Composites

Background

Weight reduction has been a critical goal since the earliest days of crewed flight. Following initial applications of wood, fabric, and wire for structural components, the aircraft industry made a major transition to aluminum and all-metal aircraft. As a result of this approach to structural design, modern civil aircraft are designed with greatly reduced aircraft operating empty weight to achieve a significant payload to weight fraction that contributes directly to aircraft flight efficiency. In the transition to aluminum components, the industry accepted the significant costs that were required to retool and modify its manufacturing processes.

In the continual quest for reduced weight, aircraft manufacturers began to introduce applications of nonmetallic materials, such as fiberglass-reinforced plastic composites. For example, initial applications of structural fiberglass parts by Boeing on commercial transports started with about 200 square feet on the Boeing 707 for the radome and small closure fairings. By the time the Boeing 747 was introduced, the application of fiberglass parts had grown to over 10,000 square feet, including the radome, wing leading- and trailing-edge panels, flaps, fairings, and control surfaces. Beginning in about 1962, composite sandwich parts made from fiberglass-epoxy materials were applied to aircraft such as the Boeing 727. Major operational issues for composite structures, such as lightning protection, were satisfied by the bonding of aluminum foil on the inner surfaces and aluminum flame spray on the outer surfaces of structural parts. The construction technique used for composites at that time consisted of tailoring the glass fabric to the required shape, pouring liquid resin onto the fabric, spreading and sweeping the resin to impregnate the fabric, vacuum bagging the part and tool, and curing in an oven or autoclave. This wet layup method was very labor intensive.

The next major advance in composites was a transition to graphite composite secondary aircraft structures, such as wing control surfaces, wing trailing and leading edges, vertical fin and stabilizer control surfaces, and landing gear doors. The obvious benefits of lightweight, strong composites have historically been tempered by issues regarding fabrication costs, potential degradation in characteristics due to environmental effects, impact damage resistance and repairability, and potential environmental effects of composites following aircraft accidents.

The transition from manufacturing aluminum aircraft components to composite structures involved the fabrication of filaments of graphite, fiberglass, or DuPont Kevlar material arranged in a matrix of epoxy, polyimide, or aluminum. The filament materials are imbedded in a matrix at specified angles in successive layers, and they can develop very high strength and stiffness. Potential weight savings come about because of the high strength-to-weight and stiffness-to-weight properties of the composite material. Cost reductions come about from the fewer number of pieces that make up the components and from the fewer number of fasteners required for assembly. The fabrication of composites was initially accomplished with hand layups similar to those used in the fiberglass construction of boats or automobiles. Currently, advanced fabrication techniques, including tape placement and stitching technology, are being applied by industry.

Research contributions of the Langley Research Center have played a key role in the widespread acceptance and application of emerging composite technology for both civil and military aircraft. Langley is the Agency’s Center of Excellence for Structures and Materials in recognition of its long history of research into innovative composites, polymers, metallics, and structures for aircraft and spacecraft. By conducting fundamental and applied research with its industry partners, Langley has accelerated the use of composites and the confidence in the safety and economic feasibility of such applications.

Langley Research and Development Activities

The Langley Research Center has been conducting composites research with industry since 1970. The initial impetus for more aggressive focused research on composite structures came in 1972 during meetings of industry, universities, and government representatives involved in a project known as RECAST. One of the highlights of the study was the recognition that a major obstacle to large-scale applications of composites technology was the high initial costs of introducing the new materials because there was no large volume of production due to limited applications; no widespread applications were being used because the total cost was too high. To break this cycle, the RECAST participants suggested three approaches. First, components of advanced composites should be fabricated and tested under realistic service conditions; second, the application of composites to new designs should be encouraged; and third, in-depth studies should be undertaken to develop the technology and provide databases for designers. In response to these recommendations, NASA included composites in its Advanced Transport Technology Program as well as other research involving spacecraft, engines, and basic research. Early leaders of the composite research at Langley included Richard R. Heldenfels, Roger A. Anderson, William A. Brooks, Jr., George W. Brooks, Robert Leonard, Richard A. Pride, and Eldon E. Mathauser. Key researchers included Marvin B. Dow, H. Benson Dexter, Michael F. Card, John G. Davis, Jr., and Martin M. Mikulas, Jr.

