COMPOSITES (is NASA puslapio).doc

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COMPOSITES (I NASA puslapio)

Background

Weight reduction has been a critical goal since the earliest days of crewed flight. Following initialapplications of wood, fabric, and wire for structural components, the aircraft industry made a majortransition to aluminum and all-metal aircraft. As a result of this approach to structural design, moderncivil aircraft are designed with greatly reduced aircraft operating empty weight to achieve a significant

payload to weight fraction that contributes directly to aircraft flight efficiency. n the transition toaluminum components, the industry accepted the significant costs that were re!uired to retool andmodify its manufacturing processes.

n the continual !uest for reduced weight, aircraft manufacturers began to introduce applications ofnonmetallic materials, such as fiberglass-reinforced plastic composites. For e"ample, initialapplications of structural fiberglass parts by Boeing on commercial transports started with about #$$s!uare feet on the Boeing %$% for the radome and small closure fairings. By the time the Boeing %&%was introduced, the application of fiberglass parts had grown to over '$,$$$ s!uare feet, including theradome, wing leading- and trailing-edge panels, flaps, fairings, and control surfaces. Beginning inabout '()#, composite sandwich parts made from fiberglass-epo"y materials were applied to aircraftsuch as the Boeing %#%. *ajor operational issues for composite structures, such as lightning protection,were satisfied by the bonding of aluminum foil on the inner surfaces and aluminum flame spray on theouter surfaces of structural parts. +he construction techni!ue used for composites at that time consistedof tailoring the glass fabric to the re!uired shape, pouring li!uid resin onto the fabric, spreading andsweeping the resin to impregnate the fabric, vacuum bagging the part and tool, and curing in an oven or autoclave. +his wet layup method was very labor intensive.

+he ne"t major advance in composites was a transition to graphite composite secondary aircraft

structures, such as wing control surfaces, wing trailing and leading edges, vertical fin and stabili ercontrol surfaces, and landing gear doors. +he obvious benefits of lightweight, strong composites havehistorically been tempered by issues regarding fabrication costs, potential degradation in characteristicsdue to environmental effects, impact damage resistance and repairability, and potential environmentaleffects of composites following aircraft accidents.

+he transition from manufacturing aluminum aircraft components to composite structures involved thefabrication of filaments of graphite, fiberglass, or u ont /evlar material arranged in a matri" ofepo"y, polyimide, or aluminum. +he filament materials are imbedded in a matri" at specified angles insuccessive layers, and they can develop very high strength and stiffness. otential weight savings comeabout because of the high strength-to-weight and stiffness-to-weight properties of the composite

material. 0ost reductions come about from the fewer number of pieces that make up the componentsand from the fewer number of fasteners re!uired for assembly. +he fabrication of composites wasinitially accomplished with hand layups similar to those used in the fiberglass construction of boats orautomobiles. 0urrently, advanced fabrication techni!ues, including tape placement and stitchingtechnology, are being applied by industry.

1esearch contributions of the 2angley 1esearch 0enter have played a key role in the widespreadacceptance and application of emerging composite technology for both civil and military aircraft.2angley is the Agency3s 0enter of 4"cellence for 5tructures and *aterials in recognition of its longhistory of research into innovative composites, polymers, metallics, and structures for aircraft andspacecraft. By conducting fundamental and applied research with its industry partners, 2angley hasaccelerated the use of composites and the confidence in the safety and economic feasibility of suchapplications.

2angley 1esearch and evelopment Activities

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+he 2angley 1esearch 0enter has been conducting composites research with industry since '(%$. +heinitial impetus for more aggressive focused research on composite structures came in '(%# duringmeetings of industry, universities, and government representatives involved in a project known as140A5+. 6ne of the highlights of the study was the recognition that a major obstacle to large-scaleapplications of composites technology was the high initial costs of introducing the new materials

because there was no large volume of production due to limited applications7 no widespreadapplications were being used because the total cost was too high. +o break this cycle, the 140A5+

participants suggested three approaches. First, components of advanced composites should befabricated and tested under realistic service conditions7 second, the application of composites to newdesigns should be encouraged7 and third, in-depth studies should be undertaken to develop thetechnology and provide databases for designers. n response to these recommendations, 8A5Aincluded composites in its Advanced +ransport +echnology rogram as well as other research involvingspacecraft, engines, and basic research. 4arly leaders of the composite research at 2angley included1ichard 1. 9eldenfels, 1oger A. Anderson, William A. Brooks, :r., ;eorge W. Brooks, 1obert 2eonard,1ichard A. ride, and 4ldon 4. *athauser. /ey researchers included *arvin B. ow, 9. Benson

e"ter, *ichael F. 0ard, :ohn ;. avis, :r., and *artin *. *ikulas, :r.

