Size can matter when it comes to prizes and, for one of last remaining competitions for human endeavor in flight, a significant boost in the purse has spurred a neck-and-neck race for the finish line.
By the time these words are read, one of the longest-standing prizes in aviation could have been won, with two teams vying for the American Helicopter Society (AHS) International's $250,000Prize for a human-powered helicopter.
The pursuit of human-powered flight is almost as old as aviation. The first authenticated flight was in November 1961 in the U.K. by Sumpac, the Southampton University Man-Powered Aircraft. The Kremer Competitions for human-powered aircraft had been established in 1959, but it was to take almost 20 years for the first of the prizes to be won.
In August 1977, a team led by designer Paul MacCready took the £50,000 prize for flying a mile-long figure-eight course with the Gossamer Condor, piloted by Bryan Allen. In June 1979, MacCready took the £100,000 prize for crossing the English Channel with the Gossamer Albatross, again flown by Allen. In May 1984, a Massachusetts Institute of Technology team led by John Langford won the £20,000 speed prize for the first flight around a 1,500-meter (4,920-ft.) course in under 3 min., with the Monarch B piloted by Frank Scarabino.
Inspired by the Kremer prizes, AHS in 1980 initiated the Igor I. Sikorsky Human-Powered Helicopter Competition, with a prize of $20,000. The key requirements were to hover for 1 min. and achieve an altitude of 3 meters while staying within a 10-meter-square box. It has proven hard to win.
Several teams tried, and students at California State Polytechnic University at San Luis Obispo built a series of machines that culminated in the Da Vinci IV. Weighing 96 lb., this had a 100-ft.-dia. reaction-drive rotor, powered by propellers at the tips of the two blades. In 1989, the Da Vinci IV flew for 8.6 sec and reached a height of 8 in. to set a record.
In Japan, students at Nihon University developed the Yuri 1—with four 33-ft.-dia. rotors and weighing 89 lb.—that in 1994 flew for 19.46 sec. to set a record. But then the competition stagnated until 2009, when Sikorsky increased the prize to $250,000. This sparked a renewed effort.
“This year marks a third of a century since the prize was offered,” says Mike Hirschberg, AHS executive director. “The prize is the motivation; the ten-fold increase made it interesting. Also, there have been huge advances in composite structures” he says. Now three teams are in active pursuit: Canada's AeroVelo with the Atlas, the University of Maryland's (UMD) Gamera II and CalPoly's Upturn II. Two of the teams planned tests last week that were likely to yield a successful attempt on the AHS Sikorsky Prize.
Atlas and Gamera II are Yuri-inspired quadrotors, a configuration which provides passive stability and puts the rotors as close to the ground as possible to maximize lift from ground effect. Upturn II has a single reaction-drive rotor, as did CalPoly's Da Vinci, but with the propellers on the tips of shorter blades at right angles to the long blades.
There are two major constraints on competitors: the power-to-weight ratio of the pilot, who must average up to 1 hp over 60 sec. of flight to win; and the need to fly the fragile machines indoors, which limits their size. For Atlas and Gamera, the weight of the truss supporting the rotors has placed a major emphasis on developing light—but stiff—structures.
The AeroVelo team is led by Todd Reichert and Cameron Roberston, both graduates of the University of Toronto, where they worked on the Snowbird human-powered ornithopter, flown in 2010. The university is sponsoring the project. The Atlas has four 33.5-ft.-dia. two-blade rotors, a maximum dimension of 162 ft. and weighs around 120 lb.
The blades have non-linear taper; the main spar is a carbon-fiber tube, and the airfoils are expanded-polystyrene ribs with balsa-wood caps, covered with Mylar film. The truss uses carbon-fiber tubes with polymer-wire bracing to share lift loads from the rotors. A modified racing bicycle is suspended by polymer line from the center of the truss. Unique to the Atlas, flight control is via canard surfaces at the blade tips.
UMD students made their first assault on the Sikorsky prize with the Gamera I, which in July 2011 set a hover duration record of 11.4 sec. piloted by Judy Wexler. The team then developed the Gamera II, with reduced airframe weight and increased rotor efficiency cutting the power required from the pilot by 44%. Uniquely, the UMD design has both hand and leg cranks to increase power by up to 20%.