One of the first efforts in the Langley composites research program was to reduce potential risk and build industry confidence through a series of contracts for the development, fabrication, and testing of aircraft secondary structures. Secondary aircraft components are relatively lightly loaded, and not critical to the safety of flight. Langley started the NASA Composites Flight Service Program in 1972 and installed over 300 experimental composite components on commercial transports and rotorcraft. The research was focused on assessing the potential environmental impacts on composite characteristics during typical flight service operations. Working under Langley contracts, Boeing designed and fabricated graphite-epoxy wing spoilers on twenty-seven Boeing 737 aircraft, Lockheed applied seven Kevlar fuselage fairing panels on three L-1011 aircraft, and Douglas applied graphite-epoxy material for the upper, aft rudder segment on ten DC-10 aircraft. Douglas also used a boron-aluminum composite skin for a panel on the aft pylon adjacent to the engine on three DC-10s. In addition to these civil applications, Langley also sponsored a boron-epoxy composite reinforcement for the aft aluminum tailcone of an Army CH-54B helicopter, and a boron-epoxy reinforcement for the aluminum center wing box of two Air Force C-130 transports. By the end of 1977, these flight experiments had involved considerable flight time and yielded valuable experience in composites technology and design. For example, the graphite-epoxy of the 108 spoilers on the 737 aircraft had accumulated over 921,000 flight hours in 4.5 years. Overall, the 142 composite components accumulated in excess of 1 million flight hours around the world with 17 operators. No significant incidents occurred nor damage detected in any of the flight components. Maintenance was reported as less than that required on similar standard aluminum parts. Several instances occurred in which the graphite-epoxy spoilers received sufficient damage in service to require repairs. The repairs were made by removing the spoilers on the aircraft, cutting out the damaged area, replacing honeycomb core as needed, and replacing graphite-epoxy plies. The repair process was straightforward and easily accomplished. The Langley program tracked the performance of these and the other composite parts in its Flight Service Program for over 15 years, and some of the parts were still in operational service at the end of the 1990s. By 1991, over 5.3 million flight hours had been achieved on 350 composite components.

When the NASA Aircraft Energy Efficiency (ACEE) Program began in fiscal year 1976, the Langley Research Center focused its composites research for aircraft into two main areas: supporting base technology efforts and the ACEE Composites Program.

Environmental Exposure Effects

In early technology studies within the Base Technology Program, Langley researchers conducted efforts on environmental effects on materials, characterization of material quality, development and validation of design and analysis methods, structural durability, impact sensitivity, and potential hazardous electrical effects of carbon fiber.

The issue of potential operational environmental effects on the behavior of composite materials and aircraft components was the subject of great concern for both airline manufacturers and airline operators. The question of long-term environmental durability for composites was viewed as the major undetermined issue for widespread acceptance and application. Led by Richard A. Pride, Langley researchers conducted extensive studies involving in-flight service experiences and ground-based outdoor exposures of composite materials at various worldwide locations. The focus of these studies was the extent of composites degradation due to ultraviolet light effects and moisture gained by diffusion. Individual composite panel specimens were mounted in racks and deployed on rooftops of airline buildings at a number of airports around the world so that maximum exposure to the airport environment occurred. The test panels were deployed domestically at Langley, Seattle, San Francisco, San Diego, Honolulu, and internationally at Frankfurt, Germany, and Sao Paulo, Brazil. After exposures of either one or three years, individual panels were removed from the racks and shipped to Langley for testing and valuation. At Langley, the panels were weighed to determine moisture absorption, and scanning electron micrographs were made to evaluate the composition of the specimen. Flexure, compression, and shear stress tests were also performed. No significant degradation was observed in residual strength tests after 3-year outdoor exposures for all panels tested. These results, presented at a major NASA conference at Langley on Advanced Transport Technology in 1978, coupled with the industry flight tests of the ACEE Program, provided significant confidence for future applications of composites. Additional tests were conducted after 5, 7, and 10 years of outdoor exposure with no significant reduction in strength.

Areas of concern for free graphite fiber dispersion
following an aircraft crash and burn incident.