6ne of the first efforts in the 2angley composites research program was to reduce potential risk and build industry confidence through a series of contracts for the development, fabrication, and testing ofaircraft secondary structures. 5econdary aircraft components are relatively lightly loaded, and notcritical to the safety of flight. 2angley started the 8A5A 0omposites Flight 5ervice rogram in '(%#and installed over


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+he issue of potential operational environmental effects on the behavior of composite materials andaircraft components was the subject of great concern for both airline manufacturers and airlineoperators. +he !uestion of long-term environmental durability for composites was viewed as the majorundetermined issue for widespread acceptance and application. 2ed by 1ichard A. ride, 2angleyresearchers conducted e"tensive studies involving in-flight service e"periences and ground-basedoutdoor e"posures of composite materials at various worldwide locations. +he focus of these studieswas the e"tent of composites degradation due to ultraviolet light effects and moisture gained bydiffusion. ndividual composite panel specimens were mounted in racks and deployed on rooftops ofairline buildings at a number of airports around the world so that ma"imum e"posure to the airportenvironment occurred. +he test panels were deployed domestically at 2angley, 5eattle, 5an Francisco,5an iego, 9onolulu, and internationally at Frankfurt, ;ermany, and 5ao aulo, Bra il. Aftere"posures of either one or three years, individual panels were removed from the racks and shipped to2angley for testing and valuation. At 2angley, the panels were weighed to determine moistureabsorption, and scanning electron micrographs were made to evaluate the composition of the specimen.Fle"ure, compression, and shear stress tests were also performed. 8o significant degradation wasobserved in residual strength tests after


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assigned responsibility for the automotive issues, the epartment of 4nergy was responsible for thevulnerability and protection of power generation, and 8A5A was charged with responsibility todetermine the impact of graphite fibers released from civil aircraft. 8A5A was also charged withmanagement support to 65+ for the program.

1esponsibility for conducting the 8A5A study was assigned to the 2angley 1esearch 0enter under itsirector, onald . 9earth. 1ichard 1. 9eldenfels, irector for 5tructures, then appointed 1obert :.

9uston as program manager of the ;raphite Fibers 1isk Analysis rogram 6ffice. Cnder 9uston3sleadership, a team of about #$ researchers worked for < years7 they ultimately determined that the issuewas not a problem. +he 2angley program investigated the problem in two areas. +he first area was to!uantify the potential problem of using composites on civil aircraft. +he work included defining theways by which carbon fibers could be released in the event of an aircraft crash and subse!uent fire, the

propagation of e"tremely fine fibers away from the fire site, and the vulnerability of electricalcomponents, especially in other aircraft and in the surrounding area. +he second research area, in

parallel with this activity, was to develop materials that alleviate or eliminate the electrical ha ard. +hematerials studies included modifications or changes in the binding system which would prevent therelease of fiber following a fire and the development of nonconductive fibers to replace graphite.

9uston was assisted by deputy program manager +homas A. Bartron, and technical element leadersWolf 4lber, srael +aback, Dernon 2. Bell, :r., 1ichard A. ride, Arthur 2. 8ewcomb, Ansel :.Butterfeild, :erry 2. 9umble, and /aren 1. 0redeur. +he rogram 6ffice sponsored and coordinated '(studies conducted by 8A5A 0enters, private contractors, the aviation industry ?including Boeing,2ockheed, and ouglas@, and other government agencies. +he responsibility of the industry was to

provide data for the analysis with the unstated objective of ensuring they were fully briefed on progressand analysis. 2angley contracts re!uired industry to deliver detailed crash data on every jet transportcrash worldwide. 6ne of the companies ?2ockheed@ then turned the data into statistical rates on the

probabilities of a crash burn incident, including where ?enroute, " miles from a major airport, etc.@,when ?time of day, takeoff or landing@, how ?crash burn, fraction of structure consumed@, and what ?si eof aircraft, fuel load@. +he 2angley team then used the supplied data in its analysis. n addition to its

technical leadership, 8A5A contributed the major funding re!uired ?about E'$ million@ for the in-houseand contracted studies from its own research funds.

