Abstract:
A test apparatus for a rotor blade of a rotor includes at least one guide rail located in proximity to the rotor blade and at least one bearing secured to the rotor blade radially outboard of the at least one guide rail. The at least one bearing is in operable communication with a radially outboard surface of the at least one guide rail to be translatable thereon. At least one force applicator is in operable communication with the at least one guide rail and is configured to exert a force radially outwardly on the at least one guide rail. The force is transferred to the rotor blade via the at least one bearing and simulates a centrifugal force on the rotor blade. Further disclosed is a method for securing a rotor blade of a rotor in a test fixture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a nonprovisonal application of U.S. Provisional Application No. 61/287,464, filed on Dec. 17, 2009, the disclosure of which is also incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to impact test fixtures. More specifically, the subject disclosure relates to impact test fixtures for rotating hardware. 
         [0003]    Rotating components, for example, helicopter rotor blades, are exposed to impact with birds, shed ice, objects on the ground and the like. Testing performed to demonstrate compliance with impact requirements is typically performed on a rotating blade assembly because the force of the impact on the rotor blades is dependent on the natural frequencies of the blade. These natural frequencies depend on the blade mass and the stiffness of the blade which itself is dependent on centrifugal force generated by the rotating blade. Spinning the rotor blade accurately produces the necessary centrifugal force, but such a test is expensive and properly timing the projectile to impact the desired location on the spinning blade is difficult. 
         [0004]    Prior art fixtures have utilized cables, pulleys and/or springs connected to a reinforced portion of the blade which pull the blade radially in an attempt to replicate centrifugal forces in a stationary blade. The reinforcement typically includes a laminate buildup on an outboard section of the blade and a cuff bolted thereto to which the cable or other pulling means would be attached. This configuration adds significant weight to the outboard end of the blade which reduces the natural frequency of the blade resulting in an un-conservative reduction in the force of impact. The art would well-receive improved testing fixtures and methods which would accurately replicate the centrifugal force while reducing the effects of the fixtures on the impact force resulting from the test. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    According to one aspect of the invention, a test apparatus for a blade of a rotor includes at least one guide rail located in proximity to the rotor blade and at least one bearing secured to the rotor blade radially outboard of the at least one guide rail. The at least one bearing is in operable communication with a radially outboard surface of the at least one guide rail to be translatable thereon. At least one force applicator is in operable communication with the at least one guide rail and is configured to exert a force radially outwardly on the at least one guide rail. The force is transferred to the rotor blade via the at least one bearing and simulates a centrifugal force on the rotor blade. 
         [0006]    According to another aspect of the invention, a method for securing a rotor blade of a rotor in a test fixture includes securing a hub of the rotor in a fixed position and locating at least one guide rail in proximity to the rotor blade. At least one bearing is secured to the rotor blade and is in operable communication with a radially outboard surface of the at least one guide rail to be translatable thereon. A radially outwardly directed force is applied to the at least one guide rail, and the force is transferred to the rotor blade via the at least one bearing. The radially outwardly directed force simulates a centrifugal force on the rotor blade. 
         [0007]    According to yet another aspect of the invention, a test apparatus for a specimen includes at least one guide rail located in proximity to the specimen and at least one bearing secured to the specimen. The at least one bearing is in operable communication with the at least one guide rail to be translatable thereon. At least one force applicator is configured to exert a force on the at least one guide rail. The force is transferred to the specimen via the at least one bearing, the force simulating an operational force on the specimen. 
         [0008]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0010]      FIG. 1  is a perspective view of an embodiment of a test fixture; 
           [0011]      FIG. 2  is partial cross-sectional view of an embodiment of a test fixture; 
           [0012]      FIG. 3  is another partial cross-sectional view of an embodiment of a test fixture; and 
           [0013]      FIG. 4  is a perspective view of another embodiment of a test fixture. 
       
    
    
       [0014]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring now to  FIG. 1 , an embodiment of an improved impact test fixture  10  for, for example, helicopter rotor blades, is shown. The test fixture  10  includes a center support  12  to which a hub  14  of a rotor  16  is fixed. The rotor  16  includes a plurality of rotor blades  18  extending radially from the hub  14 . In this embodiment, the rotor  16  shown has two rotor blades  18 , but it is to be appreciated that rotors  16  having other quantities of rotor blades  18  may be accommodated by the test fixture  10 . 
         [0016]    Referring to  FIG. 2 , each rotor blade  18  includes a strap  20  extending along a length of the rotor blade  18  which forms an interior support for the blade shell  22  which forms the exterior of the rotor blade  18 . The blade shell  22  is formed from, for example, a composite material, while the strap  20  is formed from a metal such as titanium or an alloy thereof It is to be appreciated, however that the materials described herein are merely exemplary and the use of other materials for the blade shell  22  and/or the strap  20  is contemplated within the scope of the present disclosure. Referring again to  FIG. 1 , the shell  22  is continuous along a length of the rotor blade  18  and may be secured to the strap  20  along the entire length. In some embodiments, including, for example, bearingless rotors, the shell  22  may be segmented into discrete shell segments  24  which extend partially along a length  26  of the rotor blade  18 . Each shell segment  24  includes one or more attachment points  28  along each shell segment  24  to affix the shell segment  24  to the strap  20  by, for example, a plurality of attachment bolts (not shown). 
