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 which occur on an overwing distributed propulsion system with boundary-layer ingestion. Wind tunnel experiments were performed on a NACA 64₃ -618 airfoil model equipped with an array of five ducted fans integrated across the upper-surface trailing edge. Lift, drag, and pitching moment characteristics, as well as airfoil pressure distributions and PIV velocity fields were experimentally characterized as a function of angle of attack and fan throttle setting. Additionally, various mixed-throttle cases were investigated with certain fans assumed to be inoperative. The experimental results revealed non-linearities in the changes of the lift, drag, and pitching moment polars as a function of α and throttle setting. These nonlinear variations in the airfoil forces and moments were attributed to variation in the magnitude and direction of the fan thrust vector, changes in the airfoil pressure distribution, and an induced circulation effect created by the fan exit mass flow. The ingested boundary-layer height was seen to decrease linearly with a uniform increase in throttle setting across all fans. The fan-out cases showed that an inoperative fan on the edge of the array was more detrimental to performance than an inoperative fan located within the fan array, and a highly three-dimensional flow was observed from the PIV velocity fields and surface pressure distributions in front of a fan which was adjacent to two inoperative fans.