With automated lay-up, they were produced in less than 1 man-hour per pound. Recently, one was designed to be built with composites. Metal matrix composites are, as might be expected, formed under various combinations of high temperature and pressure, and ceramic matrix composites, such as carbon-carbon, by infiltration processes such as chemical vapor deposition in a vacuum. C. Monocoque structure. to provide such data with the objectives of predicting responses to all pertinent types of loadings and states, and the total lifetimes for the structural components of interest. If well-defined and accepted methods and criteria for demonstrating airworthiness compliance are lacking at the time of aircraft development, factors of conservatism are likely to be imposed which are so large that the advantages of improved materials or structural concepts are lost. Composite materials and structures fabrication techniques constitute a major area of uncertainty for the aircraft of the future. Structural concepts that minimize parts count and can be automated are essential. This should include consideration of how compliance with airworthiness regulations can be demonstrated on a practical basis during aircraft certification programs. It is an important factor in community acceptance. An effort to develop quantitative methods for nondestructive evaluation of composite structures is clearly needed. Plate or shell-like components with polymeric resin matrices tend to be "laid up" from tape or fabric that may have been preimpregnated with resin or, in the case of thermosets, have had the resin applied "wet." If, for example, a wing or wing section can be made to adapt its shape to maximize aerodynamic performance or minimize load regardless of flight regime, this could be a significant advantage—particularly if it were to reduce the number of moving parts. Equally important is their promise for active control of internal noise and for reducing structural dynamic loads, stabilizing various aeroelastic phenomena having the potential for destructive instabilities, and improving crew and passenger comfort by reducing vibrations. It is noted, however, that before any diagnostic means for increasing structural integrity can be useful, the damage tolerance of composite materials needs to be increased substantially. - main structure or body of aircraft to which all other components are attached - provides space for crew, passengers, cargo and other equipment. Improvements in carbon fiber reinforcements for polymer matrix composites are expected to continue, based on the efforts of various suppliers; government research programs in this area are not likely to be required. They generally follow the technology requirements defined in the studies being conducted for the NASA High-Speed Civil Transport (HSCT) program. Materials and structures technology needs for subsonic commercial transport aircraft are outlined in this section. Although more experience exists with MMCs than CMCs, both are in their infancy with regard to large-scale application. For CMCs in which the matrix modulus is high relative to the fiber. Fibers can be entirely of one constituent material or used in combination. This requires an understanding of various crack geometries. Reduction in size, weight, and cost of the components constituting these systems, through fiber optics, microprocessors, and smart material sensors and actuators, will allow the redundancies necessary for operations in keeping with commercial transport safety standards. These advances could lead to their widespread use. Such techniques should also allow for choice among multiple static and dynamic analysis options (e.g., transfer matrix, finite element, and boundary integral methods) in unified procedures that ensure the balance between efficiency and accuracy at various design stages, which is requisite for application of these analyses to realistic designs. Not being able to unbuckle, the aircraft crashes, as a result of structural failure. The bore of the disk is primarily stressed in the circumferential, or ''hoop,'' direction. Uniformity of the inside surface, with tooling on the outer surface, cannot be counted on to provide good surface-to-surface conformity and, in the case of precured substructures, clamp-up stresses can cause cracking of the substructure matrix around fasteners. The use of sandwich construction is desirable, because it is very efficient and provides good heat insulation for the cabin area. This might reduce wing weight by 35 percent, since tiltrotor wing structures are sized for whirl flutter torsional stiffness requirements. Principal Structural Elements. One pound added to structural weight requires additional wing area to lift it (all other flight variables being held constant), additional thrust to overcome the associated incremental drag, and additional fuel to provide the same range. Such applied research, specific to vehicle classes discussed in other parts of this report, is dealt with in subsequent sections of this chapter. NASA should have an important role in bringing about this cultural change by conducting innovative structural design and manufacturing research for both airframes and engines in a program conducted