Abstract:
A damper for a landing gear of a helicopter having a barrel, a piston defining a first hydraulic cavity between the piston and a bottom end of the barrel and a second hydraulic cavity between the piston and a top of the barrel, a stack of disc springs disposed within the first hydraulic cavity in a compressed state between the bottom end of the barrel and the piston. Each of the disc springs having a substantially concave side and an opposing substantially convex side, the disc springs stacked in an alternating pattern such that any two adjacent washers have their like sides positioned adjacent to each other. A spring disposed within the second hydraulic cavity such that the spring is compressed between the compression member and the top of the barrel when the piston is in an extended position. A hydraulic fluid is disposed inside at least a portion of the first hydraulic cavity and at least a portion of the second hydraulic cavity. At least one valve is disposed in the piston so as to establish fluid communication between the first hydraulic cavity and the second hydraulic cavity so as to allow bilateral fluid communication between the first hydraulic cavity and the second hydraulic cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/183,127, filed Jun. 2, 2009, which is hereby incorporated herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a landing gear damper and, more particularly, but not by way of limitation, to a landing gear damper for a helicopter which allows for appropriate ground resonance performance. 
     2. Brief Description of Related Art 
     Aircraft landing gears, such as landing gears for airplanes, helicopters, and other aircraft are typically equipped with hydraulic damper systems and struts to absorb forces during aircraft landing and to help support the aircraft while on the ground. Helicopter landing gears absorb the energy from landings through damper motion and elastic deformation of struts. 
     Helicopter landing gears have unique design requirements. Military helicopter landing gears are required to function in vertical sink speeds ranging from 8 ft/sec limit landing to 42 ft/sec crash landing conditions. To provide protection to the airframe, other aircraft components, and cargo, multistage shock struts with controlled mechanical failures are utilized. Some civil helicopters utilize shock struts in combination with skid gears. Although the sink speed ranges for civil helicopters are less stringent than those for military helicopters, ground resonance, static ground position, structural and economic considerations are still relevant. 
     One critical design requirement for helicopters is static ground clearance, or the ability of the landing gear to support the helicopter&#39;s weight without “bottoming out.” Static ground clearance is one of a myriad of design requirements for helicopter landing gears, for example, energy absorption capability, load factor, ground resonance and the like. The landing gear damper is one of the key components most influenced by design requirements. 
     Static ground clearance for helicopters can be increased by reducing damper stroke. Typically, this is facilitated by utilizing a nitrogen containing damper in which the amount of nitrogen in the damper system is increased. Nitrogen containing dampers are replete with drawbacks. For example, any leakage of nitrogen (a common problem with current systems) causes adverse performance of the damper, such as ground resonance problems. 
     Thus, a need exists for a helicopter landing gear damper which reduces the need for maintenance of the same and provides for enhanced performance, and reduced ground resonance. It is to such an apparatus that the present invention is directed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a perspective view of a helicopter having a front helicopter landing gear damper and a rear helicopter landing gear damper. 
         FIG. 2  is a perspective view of a helicopter landing gear damper constructed in accordance with the present invention. 
         FIG. 3A  is a cross sectional view of the helicopter landing gear damper of  FIG. 2 . 
         FIG. 3B  is a fragmented cross sectional view of the helicopter landing gear damper of  FIGS. 2 and 3 . 
         FIG. 4A  is a perspective view of a compression member of a piston of the helicopter landing gear damper of the present invention. 
         FIG. 4B  is a top plan view of the compression member of  FIG. 4A . 
         FIG. 4C  is a sectional view taken along line  4 C- 4 C of  FIG. 4B . 
         FIG. 5A  is a bottom plan view of another embodiment of a compression member of the piston of the helicopter landing gear damper of the present invention. 
         FIG. 5B  is a sectional view taken along line  5 B- 5 B of  FIG. 5A . 
