Abstract:
Methods and apparatus are provided for making a rotor blade spar from composite material wherein a multi-component mandrel is used to form the composite spar. The mandrel is made using a number of components, which are assembled to provide a structure that is sufficiently strong to maintain the spar shape during pre-cure lay up, compaction and curing of the composite material. The multiple components used to form the mandrel can be separated from each other and easily removed from the spar either before or after curing of the composite material. The mandrel components can then be re-assembled and re-used to form additional composite spars.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to helicopter rotor blades that are made from composite materials. More particularly, the present invention is directed to the processes and apparatus that are used in the manufacture of such composite rotor blades. 
     2. Description of Related Art 
     Rotor blades are a critical component of every helicopter. The rotor blades are subjected to a complex set of rather extreme aerodynamic forces that vary continually during flight. The rotor blades function as rotating airfoils or wings that are shaped to provide the aerodynamic lift required for a given aircraft. Rotor blades typically include a spar that extends from the root of the rotor blade to its tip. The spar is a major structural element of the rotor blade that provides the blade with the structural strength needed to carry high operational loads. 
     The typical rotor blade spar is a long tubular structure around which the rest of the blade is formed. The spar tube has an elliptical cross-section that is formed to provide a forward or leading edge and rearward or trailing edge. In order to provide optimum aerodynamic performance, many spar tubes include a slight twist about the longitudinal axis. Typical twists in the spar provide rotations of the elliptical cross-section of up to 10 degrees and more as one moves from the root of the rotor blade to its tip. In addition, the elliptical shape of the spar cross-section may be varied from the spar root to the spar tip to meet a variety of aerodynamic and structural loading parameters. 
     High strength materials, such as titanium and aluminum alloys, have typically been used to make rotor blades. These high strength metal materials are particularly well suited for forming the rotor blade spar. Titanium has been routinely formed into the relatively long, tubular spar structure and machined or otherwise fabricated to provide a complex variety of twists and varying cross-sectional shapes. 
     Composite materials have also been used to form rotor blade spars. The combination of light weight and structural strength have made composites a popular choice for making not only the rotor blade spar, but the entire rotor blade. Exemplary composite rotor blades and the processes for making them are described in U.S. Pat. Nos. 4,892,462; 5,346,367; 5,755,558; and 5,939,007. 
     The typical composite spar is fabricated by applying the uncured composite material to the surface of a long cylindrical mold or mandrel that is shaped to provide the interior surface of the spar tube. After the composite material is applied to the mold or mandrel, it is compacted and cured at an elevated temperature to provide the final spar structure. A problem associated with making composite spars revolves around what to do with the mold or mandrel once the spar has been formed. The length of the mold and the variations in elliptical cross-section of the spar, as well as any twist in the spar, make it very difficult to remove the mold or mandrel after the spar has cured. 
     One approach to solving the mold/mandrel removal problem has been to make a mold out of a material that is strong enough to maintain its shape during pre-cure fabrication of the composite spar, but which disintegrates or otherwise shrinks during the cure cycle so that it can be removed from the spar cavity or simply left in place. For example, a variety of foams have been used alone or in combination with an underlying hard mandrel structure to provide a suitable spar mold. The foam melts or otherwise shrinks to a fraction of its initial size during curing at elevated temperatures. The resulting shrunken mold is sufficiently small so that it can be removed from the spar cavity or left in place. 
     Although foam molds have been used successfully in fabricating composite spars for rotor blades, it is many times difficult to find a foam or other material that has the needed structural strength to maintain critical spar dimensions during formation of the spar, while at the same time being able to deteriorate relatively rapidly during cure. In addition, the mold can only be used once, which adds considerably to the cost of spar fabrication. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, methods and apparatus are provided for making a rotor blade spar from composite material wherein a multi-component mandrel is used to form the composite spar. The mandrel is made using a number of components, which are assembled to provide a structure that is sufficiently strong to maintain the spar shape during pre-cure lay up, compaction and curing of the composite material. The multiple components used to form the mandrel can be separated from each other and easily removed from the spar either before or after curing of the composite material. The mandrel components can then be re-assembled and re-used to form additional composite spars. 