A major issue regarding the large-scale application of composites in the early 1970s was the potential effect of carbon fiber on electrical components. Laboratory tests and the accidental release of long free fibers from a carbon fiber plant had caused widespread concern that the properties of carbon fibers could have a unique adverse economic impact on the Nation. Carbon fibers are electrical conductors, and free fibers in contact with an unprotected electrical circuit can cause shorts, electrical arcing, and resistive loading. Fibers that are confined in a plastic matrix do not pose any electrical hazard. However, concern existed over ways by which free carbon fibers could be released into the atmosphere in the aftermath of an aircraft crash and fire. The uncontrolled release of carbon fibers might occur if the binding matrix material was burned away cleanly and the fibers became airborne following a crash.

Concern over the potentially disastrous effects of free graphite fibers reached the highest national levels (including the White House), and the future of composite graphite structures was suddenly examined with intensity. In view of the widespread applications and plans for greatly expanded uses of composites within the aviation, automotive, housing, leisure, and other industries, this issue posed a threat that could have terminated any application of composites. In July 1977, the Office of Science and Technology Policy (OSTP) directed that several government agencies undertake immediate studies to justify or disprove the serious concerns regarding composites. A national program on carbon fiber effects was established in 1978, and responsibilities for activities in the program were delegated to nine individual agencies for specific application areas—for example, the Department of Transportation was assigned responsibility for the automotive issues, the Department of Energy was responsible for the vulnerability and protection of power generation, and NASA was charged with responsibility to determine the impact of graphite fibers released from civil aircraft. NASA was also charged with management support to OSTP for the program.

Responsibility for conducting the NASA study was assigned to the Langley Research Center under its Director, Donald P. Hearth. Richard R. Heldenfels, Director for Structures, then appointed Robert J. Huston as program manager of the Graphite Fibers Risk Analysis Program Office. Under Huston’s leadership, a team of about 20 researchers worked for 3 years; they ultimately determined that the issue was not a problem. The Langley program investigated the problem in two areas. The first area was to quantify the potential problem of using composites on civil aircraft. The work included defining the ways by which carbon fibers could be released in the event of an aircraft crash and subsequent fire, the propagation of extremely fine fibers away from the fire site, and the vulnerability of electrical components, especially in other aircraft and in the surrounding area. The second research area, in parallel with this activity, was to develop materials that alleviate or eliminate the electrical hazard. The materials studies included modifications or changes in the binding system which would prevent the release of fiber following a fire and the development of nonconductive fibers to replace graphite.

Huston was assisted by deputy program manager Thomas A. Bartron, and technical element leaders Wolf Elber, Israel Taback, Vernon L. Bell, Jr., Richard A. Pride, Arthur L. Newcomb, Ansel J. Butterfeild, Jerry L. Humble, and Karen R. Credeur. The Program Office sponsored and coordinated 19 studies conducted by NASA Centers, private contractors, the aviation industry (including Boeing, Lockheed, and Douglas), and other government agencies. The responsibility of the industry was to provide data for the analysis with the unstated objective of ensuring they were fully briefed on progress and analysis. Langley contracts required industry to deliver detailed crash data on every jet transport crash worldwide. One of the companies (Lockheed) then turned the data into statistical rates on the probabilities of a crash burn incident, including where (enroute, x miles from a major airport, etc.), when (time of day, takeoff or landing), how (crash burn, fraction of structure consumed), and what (size of aircraft, fuel load). The Langley team then used the supplied data in its analysis. In addition to its technical leadership, NASA contributed the major funding required (about $10 million) for the in-house and contracted studies from its own research funds.

The results of the studies were reported in over 50 technical reports by NASA and other agencies. The scope of activities included probability and risk analyses, outdoor experiments, modeling of events, visits to potentially susceptible sites including hospitals, and nuclear power plants. In one study, for example, Pride directed an investigation of the realistic release of carbon fibers by burning about 45 kg of carbon fiber composite aircraft structural components in five individual large-scale, outdoor aviation jet fuel fire tests that included detailed measurements of the fiber physical and release characteristics.

The Langley investigation projected a dramatic increase in the use of carbon composites in civil aircraft and developed technical data to support the risk assessment. Personal injury was found to be extremely unlikely. In 1993, the year chosen as a focus for the study, the expected annual cost of damage caused by released carbon fibers was only about $1,000. Even the worst-case carbon fiber incident simulated (costing $178,000 once in 34,000 years) was relatively low-cost compared with the cost of a typical air transport accident. With regard to potential power distribution outages, one outage induced by carbon fiber was expected to occur for every 200,000 to 1,000,000 outages caused by lightning or tree contact.