With four 42.6-ft.-dia. two-blade rotors, and a maximum dimension of 105 ft., the Gamera II weighs just 71 lb., 35 lb. less than the Gamera I. Flight tests showed blade flexing had a big impact on ground effect, so the Gamera II has a tapered blade, which improves aerodynamics and increases root stiffness, keeping tip deflections low. The structure was redesigned into a “truss of trusses,” with micro-trusses replacing carbon-fiber tubes to reduce weight and increase stiffness.
The Upturn II was designed and built by Neal Saiki, who led CalPoly's Da Vinci team. The aircraft has two blades of 85-ft.-dia., and two 48-ft.-dia. blades with 6-ft.-dia. propellers at the tips to drive the rotor. The Upturn II weighs 95 lb., and made a 10-sec. first flight in June 2012. Saiki's company, NTS Works, subsequently donated the aircraft to CalPoly, which has focused on reducing weight and improving stability.
CalPoly hopes to resume flight tests “in about a month or so if pre-flight testing goes well,” says student lead Amber Carney. “We are hoping to have manufacturing essentially completed by the end of the month,” she says, adding “We are staying on track.” But AeroVelo and UMD are ahead.
UMD flew the Gamera II in June 2012, setting a record for flight duration of 49.9 sec. piloted by Kyle Gluesenkamp. In August and September, the team tested a refined aircraft, staying aloft for 65 sec. on one flight and reaching a height of 9 ft. on another. The Gamera II XR has 70-cm longer blades and foam-shell outboard blade sections to maintain airfoil shape. These and other modifications increase power and lift, but add about 6 lb. to the weight.
AeroVelo began flying the Atlas in August 2012, piloted by Reichert, and has routinely exceeded 30 sec. as it tests modifications to trim the vehicle and controls. A few issues remain to be ironed out, but “we know the power requirements of the helicopter are reasonable for the pilot and the controls will be sufficiently effective during the Sikorsky flight,” says Robertson. All three contenders drift in flight, making it difficult to stay within the 10 x 10-meter box to win the prize, but only the Atlas has manual flight control.
With the pilot using both hands and feet to power the aircraft, UMD faced a challenge developing a control system for the Gamera II. But an attempt to clarify the rule prohibiting energy-storage devices inadvertently opened the door to electronic controls being used on both the Gamera II and Upturn II.
AHS has closed the loophole, but both teams have until July to attempt winning the prize under previous rules. UMD tested a control system based on center-of-gravity shifting in November 2012, but it added too much weight. A new control system has been developed, but kept secret. “However, our reading of the rules does suggest that an electric system would only be allowed if the pilot were to generate the electricity used during the flight,” says project manager William Staruk.
Human-powered flight may seem a far cry from practical everyday aviation, but Kremer prizewinner MacCready founded, now a leading supplier of unmanned aircraft, while Langford formed Aurora Flight Sciences.
After the success of the Gossamer Condor and Albatross, AeroVironment developed two solar-electric aircraft. The Gossamer Penguin flew in 1980, and in 1981 the Solar Challenger made a 163-mi. flight from France to England. The company then developed a series of solar-powered stratospheric UAVs, first secretly for the Pentagon, then for, with the 100-ft.-span Pathfinder reaching 71,500 ft. in 1997 and the 247-ft.-span Helios achieving a record altitude of 96,863 ft. in 2001.
AeroVironment found fame making small hand-launched UAVs, but returned to its origins in aerodynamic and structural efficiency with the hydrogen-powered Global Observer. Designed to fly for a week at up to 65,000 ft., the 175-ft.-span UAV flew in August 2010, but crashed in April 2011.
Aurora was formed a year after MIT's Daedalus set human-powered endurance and distance records with the idea of finding practical applications for the technology. The first of these was the Perseus gasoline/liquid-oxygen-powered high-altitude UAV built forto perform climate research.
Aurora returned to its roots to build the hydrogen-powered, high-altitude Orion, which never flew, but instead morphed into a diesel-powered medium-altitude, long-endurance UAV—work on which has been stalled by lack of funds. But closest to the company's origins was its design work on the U.S. Defense Advanced Research Project Agency's now-defunct Vulture program to develop a solar-powered stratospheric UAV capable of staying aloft for five years.
Technologies developed for human-powered aircraft could also find use in Mars vehicles, but Hirschberg says one certain benefit has been the “galvanizing effect” of presenting students with a seemingly impossible task.
To watch videos of the human-powered helicopter flights and learn more about the projects, go toAviationWeek.com/innovation