Abstract:
A method of forming an aircraft nacelle nose lip segment. The method includes bending a sheet of metal into a substantially U-shaped workpiece having a spanwise axis, opposed first and second ends, and opposed first and second edges. The method further includes placing the workpiece over a substantially flexible first mandrel, stretching the workpiece in a spanwise direction between the first and second ends, and wrapping the workpiece and first mandrel together about a curved die while stretching the workpiece. The workpiece is thereby plastically deformed to have a first shape. The method may further include removing the workpiece from the first mandrel, and placing the workpiece over a substantially rigid second mandrel that substantially corresponds in shape to the first shape of the workpiece. The workpiece is stretched over the second mandrel in a chordwise direction that is substantially transverse to the spanwise axis of the workpiece, thereby further plastically deforming the workpiece.

Description:
FIELD OF THE INVENTION 
   The invention relates to methods of producing sheet metal skins having compound curvilinear shapes and large depth-to-diameter ratios, and more particularly relates to a method of stretch forming a segment of an aircraft engine nacelle inlet nose lip. 
   BACKGROUND 
   Aircraft engine nacelles provide streamlined enclosures for aircraft engines. The nacelles typically include an underlying support structure covered by a thin, aerodynamically shaped metal skin. The portion of the nacelle that surrounds an engine&#39;s inlet commonly is referred to as the nacelle inlet nose lip, or simply the noselip. The noselip has a complex shape with compound curvatures. First, the noselip has a chordwise curvature that curves from forward portions of the noselip toward aft portions of the noselip, thereby forming an aerodynamic shape. In addition, the noselip has a spanwise curvature that curves in a circumferential direction around the inlet. The noselip has a relatively large depth-to-diameter ratio. For example, the noselip may have a depth-to-diameter ratio of between about 1.0 and about 5.0. The compound curved shape of the noselip, the noselip&#39;s large depth-to-diameter ratio, and the large overall diameter of a noselip for high bypass ratio aircraft engines (up to 10 feet in diameter) can make the noselip particularly difficult to manufacture. Noselips commonly are produced in multiple arcuate segments to facilitate their manufacture and maintainability. The arcuate segments are assembled together in a conventional manner known to those skilled in the art to form a complete noselip. 
   Draw forming is one traditional method used to produce a sheet metal skin segment having a complex, multi-curved shape, and a large depth-to-diameter ratio. The draw forming process plastically deforms a sheet of metal by fixing the edges of the metal, and plunging a specially constructed die or punch into the sheet. The die has a shape corresponding to the desired shape of the formed metal. Optionally, the sheet of metal may be preheated before forming. The deep drawing process often requires multiple drawing cycles to produce a finally formed part. Unfortunately, the draw forming process is complex and time consuming. In addition, the draw dies used in the draw forming process experience substantial wear, and require periodic refurbishment or replacement. Furthermore, the tooling and equipment required to draw form a nacelle noselip, for example, can be expensive to purchase and costly to maintain. 
   Another common method of forming a complex skin segment having a large depth-to-diameter ratio is spin forming. Spin forming involves spinning a thin-walled workpiece on a rotating mandrel while heating and deforming the workpiece. Spin forming permits formation of a complete nacelle noselip in a single piece. The spin formed workpiece can be finally shaped during spin forming, or can be preformed by spin forming and finally shaped on a drop hammer die or the like. Unfortunately, the equipment and tooling required to spin form a part as large as a nacelle noselip can be expensive to purchase, and costly to maintain. 
   Thus, there is a need for an alternative, less costly, and less time-consuming process for producing metal skins having complex shapes and large depth-to-diameter ratios, such as nacelle inlet noselips. 
   SUMMARY OF THE INVENTION 
   The invention includes a stretch-forming process for producing a thin metal skin having multiple axes of curvature. The method includes forming a sheet of metal into a curved channel having a longitudinal first axis. The method further includes plastically stretching the channel in a longitudinal direction while substantially simultaneously bending the channel about a second axis. The method can further include plastically stretching the channel in a direction that is substantially transverse to the longitudinal axis. 
