Abstract:
Disclosed is an arrangement in which the main landing gear assemblies of a large commercial airplane are each joined to the airplane at four attachment points. Two of the attachment points connect the landing gear side brace and drag brace to the airplane. The two remaining attachment points are aft and forward trunnions that are mounted to the airplane wing. The aft trunnion pivotally connects one side of the landing gear shock strut to the airplane and carries both vertical and torsional loading. The forward trunnion is pivotally connected to the other side of the shock strut by a trunnion link. An actuator is coupled between the shock strut and the trunnion link. When the landing gear is down and locked the actuator is not activated and the trunnion link does not have the rigidity and stiffness required to transfer vertical loads to the forward trunnion. When the actuator is activated, the combination of the actuator and trunnion link rigidly interconnects the shock strut with the forward trunnion so that four point mounting stabilization is present during landing gear retraction, extension and stowage.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/605,424, filed Aug. 30, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to airplane landing gear assemblies, and more particularly to main landing gear for large commercial aircraft that is of a statically determinate design.  
       BACKGROUND OF THE INVENTION  
       [0003]     Large commercial jet airplanes typically include two main landing gear assemblies that are attached to the undersides of the wings near the fuselage. When deployed, the landing gear assemblies extend downwardly for supporting the airplane during landings and taxiing. When retracted, the landing gear assemblies are stowed in a wheel well in the airplane fuselage. During normal airplane operations, the landing gear assemblies are subject to several types of mechanical load. For example, when the airplane is stationary or slowing taxiing, the landing gear assemblies must withstand the static weight load of the airplane. The main landing gear assemblies are additionally subjected to large, vertical loads when the airplane touches down on the runway. Large, rearwardly directed drag loads can also be present at touchdown and when the brakes are applied. Further, during steering maneuvers, the landing gear assemblies are subjected to side loads. All of these various loading conditions are reacted by the components of the landing gear and, ultimately, by the landing gear support structures that attach the gear to the airplane. Thus, a considerable amount of design and development effort has been directed to providing landing gear assemblies in which the landing gear load path geometry distributes the loads between the wing and fuselage of the airplane.  
         [0004]     One design that has been used to successfully spread the loads between the airplane wing and fuselage uses four discrete attachment points for joining the landing gear assembly to the airplane. Although the use of four attachment points has been successfully used to distribute load in airplanes (such as the Boeing models 767 and 777), the arrangement is subject to a disadvantage and drawback. In particular, because four attachment points are used, the internal loads and attachment loads are statically indeterminate. Thus, the load distribution between the airplane wing and fuselage is a function of the stiffness of the members that join the landing gear to the wing and fuselage. As a result, designing and developing four-point landing gear attachment arrangements can present difficult tasks. In particular, during the design and development of an airplane, changes may be required that may effect the stiffness of the structure used to attach the landing gear to the four attachment points of the airplane. To accommodate these changes, it may be necessary to make changes in the four-point attachment arrangement that add additional weight or, in an extreme case, a redesign could be required.  
         [0005]     A further design consideration is that the main landing gear of large commercial airplanes is designed to breakaway from the airplane under severe overload conditions, such as off-runway excursions, high sink rate landings, etc., so that the gear will not break open fuel tanks that are contained in the wings. In some arrangements, the trunnion attachment used to join the landing gear with the airplane is very stiff as a result of a cantilever trunnion support. In such an arrangement, the trunnions typically carry very high loads, thus requiring that the trunnion pins be high strength fuse links in order to achieve landing gear breakaway. The very large design loads that result from the use of high-strength fuse pins can present disadvantages with respect to the necessary size of the airplane wing box.  
