Aurora Flight Sciences has revealed its latest concept for a low-drag, fan-in-blended-wing body design for DARPA's Speed and Runway Independent Technologies (SPRINT) program aimed at producing a high-speed VTOL aircraft for Special Forces missions.
The idea behind SPRINT is to develop the technologies needed to create a new class of aircraft that combine VTOL capabilities with high speeds. While it's true that such aircraft have been around for over half a century – the Harrier and the F-35 Lightning II, for example – these fighter planes aren't suited for commando operations or similar missions that require high speed, high agility, plus independence from conventional runways.
One of four companies chosen by DARPA to create a concept prototype, as its preliminary design phase Aurora has opted for a scalable blended wing design that incorporates three lifting fans inside of a composite fuselage. Though the latest version has an uncrewed cockpit, later, larger ones designed to carry four or more lift fans could have more traditional crewed cockpits.
According to Aurora, the blended wing concept meets or exceeds the project's objectives, with more speed than required and integrated fan covers to reduce drag while in horizontal flight. The aircraft is also capable of Short Take-Off and Vertical Landing (STOVL), Super Short Take-Off and Landing (SSTOL), and conventional take-off and landing using existing engine designs.
The company estimates that the preliminary design review will be complete in about a year, with the first flight in three years.
"Aurora and Boeing bring relevant expertise in blended-wing-body platforms, high-speed VTOL configurations, and military aircraft development," said Larry Wirsing, vice president of aircraft development at Aurora Flight Sciences. "The DARPA SPRINT program is an exciting opportunity to continue our history of advancing technology demonstrator programs that enable new capabilities for the U.S. military."
Source: Aurora
Here, use of Wright brothers-style rotor blades prevented application of the tiltrotor technology, which (otherwise) would be most suitable to satisfy the DARPA requirements of a high-speed VTOL aircraft. Stiff blades will never be able to provide maximum (or even acceptable) level of performance/efficiency, when extreme requirements are present for both low speed (VTOL), and high cruising speed regimes of operation. (BTW a norm for military hardware everywhere.)
Carrying separate propulsion systems - one for vertical lift and one for cruising flight – lacks engineering elegance and, mainly, is an expensive luxury that reduces useful load capacity. (See e.g. the above bargain on passenger numbers.)
Optimally, a tiltrotor with the variable twist feature of the blades could best match the requirements of DARPA. Also known as the stall free blade technology, quite straightforward concepts for the variable blade twist of rotors and propellers have been around for some years. Duly supported mathematically, they can provide solutions with excellent useful load ratio and other parameters of energetics. Up to about 0.8 Mach cruising speed.