         FIG. 6  is a sectional view of another embodiment of a barrel having a pressure relief member. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring now to the drawings, and more particularly to  FIG. 1 , shown therein is a pictorial representation of a helicopter  8  shown equipped with a plurality of helicopter landing gear dampers  10  constructed in accordance with the present invention. As discussed above, the landing gear dampers  10  function to absorb forces during landing operations and to help support the helicopter when on the ground. 
     Referring now to  FIGS. 2-3B , the landing gear damper  10  is provided with a barrel  12  and a piston  14 . The barrel  12  is provided with a hollow shaft  16  which defines a first hydraulic cavity  17 . The barrel  12  is fabricated from a substantially rigid material having a substantially tubular cross sectional area. Furthermore, the hollow shaft  16  is sealed on a bottom end  18  and has a top end  20  which is adapted to receive and/or retain at least a portion of the piston  14 . It will be understood that the hollow shaft  16  may be fabricated with any number of different materials, for example, an anodized metal and/or differing cross sectional areas that would be known to one of ordinary skill in the art having the present disclosure before them are likewise contemplated for use in accordance with the present invention. 
     The barrel  12  includes a plurality of cupped spring washers or disc springs  22 , hereinafter referred to as disc springs  22 , disposed within the first hydraulic cavity  17  of the barrel  12 . The disc springs  22  are provided with a body portion  24  having an inner diameter  26  and outer diameter  28  ( FIGS. 3A and 3B ). The outer diameter  28  of the disc springs  22  is smaller than the internal diameter of the barrel  12  such that when the disc springs  22  are compressed they traverse linearly within the first hydraulic cavity  17  without impinging against the inner surface  23  of the barrel  12 . Furthermore, the disc springs  22  are provided with a concave geometry, and are stacked to create a spring-like mechanism which exhibits spring-like behavior. 
     To facilitate spring-like behavior the disc springs  22  are stacked within the first hydraulic cavity  17  in an alternating pattern so that when one of the disc spring  22  is disposed with the curved side up, the curved side of the adjacently disposed disc springs  22  is disposed downwardly. Furthermore, if the curved side of the disc spring  22  is disposed downwardly, the curved side of the disc spring immediately below it is disposed upwardly. Thus, when a load is applied to the stacked disc springs  22 , the disc springs  22  compress together absorbing a portion of the energy transferred to them by the load. It will be understood that using disc springs  22  with differing inner diameters  26  produces disc spring stacks with different spring coefficients. This allows each landing gear damper  10  to be tailored to specific installations and/or applications. By way of a non-limiting example, the disc springs  22  are stacked such that the inner diameter  26  of the disc springs increases from the top of the stack to the bottom. 
     The piston  14  is at least partially positioned within the barrel  12 . The piston  14  includes a shaft  32  and a compression member  34 . The shaft  32  may have a piston cavity  33 . To further seal the landing gear damper  10 , a seal member  37  is disposed between the piston  14  and the shaft  32  of the barrel  12 . By way of a non-limiting example, the seal member  37  may include a hat-seal or an o-ring. 
     A hydraulic fluid  30  is disposed within the first hydraulic cavity  17  of the barrel  12  and operates to absorb and/or transfer energy transmitted to the hydraulic fluid  30  from the piston  14 . The hydraulic fluid is at substantially ambient pressure when the piston is in the extended position. 
     The compression member  34  may be an integral member with the shaft  32 . The compression member  34  generally has an upper stem  35 , and the compression member  34  has an outer diameter which is slightly smaller than the inner diameter of the barrel  12 , so as to allow the compression member  34  to traverse within the first hydraulic cavity  17  of the barrel  12  without impingement when the piston  14  is moved linearly within the barrel  12 . To retain the hydraulic fluid  30  within the first hydraulic cavity  17  of the barrel  12 , the compression member  34  may include an o-ring seal  36 . 