   The invention also includes a method of forming a sheet metal skin having compound curvatures. The method includes bending a sheet of metal about a first mandrel having a longitudinal axis to form a channel. The method further includes plastically stretching the channel in a longitudinal direction while substantially simultaneously bending the channel and first mandrel about a curved second mandrel, wherein the second mandrel has an axis of curvature that is non-parallel to the longitudinal axis of the first mandrel. 
   The invention further includes a method of forming an aircraft nacelle nose lip segment. The method includes bending a sheet of metal into a substantially U-shaped workpiece having a longitudinal axis, opposed first and second ends, and opposed first and second edges. The method also includes placing the workpiece over a substantially flexible first mandrel, longitudinally stretching the workpiece between the first and second ends, and wrapping the workpiece and first mandrel together about a curved die while longitudinally stretching the workpiece, whereby the workpiece is plastically deformed to have a first shape. The method further includes removing the workpiece from the first mandrel, placing the workpiece over a substantially rigid second mandrel that substantially corresponds in shape to the first shape of the workpiece, and stretching the workpiece over the second mandrel between the first and second edges in a direction that is substantially transverse to the longitudinal axis of the workpiece. Accordingly, the workpiece is further plastically deformed to have a second shape. 
   These and other aspects of the invention will be understood from a reading of the following detailed description together with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a nacelle inlet noselip segment produced by a method according to the invention;  FIG. 2  is a perspective view of a substantially flat sheet of metal used to form the noselip of  FIG. 1 ; 
       FIG. 3  is a substantially U-shaped workpiece formed from the substantially flat sheet of metal shown in  FIG. 2 ; 
       FIG. 4  is a perspective view of the U-shaped workpiece of  FIG. 3  positioned on the flexible pre-form mandrel shown in  FIG. 5 ,  FIG. 6 , or  FIGS. 7A and 7B ; 
       FIG. 5  is a perspective view of a one-piece flexible pre-form mandrel for use in pre-forming the workpiece shown in  FIG. 3 ; 
       FIG. 6  is a perspective view of segmented flexible pre-form mandrel for use in preforming the workpiece shown in  FIG. 3 ; 
       FIG. 7A  is a perspective view of a curved one-piece flexible pre-form mandrel in an unrestrained state for use in pre-forming the workpiece shown in  FIG. 3 ; 
       FIG. 7B  is a perspective view of the flexible perform mandrel of  FIG. 7A  in a restrained, non-curved state; 
       FIG. 8  is a perspective view of an end-gripping jaw for gripping and longitudinally stretching the U-shaped workpiece on the flexible pre-form mandrel shown in  FIG. 4 . 
       FIG. 9  is a perspective view similar to that of  FIG. 4 , and showing each end of the U-shaped workpiece crimped to form opposed gripping portions; 
       FIG. 10A  is a plan view showing an arrangement for initial stretch forming of the U-shaped workpiece on the flexible pre-form mandrel; 
       FIG. 10B  is a plan view showing the U-shaped workpiece being partially stretched on the pre-form mandrel and partially wrapped around the curved die; 
       FIG. 10C  is a plan view showing the U-shaped workpiece being finally stretched on the pre-form mandrel and finally wrapped around the curved die; 
       FIG. 11  is a perspective view showing the workpiece after the gripping portions have been trimed from its ends; 
       FIG. 12  is a perspective view showing a finish-form mandrel for use in finally stretch forming the workpiece; 
       FIG. 13  is a perspective view showing the workpiece positioned on the finish-form mandrel of  FIG. 12 , and showing the workpiece being stretched in a chordwise direction over the finish-form mandrel. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a nacelle inlet noselip segment  10  produced by a method according to the invention. The noselip segment  10  forms a portion of a complete noselip  200  indicated in dashed lines. As shown in  FIG. 1 , the noselip  200  and noselip segment  10  includes a spanwise axis  14  about which the noselip curves in a chordwise direction. In addition, the noselip  200  and noselip segment  10  includes a chordwise central axis  16 , about which the noselip curves in a spanwise direction. As used herein, a “chordwise axis” extends between a forward (or leading edge) position and an aft (or trailing edge) position, or extends substantially parallel to a forward-aft direction. In addition, as used herein, a “spanwise axis” extends in a direction that is substantially perpendicular to a chordwise axis, and extends along or parallel to the span of an elongated structure, or along or parallel to the circumference of a circular or semi-circular structure. In addition, as used herein, “chordwise” describes a direction or orientation that is substantially parallel to a chordwise axis, and “spanwise” describes a direction or orientation that is substantially parallel to a spanwise axis. In  FIG. 1 , the chordwise axis  16  substantially coincides with a central longitudinal axis of an associated aircraft engine, and the center of the engine&#39;s inlet. 
