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U.S. Team Pushes Boundaries of VTOL Flight, Electric Propulsion

 - June 14, 2017, 9:33 AM

A U.S. industry team headed by Aurora Flight Sciences has demonstrated that a subscale version of a novel vertical takeoff and landing (VTOL), forward-flying unmanned aircraft design actually works. The challenge now is to build into a full-scale aircraft the hybrid-electric propulsion system that will make it radically faster and more efficient than today’s helicopters.

After completing flight tests of a 20-percent scale demonstrator this spring, an Aurora team that includes engine manufacturers Rolls-Royce North America and Honeywell Aerospace have started building a full scale XV-24A “LightningStrike” aircraft under the U.S. Defense Advanced Research Project Agency (Darpa) VTOL X-plane program. Plans call for flying the 12,000-pound, tilting-wing, tilting-canard aircraft late next year.

Darpa announced a second-phase, $89.4 million contract award to Aurora to build the VTOL X-plane demonstrator in March 2016, following a competitive design phase. The company started flying the 325-pound subscale demonstrator later that month and this March completed flight tests at Webster Outlying Field, a U.S. Navy facility in St. Inigoes, Maryland.

The regime served to validate the aerodynamics and flight-control logic of the design and covered both hover and forward-flight modes. The miniature LightningStrike features a 10.7-foot wing and a forward canard surface structured from carbon fiber and 3D-printed FDM (fused deposition modeling) plastics. The replica proved the concept, but used lithium polymer batteries as its power source.

While the LightningStrike airframe is interesting, the beauty of the aircraft lies in its propulsion system. A single Rolls-Royce AE 1107C turboshaft engine mounted in the fuselage and coupled to a gearbox will drive three Honeywell one-megawatt electric generators that will power 24 ducted fans distributed across the wings and canards. The fan units will be synchronized to the generators and turn at a constant RPM with variable pitch. The wings and canards will tilt upward for vertical flight and rotate to a horizontal position for forward flight. Among other participating companies, ThinGap, of Camarillo, California, is providing the electric motors; Ingenium Aerospace of Rockford, Illinois, is providing actuators.

A propulsion system that maximizes power while minimizing weight is necessary to meet Darpa’s specifications for a VTOL aircraft that is capable of flying at 300-to-400 knots top sustained speed, with a 15 percent increase in hover efficiency and the ability to carry a useful load of at least 40 percent of its gross weight. More broadly, the work the Aurora team is performing on the Lightning Strike program is paving new ground toward electric-powered aircraft.

“From the onset Honeywell’s task in this program has been clear—to realize hybrid electric propulsion, vertical takeoff and landing (capability) and to ensure this approach plays a larger part and a more consistent role in the way humans fly in the future,” said Tom Konicki, Honeywell Aerospace director of business development. “We have no choice but to master efficient power generation for these energy-hungry aircraft.”

At 300 pounds, Honeywell’s megawatt generator is five times smaller in size and weight compared to other generators that produce similar power, said Konicki, who participated with other executives in a LightningStrike program briefing at the Navy League Sea-Air-Space conference in April.

Rolls-Royce is bringing to bear capabilities for hybrid electric power generation and distributed control systems, plus experience as an engine supplier on various VTOL platforms—the AE 1107C is the engine that powers the V-22 Osprey—as well as unmanned aircraft including the RQ-4 Global Hawk/MQ-4C Triton and MQ-8B and C Fire Scout. The advanced power generation system the manufacturer is developing for the LightningStrike features an innovative electrical distribution system that helps minimize weight, said Mark Wilson, Rolls-Royce North American Technologies chief operating officer.

“In a VTOL aircraft, obviously weight and power density is extremely important,” Wilson said. “If you were to look at trying to do this mechanically, it would be very difficult from the number of gearboxes and chassis—things we’d have to have,” he added. “We’ve come up with an electric synchronous control system that allows you not to (need) some of the conversion boxes to go from AC to DC back to AC (current) again. So we’ve been able to take quite a bit of weight out of the system, but integrating all that system into a new vehicle like this is really the big challenge.”

Rolls-Royce was “making rapid progress” and had started building elements of the LightningStrike propulsion system. It planned to conduct an “iron bird” systems integration test with the full system and a one-third set of fans later this year at its Indianapolis facility. Aurora was fabricating and assembling the full scale aircraft at its facility in Manassas, Virginia, with the aim of rolling it out by year-end. Plans then call for trucking the aircraft to NAS Patuxent River for flight testing.

“We have pursued, developed and flown an extraordinary aircraft and matured key and innovative technologies in support of the future of vertical flight,” declared Aurora founder and CEO John Langford. “This is clearly an achievement like no other, and will be surpassed only by the flight of the full-scale aircraft.