Aluminum-lithium (Al-Li) alloy systems, for example, promise evolutionary benefits in higher stiffness and lower density, with no reduction in structural life. Alternative nozzle designs being considered for the HSCT all represent a significant percentage of the installed propulsion system weight. The need to incorporate noise suppression treatment in these structures will continue in the future. Operating at extremely high temperatures over such a long portion of the flight reinforces the need to develop high-temperature materials for the HSCT combustor. 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. An economic objective of the HSCT program is to achieve an airframe weight reduction of up to approximately 30 percent relative to Concorde-generation designs. Metallic alloys continue to be used for more than 75 percent of most airframe and propulsion systems by weight. 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. Graphite/epoxy, for example, is a brittle material. Both external and internal noises are matters of concern with unducted, so-called ultrahigh bypass fan propulsion systems. This is much lower than the number of man-hours expended for metal parts. Commercial transports use advanced composites in essential secondary structures such as flaps and control surfaces and in some primary structure such as vertical fins. Various combinations offer differing advantages, depending, for example, on the thermal environment (Figure 9-1). Hybrid materials such as those having combinations of glass and graphite reinforcements show significant improvement in tensile fracture properties versus solely graphite-reinforced laminates. 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. Composite materials and structures fabrication techniques constitute a major area of uncertainty for the aircraft of the future. No similar capability exists for civilian aircraft. 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. 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. As an undergraduate studying aerospace engineering, I have to say this blog is a great resource for gaining extra history and Materials and structures technology needs for subsonic commercial transport aircraft are outlined in this section. Its cost, however, was marginal for production use at approximately 25 man-hours per pound. This class of design problem is particularly important for high-temperature applications. Disks and/or drums make up a major portion of high-pressure compressor weight. In the current metallic aircraft fleet, particular concerns are disbonds in fuselage splice joints, fatigue cracks in riveted splice joints, and airframe corrosion. It is important to note that CMC development has the potential to be one of the highest-payoff materials programs for advanced engine systems. First, fundamental test information is needed from which materials constitutive relationships can be developed that lead to reliable structural models of failure mechanisms. Experience with optimization methods to date indicates that the state of these procedures requires fundamental research and that successful application can establish major competitive advantage in the marketplace. Although fracture mechanics technology has existed for years, continued advances in understanding and capability are needed, including the ability to analyze the stress field in, and resultant fracture of, structures with multiple-site damage. Use our Career Test Report to get your career on track and keep it there. IMCs will make up many other parts of the structure. Polymer matrix composites research appropriately deals with both the constituent materials and the way they are combined to form composites. An element of growing importance in this area is continued airworthiness over the life of the aircraft, because the useful lives of aircraft have increased greatly in recent years. CMCs capable of operating to 3000°F are likely candidate materials for the combustor. For example, when an airplane is on the ground, the landing gear struts are under a constant compression stress. For propulsion systems, higher specific strength and ability to withstand higher temperatures are the principal drivers. The tool concept developed for the Airbus fin by the German firm MBB bonds precured ribs by cocuring rib shear ties to the skins. should be the development of generic design concepts accommodating combinations of materials with mismatched thermal coefficients of expansion. Research is needed to increase the reliability and efficiency of NDE techniques, such as ultrasound and phased array imaging. An important technological development for the future of composite structures, whether sandwich panels or integrally stiffened skin panels, is the incorporation of crack stoppers. Special wing configurations are likely in an advanced HSCT to minimize sonic boom footprints and provide laminar flow control. Current aeroelastics technology leads to first-stage blades with lower aspect ratios than desirable based on weight goals. This technology began, in one sense, with the so-called control-configured vehicle concept and has grown to include compliant materials and structures combined with embedded sensor/processor/actuator systems. In wet-wing applications of sandwich skins, there is concern about fuel seepage into core voids, certainly more concern than one has about the small amount of water in other structural components. This is an enabling technology for the HSCT. Their temperature requirements are modest, so that polymer matrix composites can be applied. Joining technology for these applications is not currently receiving adequate attention. Nondestructive inspection techniques for laminated composite structures are not well developed in comparison to those for metallic structures. It is not unusual to do this with uncured skins and either a cured or a partially cured ("B-stage") substructure. Substitution of CMCs for metals in engine hot sections is likely to occur in the next decade, and NASA should lead the way. Particular attention should be given to improving the understanding of failure modes in composites, increasing their damage tolerance, and advancing means of nondestructive evaluation. By way of comparison, the cost of an aluminum fuselage structure would be 22 man-hours per pound on a comparable basis (i.e., for the first prototype in both cases). The Lockheed Vega is an example of an early model aircraft with a monocoqne shell structure. The most demanding aspect of an HSCT regarding airframe structure is the fuselage. This includes sensors, sensor placement tailored to the structure, and automated scanning and interpretation of results. Ultimately, a probabilistic approach will be required with regard to operational loads, routine damage in service, and material properties in the delivered structure, to maximize the potential of many of the advanced materials. All advanced composite materials applications to aircraft structures require that design and manufacturing developments proceed hand in hand from the earliest stages. Relatively thin-walled cylindrical components are frequently wound, using continuous filaments or braids. The boxed material summarizes the primary recommendations that appear throughout the chapter, with specific recommendations given in order of priority, and the benefits that can be gained through research and technology development efforts aimed at advanced structures and materials. Right, so an example: An aerobatic aircraft goes into a manoeuvre, the back support bends, moving the pilot away from controls. A greatly expanded design data base of applied loads is now available for more complete and thorough definition of critical design conditions, thanks to the expanding use of computational fluid dynamics (CFD), advanced wind tunnel testing techniques, and increasingly comprehensive aeroelastic and structural dynamic analysis computer codes. Foreign competitors are applying composites and superplastic forming of metals aggressively and are gaining valuable experience in their use in structural design. Commuter aircraft range in size from 19-passenger turboprops to 65-passenger turboprops and 107-passenger jets. Differences in criteria should be addressed by NASA and the FAA to the extent that safety and reasonably competitive positions are ensured. Research in these areas, however, should be a continuing part of NASA's base program. In addition, special developments in the inlet, combustor, and exhaust nozzle are required for the HSCT. Damage tolerance of these materials—particularly hybrids—is not as well understood and is an area of high potential payoff. The compatibility of desired fiber/matrix volume fraction, resin viscosity, preform density limitations, and fiber wet ability are principal problems of the kind that injection cures encounter. In addition, active rotor controls can reduce vibrations generated by the rotor of tiltrotor aircraft in cruise flight, which are caused by rotor operation in the wing's nonuniform flow field. 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. 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. Additionally, the materials system selected for combustors must have good high-cycle fatigue resistance to withstand significant acoustic and vibratory loads. © 2020 National Academy of Sciences. I.e. Economics dictate that this industry concentrate on materials research and development for applications of the largest scale. At higher Mach numbers, materials with a 300–350°F temperature capability are required. 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. verifying materials developments, design concepts, and fabrication technology by producing large-scale components that can be subjected to laboratory or, preferably, full-scale service testing. Powder metallurgy also has the potential of producing aluminum base alloys with capabilities to 900ºF that could make them competitive with more costly materials, such as titanium, in both airframe and engine applications. Thus, the materials technology program required to meet HSCT requirements should focus on PMC, advanced titanium alloys, and the development of cost-efficient design concepts for titanium and hybrid laminates. 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. Numerous issues must be addressed such as the basic capability of the embedded sensors (e.g., optical fibers, piezoelastic materials, memory materials), the network of sensors and information carriers necessary, embedding techniques, and on-line analysis and assessment capabilities. Thus, the financial risks undertaken by private companies when they introduce advanced materials and structures into commercial transport aircraft go beyond liability for passenger safety—as important as those ramifications are—and can involve structural maintenance, modification, and repair of fleets worldwide. To a first approximation, the thrust generated, blade radius, helical tip speed, blade area, and number of blades determine the sound pressure levels generated. With automated lay-up, they were produced in less than 1 man-hour per pound. Life management programs generally involve discrete inspection time intervals as determined from various analysis techniques and design philosophies. Examples of Principal Structural Elements typically include: (a) Control surfaces, slats, flaps, and their mechanical systems and attachments (hinges, tracks, and fittings); (e) Skin or reinforcement around cutouts or discontinuities; (a) Circumferential frames and adjacent skin; (e) Skin and any single frame or stiffener element around a cutout; (f) Skin or skin splices, or both, under circumferential loads; (g) Skin or skin splices, or both, under fore and aft loads; (i) Skin and stiffener combinations under fore and aft loads; This site uses Akismet to reduce spam. It is an important factor in community acceptance. Active, higher harmonic rotor control, including the possibility of individual blade control, can reduce helicopter and tiltrotor vibration and rotor noise caused by blade-vortex intersections. Programs dealing with aircraft structural integrity, fleet structural management, and aircraft life cycle management and operation are important contributors in this regard, but technology advances are needed in each of these three parts of life management programs. In other respects, combustor materials needs for HSCT are similar to those of advanced subsonic commercial transport applications. 