     FIG. 2  shows a substantially flat, thin-gauge metal sheet  20  from which the noselip  10  can be formed according to the invention. In one embodiment, the sheet metal  20  is bare aluminum 2219 sheet having an initial nominal thickness from about 0.080 inch to about 0.125 inch. Other types, grades, and thickness of substantially ductile sheet metal also may be used. For example, a noselip  10  can be formed by a process according to the invention from a substantially ductile metal sheet of aerospace grade aluminum or titanium alloy having a nominal thickness between about 0.008 inch and about 0.250 inch. 
   In a process according to the invention, the metal sheet  20  can be plastically bent into a substantially U-shaped channel or workpiece  30  as shown in  FIG. 3 . The U-shaped workpiece  30  has a spanwise or longitudinal axis  32 , opposed ends  34 ,  36 , and opposed edges  38 ,  39 . The metal sheet  20  can be bent to form the U-shaped workpiece  30  by any suitable or desired bending process. 
   The U-shaped workpiece  30  is placed over a flexible pre-form mandrel  40 ,  50 ,  60  as shown in  FIG. 4 . As used herein, the terms “flexible” and “bendable” are used interchangeably to mean being capable of flexing or bending in at least one direction without substantial permanent deformation or breakage. Various embodiments  40 ,  50 ,  60  of the flexible pre-form mandrel are shown in  FIGS. 5-7 . As shown in  FIG. 5 , a first embodiment of the flexible pre-form mandrel  40  is an elongated member having a curved upper surface  42  and substantially flat ends  44 ,  46 . The curved upper surface  42  curves about a spanwise or longitudinal axis  48 . The curvature of the upper surface substantially corresponds to the desired chordwise curvature of a finally formed nacelle noselip  10 . The pre-form mandrel  40  preferably is constructed of a flexible and substantially incompressible material. As used herein, the term “incompressible” is used to refer to a material that substantially maintains its original thickness when subjected to compressive forces experienced during the stretch forming process described herein. In a preferred embodiment, the pre-form mandrel is constructed of a polymeric material, such as polyurethane, having sufficient hardness to be substantially incompressible, and being sufficiently ductile to permit sufficient flexing and bending during the stretch forming process described herein. In a preferred embodiment, the pre-form mandrel is constructed of polyurethane having a Shore A hardness of about 65. 
   A second embodiment  50  of a pre-form mandrel for use in a process according to the invention is shown in  FIG. 6 . In this embodiment, the pre-form mandrel  50  includes a plurality of articulating segments  52 . The segments  52  can be flexibly interconnected by any suitable connection means. For example, the segments  52  can be interconnected by one or more wire cables  54 , links, hooks, hinges, or the like. When interconnected, the segments  52  are capable of at least partially rotating relative to each other. Accordingly, the mandrel  50  is capable of being articulated into a bent shape. Like mandrel  40  described above, the articulated mandrel  50  has a spanwise or longitudinal axis  59 , and a curved upper surface  58  that substantially corresponds to a desired chordwise curvature of a finally formed nacelle noselip  10 . The segments  52  may be constructed of any suitable substantially incompressible material. For example, the segments  52  may be constructed of polyurethane or another suitable plastic material, metal, wood, concrete, or the like. 
   A third embodiment of a pre-form mandrel for use in a process according to the invention is shown in  FIGS. 7A and 7B . As shown in an unrestrained state in  FIG. 7A , the pre-form mandrel  60  is similar to the non-segmented mandrel  40  described above, but has a spanwise curvature around a chordwise axis  62 . In the unrestrained state shown in  FIG. 7A , the upper surface  64  of the pre-form mandrel  60  substantially corresponds in shape to a finally formed nacelle noselip  10 , like that shown in  FIG. 1 . The mandrel  60  is constructed of a flexible and substantially incompressible material such as polyurethane. The flexible material permits the mandrel  60  to be restrained in a straightened condition (like that shown in  FIG. 7B ). In this restrained condition, the mandrel  60  is substantially identical in shape to the non-segmented mandrel  40  described above. 
