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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.