It took 33 years, but perhaps the final great feat of human endeavor in aviation has been accomplished, with Canada's AeroVelo team winning the $250,000 prize for human-powered helicopter flight, offered by the American Helicopter Society (AHS) International.
Piloted and powered by Todd Reichert, AeroVelo's Atlas flew for 65 sec., reaching a height of 3.3 meters (10.8 ft.) and staying within a 10-meter box to win the prize. The flight took place June 13 at an indoor soccer center in Vaughan, Ontario. The University of Maryland's (UMD) Gamera team completed flights of its human-powered helicopter (HPH) on June 26 without fulfilling the requirements, but set an unofficial duration record of 74 sec.
“This was not about creating a practical helicopter,” says AHS Executive Director Mike Hirschberg. “It was intended to challenge engineers in the vertical flight community to harness a variety of technical skills and work as a team to meet stringent technical requirements.”
Inspired by Paul MacCready's Gossamer Condor winning the Kremer prize for human-powered aircraft flight in 1977, the HPH competition was established by AHS in 1980 after a study showed it was feasible—in theory. Named after helicopter pioneer Igor Sikorsky, the prize initially was just $10,000. But the competition heated up after 2009, when Sikorsky Aircraft pledged $250,000 for the prize.
To win, an HPH had to hover for 60 sec., reach 3 meters altitude and stay within a 10-meter box. Duration was set to “approach the limits of human endurance,” and required efficient power extraction and transmission. The height goal emphasized maximizing lift and minimizing weight, while the drift limit put a premium on controllability. “It took a third of a century of aerospace development—and a significantly larger prize—to make it realizable,” Hirschberg says.
Gordon Leishman, a professor in rotorcraft at UMD, considered the prize “all but impossible to win.” Looking at the study that set the rules, he saw “the most wildly optimistic assumptions” and concluded “there was no way anybody could do this.” Tipping his hat to both teams, he says “the science behind this means they had to get everything right.”
The first HPH to fly was California Polytechnic State University's Da Vinci III, which hovered for 8.9 sec. in 1989, while in 1994 a Japanese team at Nihon University flew the Yuri I HPH for 19.46 sec. Where Da Vinci had a single reaction-drive rotor, Yuri was a quadrotor—a stable configuration later adopted for Gamera and Atlas.
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.
Roberston says sensors on the Atlas show Reichert put out a peak of 1.1 kW of power, decreasing to 600 watts at the end. “The flight took almost everything I had,” Reichert admits. “He put out a hell of a lot of power,” acknowledges Leishman, noting a more typical figure is well below a kilowatt and that Gamera used both leg and arm power to increase its pilot's output.
UMD's biggest challenge, says Gamera team leader Will Staruk, was controlling drift. At very low disk loadings it does not take much to enter vortex ring state, the rotor equivalent of wing stall, Leishman says, and the HPHs drifted on descent. UMD added an rpm-varying electronic control system, “but did not have time to practice at altitude,” says Staruk.
AeroVelo, meanwhile, “changed its control strategy completely,” says Robertson, which had the effect of simplifying the machine and reducing weight—the opposite of what happed with Gamera. Originally, the Atlas had canard control surfaces at the tips of the blades, but these proved “untrimmable for stability and unpredictable to actuate,” he says.
While testing, they noticed how flexible the structure was and how easy it was to tilt the rotors, “like thrust vectoring,” Roberston says. With lightweight control lines connecting the bottom of the bike frame to the bottoms of the rotors, Reichert could lean the bike to pull on a line, bend the structure and tilt a rotor. This fast-reacting system overcame the problem of controlling drift.
Mark Miller, Sikorsky vice president of research and engineering, cites the “very creative way” AeroVelo took advantage of the flexible structure as an example of the “passion, drive and ingenuity” brought out by the competition. “AeroVelo used advanced materials and an inventive structural approach that was all about power-to-weight,” he says.
“Open competitions like this are just another mechanism we have used to ignite innovation in this industry and create an environment that has people taking on really tough problems,” says Chris Van Buiten, director of Sikorsky Innovations. “We've hired great talent directly out of these teams. We like this model for innovation.”
Tap the icon in the digital edition of AW&ST to watch a video of the prize-winning flight, or go to AviationWeek.com/video
Read more about the competition on our ThingsWithWings blog at ow.ly/mROKZ