The Learjet 85: Large step out of the autoclave
Bombardier’s Learjet 85 business jet flew for the first time on April 9 in Wichita, Kan. An aircraft’s first flight is a significant milestone, and good cause for celebration. However, the Learjet 85’s first foray into the air is, potentially, even more significant for the aerospace industry, and might be offering us, at least symbolically, a glimpse into the future of resin, fiber and process use in composite aerostructure manufacturing.
The Learjet 85’s significance does not lie in its composites-intensity. Almost every aircraft under development today — in commercial, business and general aviation — will fly composites in some way, and many will make extensive use of carbon fiber composites in the fuselage, wings, tail and other structures. What sets the Learjet 85 apart is how and with what Bombardier is manufacturing the plane’s composite structures.
Pierre Harter, engineering manager – M&P, technology readiness and structural certification Learjet, reported at SAMPE Tech in Wichita late last year that the wingskins and spars for the plane are manufactured in Belfast, Ireland, using an in-autoclave resin transfer infusion (RTI) process. Moreover, the fuselage and empennage are manufactured in Querétaro, Mexico, via an out-of-autoclave (OOA) vacuum-bagged process.
Infusion and OOA are not new, but their use in the manufacture of major aerostructures was, prior to the Learjet 85 program, largely unexplored territory. The production of the fuselage is particularly ambitious. It’s done with Cytec Aerosapce Material’s (Tempe, Ariz.) CYCOM 5320 prepreg, under vacuum bag in a conventional oven — at about 6,000 ft/1,829m above sea level in south-central Mexico. The altitude, of course, makes the vacuum calculations more challenging. On top of that, Harter says breathing methods, debulk cycles, dwell times and resin rheology needed special tweaking to achieve less than 1 percent void content in fuselage parts.
When asked why Bombardier is taking the time, and going to the expense and effort required to develop an OOA process for the Learjet 85, Harter said the company saw that aerostructures manufacturing was headed in this direction and wanted to be in front of the technology, not chasing it … or competitors.
Even more time, expense and effort, of course, was required to meet the most important challenge: the U.S. Federal Aviation Admin. (FAA). As Boeing and Airbus did with the 787 and A350 XWB respectively, Bombardier was required to perform extra tests to meet the FAA’s special conditions for certification. These focus on inflight flammability, post-crash flammability, crashworthiness, durability, toxicity in burn, damage tolerance and thermal expansion at interactions with metals.
The more composites are used in aircraft, the more familiar and comfortable the FAA will become with what is still, in its view, a relatively novel material. Thus, theoretically, composite aircraft certification will become easier and faster. Until then, however, airframers like Bombardier will bear the brunt of the extra scrutiny on behalf of what should be a grateful industry and will surely earn a place among composites industry pioneers. Less certain, however, is the Learjet 85’s place in the evolution of composite materials and process development. Does it mark the first large step out of the autoclave, or will it be an historical anomaly? I would wager the former, and I look forward to what the aerocomposites industry does next.
A look at the process by which precursor becomes carbon fiber through a careful (and mostly proprietary) manipulation of temperature and tension.
As composites take a larger part (and form larger parts) in the aerospace structures sector, it’s not just a make-it-or-break-it proposition.
Yes, advanced forms are in development, but has the technology progressed enough to make the business case?