“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.