A Strategic Center of Gravity—Composite Materials for Aircraft

Since the beginning of aviation history, weight reduction has been a primary goal.[108] During the time of the Wright brothers’ first flights, air­planes were constructed of various types of wood, fabric, and wires.[109] It w’as not until the 1920s that one of the first materials breakthroughs occurred, the Ford Tri-motor, dubbed the “Tin Goose.” Henry Ford began manufac­turing these aircraft in 1925, and they were unusual because of their use of metal and aluminum. The first planes used a corrugated metal shell, which surrounded a metal truss framework. In the 1930s, stressed-skin aluminum monocoque construction techniques emerged, and Langley played a key role in developing stress and strength analyses of the mate­rials. These analyses paved the way for other structural and materials advances at Langley, which included thin wings for military aircraft in the 1940s. The new aircraft were required to withstand the stresses resulting from much faster speeds and also greater dynamic loads and vibrations. Langley engineers helped to pioneer the use of higher-strength alloys that prevented the aircraft from breaking apart under these forces. In the 1960s, a new type of material emerged that would come to challenge the domi­nance of metals in the skies. These were known as composites.

In 1967, at the 50th anniversary of the birth of the Langley laboratory, engineers announced that they were on the verge of several revolutionary new aircraft concepts, one of which was in materials.2′ The size of aircraft hadin – creased dramatically since the time of the first airplanes. The Wright broth­ers’ historic first aircraft was a fragile device that weighed just 1,260 pounds. In comparison, the all-metal 747 aircraft, which first flew 1 year after this Langley celebration, weighed 750,(XX) pounds. The fuel required to lift and propel these massive, metallic beasts was immense, so any weight reduction achieved through new materials was eagerly anticipated. Langley engineers believed they were on the cusp of achieving a major advance in composites.[110] [111]

When two or more substances are combined together in one struc­ture, the resulting material is called a composite. Aircraft composites are made by bonding together a primary material that has strong fibers with an adhesive, such as a polymer resin or matrix. These are various types of graphite, glass, or other synthetic materials that can be bonded together in a polymer epoxy matrix. The composite materials are typically thin-thread cloth layers or flat tapes that can lie shaped into complex and

The corrugated shell is made from thermoplastic composite materials (February 17. 1978). (NASA Langley Research Center |NASA LaRC|.)

aerodynamically smooth shapes of virtually any size.2′ Their application to aircraft led some to imagine the “Jet Fighter Made of Thread The physi­cal properties of these materials made them extremely attractive in aircraft design because they were stronger, stiffer, and lighter than their metallic counterparts. Composites were also resistant to corrosion, a constant plague on metal aircraft. While efforts to incorporate these materials had been ongoing for several years prior to the 1970s, there were difficult hur­dles that prevented their adoption. First was the general uncertainty as to whether they would actually work and could withstand the rigors of flight. The second was the cost of research and development simply to reach the stage at which they could be flight-tested. The cost of fabrication for pro­duction applications was, and still is, a key factor. Finally, there were no [112] [113] data on their durability and maintenance requirements over time. As one observer stated, “The planned application of composites would require the development of revolutionary technology in aircraft structures.”[114]

This development became the focus of 1972 joint Air Force – NASA program known as Long Range Planning Study for Composites (RECAST). The success of these investigations led NASA to include it as one of the six main program elements of ACEE, and it became known as the Composite Primary Aircraft Structures. Langley Research Center was to coordinate the program in conjunction with its industry partners: Boeing Commercial Airplane, Douglas Aircraft, and Lockheed. Langley was the obvious choice for this program, because the Center had played a lead­ing role for decades in investigating aircraft structures and materials. The stated objective of CPAS was to “provide the technology and confidence for commercial transport manufacturers to commit to production of compos­ites in future aircraft.”-[115] The technology included the development of design concepts and the establishment of cost-efficient manufacturing processes. The confidence would come with proof of the composite s durability, cost verification, FAA certification, and ultimately its acceptance by the airlines.

The main goal was to reduce the weight of aircraft by 25 percent through the use of these new materials, thereby decreasing fuel usage by 10 to 15 percent. Using composites for the wings and fuselage promised the greatest savings, but this was also the most technically challenging because these components were so vital to aircraft safety. To overcome some of the uncer­tainties of the materials, secondary structures (upper aft rudders, inboard aileron, and elevators) were the first candidates for composite materials.

