Prediction of Ground Effects for VTOL Aircraft with Twin Lifting Jets

An empirical method has been developed for predicting ground-effect thrust losses, often called "suckdown" losses, for side-by-side rectangular-jet configurations. The method combines available flat-plate single-jet suckdown correlations (adapted to highand low-wing airplane configurations) with a new correlation for the fountain effect associated with twin jets. The fountain correlation is based on a new experimental investigation in which nozzle variables were varied parametrically. Properly configured rectangular nozzles were found to produce a favorable fountain effect that largely cancels the suckdown losses. Considerably weaker fountains were observed for twin round nozzle configurations.