     The compression member  34  includes at least one valve  38 , such as a poppet valve. The valves  38  operate to provide fluid communication of the hydraulic fluid  30  from within the first hydraulic cavity  17  of the barrel  12  into a second hydraulic cavity  40  and/or the cavity  33 . As the piston  14  moves linearly through the barrel  12  and compresses the disc springs  22  and the hydraulic fluid  30 , the size (i.e., the volume) of the second hydraulic cavity  40  increases proportionally. By allowing the hydraulic fluid  30  to at least partially transfer into the second hydraulic cavity  40  and/or the cavity  33  regulates the pressures exerted on the hydraulic fluid  30  and the disc springs  22 , thus improving the performance of the landing gear damper  10 . 
     Referring now to  FIGS. 3A ,  3 B, and  4 A- 4 C collectively, it will be understood that if the force exerted on the landing gear damper  10  during, for example, a crash landing is too great, the landing gear damper  10  may not be able to transfer fluid quickly enough from the first hydraulic cavity  17  of the barrel  12  into the second hydraulic cavity  40  and/or the cavity  33 . Such a failure to effectively transfer the hydraulic fluid  30  from the first hydraulic cavity  17  of the barrel  12  into the second hydraulic cavity  40  and/or the cavity  33  may cause the landing gear damper  10  to fail in absorbing and damping landing forces which may result in potentially significant damage to the helicopter  8 . Therefore, the compression member  34  may further include one or more pressure relief members  39 . The pressure relief members  39  are provided for crash landing speeds ranging from approximately 25 ft/sec and greater. The pressure relief members  39  may include, for example, a membrane, valve, wafer or the like. During landings speeds ranging from approximately 25 ft/sec and greater, the pressure exerted on the hydraulic fluid  30  by the compression member  34  causes the pressure relief members  39  to transfer the hydraulic fluid  30  contained within the first hydraulic cavity  17  of the barrel  12  into at least one of the second hydraulic cavity  40  and/or the piston cavity  33  at a greater volume than the hydraulic fluid  30  would transfer via the valves  38  alone. 
     By way of non-limiting example, each of the pressure relief members  39  includes a pressure operated membrane  43  covering apertures  41 . The apertures  41  may be fabricated as substantially linear apertures ( FIGS. 4A-4C ), as angled apertures ( FIGS. 5A and 5B ), or as a combination thereof. The apertures  41  allow fluid communication between the first hydraulic cavity  17  of the barrel  12  into the second hydraulic cavity  17 . During normal operation (i.e., landing speeds of 1 to 20 ft/sec) the pressure operated membranes  43  preclude hydraulic fluid from transferring through the apertures  41  and therefore allow the hydraulic fluid  30  to transfer only via the valves  38  as discussed previously. During crash landing operations, increased pressure caused by the helicopter impacting the ground at an increased landing speed causes the pressure operated membranes  43  to separate from apertures  41  in such a way that the hydraulic fluid  30  in the first hydraulic cavity  17  of the barrel  12  is rapidly transferred into at least one of the second hydraulic cavity  40  and/or the cavity  33 . The rapid transfer of the hydraulic fluid  30  creates a “softer” landing and may reduce damage to the helicopter during crash landing situations. 
       FIG. 6  shows another embodiment of a barrel  12   a  provided with a pressure relief member  45 . The pressure relief member  45  will allow further dampening in a hard landing situations, such as during a crash landing, by allowing the hydraulic fluid in the barrel  12   a  to be expelled from the barrel  12   a.    
     By way of non-limiting example, the pressure relief member  45  may include a rupture disc  47 . During normal operation (i.e., landing speeds of 1 to 20 ft/sec) the pressure relief member  45  precludes hydraulic fluid from transferring from the barrel  12   a . During crash landing operations, increased pressure caused by the helicopter impacting the ground at an increased landing speed causes the rupture disc  47  to burst in such a way that the hydraulic fluid in the first hydraulic cavity  17  of the barrel  12   a  is rapidly transferred from the barrel  12   a.    
