The Mysteries of the B2
August 20, 2002
Page 1
The Problem of Long-Range Bombing
As soon as the United States possessed the atomic bomb, the problem arose of having intercontinental delivery vehicles capable of striking deep into the territory of the supposed potential enemy: the USSR. Initially, in 1945, the Americans had the B-29 Superfortress bombers that had carried out the bombings of Hiroshima and Nagasaki, forming the 509th Bombardment Group, based in Roswell.
For bombing Soviet territory, these aircraft did not offer sufficient performance. Northrop then developed flying wings, aircraft believed to have a very long range and, moreover, stealthy—that is, barely or undetectable by the radar systems of the time. Jack Northrop initiated this project even before the war of 1939–1945. A first aircraft with contra-rotating propellers, the XB-35A, made its first flight at the American base of Edwards on June 26, 1946.
The aircraft were later given a different type of propulsion—eight turbojets—and were renamed the YB-49.
They were assembled in Hawthorne, California, in 1947. The wingspans of both aircraft were identical (52 meters). A pre-series of about a dozen aircraft was thus built. However, flight tests proved catastrophic. At the time, little was known about the aerodynamics of swept wings. In particular, it was unknown that at high angles of attack, flow separations occurring at the wingtips tended to migrate very rapidly toward the nose of the aircraft, causing an extremely abrupt dive. This phenomenon occurred repeatedly during YB-49 stall tests and even led to the loss of the aircraft and its pilot. It was impossible to put into service aircraft for which it would have been necessary to prescribe never approaching stall conditions. Furthermore, commercial competition issues arose at the time between Northrop and Boeing, and Boeing ultimately prevailed. All YB-49s were destroyed. To be precise, this catastrophic stall behavior did not seem to have occurred with the XB-35A, which had propellers, though it was never clearly understood why. The problems encountered by the YB-49 manifested with all swept wings, and the solution consisted of equipping them with "fences" (barriers). Below (a), this solution as implemented on certain aircraft (identical solution for the "Caravelle" wing):
The subsequent swept-wing aircraft, including delta-wing aircraft, used two other solutions. The first consists of creating a leading-edge notch (b), which generates a particularly strong vortex at high angles of attack, effectively acting as a natural barrier preventing the wingtip flow separations from migrating forward.
The current classic solution (Mirage III) consists of cutting a notch into the leading edge (c), which proved capable of performing the same function. It should be noted in passing that the zig-zag trailing edge of the B2 serves two purposes. The first is indeed to provide a weaker radar return (this is the widespread use of all joints or edges in "hairpin" or "hairpin" shapes). But at high angles of attack, this serrated trailing edge generates vortices that act as barriers, preventing the separation that might occur at the wingtips from migrating forward (d).
If this solution (fences extending up to the leading edge) had been known in 1947, Jack Northrop's aircraft might have been saved. The United States would thus have immediately possessed a highly stealthy aircraft. But it seems that this problem of sudden dive during stall was not the only one encountered during testing, and there were also issues with engine overheating.
Let us recall that the Germans appear to have been the first to understand the importance of stealth (they suffered heavy losses due to the early detection of their aircraft waves by British radars, the most advanced at the time), and in the final years of the war, they developed a craft designed by the Horten brothers.
This aircraft lacked any vertical tail surface and was systematically constructed using materials that weakly reflected radar echoes (as was later the case with the famous British Mosquito fighter-bomber, built entirely of wood). The reader may naturally wonder how such machines can be controlled in yaw. The answer is extremely simple. It is the same principle used on the B2, which also lacks vertical tail surfaces. It consists of simultaneously opening two flaps located at the wingtips—upward and downward—thereby locally increasing drag.
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