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Shortening the Span |
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Semi-span wing status. Lower cover panel with stringer attached. |
CAD
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Computer software that makes a direct link from three-dimensional CAD geometry to automated ply cutters and laser projection systems provided about a 50-percent time savings in the manufacture of a new composite wing design at the Boeing Company, Huntington Beach, CA. FiberSIM simulation software from Composite Design Technologies of Waltham, MA, allowed designers to define composite layup on the computer, eliminating the need for trial and error on the shop floor that normally takes up the bulk of development-cycle time. The software was part of a new composite manufacturing process based on stitched resin film infusion (RFI) that prevents delamination, allowing fabrication of full-span composite wings. Stitched RFI technology is expected to reduce wing weight by 25 percent and cost by 20 percent within 3 years. Laminated composites have emerged in recent years as a high-performance, low-weight, cost-effective replacement for metal in many aerospace applications. Yet up to now the commercial aircraft industry has made only minimal use of them to manufacture wing and aircraft structures. The primary concern has been the risk of damage from delamination caused by manufacturing-induced defects or by impact with runway debris, hailstones, or birds. Another concern is the difficulty in manufacturing the very large composite structures required for aircraft wings at a cost competitive with state-of-the-art aluminum wings. In the 1980s, researchers began investigating new manufacturing techniques, including knitting, weaving, and braiding, modeled after existing textile manufacturing technology. Stitching combined with RFI showed the greatest potential for overcoming cost and damage-tolerance barriers to wing structures. Assembling carbon fabric preforms with closely spaced through-the-thickness stitching provided essential reinforcement to prevent delamination. Also, stitching made it possible to incorporate the various elements -- wing skin, stiffeners, ribs, and spars -- into an integral structure that can eliminate thousands of metal fasteners. An Advanced Stitching Machine NASA awarded Boeing the contract to develop a machine capable of stitching contoured aircraft-wing surfaces at a very high speed. Ingersoll Milling Machine Co. of Rockford, IL, was selected to design and build the advanced stitching machine (ASM), which is capable of stitching one-piece wing-cover panels 40 feet long and 8 feet wide at a rate of 3200 stitches per minute. The ASM combines high-speed stitching with advanced automation, allowing it to stitch large, thick, complex wing structures with minimal manual intervention. As this machine was being developed, refinements were also made to the entire composite fabrication process. The traditional manual approach involves cutting raw fabric into plies using hand templates. The initial plies would be located and aligned on the stitching machine surface with the assistance of scribe lines, templates, and measurements. But many of the plies for a commercial aircraft are 40 feet or longer, making them cumbersome to handle using the conventional approach. So engineers developed an alternative that uses the latest CAD/CAM technology based on FiberSIM software, as well as automated material cutting and laser projection techniques. The new process begins with the definition of the part geometry, including the mold surface, ply boundaries, holes, splices, and helpful markers using the Unigraphics CAD/CAM system from EDS Unigraphics, Maryland Heights, MO. Boeing engineers then use simulation with FiberSIM to test the producibility of ply shapes and orientations. The software is completely integrated with Unigraphics (as well as with CATIA and Pro/ENGINEER) so that its commands appear within standard CAD menus. Engineers run the FiberSIM flat-pattern function to generate net flat patterns for the complexly curved multisurface plies that take the thickness of laminate components into account. The software automatically models the deformation mechanism that woven and unidirectional materials undergo during lay-up, changing their surface area. This process provides significant time savings and also greatly reduces the need for manual trimming of the resulting patterns. Once they have created satisfactory flat patterns, Boeing engineers use FiberSIM to export the CAD data to a numerically controlled ply cutter from GGT Cutting Edge, Marblehead, MA. A commercial supplier delivers multiaxial warp-knit fabrics stacked as specified by Boeing, and then the stacks are cut by the automated cutter into the preforms that generate the shape of the wing. Since FiberSIM produces net flat patterns with limited excess and the ply-cutting nesting software optimizes the position of the plies on the bed, material waste is reduced by 25 percent compared to the older process. Automated cutting also provides a cleaner cut and more precise ply shape while eliminating the need to make the templates required when cutting by hand, resulting in additional cost savings. The next step is to lay up the cut plies on the stitching machine