North American XB-70 Valkyrie SST

Courtesy NASA

Originally conceived as a supersonic bomber, the North American XB-70A Valkyrie instead became the world's largest experimental research aircraft, flying from September 21, 1964, until February 4, 1969.

Two experimental prototypes of the XB-70A were under construction when the bomber program was canceled. At the same time there was growing interest in an American supersonic transport (SST), and the Valkyrie seemed a perfect testbed for SST research. The two prototypes were kept alive for a joint NASA-Air Force flight research program.

The Flight Research Center (FRC -- later the Dryden Flight Research Center) had several SST studies underway in the early 1960's. Its Douglas F5D-1 was used for landing studies, a North American F-100C was modified to simulate SST handling qualities, a North American A-5A was used to develop ways an SST would operate in the air traffic control system, and a Lockheed JetStar was modified as an in-flight SST simulator. But the XB-70A was the first transport-sized aircraft capable of sustained, long-range supersonic flight. Its research programs had a significant impact on American SST efforts at the time and could influence the design of future large, supersonic aircraft.

Although intended to cruise at Mach 3, the first aircraft was found to have poor directional stability above Mach 2.5, and it never flew faster than Mach 2.55 in its flight research at the NASA FRC between 1967 and 1969. However, NASA Ames wind-tunnel studies led North American Aviation, Downey, California, to build its sister ship with an added 5 degrees of dihedral on the wings. It handled much better, and achieved Mach 3.08 on April 12, 1966. Two months later it was lost in a mid-air collision during a formation photo flight.

One of the unique features of the Valkyries were the variable outer wing panels. These were left undeflected at subsonic speeds to take advantage of the full wingspan and wing area and to increase the lift-to-drag ratio and improve takeoff and landing performance.

At supersonic speeds, adequate cruise lift-to-drag ratio could be developed with less wingspan, so the outer panels were folded down. Deflected, they reduced drag as the wingtips interacted with the inlet shock wave in the lower surface flow field. Lowering the wingtips also reduced the area behind the airplane center of gravity (cg). This phenomenon was important because as Mach number increased, the center of pressure moved rearward, so less area aft of the cg caused a reduction of trim drag. The outer panels also provided more vertical surface to improve directional stability.

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