   As shown in  FIG. 9 , in a preferred embodiment of a process according to the invention, the ends  34 ,  36  of the workpiece  30  are crimped to form substantially flat gripping portions  90 ,  92 . The gripping portions  90 ,  92  facilitate gripping the ends  34 ,  36  of the workpiece  30  during the pre-form stretching of the workpiece  30  described in detail below. Spacer blocks may be placed near the ends of the U-shaped workpiece  30  as the ends  34 ,  36  are crimped to maintain the general shape of the workpiece  30  adjacent to the gripping portions  90 ,  92  (not shown). Alternatively, the ends  34 ,  36  can be left uncrimped as shown in  FIG. 4 . 
   In an alternative embodiment, the ends  34 ,  36  of the workpiece  30  are left uncrimped. In this embodiment, gripping fixtures or jaws  80  like that shown in  FIG. 8  may be used to grip the U-shaped ends  34 ,  36  of the workpiece  30  during the pre-form stretching of the workpiece  30  that is described in detail below. Each jaw  80  includes a plurality of pairs of blocks  84  arranged in a generally U-shaped pattern on a base  82 . Each pair of blocks  84  is configured to receive a portion of an end  34 ,  36  of the workpiece  30  between the pair of blocks  84 . Each pair of blocks  84  is compressed together using threaded fasteners  86  or the like to grippingly engage a corresponding portion of an end  34 ,  36  of the workpiece  30 . The opposite side of the base  82  of each jaw  80  is provided with one or more suitable attachment elements for connection to a stretch-forming device (not shown). 
   As shown in  FIG. 9 , the workpiece  30  is placed over the flexible pre-form mandrel  40 ,  50 , or  60 . One or more anchor straps  94  or similar restraining devices may be used to maintain contact between the work-piece  30  and mandrel  40 ,  50 , or  60  during pre-form stretching. 
   One embodiment of a pre-form stretching portion of a process according to the invention is shown in  FIGS. 10A-10C . As shown in  FIG. 10A , a curved die  104  is positioned adjacent to an inside surface of the workpiece  30 . The curved die  104  has a curved surface  106  that is substantially centered along an inside surface of the workpiece  30 . The curved die  104  may be constructed of any suitable material. For example, the curved portion of the die  104  may be constructed of polyurethane or another suitable plastic material, metal, wood, concrete, or the like. In the embodiment shown in  FIGS. 10A-10C , the workpiece  3  has crimped gripping portions  90 ,  92  as described above. Opposed articulating jaws  100 ,  102  tightly grip the gripping portions  90 ,  92 . The articulating jaws  100 ,  102  are configured to withstand a tensile force “P” in a direction that is substantially coincident with the spanwise axis  14  of the workpiece  30  as the workpiece is stretch formed. The jaws  100 ,  102  preferably are connected to articulating hydraulic cylinders (not shown) as are common in known skin press machines. The hydraulic cylinders permit monitoring of the tensile force P during pre-form stretching by measurement of the cylinder pressures. 
     FIG. 10A  shows the workpiece  30  in an initial position prior to pre-form stretching. In this beginning position, an initial pre-tension P O  is applied to the workpiece  30  by articulating jaws  100 ,  102 .  FIG. 10B  shows the workpiece  30  during an intermediate stage of pre-form stretching. As shown in  FIG. 10B , the curved die  104  is advanced in a direction “T” against the inside surface of the workpiece  30  and the enclosed pre-form mandrel  40 ,  50 , or  60 . As the curved die  104  presses against the inside surface of the workpiece  30 , the central portions of the workpiece  30  and pre-form mandrel  40 ,  50 ,  60  are displaced, and the workpiece  30  and mandrel  40 ,  50 ,  60  begin to conform to the curvature of the die  104 . In addition, the workpiece  30  is stretched in a spanwise direction between the articulating jaws  100 ,  102 . The process is continued until the workpiece is substantially fully stretched around the curved surface  106  of the die  104 , and/or desired spanwise tensile forces P f  are measured at the jaws  100 ,  102 , as indicated in  FIG. 10C . In one embodiment of the process, the spanwise tensile forces P f  are about 30 tons at each end of the workpiece  30  when the workpiece is bare aluminum 2219 sheet having an initial nominal thickness from about 0.080 inch to about 0.125 inch. Under such conditions, the workpiece  30  undergoes substantial plastic strains in a direction parallel to its spanwise axis  14 . For example, the material may undergo plastic strains between about 6 percent and about 16 percent. Accordingly, when the curved die  104  is withdrawn from the workpiece  30 , the workpiece  30  substantially maintains the spanwise curvature imparted by the die  104 . 
