Last October, when a who's who of Vermont politicos and businesspeople gathered to celebrate the opening of Beta Technologies' electric aircraft factory in South Burlington, the production floor was still empty. A year later, the 188,000-square-foot space — as open and tall as an indoor arena — has come to life. Technicians tinker beneath an unpainted fuselage. Fifty-foot wings of Alia, the company's five-seat electric plane, hang from assembly mounts. Thousands of battery cells enter one end of a production line in cardboard boxes and exit the other as suitcase-size packs that can power a plane. The company's first product to go to market, tidy charging stations that can refuel the Alia in about an hour, sit in various stages of assembly. Here, at the far end of the Burlington International Airport along Williston Road, is the only place in the U.S. where you'll find a commercial-grade factory configured to produce what is the most anticipated advancement in aviation this century: an electric plane that can take off and land vertically, like a helicopter, untethered from airstrips. What you won't see — yet — are finished planes at the end of the assembly line waiting to be shipped to customers. Beta remains locked in a high-stakes race alongside companies in Silicon Valley, China, Germany and elsewhere to bring this new form of aviation technology to market. That race has entered a grueling middle stage, as companies seek to turn their prototypes into sellable products. To do that, and to operate in the U.S., each must win the approval of the Federal Aviation Administration — both for their plane designs and for their manufacturing processes. For now, the production floor in South Burlington functions as a kind of proving ground where Beta technicians are working to create and refine each step of manufacturing and assembly. It's a crucial transition for a company that has raked in nearly $1 billion in investment on the basis of its prototypes alone. "A lot of our work has been the not-as-sexy stuff — just grinding out, How do you become an aerospace manufacturing company? That's very different from being a design and R&D company," chief information officer Blain Newton said. Embracing the grind has been Beta's secret sauce. Unlike many of its competitors, the company has pitched itself as pursuing a more realistic, if somewhat less glamorous, path to an electric aviation revolution. Rather…