Wednesday, 13 March 2013

Darpa X-plane :high-speed vertical take-off and landing aircraft

Imagine an aircraft that combines a helicopter’s ability to takeoff and land from almost anywhere, with the speed and range of a fixed wing aircraft. That’s precisely what aviation enthusiasts have dreamed of building for well over 50 years. In fact, so many efforts have been made to get a vertical takeoff and landing (VTOL) aircraft off the ground that even advocates of the concept often refer to the “wheel of misfortune,” a diagram that depicts the dozens of mostly failed concepts.
Many never got off the blackboard, and only three have ever been flown operationally by the military – the main customer for these vehicles.
But now, the Pentagon’s Defense Advanced Research Projects Agency (Darpa) is taking another spin at the “wheel”. This week, the agency announced a new X-plane program, which will seek to build a prototype aircraft to demonstrate a better VTOL design. That means more than just brushing off old ideas: the agency is hoping for an entirely fresh approach.
"Strapping rockets onto the back of a helicopter is not the type of approach we're looking for,” Ashish Bagai, Darpa program manager said in a statement announcing the new programme. “The engineering community is familiar with the numerous attempts in the past that have not worked. This time, rather than tweaking past designs, we are looking for true cross-pollinations of designs and technologies from the fixed-wing and rotary-wing worlds.”
Indeed, the Pentagon agency is seeking what it calls the “elegant confluence” of different engineering designs and approaches to a VTOL aircraft.  
There are, as the wheel shows, numerous to choose from. VTOL concepts over the years have included various ingenious solutions for powering and lifting these craft including tilt-rotor aircraft, tilt-wing, vectored thrust, tilt prop and tilt jets, just to name a few. A few have even been used operationally, such as the US military’s tilt-rotor V-22 Osprey; the iconic Harrier, which relies on vectored thrust to control its movements; and the Soviet Yak-38, another vectored thrust machine.  But many more never got anywhere near the battlefield, such as a class of aircraft that stood on their tails – like rockets – for lift-off (These impractical designs included the Lockheed XFV-1 and the Convair XFY-1 Pogo.)

‘Art of the possible’

The traditional problem with VTOL aircraft has been the tremendous complexity involved in having to transition from horizontal flight to vertical flight. Many of the schemes, which combine rotors and wings, or involve tilting some component of the aircraft, make for technically elaborate designs that are often impractical to operate and fly.
That Darpa would try to tackle the VTOL problem, even after so many failures, is not surprising. It has long been the tradition of the agency to tackle significant engineering challenges, such as robotics and hypersonics, multiple times until something works. Moreover, the appeal of a VTOL aircraft is simple: helicopters offer the unique ability to take-off and land without an airstrip, providing the military with access to places where fixed-wing aircraft can’t land, while fixed-wing aircraft offer greater speed and range.
The question, now, however, is whether Darpa can come up with anything new. And on that point, not everyone is optimistic.
The number one problem in the aeronautical sense, with VTOL, is you’re trying to have it all, and in aviation, everything is a tradeoff,” says Roger Connor, curator at the Smithsonian’s Air and Space Museum in Washington DC. “This is really, really hard to do, this is harder than rocket science in a lot of ways,” says Connor. “You’re trying to do something that is counterintuitive, and it’s not obvious that you’re going to have success.”

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