On the basis of these projections, the NASA study concluded that the issue was a nonproblem—exploitation of composites should continue, additional protection of avionics was unnecessary, and development of alternate materials specifically to overcome this problem was not justified. Three independent assessments of the risk all predicted very low value damage to the public and local governments (relative to the cost of the crashed airplane itself). All three cost projections were 3 or 4 orders of magnitude under a risk level that would cause concern. The results of the study, presented in 1980 and 1981 in three public hearings, a formal NASA publication for OSTP (see bibliography), and a presentation to the Director of the Civil Preparedness Agency (now the Federal Emergency Management Agency), are regarded as a pivotal and extremely significant contribution to the Nation’s application of composite materials to civil aircraft of the 1990s. The final OSTP report concluded “The economic loss risk from the accidental release of carbon fibers is so low as to be clearly acceptable on a national basis and does not justify follow-on work to develop alternate materials.” The Langley Research Center clearly played a key role in eliminating one of the most serious obstacles to the growth and use of composite materials.

Aircraft Energy Efficiency Composites Program

Under the sponsorship of the NASA Aircraft Energy Efficiency (ACEE) Program, composite research and applications were investigated by The Boeing Company, Douglas Aircraft Company, and Lockheed Aircraft Corporation with coordination and technical oversight provided by Langley. The overall objective of the ACEE Composite Primary Aircraft Structures Program was to develop and conduct experiments that would lead to applications of composites for small, secondary aircraft components in the early 1980s followed by more complex larger scale structures through the 1990s, with the goal of providing weight reductions resulting in fuel savings over 15 percent.

In 1978, the Langley Aircraft Energy Efficiency Project Office was headed by Robert W. Leonard, and leadership for the NASA Composite Primary Structures Project Office was provided by Louis F. Vosteen. Langley planned a phased composite development research program that would incrementally lead to the design, fabrication, and test of a large-segment wing and fuselage representative of future transports. The first phase involved secondary structures, including elevators for the Boeing 727, ailerons for the Lockheed L-1011, and rudder segments for the McDonnell Douglas DC-10. Later, medium-sized primary structures were created in the second phase of the program, including a horizontal stabilizer structural box for the 737 and vertical fins for the L-1011 and DC-10.

Composite elevators in flight evaluations on Boeing 727 during ACEE Program.

Use of composite materials on Boeing 767 aircraft.

Use of composite materials on Boeing 777 aircraft.

In addition to weight reduction for aircraft components (projected to be from 10 to 30 percent), it was anticipated that a significant reduction in component parts (40 to 60 percent) such as fasteners could be obtained. The principal role of the integrated studies was to define the specific technologies that would be required in order to proceed with a large primary composite structure such as the wing of future transports. Industry’s experience in the studies would cover all critical aspects of composite applications including manufacturing, FAA certification, and flight assessments.

The research plan for the ACEE Composites Program called for the components to be flown on passenger-carrying aircraft in normal airline service; therefore, the new composite structures had to comply with FAA certification requirements; the industry also utilized production-quality tooling to manufacture the components. The benefits of composite components became evident in the ACEE Program: the DC-10 upper rudder segment saved about 30 percent in weight over the aluminum rudder; the L-1011 vertical fin saved about 25 percent in weight; and the Boeing 727 elevators used almost 50 percent fewer ribs and 70 percent fewer fasteners than conventional structures.

When Boeing started design work on the 767 in 1971, the aircraft had been conceived as an all-aluminum aircraft, but the timing of the NASA ACEE Composites Program permitted expanded experience for Boeing, which resulted in extensive applications of composites for the 767, as well as the 757. Even though no primary composite structures were utilized, the weight savings for the 767 was an impressive 2,000 lb. Applications of composites by McDonnell Douglas to its MD-80 and MD-11 transports followed. Numerous applications also occurred for derivative Boeing 737 and 747 aircraft. Stimulated by the successes of the ACEE activities, industry began efforts on advanced composite wings and primary composite structures. Boeing’s experience base and further developments of composite technology led to incorporation of an even higher degree of composites for the Boeing 777. The first major use of composites for primary structures of a U.S. commercial transport was for the empennage of the 777.

