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Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 7/98 ADVANCED COMPOSITES by Gary F. Turner Are composite materials and structures exotic? Yes and no. Few people really understand them, but composites are actually both exotic and mundane, depending on their application. They have been around a long time. Many people lose sight of the fact that the first airplanes, as well as hot air balloons preceeding them, were largely of wood, fabric and adhesive composites well before metals came onto the scene. Basically, a composite is any material made up of two or more components. This is most often in the form of a matrix reinforced by some type of fiber. Glass and carbon reinforced epoxy and phenolic plastics are usually made into laminates, ie. layered sheets, which may in turn be in monolithic or sandwich form. Monolithic if they are solid, and sandwich if the laminate sheets are seperated by a core of different material, usually of honeycomb or foam. Heavily loaded structural components are usually monolithic. Lightely loaded fairing, interior components, etc. are usually of the sandwich type. While composites have in general turned in a sparkling performance and are living up to their potential, not all has been positive. cost versus performance is the key question and performance have sometimes been less than anticipated. Composite structures also require careful, precise design and can be sensitive to heat and impact. Several airlines for example, have experienced occasional problems with them in various areas, for example engine nacelles, spoilers and cabin flooring. Although the basic materials cost more than metal alternatives, new manufacturing methods are now reducing the overall life-cycle structural cost to competitive levels. Aside from its well known strenght, stiffness and lightness, at least two other big benefits come with composites: they do not corrode, and have virtually unlimited fatigue life. In military aircraft, where weight savings are critical to mission success, development paths have been broadly similar in the US and Europe. Unfortunately, the quantity of applications, more than actual quality, have suffered from widespread cutbacks in both locations; the US perhaps more so. In commercial aircraft, General Aviation and heloicopters however, Europe has often chosen a higher profile with composites, contributing notable achievements. Airbus for example shows a logical and definite progression of ever larger and more complex applications. Composites were at first chosen mainly for weight savings, fatigue and corrosion resistance, but a major new criteria has appeared: manufacturing cost reduction. Initial employment of larger, more demanding primary composite structures at Airbus began in Germany with the single piece rudder for production model A300s and A310s as far back as 1983. The relatively straightfoward design utilized three carbon/epoxy skinned honeycomb sandwich panels assembled as a hollow triangle to form the rudder. About eight meters in lenght and varying from one to two meters in chord, weight of the composite version is 175 kilogramms. It could be directlysubstituted for the metal version without changes to the plane. Improvements over the original metal structure included weight savings of some 22 percent, component count reduced by nearly half plus reduction of details froom over 17000 down to only 4800. Hardly two years later, the much larger, heavier and far more complex vertical stabilizer was converted from metal to composite. Essentially of carbon/epoxy laminate, with some glass and honeycomb core on the leading edges, and weighing 800 kilogramms, it was for several years the largest primary composite structure flying on a production model civil transport. As with the rudder, design and manufacture is by the German partner DASA Airbus, at its highly automated Stade plant south of Hamburg, a facility specializing in composite structures. The program entailed a running re-design and replacement of the original all-metal vertical stabilizer in all Airbus production models, beginning with the A300. Two seperate manufacturing methods, matten and module, were investigated for this program, which the latter won. Module technology is a multi-step, semi-automated process whereby aluminium modules are wrapped with prepreg composite material to form integral skin stiffeners. They are then positioned by robot on the already laid up skin, where US strips are in turn placed on top of the modules to form spar caps. Most of the work at Stade except the actual module wrapping has been automated. Even prepreg is carried from the cutters to the layup area by robot vehicles employing carbon/epoxy containers or baskets made from scrap prepreg. The entire, single piece vertical stabilizer skin was then co-cured in a single step, in a large autoclave. The advantage: only one trip to the autoclave was necessary. the relatively massive attach fittings at the base of the fin are also of carbon/epoxy and integrated into the skin; only the pins and bushings are metal. The original co-curing has since been modified to simplify the tooling, reduce manufacturing risk and cost, however more trips to the autoclave are required. Moreover the massive attach fittings, normally made up of hundreds of layers of prepreg, are being considered for manufacture by another method: Resin Transfer Molding RTM. The next major composites advance in Airbus composite primary structures extended the concept of aft airframe application, important for aerodynamic efficiency, to the entire tail. employing composites manufacturing techniques based on that of the vewrtical tail, SpainÕs CASA, with assistance from DASA developed and produced a fully composite horizontal stabiliser and elevators, initially for the A320 and later for the larger models. The stabilizer, including integral fuel tank entered production as far back as 1987. Well over four hundred all composite horizontal tails have been produced; every A320, 330 and 340 flying has one. Again weight savings of about 20 percent are accompanied by massively reduced parts and details count in this partly co-bonded structure. Horizontal tails are manufactured at CASAÕs Getafe plant in Madrid, with components and sub-assemblies supplied by other Spanish subcontractors such as ICSA. Due largely to these structures, CASA has become one of the top aerospace consumers of carbon/epoxy prepreg in Europe. One of the first to build and test an airliner-sized fully composite wing in Europe was DASA Airbus, again at its specialised Stade plant. The first large German wing box was completed as early as 1991, basically intended for the onetime MPC-75 airliner proposal, but its research scales well to larger projects. Low cost manufacturing dominated design, at the sacrifice of slight weight savings. This time the skin and stringers were produced seperately and then bonded together. Interestingly, the doublers were reportedly laid up wet, then cocured to skin and stringers at the same time. An impressive test program included 50000 simulated