A FAN-EXIT CASE produced by using resin transfer moulding (RTM) is "...one of the largest and most critical jet-engine parts ever made from composites", says its manufacturer, Dow-United Technologies (Dow-UT). The component, which is an estimated 39% lighter and a projected 38% cheaper to produce than a traditional fabricated-titanium case, could not have been produced using conventional composite construction, says Dow-UT.
Consisting of hollow inner and outer rings joined by 44 hollow vanes, the 2.8m-diameter composite fan-exit case is designed as the structural attachment between the engine core and the fan case in Pratt & Whitney's projected 178kN (40,000lb)-thrust Advanced Ducted Prop very-high-bypass turbofan. The case is designed to withstand birdstrikes and large rotor-imbalance loads resulting from a fan-blade failure.
The 215kg case has been produced under a $14.8 million contract from the US Department of Defense's (DoD) Advanced Research Projects Agency. The Affordable Composites for Propulsion contract was funded under the DoD's technology re-investment programme, which seeks to adapt military technologies to commercial applications. Dow-UT is now designing an RTM fan-exit case for P&W's PW4168 commercial turbofan.
"The fan-exit case demonstrates that we have now developed a process by which composites can be used for a new class of parts," says Dow-UT president Thomas Scarpati. "This is a component we were not able to make using 'prepreg' technology - it was not a candidate for composites. Now, we have demonstrated, with RTM, not only that we can make the part cost-effectively, but in a scale - 2.8m diameter - which has surprised many."
The term "prepreg" describes the conventional approach to producing composite parts from plies of reinforcing fibre pre-impregnated with resin. The resulting laminate is cured in an autoclave, the combination of high temperature and high pressure melting the resin and forcing the fibres into the final shape.
Scarpati says that, while RTM has been used for 30 years to produce such items as cafeteria trays, Dow-UT has developed ways to increase the fibre volume and to use high-performance resins matching those used in prepreg composites. At fibre volumes of 50-60% "...advanced RTM is structurally competitive with prepreg and as competitive with metal as prepreg", he says.
In RTM, dry fibres are placed in a closed-cavity mould and resin is injected into the mould. Dow-UT says that the process relies on resin fluid pressure, which is uniformly distributed throughout the part, rather than mechanical (autoclave) pressure, which must be applied simultaneously to multiple surfaces.
Significantly more complex parts can be produced using RTM than with conventional autoclave curing of prepreg composites, the company says. In addition, tight dimensional tolerances can be maintained on all surfaces of a part by using computer-machined steel moulds.
Scarpati highlights the repeatability of the process. The 44 vanes of the fan-exit case were moulded in groups of four - dubbed "four-packs" - and 15 sets were produced with "extraordinary repeatability". Wing "sine-wave" spars for the Lockheed Martin/Boeing F-22 fighter are being produced with such "incredible predictability", he says, that consideration is being given to eliminating ultrasonic inspection of the spars for voids.
"We can achieve a level of repeatability unachievable with prepreg," Scarpati says. "We never expected the incredible laminate quality," he admits, adding: "Our process controls are such that, by the end of [resin] injection, we know if the part is good."
The key to the advanced RTM process is Dow-UT's ability to design net-size preforms - stack-ups of dry fibres which are the shape and size of the final part. Plan, contour and thickness data are developed and a machine used to stack computer-cut dry plies according to orientation and location, to assist manual assembly in the mould.
Dow-UT was formed in 1989 as a joint venture between Dow Chemical and United Technologies, with the goal of developing RTM for aerospace applications. Today, RTM parts make up less than $30 million of the company's $85 million annual sales, but Scarpati expects the process to account for 75% of Dow-UT's business within ten years.
RTM parts are already being made in production quantities for the F-22 and Lockheed Martin's F-117 fighter and P&W's PW4168 and PW4084 powerplants. Dow-UT is producing 11 shipsets of some 150 RTM parts for the F-22, as well as fan spacers for the PW4084 and thrust-reverser cascades for the PW4168 nacelle. F-117s are receiving RTM engine-inlet grids one-third cheaper than the prepreg components which they replace, with better paint adherence, erosion characteristics and dimensional fidelity, Scarpati says.
"Over the past two years, we have put in place a production capability, largely to meet the demand requirements of the F-22 programme: RTM is no longer experimental. One year ago it was considered high risk; now it is low risk," he says. The fan-exit case "...clearly shows that composites are a practical, cost-effective, alternative to metals" in jet-engine structures.
Source: Flight International