The stimulation of the ACEE Program is believed to have accelerated the application of composites to commercial transports by approximately 5 to 10 years. NASA research and development in the ACEE era (1975 to 1986) produced over 600 technical reports. In addition to technology advances in performance prediction and manufacturing processes, a significant increase in confidence was obtained regarding issues such as durability, cost verification, FAA certification, and airline acceptance. The ACEE Program was primarily responsible for the impact of Langley contributions to the application of composites to commercial aircraft of the 1990s. However, composites research activities at Langley that followed ACEE, which ended in 1985, have had a marked influence on the potential near-term applications of composites in the new millennium. In particular, now that issues regarding the environmental durability have been successfully addressed for composites, the focus of research in the late 1980s and 1990s has turned to the all-important issue of cost. Whereas military applications of composites have been very aggressive (for example, the F-22 has about 38 percent of its structural weight in composites), applications to the commercial transports area have been relatively low. The most aggressive U.S. commercial transport application to date has been the Boeing 777, which has about 10 percent of its structural weight in composites. If commercial aircraft applications are to increase, cost impact factors must be significantly improved.

Without question, the ACEE Program provided the airframe companies with important technology, but the program ended without accomplishing its original goal of developing composite primary wing and fuselage structures. Without a NASA technology program, industry lacked the confidence to proceed with production of high-risk primary structures. The barrier issues were high acquisition costs and low damage tolerance. Cost data extrapolated from the ACEE development contracts showed that wings and fuselages would cost considerably more than aluminum structures. The industry position on production commitment was that composite primary structures must be demonstrated to cost less than aluminum structures. Low damage tolerance remained a characteristic of composite structures despite major efforts to develop and use toughened matrix resins. Industry wanted robust structures able to withstand the rigors of flight service with minimal damage.

Composite upper aft rudder flown on McDonnell Douglas DC-10 in ACEE Program.

Use of composite materials on U.S. military aircraft.

Use of composites in civil commercial transports.

Advanced Composite Technology Program

By 1985, research engineers at Langley were holding conferences to explore the potential of textile composites, based on approaches similar to those used in the textile industry, to provide barrier-breakthrough technology. By 1987, funds were available for a modest expansion of the Langley composites program. A NASA Research Announcement (NRA) was issued seeking proposals for innovative approaches to cost-effective fabrication, enhanced damage tolerance designs, and improved analysis methods. Forty-eight proposals were submitted by companies and universities, and 15 proposals were selected for contracts. Then, in 1988, NASA launched its Advanced Composites Technology (ACT) Program, a major new program for composite wing and fuselage primary structures. The program incorporated the existing NRA contracts with significant increases in funding for wing and fuselage hardware developments. A Structures Technology Program Office at Langley provided management for the ACT Program. Under the direction of Charles P. Blankenship, John G. Davis, Jr., was the Program Manager of ACT, and leading researchers included James H. Starnes, Jr., Marvin B. Dow, H. Benson Dexter, and Norman J. Johnston. The 15 previously mentioned contracts were awarded by Langley in 1989 to commercial and military airframe manufacturers, materials developers and suppliers, universities, and government laboratories. The program approach was to develop materials, structural mechanics methodology, design concepts, and fabrication procedures that offered the potential to make composite structures cost-effective compared with aluminum structures. Goals for the ACT program included 30–50 percent weight reduction, 20–25 percent acquisition cost reduction, and the scientific basis for predicting materials and structures performance.

Phase A of the Program, conducted from 1989 to 1991, focused on the identification and evaluation of innovative manufacturing technologies and structural concepts. Industry participants included Northrop, Lockheed Corporation, McDonnell Douglas Corporation, The Boeing Company, and Grumman Aerospace. At the end of Phase A, the leading wing and fuselage design concepts were selected for further development in Phase B of the Program from 1992 to 1995. Two major fabrication technologies emerged from Phase A as the most promising approaches to manufacturing cost-effective composite primary structures. These two approaches were the stitched textile preform and automated tow placement manufacturing methods. Each method emphasized rapid fiber placement, near-net-shape preform fabrication, part count minimization, and matching the technologies to the specific structural configurations and requirements. The objective of Phase B was to continue the evolution of design concepts by using the concurrent engineering process; selecting the leading structural concept; and designing, building, and testing subscale components. In this phase, Boeing and Lockheed focused on fuselage technology, while McDonnell Douglas focused on wing technology. Phase C of the ACT Program, begun in 1995, was to design, build, and test major components of the airframe and to demonstrate the technology readiness for applications in the next generation of subsonic commercial transport aircraft. The original program plan called for the contribution of Boeing to be a complete fuselage barrel with a window belt and a wing box at the wing-fuselage intersection. The structure was to have been pressure tested as part of the engineering verification process. Unfortunately, the funding for ACT was reduced and forced cancellation of the composite fuselage studies. McDonnell Douglas, meanwhile, focused on the successful development, fabrication, and testing of an advanced composite wing, as discussed later. The ACT Program ended in fiscal year 1997.

