Second Prototype Lost In April Accident

A lack of low-speed stability and control authority -- that manifested itself in the loss of two flying prototypes -- was enough to convince the Defense Advanced Research Projects Agency (DARPA) to axe Boeing's Dragonfly X-50A unmanned aerial vehicle project recently.

The decision marks the end of the $51.8 million program, with Boeing using the leftover funds to compile a report on just what went wrong with the UAV, according to media sources.

Utilizing a design called a Canard Rotor/Wing (CRW), the X-50A Dragonfly was designed for vertical takeoff using a main rotor similar to a helicopter -- but using vectored turbojet thrust to turn the blades, instead of a conventional mechanical linkage. The craft was to then transition to forward flight by stopping the wide rotor (which then served as a wing) and directing thrust over the aircraft's tail... with the process reversed for landing.

Alas, that kind of "conversion" flight was never fully achieved -- as both Dragonfly prototypes were lost in early flight testing. The first crash, in March 2004, led to a joint investigation by both parties. DARPA and Boeing later ruled cross-coupling of the rotor controls as the main cause of the mishap -- resulting in improvements to the second prototype.

As Aero-News reported, the second Dragonfly prototype flew for the first time in December 2005. It progressed as far as slow forward flight... but 18 minutes into a test flight on April 12, 2006, the prototype once again lost control during a transition attempt.

DARPA says the second prototype was lost due to poor low-speed control authority, as well as extreme sensitivity to wake strength off the vehicle's rotor.

The agency states the accident occurred after rotor wake hit the fuselage, and caused the Dragonfly's nose to pitch up violently -- and in excess of the abilities of the control system to recover.

FMI: www.darpa.mil, www.boeing.com