       SUMMARY OF THE INVENTION  
       [0006]     In accordance with this invention, a main landing gear assembly is attached to the airplane at four points. A folding drag brace and a folding side brace extend between the landing gear shock strut and the airplane wing and fuselage structure in a conventional manner to form two of the landing gear attachment points. The other two attachment points are aft and forward trunnions that are located on a cantilever support that extends from the rear wing spar. In the currently preferred embodiment of the invention, the aft trunnion provides an attachment for a trunnion lug that longitudinally extends from the upper end of the landing gear shock strut. The forward trunnion is spaced apart from the upper end of the shock strut and pivotally receives one end of a trunnion link. The other end of the trunnion link is pivotally connected to a trunnion that extends radially from the upper end of the landing gear shock strut. An actuator extends between the upper portion of the shock strut and the central region of the trunnion link. In the currently preferred embodiment, one end of the actuator is trunnion mounted to the shock strut directly below the trunnion link and the other end is trunnion mounted to the central region of the trunnion link. In operation, the actuator serves to, in effect, lock and unlock the forward trunnion arrangement so that the landing gear has three attachment points when the side brace and drag brace are in their locked position, i.e., when the landing gear is down and locked, and in effect, has four attachment points during deployment, retraction and stowage. Thus, when the aircraft is landing and is on the ground, vertical load is transferred to the airplane through the aft trunnion, with the forward trunnion reacting at least almost entirely to torsional loads. Accordingly, from the standpoint of primary design considerations, the landing gear employs a three-point attachment system that is statically determinate. As is known in the art, the invention thereby provides design predictability and repeatability in manufacturing that is not subject to the stiffness of the attachment components which can be difficult to determine and which can change significantly with changes that occur in the design and development of an airplane.  
         [0007]     On the other hand, the actuator, in effect, locks the trunnion link to the aft trunnion whenever the landing gear drag brace and side brace are not locked. Thus, there are four main landing gear attachment points to stabilize the gear when it is stowed in the wheel well or is being extended or retracted. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0009]      FIG. 1  is a perspective view of the upper portion of a main landing gear that is configured in accordance with the invention for locking and unlocking on of the two trunnions that couple the upper end of the shock strut to the airplane wing structure;  
         [0010]      FIG. 2  is a partial perspective view of the upper end of the landing gear shock strut looking downwardly and outwardly to further illustrate attachment of the shock strut to a wing cantilever support; and  
         [0011]      FIG. 3  is an enlarged view of the mounting arrangement between the end of the trunnion link and its mounting that is located at the upper end of the shock strut to show features of the trunnion link that relate to failure of the forward trunnion during landing gear breakaway. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0012]     The upper portion of a main landing gear that is configured in accordance with this invention is shown in  FIG. 1 . The depicted components include a shock strut  10 , a side brace  12 , a drag brace  14 , and a cantilever support  16  for attaching the upper end of the shock strut  10  to the airplane employing the depicted main landing gear. In  FIG. 1 , the depicted main landing gear is on the left-hand side of an airplane with the view being generally in the outboard direction.  
         [0013]     Various main landing gear components that are conventional in type and arrangement are not shown in  FIG. 1 . For example, as is known in the art, a landing wheel truck having an appropriate number of wheels and tires is journaled to the lower end of the shock strut  10 . Also not shown in  FIG. 1 , are the mechanisms for locking the side brace  12  and drag brace  14  when the main landing gear is down and locked (i.e., when the shock strut  10  extends downwardly as depicted in  FIG. 1 ).  
         [0014]     As can be seen in  FIG. 1 , the landing gear of the invention is attached to the airplane at four points, two of which are provided by side brace  12  and drag brace  14  with the two remaining attachment points joining the upper end of shock strut  10  to a cantilever support  16  that extends in the aft direction from the rear spar  24  of an airplane employing the invention.  
         [0015]     In the arrangement shown in  FIG. 1 , side brace  12  is constructed and arranged in a conventional manner. Specifically, side brace  12  includes an upper link  18  and a lower link  18  that are journaled together for alignment with one another when the landing gear is deployed and so that the upper and lower links swing into a folded condition when the landing gear is retracted and stowed. To allow the required movement of side brace  12  between the landing gear down and stowed positions, the lower terminus of side brace lower link  20  is journaled to shock strut  10 . Located at the upper terminus of side brace  12  is a conventionally configured attachment link  22 . Attachment link  22  is joined to an appropriate attachment point of the airplane so that adequate reaction to landing gear side forces is provided during landing and taxiing maneuvers that are within the airplane design range.  
         [0016]     Drag brace  14  also is of conventional type and function. Like side brace  12 , drag brace  14  includes journaled together upper and lower links  24  and  26 , respectively, with the lower terminus of drag brace lower link  26  being journaled to shock strut  10  so that the upper and lower links  24  and  26  can be locked down in alignment with one another when the landing gear is deployed and folded with respect to one another during retraction, stowage, and deployment. As also is the case with respect to side brace  12 , the upper terminus of drag brace upper link  24  includes an attachment link  28 . As is known in the art, attachment link  28  of drag brace  14  attaches drag brace  14  to the appropriate part of the airplane so that drag brace  14  reacts to fore and aft loading that is encountered during landing and taxiing operations that are within the airplane design range.  
