Abstract:
A method of assembling a rotor blade assembly includes determining a first spanwise moment of a first component of the rotor blade assembly and comparing the first spanwise moment to a target first spanwise moment. The first spanwise moment of the first component is adjusted based on a result of the comparison. A second spanwise moment of a second component of the rotor blade assembly is determined and compared to a target second spanwise moment. The second spanwise moment of the second component is adjusted based on a result of the comparison. The first component is assembled to the second component, resulting in a rotor blade assembly meeting a target spanwise moment of the rotor blade assembly.

Description:
GOVERNMENT RIGHTS STATEMENT 
     This invention was made with Government support under N00019-06-C-0081 awarded by the Department of the Navy. The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     The subject matter disclosed herein generally relates to rotors for aircraft use. More specifically, the subject disclosure relates to balancing of main rotor blades of rotor craft. 
     For the rotor blade to operate properly in the dynamic environment in which it is used, the blade must meet requirements for balance so as to not result in excessive vibration levels when used in the rotorcraft. A typical rotor blade for a rotor craft, such as a helicopter or dual coaxial rotor rotorcraft is formed from several components including a spar with counterweights, a trailing edge pocket assembly including one or more skins, such as upper and lower skins, with a core therebetween, and a leading edge assembly including a leading edge sheath and other components. These components are typically secured to each other by a structural film adhesive bond and/or other fastener resulting in a blade assembly. Manufacture of the various components, and their assembly, introduces some variation in weight and weight distribution into the blade assembly. This variation is observed by measuring the weight and moments of the completed blade assembly via a balance procedure, often including a whirl fixture, in which the blade assembly is spun with a master rotor blade, having a selected weight and weight distribution. The rotation of the blade assembly is observed and compared to the master rotor blade, and any variation between the two is indicative of variation in weight or weight distribution of the blade assembly compared to the master rotor blade. Such variation is corrected in the blade assembly by removing material or counterweights, up to certain acceptable or feasible limits. The limits constrain how severe of an out of balance condition of the blade assembly can be corrected, resulting in potential of scrapping costly rotor blade assemblies. 
     BRIEF DESCRIPTION 
     In one embodiment, a method of assembling a rotor blade assembly includes determining a first spanwise moment of a first component of the rotor blade assembly and comparing the first spanwise moment to a target first spanwise moment. The first spanwise moment of the first component is adjusted based on a result of the comparison. A second spanwise moment of a second component of the rotor blade assembly is determined and compared to a target second spanwise moment. The second spanwise moment of the second component is adjusted based on a result of the comparison. The first component is assembled to the second component, resulting in a rotor blade assembly meeting a target spanwise moment of the rotor blade assembly. 
     In another embodiment, a method of assembling a rotor blade assembly includes determining a trailing edge pocket assembly spanwise moment of a trailing edge pocket assembly of the rotor blade assembly and comparing the trailing edge pocket assembly spanwise moment to a target trailing edge pocket assembly spanwise moment. The trailing edge pocket assembly spanwise moment of the trailing edge pocket assembly is adjusted based on a result of the comparison. A spar assembly spanwise moment of a spar assembly of the rotor blade assembly is determined and compared to a target spar assembly spanwise moment. The spar assembly spanwise moment of the spar assembly is adjusted based on a result of the comparison. The trailing edge pocket assembly is assembled to the spar assembly, resulting in a rotor blade assembly meeting a target spanwise moment of the rotor blade assembly. 
     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 
       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: 
         FIG. 1  is a schematic view of an embodiment of a rotary wing aircraft; 
         FIG. 2  is a cross-sectional view of an embodiment of a rotor blade; 
         FIG. 3  is a plan view of an embodiment of a trailing edge pocket assembly for a rotor blade; 
         FIG. 4  is a cross-sectional view of an embodiment of a trailing edge pocket assembly for a rotor blade; 
         FIG. 5  is another plan view of an embodiment of a trailing edge pocket assembly for a rotor blade; 
         FIG. 6  is a cross-sectional view of an embodiment of a spar assembly for a rotor blade; and 
         FIG. 7  is a cross-sectional view of an embodiment of a leading edge assembly for a rotor blade. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION 
     Shown in  FIG. 1  is a schematic view of an embodiment of an aircraft, in this embodiment a helicopter  10 . The helicopter  10  includes an airframe  12  with an extending tail  14  and a tail rotor  16  located thereat. While the embodiment of a helicopter  10  described herein includes an extending tail  14  and tail rotor  16 , it is to be appreciated that the disclosure herein may be applied to other types of rotorcraft, such as dual coaxial rotor rotorcraft. A main rotor assembly  18  is located at the airframe  12  and rotates about a main rotor axis  20 . The main rotor assembly  18  is driven by a drive shaft  22  connected to a power source, for example, an engine  24  by a gearbox  26 . The main rotor assembly  18  includes a rotor hub  28  located at the main rotor axis  20  and operably connected to the drive shaft  22 . A plurality of blade assemblies  30  are connected to the rotor hub  28 . 
     Referring now to  FIG. 2 , an embodiment of the rotor blade assembly  30  includes a number of subassemblies arranged along a blade chord  32  extending along a blade assembly length  34  (shown in  FIG. 1 ). The subassemblies include a center subassembly (i.e., a spar assembly  36 ), a leading edge subassembly  38 , and a pocket subassembly  44 . The spar assembly  36  includes a plurality of counterweights  42 . The leading edge subassembly  38  includes a leading edge sheath  40 . The pocket subassembly  44  includes a core  46 , an upper skin  48  and a lower skin  50 . While shown as three distinct assemblies, the number of assemblies and the construction thereof is not specifically limited. 
     To ensure the rotor blade assembly  30  is balanced within selected mass and moment requirements as a finished assembly, a procedure and apparatus is described herein to achieve a balance condition of each subassembly  36 ,  38 ,  44  such that when finally assembled into rotor blade assembly  30 , no further balance procedures, such as a typically used whirl balance, is not necessary. 
     First, referring to  FIGS. 3 and 4 , the pocket subassembly  44  is balanced. The trailing edge pocket assembly  44  is loaded onto a pocket balance fixture  52 , which is in contact with a plurality of load cells  54 . In the embodiment shown, the loads cells  54  are secured to a balance table  56  to which the pocket balance fixture  52  is installed. Further, in this embodiment, three load cells  54  are utilized, but it is to be appreciated that other quantities of load cells  54  may be used to obtain the desired information. Referring to  FIG. 4 , the load cells  54  are arranged such that a first load cell  54   a  is radially inboard from a second load cell  54   b  and a third load cell  54   c . The second load cell  54   b  and third load cell  54   c  are positioned at the same radial location, and are spaced in a chordwise direction. A moment of the trailing edge pocket assembly  44  is calculated from readings of load cells  54   a ,  54   b ,  54   c . While not required in all aspects, the read out of the load cells  54   a ,  54   b ,  54   c  can be done by a computer connected to the load cells  54   a ,  54   b ,  54   c  through wired and/or wireless protocols, or read and manually entered into the computer. 
     In some embodiments, the moment is calculated on the computer or manually by Equation (1):
 
