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3 Discussion and Findings AIRCRAFT DIFFERENCES There are appreciable differences in the structural requirements and usage of the four classes of aircraft addressed: large transports, high-performance military aircraft, rotorcraft, and general aviation aircraft. Large commercial transports are designed to a limit-load factor of 2.5 g, compared to 9 g for high-performance military aircraft. Large commercial transports fly 10 or more hours a day and experience thousands of takeoffs and landings through their lifetime. As a result their pressurized fuselages experience loads approaching limit load thousands of times. High-performance military aircraft fly only 20 to 40 hours a month during peacetime and reach or exceed limit load relatively few timesâin the hundredsâ during their lifetime. The design longevity of a transport is upward of 40,000 flight hours whereas high-performance military aircraft have a design life of some 6,000 to 8,000 flight hours. Rotorcraft, both military and civil, are designed for relatively low limit-load factors of 2.5 g to 3.0 g and are often flown at or close to these limits. The rotorcraft design problem is complicated by the wide spectrum of vibratory loads imposed by different speed regimes and associated design limitations as well as the high degree of maintenance required. General aviation aircraft, the Federal Aviation Administration's (FAA) category for aircraft whose takeoff gross weight is under 12,500 pounds, are lightly loaded and are maintained by an infrastructure that is much different from large transports or military aircraft. Their structural design is dominated by stiffness rather than strength. These factors as well as others lead to structural configurations and design detail that are unique for each of the four classes of aircraft. Thus, for example, it is
basically not practical to scale up geometrically a general aviation aircraft into a large transport or vice versa. Despite these differences, there are similarities in the potential benefits, inhibiting factors, needs for technology development, and possible government actions with respect to advanced organic composite material research and technology. MEASURES OF PERFORMANCE Range and maneuverability are two of the traditional measures of aircraft per- formance. The benefits of a lower structural weight fraction are quantified by the Breguet range and specific excess-power equations. Both of these equations contain only aircraft performance variables. For example, the Breguet equation will show either the increase in range attendant to reduced structural weight for the same gross-weight airplane or the same range for an airplane of less gross weight. Previous advanced composite research, technology, and development programs have focused on improvements in these kinds of aircraft performance parameters. Neither the Breguet range equation nor the specific excess-power equation addresses improved aircraft system capability. Here, for example, structural weight savings can be used for increasing mission capability, such as adverse weather flight, wind-shear warning, collision avoidance, category 3 landings, and air-freight adaptation, and for modifying military aircraft with equipment to cope with increasingly sophisticated enemy defenses. Thus, more and more avionics are being put into all classes of airplanes. Structural weight savings for future military aircraft can be expected to allow multipurpose capability; for example, the same basic airplane could be called upon to fulfill attack, air defense, and interdiction missions. Additionally, stealth, a future requirement, places special demands upon the application of organic materials. For civil aircraft, structural weight savings can be translated into reduced direct operating costs resulting in lower passenger seat-mile or cargo ton-mile costs. Structural integrity directed at providing greater absolute safety is another evolving factor that requires increased attention to design detail. An example is the recent addition of the damage tolerance concept to the federal aviation regula- tions. This new regulation could result in more structural weight as well as many more engineering hours for design and testing. These aircraft system requirement trends tend to increase takeoff gross weight, although the traditional performance requirements (measures), such as range, takeoff distance, altitude, and cruise speed, remain the same or call for improvements. Unless new technology is forthcoming, these more capable aircraft will be larger, heavier, and require more propulsive power, thereby becoming less productive. It is for this reason that advanced composites of all kindsâmetals as well as organics and combinationsâhave a unique future role. They can provide the designer with the ability to reduce structural weight significantly, allowing the addition of safety and operational improvements while holding aircraft to reasonable sizes and gross weights.
