It is fitting that, in Sikorsky Aircraft’s 90th year, the prize for human-powered helicopter flight established 33 years ago by the American Helicopter Society in the name of company founder and industry pioneer Igor Sikorsky should finally be won.
But, after all this time, how did Canada’s AeroVelo team do it?
June 13 prize flight (Photo: Martin Turner, Visiblize.com)
First, the prize itself. Inspired by Paul MacCready’s Gossamer Condor winning the Kremer prize for human-powered flight, AHS studied whether a human-powered helicopter would be possible and decided it was theoretically feasible. So the Sikorsky prize was established in 1980 with a purse of just $10,000. This edged up over the year until 2009, when Sikorsky pledged $250,000 as the prize.
To win, a human-powered helicopter (HPH) had for fly for 60sec, reach 3m altitude and stay within a 10m box. The duration was chosen 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 – something all the teams were challenged by.
Arguing the rules were based on “the most wildly optimistic assumptions,” Gordon Leishman, a respected professor in rotorcraft at the University of Maryland (UMD), in March declared the prize “all but impossible to win”. UMD’s own Team Gamera came tantalizingly close, and ended tests in June having achieved the requirements for the prize – but all not on one flight.
AeroVelo, meanwhile, was formed by two former students of the University of Toronto Institute of Aerospace Sciences (UTIAS). Todd Reichert is chief aerodynamicist and a racing cyclist; Camerson Roberston is chief structural designer. While at UTIAS they built and flew the Snowbird, the first human-powered flapping-wing ornithopter to achieve sustained flight.
AeroVelo’s Atlas is a quadrotor, a stable configuration first used by Nihon University’s Yuri I HPH, which flew in 1994. UMD’s Gamera is also a quadcopter, but where UMD sized their HPH to fly inside a sports stadium, AeroVelo decided to let their HPH be as big as it needed be, then worry about where to fly it. And it is massive – luckily there was an indoor soccer field nearby large enough in which to fly.
Making Atlas very big, but very light is probably the first key to Aerovelo’s success, as this minimized disk loading and reduced the lift-induced component of the power required. But it required some creative thinking to minimize weight, using a wire-braced carbon-fiber trust structure. Atlas has a rotor diameter of 66.2ft, a total span of 153.9ft, but weighs barely 122lb empty and 282lb with Reichert on board.
The basic structure is a X-shaped truss supporting the rotors and braced by multiple Vectran lines. The pilot hangs from the truss on more Vectran lines. The rotor blades have tapered carbon-fiber spars, balsa and polystyrene ribs, wrapped in Mylar film to create the aerofoil, with expanded-polystyrene leading edges and Kevlar training edges. Pedaling pulls Vectran drive lines from spools under the rotors to a drive spool on the bike frame.
The second key was Reichert himself. To win the prize, he trained to sprint for 15sec to achieve the height goal, then sustain power for 60sec while gradually descending to meet the duration goal – which is not how a cyclist would train, he notes. The rotors are located as low as possible, to maximize the ground effect, which increases lift, but this advantage reduces rapidly with height, so the initial sprint “is like climbing a hill that gets increasing steep, very quickly,” says Leishman.
A typical human power output would be muchless than a kilowatt, but on the prize flight Reichart put out a peak of 1.1kW and was still producing 600W at the end of the flight. Several observers attribute a significant part of Aerovelo’s success to his athleticism. The prize flight “took almost everything I had, I maybe could have gone another 10-15sec,”, says Reichert.
A third key for AeroVelo was its control strategy. Will Staruk, head of the UMD Gamera team, says their greatest challenge was getting control of the drift. They eventually added a pilot-powered electronic control system that varied individual rotor rpm to steer the helicopter, but it added weight and they were never able to get to altitude with the system installed.
On descent at very low disk loading it does not take much to enter vortex ring state, the rotor equivalent of wing stall, and AeroVelo had a couple of crashes from altitude caused by VRS. This led to a change of control strategy. Originally the rotors had canard control surfaces at their tips, but these provide unpredictable and untrimmable so they were removed, saving weight.
Instead, the team noted the flexibility of the structure and how easy it would be to bend the truss and tilt a rotor to vector its thrust. So the attached control lines from the bottom of the bike to the bottoms of the rotors. The counter drift on descent, Reichert would mean the bike to pull on a line, bend the structure and tilt a rotor. It worked fast and overcame the drift issue.
But the final, and perhaps crucial, key to Aerovelo’s success, observers says, was the creativity of the team, which included students from UTIAS, in coming up with innovative engineering solutions to the challenges of human-powered helicopter flight. They give similar praise to the UMD Gamera team, and Sikorsky says it has already hired members of both teams.
For AeroVelo, if is not over. The Atlas will fly again in the fall to give other members of the team an opportunity to fly. Another small team has begun work on a high-speed land vehicle, with which AeroVelo hopes to set a human-powered land speed record in the fall. There are also two more human-powered aircraft in the works, says Robertson.
Longer term, they plan to establish an experiential learning program that will allow new engineering students to create innovative designs and learn how to “do more with less,” he says, “and show them how innovation and creativity are key to achieving the next step in every field, so that they will carry that passion with them into industry.”