 M   TE   =R   1   ×F   A1   +R   2 ×( F   A2   +F   A3 )  (1)
 
     Where: 
     M TE  is the spanwise moment of the trailing edge pocket assembly  44 ; 
     R 1  is a radial distance of the first load cell  54   a  from the main rotor axis  20 ; and 
     F A1  is a force applied by the trailing edge pocket assembly  44  to the first load cell  44   a.    
     Similarly, R 2  is a radial distance of the second load cell  54   b  and the third load cell  54   c  from the main rotor axis  20 ; and 
     F A2  and F A3  are forces applied by the trailing edge pocket assembly  44  to the second load cell  54   b  and third load cell  54   c , respectively. 
     In Equation (1), the second load cell  54   b  and third load cell  54   c  are located at the same radial distance R 2  whereas the first load cell  54   a  is located at a different radial distance R 1 . However, it is understood that the invention is not so limited and each of the load cells could be located at the same radial distance, or at three different radial distances as need be. 
     The calculated moment, M TE , is compared to a target value M TE-T . Depending on the results of the comparison, one or more actions are taken to add or remove mass of portions of the trailing edge pocket assembly  44  to counteract M TE . Referring to  FIG. 5 , in some embodiments, the added mass is in the form of one or more structural film adhesive layers  58  to the trailing edge pocket assembly  44 , which are then cured into the blade assembly  30 . Each structural film adhesive layer  58  may be, for example, 0.006″ in thickness and 15″ in chordwise width. It is to be appreciated that, in other embodiments, other thicknesses and widths of structural film adhesive layers  58  may be used. A number of structural film adhesive layers  58  utilized and a spanwise length of each structural film adhesive layer  58  is determined by the result of the comparison between M TE  and M TE-T . 
     For example, ΔM TE  is defined as a M TE  subtracted from M TE-T . If ΔM TE  is within a first range, in some embodiments, between 0 and 35 inch-pounds, the trailing edge pocket assembly  44  is sufficiently balanced and no modification is required. If ΔM TE  is within a second range, in some embodiments between 35 and 288 inch-pounds, one structural film adhesive layer  58  is applied to the core  46 . The structural film adhesive layer  58  has a spanwise layer length, L TE , proportional to ΔM TE . In some embodiments, L TE  is expressed as Equation (2):
 
