General Electric – Variable Geometry

General Electric’s approach to solving the high pressure-ratio compressor problem, by contrast, was to stay with the single spool design they had employed on their highly successful earlier engines, and to adopt “variable geometry” in the forward stages of the compressor in order to modulate the flow at off-design operation. Specifically, the stationary blades, or “stator vanes,” in the forward stages were rotated to different stagger angles, depending on the operating point, thereby altering the flow area in these stages in order to maintain favorable incidence angles on the blades at different conditions.21 The first flight-qualified engine GE designed with variable stator vanes was the J-79, which powered the Mach 2.2 B – 58 bomber and several Mach 2.2 fighters, including the F-104 and the F-4H.22 The design that evolved into the J-79 was begun in 1951, with the first flight test of the engine in 1955. The J-79 produced 12,000 pounds of thrust without afterburner and 17,000 pounds of thrust with afterburner. Its 17-stage compressor had variable stator vanes in the first 6 stages, as well as variable inlet guide vanes; its overall compressor pressure-ratio was 12 to 1 (for an average pressure-ratio just below 1.16 per stage).23