Textile Composites

In the 1980s, researchers looked to textile composites as breakthrough technology. Supporters argued for new concepts that would use knitting, weaving, braiding, and through-the-thickness stitching for reinforcement and use existing U.S. textile manufacturing technology for cost-efficiency. An outstanding summary by Dow and Dexter of progress and details of textile composite research by NASA during the period from 1985 to 1997 is recommended to the reader (see bibliography).

Under the leadership of Marvin B. Dow, Langley conducted and sponsored extensive research on woven, braided, knitted, and stitched (textile) composites in the NASA ACT Program in the period from 1985 to 1997. The major objective of the studies was to develop textile composites technology approaches that would provide a paradigm shift in cost and damage tolerance to overcome barrier issues. One such barrier issue is the impact performance of textile composites. Low-velocity impacts from tools, hail, runway debris, and ground equipment can damage resin matrix composites with carbon fibers. With sufficient kinetic energy, these impacts can damage the composite without readily visible evidence and can significantly reduce the strength. Current regulations require composite structures to carry ultimate load with nonvisible impact damage. Textile composites are potentially more resistant to impact damage than traditional laminated composites fabricated using prepreg unidirectional tape. In 1994, Clarence C. Poe, Jr., of Langley conducted studies of conventional tape laminates and textile composites, providing detailed design information on their characteristics.

Research by H. Benson Dexter in 1994 on braided composite materials demonstrated that a braided-woven stiffener wing concept could meet damage tolerance goals and be designed and fabricated with a cost-effective process. Braiding is an automated process for obtaining near-net-shape preforms for fabrication of components for structural application. Stiffeners, wing spars, floor beams, and fuselage frames are examples of potential applications of cost-effective braided composites. Test results on wing panels fabricated from stitched skins and stitched-stiffener preforms obtained at Langley and McDonnell Douglas indicated that damage-tolerance requirements could be met. Accordingly, stitched panels with braided stiffeners were tested to assure that braided stiffeners also satisfied damage requirements.

Braid-stiffened wing-panel preforms were fabricated by Langley from dry-stitched skin and braided stiffeners obtained from Fiber Innovations, Inc., Norwood, Massachusetts, followed by a resin film infusion (RFI) process by McDonnell Douglas. Wing panels were intentionally impacted on the skin side midway between stiffeners, directly beneath a stiffener, or at the flange edge of a stiffener. Impact energies were selected to produce the onset of visual damage. All impacted panels exceeded the impact design goal and failed without any skin-stiffener separation.

One major breakthrough in Dow’s program was the use of advanced stitching methods to fabricate large composite structures. Various types of textile composites were thoroughly tested, but it was stitching, combined with RFI, that showed the greatest potential for overcoming the cost and damage tolerance barriers to wing structures. Assembling carbon fabric preforms (precut pieces of material) with closely spaced through-the-thickness stitching provided essential reinforcement for damage tolerance. Also, stitching made it possible to incorporate the various elements—wing skin, stiffeners, ribs and spars—into an integral structure that would eliminate thousands of mechanical fasteners. Although studies showed that stitching had the potential for cost-effective manufacturing, the critical need was for machines capable of stitching large wing preforms at higher speeds.

