Abstract:
A system of adjustable balancing weights is provided for a helicopter main rotor blade. The trailing edge of the rotor blade is formed with a plurality of recesses that each receive an aerodynamically shaped weight to adjust and balance the weight of the rotor blade. A forward weight recess and a forward weight are also provided on the rotor blade adjacent the rotor blade leading edge. The lateral spacing between the weight adjacent the rotor blade leading edge and the weights adjacent the rotor blade trailing edge provides greater flexibility in balancing the mass of the rotor blade.

Description:
This invention was developed in the course of work under U.S. Government Army Contract DAAB07-97-9-J046. The U.S. government may possess certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention pertains to an adjustable weight system for a rotor blade of a rotary wing aircraft. Specifically, the present invention pertains to an adjustable weight system for a helicopter rotor blade, where the trailing edge of the rotor blade is formed with a plurality of recesses that each receive an aerodynamically shaped weight to adjust and balance the weight of the rotor blade. 
   (2) Description of the Related Art 
   For rotary wing aircraft, for example helicopters, it is necessary that the rotor blades of the aircraft have a mass balance adjustment capability at the tip end or distal end of each rotor blade. Balancing the weight of a rotor blade enables a set of the balanced rotor blades to be operated with reduced vibration. The mass balance adjustment of the rotor blade is provided for both the spanwise direction, i.e. the longitudinal length of the rotor blade, and the chordwise direction, i.e. the lateral width of the rotor blade. This is typically achieved by a helicopter rotor blade balance system that has a pair of adjustable weights positioned adjacent the leading edge of the rotor blade and the distal end of the rotor blade. The two adjustable weights are spaced from each other in the chordwise or lateral direction on the rotor blade. The weights are positioned in a pair of mechanical pockets provided in the surface of the rotor blade. The pockets are typically constructed of metal to withstand the load created by the weights contained in the pockets and to transfer the load to the rotor blade structure. 
   A typical construction of a prior art rotor blade mass balance system is shown in  FIG. 1 .  FIG. 1  shows a section of a rotor blade  10  adjacent the blade distal end. A first forward pocket  12  is positioned in a complementary shaped cavity in the rotor blade adjacent the blade leading edge  14 , and a second forward pocket  16  is positioned in a complementary shaped cavity in the rotor blade adjacent the first forward pocket  12 . The first pocket  12  and second pocket  16  are typically secured to the rotor blade structural spar tube  22 . This positioning of the pair of pockets  12 , 16  provides sufficient structural strength to the pockets in the blade. 
   A pair of weights  24 ,  26  are positioned in the pockets  12 ,  16  in adjusting the mass balance of the rotor blade. The weights  24 ,  26  are secured in the pockets  12 ,  16  by a plurality of mechanical fasteners  28 , for example, self-locking screws. A cover plate  32  is provided to cover over the pair of weights  24 ,  26 . The cover plate  32  fits into a complementary shaped recess  34  in the surface of the blade  10 . A plurality of mechanical fasteners  36 , for example self-locking screws, are provided to secure the cover plate  32  to the surface of the blade  10 . 
   The above described mass balancing system of the prior art is disadvantaged in that the two weight pockets  12 ,  16  cannot be spaced any further apart in the chordwise or lateral direction of the blade  10  beyond the lateral width of the rotor blade spar tube  22 . The structure of the tube  22  is needed to secure the pockets  12 ,  16  and weights  24 ,  26  in the rotor blade  10 . The portion of the rotor blade  10  that extends aft of the spar tube  22  is typically constructed of lightweight composite material skins or layers that extend over the opposite sides of a core material of the blade. This portion of the blade is typically configured and dimensioned to maintain an aerodynamic shape. The thickness dimensions of this portion of the blade are not sufficiently large to retain the weight pockets and their associated weighs, without adding significant reinforcement, which unacceptably adds weight to the blade. 
   SUMMARY OF THE INVENTION 
   The present invention provides a trailing edge adjustable weight system for helicopter main rotor blades that overcomes the disadvantages associated with the prior air system of mass balancing rotor blades. The system of the invention enables the addition of a sufficient amount of mass or weight to the rotor blade for the required mass balancing of the rotor blade. The weights are easily accessible, and the impact of the weights on the surrounding structure of the rotor blade is minimized. The modification required of the rotor blade to implement the adjustable weight system is inexpensive, and does not significantly impact the aerodynamics of the rotor blade outer mold line. 