     Normal category and military helicopters are designed to auto rotate (engine inoperative). The “crush box” (seats and belly structure) are designed to absorb approximately 10 G forces upon an auto rotation (engine inoperative), and is calculated to make survival possible for the crew and passengers. This computes to approximately 16 feet per second rate of descent. Test has also shown the a gross or over gross helicopter can and has a descent rate of 32 feet per second. This, of course, is fatal to all passengers and crew. Accordingly, the pressure relief member  45  may, by way of example, be adjusted to relieve the pressure at approximately 10 G forces. 
     Referring again to  FIGS. 3A and 3B , the landing gear damper  10  also includes a compression spring  42  for regulating the upward linear movement of the piston  14  relative to the top end  20  of the barrel  12 . It will be understood that the compression spring  42  may include, for example, a coiled compression spring. The compression spring  42  is positioned around the compression member  34  and is at least partially positioned within the second hydraulic cavity  40 . To regulate upward linear movement of the piston  14 , the upper flange  35  of the compression member  34  of the piston  14  operates to compress the compression spring  42  against the top section  44  of the barrel  12 . As the piston  14  moves in an upward linear direction, the hydraulic fluid  30  contained within the second hydraulic cavity  40  and/or the cavity  33  is transferred back into the first hydraulic cavity  17  of the barrel  12 . Although the compression spring  42  has been disclosed as including a coiled compression spring, any number of different elastomeric materials and/or components that would be known to one of ordinary skill in the art having the present disclosure before them are likewise contemplated for use in accordance with the present invention. 
     Referring now to FIGS.  1  and  3 A- 3 B, the helicopter  8  includes a pair of skids  48 . The skids  48  are adapted to contact the ground when the helicopter  8  is landing or at rest. Each of the landing gear dampers  10 A and  10 B is secured to the skid  48  by a bottom bushing  50 . Each of the landing gear dampers  10 A and  10 B is further secured to the helicopter  8  via a top bushing  52 . The securement of the top bushing  52  to the helicopter  8  may occur at any location on the helicopter (i.e., externally or internally). 
     In operation, when the helicopter  8  contacts the ground, the weight of the helicopter  8  is distributed over downwardly through the skids  48  and actuates the landing gear damper  10 . The load is applied to the top bushing  54  and acts to move the piston  14 , and therefore the compression member  34  linearly downward into the first hydraulic cavity  17  of the barrel  12 . The linear movement both compresses the disc springs  22  and transfers at least a portion of the hydraulic fluid  30  contained within the first hydraulic cavity  17  to the second hydraulic cavity  40  and/or the piston cavity  33 . When the helicopter  8  takes off, the piston  14  moves linearly upward and transfers at least a portion of the hydraulic fluid  30  contained within the second hydraulic cavity  40  and/or the piston cavity  33  into the first hydraulic cavity  17 . To regulate the movement of the piston  14  in the upward direction, the compression member  34  compresses the compression spring  42  against the top section  44  of the barrel  12 . 
     It will be understood that the helicopter  8  may include a pair of forward landing gear dampers  10 A and a pair of rear landing gear dampers  10 B. For illustrative purposes, only one half of the landing gear dampers  10 A and  10 B are shown in  FIG. 1 . When the helicopter  8  is in a stored position (not shown), for example, when the helicopter&#39;s rotors  60  are disposed in a swept back configuration (also not shown), the center of gravity of the helicopter  8  is transferred rearwardly to the rear landing gear dampers  10 B. Unlike typical dampers which utilize a gas that may leak from the damper under constant strain, the hydraulic fluid  30  bears the added weight exerted on the rear landing gear dampers  10 B without losses. 
     From the above description it is clear that the present invention is well adapted to carry out the objects and to attain the advantages mentioned herein as well as those inherent in the invention. While presently preferred embodiments of the invention have been described for purposes of this disclosure, it will be understood that numerous changes may be made which will readily suggest themselves to those skilled in the art and which are accomplished within the spirit of the invention disclosed and as defined in the appended claims.