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. This background of good experience accumulated by Boeing Helicopters and others with composite honeycomb sandwich structures is still apparently unable to overcome resistance to its widespread use on the part of a large segment of the industry. 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. PSE’s are those elements of primary structure which contribute significantly to carrying flight, ground, and pressurization loads, and whose failure could result in catastrophic failure of the airplane. 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. The introduction of metal matrix composites into high-pressure compressor disks deserves major emphasis in NASA's engine programs for the nearer term. The metal blades had aluminum honeycomb structure aft of the spar, and the composite blades had NOMEX® honeycomb in the same application. All test panel failures were within the scatterband of the original, hand lay-up fabric design. This will require analytical methods for predicting noise generation and propagation characteristics reliably, as well as research on human reaction to noise, including sonic boom. In single engine aircraft, it also houses the powerplant. Competitive designs for advanced rotating parts will depend on such exploitation and on improved understanding of flutter and resonance stress problems and application of magnetic bearing technology. There is also a need for extensive data bases adequate to ensure substantiation. 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. Both aluminum and titanium matrix composites with silicon carbide type reinforcements (particulate, fiber, ribbon), for example. Their impact, taken together with applications of automatic feedback control techniques, particularly in providing solutions to aeroelastic instability problems, will be continually increasing. Integrating the disciplines of material sciences, mechanics, structural, design, and manufacturing process development will be essential to the success of this enabling technology. The weight savings possible with composite structural materials are limited by inspection capabilities and damage design criteria. Technology advances in materials and structures applicable to commercial transport are, for the most part, transferable to other subsonic aircraft systems. Adjustment normal to the surface of the position of skin surfaces, with rib height, prevents prestressing at assembly. Although not as high as those routinely experienced by engine hot-section parts, portions of the HSCT airframe will be subjected to temperatures beyond all commercial transport airframe experience to this time (except, possibly, the Concorde). The viewpoint taken in this report is that as long as cure of one of the components being joined to another occurs simultaneously with the joining, the part is integrally stiffened. high payoff for other sections of advanced subsonic engines, once feasibility has been established. Not being able to unbuckle, the aircraft crashes, as a result of structural failure. There will be an ongoing need for the evolutionary development of conventional metals for the particular requirements of gas turbine engine applications. However, the magnitude of the potential benefits from these materials for higher-temperature applications, such as uncooled turbine engine components, justifies major research efforts. These prohibitions were based on experience with poorly executed designs. Advanced joining techniques should be exploited to eliminate conventional but inefficient dovetail attachments and to exploit more fully the capabilities of advanced blade and disk materials. These needs will also require innovative solutions by the structures community. Components with roughly equal three dimensionality are candidates for woven preforms of fiber that may later be injected with resins in a liquid state. Improvements in engine noise for commercial high-subsonic transport aircraft have reached the point, thanks largely to higher bypass ratios and duct absorption systems, at which noise generated by the airframe is an important consideration. Also, more robust joining procedures are needed. focused technology programs in materials and structures to address specific aircraft system requirements (e.g., subsonic fixed and rotary wing, and supersonic transport aircraft). The materials systems being considered currently have low ductility in general and, thus, may be difficult to fabricate. Although the drive for a low structural weight fraction places PMC materials in the lead role, advanced titanium is competitive in compression applications. Applications where weight savings, fatigue life, and corrosion resistance override cost considerations have been limited VTOL and combat aircraft. Expansion of structural synthesis, analysis, and testing capabilities and the widening options available are making the choice of materials for both the airframe and the engine one that is intrinsically woven into the structural concept, detailed part design, and manufacturing process selection. It was build with molded plywood, featuring multiple layers that cover a plug