Analysis of Spread Multi-Jet VTOL Aircraft in Hover

An investigation of VTOL aircraft lift losses in hover has been conducted to evaluate a method for a simplified test technique. Three flat plate models were tested to determine their usefulness in predicting hover characteristics by comparing results between plate and three­ dimensional models. Attempts to correlate the out of ground effect results for the plate models with the empirical expression for calculating jet-induced loads were successful for the models which used engine simulators producing efflux characteristics similar to the three­ dimensional model engines. Data for the model using engine simulators with characteristics unlike those of the three-dimensional model simulators could not be correlated in this manner. In ground effect, comparisons of induced lift loads indicated correlation between the plate and three-dimensional models were within 2 percent of thrust in the height range dominated by the fountain effect. However, small outward deflection of lift-engine exhaust was found to cause a decrease in beneficial fountain on the order of 5 to 10 percent -of thrust. The engine arrangement of a midspan pod VTOL lift-fan transport model showed a strong adverse effect on lift loss due to engine deflection. Dashpots proved useful in eliminating data scatter caused by flow-induced model vibration. Data obtained for the plate models could be correlated to three-dimensional results by the application of a geometrical equivalent height correction factor ∆h/De. The correlation of plate and tunnel models indicated that lift losses in ground effect were essentially independent of the efflux characteristics for the engine simulators.