July 15, 2013
When Wichita thought of tearing down a former concert and sports arena on the north end of town, John Tomblin, executive director of the National Institute for Aviation Research (NIAR), told city leaders there was a good reason to save the 56,000-sq.-ft. structure. Its domed roofline and big flat floor were just what the institute needed for an aircraft test center. So out came the concert seats and in went a hangar door.
When the conversion was completed last February, NIAR already had its first client. Learjet, one of the institute's early backers among Wichita's diverse aviation manufacturing community, brought in the static test article for its first composite aircraft, the Learjet 85. Composites are one of NIAR's main research subjects and the new 8-place mid-sized business jet offers an early chance to test the fatigue life of an unusual set of wings.
Learjet and its parent, Canada's Bombardier Aerospace, have developed a proprietary resin transfer injection (RTI) technology aimed at eliminating the need to pre-impregnate (prepreg) carbon fiber reinforced plastic (CFRP) before the fabric is wound into or laid up in the build process. Prepreg is the industry's dominant method for creating composite parts and assemblies, especially large structures, but it is time-consuming and expensive.
In RTI, the carbon fiber is laid up dry. The resin that will bind the multiple fabric layers that make up a composite structure is not injected until the completed structure is in the autoclave. Although the method has not been applied universally, it works particularly well on flat surfaces that can be formed horizontally, such as wings and spars, and it allows skins and stringers to be formed in a single piece.
RTI is favored as it gives greater control over forming complex shapes, reduces part counts and provides better surface finish than traditional prepreg infusion methods, says Learjet 85's structures director, Pierre Harter.
“Prepreg is clock-limited because the resin cures, so it has to be stored properly,” he says. When fabric is resin-infused it has to be stored cold because it begins curing in warm air. Because the dry-fabric RTI process does not receive its epoxy infusion until curing is ready to begin, it automatically assures greater flexibility in the manufacturing process, Harter says.
But Bombardier's patent-pending process, which the company describes only in general terms, does not work on every large structure, such as fuselage barrel formation because gravity prevents the non-adhesive dry tapes from staying in place on the undersides of structures. The Learjet 85 has a 32-ft.-long carbon fiber barrel section produced in the company's Queretaro, Mexico, factory, but it is created with traditional pre-preg, not RTI.