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When the jet stream from a fan or jet powered STOVL aircraft impinges on the ground during STOL operation, the part of the impinging flow that is directed forward is opposed by the free stream (fig. 1) and rolled up into a horseshoe shaped ground vortex. This ground vortex induces pressures on the lower side of the model that can significantly affect the forces and moments experienced in STOL operation. Available methods for estimating the lift and moment induced by the ground vortex (ref. 1 - 2) are based primarily on force data. This paper presents a review of an ongoing analysis of the effects of the ground vortex that is based on detailed lower surface pressure distributions (ref. 3). The estimating method that is being developed is demonstrated using some comparisons with experimental data.

The flow fields encountered by jet- and fan-powered vertical/short takeoff and landing (V/STOL) aircraft when hovering in ground effect are reviewed and their effects on the aerodynamic characteristics are discussed. The ground effects considered include the suckdown generated by the flow from a single nozzle, the fountain effects generated by multiple-nozzle configurations, and the additional suckdown associated with the fountain flow generated by multiple-nozzle configurations. Our current understanding of the flow fields involved and the capability and limitations of available methods for estimating the effects of ground proximity are reviewed and the areas where additional work is needed are discussed.