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For the past 60 years, helicopters have provided essential vertical takeoff and landing (VTOL) capabilities–omnidirectional maneuverability, hovering, landing on almost any flat surface–for countless military operations. Even as VTOL aircraft technology continues to advance, however, one key goal still remains elusive: improving top speed beyond 150 kt-170 kt. Faster VTOL aircraft could shorten mission times and increase the potential for successful operations, while reducing vulnerability to enemy attack. Unfortunately, new VTOL designs so far have been unable to increase top speed without unacceptable compromises in range, efficiency, useful payload or simplicity of design.

Historically, attempts to develop VTOL aircraft with high speed capability (=250kts) have repeatedly encountered difficulty in meeting speed, range and payload targets, owing to excessive power and/or fuel requirements, high weight-empty fraction, or poor cruise efficiency. Ongoing technology developments are supporting efforts to overcome these problems and develop advanced vertical takeoff aircraft with high speed capability by using a wide range of lift and propulsion concepts, moving beyond conventional helicopter designs to various forms of compound rotorcraft or other VTOL variants.

Ducted propellers and fans have been among the design features used in VTOL aircraft seeking to attain high speed, and these elements are being considered in several current concept studies, either for propulsion or for a mixed lift/propulsion role. Low to moderate solidity ducted propellers have also been featured in several recent design studies, as well as in past experimental aircraft such as the Bell X-22 and the Piasecki X-49. It is well known that high-solidity ducted fans can offer both lift and propulsion capability, though typically with relatively poor efficiency in hover. Use of one or more low-to-moderate solidity rotors or ducted propellers would in principle allow hover efficiency higher than prior “fan-in-wing” concepts, though they would exhibit significant drag levels in high speed forward flight unless the lifting rotors are stopped and the duct cavity faired over (e.g., the Ryan XV-5), or a tilting propulsion system is used (as with Bell X-22 or the more recent AgustaWestland Concept Zero aircraft).

DARPA’s VTOL experimental plane, or VTOL X-Plane, program seeks to overcome these challenges through innovative cross-pollination between the fixed-wing and rotary-wing worlds, with the goal of fostering radical improvements in VTOL flight. Rather than tweaking past designs and technologies, VTOL X-Plane challenges industry and innovative engineers to create a single hybrid aircraft that would concurrently push the envelope in four areas:

  1. Speed: Achieve a top sustained flight speed of 300 kt-400 kt
  2. Hover efficiency: Raise hover efficiency from 60 percent to at least 75 percent
  3. Cruise efficiency: Present a more favorable cruise lift-to-drag ratio of at least 10, up from 5-6
  4. Useful load capacity: Maintain the ability to perform useful work by carrying a useful load of at least 40 percent of the vehicle’s projected gross weight of 10,000-12,000 pounds

DARPA’s VTOL Experimental Plane (VTOL X-Plane) program aims to overcome these challenges through innovative cross-pollination between fixed-wing and rotary-wing technologies and by developing and integrating novel subsystems to enable radical improvements in vertical and cruising flight capabilities.

DARPA awarded prime contracts for Phase 1 of VTOL X-Plane to four companies:

  1. Aurora Flight Sciences Corporation
  2. The Boeing Company
  3. Karem Aircraft, Inc.
  4. Sikorsky Aircraft Corporation
The inclusion of Aurora and Karem were deliberate attempts by the DoD, the author believes, to introduce an element of friction and competition into this S&T process. “We were looking for different approaches to solve this extremely challenging problem, and we got them,” said Ashish Bagai, DARPA program manager. “The proposals we’ve chosen aim to create new technologies and incorporate existing ones that VTOL designs so far have not succeeded in developing. We’re eager to see if the performers can integrate their ideas into designs that could potentially achieve the performance goals we’ve set.”

All four winning companies proposed designs for unmanned vehicles, but the technologies that VTOL X-Plane intends to develop could apply equally well to manned aircraft. Another common element among the designs is that they all incorporate multipurpose technologies to varying degrees. Multipurpose technologies decrease the number of systems in a vehicle and its overall mechanical complexity. Multipurpose technologies also use space and weight more efficiently to improve performance and enable new and improved capabilities. The next major milestone for VTOL X-Plane was scheduled for late 2015, when the four performers are required to submit preliminary designs. At that point, DARPA planned to review the designs to decide which to build as a technology demonstrator, with the goal of performing flight tests in the 2017-18 timeframe.

In an important step, in March 2016 DARPA awarded the Phase 2 contract for VTOL X-Plane to Aurora Flight Sciences. “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 Ashish Bagai, DARPA program manager. “This is an extremely novel approach,” Bagai said of the selected design. “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.”

Aurora’s Phase 2 design for VTOL X-Plane envisioned an unmanned 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 megawatts (4,000 horsepower) 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.

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