     As a feature of the present invention, a multi-component mandrel is provided for use in molding a helicopter blade wherein the rotor blade includes a spar that extends parallel to the longitudinal axis of the rotor blade. The spar that is being formed includes interior surfaces that form a spar cavity that also extends longitudinally from the root of the blade to the tip. The spar interior surfaces include a leading edge surface that is composed of an upper leading edge portion and a lower leading edge portion. The spar interior surfaces further include a trailing edge surface that is composed of an upper trailing edge portion and a lower trailing edge portion. The spar interior surfaces also include an upper surface that extends between the leading edge upper portion and the trailing edge upper portion, as well as a lower surface that extends between the leading edge lower portion and the trailing edge lower portion. 
     The mandrel is made up of a forward component that includes an exterior surface that is shaped to provide the leading edge surface of the spar interior surface. The forward component includes an upper rear edge that has an outer surface, which is shaped to provide the upper leading edge portion of the spar interior surfaces. The forward component also includes a lower rear edge that is shaped to provide the lower leading edge portion of the spar interior surfaces. The mandrel also includes a rearward component that is shaped to provide the trailing edge surface of the spar interior surfaces. The rearward component includes an upper forward edge that is shaped to provide the upper trailing edge portion of the spar interior surfaces. The rearward component also includes a lower forward edge that is shaped to provide the lower trailing edge portion or the spar interior surfaces. 
     The forward and rearward components of the mandrel are connected together by an upper component and a lower component. The upper component is shaped to provide the upper surface of said spar interior surfaces. The upper component includes a forward edge that is connected to the upper rear edge of the forward component and a rearward edge that is connected to the upper forward edge of the rearward component. The lower component is shaped to provide the lower surface of said spar interior surfaces. The lower component includes a forward edge that is connected to the lower rear edge of the forward component and a rearward edge that is connected to the lower forward edge of said rearward component. 
     The final component of the mandrel is a support structure that is located between the upper component and the lower component. The support structure provides reinforcement for the upper and lower components and also holds them in place against the forward and rearward components. As a feature of the present invention, the support structure is collapsible when a tensioning force is applied to the support structure along the longitudinal axis of said rotor blade. The collapsing of the support structure allows it to be removed from the spar cavity. Once the support structure is removed, the upper and lower components of the mandrel can be disconnected from the forward and rearward components. The components can then be removed individually from the spar cavity. 
     The present invention also covers methods for making the multi-component mandrel as well as the methods for molding composite rotor blade spars using the multi-component mandrel and the resulting rotor blade spar. The multi-component mandrel of the present invention provides a number of advantages over existing methods for making composite rotor blades. These advantages include the ability to withstand the forces applied to the mandrel during fabrication of the composite blade in order to avoid any undesirable variations in blade shape. In addition, the mandrel can be used to form complex spar shapes including spars with varying degrees of twist and changes in elliptical cross-sectional geometry. A further advantage is that the mandrel can be re-assembled and used repeatedly. 
     The above described and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial perspective view of an exemplary composite helicopter rotor blade that includes a spar that can be made using the multi-component mandrel in accordance with the present invention. 
         FIG. 2  is a perspective view of a preferred exemplary multi-component mandrel in accordance with the present invention. 
         FIG. 3  is a sectional of view of a portion of  FIG. 2  taken in the  3 - 3  plane 
         FIG. 4  is the same view as  FIG. 3  except that the composite material spar is shown in place on the exterior surface of the mandrel. 
         FIG. 5  is a cross-sectional view of the mandrel depicting how the components are separated for removal from the spar after compaction and/or curing of the composite spar. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred exemplary multi-component mandrel in accordance with the present invention for use in molding a helicopter rotor blade from composite material is shown generally at  10  in  FIG. 2 . An exemplary helicopter rotor blade that can be molded utilizing the mandrel  10  is shown in a simplified form in  FIG. 1  at  12 . The rotor blade  12  includes a spar  14  that extends parallel to the longitudinal axis  16  of the rotor blade  12 . The spar  14  typically extends from the root of the rotor blade (not shown) to the tip  18 . The spar  14  is a tubular structure that has an elliptically shaped cross-section as shown in  FIG. 1 . The spar  14  includes a number of interior surfaces that are formed by the mandrel  10 . These interior spar surfaces define the spar cavity  20 . 
     Referring to  FIG. 1 , the spar interior surfaces are composed of a leading edge surface  22 , trailing edge surface  24 , an upper surface  26  and a lower surface  28 . The leading edge surface  22  includes an upper leading edge portion  30  and a lower leading edge portion  32 . The trailing edge surface  24  includes an upper trailing edge portion  34  and lower trailing edge portion  36 . The upper surface  26  extends between the upper leading edge portion  30  and the upper trailing edge portion  34 . The lower surface  28  extends between the lower leading edge portion  32  and the lower trailing edge portion  36 . 