ADVANCED COMPOSITES AND ADVANCED STRUCTURES Advanced composites coupled with various, possibly new, structural concepts will further reduce the structural weight fraction of the airframe. The enhanced reductions can then be used by designers to provide aircraft system improvements beyond those available through material improvement alone. New, higher-performing aircraft will be smaller and more productive for the same mission. At a minimum, for example, these aircraft will takeoff and land from the same airports or aircraft carriers, use the same gates at airports, cruise at the same altitudes, and have the same or greater operational capability. For the same gross weight, they will have greater range and/or operational flexibility. Through new design with lower structural weight, they may be able to perform entirely new missions. Cost Issues Every constituency (transport, fighter, rotorcraft, and general aviation) and every government agency (NASA, Army, Air Force, Navy, and FAA) listed cost as a major inhibiting factor to the more widespread application of advanced composites. Early in the development of advanced composites, system "effectiveness" was prom- ulgated as the justification for using a material that cost $100 or more per pound. Aluminum alloys could be purchased for $1 or $2 per pound. Although significant reductions in cost have been realized, there is still an order- of-magnitude difference in the cost of carbon-epoxy compared to aluminum. Some consider material cost not a dominant cost factor. However, material cost is impor- tant in commercial aircraft and a concern in military aircraft. At present, cost issues run the gamut from materials to certification, tooling, and other facets of manufac- turing as well as the retraining of engineers and shop personnel whose expertise is in metal technology. Manufacturing costs are identified as a significant cost. This involves not only the placement but the distribution and processing of material to optimize manufacturing from cost considerations. While grappling with the wide range of issues associated with costs, the commit- tee noted that many people believe that costs play a dominant role in the selection of the technology used in a new aircraft design. There is some concern that system costs have been used as an argument for inaction, both with respect to the development of advanced composites and the development of new airplanes using composites. If all other factors were the same, lower costs alone would encourage the fuller use of advanced composites. But these factors are not the same. The committee found other significant technical inhibitors to the use of advanced composites, inhibitors that can be overcome by basic research and technology development. Other Inhibiting Factors Presently, designers cannot design complete composite structures with the same
level of confidence with which a metal structure can be designed without planning for extensive testing. The composite designer has neither the comparable metallic data base nor methodology to address fully such structural integrity factors as strength, longevity, damage tolerance, lightning strikes, and durability. There is extant a very large investment in machine tools to fabricate metal components as well as a work force with years of experience in "cutting" metal. The lack of an engineering data base in conjunction with an immature manufacturing capability tips the scale toward metal technology and/or forces designers to be so conservative that the true potential of advanced composites is not realized. Also, the owners/operators of composite aircraft have concerns with respect to serviceability, maintenance, and repairability because of the relatively narrow service experience with advanced composites. Government R&T Role The committee recognizes the need for tough budget decisions. These decisions, in particular, have adversely affected the levels of funding available to NASA and the other government agencies for their aircraft structures' advanced organic composite research and technology development (R&T) activity. The result, in the view of the committee, has been a general sense of drifting in the NASA program resulting, in particular, in a loss of R&T program leadership. The committee believes the nation cannot afford this loss. There is an important role for NASA and the other government agencies to play in providing resources for needed R&T, in coordinating the attack on the factors that inhibit the beneficial application of composites and in assisting the United States in retaining a leadership role in aeronautical systems development and sales. Regarding the role of government in future technology development, the com- mittee agrees with earlier studies that the government has a vital role in aeronautical R&T, including advanced composite material for aircraft structures.* This unique role stems from the importance of aeronautics R&T in social, economic, and defense affairs and from the diverse nature of the industry itself. Industry cannot provide (and cannot be expected to disseminate among itself) the technology developments needed in industry for design, development, and manufacture, and by government user agencies (U.S. Department of Defense and FAA) for advanced aircraft system specification, definition, and certification. The advanced composite material R&T addressed in this report has been identi- fied as important to aeronautical developments through the year 2000 and beyond.13'15 It is particularly important to the first of the three major aeronauti- cal R&T policy areas (subsonic, supersonic, and transatmospheric) identified by the President's Office of Science and Technology Policy (OSTP)1'14'17 in their studies *See items 1, 9, 12, 13, 14, 15, and 17 in the bibliography listing. The following document, published after the work of this study was completed, also relates to the role of government in research, technology, and development: "National Aeronautical R&D Goals: Agenda for Achieve- ment," Executive Office of the President, Office of Science and Technology Policy, Washington, D.C., 1987.
of aeronautical R&T policy. The subsonic goal (to which most of the committee's comments apply) identified by OSTP notes that the United States should Build trans-century (civil) renewal through new technology, affordable aircraft, a mod- ernized air space system, and key technology advances for 1995 readiness. This activity will support military aircraft development and supersede foreign technology challenges. Although the committee did not address the details of a possibile R&T program, the committee firmly believes that the appropriate government agencies should do so, led by NASA. The effort should be aimed at understanding the fundamental knowledge needed to build composite aircraft structures for the twenty-first century. This planning, of course, must include consideration of advanced metals and metal- composite mixes. FINDINGS In summary the committee has arrived at the following major findings: 1. Technology MaturationâAdvanced organic composites need to proceed through a technology maturation phase that includes manufacturing. The tech- nology has reached an application plateau far below its potential height. An order- of-magnitude increase in resources devoted to the development of basic knowledge, requiring both analyses and experiments, is justified, in the view of the committee, on the basis of the aircraft performance and cost gains to be realized. 