   The system of the invention comprises a plurality of depressions or aft recesses formed into the lower surface of the rotor blade adjacent the blade trailing edge. In the preferred embodiment, three aft recesses are formed into the rotor blade lower surface. A fastener hole is provided in each of the recesses. The fastener hole extends through the rotor blade. 
   A forward weight pocket is also formed into the rotor blade lower surface adjacent the leading edge of the rotor blade, as was done in the prior art. However, because providing weights adjacent the rotor blade trailing edge provides greater flexibility in mass balancing in the chordwise or lateral direction of the rotor blade, only one forward weight pocket is formed in the rotor blade. A balancing weight and a cover plate are provided for the forward weight pocket in a similar manner to that of the prior art. 
   In addition to the forward pocket weight, the system of the invention comprises a plurality of aft weights. Each of the aft weights has a configuration that is complementary to the configuration of the aft recesses. Each of the aft weights has the same configuration or shape, however each of the aft weights can have a different mass. In this manner, the system of the invention provides a great deal of flexibility in adding weight to the trailing edge of the rotor blade. 
   Each of the aft weights are comprised of a molded plastic body having a metallic core. The bodies of the weights are identical in size and shape. Weight variability is achieved by utilizing different size and/or density metallic core inserts in the plastic bodies of the weights. The metallic inserts also incorporate a threaded hole. A retention screw is passed through the hole in the blade recess from the upper surface of the blade and is screw threaded into the threaded hole of the weight to secure the weight in the recess. 
   The system of the invention simplifies the mass balancing of the rotor blade by allowing the spanwise or longitudinal length adjustments to the blade to be performed by removing or adding the forward weight to the single forward weight pocket. Secondly, the system minimizes the quantity of weight required to make a chordwise or lateral weight adjustment. This is due to the greater distance provided between the forward weight and the aft adjustable weights along the lateral width of the blade. The system of the invention also provides the benefit of reducing the size of the aft weights secured in the weight recesses due to the large lateral offset distance between the forward weight pocket and the aft weight recesses. The offset distance between the two sets of weights also enables the elimination of the second forward weight pocket of the prior art. 
   Thus, the system of the invention reduces the total system weight required to achieve the desired mass balance adjustability of the rotor blade. The system also reduces the fabrication costs of the rotor blade assembly by eliminating the necessity for the second forward adjustable weight pocket that was required by the prior art. Still further, the system reduces the level of effort required to perform a mass balancing operation on the rotor blade at the initial manufacturing of the blade as well as throughout the life of the rotor blade assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features of the rotor blade weight adjustment system of the invention are set forth in the following detailed description of the preferred embodiment of the invention and in the drawing figures wherein: 
       FIG. 1  is a partial view of a prior art rotor blade weight balancing and adjusting system; 
       FIG. 2  is a partial plan view of the rotor blade weight adjusting and balancing system of invention; 
       FIG. 3  is a cross section view of the rotor blade of  FIG. 2 , taken along the line  3 — 3  of  FIG. 2 ; 
       FIG. 4  is a partial assembly view of a part of the weight adjusting and balancing system of the invention; 
       FIG. 5  is a partial view of the rotor blade and the aft weight recesses of the invention; 
       FIG. 6  is a partial view similar to that of  FIG. 5 , but showing the aft weights secured in the aft weight recesses; and 
       FIGS. 7(   a ) and  7 ( b ) show two variant embodiments of the aft weights of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 2 and 3  show a distal end portion of a helicopter main rotor blade  42  incorporating the adjustable weight system of the invention. In the description of the invention to follow, the weight adjustable system is described as being attached to a composite material rotor blade  42  of a helicopter. However, it should be understood that the weight adjustable system of the invention may be employed in other operative environments, and the environment of the composite material helicopter rotor blade  42  used herein is illustrative only and should not be interpreted as limiting. 
   Only a portion of the distal end of the rotor blade  42  is shown in  FIG. 2 . The rotor blade  42  has a construction that is typical of composite material rotor blades. The blade  42  has a longitudinal length with opposite proximal  44  and distal  46  ends. The blade  42  has a lateral width between a leading edge  48  and a trailing edge  52  of the blade. As best seen in  FIG. 3 , the thickness of the rotor blade  42  decreases as it extends from the leading edge  48  to the trailing edge  52 . 
   A tubular spar  54  extends longitudinal through the interior of the rotor blade along the rotor blade length. A core material  56  fills a portion of the rotor blade interior in front of the spar  54  and fills a portion of the rotor blade interior behind the spar. As best seen in  FIG. 3 , the core material  56  behind the rotor blade spar  54  tapers as it extends to the rotor blade trailing edge  52 . 
