The goal of the program is to demonstrate an aircraft design that can take off vertically and efficiently hover, while flying faster than conventional rotorcraft. There have been many previous attempts, most of them unsuccessful.
A helicopter with a conventional rotor layout has a theoretical top speed of 200 kn (230 mph; 370 km/h), after which it suffers from dissymmetry of lift. Some designs have successfully created hovering and high-speed aircraft, including the V-22 Osprey tiltrotor that can fly at 275 kn (316 mph; 509 km/h) and the Sikorsky X2 compound helicopter that flew at 260 kn (300 mph; 480 km/h), but both made significant aerodynamic compromises to hovering efficiency or range. DARPA's goal is to demonstrate a VTOL aircraft that can achieve a sustained top speed of 300 to 400 knots (345 to 460 mph (555 to 740 km/h)).
All competitors for the program opted to demonstrate their concepts using an unmanned aerial vehicle even though it was not required, but the technologies are intended to be applied to manned aircraft as well.
Requirement and programme
DARPA announced the programme in February 2013 with a requirement to create a new aircraft that uses the best features from both vertical take-off landing technology and that used for conventional aircraft. The hybrid aircraft will try to improve on four areas:
- Speed - achieve a top speed of at least 300 kts.
- Hover - increase hover efficiency to at least 75 percent.
- Cruise efficiency - achieve a lift-to-drag ratio of at least 10.
- Load - ensure the solution can carry a useful load of at least 40% of the projected gross-weight.
Phase One - Preliminary design study
The first two companies to be involved were announced in December 2013 when Sikorsky Aircraft was awarded a US$14.4 million contract and Aurora Flight Sciences was given US$14 million for preliminary design studies as part of the $47 million Phase One budget.
On 18 March 2014, DARPA announced that Sikorsky, Aurora Flight Sciences, Boeing, and Karem Aircraft had been selected to compete for the VTOL X-plane. The four companies have based their designs on unmanned aircraft and will compete over the next 20 months. The name of Aurora's submission was revealed as the LightningStrike in February and although the design is unknown, the company has a history of producing ducted fan and hybrid propulsion aircraft. Karem Aircraft is expected to propose a tiltrotor aircraft with an optimum speed rotor. The Boeing PhantomSwift embeds twin lifting fans inside the fuselage with tilting ducted fans mounted on wingtips for lift and forward thrust; a scale demonstrator was built and flown by the company in 2013. Sikorsky has teamed with Lockheed Martin and has a "low complexity" design that combines fixed wing aerodynamics and advanced rotor control. A single design will be selected in 2015 for a demonstrator to be built in phase 2.
- Rotor Blown Wing - Sikorsky and Lockheed Martin teamed to develop their unmanned rotor blown wing concept. Few details have been revealed, but they claim it integrates fixed wing aerodynamics and advanced rotor control to provide a low complexity configuration. An artist depiction of Sikorsky's Rotor Blown Wing bears visual resemblance to the Boeing Heliwing, an unmanned tailsitter intended to reach 180 kn (210 mph; 330 km/h) in forward flight; it first flew in April 1995 but crashed in July and the project was shelved. Sikorsky has confirmed that the Rotor Blown Wing will be a tailsitter, with its name suggesting that the wing remains aligned into the proprotor wash during transition between vertical and forward, reducing download on the wing in hover mode.
- LightningStrike - Aurora Flight Sciences' LightningStrike will achieve high overall efficiency by integrating the propulsion into the air vehicle's aerodynamic design. The company has experience with ducted-fan designs through Goldeneye series aircraft, also under DARPA programs, and with hybrid-electric propulsion with the Excalibur unmanned aerial vehicle proof-of-concept aircraft to create a design providing vertical takeoff and landing with high-speed horizontal flight.
- Karem tiltrotor - Karem Aircraft is submitting a tiltrotor concept employing a slender-bodied fuselage with a high-aspect-ratio gull wing. The outer sections of the wing, together with two large rotor/propellers, rotate through 90 degrees for transition between hover and forward flight.
- PhantomSwift - Boeing's PhantomSwift concept has two large internal fans to provide lift with two wingtip fans that provide stability while hovering. In forward flight, the internal fans stop supplying power and the wingtip fans provide thrust. Boeing claims that this configuration is 50 percent more efficient in the hover than a typical helicopter, and expected to have a top speed of 740 km/h (460 mph; 400 kn). The full-size version will have a wingspan of 50 ft (15 m), a fuselage length of 44 ft (13 m), and weigh 12,000 lb (5,400 kg). Boeing believes the PhantomSwift configuration can be replicated to create a whole new developmental series of aircraft. The demonstrator will be powered by a General Electric CT7-8 engine, but a production version will be powered by an all-electric drive.
In June 2014, Sikorsky Aircraft and Aurora Flight Sciences were awarded Phase 1B program contracts, which include preliminary design, technology maturation, modeling and simulation, aircraft configuration, and component-level testing for their submissions. Boeing was awarded a Phase 1B contract in August 2014, Aurora was awarded its Phase 1B funding in September 2014, and Karem was issued Phase 1B funding in December 2014.
- DARPA Awards Contracts in Search of a 460 MPH Helicopter - News.USNI.org, 19 March 2014
- Warwick, Graham. "High-Speed VTOL -- DARPA Tries ... Again" Aviation Week & Space Technology, 24 March 2013. Accessed: 25 May 2014.
- DARPA’s Selects Four Developers for new X-Plane Program - Defense-Update.com, 18 March 2014
- "DARPA annonuces new X-plane programme". Flightglobal. 25 February 2013. Retrieved 22 December 2013.
- "Vertical Takeoff and Landing Experimental Plane (VTOL X-Plane)". DARPA. Retrieved 22 December 2013.
- "Sikorsky wins contract to build experimental VTOL aircraft". Flightglobal. 20 December 2013. Retrieved 22 December 2013.
- Niles, Russ (23 December 2013). "DARPA Wants 400-Knot VTOL". AVweb. Retrieved 26 December 2013.
- DARPA selects four bidders for high-speed VTOL X-Plane - Flightglobal.com, 18 March 2014
- Sikorsky's Rotor Blown Wing -- Look Familiar? - Aviationweek.com, 20 December 2013
- Aurora Wins DARPA VTOL X-Plane Program Contract - Aurora press release, 4 February 2014
- Darpa Awards Final VTOL X-Plane Contracts - Ainonline.com, 21 March 2014
- Boeing Phantom Swift Joins VTOL X-Plane Comp - Armedforces-Int.com, 25 June 2014
- Boeing Gets $9M More to Develop Phantom Swift X-Plane - Defensetech.org, 28 August 2014
- Sikorsky moves forward with DARPA VTOL X-Plane project to design new military tiltrotor aircraft - Militaryaerospace.com, 9 June 2014
- Aurora Flight Sciences joins Sikorsky in next phase of project to design new tiltrotor - Militaryaerospace.com, 10 June 2014
- Boeing proceeds into next phase of DARPA-sponsored Phantom Swift
- Aurora Awarded Funding to Continue Development of DARPA VTOL X-Plane Program - sUASNews.com, 30 September 2014
- Karem progresses to next phase of VTOL-X project - Shephardmedia.com, 18 December 2014
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