CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/568,170, filed 13 Nov. 2006, titled “Dual Spring Rate Damper,” which issued as U.S. Pat. No. 8,181,755 on 22 May 2012, which claims the benefit of International PCT Application No. PCT/US04/27964, filed 27 Aug. 2004, titled “Dual Spring Rate Damper,” which claims the benefit of Provisional Application No. 60/498,073, filed 27 Aug. 2003, titled “Soft In-plane Tiltrotor Hub,” all of which are hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. VGART NCC2-99086. 
    
    
     BACKGROUND 
     1. Field of the Present Description 
     The present invention relates in general to the field of rotor hubs for aircraft. In particular, the present invention relates to a dual spring rate damper for soft in-plane rotor hubs. 
     2. Description of Related Art 
     Many aircraft rotors, especially those for helicopters and tiltrotor aircraft, include a lead/lag hinge designed to allow in-plane motion of a blade about an axis generally normal to the plane of rotation, such that the blade “runs in” or “gets behind” with respect to other blades. This is mainly to compensate for the extra rotational speed that comes with “blade flapping” and to compensate for differences in blade aerodynamic drag encountered at various moments of one rotational cycle. 
     To prevent excessive motion about the lead/lag hinge, dampers are normally incorporated in the design of this type of rotor system. The purpose of the dampers is to absorb the acceleration and deceleration of the rotor blades and maintain the frequency of the lead/lag motion within a desired range. Often, the damper is an elastomeric damper. Normally, the spring rate chosen for a lead/lag damper is a compromise between the value required for the desired in-plane stiffness and a value that reduces load and fatigue on the rotor and other aircraft components. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention taken in conjunction with the accompanying drawings in which like numerals identify like parts, and in which: 
         FIG. 1  is a perspective view of a four-blade aircraft rotor hub according to the present invention; 
         FIG. 2  is an exploded perspective view of a three-blade aircraft rotor hub according to the invention; 
         FIG. 3  is a partially sectioned perspective view of the rotor hub of  FIG. 2 ; 
         FIG. 4  is a cross-sectional plan view of a portion of the rotor hub of  FIG. 2 ; 
         FIG. 5  is a cross-sectional plan view of a dual-spring-rate damper for use in the rotor hubs of the present invention; 
         FIG. 6  is a cross-sectional plan view of the damper of  FIG. 5 , the damper being configured for a softer spring rate; and 
         FIG. 7  is a cross-sectional plan view of the damper of  FIG. 5 , the damper being configured for a stiffer spring rate. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1  in the drawings, a soft in-plane rotor hub  11  according to the present invention is illustrated. As shown, hub  11  is configured as a four-blade hub for use as a proprotor hub of a tiltrotor aircraft. Rotor hubs according to the invention may have more or fewer blades and may also be configured for use on other rotary-wing aircraft, including helicopters. 
     Hub  11  has a central member  13  which is adapted to fixedly receive mast  15 . Mast  15  is rotated by torque from a drive unit, which may be routed through a transmission (not shown), and the torque is transferred through mast  15  to central member  13  for rotating hub  11 . Blades (not shown) are attached to hub  11  with blade attachment assemblies  17 , each assembly  17  comprising a blade attachment strap  19  and a blade grip  21 . Straps  19  are circumferential and oriented vertically to extend out of the plane of rotation. Straps  19  are hingedly connected to central member  13  at flapping hinges  23 , and blade grips  21  are rotatably and pivotally attached to the outer end of straps  19 . Flapping hinges  23  allow for out-of-plane flapping motion of each blade about an axis generally parallel to the plane of rotation of hub  11 . Blade grips  21  rotate relative to straps  19  about radial pitch axes that are generally parallel to the plane of rotation of hub  11 , and a pitch horn  25  extends from the leading edge of each grip  21  for controlling the pitch of the associated blade. Pitch horns  25  combine with the associated flapping hinge  23  to yield the desired delta-3 pitch-flap coupling. In addition, each blade grip  21  is connected to strap  19  with a lead/lag bearing (not shown), and the grip  21  pivots relative to the associated strap  19  about a lead/lag axis generally normal to the plane of rotation of hub  11 . This provides for chordwise, lead and lag motion of the blades in the plane of rotation of hub  11  about the lead/lag axis. Both the bearing for flapping hinge  23  and the lead/lag bearing are located within strap  19 . The flapping hinge axis is located inboard, and the lead/lag axis is located outboard, the axes being non-coincident. Blade roots  27  are shown installed within the outer ends of grips  21 . 