in the mold. used on aircraft honeycomb structures and of additional sealing methods is necessary to identify and substantiate the best sealing method for any application. Second, NDE is an area of great need and promise. Carbon-carbon composites have high specific strength and stiffness and adequate temperature capability, but exhibit poor oxidation resistance uncoated. procedures now make substantial advances possible. This will be especially important as new failure theories are developed consistent with the way composite materials behave. They generally follow the technology requirements defined in the studies being conducted for the NASA High-Speed Civil Transport (HSCT) program. Frames were placed only where major loads entered the structure, resulting in frame spacings up to 6 feet. Damage tolerant structure. Thus, a successful airframe and engine structural design/manufacturing team will cover a spectrum of sub disciplines, consisting of. Means of doing this most cost-effectively need to be investigated. An appropriate program of this kind should be guided by needs that arise in the development of generic aircraft types; it also should, by its results, change the direction of generic aircraft developments. Among the attributes mentioned earlier, low structural weight fraction, long life, and low costs are the principal drivers for the airframe structures of future aircraft systems described in this report. Special attention needs to be given, however, to structures that are so lightly loaded that problems of minimum gauge arise for skin material and for ultralow-density sandwich core material. Alternate rotor hub designs taking full advantage of composites technology for tiltrotors and helicopters promise to significantly reduce drag and weight and improve rotorcraft reliability and maintenance. The increased use of composites and the combining of materials should make airmen vigilant for wings spars made from a variety of materials. Both ceramic matrix and ceramic fiber technologies need to be pursued, along with an emphasis on improving fabrication technology. Rotorcraft vibrations can be reduced through aeroelastic tuning of the rotor, but this very complex procedure has not been entirely mastered. However, recent advances in powder metallurgy aluminum alloys show excellent potential for achieving the higher-temperature capability. Furthermore, composite materials exhibit a number of damage modes, all of which may not be detectable if NDE is limited to one technique. warrant substantial continuing research and development. Automated lay-ups and filament winding are probably unsuitable, so such parts may require more innovative systems of automation. This is yet another example in which effectively integrating structural design efforts with both. Click here to buy this book in print or download it as a free PDF, if available. 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. Flight operations per aircraft average roughly 3,000 hours annually and close to 60 hours per week. Application of composite materials to engine static structures will be highly dependent on the ability to design and manufacture these complex structural shapes and to provide means for determining their remaining life after years of use. Hybrid composite construction does promise the means to do this, with bundles of highstrain-allowable fibers interspersed at intervals among the high-modulus fibers that provide the bulk of structural properties. It is equally important, on the other hand, that appropriate noise information, including subjective response surveys, be available from unbiased authority to help ensure that evolving noise regulations are established on sound technical and environmental bases can be met with practical configurations and without incurring unacceptable costs. In the monocoqne shell structure, the fuselage is designed within the aircraft’s primary structure. 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 design tasks are usually multi-criteria. Here, too, conventional blade attachment "firtrees" must be replaced with mechanical schemes that exploit the directionality of engineered material systems. Structural concepts that minimize part count and can be automated are essential to achieving an economically competitive airframe. Furthermore, the time between conception and application of new structural materials is very long, largely because ultraconservatism must be exercised by responsible structural designers. A paraglider is a lightweight, free-flying, foot-launched glider aircraft with no rigid primary structure. View our suggested citation for this chapter. Uncertainty regarding the integrity of bond lines made outside of cocuring facilities has mitigated against bonded joints. The progressive substitution of ceramics and CMCs for metals in the hot section of aircraft engines could begin late in the 1990s and continue for the next few decades. The form of the precured material, the manner in which it is put together to form the desired component, its "cure," and means of assembly into the final structure all are involved. Click to share on Twitter (Opens in new window), Click to share on Facebook (Opens in new window), Living in the age of Airplanes – National Geographic, SROV – Service Ready Operational Validation. Sign up for email notifications and we'll let you know about new publications in your areas of interest when they're released. 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. Also, you can type in a page number and press Enter to go directly to that page in the book. This program is seen as involving large-scale test components and, preferably, full-scale service testing. Both airframe and propulsion systems could benefit substantially from the high strength-to-weight potential of these more unusual alloy systems. 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