     Referring to  FIG. 2 , the mandrel  10  includes a forward component  38  that has an exterior surface, which is shaped to provide the spar interior leading edge surface  22 . The mandrel forward component  38  includes an upper rear edge  40  that has an exterior surface, which is shaped to provide the upper leading edge portion  30  of the spar. The mandrel forward component  38  also includes a lower rear edge  42  that has an exterior surface, which is shaped to provide the lower leading edge portion  32  of the spar. 
     The mandrel  10  also includes a rearward component  44  that has an exterior surface, which is shaped to provide the spar interior trailing edge  24 . The mandrel rearward component  44  includes an upper forward edge  46  that has an exterior surface, which is shaped to provide the upper trailing edge portion  34 . The mandrel rearward component  44  also includes a lower forward edge  48  that has an exterior surface, which is shaped to provide the lower trailing edge portion  36 . 
     The mandrel  10  further includes an upper component  50  that has an exterior surface, which is shaped to provide the spar upper interior surface  26 . The upper component  50  includes a forward edge  52  that is connected to the upper rear edge  40  of the forward component  38 . The upper component  50  also includes a rearward edge  54  that is connected to the upper forward edge  46  of the rearward component  44 . The mandrel  10  also includes a lower component  56  that has an exterior surface, which is shaped to provide the spar lower interior surface  28 . The lower component  56  includes a forward edge  58  that is connected to the lower rear edge  42  of the forward component  38 . The lower component  56  also includes a rearward edge  60  that is connected to the lower forward edge  48  of the rearward component  44 . 
     The final component of mandrel  10  is a collapsible support structure, which is shown in  FIG. 2  as corrugated support strips  62  and  64 . The corrugated support strips  62  and  64  extend longitudinally within the mandrel  10  (i.e. parallel to the longitudinal axis  16  of the spar). The corrugated support strips  62  and  64  are located within the mandrel cavity so as to provide an outward bias against the upper component  50  and lower component  56  and to provide support for these two components along their entire lengths. As can be seen from  FIG. 2 , the forward and rearward edges  52  and  54  of the upper component  50  are shaped so that they overlap on the inside of the upper rear edge  40  of the forward component  38  and upper forward edge  46  of the rearward component  44 , respectively. This overlapping arrangement provides for a secure, but releasable, connection between the upper component  50  and the forward and rearward components  38  and  44 . Likewise, the forward and rearward edges  58  and  60  of the lower component  56  are shaped so that they overlap on the inside of the lower rear edge  42  of the forward component  38  and lower forward edge  48  of the rearward component  44 , respectively. This overlapping arrangement also provides for a secure, but releasable, connection between the lower component  56  and the forward and rearward components  38  and  44 . 
     The outward bias provided by the corrugated strips  62  and  64  against the upper and lower mandrel components  50  and  56  provides compression connections along the four locations where the mandrel components overlap as described above. These compression connections keep the mandrel in the form of a single relatively strong structure during fabrication of the composite spar. Upon removal of the corrugated support strips  62  and  64 , the upper and lower components  50  and  56  may be move toward each other and disconnected from the forward and rearward components  38  and  44 . 
     The collapsible support structure must be sufficiently strong to prevent the upper and lower components from collapsing together during lay up, compaction and curing of the composite spar. At the same time, the support structure must be able to collapse laterally when a tensioning force is applied to it along the longitudinal axis of the rotor blade. The term “collapsible” is used herein to mean that the structure must, as a minimum, be able to collapse a sufficient amount to reduce the friction between the support structure and the upper/lower components, so that the support structure may be pulled longitudinally from the root end of the spar cavity. In other words, the support structure must be able to collapse enough, when pulled from one end, to unlock the support structure from its friction fit against the upper and lower components. 
     Corrugated strips, as shown in  FIG. 2  are the preferred collapsible support structure. However, other types of materials may be used provided that they meet the above criteria with respect to lateral shrinkage when a longitudinal tensioning force is applied. It should be noted that two corrugated strips  62  and  64  are shown in  FIG. 2  as being preferred. However, a single corrugated strip may be used, especially in situations where the geometry of the spar cavity is not complex. Alternatively, more than two strips may be used, if desired. 