+he results of the studies were reported in over =$ technical reports by 8A5A and other agencies. +hescope of activities included probability and risk analyses, outdoor e"periments, modeling of events,visits to potentially susceptible sites including hospitals, and nuclear power plants. n one study, fore"ample, ride directed an investigation of the realistic release of carbon fibers by burning about &= kgof carbon fiber composite aircraft structural components in five individual large-scale, outdoor aviation

jet fuel fire tests that included detailed measurements of the fiber physical and release characteristics.

+he 2angley investigation projected a dramatic increase in the use of carbon composites in civil aircraft

and developed technical data to support the risk assessment. ersonal injury was found to be e"tremelyunlikely. n '(($ and '(>' in three public hearings, a formal 8A5A publication for 65+ ?see bibliography@, and a

presentation to the irector of the 0ivil reparedness Agency ?now the Federal 4mergency

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*anagement Agency@, are regarded as a pivotal and e"tremely significant contribution to the 8ation3sapplication of composite materials to civil aircraft of the '(($s. +he final 65+ report concluded +heeconomic loss risk from the accidental release of carbon fibers is so low as to be clearly acceptable on anational basis and does not justify follow-on work to develop alternate materials.G +he 2angley1esearch 0enter clearly played a key role in eliminating one of the most serious obstacles to the growthand use of composite materials.

Aircraft 4nergy 4fficiency 0omposites rogram

Cnder the sponsorship of the 8A5A Aircraft 4nergy 4fficiency ?A044@ rogram, composite researchand applications were investigated by +he Boeing 0ompany, ouglas Aircraft 0ompany, and 2ockheedAircraft 0orporation with coordination and technical oversight provided by 2angley. +he overallobjective of the A044 0omposite rimary Aircraft 5tructures rogram was to develop and conducte"periments that would lead to applications of composites for small, secondary aircraft components inthe early '(>$s followed by more comple" larger scale structures through the '(($s, with the goal of

providing weight reductions resulting in fuel savings over '= percent.

n '(%>, the 2angley Aircraft 4nergy 4fficiency roject 6ffice was headed by 1obert W. 2eonard, andleadership for the 8A5A 0omposite rimary 5tructures roject 6ffice was provided by 2ouis F.Dosteen. 2angley planned a phased composite development research program that would incrementallylead to the design, fabrication, and test of a large-segment wing and fuselage representative of futuretransports. +he first phase involved secondary structures, including elevators for the Boeing %#%,ailerons for the 2ockheed 2-'$'', and rudder segments for the *c onnell ouglas 0-'$. 2ater,medium-si ed primary structures were created in the second phase of the program, including ahori ontal stabili er structural bo" for the %


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Cse of composite materials on Boeing %)% aircraft.

Cse of composite materials on Boeing %%% aircraft.

n addition to weight reduction for aircraft components ?projected to be from '$ to $ and * -''transports followed. 8umerous applications also occurred for derivative Boeing %


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composite technology led to incorporation of an even higher degree of composites for the Boeing %%%.+he first major use of composites for primary structures of a C.5. commercial transport was for theempennage of the %%%.

+he stimulation of the A044 rogram is believed to have accelerated the application of composites tocommercial transports by appro"imately = to '$ years. 8A5A research and development in the A044era ?'(%= to '(>)@ produced over )$$ technical reports. n addition to technology advances in

performance prediction and manufacturing processes, a significant increase in confidence was obtainedregarding issues such as durability, cost verification, FAA certification, and airline acceptance. +heA044 rogram was primarily responsible for the impact of 2angley contributions to the application ofcomposites to commercial aircraft of the '(($s. 9owever, composites research activities at 2angleythat followed A044, which ended in '(>=, have had a marked influence on the potential near-termapplications of composites in the new millennium. n particular, now that issues regarding theenvironmental durability have been successfully addressed for composites, the focus of research in thelate '(>$s and '(($s has turned to the all-important issue of cost. Whereas military applications ofcomposites have been very aggressive ?for e"ample, the F-## has about percent of its structuralweight in composites@, applications to the commercial transports area have been relatively low. +hemost aggressive C.5. commercial transport application to date has been the Boeing %%%, which hasabout '$ percent of its structural weight in composites. f commercial aircraft applications are toincrease, cost impact factors must be significantly improved.

