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Aviation History
1990
1990 - 0034.PDF
Composites require new inspection . . . LHX will make extensive use of carbon- fibre-reinforced plastic—"for virtually the entire fuselage", says Linden. The First Team has also settled on composites for the trans mission housing, bearingless main rotor and blades. Metals may be limited to the main rotor shaft (which might be composite if the design proves simple) and some landing gear components. While reduced weight is the most obvious advantage of using composites in helicopters, others include the abililty to make a stiff fuselage, avoiding resonance with the main rotor and eliminating the need for heavy anti- vibration devices, says Linden. Obstacles to the use of composites include meeting damage-tolerance and crashworthiness requirements. Helicopter airframe loads are rarely unidi rectional, says McDonnell Douglas' Andy Logan, and composite structures can be subjected to prying loads trying to lift lami nates and compressive loads trying to buckle laminates. To prevent delamination, there fore, designers are looking at stitching lami nations together. This is an additional fabri cation process, however. Crashworthiness requirements are unique to helicopters: "You have to be very creative to make a composite believe it is a metal and yield to absorb 13m/s impact energy," says Logan. "Both materials absorb impact energy, but where metal bends, composite fractures," says Linden, noting that Sikorsky (and Bell, McDonnell Douglas' partner in the LHX 'SuperTeam') successfully demonstrated the crashworthiness of an all-composite helicop ter airframe. With the US Army planning to buy 2,100 LHX helicopters, the challenge facing con tractors is how to manufacture at high rate and low cost. Logan identifies the potential for automation, not only in tape laying, but in winding of blades and airframe sections, and robotic cutting, kitting and drilling. Major advances have already been achieved in production of the largely com posite airframe of Northrop's B-2 flying-wing bomber. Rockwell Industrial Tools devel oped an adaptive-control drill able to cut through mutiple layers of dissimilar materials and reduce hole preparation time by two- thirds. Drilling through a carbonfibre/titani- um/aluminium laminate, the adaptive drill automatically adjusts spindle speed and pen etration rate as the bit passes through stacked materials of varying hardness. Northrop has achieved a 75% reduction in hole drilling time and a 300% increase in cutter life. ROBOTIC DRILLING Building the B-2 center wing section under subcontract to Northrop, LTV is using a robot to drill holes and install fasteners in a fixed composite/metal assembly in a single pass, saving on manufacturing and inspec tion time. The machine is able to work on aluminium/composite/aluminium sand wiches up to 25mm thick, presented vertical ly on an air-bearing fixture. Northrop, meanwhile, is using a Design Technologies ultrasonic knife to cut compos ite plies with twice the speed and eight times the accuracy of conventional reciprocating cutters. Subcontractor Boeing has worked with Cincinatti Milacron to develop an auto matic tape-laying machine capable of produc ing channel stringers up to 20m long, cutting . . . the result can be worth the expense labour content by 65%, and a programmable tape layer capable of producing contoured components. Sikorsky's Linden identifies tooling op tions (producing large assemblies in one operation or bonding together small assem blies) and new production techniques, such as resin transfer moulding (RTM) as possible ways of reducing LHX manufacturing costs. Already widely used by industries other than aerospace, RTM couid hold the key to wider use of composites in transport-aircraft primary structures, believes Douglas Air craft's Max Klotzshe. "Composites are a state-of-the-art material suitable for secondary structures, where the weight savings they show are such that their excess cost over metal parts is tolerated," says Klotzshe. "In primary structures, we cannot take full advantage of the weight saving, so composites are not cost-competitive with metal," he argues. The barrier to composite primary struc tures, Klotzshe believes, is that the raw material costs more than the finished metal parts, a premium that has so far prevented their use in either Boeing or Douglas airlin ers. Airbus Industrie, which uses a composite fin on the A310 and fin and tailplane on the A320, acknowledges the additional cost, which it believes the weight saving justifies. Douglas' 12-year search for alternative processing methods which break down the barrier to composite primary structures cen tres on producing a high-strength, damage- tolerant structure without incurring the un- acceptably high cost of toughened resins and high-modulus carbonfibres, says Klotzshe. In the selected process, Douglas buys in
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