Once these investigations were successful, then the development of medium-size primary structures (vertical stabilizer, vertical fin, horizontal stabilizer) would begin. In the meantime, some preliminary wing work would be explored, followed by work on the fuselage. Louis F. Vosteen headed the program at Langley.[116]

Composite elevators in flight evaluations on Boeing 727 during ACEE program. Courtesy of Joseph Chambers.

Secondary structures are those that have light loads and are not critical to the safety of the aircraft. The upper aft rudder on the Douglas DC-10 was one of the first of the secondary structures to be studied.3" The rudder is a mov­able vertical surface on the rear of the vertical tail and is used for coordinating turning maneuvers and trimming the aircraft following the loss of an engine. Work to construct composite upper aft rudders actually began in 1974 but was completed as part of the ACEE program. Twelve units were put into sen ice, and ACEE engineers estimated that manufacturing would cost less than metal after 50 to 100 units were installed. These units resulted in a 26.4-percent weight savings over the traditional aluminum alloy previously used for the rudder. Elevators were the next secondary structural components designed. Located at the rear of the fixed horizontal surfaces, elevators are movable sur­faces used for controlling the longitudinal attitude of the airplane. Ten units were designed for the Boeing 727, and flight-testing began in March 1980." [117] [118]

Composites technology was applied to other projects as well. The Rutan Model 33 VariEze was built by the Model and Composites Section of Langley and then tested in a tunnel. (July 17. 1981). (NASA Headquarters-Greatest Images of NASA (NASA HQ GRIN].)

With a 23.6-percent decrease in the plane’s weight, Boeing considered the production a success and approved the elevators for use on the 757 and 767. The final secondary structures were the inboard ailerons, move – able surfaces located on the edges of the wing/2 Working in conjunction [119]

NASA’s Boeing 737 in front of the hangar after its arrival in July 1973. Much ACEE work was performed on the 737 in later years. (NASA Langley Research Center [NASA LaRC|.)

with the rudder, in-board ailerons are used for banking the airplane during high-speed turning maneuvers. Installed on a Lockheed L-1011 airplane, eight units began flight-testing in 1982. These were a significant improve­ment over the aluminum ailerons, reducing the weight by 65 pounds, the number of ribs from 18 to 10. and the number of fasteners from 5.253 to 2,564.” Taken together, these 3 secondary structures made with graph­ite epoxy materials weighed 1.500 pounds and represented a 450-pound weight reduction over the aluminum components.54

Three other medium primary structures were designed for the ACEE program: the vertical fin. the horizontal stabilizer, and vertical stabilizer. The “medium primary" classification meant that other components were attached to them (and they provided the aircraft with stability), so they were more critical to a safe flight than the secondary structures were. The vertical fin is at the rear of the airplane, where it contributes aerodynamic directional stability.

Design of a composite vertical fin for the Lockheed L-10I1 started in 1975, and the project was then transferred to the ACEE program once [120] [121] underway in 1976.15 The development was plagued by several problems when the composite materials failed prior to reaching ultimate load, and as a result, it never progressed beyond the static testing stage under ACEE. Though it never took flight, this was a 7-foot by 23-foot structure and, at 780 pounds, represented a 22.6-percent weight savings. The next medium primary structure designed was the horizontal stabilizer. This is a fixed surface at the rear of the airplane that provides longitudinal sta­bility.*6 Designed for a Boeing 737, it too experienced structural failures during ground tests, but these were corrected, and the FAA certified the component in August 1982. On April 11. 1984. the first composite primary structure went into service, representing 28.4-percent weight savings.[122] [123] [124] [125] [126] The final medium primary structure was the vertical stabilizer. Located at the back of the airplane, it is used to control yaw, or the rotation of the vertical axis.3*1 Designed for the Douglas DC-10, the vertical stabi­lizer provided a 22.1-percent reduction in weight, but it too experienced several production problems and failed a ground test. After the failure, engineers incorporated a different structure, and though it took much more time to develop than expected, the FAA certified it in 1986. and commer­cial flight commenced in January 1987.’9