   The workpiece  30  is removed from the flexible mandrel  40 ,  50 ,  60 , and the gripping portions  90 ,  92  are removed to form a pre-formed workpiece  110 , as shown in  FIG. 11 . Preferably, the workpiece  30  is thermally treated before final stretch forming (described below) to at least partially relieve stresses within the skin and to stabilize the stretch-formed shape of work-piece  30 . For example, when the workpiece is fabricated from bare aluminum 2219 sheet having an initial nominal thickness from about 0.080 inch to about 0.125 inch, the workpiece maybe heat treated at about 995 degrees F. for about 40 minutes. 
   As shown in  FIG. 13 , the pre-formed workpiece  110  is placed over a finish-form mandrel  120 . As shown in  FIG. 12 , the finish-form mandrel  120  may include a forming portion  124 , a frame  122 , and a base  128 . The forming portion  124  includes an upper surface  126  that substantially corresponds in shape to a completed nacelle inlet noselip  10  like that shown in  FIG. 1 . As shown in  FIG. 13 , the edges  38 ,  39  of workpiece  110  are grippingly engaged by gripping jaws  130 . The gripping jaws  130  include a plurality of vice-like blocks that tightly grip the edges  38 ,  39  of workpiece  110 , and are fixed to a stationary foundation or structure. The final form mandrel  120  is advanced in direction “A” against the resistance of the gripping jaws  130  (indicated by downwardly extending arrows), thereby stretching the workpiece  110  in a chordwise direction over the mandrel  120 . The process is continued until a sufficient degree of chordwise plastic strain is induced in the workpiece  110 . For example, the skin of workpiece  110  may be stretched to produce plastic strains ranging from about 6 percent to about 16 percent in bare aluminum 2219 sheet having an initial nominal thickness from about 0.080 inch to about 0.125 inch. 
   The stretch forming operations described above may be performed on a conventional skin press machine. For example, the stretch forming operations may be performed on a numerically controlled sheet stretch form press, such as a Sheridan Model No. LV-300-72-22 150-ton sheet stretch press. Of course, other types of skin press or stretch forming devices, or other specially designed equipment also may be used in a process according to the invention. 
   After final stretch forming is completed, the jaws  130  are disengaged from the workpiece  110 , and the workpiece  110  is removed from the final-form mandrel  120 . Excess material is trimmed from the workpiece to a form a complete nacelle inlet noselip segment like that shown in  FIG. 1 . If necessary, the workpiece  110  may be hand worked or otherwise further shaped to have the desired contours of the finished noselip segment  10 . The workpiece  110  may be age hardened to yield desired material properties. For example, a workpiece constructed of bare aluminum 2219 sheet having an initial nominal thickness from about 0.080 inch to about 0.125 inch may be age hardened at about 360 degrees F. for about 36 hours. 
   The above descriptions of various embodiments of the invention are intended to describe and illustrate various aspects of the invention. Persons of ordinary skill in the art will recognize that various changes or modifications may be made to the described embodiments without departing from the scope of the invention. For example, though the processes described above primarily have been described regarding production of a nacelle inlet noselip for an aircraft engine, persons of ordinary skill in the art will recognize that the described methods also can be used to produce other complex curved skin structures having large depth-to-diameter ratios. In addition, whereas the stretch-forming operations are described herein as including substantially stationary gripping jaws and movable forming fixtures, the stretch forming operations may be performed equally well using stationary fixtures and movable gripping jaws. All such changes and modifications are intended to be within the scope of the appended claims.