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COMPETITIVE ASSESSMENT OF TECHNOLOGY 110 Flight Controls Active control systems that could allow reduced static longitudinal stability are conceptually possible for transport aircraft, with resulting reductions in drag and weight due to reduced tail and wing areas. However, further research and development effort is required for large-scale applications. The next generation of 150-seat transports is expected to use augmented stability systems to provide a tail designed to accommodate a center of gravity located aft, thus minimizing trim drag. U.S. manufacturers and Airbus appear to be approximately equal in this technology. Full exploitation of this technology will require another round of aircraft development. The use of limited active controls for wind-gust and maneuverload alleviation has already been incorporated in the Lockheed L-1011-500. This technology can allow reductions in wing structural weight or further increases in wing aspect ratio to improve performance without weight increase. Flutter suppression modes offer further improvements for more advanced aircraft. Application of this technology is already being considered by Airbus for stretched versions of the A300 as well as for later versions of the A320, which is to have what is called a fly-by-wire control system. The Concorde was the first certificated commercial aircraft to rely principally on a fly-by-wire control system. It also contains a highly integrated stability augmentation control system. In this area of technology, the United States and Europe can be judged to be about equal in current capability. General aviation airplanes tend to follow large aircraft in adopting advanced flight controls. In rotorcraft, the United States is thought to have the lead in flight control technology. Advanced Structures Recently, new high strength-to-weight aluminum-lithium alloys have shown potential for additional significant weight savings, but much work remains to be done in qualifying the material and scaling up its production in sheet, plate, and extruded forms before widespread application in aircraft manufacture can take place. Another emerging structural concept that shows much promise is superplastically formed, diffusion-bonded metals (notably titanium but also possibly aluminum). Improved aluminum alloys are now being incorporated by Airbus in the A310. However, the "economic repair life" of the A310 is estimated by Airbus to be 40,000 cycles compared to Boeing's estimated life of 60,000 cycles for the 767. Comparisons of life, however, are dependent on the stress level chosen by the
COMPETITIVE ASSESSMENT OF TECHNOLOGY 111 designer for the structure in question. Newly developed aluminum-zinc alloys with thermomechanical treatment for increased compression strength and better fracture properties are planned for the A320. More extensive use of titanium is also planned for highly stressed parts. In these respects, Airbus metallic structure technology is fully competitive with current U.S. technology. Its research on advanced alloys of the aluminum-lithium type and superplastic-formed, diffusion-bonded titanium is approximately the same as in the United States. To date, current and planned aircraft are minimizing use of adhesive bonding due to poor early experience. Metal-to-metal bonding technology applications in Europe are at least equivalent to those in the U.S. where application to fuselage structure, including compound-curved panels, is fully accepted, certified, and demonstrated in extended operation of wide-bodied transports. The largest single opportunity in airframe materials lies in composite materials, including metal matrix composites. The combination of thermoplastic or thermoset composites with the attendant means of processing and fabricating technology is a rapidly expanding field with very large potential payoff. Both United States and European developers are active. European research capabilities are almost equal to those in the U.S. During the past few years, great strides have been made in the use of advanced nonmetallic composite structural elements. These composite structures have high stiffness and extremely light weight when compared with conventional metal structures and offer the promise of significant increases in performance, due to the reduction in weight and the promise of extended life in overall aircraft performance and efficiency. The latest U.S. aircraft, such as the Boeing 767, incorporate significant amounts of composites in secondary structures. More advanced designs, such as the proposed McDonnell Douglas D-330 series, extend composites to more wing components, cabin floor beams, the entire nacelle, and the tail cone (Figure 5-2). European research and development efforts on composite materials are extensive and continue to accelerate. Many European aerospace companies have been working with composite materials for up to 15 years. These companies believe they have a basic scientific understanding of the materials, which they are now converting into practice. Airbus Industrie has a program for the progressive introduction of composite components on the A300 and the A310 (Figure 5-3). The A320 will add composite elevators, fin and tail-plane trailing edges, floor panels, cowl components, wing-to-body fairings, and carbon-composite wheel brakes (Figure 5-4). At present, relatively small elements such as rudders, ailerons, and spoilers have been produced. In the long term, the full poten
COMPETITIVE ASSESSMENT OF TECHNOLOGY 112 tial of composite materials will be realized with their incorporation into primary aircraft structures such as wings, stabilizers, and fuselages. Benefits would potentially include a 15 to 25 percent reduction in structural weight, a 7 to 15 percent improvement in fuel efficiency, and resulting 4 to 8 percent reductions in direct operating cost. Projections of the latter are more uncertain because the manufacturing costs for composites and future costs for fuel are very uncertain. Figure 5-2 McDonnell Douglas D-3300 Advanced Material Applications Source: McDonnell Douglas. In the United States a composite primary structure program was initiated by NASA in 1976, with the objective of developing the technology and confidence to permit commercial transport manufacturers to use composites extensively in the primary structure of production aircraft. The original plan to build and flight test a full-scale wing was regarded as too expensive. The program has been scaled back to build and test key components. Further specialized tests and the establishment of a resulting data base are still necessary to develop confidence in the application of composites to primary structures. NASA plans still call for fuselage design studies to begin this year, followed by a six-year fuselage test program. Similar plans, directed at demonstration of such structures for airline evaluation and assurance that certification is valid, have consistently been terminated during budget negotiations with OMB.
COMPETITIVE ASSESSMENT OF TECHNOLOGY 113 Figure 5-3 A300-600 Application of Advanced Composites Source: Airbus Brochure. Figure 5-4 A320 Advanced Composite Materials Source: Airbus Brochure.
