High-Performance Composites

MAY 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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APPLICATIONS M A Y 2 0 1 4 | 6 7 APPLICATIONS Carbon landing gear leg cushions UAV landing Using its Lost Core Resin Transfer Mold- ing (LCRTM) system, Wichita, Kan.-based Fiber Dynamics Inc. produces complex, hollow structures in a single molding op- eration. For the past 10 years, the com- pany has used the process, developed in- house, to manufacture the main landing gear strut for the Predator B MQ-9 Reaper unmanned aerial vehicle (UAV), pro- duced by General Atomics Aeronautical Systems (San Diego, Calif.). "Conventional prepreg processes were unable to economically provide the structural performance required, and metallic options would typically weigh and cost more," explains Fiber Dynamics' CEO Darrin Teeter. "In comparison, our LCRTM system reduces weight and cost, and improves structural integrity through a unitized structure." More than 600 of these struts — func- tionally, hollow leaf springs — have been made using S-2 Glass fber from AGY (Aiken, S.C.) with a carbon fber center stiffener (see photo, top right). Now, Fi- ber Dynamics is developing a prototype strut for another as yet unidentifed UAV (image below). "This one uses a shock absorber to cushion the landing instead of the spring action of the strut itself," Teeter says. To meet the higher stiffness and load requirements of this design, IM7 unidirectional carbon fber from Hexcel (Stamford, Conn.), aligned in the primary load-bearing direc- tion (longitudinal), is woven together with a plain-weave carbon fber fabric, to Fiber Dynamics' specifcations, by Fabric Development Inc. (Quakertown, Pa.). Fiber Dynamics begins the manufacturing process by casting the core from its proprietary Thermocore material. Tailored for low- melt viscosity and minimal thermal expansion, the ma- terial "behaves like a ther- moplastic," Teeter says. The cast core is then machined to the exact shape of the inside surface of the part's hollow cavity. It serves as a male mold for formation for the carbon preform and as a sacrifcial mandrel during molding. Dry fabric for the preform is cut into preprogrammed ply patterns on a CNC cutting system from Eastman Machine Co. (Buffalo, N.Y.), and the plies are layed up directly on the core mandrel — along with doublers and fttings — using customer-approved epoxy tackifers. Ti- tanium doublers are interleaved with the carbon fabric around the primary load- bearing bushing, which is molded-in and hinged to enable landing gear deploy- ment and retraction. "The titanium plates ft very tightly against the bushing to help mitigate and trans- fer the high loads into the laminate — especial- ly the high impact loads created on landing the more than 4,000 lb [1,814 kg] UAV," Teeter says. The dry preform is then con- solidated, either by vac- uum or mechanical com- pression. "We are able to compress it to the net shape of the mold cav- ity," Teeter explains, not- ing that the preform vol- ume, then, is "60 percent fber, and 40 percent air," a match for the part's expected 60/40 f- ber/resin ratio. The net-shape preform and core are enclosed in the female closed mold that defnes the part's outer geometry, and then it is infused with an unnamed ep- oxy resin system supplied by Momentive Specialty Chemicals (Columbus, Ohio), using standard RTM techniques. The part is cured at a temperature below the melt- ing temperature of the Thermocore man- drel. After demolding, the part is CNC- trimmed and holes are drilled for fttings and fasteners, which, synergistically, pro- vide drainage paths for the mandrel when the part is postcured at a proprietary tem- perature that melts out the Thermocore. After postcure, the part is cleaned to re- move residual mandrel material. Still in the test phase, Fiber Dynamic's carbon composite LCRTM landing gear is proving its ability to absorb the landing loads imposed by the UAV. "Based on test results, we expect the landing gear leg to be fully implemented when the UAV be- gins production," Teeter says. When that will be has not yet been determined. Teeter points out that the UAV mar- ket provides opportunities to try new processes and produce prototypes that would be much more diffcult to attempt on a manned aircraft. However, he adds, "that will probably change as UAVs start entering public air space." A test program is underway to safely implement integra- tion of unmanned aircraft systems into U.S. airspace in 2015. Glass & carbon landing strut A fiberglass leaf spring made by LCRTM for the Predator B MQ-9 Reaper UAV. Its carbon fiber stiffener strengthens the attach point to the airframe (lower right). Wheel ftting bonded into this end Shock absorber mount Retract link mounting holes Main pivot bushing with titanium interleaving All-carbon landing strut Carbon composite prototype shock-absorbing UAV landing gear made by LCRTM. Source: General Atomics Source: Fiber Dynamics 0514HPC Application-OK.indd 67 4/22/2014 3:32:55 PM

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