     The corrugated strips may be made from any material that provides the desired combination of strength and flexibility. It is preferred that the corrugated strips be sufficiently resilient that they are not deformed when they are collapsed and removed from the mandrel cavity by application of longitudinal tension. This allows the strips to used more than once as the collapsible support structure. A wide variety of metals are available that have the necessary strength and flexibility to function as a support structure. However, it is preferred that the corrugated strips be made from composite material. Composite materials provide the strength necessary to keep the upper and lower components from collapsing during application of external pressure during the compaction and curing processes. In addition, corrugated strips made from composite materials have sufficient flexibility and resiliency to collapse the desired amount when longitudinal tension is applied and then spring back to their original shape. 
       FIG. 3  is a sectional view that shows a portion of the corrugated strip  64  in place between the upper and lower components  50  and  56  of the mandrel  10 . The corrugated strip  64  has ridges  66  which extend substantially perpendicular to the longitudinal axis  16  of the spar  14 . Spacers  68  may be placed between the ridges  66  of the corrugated strip and the upper and lower components  50  and  56  to facilitate removal of the corrugated strip  64  from the mandrel. Use of spacers  68  to enhance release of the ridges  66  from the upper and lower components  50  and  56  is preferred, but not required. The spacers  68  may be used on all or some of the ridges  66 . The spacers  68  may be in the form of single elongated strips or they may be in the form of multiple washers that are spaced along the ridges. The spacers  68  made from fiber reinforced polyamide or phenolic are preferred. However, the spacers can be made from a variety of materials provided that the surface tension is sufficiently low so that the washers are released from the mandrel surfaces when the corrugated strip is removed. Materials, such as polytetrafluorethylene and other non-sticking substances, may be used, but are not particularly preferred because they can be difficult to keep in place on the ridges of the corrugated strips during mandrel assembly. 
     The washers are preferably kept in place on the ridges using a small amount of glue or other binder material that is sufficient to hold the washer in place during mandrel fabrication, but which allows the washers to be released from the ridges when the mandrel is removed from the spar cavity. In addition, it is preferred that the ridges be machined slightly in order to form platforms for seating the washer. The degree of machining depends upon the size of the washer and the thickness of the corrugated support material. The washers are typically on the order of a few thousandths of an inch thick to a few tenths of an inch thick or even thicker depending upon the particular mandrel dimensions and the degree to which the corrugated ridges are machined to accept the washer. 
     As shown in  FIG. 3 , it is preferred that the spacers  68  are located on only one side of the corrugated strip. The spacers  68  are shown located on the top ridges in  FIG. 3  for demonstrative purposes only. The spacers  68  could alternatively be located on the bottom ridges. During assembly of the mandrel, it is preferred that the corrugated strip  64  be temporarily attached to the lower component  56  using removable fasteners. The removable fasteners, such as clecos, are used to connect the lower ridges of the corrugated strip to the lower component. The washers  68  are placed on the upper ridges of the corrugated strip  68  and then the upper component  50  is put in place. In order to hold the assembly of components together, it is preferred to wrap the completed assembly, as shown in  FIG. 3 , in shrink-wrap or other suitable tape. 
       FIG. 4  shows a partial cross-sectional view of the mandrel  10  located within the composite spar  14  just after compaction and/or curing of the composite material. Arrow  70  depicts the application of a tensioning force (i.e. pull) along the longitudinal axis of the rotor blade. Arrows  72  depict the collapsing of the corrugated strip  64  that results from the longitudinal pull on the strip. As is apparent, the ridges  66  only need to collapse in the direction of arrows  72  a sufficient amount to release washers  68 , so that the strip  64  may be pulled longitudinally from the mandrel. This allows for the corrugated strip  64  to be deformed the minimum amount so that it can be reused a number of times. In some situations, especially with very complex spar geometries, it may be necessary to apply sufficient tension  70  in order to substantially collapse the corrugated strip in order to be able to remove it from the mandrel. 
       FIG. 5  is a simplified side cross-sectional view that shows the mandrel  10  in place after the corrugated supports strips  62  and  64  have been removed. The upper and lower components  50  and  56  are moved inward towards each other, as represented by arrows  74 , so that they can be removed from the spar cavity  20 . The forward and rearward components  38  and  44  are also moved inward towards each other, as represented by arrows  76 , so that they also can be removed from the spar cavity  20 . 