         [0017]     Having described two of the landing gear attachment points, reference is now taken to the interconnection of the upper end of shock strut  10  with cantilever support  16 .  
         [0018]     In the depicted arrangement, an attachment lug  34  is integrally formed at the upper end of shock strut  10 , extending longitudinally therefrom and being in a plane that is substantially parallel to a plane that includes the longitudinal center line of shock strut  10 . Attachment lug  34  passes between the parallel, spaced apart bearing arms  38  of an aft trunnion  36 . Similar arrangements are known in the art for joining the upper end of a landing gear shock strut to the wing of an airplane. For example, in one relatively common arrangement, two parallel, spaced apart attachment lugs extend longitudinally from the upper end of a landing gear shock strut with the spaced apart lugs being rotatably joined with appropriately positioned trunnions. As is known in the art, in such arrangements, a trunnion pin passes through the spaced apart trunnion bearing arms and a suitable opening in the shock strut attachment lug. In the arrangement of  FIG. 1 , trunnion pin  42  preferably is a fuse pin (i.e., a pin that shears when subjected to a predetermined force). As shall be described in additional detail, employing a fuse pin as the trunnion pin  42  of aft trunnion  36 , is one of the features that can be incorporated in the invention to provide landing gear breakaway under load conditions that exceed the design capability of the airplane (e.g., off runway excursions, excessive sink rate landings, etc.)  
         [0019]     The upper end region of shock strut  10  that is diametrically opposed to shock strut attachment lug  34  is attached to cantilever support  16  by means of a trunnion link  44 . As is best seen in  FIG. 2 , the end of trunnion link  44  that is located adjacent the upper end of shock strut  10  includes two spaced apart lugs  48  that include openings for pivotally mounting trunnion link  44  to shock strut  10 . In the arrangement of  FIG. 2 , the trunnion link lugs  48  are positioned adjacent spaced apart lugs that are integrally formed in shock strut  10  and project in the forward direction relative to the airplane. As also is shown in  FIG. 2 , trunnion link  44  includes two sidewalls  50  that converge inwardly to form the forward terminus of trunnion link  44 . Extending from the forward terminus of trunnion link  44  and toward cantilever support  16  is a lug  52  for pivotally connecting trunnion link  44  with a forward trunnion  46  that is integrally formed in cantilever support  16 . As can be seen in both  FIGS. 1 and 2 , aft trunnion  36  and forward trunnion  46  are in alignment with one another to allow shock strut  10  (and hence the landing gear) to be swung between a down and locked position and a stowed position.  
         [0020]     Continuing with the description of trunnion link  44 , a relatively thin web  52  spans the substantially “V-shaped” region defined between trunnion link sidewalls  50 . As is best seen in  FIG. 1 , a small trunnion  54  extends downwardly from the central region of trunnion web  54 . Extending between trunnion  54  and a small trunnion  58  that is located on shock strut  10  is an actuator  56 .  
         [0021]     Trunnion link  44  and actuator  56  are configured and arranged to function in a manner that effectively attaches the gear assembly to the airplane at three points when the landing gear is down and locked and attaches the landing gear at four points when the landing gear is being deployed, retracted, or is stowed in the wheel wells. In this regard, when the landing gear assembly of  FIG. 1  is down and locked, the vertical loads are transferred to aft trunnion  36  by means of shock strut  10 . However, as will be described below, with the gear down and locked, trunnion link  44  carries loads from torsion that is applied to the landing gear, but transfers substantially no vertical loads to forward trunnion  46  and cantilever support  16 . Thus, when the gear is down and locked, three point attachment is achieved. On the other hand, when the gear is not down and locked (during deployment, retraction and storage), actuator  56  causes trunnion link  44  to assume a condition under which trunnion link  44  transfers loads that would be vertical loads if the airplane were landing or taxiing. (The gear is in a state equivalent to four point attachment) As previously mentioned, this allows stabilization of the landing gear during gear extension, retraction, and stowage (i.e., at all times when side brace  12  and drag brace  14  are unlocked). As also mentioned, because the landing gear uses three attachment points during touchdown, and at all times the airplane is on the ground, the main landing gear interface loads are statically determinant and predictable, which is highly advantageous from the standpoint of airplane design and development. Moreover, in the arrangement of the invention, a single trunnion attachment carries the vertical load (e.g., aft trunnion  36  in the arrangement of  FIG. 1 ) which means that a single fuse pin can be used to provide landing gear breakaway under vertical overload conditions (e.g., significantly excessive sink rate at touchdown).  