   The exterior surfaces of the rotor blade are constructed of layers of composite material. One or more layers of the composite material form the upper surface  58  of the rotor blade and the lower surface  62  of the rotor blade. 
   A forward weight pocket  64  is formed in the lower surface  62  of the rotor blade adjacent the rotor blade leading edge  48 . The forward weight pocket  64  is constructed as prior art forward weight pockets and includes a metal sidewall having an oblong shape. The weight pocket sidewall  64  extends into the interior of the rotor blade tubular spar  54 . A plurality of fastener posts  66  are positioned along the center line of the forward weight pocket  64 . 
   A forward weight  68 , having a shape complementary to that of the forward weight pocket sidewall  64 , is positioned inside the pocket sidewall. A plurality of threaded fasteners  72  secure the weight  68  in the interior of the forward pocket sidewall  64 . A cover plate  70  is secured over the forward weight pocket  64  and the forward weight  68  by threaded fasteners (not shown), as is conventional in the prior art. 
   The construction of the forward pocket  64  of the invention shown in  FIG. 3  differs from that of the prior art in that only a single weight  68  is secured in the pocket. This eliminates the second, forward weight of the prior art, and its associated manufacturing and assembly costs. 
   A plurality of depressions or aft recesses  74  are formed into the composite material layers of the rotor blade lower surface  62  adjacent the blade trailing edge  52 . In the preferred embodiment, three aft recesses  74  are formed in the rotor blade. Each of the aft recesses  74  has the same aerodynamic, triangular configuration. An apex area  76  of each recess is positioned in a forward area of the recess toward the rotor blade leading edge  48 . A wider, base area  78  of each recess is positioned in the recess toward the rotor blade trailing edge  52 . Each recess  74  has a greater depth dimension toward the apex area  76  of the recess, as seen in  FIGS. 3 and 5 . As each recess  74  extends laterally from the apex area  76  toward the base area  78  and the rotor blade trailing edge  52 , the thickness of the recess tapers and decreases. 
   A fastener hole  82  is positioned in a central portion of each aft recess  74 . The fastener hole  82  extends completely through the rotor blade  42  adjacent the rotor blade trailing edge  52 . 
   The system of the invention also comprises a plurality of aft weights  92 ,  92 ′ that are removably attached in the aft recesses  74  of the rotor blade  42 . Examples of two of the aft weights  92 ,  92 ′ of the invention are shown in  FIGS. 7   a  and  7   b . Each of the aft weights  92 ,  92 ′ is comprised of a body  94 ,  94 ′, preferably of a plastic material, and a metallic core  96 ,  96 ′. Other types of materials may be employed in constructing the core  96 ,  96 ′, with the material of the core primarily providing the weight to the aft weight  92 ,  92 ′. 
   Each aft weight body  94 ,  94 ′ has an aerodynamic, triangular configuration that is dimensioned to closely match the configuration of one of the aft recesses  74 . The aft weight body  94 ,  94 ′ is formed with an apex portion  98 ,  98 ′ that is received in the apex area  76  of an aft recess, and a base portion  102 ,  102 ′ that is received in the base area  78  of the recess. The exterior configurations of the aft weights are substantially the same. 
   Each of the aft weight cores  96 ,  96 ′ has an internally threaded screw hole  104 . The screw hole  104  is positioned on the weight where it will align with the fastener hole  82  of the aft recess  74  in which the aft weight is positioned. The plurality of aft weights  92 ,  92 ′ differ from each other in the different sizes and masses of the weight cores  96 ,  96 ′, as shown in  FIGS. 7   a  and  7   b . By providing the aft weights  92 ,  92 ′ with different size and mass cores  96 ,  96 ′, a variety of different combinations of weights can be secured in the aft recesses  74  of the rotor blade  42  in balancing the rotor blade. 
   With the aft weight  92  positioned in an aft recess  74 , a threaded fastener  106  can be inserted through the recess fastener hole  82  and screw threaded into the aft weight fastener hole  104  in removably attaching the weight to the rotor blade trailing edge  52 . With the aft weight  92  secured in the aft recess  74 , the exterior surface of the aft weight is flush with the rotor blade lower surface  62 . In this manner, the weight of the rotor blade  42  can be adjusted and balanced without appreciably altering the aerodynamics of the rotor blade. 
   Although a specific embodiment of the system of the invention has been described above, it should be understood that variations and modifications to the system of the invention may be arrived at by the ordinary skilled artisan without departing from the intended scope of the following claims.