Hovering ten feet off the ground doesn't sound so hard if you've got a helicopter—but if the helicopter is powered by a human, then that hover has confounded inventors since Leonardo da Vinci.
Now, the Canadian team AeroVelo has finally done it with the Atlas, a copter that looks more like a gigantic ceiling fan than an aircraft.
Hot on their heels was a team from the University of Maryland, both gunning for the $250,000 Sikorsky Prize and the dream of human-powered flight. (See: "If We Only Had Wings: The Daring Dream of Personal Flight.")
Decades of Competition
The challenge sounded simple enough: Fly three meters off the ground for 60 seconds while keeping the cockpit within a ten-square-meter area. But in the nearly three decades since the American Helicopter Society launched its Igor I. Sikorsky Human-Powered Helicopter Competition, only five such craft have managed to get off the ground.
Launching a helicopter is much more difficult than launching a winged craft, aeronautical engineers say.
"Aerodynamically, it takes a lot of power to hover," explained Bill Patterson, an emeritus professor of engineering at Cal Poly San Luis Obispo who led the effort to build Da Vinci III, the first human-powered helicopter to lift off. "Flying forward, you're grabbing a lot more air because of your forward velocity."
The Problem of Size
The Cal Poly students set their sights on the prize shortly after it was announced in 1980, but it took until 1989 for them to get a craft aloft. Even then, the Da Vinci III hovered just 20 centimeters off the ground for seven seconds. Powered by a student pedaling a modified bike chain drive, it resembled a traditional helicopter, with two rotor blades and a 50-foot radius.
"We knew, and everyone knew, you had to kill it with size," Patterson says now.
Bigger rotors mean more lift, but they also mean more weight. Balancing size, weight, and control have proved a struggle since the days of Icarus, the mythical Greek who died after an overenthusiastic test flight of his wax-and-feather wings.
The Cal Poly team could never figure out how to keep its craft from either slipping sideways outside the ten-square-meter box or crashing, Patterson said. At the time, he called it an "untamed beast."
But a Japanese student team figured out how to calm the beast. They created the Yuri, a 20-square-meter machine with an X-shaped fuselage and a five-meter-wide rotor blade whirring on the end of each X. The huge size made it possible to generate more lift, and the four rotors counterbalanced each other. It first flew in 1994.
All the human-powered copters since, including the Canadian Atlas and Maryland's Gamera, have used the four-truss design pioneered by the Japanese. The University of Maryland Department of Aerospace Engineering launched its Gamera project in 2009. And while Gamera was cooking, a group of engineering students at the University of Toronto (two of whom later worked on the Atlas) built a human-powered ornithopter, a craft that flies by flapping its wings. (See photo gallery: "Amazing Transportation Inventions.")
Landing on a Winner
Eager to take on another craft and inspired by Maryland's success, the Canadian team started working on Atlas in January 2012.
This spring the competition intensified, with both the Canadians and the Marylanders nailing the Sikorsky Prize's three requirements—just not all in the same flight.
The Atlas finally hit all three goals in one flight on June 13, 2013, and was officially declared the winner of the Sikorsky Prize on July 11.
In the end it came down to control, according to Cameron Robertson, the chief structural engineer for the Atlas. "We realized it was not simply getting to three meters or flying for 60 seconds, but staying inside the box." The first control system they designed for the Atlas failed to keep the rotors in balance. On one flight, two rotors broke—an expensive setback for a project partially funded by KickStarter contributions.
Then Robertson and his colleagues realized that they could use the craft's flexibility as an asset, rather than a liability. They ran lines from the pilot's seat in the center of the craft to the ends of the trusses. When the pilot leaned, the rotors tilted.
"It was very intuitive, a very fast response," Robertson said. "It also allowed us to save 8 percent of the helicopter weight."
It also meant they didn't have to spend quite so many hours making minute adjustments to the rotor blades to keep them balanced each time they set up the craft in an indoor soccer stadium.
Yes, these craft are so delicate that they have never flown outdoors. But that doesn't mean that inventors aren't imagining a day when human-powered aircraft will be used for Olympic competition. The AeroVelo team is already looking ahead to other challenges: The British Royal Aeronautical Society has called for a human-powered craft that can fly 26 miles in an hour, and for one sturdy enough to fly outside and be suitable for sport competition.
Want to see for yourself? For the next two weeks, the Royal Society's Icarus Cup is luring aviators and engineers eager to compete in human-powered craft of all kinds to Sywell, England. No word yet on whether any of the competitors are using feathers and wax.