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Out of the black: how Bird of Prey advanced stealth

GRAHAM WARWICK / ST LOUIS

Technology and design tools from low-observable testbed are already being used on unmanned aircraft

Boeing has had to wait six years to take the wraps off its Bird of Prey, but the stealth technology demonstrated by the highly classified aircraft has already been incorporated into the X-45 unmanned combat air vehicle and other designs the company is not ready to reveal.

As the first major project for the Boeing Phantom Works, the Bird of Prey also validated rapid prototyping tools later used during construction of the X-32 Joint Strike Fighter (JSF) concept demonstrators. Although Boeing was unable to unveil the aircraft, it was allowed to cite the experience gained developing the stealth testbed in its JSF proposal, says George Muellner, Boeing Integrated Defense Systems (IDS) senior vice-president, air force systems.

"The Bird of Prey programme had two objectives: to pioneer breakthrough low-observable technologies and revolutionise the aircraft design process," says Muellner. The aircraft met its signature goals and also validated the Phantom Works' rapid prototyping tools. It was designed and built in four years. "It would have taken at least twice that [using conventional processes]," says Muellner.

Three-dimensional design tools and virtual-reality assembly simulations were used. The 14.8m (48.6ft)-long, 6.9m-span aircraft was built using low cure-temperature composites, on low-cost disposable tooling pioneered by the boat industry, says Muellner.

Stealth features include large single-piece composite structures, minimising the panels, joins and fasteners that make manufacturing and maintaining low-observable aircraft lengthy and costly processes. The upper and lower halves of the chined fuselage were essentially single pieces, Muellner says. The only visible panels are the landing gear and engine doors.

The cockpit completely shields the intake when viewed from the front. A similar dorsal inlet is used on the X-45, which also adapts the stealth demonstrator's planform and nozzle shape. Signature and stability data collected during the Bird of Prey flight tests were used in design of the unmanned combat air vehicle, says Muellner.

The Bird of Prey is believed to have tested ways to minimise the radar reflectivity of control surfaces, with flexible strips covering the hinge lines of the flaperons and rudderons. The unusual gull wing design is only partly for signature reasons, says Muellner. The downturned tips were also required to give the tailless aircraft adequate directional stability and control. A small ventral strake was fitted for initial power approach tests, but removed for stealth flights.

The tailless X-45 does not need a gull wing because it uses thrust vectoring for directional control, says Muellner. Low-observable thrust vectoring was developed by the Phantom Works for NASA's X-36 tailless fighter agility demonstrator, a subscale unmanned aircraft which was designed after the Bird of Prey.

Despite its space-age appearance, the Bird of Prey is a simple aircraft, designed to be built quickly and cheaply, with mechanical flight controls, general-aviation cockpit instruments and landing gear from a Beech King Air. A 3,000lb-thrust (13.4kN) Pratt & Whitney Canada JT15D-5C turbofan, from a Cessna Citation business jet, gave the 3,350kg (7,400lb) aircraft only modest performance: a 260kt (480km/h) top speed and a ceiling of 20,000ft.

The $67 million company- funded programme began in 1992 and the aircraft completed 32 flights between 1996 and 1999. Three test pilots, two Boeing and one US Air Force, flew the aircraft. "It flew relatively nicely," says senior Boeing test pilot Joe Felock. No-one is revealing where the flights took place, but photographs suggest the Groom Lake test site in Nevada. The US Air Force acknowledges it provided "facilities and security" for the programme.

Boeing hopes the Bird of Prey is headed for the US Air Force Museum and display alongside Northrop Grumman's Tacit Blue, which demonstrated stealth technology for the B-2 bomber. "The Bird of Prey demonstrated low-observable manufacturing and supportability technologies for aircraft of the future," says Boeing IDS chief executive Jim Albaugh. "The aircraft will go into a museum, but the technologies will live on."

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