         [0022]     The manner in which trunnion link  44  and actuator  56  implement the “locked” and “unlocked” trunnion conditions is as follows. Trunnion link  44  is designed so that, standing alone, it will not transfer vertical loads from shock strut  10  to forward trunnion  46  (and hence, the airplane wing structure). However, when actuator  56  is activated to rigidly extend between shock strut  10  and trunnion link  44 , axial loading of shock strut  10  (vertical loading if the gear is down and locked) is transferred to forward trunnion  46  (and hence, the airplane wing structure). Thus, by maintaining actuator  56  in a deactivated state when the landing gear is down and locked (e.g., when the side brace  12  and drag brace  14  are locked), only landing gear pivot torque is transferred to forward trunnion  46 . On the other hand, activating actuator  56  when the landing gear is not down and locked (e.g., when side brace  12  and drag brace  14  are unlocked) forms a rigid interconnection between the upper end of shock strut  10  and forward trunnion  46 . In effect, the landing gear then becomes a four point attachment arrangement. More importantly, rigidly interconnecting the upper end of shock strut  10  with forward trunnion  46  provides structural stability to the landing gear that is required in order to swing the landing gear between the gear down and gear up configurations against various forces such as wind forces that tend to push the gear back toward the stowed position during gear deployment.  
         [0023]     Those of ordinary skill in the art will recognize that various arrangements can be used as actuator  56  to establish stable structure when the gear is not down and locked. For example, a positioning actuator similar to those used for positioning a landing gear truck can be used, as can a hydraulic lock, an air spring, or a mechanical stop arrangement similar to side and drag brace locking mechanisms. Those skilled in the art will also recognize that other arrangements can be used to provide a mechanism that is operationally equivalent to the combination of trunnion link  44  and actuator  56 . In that regard, structure equivalent to trunnion link  44  can be configured in various manners as long as that structure physically couples the upper end of shock strut  10  to trunnion  46  and, in addition, lacks rigidity or stiffness that is required in order to transfer vertical loads to trunnion  46  when the landing gear is down and locked. In such an arrangement, actuator  56  can be replaced by mechanical linkage, or hydraulically or pneumatically operated mechanisms that extend between the upper region of shock strut  10  and any portion of the structure that substitutes for trunnion link  44  (or even forward trunnion  46  or cantilever support  16 ) as long as the mechanism equivalent to actuator  56  structurally locks shock strut  10  to forward trunnion  46  when the landing gear is not down and locked.  
         [0024]     As previously mentioned relative to the arrangement shown in  FIGS. 1 and 2 , landing gear breakaway is facilitated by the use of a fuse link as trunnion pin  42 , which joins shock strut attachment lug  34  to aft trunnion  36 .  FIGS. 2 and 3  depict additional features of the currently preferred embodiment of the invention relative to providing landing gear breakaway. In that regard, in order for the gear to breakaway from the airplane, failure must be induced in both aft trunnion  36  and forward trunnion  46 . In the arrangement depicted in  FIGS. 2 and 3 , failure of the forward trunnion joint is achieved by configuring the trunnion link lugs  48  with inwardly extending stops  60  that are forced against stop surfaces  62  that are included in the lugs of shock strut  10  that pivotally attach shock strut  10  to trunnion link  44 . This arrangement short couples the pin that attaches trunnion link  44  to shock strut  10  which causes trunnion link  44  to break at the pin location due to excessive rotation at aft trunnion  36  that occurs under breakaway conditions.  
         [0025]     With the arrangements shown in  FIGS. 2 and 3  arranged in the described manner, a vertical overload that causes the landing gear to breakaway first fuses the aft trunnion pin  40  and then causes the forward trunnion joint to fail. With respect to landing gear breakaway caused by drag overload, the breakaway sequence is: a fuse pin that is installed in the conventional manner in the drag brace fails; the forward trunnion attach pin fails as described above; and the aft trunnion lug then fails due to excessive gear rotation and the landing gear assembly collapses.  
         [0026]     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.