jointly with industry. Improved resonance stress prediction capability is also needed for such advanced designs. The increased use of composites and the combining of materials should make airmen vigilant for wings spars made from a variety of materials. Manufacturing technology programs conducted for composite structures by both the Air Force and NASA have proved to be of great benefit to our national competitive position. Although more are in service, today only two commuter types are being manufactured in the United States, the 19-passenger Beech 1900 and the Fairchild Metro. The stress field in a disk varies considerably from the bore inner diameter to the outer rim area that retains the blades, due to the effects of centrifugal loads and radial thermal gradients. NASA should lead a coordinated national program to address longevity and durability issues for composite structures. The mean time to unscheduled removal to depot was increased from 800 flight hours with metal rotor blades to 10,000 hours with composite blades. Fan exit frames, for example, incorporate large-diameter rings interconnected by aerodynamically shaped struts that, in turn, are attached to inner rings forming the flow path for exit of the engine fan stream. In either case, it is essential that the engines satisfy low nitrogen oxide (NOx) requirements. A very wide range of maximum temperatures and a wide range of specific strength requirements will be encountered, depending on which part of the nozzle is considered. applications. These result in skins that are too thick to be good candidates for sandwich construction. Aeroelasticity considerations in fan blade design continue to pace the technology. Low-weight composite and/or superplastically formed metallic airframe structures, with costs substantially below those of aluminum structures, could provide a competitive edge, helping U.S. manufacturers to compete in the short-haul market. Aircraft EWIS Practices Job Aid 2.0 UNCONTROLLED COPY WHEN DOWNLOADED 10 Federal Aviation Administration 10 Aircraft EWIS Best Practices Job Aid 2.0 Coil and Stow In-Service Example / Result This slide shows an example of improper termination of unused wires. Because finding an effective means to seal sandwich panels has been a particular challenge and concern, an evaluation of existing edge and surface sealing methods. Boeing helicopter operational experience with composite honeycomb rotor blade structures on U.S. Army aircraft has been excellent. Stiffness and strength at moderate temperature are required to carry the heavy ''hoop tension'' created by centrifugal and thermal loads, and light weight is always particularly important in rotating machinery. Powder metallurgy technology is another area in which continued research efforts are warranted. Thus, it appears that with proper design, remarkable cost reductions can be realized in composite part production by introducing automation to replace hand lay-up. The commercial fleet today is made up primarily of high-bypass ratio, turbofan-powered aircraft, whereas the next generation of commercial aircraft will be powered by advanced ducted engines characterized by very high bypass ratios. The advantages of composite materials, as exemplified by their greater strength and stiffness per unit weight, superior fatigue and corrosion resistance for many applications, and potential for lower manufacturing costs through reduced part counts and tooling expenses, make their wide application to U.S. aircraft designs a compelling need. The associated propulsion systems in the 2000–2020 time frame have no substantial materials and structures problems that differ from those of other subsonic aircraft. The basic structure of an aircraft and is designed to withstand all aerodynamic forces, as well as the stresses imposed by the weight of fuel, crew and payload. However, for maximum benefit in case applications, the details of the design and the orientation of fibers may well require specialized development. These needs will also require innovative solutions by the structures community. Adhesive bonding of aircraft primary structures has been in use for over 50 years and is still in use on current aircraft projects as a direct alternative to riveting. Reduced susceptibility to corrosion when moisture invades core voids offer another. A fundamental aspect, of course, is knowledge of the physical properties of these materials. All others are of foreign design and manufacture. Similar advantages appear in lifting surface manufacture. A typical early form of this was built using molded plywood. These may encompass, for example, stabilizing aeroelastic phenomena, internal noise suppression, and rotorcraft vibration reduction. NASA should lead the nation, with program levels reflecting the importance of noise to civil aviation, in aeronautical acoustics research to (1) improve the understanding of its sources; (2) accumulate the knowledge required for application of noise control methods; (3) develop analysis methods for predicting noise generation and propagation and for evaluating noise reduction methods; (4) improve understanding of human reaction to noise; (5) arrive at reasonable criteria; and (6) develop active noise control techniques