Undeterred by past failures, the U.S. Defense Advanced Research Projects Agency (Darpa) is launching another attempt to develop a high-speed vertical-takeoff-and-landing (VTOL) aircraft with the hover capability of a helicopter.
The VTOL X-Plane program is a 52-month, $130 million effort to fly an experimental aircraft capable of exceeding 300 kt., but with a hover efficiency of 75% or better and a cruise lift-to-drag ratio of 10 or more. By comparison, according to Darpa, today's conventional helicopters are capable of 150-170 kt., with a hover efficiency of 60% and a cruise L/D of 4-5.
High-speed compound helicopters such as Sikorsky's X2 are capable of 240-260 kt., while the Bell Boeing V-22 tiltrotor can exceed 275 kt., but with low hover efficiency.
"We have a simple objective: to fly much faster, with improved efficiency in hover and forward flight, without sacrificing the ability to do useful work," says Ashish Bagai, Darpa program manager and former Sikorsky principal engineer.
Useful work is defined as the difference between empty weight and maximum gross weight, he says. The program is targeting a useful load of at least 40% of gross weight, which is expected to be 10,000-12,000 lb. for the X-plane demonstrator.
This compares with a useful load of 35-40% of gross weight for a state-of-the-art helicopter, according to Darpa.
Bagai says Darpa is looking for "elegant" designs combining the attributes of fixed- and rotary-wing aircraft, that are "not overly complex and not brute-force approaches," he says. A broad agency announcement has been released, and Darpa is hoping to attract proposals from nontraditional companies and "exotic ideas from smaller companies on technologies, agile teaming and rapid design," he says.
This is not the agency's first attempt to develop a high-speed VTOL aircraft. The canard rotor/wing Boeing X-50A Dragonfly was designed to demonstrate a circulation-control rotor that could be stopped in flight to act as a fixed wing. Both subscale unmanned demonstrators crashed before transition could be achieved and the program was terminated in 2006.
The Groen Heliplane was a high-speed gyroplane design with a rotor that was powered by tipjets for vertical takeoff and landing and autorotated in forward flight at speeds up to 350 kt. The program encountered technical challenges with noise from the tipjets and was terminated in 2008.
-- This article first appeared in Aerospace Daily & Defense Report.