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The direct relation between subcriticai jet efflux characteristics and propulsion induced changes in STOVL aircraft forces and moments is described in this paper. Subcriticai jets may have core lengths which range up to 6 diameters. The shorter core length jets tend to entrain ambient air more rapidly, inducing larger hover lift losses, and decay more rapidly, reducing adverse ground erosion. In transition flight, shorter core length jets show a decrease in the lift loss. Supercritical pressure ratio jets are also described; they tend to have a longer, higher pressure core. For both subcriticai and supercritical pressure ratio jets, the dynamic pressure decay is similar. In ground effect the supercritical jet induces an oscillating pressure distribution on the ground with reflected shocks and expansions which can increase ground erosion and at low ground heights cause non-monotonic lift loss variations.

Hover and ground-effect tests were conducted with the Lockheed-Martin Large Scale Powered Model (LSPM) during June-November 1995 at the Outdoor Aerodynamics Research Facility (OARF) located at NASA Ames Research Center. This was done in support of the Joint Strike Fighter (JSF) Program being lead by the Department of Defense. The program was previously referred to as the Joint Advanced Strike Technology (JAST) Program. The tests at the OARF included: engine thrust calibrations out of ground effect, measurements of individual nozzle jet pressure decay characteristics, and jet-induced hover force and moment measurements in and out of ground effect. The engine calibrations provide data correlating propulsion system throttle and nozzle settings with thrust forces and moments for the bare fuselage with the wings, canards, and tails removed. This permits measurement of propulsive forces and moments while minimizing any of the effects due to the presence of the large horizontal surfaces.