DARPA pulls plug on X-50 stopped-rotor concept

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By Graham Warwick in Washington and Helen Massy-Beresford in London

Loss of second Dragonfly demonstrator due to lack of low-speed control, says agency

The latest attempt to develop a stopped-rotor aircraft has been scrapped, with the US Defense Advanced Research Projects Agency (DARPA) withdrawing support for Boeing's canard rotor/wing (CRW) concept following the April loss of the second X-50A Dragonfly unmanned demonstrator.

The crash investigation has concluded that the aircraft was lost due to insufficient low-speed control authority. The crash of the first X-50, in March 2004, was caused by a cross-coupling control phenomenon. Neither vehicle had progressed beyond hover and low-speed manoeuvres in the brief flights completed before they crashed.

The CRW was intended to take off like a helicopter, transition to forward flight and stop the rotor, which would then act as a fixed wing. Investigators concluded the X-50's fuselage aerodynamic pitch moment was extremely sensitive to airspeed and wake strength. "At very low speeds, the rotor wake impact on the fuselage resulted in a strong pitch-up moment that used up the available control margin," says DARPA. As a result, the vehicle was unable to recover from an aerodynamic disturbance during its last flight.

Boeing is using remaining DARPA funds to complete a study later this year to "draw together final ideas on the potential of the stopped-rotor concept", says the agency. Stopped rotor has long been eyed as a potential high-speed rotorcraft configuration, but previous efforts have also failed to produce a viable design.

Italian aerospace research agency CIRA, meanwhile, is to invest in feasibility studies on a vertical take-off and landing flying car that would stop its rotor in forward flight and stow the rotor while on the road. An initial prototype may be completed by January and production vehicles could be flying in around six years, says CIRA.

The two-seat Aviocar would be built with the help of Italian propulsion company Avio, with development and preparation for production estimated to cost around €60 million ($76 million), according to CIRA president Sergio Vetrella.

After a vertical take-off the rotor would stop in flight to become a fixed wing, and the rotor would stow itself lengthwise when the vehicle was being driven on roads. CIRA says advanced navigation, control and communications technology would allow for the consistent and safe management of private traffic, both on the ground and in the air.