second links  30   a ,  30   b . As the inner cylinder  16  of the shock strut  12  is extended by a greater distance than may be accommodated by the opening of the semi-levered landing gear linkage  30 , the semi-levered landing gear linkage limits the downward travel of the forward end  18   a  of the truck beam  18  in response to the extension of the inner cylinder  16  of the shock strut  12  such that the aft end  18   b  of the truck beam  18  is lowered relative to the forward end of the truck beam. 
     Following takeoff, the truck beam  18  remains in the toes-up attitude with the shock strut  12  extended as shown in block  124  of  FIG. 4 . However, the landing gear  10  may not generally be stowed within a wheel well while in a toes-up attitude. Instead, the truck beam may need to be repositioned to another attitude so as to be stowed within the wheel well. In accordance with configurations of the present disclosure, when the landing gear is commanded by the pilot, flight control system or the like to retract to the stow mode as shown in block  122  of  FIG. 4 , a command is correspondingly issued to the truck pitch actuation system  40  so as to cause, for example, the truck pitch actuator to extend, such as by extending the inner cylinder  44  relative to the outer cylinder  42  (see  FIG. 9 ). As shown in  FIG. 9  in which the landing gear is in an intermediate position between the toes-up attitude and the toes-down attitude, the extension of the truck pitch actuator causes the first and second lock links  36   a ,  36   b  to be unlocked and to fold relative to one another. However, the biasing device, such as spring  34 , of the semi-levered landing gear linkage maintains the first and second links  30   a ,  30   b  in a fully opened position. The extension of the truck pitch actuator causes the third pivot to move to the second position with the third link  24  being caused to rotate in a counterclockwise direction about the fourth pivot pin  26  and correspondingly causes the truck beam  18  to rotate in a counterclockwise direction about the pivot pin that connects the truck beam to the shock strut  12  such that the forward end  18   a  of the truck beam is lowered relative to the aft end  18   b  of the truck beam. Further extension of the truck pitch actuator causes the pair of lock links  36   a ,  36   b  to further fold so as to define a smaller acute angle therebetween. The continued movement of the pivot link also causes the first lock link  36   a  to rotate in a counterclockwise direction about the third pivot pin  28  and causes the second lock link  36   b  to rotate in a clockwise direction about the sixth pivot pin  38 , as shown in  FIG. 10 . 
     The extension of the truck pitch actuator  40  may continue until the first and second lock links  36   a ,  36   b  lock in a fully open or extended position. As described above, a pair of lock links may include respective lock stops  54 ,  56  (see  FIG. 12 ) for limiting the relative movement of the first and second lock links  36   a ,  36   b  and defining a fully opened or extended position, as shown in  FIG. 11 . In this regard,  FIG. 12  provides an illustration of the pair of lock links  36   a ,  36   b  in a fully extended position with the respective lock stops  54 ,  56  defining the fully extended position to be over-center. By appropriately attaching the truck pitch actuation system  40  relative to the shock strut  12  and to the pair of lock links  36   a ,  36   b , a single truck pitch actuation system, such as a single truck pitch actuator, may advantageously move the remainder of the semi-levered landing gear mechanism between an up and locked position and a down and locked position. Additionally, it is noted that loads to the third pivot pin  28  are distributed through the truss created by the third link  24  and the pair of lock links  36   a ,  36   b  to the outer cylinder  14  of the shock strut  12  such that the truck pitch actuation system  40  is not in the load path. Once fully extended, the landing gear  10  is in a stowed position with the forward end  18   a  of the truck beam  18  in a lower position relative to the aft end  18   b  of the truck beam. Once in the stowed position as shown in block  126  of  FIG. 4 , the landing gear may be retracted and stowed within a wheel well during the course of a flight. 
     Landing gear extension, such as in advance of the landing of an aircraft, may be performed in the inverse sequence of operations described in conjunction with the retraction of the landing gear  10  including a return of the third pivot to the first position. In this regard, once the landing gear has been removed from the wheel well with the shock strut  12  extended, the truck pitch actuation system  40 , such as a truck pitch actuator, may be retracted so as to return the landing gear to a toes-up attitude, as shown in  FIG. 8 . See also blocks  128 ,  130  and  132  of  FIG. 4 . Alternatively, the truck pitch actuation system may be only partially retracted such that, for example, the inner cylinder  44  of the truck pitch actuator has an intermediate position between the fully extended and fully retracted positions relative to the outer cylinder  42  of the truck pitch actuator. In this configuration, the forward end  18   a  of the truck beam  18  may be raised relative to the aft end  18   b  of the truck beam, but not to the same degree as in the toes-up configuration of  FIG. 8 . By only partially retracting the truck pitch actuation system, the truck pitch actuation system can then act as a truck pitch dampener upon landing. 
