ONE OF THE IMPORTANT applications of technological progress to modern aircraft is the use of composite materials in their construction. Moulded into an epoxy resin matrix, they have produced extremely tough and stable materials that are replacing aluminum and aluminum alloys. This has a significant effect on performance, weight, design and cost.

Advances in technology have had an enormous impact on the shape, performance, reliability and composition of modern aircraft and fly-by-wire flight control systems (FCS) and sophisticated avionics suites have enhanced the utility and performance of military aircraft dramatically. Propulsion systems also have improved and advances in structural technology have influenced the way military aircraft are designed, produced and maintained.

Until the late 1960s, almost all tactical aircraft were composed primarily of aluminum and its alloys. High-speed aircraft such as the Lockheed SR-71 used a sizeable amount of titanium, but high cost and the demanding production requirements of this material limited it to moderately high temperature applications. Consequently the latest tactical aircraft incorporate many non-metallic composite materials. Sixteen per cent of the structural weight of the Boeing F/A-18E/F and Lockheed F/A-22 are made up of about 20 per cent composite material while 26 per cent of the AV-Harrier II's empty weight is composite structure. Future military aircraft such as the F-35 joint strike fighter are expected to have a composite content of at least 35 per cent.

Composite material is made up of two or more separate components that when combined result in property changes that differ from the original materials. Composites most widely used in combat aircraft are composed of high-strength fibres of glass, boron, plastic or carbon that are embedded in an epoxy resin matrix. The fibres have very high strength, a uniform structure and lack flaws. The epoxy resin bonds with the fibres in the curing process to produce an extremely tough and stable material.

The most widely used composite material in tactical aircraft is a carbon fibre/epoxy mix. Carbon epoxy has eclipsed boron-based composites because it is much cheaper to produce, easier to machine and drill, and can be formed into complex shapes to produce structural members such as spars and ribs. Other fibres typified by Dupont's Kevlar also are being used in aircraft production. Kevlar is less dense than carbon fibres but has inferior mechanical properties. It is used in pressure vessels, for ballistic protection and as lightweight fibreglass non-structural parts.

Composites have displaced conventional materials such as aluminum because they have several advantages. They have lower density and greater strength and stiffness than aluminum, therefore a smaller lighter structure can carry the same load. Studies conducted by Boeing indicate that a 38 per cent composite structural weight can result in a 40 per cent reduction in empty weight, 39 per cent reduction in wing area and a 33 per cent fuel saving for the same mission profile when compared to an aircraft of conventional metal structure.

Another big advantage is that composites are relatively insensitive to flaws. Fatigue testing of composite structures demonstrated their high resistance to cracking and that fractures generally do not propagate. Composite materials are very stable and so are not subject to corrosion as are metallic structures. However, in the design process, careful attention must be paid to composite/metal interaction because through galvanic action some metals will corrode when in contact with carbon fibre/resin laminate.

Design impact Composites have had a significant impact on the design process. Metal parts start as a solid piece, usually machined down to a specified size and thickness. Multiple parts are fastened or riveted together to form structures. Using composites, a designer has much greater flexibility because the strength and stiffness of structures can be tailored. The material can be stacked with one ply running in one direction and the next at a 45 or 90-degree angle.

To increase strength or stiffness in a localised area, a larger number of plies may be overlaid, each with a different shape and orientation. Tailorable strength enables designers to optimise aerodynamics such as in a forward-swept wing aircraft design. The weight of metal structures would prohibit such a prospect. Beech Aircraft is filament-winding entire business aircraft fuselages and the Vantage business jet is an all-composite aircraft. These new technologies also are expected to be incorporated into military aircraft and missiles.

Metallic fasteners, leading edge sections, spars and honeycomb materials can be combined with composites to form very strong and lightweight structures. Carbon epoxy composite materials are cured at up to 350^o and pressures of up to 150lbs/in². This changes the epoxy in the material from a soft resin to a hard, strong, solid material.

However, composites do require new skills. Design, production and quality-control personnel have had to adjust to the way they operate in order to take full advantage of the potential of these materials and to produce it economically. The computer has been a major ally in the move to composites. Computer-aided design (CAD) has made it much easier to develop composite structures and to understand their relationship with other elements of an aircraft more thoroughly.

