Abstract:
The disclosure is generally directed to a modified blade stiffener having enhanced mechanical properties. An illustrative embodiment of the modified blade stiffener includes a stiffener-forming material shaped into a pair of coupled blades having first and second ends, a filler cavity provided at the first end of the blades, a blade filler provided in the filler cavity and a pair of blade flanges provided at the second end of the blades.

Description:
TECHNICAL FIELD 
     The disclosure relates to blade stiffeners for aircraft structures. More particularly, the disclosure relates to a modified blade stiffener having enhanced mechanical properties and a method of fabricating a modified blade stiffener which can be fabricated as a single piece without the use of fasteners. 
     BACKGROUND 
     In the fabrication of some structures such as aircraft, for example, it may be desirable to use composite panels due to the panels&#39; high strength-to-weight ratio. Stiffening elements may be attached to the composite panels at various locations to reinforce and impart structural strength to the panels. These stiffening elements may include, for example, hat stiffeners; I-beams; blade stiffeners; honeycomb cores; and foam cores. 
     The conventional types of stiffening elements used to stiffen composite panels may have various disadvantages. For example, hat stiffeners may occupy a relatively large area on the panel and may require internal support for cure. I-beams may need to be fabricated from multiple pieces and may require complex tooling during cure to maintain their profile. Furthermore, the tooling may be difficult to remove after cure. Traditional blade stiffeners may not be as structurally efficient as other shapes. Honeycomb and foam core may be heavier and may have a tendency to be crushed during cure and to retain water in service. 
     What is needed is a modified blade stiffener having enhanced mechanical properties and a method of fabricating a modified blade stiffener which can be fabricated as a single piece without the use of fasteners. 
     SUMMARY 
     The disclosure is generally directed to a modified blade stiffener having additional reinforcement at the top of the blade to increase the mechanical performance. An illustrative embodiment of the modified blade stiffener includes a stiffener-forming material shaped into a pair of coupled blades having first and second ends, a filler cavity provided at the first end of the blades, a blade filler provided in the filler cavity and a pair of blade flanges provided at the second end of the blades. 
     The disclosure is further generally directed to a method of forming a modified blade stiffener. An illustrative embodiment of the method includes providing a blade-forming assembly having a pair of tool dies, a tool gap between the tool dies and a forming cavity in each tool die; providing a layup charge of stiffener-forming material; bridging the tool gap between the tool dies with the layup charge; placing a blade filler on the layup charge at the tool gap; pressing the blade filler through the tool gap between the tool dies until the blade filler reaches and extends into the forming cavities in the tool dies; and pressing the tool dies together to substantially eliminate the tool gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a modified blade stiffener attached to a composite panel, with cure tools engaging the modified blade stiffener. 
         FIGS. 2-7  are cross-sectional views which illustrate sequential fabrication of a modified blade stiffener according to a method of fabricating a modified blade stiffener. 
         FIG. 8  is a cross-sectional view of a modified blade stiffener fabricated according to the method shown in  FIGS. 2-7 . 
         FIG. 9  is a cross-sectional view of a composite panel, with multiple modified blade stiffeners attached to the composite panel. 
         FIG. 10  is a flow diagram which illustrates an illustrative method of fabricating a modified blade stiffener. 
         FIG. 11  is a flow diagram of an aircraft production and service methodology. 
         FIG. 12  is a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIGS. 1 ,  8  and  9 , an illustrative embodiment of the modified blade stiffener is generally indicated by reference numeral  1 . The modified blade stiffener  1  may include a formed sheet of stiffener-forming material  3  which may include a pair of generally elongated, adjacent, parallel blades  4 . The stiffener-forming material may be composite plies, prepreg material or resin-infused dry fabric preform, for example and without limitation. A pair of blade flanges  8  may extend outwardly from the respective blades  4  at a pair of respective blade/flange junctions  9 . The plane of each blade flange  8  may be disposed in generally perpendicular relationship with respect to the plane of the corresponding blade  4  from which the blade flange  8  extends. 
     A pair of blade arms  5  may extend from the respective blades  4  opposite the blade flanges  8 . The plane of each blade arm  5  may be oriented at an obtuse angle or radius of curvature with respect to the plane of the corresponding blade  4  from which the blade arm  5  extends. A pair of straight or curved arm portions  5   a  may extend from the respective blade arms  5 . A blade arm connecting portion  6  may extend between the straight or curved arm portions  5   a  of the blade arms  5 . The blade arms  5 , straight or curved arm portions  5   a  and blade arm connection portion  6  may together define a filter cavity  7  which in fabrication of the modified blade stiffener  1  may contain a blade filler  16  which may be an uncured, partially cured, or fully cured formed composite, for example. The blade filler  16  may be used to form the shape of the blade arms  5 , the straight or curved arm portions Sa and the blade arm connecting portion  6  during fabrication of the modified blade stiffener  1 , as will be hereinafter described. As shown in  FIG. 1 , a flange filler  14 , which may be an uncured formed composite, for example, may be provided between the blades  4  at each blade/flange junction  9 . 
