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Aviation History
1991
1991 - 1597.PDF
B-2'S CHANGING SHAPE When the US Air Force changed the B-2's mission, Northrop was forced into a $1 hillion redesign of its stealth bomber. Guy Norris reports. J ust days before the opening of the Paris show, the Northrop B-2 bomber made its first air-show appearance at Andrews AFB near Washington DC. Visitors to the "stealth week" show also saw Lock heed's F-117A and YF-22A. Those seeing the B-2 for the first time could hardly guess at the complex genesis of this dramatic aircraft. Design of the B-2 was driven by stiff requirements for long range, high payload and low observability. Northrop's first sketches of the Advanced Technology Bomber (ATB), as it was then known, were made in 1979. Hal Markarian produced the first flying-wing drawings under the direction of stealth specialists Irv Waaland and John Cashen. In spite of the B-2's resemblance to Northrop's YB-49 flying wing of 1947, the company says that aircraft "...had little influence on the decision to pursue an all-wing solution for the B-2 other than having proven that a flying-wing works. "The all-wing approach was derived from its potential for an exceptionally clean configuration for minimising radar cross- section, including freedom from vertical fins, as well as the known benefits of span-loading structural efficiency and high lift-to-drag for efficient cruise," the comp any adds. The original ATB planform, as drawn by Markarian, had a diamond-shaped cen- trebody long enough to accommodate the crew, engines and weapons with a fineness ratio suited to transonic speeds. Outboard wing panels were added for longitudinal balance, to increase lift/drag ratio and to provide sufficient span for pitch, roll and yaw control. Leading-edge sweep was selected for bal ance and transonic aerodynamics while the overall planform was designed to have Today's B-2 looks similar to 1979's ATB (inset) but internal changes were significant and costly neutral longitudinal (pitch) static stability. Because of its short length, the aircraft had to produce stabilising pitch-down moments beyond the stall for positive recovery. The ATB used split drag rudders for roll, pitch and yaw control and for directional stability. Where conventional aircraft need a fin to overcome unstable yaw generated by the forward fuselage, an all-wing design has neutral directional stability, Northrop found. In addition, the split rudders were used as speedbrakes. The flying-wing configuration resulted in an aerodynamic efficiency (a function of lift/drag ratio, wing span and wetted area) greater than the Boeing B-52's but less than that of the Lockheed U-2 with its high aspect-ratio wing. The YB-49's high ratio of span to wetted area was offset by an excess of protuberance drag, Northrop notes. As its engines are buried within an otherwise aerodynamically clean structure, the B-2 offers higher ef- FLIGHT INTERNATIONAL 12- 18 June,'1991 101
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