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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.

A series of tests has been completed in which suckdown and fountain forces and pressures were measured on circular plates and twin-tandem-jet generic STOVL (short takeoff and vertical landing) configurations. The tests were conducted using a small-scale hover rig, for jet pressure ratios up to 6 and jet temperatures up to 700°F. The measured suckdown force on a circular plate with a central jet was greater than that found with a commonly used empirical prediction method. The present data showed better agreement with other sets of data. The tests of the generic STOVL configurations were conducted to provide force and pressure data with a parametric variation of parameters so that an empirical prediction method could be developed. The effects of jet pressure ratio and temperature were found to be small. Lift improvement devices were shown to substantially reduce the net suckdown forces. Paper to be presented at SAE Aerospace Meeting, Dayton, Ohio, April 24-27, 1990

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.