When I first saw Sikorsky's Unmanned Rotor Blown Wing concept for DARPA'S VTOL X-Plane program, it reminded me of something. But I couldn't remember what, so I sought expert help. And the answer? It's Boeing's Heliwing - a tailsitter VTOL unmanned aircraft that flew, briefly, in 1995.
Heliwing was funded by the Pentagon's UAV Joint Program Office (remember that?). It was a 17ft-span, 1,450lb gross-weight UAV with two 7ft-diameter proprotors powered by a single 240shp Williams WTS117 turboshaft. The Heliwing was intended to reach 180kt in forward flight and carry a 200lb payload for 5hr. The sole example first flew in April 1995, but crashed in July when the engine flamed out and the program was shelved.
Sikorsky is working on conceptual and preliminary design of the Unmanned Rotor Blown Wing with Lockheed Martin Skunk Works under a 22-month, $14.4 million VTOL X-Plane Phase 1 contract. DARPA is expected to award up to four Phase 1 contracts. Aurora Flight Sciences has one, but isn't saying what its design looks like. And Boeing is tipped to get one for its Phantom Swift ducted-fan design (seen below). DARPA plans to pick one design to fly in 2017.
DARPA's goal for VTOL X-Plane is to fly a demonstrator capable of 300-400kt forward speed, but with a high hover efficiency (within 25% of ideal power loading) in vertical flight and high cruise efficiency (lift-to-drag ratio better than 10) in horizontal flight. In seeking proposals, the agency said it was looking for simplicity and elegance in design and encouraged innovative proposals and not simply improvements to existing designs.
Sikorsky isn't saying much about the Rotor Blown Wing concept except to confirm what the artist's impression strongly suggests -- it is a tailsitter like the Heliwing. The name suggests the wing remains aligned into the proprotor wash throughout transition between vertical and forward. That would reduce download on the wing in hover mode. But details beyond the five-blade proprotors and lack of horizontal tails are scarce.