Effects of gust on the aerodynamics of multi-rotors

Understanding the aerodynamics of multi-rotor systems under temporally varying discrete gusts is the focus of this work. Axial and edgewise 1-minus-cosine profiles of varying amplitude (0.1ΩR–0.3ΩR) and gust durations (0.3–15 s) were studied for the rigid, unarticulated Harrington single and coaxial rotors, and the Advanced Precision Composites (APC) 12x6E (a commercial off-the-shelf propeller) quad-rotor in hover. An in-house free vortex method (FVM) was utilized to study the time and frequency domain of thrust, span-wise angle of attack distribution, and the rotor wake structure across the gust event. It was observed that edgewise gusts produced higher frequency components in the thrust signal compared to axial gusts for all rotor systems. Strong wake–wake and blade–vortex interactions (BVI) were observed particularly for edgewise gusts, leading to strong variations in the sectional angle of attack and the thrust values. The wake interactions are more pronounced in the APC quad-rotor system, which operates at a low reduced frequency, essentially resulting in a quasi-steady response of the wake to the gust, resulting in a considerably higher number of BVI events, increased peak rotor loads, and higher frequency content. This study can aid in the understanding of the gust response of small- and medium-scale rotors.