Langley engineers wrote a number of computer programs to aid in the design and analysis of these composites. PASCO analyzed compos­ite panels and helped determine their material strength. V1PASA provided data on buckling and vibration and worked in conjunction with PASCO. CONMIN was a nonlinear mathematical programming technique that assisted in sizing issues.4’* Three years later, another program, POSTOP, assisted in the design of composite panels by analyzing compression, shear, and pressure on the materials.[127] [128] Temperature effects were also included. Other design and analysis studies used traditional mathematics and experi­mentation. Extensive failure studies were undertaken to help ensure the durability of the composite structures. One type of study analyzed what happened when surfaces cracked and how that compromised the safety of the airplane.[129] Resulting experiments looked at repair techniques for these composite structures when cracks and other tears appeared.[130]’

Engineers also designed long-term environmental studies to deter­mine the possible effects of environmental exposure on the composites. One concern was that the composites would degrade over time because of ultraviolet light. Another concern was whether they would absorb moisture. Tests included composite panels placed on airport rooftops at Langley and in San Diego, Seattle. Sao Paulo, and Frankfurt. These took into account geographical location, solar heating effects, ultraviolet deg­radation. and test temperatures.[131] Other studies evaluated components in flight. Richard A. Pride, who headed the program at Langley, found that after 3 years, “No significant degradation has been observed in residual strength.”[132] Longer-term studies, up to 10 years, indicated that composites did not degrade over time given normal use and environmental exposure.[133]

Despite the success of these studies, there was one important envi­ronmental concern that threatened to halt the composites program and for a time did ground all composite flight-testing. Because carbon libers were a main component of these composites, flight over population centers was an environmental issue. The risk was to everyday electrical systems that could potentially be damaged through exposure to the accidental release of carbon fibers into the air through an accident or crash. There was a possibility that libers released from composite aircraft materials could interfere with electri­cal systems on the ground (because the libers can conduct electricity), caus­ing them to fail. The concern spanned from the mundane—a toaster or televi­sion—to the critical—air traffic control equipment or nuclear powerplants. The fibers were so light that they could be easily blown and distributed in the air by an explosion, affecting a wide area. Moderate winds could spread them tens of miles. The airline industry was concerned because it would then be liable for replacing all the failed electronics equipment.[134] A major ACEE investigation, the Carbon Fiber Risk Assessment, was launched to determine the significance of this threat.[135]* It was headed at Langley by Robert J. I luston, the Program Manager of the Graphite Fibers Risk Analysis Program Office.

After extensive research at Langley, engineers concluded that the threat would be negligible.[136] [137] For example. 0.00339 televisions out of 100 would fail. Only 0.(X) 171 toasters would be affected out of 100. For more critical equipment, the predictions were also low, only 0.005 out of 100 types of air traffic control equipment, or 0.016 out of 100 ground computer installations. After more than 50 technical reports, NASA predicted that carbon fiber accidents would only cause SI,000 worth of damage in 1993, and the absolute worst-case scenario would be a $178,000 loss occurring every 34,000 exposures.4′ Compared with other possible air transportation threats, the carbon fiber risk was simply nonexistent.

While the ACEE composites program lasted 10 years, from 1976 to 1985, it ended before achieving its major goal of developing wings and fuselages with composite materials, the stated goal of the program, because the wing and fuselage represented 75 percent of the weight of the airplane. Wings and fuselages made of composites would have achieved signifi­cant weight savings and fuel economy.[138] [139] [140] There were several reasons that these were never developed by ACEE. First was the amount of time and resources devoted to the Carbon Fiber Risk Assessment. Unanticipated at the start of the project, this potential problem became a serious threat to the use of composites. Therefore, it was necessary to prove that there was little risk in their use. After this setback. NASA was finally able to devote all of its attention to wings and fuselages in 1981, but engineers took a dif­ferent approach to their development than they did with the previous com­ponents. Whereas before NASA had developed composites that replaced entire metal components on aircraft, it now decided to try to incorporate composite pieces into the fuselage (a section barrel) and wing (short-span wing box). Boeing studied the damage tolerance of composite wings, the threat posed by lightning strikes, and an evaluation of their fuel sealing capabilities.5’ Lockheed examined acoustic issues, such as how noise was transmitted through flat, angular, composite panels and how to reduce it/" By this time, the ACEE program and its funding were nearly at its end, so the ultimate goal of composite wings and fuselages was never attained.[141]