bed using a laser projection system (LPS). Since the FiberSIM software creates both the shapes to be cut and the ply boundaries projected by the LPS, two-dimensional patterns can be cut with the assurance of three-dimensional tool conformity. Consequently, Boeing engineers used FiberSIM software to produce three-dimensional ply boundary, hole, splice, and marker data for the LPS, made by General Scanning, Watertown, MA. Data generated by FiberSIM was read by the LPS, and the laser heads projected ply data on the stitching machine bed in the exact location where the material should be placed. Lay-up technicians then placed the precut plies between the continuous laser lines in a predetermined, preprogrammed sequence. When generating laser projection data, FiberSIM automatically accounted for material thickness and offset due to ply buildup. This eliminated parallax errors due to accumulation of ply offset from the tool surface that could arise when manually programming the LPS. Preprogramming the lay-up sequence with FiberSIM cut lay-up time by about half. An additional benefit was the ability to use the laser projection system to check the lay-up for accuracy. |
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A Matter of Control Since FiberSIM generated data for both the LPS and the cutting system directly from the CAD model, engineers were able to close the loop between the lay-up station and the 3D CAD model. Any discrepancies between the CAD model and manufacturing data were easily detected, since net plies were expected to fit precisely within the projected laser lines. This gave the engineering staff complete control over ply layouts. Furthermore, the direct link that FiberSIM provided from the CAD model to the manufacturing equipment drastically reduced the amount of time required on the shop floor to prepare plies for cutting and also insured that manufacturing would build exactly what the designer had designed into the 3D CAD model. |
FiberSIM-generated fiber orientations |
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The accuracy and seamless nature of this link was assured by the fact that Composite Design Technologies maintained close contact with equipment manufacturers to insure that FiberSIM output was compatible with composite manufacturing equipment. Once the fabric pieces were arranged in the proper position, the ASM stitched the stacks to make a solid wing preform. The stiffeners and rib clips for wing covers were made using a braiding process that made it easier for them to conform to the contours of the wing. Braided tubes were collapsed and stitched to make blade-shape stiffeners and rib clips. In a final step, the ASM stitched the stiffening elements to the skin preform. The result was an integral wing-cover preform, shaped to the wing contours, ready for the RFI process. The still-flexible wing-skin panel was put into an outer mold line tool that was the shape of the outside surface of the wing. A film of resin was laid on the form, followed by the stitched skin preform and the tools that would define the inner mold line. These elements were put into a plastic bag from which the air was drawn out to create a vacuum. The materials were then placed in an autoclave, where heat and pressure were applied to let the resin spread throughout the carbon fiber material. After heating to 175 degrees C for two hours, the wing-skin panel took on its final hardened shape. Time and Cost Savings The new RFI process eliminates the cost of conventional prepregging and its time-consuming setup. Stitching materials requires less manual labor than drilling holes and assembling the 80,000 metal fasteners used in an aluminum wing. Wing cover panels can be stitched in one two-shift operation, compared to the several days required for conventional composite fabrication processes. Removal of this excess metal also decreases the weight of the wing and eliminates the problems of fatigue and corrosion of metal fasteners. Panels now being stitched will be used as test articles in full-scale ground testing next year to assure that the stitched structure meets Federal Aviation Administration standards. Ultimately, engineers expect this new technology will find wide usage early in the next decade, helping to meet NASA's goal of reducing the costs of air travel by 25 percent within ten years. This application provides a dramatic example of how CAD/CAM technology can help to develop innovative new composite manufacturing processes at a competitive cost and a reasonable cycle time. By using software tools such as FiberSIM that close the loop between design and manufacturing, Boeing was able to nearly eliminate trial and error on the shop floor while insuring that the finished product perfectly matched the design intent. Boeing is using FiberSIM technology to create nose fairing for the Delta III rocket and in other programs such as the F-22, V-22, F-18, Apache helicopter, and the 737, 767, and 777 aircraft. This article was prepared by Mike Karal, deputy project engineer,
and Patrick Thrash, technical specialist, at the Boeing Company, Huntington Beach, CA. For more information,
contact Composite Design
Technologies Inc., 235 Wyman St., Suite 110, Waltham, MA 02154;
(781) 290-0506, ext. 223; fax: (781) 290-0507.
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