Airliner Manufacture - Composite Creations

Using composites in airliner construction reduces the airframe weight which enables lower fuel burn and therefore reduces operating costs

Think “airliner” and the image of a shiny, all-metal machine comes to mind. Yet about a century ago, wood, canvas and wire were the materials of choice. They were strong, light and simple to use for fabricating small aircraft. More importantly, they were easy to repair. This was crucial as frequent mishaps necessitated minor or major repairs; the quicker the better. But the large aircraft that were straining to fly off the drawing boards in the 1920s could not possibly have been held together by wood and fabric. Metal was the obvious solution. The hunt for a more appropriate aircraft manufacturing material ended with aluminium, mainly due to its vastly superior strength and resistance to corrosion. Of course, the weight of aluminium was an inevitable penalty (and even worse, steel) but it was much lighter than steel. Also, to compensate for overall higher weight, the new metal aircraft had to be fitted with more powerful engines.

Even today, weight is an anathema for an actor as well as an aircraft. Aeronautical engineers are forever on the lookout for ways of reducing weight; prepared to give an arm and a leg for a truly robust but lightweight airframe material. With green issues coming to the fore, decreasing fuel burn and emissions have become sacred. Low weight thus assumes added importance but not at the cost of durability or safety. Composite materials, both lightweight and strong, provide the panacea by reducing fuel burn and enhancing operational efficiency. This helps in reducing the adverse impact of aviation on environment. No wonder, composites are rapidly emerging as the most important material in the manufacture of aircraft since the use of aluminium became widespread. Is it possible that aluminium itself is on the way out?

Composite Culture

A basic composite consists of at least two materials—one which acts as a “matrix” to hold everything together while the other serves as reinforcement, in the form of fibres embedded in the matrix. Very simplistically, by combining materials with complementary properties, a composite with advantages such as high strength, stiffness, toughness and low density, and few or none of the weaknesses of the individual materials, can be obtained. Fibreglass, a composite which consists of glass fibres embedded in a resin matrix, became familiar in the construction of boats and cars half a century ago. Most cars today have fibreglass bumpers over a steel frame. Some of the most commonly used matrix materials are thermosetting materials such as epoxy, bismaleimide or polyimide. The reinforcing materials may be glass fibre, carbon fibre, boron fibre or other more exotic mixtures. Composites may also consist of more than two components, organic or inorganic.

Thermoplastics are now replacing thermo sets as the matrix material since they are easier to produce. They are also more durable and tougher than thermo sets, particularly for light impacts. For instance, a hammer accidentally dropped on a wing could easily crack a thermo set material but would just bounce off a thermoplastic composite. A distinction can also be made in advanced composites. Materials such as fibres, resin systems and cores which when combined have strengths and other desirable properties far in excess of ordinary composites. They are superior to common composites such as fibreglass.

Why are advanced composite materials rapidly replacing metals in aircraft manufacture? Their exceptional strength and stiffness-to-density ratios together with superior physical properties such as lightness, enhanced durability, very high resistance to fatigue, and low susceptibility to corrosion (meaning reduced inspection and maintenance requirements) make them irresistible. Aircraft manufacture is particularly aided by the capability to produce very complex shapes in a single piece using advanced carbon fibre composites (CFC). Carbon fibre reinforced plastics (CFRP) and glass fibre reinforced plastics (GFRP) are becoming increasingly important. They consist of carbon or glass fibres, both of which are stiff and strong due to their density, but brittle, embedded in a polymer matrix, which is tough but neither particularly stiff nor strong. The result is a family of miracle materials.

Composites have been in use in the aircraft manufacturing industry for decades although they were probably used first on combat aircraft. Around three decades ago, a boron-reinforced epoxy composite was used for the skins of the empennage of the Grumman F-14 Tomcat and McDonnell Douglas/Boeing F-15 Eagle. From these humble beginnings, today almost 24 per cent of the structural weight of the Lockheed Martin/Boeing F-22 Raptor consists of composites. And as knowledge and development of various composite materials has improved, their use in the primary structures such as wings and fuselages is also growing.

While combat aircraft employ composite materials for increased strength and performance, composites used in the construction of commercial transport aircraft actually make them more expensive than traditional metallic structures. The reason for their increasing employment, however, is because they improve the operating economy of the airliners. Using composites in airliner construction reduces the airframe weight which enables lower fuel burn and therefore reduces operating costs. The Boeing B787 Dreamliner has more composites than any previous airliner —an amazing 50 per cent of its airframe, including the fuselage, will be lightweight CFC. Composites represent around a quarter of the Airbus A380’s structural weight and will account for more than 50 per cent of the Airbus A350 XWB, currently under development. In the future Bombardier CSeries airliners, composites will account for some 46 per cent of the structure.