     In yet another configuration, the semi-levered landing gear  10  may support an alternate extension landing in instances in which the landing gear is not functioning normally and, therefore, is not fully operational, as shown in block  134  of  FIG. 4 . In this regard, the landing gear may be extended without power in some situations. For example, an alternate extension landing system may include a dedicated battery to release the landing gear and locks associated with the doors of the wheel wells in which the landing gear is stowed. The landing gear may then extend by gravity with the shock strut  12  continuing to be extended as shown in block  136  of  FIG. 4 , but the alternate extension landing system may not have sufficient power to reposition the landing gear from the toes-down stowed position to the toes-up landing position. In this configuration, the semi-levered landing gear is configured to back-drive the semi-levered landing gear mechanism and to unlock the pair of lock links  36   a ,  36   b , thereby preventing damage to the semi-levered landing gear in the event of an alternate extension landing. In an alternate extension landing, the landing gear unfolds from the wheel well, but remains in a stowed or toes-down configuration with the truck pitch actuation system  40  extended and the third pivot in the second position, as shown in  FIG. 11 . See also block  138  of  FIG. 4 . Upon contact with the ground and the application of an upwardly directed force to the tire  22   a  carried by the forward end  18   a  of the truck beam  18 , the semi-levered landing gear linkage  30  folds as shown in  FIG. 13 . The folding of the semi-levered landing gear linkage continues until a stop feature  60  of the second link  30   b  contacts the corresponding stop feature  62  of the first lock link  36   a , as shown in  FIG. 14 . As shown in  FIG. 15 , further folding of the semi-levered landing gear linkage  30  causes the pair of lock links  36   a ,  36   b  to unlock and fold. The semi-levered landing gear linkage continues to fold until the semi-levered landing gear linkage reaches a minimum fold angle, as defined by corresponding stops  54 ,  56  carried by the first and second lock links  36   a ,  36   b  (see  FIG. 14 ). In this regard, the semi-levered landing gear linkage is configured to define a minimum fold angle so as to prevent collision between the second link and the truck pitch actuation system, such as the truck pitch actuator. When the semi-levered landing gear linkage  30  reaches its minimum fold angle, the stops of the second link  30   b  and the first lock link  36   a  will disengage or unlock as a result of a force applied to the third link  24  by the semi-levered landing gear linkage  30  that causes the third link to rotate clockwise, as shown in  FIG. 16 . The pair of lock links  36   a ,  36   b  are, in turn, driven by the clockwise motion of the pivot link which, in turn, causes the truck pitch actuation system to be back-driven in order, for example, to cause the inner cylinder  44  of the truck pitch actuator to be retracted within the outer cylinder  42 . This process continues until a position as shown in  FIG. 17  is reached with the aircraft having landed on the ground. The semi-levered landing gear linkage will then remain in this position until hydraulic pressure is restored to the system, at which time the truck pitch actuation system will further retract, causing the pair of lock links  36   a ,  36   b  to open and to lock in a fully open or extended position, as shown in  FIG. 8  in preparation for the next takeoff. 
     As described above, the semi-levered landing gear  10  of configurations of the present disclosure is advantageously passive so as to lock automatically upon takeoff and to unlock automatically upon landing. The semi-levered landing gear of one configuration also advantageously repositions the truck beam  18  from the takeoff position to the stowed position with a single truck pitch actuator  40  so as to eliminate requirements for additional systems that otherwise add to the complexity, weight and cost of the landing gear. The weight-carrying capacity of the airplane may be increased. Further, the semi-levered landing gear of one configuration removes the truck pitch actuation system from the semi-levered landing gear load path in order to reduce the size and complexity of the truck pitch actuation system. The semi-levered landing gear of configurations of the present disclosure advantageously utilize the same system to position the landing gear in one of various configurations. Examples of such configurations include a toes-up attitude (for takeoff/landing) and a toes-down attitude (for stowage in the wheel well or for an alternate extension landing). 
     Many modifications and other configurations of the disclosures set forth herein will come to mind to one skilled in the art to which these disclosures pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosures are not to be limited to the specific configurations disclosed and that modifications and other configurations are intended to be included within the scope of the appended claims. For example,  FIG. 18  depicts an alternative configuration of a semi-levered landing gear that includes three axles. Although a number of components of the semi-levered landing gear are the same in construction, interconnection and function as described above, several aspects of the semi-levered landing gear of  FIG. 18  have been modified. In this regard, while the third link  24  remains pivotally connected to the shock strut  12 , the third link of this configuration is pivotally connected to a lug  60  that is connected to the shock strut, such as the outer cylinder  14  of the shock strut. As will be noted, the lug  60  is positioned above the sixth pivot pin  38  and is spaced apart from the fourth pivot pin  26  by which the torsion links  20  are pivotally connected to the shock strut. Also, the truck pitch actuation system  40  of the configuration of  FIG. 18  is pivotally connected to the pair of lock links  36   a ,  36   b  at the fifth pivot point  37 , as opposed to being connected to the first lock link  36   a  intermediate of the third and fifth pivot pins  28 ,  37  as in the above-described configuration. Additionally, the biasing device, such as spring  34 , of the semi-levered landing gear linkage is positioned differently than that described above, namely, the spring extends between an intermediate portion of the first link  30   a  and a lug carried by the forward end  18   a  of the truck beam  18 . Thus, although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.