Two experimental flight teams are down to the wire in a race to win the American Helicopter Society’s Sikorsky Prize. Established in 1980 in honor of helicopter pioneer Igor Sikorsky, the contest will reward the first person or team to build a human-powered helicopter. It must lift off and hover for at least one minute, achieve a height of three meters and remain within a 10-square meter space during flight.

Student teams from the University of Maryland and the University of Toronto are two prime contenders for the $250,000 prize, pledged by Sikorsky Aircraft Corporation, Stratford, Conn. Both schools have created and tested helicopters made from carbon fiber.

A team of 40 graduate and undergraduate students from the University of Maryland’s A. James Clark School of Engineering built and tested the Gamera I helicopter in 2010-2011. It was the third human-powered helicopter to ever fly and stayed airborne for more than four seconds, says William Staruk, project manager and a graduate student. The team designed a lighter and more efficient helicopter – the Gamera II – in June 2012.

The Gamera II relies on carbon fiber trusses for its main structural elements. “These structures are really effective at bending loads that we see on a helicopter,” says Staruk. “Typical helicopter blades use a tubular spar, D-spar or box spar. But those tense up from local buckling. A truss-based design avoids that local buckling issue.” The team also created a “truss-of-trusses” design so the most critically loaded members in the airframe were replaced with filament wound micro trusses, achieving significant weight savings.

The Gamera II has a rotor at each of the four ends of its X-shaped frame, with the pilot’s module suspended in the middle. Each crossbar of the frame is 60 feet long, while each rotor is 42.6 feet in diameter. But the vehicle weighs only 82 pounds thanks to lightweight materials such as balsa, foam, Mylar and carbon fiber.

The pilot adds the greatest weight by far: Gamera II’s test pilots weigh between 122 and 135 pounds. They power the helicopter through hand and foot pedaling, with chains, gears and lightweight string transmitting power to the rotors.

The helicopter has come tantalizingly close to meeting all of the Sikorsky Prize requirements. Last June, the Gamera II set an official U.S. record for a 49.9-second flight, verified by the National Aeronautic Association. On a subsequent flight in August, the helicopter hovered for 65 seconds, while staying within the 10 square-meter area. That flight is under review by the association. Gamera II also reached an altitude of 9.4 feet in yet another flight test. But combining all the requirements in one flight has proven elusive so far.