Fluid Dynamics of Rotors Through Dynamic Transition

Urban Air Mobility requires vehicles (UAMs) that operate in close proximity to buildings, particularly in take-off and landing phases. Such operation results in the aircraft encountering significant unsteadiness in the form of large amplitude gusts. Control in these transitional areas of flight is paramount to safety, and therefore rotor performance during transition must be understood. To this end, the Bath University Low-speed eVTOL Test-Rig (BULLET) has been significantly upgraded to deliver transition rates up to 10pi/9 rad/s, designed to probe the dynamic behavior of rotors through dynamic transitions and gust encounters. To the authors' knowledge these represent the fastest experimental transitions in the literature. Coupled with stereoscopic Particle Image Velocimetry, this study presents for the first time flow visualization of a rotor undergoing dynamic motion. The inflow PIV indicates an asymmetry of blade loading during transition, which is magnified by rapid transition despite the net thrust remaining unchanged. Blade angle of attack distributions are observed varying by up to 0.5deg. However, force decomposition demonstrates a lack of any unsteady overall thrust response from the rotor in response to the dynamic motion. This is corroborated by PIV analysis of both the wake and inflow to the rotor disc which show no introduction of unsteady effects.