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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.

NASA is exploring a research activity to identify the technologies that will enable an Extreme Short Take-Off and Landing (ESTOL) aircraft. ESTOL aircraft have the potential to offer a viable solution to airport congestion, delay, capacity, and community noise concerns. This can be achieved by efficiently operating in the underutilized or unused airport ground and airspace infrastructure, while operating simultaneously, but not interfering with, conventional air traffic takeoffs and landings. Concurrently, the Air Force is exploring ESTOL vehicle solutions in the same general performance class as the NASA ESTOL vehicle to meet a number of Advanced Air Mobility missions. The capability goals of both the military and civil vehicles suggests synergistic technology development benefits. This paper presents a summary of the activities being supported by the NASA ESTOL Vehicle Sector.

Takeoff predictions for powered lift short takeoff (STO) aircraft have been added to NASA AMES Research Center's aircraft synthesis (ACSYNT) code. The new computer code predicts the aircraft engine and nozzle settings required to achieve the minimum takeoff roll. As a test case, it predicted takeoff ground rolls and nozzle settings for the YAV-8B Harrier that were close to the actual values. Analysis of takeoff performance for an ejector-augmentor design and a vectoring-nozzle design indicated that ground roll can be decreased, for either configuration, by horizontally moving the rear thrust vector closer to the center of gravity, by increasing the vertical position of the ram drag-vector, or by moving the rear thrust vector farther below the center of gravity.