The U.S. Defense Advanced Research Projects Agency (Darpa) is soliciting proposals from industry for a vertical takeoff and landing experimental aircraft (VTOL X-plane) that would demonstrate “radical” improvements over the current state of VTOL flight. In late February, Darpa issued a broad agency announcement seeking proposals by May 1.

The announcement calls for a demonstrator aircraft designed to have a max takeoff weight of 10,000 pounds to 12,000 pounds, a payload capacity of at least 12.5 percent of the max weight and the ability to fly at sustained speeds of 300 to 400 knots, with hover efficiency within 25 percent of the ideal power loading at sea level.

Current VTOL aircraft in the 4,000-pound to 24,000-pound weight classes “are nearly exclusively large open rotor systems” that are limited to flight speeds of about 170 knots and average hover efficiency about 60 percent of the theoretical ideal, according to Darpa.

The X-plane would overcome fundamental factors that limit conventional helicopters, including “retreating blade stall, high parasite drag, low power loadings, inefficient lift in translational flight, high empty weight fractions, vertical download, losses due to interactional aerodynamics (and) power losses,” the agency said.

The agency envisions the VTOL X-plane as a three-phase, $130 million program extending over 52 months. Darpa’s broad agency announcement seeks proposals for the first-phase effort, which has a budget of $47 million and covers conceptual design and technology maturation. “The design of the demonstrator as a manned, unmanned or optionally piloted aircraft is left to the proposers,” the announcement states. Darpa expects to award a single contract for the program’s second and third phases, covering development and flight-testing. Two test articles will be built.

“The objectives are intended to enable radical improvements in VTOL speed, hover time, range and useful loads applicable to future systems,” the agency said. The program “is not intended to create a pre-production type aircraft for any specific operational effort.”