Elektiriske fly - DARPA prosjekt demonstrerer utfordringer - AW&ST

LightningStrike Cancellation Underlines Electric Propulsion Challenges Graham Warwick No one expected the electrification of aircraft propulsion to be easy, but DARPA’s decision to cancel an ambitious hybrid-electric flight  demonstrator is a harsh dose of reality for the industry. Aurora Flight Sciences’ highly unconventional XV-24A LightningStrike had become a flagship for the much-vaunted new era in aviation. In pulling the plug on the hybrid-electric, distributed-propulsion vertical-takeoff-and-landing (VTOL) X-plane, the Pentagon’s advanced research agency cites the strong commercial interest in the technology that has emerged since the program was launched in 2013 and the lack of a service partner to which the technology could be transitioned on completion of the program. But the flight demonstrator was already facing challenges, among them development of the powerful generator at the core of the hybrid-electric drivetrain. This is a fraction of the size of existing megawatt-class generators, with higher efficiency—98% versus a more usual 92%. But, crucially, it has five times the power density of existing generators, enabling megawatt-class power levels on electric aircraft.

Credit: Aurora Flight Sciences

The LightningStrike was to be powered by a Rolls-Royce AE1107 turboshaft driving a gearbox mounting three of these generators. They were to power 24 variable-pitch ducted fans, 18 embedded in the aft tilting wing and six in the tilting canard foreplane. AC power flowed directly to the motors, rather than being converted to and from DC for distribution—a unique synchronous power approach that saved weight. Honeywell acknowledges the challenges in developing such a power-dense generator but says the prototype is scheduled to be tested at full 1-megawatt power at Florida State University in July and is on track to achieve the planned technology readiness level 6. The company is adamant the extra work required to overcome heat management issues was not a driving factor behind DARPA’s decision. This is not the first ambitious electric-aircraft X-plane to encounter challenges. In December 2016, the powerful lithium-ion battery for NASA’s X-57 Maxwell electric-propulsion demonstrator destructively failed a thermal runaway test. The battery had to be redesigned to improve containment so a short-circuit in one cell cannot propagate to other. The redesigned battery passed the safety test in late 2017 and the X-57 is on track to fly this year, but valuable lessons were learned. Aurora did fly a subscale demonstrator (SSD) under DARPA’s VTOL X-Plane program, which the company says achieved several key milestones, including demonstrating distributed-electric ducted-fan propulsion, synchronous electric power and tiltwing/tiltcanard VTOL. Aurora now plans to apply those technologies to its commercial electric-VTOL (eVTOL) development work in partnership with Uber. Flown in April 2017, the 20%-scale SSD proved both a blessing and a curse for the LightningStrike. It validated the propulsion system concept, its hover and cruise efficiency and the flight control system planned for the full-scale aircraft. But its success allowed DARPA to declare that “the major objectives of the VTOL X-Plane program were achieved with the subscale demonstrator.” Experience gained flying the subscale aircraft did enable Aurora to quickly fly a similar-sized demonstrator of its simpler eVTOL concept for Uber. And under the terms of the LightningStrike termination, the company gets to keep the SSD, which is expected to fly again in support of its eVTOL development. The program also faced hurdles that are uniquely DARPA’s. First, the agency’s priorities are constantly changing and have shifted toward areas such as hypersonics, so terminating VTOL X-Plane freed up funds. Second, DARPA programs normally require a service partner to proceed into flight testing, but VTOL X-Plane deliberately avoided specifying a mission set to encourage the most innovative ideas. And that made finding a transition partner more difficult. But it is challenges faced in developing a megawatt-class hybrid-electric propulsion system that provide a lasting lesson. Some key startups in the electric-aircraft market are developing their own motors and generators because a supply chain does not yet exist. But getting to megawatt-class, and beyond, may not prove as easy as they hoped. Enthusiasm for electric propulsion may not be reduced by the premature burnout of the LightningStrike, but an edge of realism has been introduced.