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Aurora Flight Sciences won DARPA’s Vertical Takeoff and Landing Experimental Plane (VTOL X-Plane) program competition. The program seeks to take advantage of cross-pollination between fixed-wing and rotary-wing technologies.

Aurora developing distributed hybrid-electric propulsion system for VTOL X-Plane

Aurora Flight Sciences has officially made it to Phase 2 of DARPA’s Vertical Takeoff and Landing Experimental Plane (VTOL X-Plane) program and will be building a demonstrator aircraft, with its design being the winning one during Phase 1 competition.

In making the original solicitation announcement in February 2013, DARPA made clear that while engineers have improved the speed of fixed-wing aircraft—achieving two and three times the speeds of jets designed since the 1960s—attempts to increase efficient VTOL aircraft speed have not exactly moved at the same pace.

"For the past 50 years, we have seen jets go higher and faster while VTOL aircraft speeds have flat-lined and designs have become increasingly complex," said Ashish Bagai, DARPA Program Manager. To address such issues, DARPA launched the VTOL X-Plane program in 2013 to challenge “industry and innovative engineers” to push the envelope in four areas: speed, hover efficiency, cruise efficiency, and useful load capacity.

"We have not made this easy," said Bagai at the time. "Strapping rockets onto the back of a helicopter is not the type of approach we're looking for. The engineering community is familiar with the numerous attempts in the past that have not worked. This time, rather than tweaking past designs, we are looking for true cross-pollinations of designs and technologies from the fixed-wing and rotary-wing worlds. The elegant confluence of these engineering design paradigms is where this program should find some interesting results."

As Bagai suggested, dozens of attempts by VTOL aircraft designers have been made to increase top speed without sacrificing range, efficiency, or the ability to do useful work, “with each effort struggling or failing in one way or another.”

DARPA’s VTOL X-Plane program goal is to overcome challenges that thwarted attempts in the past, “by developing and integrating novel subsystems to enable radical improvements in vertical and cruising flight capabilities.” It is with that in mind that DARPA announced it had awarded the Phase 2 contract for VTOL X-Plane to Aurora.

“Just when we thought it had all been done before, the Aurora team found room for invention—truly new elements of engineering and technology that show enormous promise for demonstration on actual flight vehicles,” said Bagai during the announcement in early March 2016. “This is an extremely novel approach. It will be very challenging to demonstrate, but it has the potential to move the technology needle the farthest and provide some of the greatest spinoff opportunities for other vertical flight and aviation products.”

Besides Aurora, three other companies were awarded prime contracts for Phase 1 of the VTOL X-Plane, including Boeing, Karem Aircraft, and Sikorsky. During Phase 1, those four companies were tasked to develop a technology demonstrator that could achieve a top sustained flight speed of 300 to 400 knot, raise aircraft hover efficiency from 60% to at least 75%, present a more favorable cruise lift-to-drag ratio of at least 10 (up from 5 to 6), and carry a useful load of at least 40% of the vehicle’s projected gross weight of 10,000-12,000 lb.

Aurora’s says its winning design, named LightningStrike, looks to provide an approximately 50% increase in speed over existing VTOL aircraft designed for comparable mission applications.

“If successful, VTOL X-Plane’s radically improved flight capabilities could lead to revolutionary advancement of the U.S. military’s future mission capabilities,” said Dr. John Langford, Chairman and CEO of Aurora.

The technology demonstrator concept was designed in collaboration with Aurora’s team members, Rolls-Royce and Honeywell. The aircraft a Rolls-Royce AE 1107C turboshaft engine that would power three Honeywell generators, and 24 ducted fans distributed on both the wings and canards.

The aircraft’s electric distributed propulsion (EDP) system would consist of highly integrated, distributed ducted fans that, combined with the synchronous electric drive system, would enable the design’s “potentially revolutionary” hover efficiency and high-speed forward flight.

Mark Wilson, Chief Operating Officer of Rolls-Royce LibertyWorks said the company is “contributing both proven components and new technologies in turbo-electric distributed propulsion” to the program. LibertyWorks is an advanced aerospace technology research and design unit within Rolls-Royce.

“Our goal is to demonstrate technology that significantly improves electric power-generation efficiency, power density, and size,” said Carey Smith, President of Honeywell Defense and Space. “The improvements are needed to meet the aggressive goals for this aircraft, and would raise the bar for electric power-generation efficiency that would enable this advance in hybrid-electric aircraft propulsion.”

The demonstrator’s flight control system (FCS) will build on the work Aurora has done for the Centaur and Orion optionally manned and unmanned aircraft platforms. The FCS system runs on a triplex-redundant design to ensure detection and correction of flight anomalies in both vertical and forward flight.

Aurora’s Phase 2 design for VTOL X-Plane essentially will consist of an aircraft with two large rear wings and two smaller front canards—short winglets mounted near the nose of the aircraft. A turboshaft engine—one used in V-22 Osprey tiltrotor aircraft—mounted in the fuselage would provide 3 MW of electrical power, the equivalent of an average commercial wind turbine. The engine would drive 24 ducted fans, nine integrated into each wing and three inside each canard. Both the wings and the canards would rotate to direct fan thrust as needed: rearward for forward flight, downward for hovering and at angles during transition between the two.

The design is hoped to be the successful manifestation of an aircraft that could fly fast and far, hover when needed, and accomplish a variety of missions without the need for prepared landing areas. While the technology demonstrator would be unmanned, the technologies that VTOL X-Plane intends to develop could apply equally well to manned aircraft.

Aurora’s design is possible through advances in technology such as air vehicle and aeromechanics design and testing, adaptive and reconfigurable control systems, and highly integrated designs. According to Bagai, the design would also be impossible with the classical mechanical drive systems used in today’s vertical lift aircraft.

The Phase 2 design addresses many longstanding technical obstacles, the biggest of which DARPA believes is the fact that the design characteristics that enable good hovering capabilities are completely different from those that enable fast forward flight. Among the other design advances to be incorporated in the technology demonstrator:

•  Electric power generation and distribution systems to enable multiple fans and transmission-agnostic air vehicle designs,

•  Modularized, cellular aerodynamic wing design with integrated propulsion to enable the wings to perform efficiently in forward flight, hover, and when transitioning between them, and

•  Overactuated flight control systems that could change the thrust of each fan to increase maneuverability and efficiency .

“This VTOL X-plane won’t be in volume production in the next few years but is important for the future capabilities it could enable,” Bagai said. “Imagine electric aircraft that are more quiet, fuel-efficient, and adaptable and are capable of runway-independent operations. We want to open up whole new design and mission spaces freed from prior constraints, and enable new VTOL aircraft systems and subsystems.”

Pending successful completion of key program milestones to be achieved in close collaboration with DARPA, Aurora plans to conduct the first flight tests of the technology demonstrator in the 2018 timeframe.

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