While the radar cross section is estimated to be as small as a mosquito, engineers also buried the engine deep within the fuselage to minimize the infrared signature and even carefully designed the paint shading to visually mask the actual fuselage shapes in daylight a measure not utilized by other stealth aircraft such as the F-117 and B-2, Flyingmag.com reported, encapsulating the extreme design of the YF-118G Bird of Prey to low observability.
Conceived in secrecy during the 1990s, the Bird of Prey was never intended to break speed records or be the best air-to-air fighter. Instead, it was a cradle of stealth technology, a test bed whose concepts would be transferred to the creation of the F-22 Raptor and F-35 Lightning II. Its distinctive shape referred to as a “Klingon spaceship” was as much more than a styling stroke as it was a deliberate exercise in form, material, and engineering constraint, intended to lose every visible signature. The mission was plain: create mature stealth concepts cheaply and quickly, not make an operational fighter.
The Bird of Prey as a vehicle was a cost-performance compromise masterclass. For a total program cost of just $67 million a fraction of the cost of purchasing two commercial aircraft at the time Boeing’s Phantom Works division built a demonstrator that made stealth its top priority. The aircraft was powered by a single Pratt & Whitney JT15D-5C turbofan, the same that powered business planes like the Cessna Citation V. At just 3,190 pounds of thrust, the engine gave the aircraft a top speed of 300 mph and a service ceiling of 20,000 feet, both unimpressive by fighter plane standards. Such was not by accident, though. The focus had been on demonstrating stealth technologies, rather than high performance.
To keep costs down, the Bird of Prey utilized off-the-shelf components wherever possible. The landing gear was adapted from Beechcraft turboprops, the ejection seat from a Harrier, and flight controls were pieced together from other tactical aircraft. Even the control system was fully manual, eschewing expensive fly-by-wire technology for simple hydraulics. Test pilot Colonel Doug Benjamin quipped, the clock was from Wal-Mart and the environmental control system was essentially a hairdryer.
What actually made the Bird of Prey special was its stealth design. The blended wing-body configuration of the aircraft, “gapless” control surfaces and single-piece composite structures were all aimed at minimizing radar reflections from every angle. The engine was recessed deep within the body with the intake completely shielded from the front side, reducing radar and infrared signatures. Even the coloration scheme was created to break up visual lines of vision under daylight, a subliminal yet powerful camouflage not on earlier stealth aircraft. All these were flight-tested and validated in 38 test flights from 1996 through 1999, each mission refining the symbiosis of shape, material, and signature control.
The Bird of Prey emerged when there was accelerated progress in digital engineering. The application of virtual reality and computer-aided design (CAD) software made it possible for engineers at Boeing to test and optimize the performance of parts prior to the cutting of any metal. Not only did this computer-centric approach accelerate prototyping, but it also reduced the number of costly physical iterations, opening the door to subsequent programs like the X-32 Joint Strike Fighter and X-45 Unmanned Combat Air Vehicle.
The Bird of Prey’s stealth technologies were not created in isolation. They built upon a heritage of signature reduction techniques developed by previous programs, including the F-117 Nighthawk and B-2 Spirit. Alan Brown, the inventor of stealth technology, explained, “There are two basic approaches to passive radar cross section reduction: shaping to minimize backscatter, and coating for energy absorption and cancellation.” The Bird of Prey reflected this philosophy, utilizing both sophisticated shaping and selective use of radar-absorbing materials to create an equilibrium, multi-directional decrease in detectability. Its consequences are observed in the F-22’s planform alignment, deeply recessed engine faces, and sawtoothed surface interfaces features that have since become hallmarks of stealth design across the industry.
While the Bird of Prey’s flight capabilities were deliberately modest, its legacy is not. The fact that stealth could be made on a shoestring budget, with quick prototyping, computer-aided design, and smart engineering decisions, was demonstrated by the success of the demonstrator. Its techniques are today standard practice, shaping the design of the world’s most advanced warplanes. The Bird of Prey is today housed in the National Museum of the U.S. Air Force a silent testament to the power of focused ingenuity and the timeless virtues of “cheap stealth.”.