     To control the chordwise motion of blades about the lead/lag axis, a damper  29  is installed in each strap  19  and is operably connected to the associated blade grip  21 . Dampers  29  are each preferably selectively switchable between at least two spring rates, allowing for hub  11  to be readily configured to have selected in-plane stiffness values. The advantage of selectable in-plane stiffness is that hub  11  can be made stiff enough to prevent ground-resonance conditions when the aircraft is resting on a surface, yet hub  11  can be made softer during flight for minimizing loads and fatigue on components of hub  11  and other components of the aircraft. Dampers  29  are preferably switched through electric actuation, though other types of actuation may alternatively be used, and the switching of dampers  29  is preferably automatically controlled by aircraft control systems. For example, the aircraft control systems may switch dampers  29  to a stiffer setting upon a signal that the aircraft is within a selected proximity of the ground or upon a signal generated by sensors indicating contact of the landing gear with the ground. 
       FIGS. 2 through 4  show a simplified, three-blade alternative embodiment of a rotor hub of the invention.  FIG. 2  is an exploded view,  FIG. 3  is a partial cutaway of the assembly, and  FIG. 4  is a cross-sectional plan view of the assembly. Referring to the these figures, hub  31  includes central member  33 , blade straps  35 , and blade grips  37 . Central member  33  is adapted to fixedly receive mast  34 . Straps  35  are circumferential and are hingedly connected to central member  33  at flapping hinge  39 . This allows for out-of-plane flapping motion of blades (not shown) attached to blade grips  37 . Each blade grip  37  receives the root end of a blade in the outer end of grip  37 , and the inner end of each grip  37  is connected to the outer end of the associated strap  35  with pitch horn brackets  41 . Each grip  37  can rotate about an associated pitch axis  43 , and the pitch for the blades is controlled using pitch horns  45  on brackets  41 . An elastomeric bearing  47  is received within a recess  49  of each bracket  41  to provide for in-plane, chordwise pivoting of brackets  41  and grips  37  about a lead/lag axis  51  passing through the focus of each bearing  47 . Both elastomeric bearing  47  and flapping hinge  39  are located within strap  35 , with the axes for these hinges being non-coincident. This configuration allow for better packaging of the components of hub  31 , especially in tiltrotor applications. 
     As hub  31  is rotated by mast  34 , centrifugal loads from the blades are transferred through grips  37  into brackets  41  and from brackets  41  into bearings  47 . The loads are then transferred into straps  35  from bearings  47  and into central member  33  from straps  35 . A post  53  protrudes from the inner end of each bearing  47 , with post  53  extending through a bore  55  in recess  49  of the corresponding bracket  41 . The inner end  57  of post  53  engages a multiple-spring-rate damper  59 , post  53  extending into an opening  61  in the outer wall  63  of damper  59  and engaging piston  65 . Though shown with an elastomeric bearing  47 , hubs of the invention may be constructed in any appropriate configuration, including hubs using pins or similar connections for the lead/lag hinge. 
     In-plane motion of a blade about the associated lead/lag axis  51  causes a proportional in-plane motion of post  53 . Because post  53  is located inward of axis  51 , the in-plane motion of post  53  is in the direction opposite the movement of the blade. This motion causes displacement of piston  65  along axis  67 , which is resisted by the bulging and/or shearing deflection of elastomeric seals  69 ,  71 . Each damper  59  is selectively switchable between at least two spring rates, including while hub  31  is in use, allowing hub  31  to be switched between at least two values of in-plane stiffness. 
     Damper  59 , as shown in  FIG. 4 , is one example of a switchable, multi-spring-rate damper according to the present invention that can be used in hubs of the present invention, though other types of selectively switchable, multiple-spring-rate dampers may be used. A more detailed view of damper  59  is shown in  FIGS. 5 through 7  and described below. 
     Referring to  FIG. 5 , damper  59  is shown in a cross-sectional plan view Elastomeric seals  69 ,  71  are fixedly mounted to inner surface  73  of housing  75  and fixedly mounted to outer surface  77  of piston  65 . Seals  69 ,  71  are preferably formed as “sandwich” structures, with alternating layers of an elastomeric material  79  and a rigid, non-elastomeric material  81 , such as a metal. This type of structure is nearly incompressible in a direction generally normal to the layers, but the structure allows for a limited amount of shearing motion. 
     Each seal  69 ,  71  sealingly engages inner surface  73  and outer surface  77  to form fluid chambers  83 ,  85  within housing  75 . Each fluid chamber  83 ,  85  is adjacent an end of piston  65  and contains a preferably incompressible fluid, such as a hydraulic fluid or an oil. The fluid may flow between chambers  83 ,  85  through passages  87 ,  89 ,  91 ,  93  formed in piston  65  and extending from one end of piston  65  to the other end of piston  65 . A bore  95  is located on outer surface  77  for receiving inner end  57  of post  53 , which extends from elastomeric bearing  47  ( FIG. 2 ). 