Without !uestion, the A044 rogram provided the airframe companies with important technology, butthe program ended without accomplishing its original goal of developing composite primary wing andfuselage structures. Without a 8A5A technology program, industry lacked the confidence to proceedwith production of high-risk primary structures. +he barrier issues were high ac!uisition costs and lowdamage tolerance. 0ost data e"trapolated from the A044 development contracts showed that wingsand fuselages would cost considerably more than aluminum structures. +he industry position on

production commitment was that composite primary structures must be demonstrated to cost less thanaluminum structures. 2ow damage tolerance remained a characteristic of composite structures despite

major efforts to develop and use toughened matri" resins. ndustry wanted robust structures able towithstand the rigors of flight service with minimal damage.

0omposite upper aft rudder flown on *c onnell ouglas 0-'$ in A044 rogram.

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Cse of composite materials on C.5. military aircraft.

Cse of composites in civil commercial transports.

Advanced 0omposite +echnology rogram

By '(>=, research engineers at 2angley were holding conferences to e"plore the potential of te"tilecomposites, based on approaches similar to those used in the te"tile industry, to provide barrier-

breakthrough technology. By '(>%, funds were available for a modest e"pansion of the 2angleycomposites program. A 8A5A 1esearch Announcement ?81A@ was issued seeking proposals forinnovative approaches to cost-effective fabrication, enhanced damage tolerance designs, and improvedanalysis methods. Forty-eight proposals were submitted by companies and universities, and '=

proposals were selected for contracts. +hen, in '(>>, 8A5A launched its Advanced 0omposites+echnology ?A0+@ rogram, a major new program for composite wing and fuselage primary structures.+he program incorporated the e"isting 81A contracts with significant increases in funding for wingand fuselage hardware developments. A 5tructures +echnology rogram 6ffice at 2angley provided

management for the A0+ rogram. Cnder the direction of 0harles . Blankenship, :ohn ;. avis, :r.,was the rogram *anager of A0+, and leading researchers included :ames 9. 5tarnes, :r., *arvin B.

ow, 9. Benson e"ter, and 8orman :. :ohnston. +he '= previously mentioned contracts wereawarded by 2angley in '(>( to commercial and military airframe manufacturers, materials developersand suppliers, universities, and government laboratories. +he program approach was to developmaterials, structural mechanics methodology, design concepts, and fabrication procedures that offeredthe potential to make composite structures cost-effective compared with aluminum structures. ;oals for the A0+ program included


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fuselage barrel with a window belt and a wing bo" at the wing-fuselage intersection. +he structure wasto have been pressure tested as part of the engineering verification process. Cnfortunately, the fundingfor A0+ was reduced and forced cancellation of the composite fuselage studies. *c onnell ouglas,meanwhile, focused on the successful development, fabrication, and testing of an advanced compositewing, as discussed later. +he A0+ rogram ended in fiscal year '((%.

+e"tile 0omposites

n the '(>$s, researchers looked to te"tile composites as breakthrough technology. 5upporters arguedfor new concepts that would use knitting, weaving, braiding, and through-the-thickness stitching forreinforcement and use e"isting C.5. te"tile manufacturing technology for cost-efficiency. Anoutstanding summary by ow and e"ter of progress and details of te"tile composite research by

8A5A during the period from '(>= to '((% is recommended to the reader ?see bibliography@.

