Aero-propulsive and propulsor cross-coupling effects on a distributed propulsion system

This study aims to characterize the complex propulsive-airframe and cross-propulsor interactions that occur on an overwing distributed propulsion system with boundary-layer ingestion. Wind tunnel experiments were performed on aNACA643-618 airfoil model with an array of five ducted fans integrated across the upper-surface trailing edge. Net forces lift, drag, and pitching moment characteristics, as well as pressure distributions and particle image velocimetry (PIV) velocity fields, were experimentally characterized as a function of fan throttle setting. Additionally, various mixed-throttle cases were investigated with certain fans inoperative windmilling. The experimental results revealed nonlinearities in the changes of the streamwise and stream-normal forces lift, drag, and pitching moment polars as a function of α and throttle setting. These nonlinear variations in the airfoil model forces and moments were attributed to variation in the magnitude and direction of the fan thrust vector, which is tied to changes in the airfoil pressure distribution and an induced circulation effect. The ingested boundary-layer height was seen to decrease linearly with a uniform increase in throttle setting. The fan-out cases showed that a windmilling inoperative fan on the edge of the array was more detrimental to performance than a windmilling inoperative fan located within the fan array, and a highly three-dimensional flow was observed from the PIV velocity fields and surface pressure distributions.