Nevertheless, the success ACEE had with secondary components was called “almost revolutionary.” One observer said this 10-year period represented the “golden age of composites research in the United States.”[142] ACEE became a “strategic center of gravity” in this golden age, and its achievements in secondary structures were vitally important in introducing a new type of material as an alternative to the traditional metal and aluminum used in airplanes. The Composite Primary Aircraft Structures program had several very significant results over its lifespan.[143] It produced 600 technical reports and provided a cost estimate for develop­ing these materials and a confidence in their durability and long-term use. Composites received certification by the FAA. as well as general accept­ance by the airline industry. Overall, its estimated that the ACEE program was responsible for accelerating the use of composites in the airline indus­try by at least 5 to 10 years. Langley continued to track the composites it developed even after the ACEE program concluded, and 350 composites reached 5.3-million flight-hours in 1991 and were still operational.

According to Herman Rediess, one of the initial ACEE task force mem­bers. “Many of things that we were talking about at the time are now just so standard that people hardly even remember that they came out of ACEE.” Prior to the ACEE program, aircraft manufacturers were reluctant to investigate the opportunities these composites offered because of costs and unknown performance capabilities. But, as Rediess now reflects. “It’s a major, major aspect of our commercial transports. It has really paid off in terms of weight savings, and in that weight is fuel.”[144] [145] [146] By the 1990s, these composite materials resulted in a fuel efficiency savings of 15 percent.54 As one observer concluded at a 1990 conference on composite materials, “The NASA Aircraft Efficiency Program provided aircraft manufacturers, the FAA, and the airlines with the experience and confidence needed for extensive use of composites in… future aircraft.”50

Since the end of the ACEE program, the use of composites has increased, though not as dramatically as first imagined. While the weight savings and fuel efficiency were undeniable, their mass implementa­tion was offset by the cost of producing them, compared with metal and

This X-29 research aircraft in flight over California’s Mojave Desert shows its striking forward – swept wing and canard design. The X-29 demonstrated the use of advanced composites in aircraft construction. Two X-29 aircraft flew at the Ames-Dryden Right Research Facility from 1984 to 1992. (NASA Dryden Flight Research Center Photo Collection.)

aluminum structures. They are also more expensive to certify for flight readiness.[147] As fuel costs increase in the 21st century, however, the economic returns for lighter aircraft will become more valuable, and composites will take on greater significance. Today, the military has sur­passed commercial aviation in the use of composites. For example, com­posites account for 38 percent of the weight of an F-22 but only 10 per­cent of a Boeing 777, which has the highest composite percentage of any commercial aircraft.[148]

The new Boeing 787 Dreamliner may become the first major com­mercial aircraft with composites comprising the majority of its materials, as the company is planning for 50 percent of primary structures, including

Fixed upper l. e. skin panels

The above graphic demonstrates the composite components of the Boeing 767. Courtesy of Joseph Chambers.

The above graphic demonstrates the composite components of the Boeing 777. Courtesy of Joseph Chambers.

Composite aircraft Laneair Columbia and Cirrus SR20. Courtesy of Joseph Chambers.

fuselage and wing, to be composites/’- The general aviation community has also benefited from composites. For example, small personal-owner aircraft and homebuilt aircraft, with designer Burt Rutan taking the lead, have taken advantage of composites technology. Business-class aircraft such as Beech Aircraft (now Raytheon Aircraft Company) has developed an all-composite aircraft known as the Laneair Columbia 3(X) and the Cirrus SR20.

The ACEE composites program was a success because, according to Jeffrey Ethell, it “demolished the fear factor surrounding the new mate­rials, which have entered the real world of transport aviation.”6′ ACEE served as an encouraging point of departure for industry entering the world of composites. The program took materials that were untested, unusual, and exotic, and it transformed them into certified and usable structures on commercial and military aircraft. According to Joseph Chambers, "The legacy of the ACEE Program and its significant contributions to the [149] [150] 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/4"1