to the point at which reliable trade-offs can be made at the design stage. Structural weight is the single largest item in the empty weight of an aircraft and is, therefore, a major factor in the original acquisition and operating cost and in establishing operational performance. Aircraft structural members are designed to carry a load or to resist stress. NASA should play a major role in developing adaptive or smart structure concepts. Materials with high specific damping, capable of functioning at moderate and high temperatures, are required to ensure inlet and exhaust structure durability and reduce noise transmission. This approach will produce a thinner disk bore with a faster thermal response characteristic, thereby reducing the critical bore-to-rim transient thermal gradient and associated stresses. Engine efficiency improvements will require compressor exit temperatures higher than 1300°F and maximum turbine temperatures (uncooled) of more than 3000°F. The current methods used by the airlines to repair damage to aircraft composite structure (secondary structure and primary flight controls) depend on the extent of damage, the time available to perform the repair, and the time until the next scheduled maintenance visit. Benefits of Research and Technology Development in Structures and Materials, Aircraft and Engine Design and Development, Improved computational capabilities for materials and structures, Improved testing facilities for materials and structures. To bring this about, it will be necessary to create a technology base to improve ceramic and CMC material reliability and producibility, while developing the concomitant design methodologies and life prediction systems. Essential to the realization of reduced manufacturing costs with composite structures is a reduction in labor costs sufficient to offset higher materials costs. In both areas, NASA should pursue means of reducing manufacturing costs—particularly as regard to new techniques of joining, both in built-up rotor stages and in joining blades to disks—and ensuring long-term reliability, as well as increased temperature capabilities and reduced weight. The primary function of the engine is to provide the power to turn the propeller. Each of these aspects must be considered and dealt with concurrently if modern structural designs for aircraft are to approach optimum configurations and, thereby, success in international, commercial competition. The Boeing Model 360 research helicopter demonstrated a large cost reduction over equivalent metal semimonocoque construction by using sandwich composite structure and wide spacing of stiffening members. The introduction of metal matrix composites into high-pressure compressor disks deserves major emphasis in NASA's engine programs for the nearer term. You are actually just avoiding the question. New systems will also incorporate electric starters/generators, on the high-speed rotor and feature all-electric accessories. Improvements in silicon carbide fiber capability are needed to increase high-temperature strength retention and composite structure creep resistance. The HSCT is a high-performance aircraft in which weight is a key factor. Not a MyNAP member yet? Several materials are available for compressor blades. Research by NASA emphasizing composites with discontinuous reinforcements is recommended, based on the belief that such materials are likely to simplify fabrication. Used in the wings of aircraft, the bodies of performance automobiles and sporting goods. Close and frequent contact between NASA materials and structures researchers or technologists and airworthiness engineers from the FAA is clearly required and could pay great dividends in the speed and efficiency with which new materials and structures approaches can be put into production and service. Materials and structures research and development effort in support of the HSCT needs to be directed toward. The longevity requirements of commercial products will typically be 15,000 hours for cold-section parts and 20,000 hours for hot-section parts. Continued research into metallics is strongly recommended, emphasizing tailoring of alloy systems to provide significant advances in such traditional areas as weight reduction and environmental resistance. The use of high-speed, large-memory computers permits, in turn, more detailed internal structural loads analysis for each of the many loading conditions and design alternatives, with fine grid analysis determining more precise load paths, stress distributions, and load deflection characteristics for subsequent aeroelastic analysis. Frames were placed only where major loads entered the structure, resulting in frame spacings up to 6 feet. This is yet another example in which effectively integrating structural design efforts with both. The rotor gearbox transmits all flight loads from the rotor to the airframe. constituting from 40 to 60 percent of the airframe weight (AV-8B and V-22, respectively.) Weight was reduced by 20 percent and cost by 10 percent, compared with the metal design it replaced. Aerospace industry - Aerospace industry - Secondary and tertiary aerospace systems: The secondary product line of the aerospace industry comprises the numerous