flights, with another 37000 successfully carried out after deliberate damage was inflicted on the structure. This program ended in 1994 with determination of final residual strenght. Meanwhile in France, Aèrospatiale has built on experience with the ATR-72 production outer wing box, in a research program to develop a highly loaded composite wing box for an 80-120 seat jet airliner. Targets are 20 percent weight savings at equal manufacturing costs to a metal wing. Begun in 1991, the program was intended to last six years and involve fabricating a full scale wing box. RTM construction methods are included, plus second generation resins and fibers, together with metal ribs and fittings to deal with load levels three times higher than those in the ATR-72 outer wing. Not to be outdone, in the UK British Aerospace, together with several other industry and academic participants, quietly assembled its own composite wing building program known as AMCAPS (Affordable Manufacturing of Composite Aircraft Primary Structures). It represents a very ambitious, high tech approach to lower manufacturing costs, combining elements of prepreg and RTM, plus Z-axis stitching, to boost structural strenght and lower production cost. One impressive feature is a seamless, rivetless, boltless, very smooth outer surface which promotes low drag air flow. The big wing resembles the glassy finish seen on sailplanes. It appears that the question of who would actually build such a large composite Airbus wing remains open, controvercial and in some ways political. BAe fabricates all current Airbus wings predominantly of metal. Regardless of the outcome, all three of the above composite R&D programs are stepping stones toward either the FLA or other substantial aircraft programs. Other examples for the application of composites in Europe are the ATR 42 and 72 twin turboprop regional airliners. The popular 40-70 seaters are produced by Avions Transport Regionale. Aèrospatiale is responsible for the partly composite wing, including nacelles built at Nantes, while Alenia fabricates the fuselage and tail with composites in Naples. The smaller ATR 42 was the first in the series and therefore more basic, it still boasts nearly 1000 kilograms of composites in numerous secondary structures and fairings. This is of course in addition to the phenolic-glass and honeycomb composite interior panels. All control surfaces are carbon or aramid sandwich, except for monolithic carbo/epoxy ailerons. Engine nacelles are predominantly carbon or Kevlar/Nomex sandwich, while the large fairings for wing to fuselage, plus landing gear are of aramid/honeycomb. Both propeller blades as well as brakes are carbon composite, with phenolic resin composites employed throughout the interior and flooring. Notably, some of the door and side panels employ unusual fiberglass/carbon hybrid fabric. Even by today's standards this is substantial composites content for commuter aircraft, though the ATR-42 was launched 15 years ago. Advanced RTM parts have been developed for both planes. However it is the ATR-72 where composites applications really become noteworthy. Both planes have essentially the same basic nonmetal applications with one major exeption: the newer ATR-72 incorporates fully composite outer wing sections in its primary structure, another first for commercial aircraft. For this reason about 22 percent of its airframe weight, not counting the interior, is compromised of advanced composites, compared to 15 percent for the ATR-42. The ATR-72 wing box sections, built by Aèrospatiale's Nantes facility, measure circa 8 x 1,2 meters a piece, weighing over 600 kilograms together. Each outer wing torsion box is compromised of integrally stiffened carbon/epoxy skins, two composite spars and aluminium ribs, part of it serving as afuel tank. Total structural composites applications on the ATR-72 approach 1700 kilograms, for a savings of 360 kilogram or about the weight of four passengers. The pace-setting carbon/epoxy outer wing boxes have been in full commercial service since 1987, with no reported problems and hardly any maintainance required. The ATR-72 also serves in A_rospatialeÕs RTM composite developments: a thick-section, heavily loaded wing joining plate has been produced and is being tested. Even the new, six-blaeded propellers are composite, produced by the French Ratier-Figeac. In addition, a fully composite straight wing was developed by Aèrospatiale for this plane along with a larger, composite swept wing for a hypothetical 100-120 seat jet airliner on an experimental basis. Moreover, in 1997 composite applications on both the ATR 42 and 72 models took another large leap forward, when the entire empenage was converted to an all-composite structure. This was not as difficult as might be expected, since with the exeption of the primary troque box structure, it was already highly composite. Notably, while the planes make extensive use of Hexcel composite materials, this work was carried out with Cytec-fiberite prepregs. The German Fairchild Dornier 328 advanced regional transport in the 30-seat category makes extensive as well as some highly unusual applications of advanced composites, For one, the entire empennage complete with rear fuselage section is of carbon and aramid/epoxy composite, produced by the former Dornier plant at Munich/Neiaubing. This facility became independent in 1996, operating as ADIS (Advanced Integrated Structures), and was acquired by the large UK aerospace subcontractor GKN Westland at teh end of 1997. The large wing to fuselage and landing gear fairings on the 328 are either of aramid or carbon/aramid hybrid skins over honeycomb core, as is nearly the entire wing trailing edge, including flaps and airlerons. Moreover the complete fuselage aft pressure bulkhead is made of Kevlar/epoxy, another first for any production aircraft. The 328 also possesses innovative smaller composite applications, such as thermoplastic (carbon/PEI) flap ribs, another small but meaningful first chosen for cost reduction. The plane also features novel composite ice pürotection amor adjacent to the propellers. Composite propeller blades are now more or less standrad on most new commuters, as are carbon brakes on larger aircraft. Not counting props, brakes or the interior, about 800 kilogramms of advanced composites are part of the 328's airframe: seven percent aramid and twelve percent carbon reinforced, for a reported total of 19 percent at present. This is expected to be the same on the 328JET, except for the engine nacelles and mounting pylons. Composites have been in aerospace since aviation began, but advanced, high-performance carbon-based composites are indeed relatively new. Such materials are in quantity use in commercial aircraft such as those reviewed here. Europe has held its own and in some ways is leading these exciting developments. From page 56 of FLUG REVUE 7/98 Home | Update | LATEST ISSUE | Gallery | FR Profile | Datafiles | FR 7/98 Copyright 1998 by Motor-Presse Stuttgart. All rights reserved. Last updated June 9, 1998 FLUG REVUE, Ubierstr. 83, 53173 Bonn, Germany