     The mandrel  10  may be removed from the spar cavity  20 , as described above, either after compaction of the uncured spar composite material around the mandrel or after the compacted composite spar has been cured. It is preferred to remove the mandrel prior to curing in order to maximize the number of times it can be re-used. The mandrel should be able to withstand the pressures that are present during normal procedures for molding helicopter rotor blade spars. Typically, the mandrel should be able to withstand external pressures on the order of 10 to 15 inches of Hg and higher. The mandrel should also be able to withstand the temperatures at which the composite materials used to make the spar are cured. Typically, such composites are cured at temperatures in the range of 120° C. to 200° C. and even higher. 
     The materials that are used to make the four external components of the mandrel  10  may also be any of the metals typically used for making mandrels for molding composite materials. However, as was the case with the corrugated support strips, composite materials are a preferred material for making the external mandrel components that actually come in contact with the spar during rotor blade fabrication. The external surfaces of the mandrel or the shrink-wrap (if used) may be coated with a suitable release agent, if desired. 
     The composite materials that may be used to make the mandrel components include those containing glass or carbon fibers. The fibers may be in the form of woven fabric, unidirectional fibers or randomly oriented fibers. Any of the various thermosetting resins that are suitable for use in relatively high temperature molding operations may be used as the matrix material. Exemplary resins include epoxies, phenolics, bismaleimides and polyester. A preferred material is an isotropic composite material that is composed of randomly oriented chips of unidirectional fibers in an epoxy matrix. This type of mandrel material is available from Hexcel Corporation (Dublin, Calif.) under the tradename HexMC®. An alternate preferred material for use in making the mandrel components is carbon fabric/epoxy prepreg, such as HEXCEL 8552, which is also available from Hexcel Corporation (Dublin, Calif.). Both of these materials are supplied as uncured prepregs, which can be formed into the desired mandrel component and cured according to conventional methods for fabricating and curing epoxy-based composite structures. 
     As an example, the mandrel of the present invention may be used to mold the spar of a helicopter rotor blade where the spar is on the order of 20 to 25 feet long or even longer for large helicopters. The distance between the leading edge and trailing edge of the spar at the blade root ranges from a few inches to two feet or more. This distance tapers down to a few inches to a foot or more at the blade tip. The thickness of the spar at the blade root ranges from an inch to a foot or more and tapers down to less than an inch or up to a few inches at the blade tip. The spar has a twist on the order 10 degrees about its longitudinal axis from the root of the spar to its tip. The various external components of the mandrel (forward component, rearward component, upper component and lower component) are made to match the internal shape of the spar. They are fabricated as four individual components that are each 20 to 25 feet long. Each component is made from a sufficient number of plies of Hexcel 8552 carbon/epoxy prepreg or HexMC® to make components that are from 0.01 inch thick to 0.5 inch thick or more depending upon the size of the mandrel. The components are cured according to conventional curing procedures. 
     For the exemplary mandrel described herein, two corrugated support strips are used that are both 20 to 25 feet long to match the length of the other mandrel components. The corrugated support strips are sized so that the lateral distances between the ridges match the thickness of the mandrel as it varies from root to tip and from leading edge to trailing edge. The two corrugated support strips are positioned inside the mandrel cavity so that they apply the proper outward bias force against the upper and lower components over the entire length of the mandrel. The longitudinal distance between the individual upper ridges on the corrugated strip should be sufficeint to provide the needed support for the upper and lower components. The longitudinal distance between the lower ridges on the corrugated strip should be about the same as the distance between the upper ridges. The longitudinal distances between the ridges may vary form the root to the tip. For example, it may be desirable to make the ridges closer together nearer the root of the mandrel in order to provide added support where the spar cavity has the largest cross-sectional area. 
     The corrugated strips are also made from HEXCEL 8552 carbon/epoxy prepreg or HexMC®, which are also formed into the required corrugated shape and compacted and cured according to conventional curing procedures. The resulting corrugated strips should be sufficiently thick to provide support for the upper and lower components during compaction and curing, if desired. The composite corrugated strips should be made from a sufficient number of prepreg plies to provide corrugated strips that are strong enough to withstand the pressures to which the spar composite material and underlying mandrel are subjected to during standard compaction procedures and curing. 
     Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the within disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited by the above-described embodiments, but is only limited by the following claims.