Composites in aircraft

Where composites sit on an airframe

Composite fairings, spoilers and flight controls were first developed in the 1960s to save weight over aluminium. Today the list runs from fairings, control surfaces, landing-gear doors, floor beams and interiors to the primary stabiliser structure of large aircraft and, on new-generation large aircraft, the primary wing and fuselage structure, as well as turbine fan blades and propellers. The reasons given are high strength, low weight and corrosion resistance.^[1]

Carbon-fibre-reinforced polymer made that journey in steps: from secondary structures where a failure would not be catastrophic, through military aircraft, to extensive use in the Boeing 787 and several Airbus models, where the weight saved justified a large premium in cost per kilogram.^[1]

Laminates: plies, orientation and prepreg

A structural laminate is built ply by ply, and its stiffness and strength follow its stacking sequence: a unidirectional ply is strong only along its fibres, and a quasi-isotropic stack of plies at 0°, ±45° and 90° approximates the uniform behaviour of a metal. In aerospace, laminates laid up from prepreg — fibre held in a partly cured resin that is kept frozen until use — are usually cured in an autoclave.^[1,2]

Repairs are often made by wet lay-up instead, impregnating dry carbon, glass or aramid fabric with resin just before use. It stores better and needs no autoclave, at the price of lower properties than prepreg.^[1,2]

Sandwich panels: two skins and a core

Much composite secondary structure — spoilers, fairings, ailerons, flaps, nacelles, floor boards, rudders — is sandwich construction: two thin laminate face sheets bonded to a thick, light core that keeps them from buckling and carries shear. A thicker core raises bending stiffness sharply for very little added weight, so such panels need no stringers or frames.^[1,2,3]

The usual aircraft core is a honeycomb of aramid paper, which is flame-resistant, insulating and formable; fibreglass honeycomb is used where more strength is needed, and aluminium honeycomb gives the best strength for its weight — but never under carbon face sheets, because the pair corrodes the aluminium. Panels are cured in an autoclave, a press or under a vacuum bag, with the skins either cured first and bonded to the core or cured together with it.^[1,2,3,4]

Light and homebuilt aircraft use foam cores instead — polystyrene, polyurethane, PVC, polymethacrylimide — to give strength and shape to wings, wing tips, control surfaces and fuselage sections. Foam is heavier and weaker than honeycomb and is rarely used on commercial aircraft. Balsa, a closed-cell natural wood, is the heaviest of the structural cores.^[1,2,3]

Layers of adhesive

Skins, cores and repair patches are joined with structural adhesives, mostly epoxy. The workhorse is a thin, rubber-toughened epoxy film on release paper, stored at −18 °C and often carried on a light fibre scrim that controls how thick the bond line ends up; its upper service temperature, 121–177 °C, is set by how much it has been toughened.^[1]

Paste adhesives keep at room temperature but make the bond line hard to control. Foaming adhesives, sheets of part-cured epoxy that expand as they cure, splice pieces of honeycomb together and bond repair plugs into existing core.^[1]

Lightning, corrosion and impact

An aluminium airframe carries a lightning strike away by conduction. A composite one cannot: carbon fibre is about a thousand times more resistive than aluminium, and epoxy about a million times. External composite parts therefore carry a conductive ply — copper or aluminium mesh, nickel-coated graphite cloth, aluminised fibreglass or conductive paint — and a repair has to restore it.^[1]

Carbon fibre also drives galvanic corrosion of the metals it touches, which is why it is kept away from aluminium honeycomb and needs care around metal fasteners. And the thin skins of sandwich control surfaces, sometimes only three or four plies, are reported in service to have poor impact resistance.^[1,2,3]

Commonly misunderstood