The richer purse broke a lull in activity, and in 2011 graduate students from UMD flew their Gamera I for 11.4 sec. The vehicle was then redesigned, the rotors enlarged, and in August 2012 the Gamera IIA flew for 65.1 sec., setting a world duration record for HPH flight.
“We knew of the prize, but were not fully aware of its value until we saw UMD doing their tests,” says Reichert. While students at the University of Toronto, Reichert and Cameron Robertson built and flew in 2010 the first human-powered flapping-wing ornithopter, the Snowbird, and saw the AHS Sikorsky prize as “a monumental achievement” to go after.
Leishman says the need to maximize rotor lift and minimize pedal power drives the design to low airframe and pilot weights and large rotor sizes. “The key to their success is they have four very large rotors and very low disk loading,” he says. This reduces the lift-induced component of power required, while keeping rotor speed low reduces the profile component.
“We decided to make it as large as it needed to be, and knew it would have to be absolutely massive, but instead of being able to fly inside a gym we wanted to build it for minimum power requirements,” he says. “At the same time, we used a lot of creative ideas to be as light as possible, including the truss structure and rotor spools.”
Atlas has a rotor diameter of 66.2 ft., total span of 153.9 ft. and disk area of 13,730 sq. ft.—making it larger than any production helicopter. By comparison UMD's final Gamera IID version had a rotor diameter of 23.6 ft. and total span of 98 ft. Despite its size, the Atlas has an empty weight of 122 lb., compared with 90 lb. for the smaller Gamera.
AeroVelo drew on experience with the Snowbird in designing the Atlas. “We wanted to win in as quick a time as possible, because UMD was getting close, so we made improvements from the Snowbird but we did not totally reinvent things,” says Reichert. While designed to minimize risk, the wire-braced truss structure “is highly optimized,” says Robertson.
Both Atlas and Gamera, like Yuri before them, have the rotors located as low as possible to benefit from ground effect, which increases lift while reducing drag. But the benefit decreases rapidly with increasing height above ground, making the pilot's initial sprint to altitude “like climbing up a hill that is getting increasingly steep, very quickly,” says Leishman.
Atlas benefited from its larger rotors, as ground effect depends on rotor diameter, but longer blades deflect more and the tips move out of ground effect so both teams evolved ways to make them stiff and light enough. “They were very innovative in optimizing the structure for stiffness and lightness, and there may be a possible spin-off there,” says Leishman.
Power-to-weight is everything in a helicopter, and AeroVelo took an engineering approach to optimizing the performance of Reichert, a competitive cyclist, as the powerplant. “I trained very specifically for this, and not the way a racing cyclist would,” he says. The flight involved a 15-sec. sprint to reach altitude, then sustained power for up to 60 sec. to allow a gradual descent.