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EDO Corporation has manufactured a prototype engine-inlet duct using a low-cost VaRTM (Vacuum-assisted Resin Transfer Molding) and braiding process under the auspices of the Air Force Composite Affordability Initiative (CAI) program.
The advanced-design duct features a foreshortened, compact geometry with low inlet loss. The complicated shape creates manufacturing challenges, and the objective of the process demonstration was to achieve required performance and quality with the most affordable manufacturing process.
EDO manufactured the compact duct for Lockheed Martin Aeronautics Company, prime contractor of the F-35 Joint Strike Fighter. According to Shaw Lee, Lockheed Martin’s CAI program manager, “we looked to EDO for a braided/VaRTM manufacturing approach because unit costs with this process are significantly lower than conventional methods.” VaRTM is a flight-qualified, low-cost, high-quality infusion and moulding process already used by EDO to produce airframe structures for the JASSM missile fuselage.
Braiding was performed over a large mandrel, segmented into 35 removable pieces to enable removal after part cure. The mandrel was braided with five layers of continuous carbon fibre, with local buildups up to eight plies thick. Braiding is an automated circular weaving process that applies tensioned fibres to the part surface. EDO used its 288-carrier braiding machine, the largest of its type in the United States. This allowed the duct to be produced with the desired fibre coverage without any gaps between yarns.
The duct design includes honeycomb core inserts to produce a stiffened duct wall. The honeycomb core is filled with closed cell foam to assure that resin will not fill the cell walls. Reinforcing keel straps were also selectively laid up into the duct structure. A high performance epoxy resin was then infused into the braided part using the VaRTM process. After the epoxy cures, the internal mandrel is removed from the moulded part, one segment at a time. Stiffeners are then bonded to the duct exterior to reinforce the structure at key locations. The finished duct assembly is currently being pressure tested and exposed to hammershock tests that simulate the pressure spike associated with an engine shock wave.
The CAI program was established to dramatically reduce the cost of composite technologies in airframe structures.
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