Cnder the leadership of *arvin B. ow, 2angley conducted and sponsored e"tensive research onwoven, braided, knitted, and stitched ?te"tile@ composites in the 8A5A A0+ rogram in the periodfrom '(>= to '((%. +he major objective of the studies was to develop te"tile composites technologyapproaches that would provide a paradigm shift in cost and damage tolerance to overcome barrierissues. 6ne such barrier issue is the impact performance of te"tile composites. 2ow-velocity impactsfrom tools, hail, runway debris, and ground e!uipment can damage resin matri" composites withcarbon fibers. With sufficient kinetic energy, these impacts can damage the composite without readilyvisible evidence and can significantly reduce the strength. 0urrent regulations re!uire compositestructures to carry ultimate load with nonvisible impact damage. +e"tile composites are potentiallymore resistant to impact damage than traditional laminated composites fabricated using prepregunidirectional tape. n '((&, 0larence 0. oe, :r., of 2angley conducted studies of conventional tapelaminates and te"tile composites, providing detailed design information on their characteristics.

1esearch by 9. Benson e"ter in '((& on braided composite materials demonstrated that a braided-woven stiffener wing concept could meet damage tolerance goals and be designed and fabricated with a

cost-effective process. Braiding is an automated process for obtaining near-net-shape preforms forfabrication of components for structural application. 5tiffeners, wing spars, floor beams, and fuselageframes are e"amples of potential applications of cost-effective braided composites. +est results on wing

panels fabricated from stitched skins and stitched-stiffener preforms obtained at 2angley and*c onnell ouglas indicated that damage-tolerance re!uirements could be met. Accordingly, stitched

panels with braided stiffeners were tested to assure that braided stiffeners also satisfied damagere!uirements.

Braid-stiffened wing-panel preforms were fabricated by 2angley from dry-stitched skin and braidedstiffeners obtained from Fiber nnovations, nc., 8orwood, *assachusetts, followed by a resin filminfusion ?1F @ process by *c onnell ouglas. Wing panels were intentionally impacted on the skin

side midway between stiffeners, directly beneath a stiffener, or at the flange edge of a stiffener. mpactenergies were selected to produce the onset of visual damage. All impacted panels e"ceeded the impactdesign goal and failed without any skin-stiffener separation.

6ne major breakthrough in ow3s program was the use of advanced stitching methods to fabricatelarge composite structures. Darious types of te"tile composites were thoroughly tested, but it wasstitching, combined with 1F , that showed the greatest potential for overcoming the cost and damagetolerance barriers to wing structures. Assembling carbon fabric preforms ?precut pieces of material@with closely spaced through-the-thickness stitching provided essential reinforcement for damagetolerance. Also, stitching made it possible to incorporate the various elements wing skin, stiffeners,ribs and spars into an integral structure that would eliminate thousands of mechanical fasteners.Although studies showed that stitching had the potential for cost-effective manufacturing, the criticalneed was for machines capable of stitching large wing preforms at higher speeds.

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A primitive single-needle stitching machine, resembling a scaled-up version of a household sewingmachine, was the first prototype used by 2angley to determine the benefits of stitched composites. +hisinitial research identified that stitched composites offered better levels of damage tolerance thanconventional laminated composites. +his single-needle sewing machine was used in e"ploratoryresearch on stitched composites. n '((&, a computer-controlled single-needle stitching machinecapable of stitching dry high-performance te"tile materials ?such as graphite and glass@ was designedand built for the *aterials ivision at 2angley. +he stitching machine was capable of stitching a

planform area of & by ) ft with thicknesses greater than '.= in. using a lock stitch, and programmingstitching in any direction ?including curves@ within the planform area. +he machine was capable ofstitching with a wide variety of needle and bobbin threads, such as polyester, nylon, u ont /evlar,and carbon. A wide variety of preform si es were fabricated and delivered to *c onnell ouglas for1F processing to produce test specimens for evaluation at 8A5A 2angley.

2ower-stitched wing cover for -ft-span structural test wing.

n the stitched-1F process, layers of dry carbon fabric are stacked to form the wing structural elementsand are stitched with through-the-thickness /evlar threads. 1F of the preform with epo"y resinfollowed by autoclave curing completes the process of making an integral wing cover.

1esults obtained with test panels and a small wing-bo" test article indicated that the process producedcomposite aircraft parts with outstanding damage tolerance. +he process has the potential for majorreductions in the labor content of manufacturing composite wing primary structures. 9owever,demonstrating a stitching machine with the si e and speed re!uired for cost-effective fabrication of full-scale composite wings for commercial transport aircraft was critically important.