Labour was one of the largest expenses in early composite fabrication. Lay-up time was long, production man-hours high and there was a sizeable amount of wasted material. CAD has assisted production planners in making efficient use of raw materials. Highly automated composite lay-up machines developed by Boeing and Vought for use in producing the B-2 stealth bomber, have set the standard for size and speed. Computer-controlled laser, water-jet and knife cutters and inspection machines have speeded the production process, increased efficiency and reduced the cost of composite fabrication.

Non-metallic parts have required the development of new testing and quality control techniques. After removal from the autoclave, composite parts are examined for flaws with computer-controlled ultrasonic and x-ray equipment. A recent advance pioneered by Lockheed Martin are the Laser ultrasound testing systems that are much faster and more accurate than older, water-based systems. Computerised design, production and quality processes, learning curve benefits and increased competition have reduced the cost of composite parts significantly.

Research has shown the potential of a wide variety of new composite materials, carbon and graphite fibres have been combined with a matrix of metal rather than epoxy. This material has improved strength at higher operating temperatures than current widely used resin matrix composites. Hot sections, such as engine exhaust, are a unique composite application.

Carbon-carbon applications were designed for the National Aerospace Plane (NASP) programme, a technology now being pursued by engine companies. Ceramic matrix composites are an alternative material also being developed for high temperature applications. However, these technologies have yet to be perfected, they have proven to be brittle and difficult to fasten together.

Fibrereinforced thermoplastic composites produced by melting resins and combining them with re-inforcing fibres under high pressure in a mold, are another promising new area. Tests have shown these composites to be highly resistant to damage, able to be reshaped and quickly fabricated. Compared to carbon epoxy, fibre-reinforced thermoplastics are equal in density, equivalent in strength and part production may be less expensive. The USAF and several other air arms, material suppliers and a multitude of contractors are developing thermoplastic composites.

One area where composites have a significant advantage over most metallic structures is in radar cross-section reduction. Composites generally reflect less radar energy than metallic structures and advanced composite materials offer this benefit with structural, heat-resistance and configuration advantages. Aircraft can be formed with smoother lines, fewer areas where different materials merge and in complex shapes required for reduced signature requirements. The F/A-22, B-2, F/A-18E/F and unmanned aerospace vehicles such as the X-45 UCAV include a significant amount of composite materials.

Tactical aircraft now in advanced development in Europe also incorporate a sizeable amount of composite materials. Eurofighter's Typhoon makes extensive use of carbon/epoxy material, as does the Saab Grippen multi-role fighter. Composites account for about 25 per cent of the Dassault Rafale's structural weight with boron, carbon and Kevlar composites used in its front and centre fuselage sections, integral fuel tank, lower rear fuselage, wing, canards, rudder and many access panels.

Boeing recently unveiled its Bird Of Prey aircraft that was designed, built and tested in the 1990s as a demonstrator for rapid prototyping and advanced composite concepts. McDonnell Douglas, that merged with Boeing in 2000, self-funded the $67m programme to provide design engineers, production personnel and pilots an opportunity to test new concepts. Another goal of the programme was the testing of low-cost disposable tooling, rapid prototyping and 3-D virtual reality design and assembly.

The strike fighter design featured a low observable configuration with a gull-wing, sharp angles and a spine air intake. Large wing and fuselage sections were made of low-temperature carbon-composite structures that, with a reduced number of connectors and seams and the angled design, significantly reduced the radar cross section (RCS) of the Bird of Prey.

This aircraft has a wingspan of 23ft, is 47ft long, weighs only 7,400lbs and is propelled by a Pratt and Whitney JT15D-5C turbofan. Boeing and USAF test pilots flew the Bird of Prey 38 times at the Groom Lake Test Range in Nevada in 1996 and 1997. Lessons from these design efforts have been incorporated in the recently flown Boeing X-45 UCAV and other advanced systems.

Composites already have had a major impact on military aircraft design and manufacture concepts and also have been used extensively in the latest generation of commercial aircraft. As this technology continues to expand its applications metal aircraft and missiles will be seen as a throwback to an earlier era. New techniques call for new skills and computer and materials science now lead the way in aerodynamics. Just as metal planes replaced wire and wood, designers are adjusting to the new realities and possibilities available with computers and composite materials.