     Primary damping passage  87  has valve means, such as rotary valve  97 , for controlling the flow of fluid through primary passage  87 . As shown in  FIG. 5 , valve passage  99  of valve  97  can be aligned with primary passage  87  for allowing fluid to freely flow between chambers  83 ,  85  through primary passage  87 . Valve  97  can be rotated between this “open” and a “closed” position, in which valve passage  99  is rotated out of alignment with primary passage  87 , preventing fluid from flowing through passage  87 . A secondary passage  89 , which is preferably smaller in cross-sectional area than passage  87 , extends through piston  65  for communicating chambers  83 ,  85 . Secondary passage  89  does not have valve means, so fluid is allowed to flow between chambers  83 ,  85  at all times through secondary passage  89 . Bypass passages  91 ,  93  also extend through piston  65  and communicate chambers  83 ,  85 . Each bypass passage  91 ,  93  has a one-way, spring-biased check valve, items  101  and  103 , respectively, for allowing fluid flow through bypass passages  91 ,  93  only when an overpressure occurs in one of chambers  83 ,  85 . An overpressure in a chamber  83 ,  85  will overcome the force of the spring in the opposing check valve  101 ,  103 , forcing valve  101 ,  103  from a seated position in bypass passage  91 ,  93 . Fluid then flows through bypass passage  91 ,  93  until the overpressure subsides enough to allow bypass valve  101 ,  103  to seat, stopping the flow of fluid. 
       FIGS. 6 and 7  illustrate damper  59  in operation. Referring to  FIG. 6 , damper  59  is shown reacting to a movement of post  53  in the direction shown by arrow  105  when damper is switched to the softer of the two available spring rates. Rotary valve  97  is in the open position, in which valve passage  99  is aligned with passage  87 , and this allows fluid to flow between fluid chambers  83 ,  85  through passage  87 . Fluid can also flow between chambers  83 ,  85  through passage  89 . When movement of post  53  causes piston  65  to move relative to housing  75  and toward chamber  85 , as is shown in the figure, the movement is resisted by the shear force required to deflect seals  69 ,  71 , which are fixedly attached to housing  75  and to piston  65 . The shear force provides a spring rate, k shear , for damper  59 . In addition, the end of piston  65  adjacent chamber  85  applies pressure to the fluid in chamber  85 , forcing the fluid to pass through passages  87 ,  89 , which act as a fluid restriction for damping oscillations of piston  65 . 
     Referring to  FIG. 7 , damper  59  is shown reacting to a movement of post  53  in the direction shown by arrow  105  when damper is switched to the stiffer of the two available spring rates. Rotary valve  97  is in the closed position, in which valve passage  99  is out of alignment with passage  87 , and this prevents fluid flow between fluid chambers  83 ,  85  through passage  87 . Fluid can flow between chambers  83 ,  85  through passage  89 . When movement of post  53  causes piston  65  to move relative to housing  75  and toward chamber  85 , as is shown in the figure, the movement is resisted by the force required to bulgingly deflect seals  71 , as shown. Because fluid in chamber  85  is restricted to flowing through only secondary passage  89 , the fluid pressure caused by piston  65  on the fluid in chamber  85  causes the central portion of seal  71  to bulge outward. The force required provides a spring rate, k bulge , for damper  59 , k bulge , being a higher value than k shear . The flow restriction to fluid flowing through passage  89  damps oscillations of piston  65 . 
     Dampers of the invention may have one piston, such as damper  59  ( FIG. 4 ), or may have more than one piston, such as damper  29  ( FIG. 1 ). Dampers  29 ,  59  preferably have a stroke of approximately +/−1.00 in., though dampers  29 ,  59  may be constructed in any appropriate size for the particular application. Dampers of the invention are shown as having passages extending through the piston, though passages routed through the damper housing may alternatively be used. 
     The damper of the invention has several advantages, including: (1) providing selectively switchable spring rates for lead/lag damping; (2) providing a small, lightweight switchable damper for use in the rotor hubs of the invention; and (3) providing a method of preventing ground resonance conditions while minimizing loads and fatigue on aircraft components. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description.
A rotor hub and method to dampen a force exerted on a rotor hub by a rotor blade during flight. The rotor hub including a central member rotatably coupled to a rotor mast, a blade grip rigidly attached to a blade and movably coupled to the central member, and an adjustable damper operably associated with the blade grip. The method includes damping the force exerted on the rotor hub with the adjustable damper and selectively adjusting the adjustable damper between a first spring rate and a second spring rate.