A primitive single-needle stitching machine, resembling a scaled-up version of a household sewing machine, was the first prototype used by Langley to determine the benefits of stitched composites. This initial research identified that stitched composites offered better levels of damage tolerance than conventional laminated composites. This single-needle sewing machine was used in exploratory research on stitched composites. In 1994, a computer-controlled single-needle stitching machine capable of stitching dry high-performance textile materials (such as graphite and glass) was designed and built for the Materials Division at Langley. The stitching machine was capable of stitching a planform area of 4 by 6 ft with thicknesses greater than 1.5 in. using a lock stitch, and programming stitching in any direction (including curves) within the planform area. The machine was capable of stitching with a wide variety of needle and bobbin threads, such as polyester, nylon, DuPont Kevlar, and carbon. A wide variety of preform sizes were fabricated and delivered to McDonnell Douglas for RFI processing to produce test specimens for evaluation at NASA Langley.

Lower-stitched wing cover for 42-ft-span structural test wing.

In the stitched-RFI process, layers of dry carbon fabric are stacked to form the wing structural elements and are stitched with through-the-thickness Kevlar threads. RFI of the preform with epoxy resin followed by autoclave curing completes the process of making an integral wing cover.

Results obtained with test panels and a small wing-box test article indicated that the process produced composite aircraft parts with outstanding damage tolerance. The process has the potential for major reductions in the labor content of manufacturing composite wing primary structures. However, demonstrating a stitching machine with the size and speed required for cost-effective fabrication of full-scale composite wings for commercial transport aircraft was critically important.

One of the first demonstration sections was a 12-ft-long wing stub box that was fabricated by McDonnell Douglas and tested at the Langley Research Center in July 1995. The wing stub box demonstrated that the stitching-RFI concept could be used to make the thick composite structures needed for heavily loaded wings. The successful test of the stub box proved the structure and damage tolerance of a stitched wing.

NASA awarded Boeing (subsequent to the merger of Boeing and McDonnell Douglas) a contract to develop a large machine capable of stitching entire wing covers for commercial transport aircraft. This high-speed, multineedle machine, known as the Advanced Stitching Machine (ASM), was designed and built under the NASA ACT Wing Program. Under subcontract to Boeing, Ingersoll Milling Machine Company, Rockford, Illinois, was selected to design and build the ASM. The advanced stitching heads of the ASM were designed and built by Pathe Technologies, Inc., Irvington, New Jersey. Concurrent with the development of the large stitching machine, NASA and Boeing proceeded with a building block approach to demonstrate the design and manufacture of stitched-RFI wing structures.

Ingersoll’s machine was capable of stitching a contoured wing preform 50-ft long and 8-ft wide. Following extensive checkout tests, the machine was dismantled, moved, and reassembled at the McDonnell Douglas stitching facility in Huntington Beach, California. When the stitching was completed on the machine, the still flexible wing skin panel was put into an outer mold line (OML) tool that provided the shape of the outside surface of the wing. A film of resin was laid on the OML form, followed by the composite skin panel and the tools that defined the inner mold line. These elements were put into a plastic bag from which the air was drawn out, creating 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 350∞ F for 2 hours, the wing skin panel took on its final hardened shape.

Panels were stitched on the ASM to be used as test articles in a full-scale ground test of a composite wing representative of a transport aircraft. The Stitched-RFI Composite Wing Program was successfully completed with ground testing of a 42-ft-long wing box. The box was tested in the Langley Structures and Materials Laboratory under the leadership of Dawn Jegley in 2000, and the box failed at 97 percent of design ultimate load (145-percent design limit load). Boeing is seriously considering using this technology in the next generation of aircraft.

Pretest photograph of 42-ft-span wing structure at Langley.

Tests of 42-ft-long wing box.

Boeing named its new Stitched Composite Development Center after NASA Langley researcher Marvin B. Dow in honor of his contributions to stitched composites research and, specifically, to the ASM. Dow spent the last 25 years of his 40-year NACA/NASA career in pursuit of the application of advanced composite materials on commercial transport aircraft. He is the first NASA employee honored in the naming of a corporate facility. His work on composites led to the early flight testing of graphite-epoxy rudders on the McDonnell Douglas DC-10 commercial transport aircraft, the ACEE structures for the DC-10, Boeing 737, and C-130 aircraft, and his pioneering and visionary research on textile reinforcement concepts such as weaving, braiding, knitting, stitching, and resin transfer processes introduced the world to innovative new fabrication techniques. The ASM—made possible by Dow’s long-term dedication—is expected to revolutionize the way aircraft wing structures are fabricated in the future.