onboard subsystems required by the designs of the various flight vehicles. However, no such programs exist for civilian. Graphite/epoxy, for example, is a brittle material. This class of materials is, in general, very large; it includes polymer matrix, metal matrix, and ceramic matrix composites (CMCs), as well as continuous and discontinuous fibers. (13) Replacing side windows where that work does not interfere with the structure or any operating system such as controls, electrical equipment, etc. This applies to acoustic sources of all kinds—aerodynamic, propulsive, and those generated by dynamic system components—and to both interior and exterior noise. Materials processing is a critical part of advanced CMC development, and it must be addressed concurrently with combustor materials selection and evolution of the design. In an aircraft structure, shear is a stress exerted … The challenges resulting from this trend involve higher rotor speeds, smaller disk bores, restrictions on maximum low-pressure shaft diameters, and very high-speed bearings. Some seven to eight years of testing is required just to validate the 60,000-hour life capability of a material under HSCT airframe thermal and mechanical loads and real-time temperature conditions. D. Semi-monocoque structure… This includes a highly reliable structure that requires minimum maintenance and is durable under all applicable environmental influences. The tool concept developed for the Airbus fin by the German firm MBB bonds precured ribs by cocuring rib shear ties to the skins. A. The materials systems being considered currently have low ductility in general and, thus, may be difficult to fabricate. Better metals, new families of engineered materials, and techniques for achieving aeroelastic stability and vibration reduction, including smart structures, all have sufficient promise to demand attention. Since turbine-powered aircraft entered commercial service, temperature capability at the turbine inlet has been increasing steadily. First, the current cost of producing composite structures is on the order of two to three times that of comparable metal designs; second, durability, maintenance, and repair present a number of uncertainties that could appreciably affect operating cost. In single engine aircraft, it also houses the powerplant. Thus, hybrids that incorporate fibers with high failure strains should be pursued to achieve higher damage tolerance. The simplicity of the structure produced an 86 percent reduction in the number of parts and a 93 percent reduction in the number of fasteners. In addition, special developments in the inlet, combustor, and exhaust nozzle are required for the HSCT. Most likely, a major breakthrough in resin technology will be required to achieve the combined technical performance with the ease of fabrication necessary to produce cost-effective airframe structures. a lead role in stimulating innovative structural design and manufacturing research for both airframes and engines in a program conducted jointly with industry. A successful, economically competitive structural design will involve a combination of materials in the airframe. Additionally, virtually all repairs (other than straight parts replacement) on transport aircraft toÂ Principal Structural Elements or Primary structures are classified as Major Repairs thus necessitating the use of Approved rather than Accepted data to accomplish the repair. The concept is particularly applicable to composite structures, because the necessary network of sensors can be embedded during the manufacturing process. Integrated analysis techniques that couple structural, thermal, dynamic, aeroelastic, and control technologies are required to truly optimize a design. Standardization of test techniques unique to composite construction should continue to be pursued. It is important to note that CMC development has the potential to be one of the highest-payoff materials programs for advanced engine systems. These include the possibility of panel flutter, large temperature gradients across airframe structures during acceleration and deceleration, and very thin wing sections. Replacing skin-stringer construction with sandwich skin. Various combinations offer differing advantages, depending, for example, on the thermal environment (Figure 9-1). In addition to the environmental aspects of noise reduction, techniques must be developed for dealing with the acoustic loads produced in inlet and exhaust structures. To make further significant increases in overall temperature capability, even greater increases in bulk material temperature capability must occur. An important technological development for the future of composite structures, whether sandwich panels or integrally stiffened skin panels, is the incorporation of crack stoppers. Microalloying and particulate reinforcement are promising approaches to make ingot aluminum alloys satisfactory for certain HSCT applications. Is is in contrast to the techniques for aircraft primary structures such as ultrasound and array. Processing, microstructure, and CMC-type materials these areas, however, large gradients. 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2020 aircraft primary structure example