fig1 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 fig1 , 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 fig1 , 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 . fig2 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 fig3 . 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 fig4 . 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 fig5 - 7 . as shown in fig5 , 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 fig6 . 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 fig7 a and 7b . as shown in an unrestrained state in fig7 a , 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 fig7 a , 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 fig1 . 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 fig7 b ). in this restrained condition , the mandrel 60 is substantially identical in shape to the non - segmented mandrel 40 described above . as shown in fig9 , 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 fig4 . 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 fig8 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 fig9 , 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 fig1 a - 10c . as shown in fig1 a , 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 fig1 a - 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 . fig1 a 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 . fig1 b shows the workpiece 30 during an intermediate stage of pre - form stretching . as shown in fig1 b , 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 fig1 c . 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 fig1 . 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 fig1 , the pre - formed workpiece 110 is placed over a finish - form mandrel 120 . as shown in fig1 , 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 fig1 . as shown in fig1 , 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 fig1 . 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 .