Helicopters and other rotorcraft are impressive bits of technology, but the only place where they match the performance of fixed-wing aircraft is in bad 1980s television. That may soon change with DARPA announcing the selection of four companies to compete in the next phase of its Vertical Take Off and Landing Experimental Plane (VTOL X-Plane) program. The four companies are tasked with developing unmanned demonstrator aircraft designed to increase the performance of VTOL rotor aircraft while reducing their complexity.

Rotorcraft can move in any direction, hover, and land on almost any flat surface. The problem is their performance is painfully limited with top speeds for operational craft below 150 kt (172 mph, 278 km/h). While it is possible for rotorcraft to set much higher speed records, it’s at the cost of range, efficiency, useful payload or simplicity of design. While some innovative designs that blend fixed-wing and helicopter platforms have emerged in recent years (most notably AgustaWestland's Avatar-esque electric tilt-rotor concept – Project Zero), attempts to make fundamental improvements to basic helicopter design and develop hybrid approaches have also proven difficult, as shown by the example of the MV-22 Osprey, whose history has been plagued by delays, technical problems, and orders of magnitude leaps in cost overruns.

DARPA’s US$130 million VTOL X-Plane program hopes to break this pattern and revolutionize VTOL flight by radically improving capabilities in vertical and cruise flight, with hopes of lowering mission times and improving the chances of military missions by reducing vulnerability to enemy attack. The plan it to achieve this by “cross-pollination” between the fixed-wing and rotary-wing technology. The idea is to reduce the complexity of the aircraft by using multi-purpose systems rather than separate systems for each function. This reduces the chances of failure, streamlines development, reduces costs, and saves space and payload weight.

The four companies chosen by DARPA to go forward with designing the new demonstrator aircraft are Aurora Flight Sciences, Boeing, Karem and Sikorsky. They have been tasked to create a demonstrator that can maintain a sustained speed of 300 kt to 400 kt (345 mph, 555 km/h to 460 mph, 740 km/h), reach a hover efficiency of at least 75 percent, attain significantly better cruise lift-to-drag ratio and have the ability to carry a useful load of at least 40 percent of the vehicle's gross weight of 10,000 to 12,000 lb (4,535 to 5,443 kg).

"Designing an aircraft to perform a vertical takeoff, while maintaining adequate low-speed control, is challenging," says Dan Newman, Boeing Phantom Works Advanced Vertical Lift capture team lead. Sustaining efficient hover is also difficult, and adding a high cruising speed is even more challenging."

The companies have already submitted designs and DARPA points out that though the project is concentrating on unmanned aircraft, the technology can also be applied to manned aircraft. In 2015, the next program milestone will see the four competing companies submit preliminary designs for evaluation with the aim of construction and performance testing sometime in 2017-18.

VTOL X-Plane design proposals from Boeing (top), Karem and Sikorsky

“We were looking for different approaches to solve this extremely challenging problem, and we got them,” says Ashish Bagai, DARPA program manager. “The proposals we’ve chosen aim to create new technologies and incorporate existing ones that VTOL designs so far have not succeeded in developing. We’re eager to see if the performers can integrate their ideas into designs that could potentially achieve the performance goals we’ve set.”

The video below shows Boeing’s Phantom Swift design for the competition.

Sources: DARPA, Boeing

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