2. National NeedâThe sale of aircraft is presently the major contributor to a positive balance of payments for industrial products, but foreign competition is becoming stronger. Looking to the year 2000, aircraft primary structural weight can be reduced by some 20 to 25 percent and possibly by as much as 50 percent compared to an all metal structure. Costs can also be reduced by this magnitude, providing the United States with a competitive posture in aircraft sales against strong and growing foreign competition. 3. Technology PotentialâAdvanced organic composites are an enabling tech- nology for achieving the nation's subsonic goal of transcentury leadership in subsonic aircraft. This is a primary technology for allowing significant reductions in structural weight fraction. 4. Weight-Saving ImplicationsâApplications of advanced organic composites have verified the predictions of lower structural weight, and the performance ad- vantages of reduced structural weight have been demonstrated. Advanced organic composites have been and will continue to be used to improve aircraft range and takeoff gross weight through weight saving. A lighter structure permits the addi- tion of fuel for greater range or airplane downsizing to achieve the same range and payload or to allow new capability. 5. New CapabilityâThe unique characteristics of advanced organic composites make it possible to build new types of aircraft such as highly maneuverable, high altitude, vertical and short takeoff and landing vehicles and enabled the realization of the around-the-world Voyager, which, in all probability, if constructed of metal
10 would not have useful range and payload. The ability of the designer to tailor structural properties, for example, makes possible the design of structurally efficient forward swept wings while avoiding serious aeroelastic problems, and to fabricate unique structural shapes and configurations. Organic composite material may offer an opportunity for enhancing the low observable characteristics of military aircraft. 6. Flight SafetyâGreater flight safety can be achieved by using some of the reduction in structural weight fraction to increase current levels of structural crash- worthiness and to accommodate increasing amounts of avionics for providing such capability as blind landing, collision avoidance, wind-shear warning, and fault toler- ant control. 7. ProductivityâGreater productivity is also possible for civilian and military aircraft. For the military, the structural weight reduction can be used to increase payloads, whether passengers or cargo, for transport aircraft, or to allow an aircraft to serve dual functionsâair superiority and attack. 8. Lower-Cost ManufacturingâThere is the potential, while largely unproven, of significant cost gains through low-cost manufacturing using such techniques as filament winding, protrusion, and hot forming, as well as integrated-structure fabri- cation of fuselages and wings. Reduced costs here will remove an application barrier and enhance the competitive position for U.S. aircraft. 9. SupportâIssues pertaining to maintenance, serviceability, repairability, and supportability will require continuing diligence but do not appear to be insurmount- able. There are some nagging concerns about repair, nondestructive evaluation techniques, and environmental effects, but the recommended R&T should help allay and resolve these concerns and lead to an improved ability to apply composites. 10. Inhibiting FactorsâA partial list of factors that inhibit the more aggressive application of advanced organic composites, and need to be resolved, are: (a) a small data base, much smaller than available for metals, e.g., there is no document comparable to MIL Handbook 5 for composite materials due to the difficulty of producing appropriate data. In general, the design data base must be larger for composites due to material anisotropy and the lack of well-defined failure theories. (b) the relative lack of knowledge of the behavior of mechanically fastened joints, (c) a concern in some quarters about the lack of reliability of bonded joints and sandwich construction, (d) a much less complete and poorer understanding of fracture and failure modes and behavior under cyclic loads, especially for rotorcraft, e.g., there is no an- alytic methodology (discipline) for composites comparable to linear elastic fracture mechanics for metals, (e) a lack of verified methodologies, based on the physics of filamentary com- posite structural behavior; composite designers are not able at this this time to design with the same degree of confidence for, longevity, damage tolerance, durability, and other aspects of structural integrity including fracture as they can with metals; as
11 an example, fracture toughness, a rigorously denned and measurable characteristic of metals, is not well denned nor is there an agreed-upon, measurable characteristic for advanced organic composites, (f) high production costs requiring improved manufacturing technology, (g) the adverse effects of lightning strikes on structural integrity, and (h) the potential for smoke and toxicity from fires. 11. Technology ApplicationâThe technology in this study, while restricted to ad- vanced organic composites in support of the subsonic national aeronautics goal, will support the other national aeronautical goals, the supersonic cruiser, and the trans atmospheric vehicle. One example, the organic composite methodologies to assess fracture, longevity, damage tolerance, and durability will provide the foundation for the methodologies to address the additional complexities of the high temperatures of high supersonic and hypersonic flight. These methodologies would be generally applicable to matrix materials other than organics and may offer attractive potential for high-temperature structures, i.e., metal matrix and carbon-carbon. 12. Large-Scale TestsâLarge-scale tests of composite structures are considered essential to the full development of composite technology. Such tests provide impor- tant information related to design, tooling, manufacturing, and testing. However, for a given program of necessity the data are restricted to selected materials and a selected structural design and do not extrapolate easily to the broad range of composite materials and structural configurations available to designers. Thus, to be effective, technology development programs need to address composite built-up structural elements as well as components. The committee believes that the technical issues identified above can be resolved through appropriate R&T. Cost is an issue but it is not separable from the technical issues. The committee believes affordable aircraft will be forthcoming if its recom- mendations for R&T are implemented. A major potential barrier is an attitude in government circles that government support is no longer necessary or justifiable. The committee does not agree with this position. The committee concludes that the government must consider the development of a new advanced organic composite R&T structures program for aircraft.