Fundamental Research and Technology in Composites

In addition to the foregoing activities, Langley’s contribution to understanding composite failure mechanisms has been widely recognized. In the late 1970s, a series of landmark tests of composite panels under impact were conducted by Marvin D. Rhodes in the Structures and Materials Laboratory. The results stimulated design, analysis, and test activities, led by James H. Starnes, to reduce impact sensitivity and understand new failure modes in stiffened plates and shells. Langley was also a key contributor to toughened resin research led by Norman J. Johnston and Terry St. Clair. Contractual work, sponsored by Langley, led to industry development of resins used in today’s structural applications. In yet another area of research, Charles E. Harris and Clarence C. Poe, Jr., led critical research efforts on fracture mechanics that included analysis methods and databases. Much of the work just described has been recognized by citations to Langley throughout Military Handbook 17 on composites.

Composites for General Aviation

The general aviation community has long been a user of composites technology, especially for small personal-owner aircraft and home-built aircraft. Led by innovative designers such as Burt Rutan, this sector of aviation has enthusiastically embraced the benefits of composites technology, and although NASA research has not been directed specifically at this class of aircraft before the AGATE program, Langley has ensured that appropriate communications with the small aircraft community regarding NASA technology has occurred through briefings at national meetings, such as the Experimental Aircraft Association’s Oshkosh convention.
Beech Aircraft (now Raytheon Aircraft Company) made extensive use of composites in the Beech Starship, as well as new business aircraft, the Premier I and the Horizon. These applications have made use of information and results from the ACEE, ACT, and AGATE Composite Programs. The general aviation industry is now the leader in the use of composites in production aircraft. The FAA is currently in the certification process for 19 new aircraft with significant use of composites. The Lancair Columbia 300 and the Cirrus SR20, which have benefited from Langley’s AGATE Program, are the two most recent all-composite GA aircraft to receive FAA certification. The NASA Small Business Innovation Research (SBIR) Program has played a major role in numerous technologies used on these aircraft, including the development of low-cost composite manufacturing processes.

Led by the efforts of Bruce J. Holmes and William T. Freeman, Jr., one of the most significant accomplishments of general aviation research at Langley was the 1998 publication entitled Material Qualification Methodology for Epoxy-Based Prepreg Composite Material Systems. This publication documents the breakthrough process that allows airframe manufacturers to procure certified composite materials from vendors in the same manner that they were able to procure metals for decades.

In the decades since the introduction of synthetic composite materials for use in aerospace applications, the cost of materials qualification has inhibited expanded uses. Much of this cost has resulted from the extensive testing required by the FAA. Each airframe manufacturer intending to apply a composite material to a product has been required to submit detailed materials property reports to the FAA, regardless of whether they or other manufacturers had previously certified the same materials. Long ago, resulting from sustained statistical confidence in the ability of materials suppliers to meet common production standards, industry and FAA dispensed with such testing requirements for aluminum and other metals.

The previously mentioned NASA publication outlines a materials qualification method that has been accepted by the FAA and eliminates the need for repeated tests to qualify composite materials. Specifically, it provides the method by which composite material vendors can market composites that comply with FAA certification requirements. This qualification process eliminates the need for airframers to qualify composite materials for their aircraft certification programs. Airframe manufacturers dramatically reduce the costs associated with using composite materials by buying materials that are already certified or approved by the FAA. As a result, the cost of FAA certification of new composite airframes is reduced by more than $500,000 per material, and the time required for certification of a new airplane is reduced by more than 2 years. These reductions in certification time and costs will make the use of composite materials a viable choice for small and large companies and can help generate the market forces necessary to foster the revitalization of the general aviation industry.

Certified composite aircraft, Lancair Columbia and Cirrus SR20.

Applications

The legacy of the ACEE Program and its significant contributions to the acceleration, acceptance, and application of advanced composites has become a well-known example of the value of Langley contributions to civil aviation. In the best tradition of NASA and industry cooperation and mutual interest, fundamental technology concepts were conceived, matured, and efficiently transferred to industry in a timely and professional manner. With the participation and guidance of Langley, industry was able to address numerous high-risk issues that posed serious obstacles to advances in the state of the art and applications. Widespread use of composites today by military aircraft and the continuing increase of composites used by civil aircraft are very visible reminders of the impact of this important technology contribution by the Langley Research Center.