         [0017]    The fixture  10  includes a guide assembly  30  located along the rotor blade  18  to be tested. The guide assembly  30  is located relative to the hub  14  by affixing the guide assembly  30  to, for example, a plate  32 , via at least two guide brackets  34 . At least one guide bracket  34  is located at each lateral side  36  of the rotor blade  18 . Two guide rails  40  extend across the rotor blade  18  between the guide brackets  34  with the rotor blade  18  located in a gap  42  (shown in  FIG. 2 ) between the two guide rails  40 . Each guide rail  40  is supported via at least one guide rod  44  extending from each guide bracket  34 . In the embodiment of  FIG. 1 , the guide rods  44  extend parallel to a length of the rotor blade  18 . The guide rails  40  include rail holes  46  (shown in  FIG. 2 ) extending therethrough through which the guide rods extend to support the guide rails  40 . 
         [0018]    An outboard surface  48  of each guide rail  40 , located nearest a blade tip  50 , includes a curvilinear portion  52  that has a center of curvature  54  at a center of the hub  14 . At least one bearing  56  is affixed to the rotor blade  18  such that the bearing abuts the curvilinear portion  52 . In some embodiments, as shown in  FIG. 3 , the two bearings  56  are utilized, one bearing  56  abutting the curvilinear portion  52  of each of the two guide rails  40 . The bearing  56  is affixed to the strap  20 . In some embodiments, the bearing  56  is affixed to the rotor blade  18  at an existing attachment point  28  utilized to affix the shell segment  24  to the strap  20 . In some embodiments, the bearing  56  is located at a center of gravity of the rotor blade  18 . Locating the bearing at the center of gravity of the rotor blade  18  allows for more accurate representation of a rotating rotor blade  18  with regard to both loads on and stiffness of the rotor blade  18 . Utilizing the attachment point  28  eliminates the need to add additional attachment points/structure to accommodate the bearing  56 . Referring again to  FIG. 1 , the bearing  56  is a low friction bearing and is configured to move along the curvilinear portion  52  in both an in-plane direction  58  and an out-of-plane direction  60  thus allowing movement of the attached rotor blade  18  in the in-plane direction  58  and the out-of-plane direction  60 . 
         [0019]    To apply a desired simulated centrifugal force to the rotor blade  18 , a force is applied to each guide rail  40  in a direction along each guide rod  44  toward the blade tip  50 . The force is applied via stacks of Belleville washers  62  which are precompressed and placed on each guide rod  44 . The washer  62  stacks are retained on each guide rod  44  by, for example a retaining nut  64 . The washer stacks  62  exert a force on the guide rails  40  which is transferred, via the bearings  56 , to the rotor blade  18  and acts in a direction toward the blade tip  50 . The washer stacks  62  are long to reduce spring rate, and highly compressed to increase the force. In some embodiments, the desired centrifugal force is 12,000 pounds. In some embodiments, alternatives to washer stacks  62  may be utilized to apply the force. For example, a spring (not shown) located at each guide rod  44  may be used. 
         [0020]    The opposing rotor blade  18  is fixed to react the force applied to the rotor blade  18  to be tested. This may be accomplished by securing the opposing rotor blade  18  to a retaining bracket  66  which is then fixed to the plate  32 . The existing attachment points  28  may be utilized to secure the opposing rotor blade  18  to the retaining bracket. In such embodiments, the opposing rotor blade  18  is first secured to the plate  32  via the retaining bracket  66 . The simulated centrifugal force is then applied to rotor blade  18  to be tested by, pulling the rotor blade  18  to be tested away from the opposing rotor blade  18 . This is accomplished by assembly the washer stacks  62  to exert a radial force on each guide rail  40  which in turn exerts a radially-directed force on the rotor blade  18  through the bearing  56 . Finally, the hub  14  is secured to the plate via a slotted plate attachment (not shown). 
         [0021]    As shown in  FIG. 4 , in some embodiments, the retaining bracket  66  is not utilized. In these embodiments, a second guide assembly  30  is located at the opposing rotor blade  18 , with the same structure as described above. As shown, the guide rods  44  may extend from the first guide assembly  30  to the second guide assembly  30 . It is to be appreciated, however, that separate guide rods  44  may be utilized in each guide assembly  30 . 
         [0022]    During impact testing utilizing the testing fixture  10 , the simulated centrifugal force is applied to the rotor blade  18 , but because of the configuration of the bearings  56  and the guide rails  40 , the rotor blade  18  has freedom of motion, subject to the centrifugal force, in in-plane (lead/lag) and out-of-plane (flap) directions which effectively simulates the motion of a rotating rotor blade  18 . The rotor blade  18  is free to react to the impact in virtually any direction. Such movement of the rotor blade  18  is possible due to the fact that the washer stacks  62  are independent allowing skew of the position of each guide rail  40  relative to the rotor blade  18 . Further, each guide rail  40  is independent allowing for a change in position of one guide rail  40  relative to the other guide rail  40  in reaction to rotor blade  18  forces from the impact of the projectile during testing. Also, the curvilinear shape of each guide rail  40  maintains the simulated centrifugal force in a direction directly radially outwardly from the center of the hub  14 , which is representative of a rotating rotor blade  18 . Such a test fixture  10  provides representative impact testing of a stationary rotor blade  18  to eliminate the need for a costly and complex rotating test. By contrast, use of a flat guide rail  40  would result in changes in direction and changes in magnitude of applied force as the rotor blade  18  moves in-plane along the guide rail  40 . Further, the use of a curvilinear guide rail  40  has advantages over the cable systems of the prior art in that as the rotor blade moves after impact during testing, the cable would impart an unrealistic restorative force on the rotor blade attempting to recenter the blade. 
         [0023]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.