     As shown in  FIG. 9  and will be hereinafter described, in typical application the blade flanges  8  of a series of multiple modified blade stiffeners  1  may be attached to a composite panel  2  to stiffen and reinforce the composite panel  2 . The composite panel  2  may form a part of an aircraft structure (not shown) or other structure. In  FIG. 1 , a pair of cure tools  12  is shown engaging the respective sides of the modified blade stiffener  1 . 
     Referring next to  FIGS. 2-7 , sequential steps of an illustrative method of fabricating the modified blade stiffener  1  using a blade-forming assembly  18  is shown. In  FIG. 2 , a pair of tool dies  20  of the blade-forming assembly  18  is initially positioned in spaced-apart relationship with respect to each other. The tool dies  20  may include a pair of respective initially spaced-apart and facing blade shaping surfaces  21  and a pair of blade flange shaping surfaces  22  which may extend in generally perpendicular relationship with respect to the respective blade shaping surfaces  21 . Each tool die  20  may also have a tool die base  25 . A forming cavity  25   a  may extend into the blade shaping surface  21  and tool die base  25  of each tool die  20 . The forming cavities  25   a  may have a shape which is generally complementary to that of the blade filler  16 . A surface junction  24  may be defined between each blade shaping surface  21  and the corresponding blade flange shaping surface  22 . The blade shaping surfaces  21  of the respective tool dies  20  may face each other on opposite sides of a tool gap  23  between the tool dies  20 . 
     As further shown in  FIG. 2 , a layup charge of the sheet of stiffener-forming material  3  may be initially placed on the blade flange shaping surfaces  22  of the respective tool dies  20  and extends or bridges over the tool gap  23 . A forming bar  26  may include a generally elongated forming bar shaft  27  having a generally V-shaped filler notch  28  in one end thereof. A blade filler  16  may be placed on the segment of the composite plies sheet  3  which spans or bridges the tool gap  23  between the tool dies  20 . The filler notch  28  of the forming bar  26  may engage and apply force against the blade filler  16  along the longitudinal axis of the forming bar shaft  27 . Accordingly, the blade filler  16  deforms the portion of the composite plies sheet  3  which spans the tool gap  23  as that portion of the composite plies sheet  3  may be pushed into the tool gap  23  and between the blade shaping surfaces  21  of the respective tool dies  20 . That portion of the composite plies sheet  3  which may be pushed into the tool gap  23  may eventually form the blade arm connecting portion  6  of the modified blade stiffener  1 . The portions of the composite plies sheet  3  which engage the respective blade shaping surfaces  21  at the point in the fabrication process shown in  FIG. 2  may eventually form the respective straight or curved arm portions  5   a  in the modified blade stiffener  1 . 
     As shown in  FIGS. 3-6 , the forming bar  26  may continue to push the composite plies sheet  3  through the tool gap  23  between the tool dies  20  to form the blades  4  of the modified blade stiffener  1  until the blade arm connecting portion  6  reaches the base  25  of each tool die  20 , as shown in  FIG. 6 . During extension of the composite plies sheet  3  through the tool gap  23 , the blade shaping surfaces  21  of the respective tool dies  20  may engage and shape the respective blades  4 , whereas the blade flange shaping surfaces  22  of the respective tool dies  20  may engage and shape the respective blade flanges  8 . Extension of the composite plies sheet  3  into the tool gap  23  may continue until the entire portion of the composite plies sheet  3  which will become a blade  4  of the modified blade stiffener  1  engages and extends along the blade forming surface  21  of the corresponding tool die  20 , as shown in  FIG. 6 . At that point, each blade/flange junction  9  of the modified blade stiffener  1  may be located at the surface junction  24  of each corresponding tool die  20 . 