6ne of the first demonstration sections was a '#-ft-long wing stub bo" that was fabricated by*c onnell ouglas and tested at the 2angley 1esearch 0enter in :uly '((=. +he wing stub bo"demonstrated that the stitching-1F concept could be used to make the thick composite structuresneeded for heavily loaded wings. +he successful test of the stub bo" proved the structure and damagetolerance of a stitched wing.

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8A5A awarded Boeing ?subse!uent to the merger of Boeing and *c onnell ouglas@ a contract todevelop a large machine capable of stitching entire wing covers for commercial transport aircraft. +hishigh-speed, multineedle machine, known as the Advanced 5titching *achine ?A5*@, was designed and

built under the 8A5A A0+ Wing rogram. Cnder subcontract to Boeing, ngersoll *illing *achine0ompany, 1ockford, llinois, was selected to design and build the A5*. +he advanced stitching headsof the A5* were designed and built by athe +echnologies, nc., rvington, 8ew :ersey. 0oncurrentwith the development of the large stitching machine, 8A5A and Boeing proceeded with a building

block approach to demonstrate the design and manufacture of stitched-1F wing structures.

ngersoll3s machine was capable of stitching a contoured wing preform =$-ft long and >-ft wide.Following e"tensive checkout tests, the machine was dismantled, moved, and reassembled at the*c onnell ouglas stitching facility in 9untington Beach, 0alifornia. When the stitching wascompleted on the machine, the still fle"ible wing skin panel was put into an outer mold line ?6*2@ toolthat provided the shape of the outside surface of the wing. A film of resin was laid on the 6*2 form,followed by the composite skin panel and the tools that defined the inner mold line. +hese elementswere put into a plastic bag from which the air was drawn out, creating a vacuum. +he materials werethen placed in an autoclave, where heat and pressure were applied to let the resin spread throughout thecarbon fiber material. After heating to


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+ests of -ft-long wing bo".

Boeing named its new 5titched 0omposite evelopment 0enter after 8A5A 2angley researcher*arvin B. ow in honor of his contributions to stitched composites research and, specifically, to the

A5*. ow spent the last #= years of his &$-year 8A0AJ8A5A career in pursuit of the application ofadvanced composite materials on commercial transport aircraft. 9e is the first 8A5A employeehonored in the naming of a corporate facility. 9is work on composites led to the early flight testing ofgraphite-epo"y rudders on the *c onnell ouglas 0-'$ commercial transport aircraft, the A044structures for the 0-'$, Boeing %


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+he general aviation community has long been a user of composites technology, especially for small personal-owner aircraft and home-built aircraft. 2ed by innovative designers such as Burt 1utan, thissector of aviation has enthusiastically embraced the benefits of composites technology, and although

8A5A research has not been directed specifically at this class of aircraft before the A;A+4 program,2angley has ensured that appropriate communications with the small aircraft community regarding

8A5A technology has occurred through briefings at national meetings, such as the 4"perimentalAircraft Association3s 6shkosh convention.Beech Aircraft ?now 1aytheon Aircraft 0ompany@ made e"tensive use of composites in the Beech5tarship, as well as new business aircraft, the remier and the 9ori on. +hese applications have madeuse of information and results from the A044, A0+, and A;A+4 0omposite rograms. +he generalaviation industry is now the leader in the use of composites in production aircraft. +he FAA is currentlyin the certification process for '( new aircraft with significant use of composites. +he 2ancair0olumbia


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0ertified composite aircraft, 2ancair 0olumbia and 0irrus 51#$.

Applications

+he legacy of the A044 rogram and its significant contributions to the acceleration, acceptance, andapplication of advanced composites has become a well-known e"ample of the value of 2angleycontributions to civil aviation. n the best tradition of 8A5A and industry cooperation and mutualinterest, fundamental technology concepts were conceived, matured, and efficiently transferred toindustry in a timely and professional manner. With the participation and guidance of 2angley, industrywas able to address numerous high-risk issues that posed serious obstacles to advances in the state ofthe art and applications. Widespread use of composites today by military aircraft and the continuingincrease of composites used by civil aircraft are very visible reminders of the impact of this importanttechnology contribution by the 2angley 1esearch 0enter.

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