The featured speaker on the last day of the 2013 SAMPE Technical Conference (Oct. 20-24, Wichita, Kan.) was Pierre Harter (Bombardier Business Aircraft, Montréal, Quebec, Canada). As the engineering manager – M&P, technology readiness and structural certification for Bombardier’s Learjet business unit, he provided a wealth of new information about the composite materials and manufacturing processes used to fabricate aerostructures for the forthcoming Learjet 85 business jet.
Wingskins and spars for the plane are manufactured in Belfast, Northern Ireland, using an in-autoclave resin transfer infusion (RTI) process. The fuselage and empennage are manufactured in Querétaro, Mexico, via an out-of-autoclave (OOA) process. Harter reported that the plane, which, at this writing was expected to make its first flight in late 2013 or early 2014, makes use of composites not only to reduce airframe weight and increase fuel economy, but also to significantly reduce the part count because composites have enabled Learjet to produce large, integrated structures. In the process, Harter says, Bombardier has learned much about materials characterization, materials management, process development and certification.
Harter said the RTI process uses dry carbon fiber noncrimp fabric (NCF) that can be quickly cut to size on flatbed machinery from Gerber Technology Inc. (Tolland, Conn.). Efficiently layed up, the NCF fabric incorporates a binder that facilitates preforming on a male tool, followed by infusion in a female tool. The tool is preheated, the part is bagged and Cycom 890 RTM resin, supplied by Cytec Aerospace Materials Tempe, Ariz.), is injected during autoclave cure. Stringers are cocured with the upper and lower wingskins. Harter said the process/material combination is certified by the U.S. Federal Aviation Admin. (FAA) for the application.
More challenging, said Harter, was the OOA prepreg material and process development for the fuselage, done in close cooperation with Cytec, which provided Cycom 5320 resin for this application. Harter described Cytec’s assistance as crucial to the plane’s development. The fuselage is manufactured in three sections: the nose, the main fuselage and the tail. The main fuselage is 30 ft/9.1m long and represents one of the largest OOA parts of its kind. Challenges included the fact that the Bombardier facility in Querétaro is more than 6,000 ft/1,829m above sea level, which reduces the available vacuum pressure compared to lower altitudes. Further, Harter said that although Bombardier is adept at developing composite material parameters for autoclave cure, the OOA process challenged many of the company’s cure management assumptions. Compaction, air removal, resin flow and ply placement all had to be adjusted, managed and fine-tuned much differently than they would have been for autoclave curing. In addition, the OOA process was unexpectedly sensitive to difficult design features.
Bombardier also had to evaluate breathing methods, bulk and debulk cycles, dwell times and rheology to achieve the desired void content. Curing the part too quickly generated many small voids in the composite. Combating porosity required perfecting the management of resin viscosity over time, Harter said, and was achieved only after extensive trial and error. “Legacy flow and gel times are not adequate,” he noted. Bombardier eventually developed an OOA manufacturing process that produced voids of less than or near 1 percent.
Harter also related that after the processes were optimized, Bombardier faced the daunting task of achieving FAA certification. As was true with the Boeing 787 and the Airbus A350 XWB, the intensive use of composites on the Learjet 85 meant that Bombardier had to perform extensive testing to satisfy the FAA’s special conditions for certification. Much of this, Harter said, focused on inflight flammability, postcrash flammability, crashworthiness, durability, toxicity in burn, damage tolerance and thermal expansion at interactions with metals. The results across the board were positive. In fact, noted Harter, composite materials on the Learjet 85 outperformed aluminum in flammability and crashworthiness tests — a fact that he believes needs to be emphasized more by aerospace composites professionals. The composite readiness testing is now complete, and certification testing has begun.