     As shown in  FIGS. 4 and 5 , as the forming bar  26  pushes the composite plies sheet  3  through the tool gap  23  toward the tool die bases  25 , the tool dies  20  may be gradually moved toward each other. The forming bar  26  may then be disengaged from the blade filler  16  and removed from the tool gap  23 , as shown in  FIG. 5 . Eventually, the blade shaping surfaces  21  of the respective tool dies  20  may press the blades  4  of the modified blade stiffener  1  against each other, as shown in  FIG. 7 . As shown in  FIGS. 5 and 6 , during movement of the tool dies  20  toward each other, the forming cavities  25   a  of the respective tool die  20  receive the complementary-shaped blade filler  16 . Therefore, the composite plies sheet  3  may be pressed between the blade filler  16  and the interior surfaces of the forming cavities  25   a , thus forming the composite plies sheet  3  into the blade arms  5  and the straight arm portions  5   a  of the modified blade stiffener  1 . As shown in  FIG. 6 , the blade filler  16  may remain in the filler cavity  7  which may be defined by the blade arms  5 , the straight or curved arm portions  5   a  and the blade arm connecting portion  6 . As shown in  FIG. 7 , the flange filler  14  may be provided between the blades  4  at each blade/flange junction  9 . The flange filler  14  and the blade filler  16  may be located at the farthest points from the center of the modified blade stiffener  1  to maximize the stiffness and structural efficiency of the modified blade stiffener  1 . Finally, the tool dies  20  may be disengaged from the modified blade stiffener  1 . It will be appreciated by those skilled in the art that the modified blade stiffener  1  may be fabricated as a single piece with no fasteners or secondary bonding. 
     Referring next to  FIGS. 1 and 9 , in typical application multiple modified blade stiffeners  1  may be bonded or co-cured to the composite panel  2  in a selected pattern, spacing and density to customize local stiffness of the composite panel  2 . Accordingly, the blade flanges  8  of each modified blade stiffener  1  may be attached to the surface of the composite panel  2  according to the knowledge of those skilled in the art. As shown in  FIG. 1 , cure tools  12  may be placed in proximity to respective sides of the modified blade stiffener  1  for curing of the composite plies sheet  3 . 
     It will be appreciated by those skilled in the art that the shape of the modified blade stiffener  1  allows the use of cure tools  12  ( FIG. 1 ) that automatically center on the modified blade stiffener  1  and can be quickly and easily removed after cure without the use of special equipment. Due to its small footprint on the composite panel  2  and the compact nature of the cure tool  12  ( FIG. 1 ), the modified blade stiffeners  1  can be placed more closely together on the composite panel  2  than is the case with typical hat-shaped stiffeners to allow specific tailoring of the performance of the overall structure. The compact or low-profile nature of the modified blade stiffener  1  may also allow it to be used in combination with other types of stiffeners such as hat-type stiffeners (not shown) to augment panel stiffness locally where there may be insufficient space to add additional hat-type stiffeners. Furthermore, the modified blade stiffener  1  may not require internal tooling such as the bladders which may be necessary for the fabrication of hat-type stiffeners; therefore, there may be no treatments or tools that must be prepared before use or extracted from the modified blade stiffener  1  after cure. The modified blade stiffeners  1  may be more structurally efficient than standard blade stiffeners and may be located more densely than hat-type stiffeners and may be easier to customize for structural requirements. The blade forming assembly  18  may fabricate finished modified blade stiffeners  1  that can be installed directly on the composite panel  2  without the need for additional assembly steps. In some applications, the bulbous end of the modified blade stiffener  1 , defined by the blade arms  5 ; the straight or curved arm portions  5   a ; and the blade arm connecting portion  6 , may be inserted in a receiving slot (not shown) provided in a frame (not shown) to reduce or eliminate the number of fasteners needed to couple a stringer (not shown) to the frame. 
     Referring next to  FIG. 10 , a flow diagram  1000  which illustrates an illustrative method of fabricating a modified blade stiffener is shown. In block  1002 , a blade-forming assembly having tool dies with a tool gap between the tool dies and a forming cavity in each tool die may be provided. In block  1004 , a layup charge of a stiffener-forming material may be provided. In block  1006 , the tool gap between the tool dies may be bridged with the layup charge. In block  1008 , a blade filler may be placed on the layup charge above the tool gap. In block  1010 , the blade filler may be pressed, using a forming bar, through the tool gap between the tool dies until the blade filler reaches and extends into the forming cavities in the tool dies. The forming bar is then removed in block  1011 . In block  1012 , the tool dies may be pressed together to substantially eliminate the tool gap. In the final block  1013 , the tool dies are removed. 
     Referring next to  FIGS. 11 and 12 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  78  as shown in  FIG. 11  and an aircraft  94  as shown in  FIG. 12 . During pre-production, exemplary method  78  may include specification and design  80  of the aircraft  94  and material procurement  82 . During production, component and subassembly manufacturing  84  and system integration  86  of the aircraft  94  takes place. Thereafter, the aircraft  94  may go through certification and delivery  88  in order to be placed in service  90 . While in service by a customer, the aircraft  94  may be scheduled for routine maintenance and service  92  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  78  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 12 , the aircraft  94  produced by exemplary method  78  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  78 . For example, components or subassemblies corresponding to production process  84  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  94  is in service. Also, one or more apparatus embodiments may be utilized during the production stages  84  and  86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  94  is in service, for example and without limitation, to maintenance and service  92 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.