 L   TE =(144×Δ M   TE )/( t×w×r ),   (2)
 
     where 
     t=structural film adhesive layer  58  thickness; 
     w=structural film adhesive layer  58  width; and 
     r=radial location of a spanwise center of gravity of the blade assembly  30 . 
     The prescribed structural film adhesive layer  58  is then assembled to the core  46 , centered about the radial location, r. 
     Further, if ΔM TE  is greater than 288 inch pounds, but less than or equal to 576 inch pounds, two structural film adhesive layers  58  are applied to the core  46 , with L TE  expressed as Equation (2) above. While described in terms of composite layers  58 , it is understood that the invention is not so limited and that the mass can include weight cups in addition to or instead of the layers  58 . 
     Referring to  FIG. 6 , the spar assembly  36  is similarly balanced. The spar moment, M S , is obtained, either by using a spar balance fixture  62  or via other means. ΔM S  is calculated as a difference between a target spar moment, M S-T , and M S . In some embodiments, if ΔM S  is greater than or equal to zero, but less than 40 inch pounds, no structural film adhesive layers  58  are required to balance the spar assembly  36 . If ΔM S  is greater than or equal to 40 inch pounds but less than or equal to 234 inch pounds, an structural film adhesive layer  58  is applied to a lower face  64  of the spar assembly  36 . In some embodiments, the structural film adhesive layer has a width, w, of 10 inches. The length of the spar structural film adhesive layer, L S , is expressed as Equation 3 below:
 
 L   S =(144×Δ M   S )/( t×w×r )  (3)
 
     The prescribed structural film adhesive layer  58  is then assembled to the lower face  64 , centered about the radial location, r. 
     If ΔMs is greater than 234 inch pounds, but less than or equal to 468 inch pounds, two structural film adhesive layers  58  with lengths L S  are applied to the spar assembly  36 , one structural film adhesive layer  58  at the lower face  64  and one structural film adhesive layer  58  at an upper face  66 . Finally, if ΔM S  is greater than 468 inch pounds, but less than or equal to 937 inch pounds, two structural film adhesive layers  58  with lengths L S  are applied to each of the lower face  64  and the upper face  66 . 
     Similarly, referring to  FIG. 7 , the leading edge assembly  38 , including the leading edge sheath  40  and the plurality of leading edge counterweights  42  is balanced. The sheath  40  is loaded onto a sheath balance fixture  68 , which is in contact with the plurality of load cells  54 . In the embodiment shown, the loads cells  54  are secured to the balance table  56  to which the sheath balance fixture  64  is installed. Further, in this embodiment, three load cells  54  are utilized, but it is to be appreciated that other quantities of load cells  54  may be used to obtain the desired information. A moment of the sheath  40  is calculated from readings of load cells  54   a ,  54   b ,  54   c . In some embodiments, the moment is calculated by Equation (4) below:
 
 M   SH   =R   1   ×F   A1   +R   2 ×( F   A2   +F   A3 )  (4)
 
     Where M SH  is the spanwise moment of the sheath  40 . 
     Next, a total moment of the leading edge assembly  38  is calculated using M SH  and moments of the leading edge counterweights  42 , as installed to the spar  36  as Equation 5 below:
 
 M   LE   =M   SH   +M   CW   (5)
 
     Where M LE  is the total moment of the leading edge assembly  38  and M CW  is the moment of the leading edge counterweights  42 . ΔM LE  is calculated as a difference between a target leading edge assembly moment, M LE-T , and M LE . Depending on the value of ΔM LE , leading edge counterweights  42  are removed from the spar assembly  36  to balance the leading edge assembly  38 . 
     With each of the leading edge assembly  38 , spar assembly  36  and pocket subassembly  44  balanced as individual components, they are assembled to form the rotor blade assembly  30 , and since they are pre-balanced at the component stage, the rotor blade assembly  30  meets a target spanwise moment for the rotor blade assembly  30 , and no further balancing of the rotor blade assembly  30  is required. 
     Further, as will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method, or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied thereon. 
     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. For instance, while described in the context of a composite rotor blade, it is understood that aspects could be used in non-composite material rotor blades, blades for wind turbines, ship propellers, and other like objects made of subassemblies and needing to be balanced. 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.