Experimental Evaluation of Multi-Rotor Aerodynamic Interactions

An investigation was performed into the aerodynamic interactions between the rotors of multi-rotor vehicles in different configurations. The effects of these interactions on the thrust and torque of all individual rotors were quantified in wind tunnel tests. Flow visualization and a limited number of particle image velocimetry measurements were conducted to get further insight into the aerodynamic interactions. The effects of the changes in hub spacings, rotor rotational speeds, and wind speeds were investigated for isolated, tandem, quad-rotor plus and X configurations. The maximum and minimum tip chord Reynolds numbers were 118,000 and 73,000, respectively. Results showed that the aft rotors experienced detrimental aerodynamic interactions in all configurations. In all examined multi-rotor configurations, an increase in the hub spacing caused a decrease in the thrust deficit between the aft rotor and the isolated rotor. However, the differences in the configurations also affected the measured loads. In the tandem configuration, the aft rotor experienced up to 24% reduction in the thrust coefficient at a hub spacing of 2.1R when compared to the isolated rotor at the same rotational frequency and wind speed. The aft-most rotor in the plus configuration experienced as large as a 28% decrease in the thrust coefficient when compared to one of the aft rotors in the X configuration for the same hub spacing and flight conditions. Good correlation was found between these wind tunnel tests and flight tests for the front and side rotors in X and plus configurations (7.9-14.2% difference), but a larger difference of 30-41.9% was found for the aft rotors, which is likely due to the different rotor trim conditions.