Induced flow measurements and analytical tonal noise predictions for a sub-scale eVTOL rotor

Tonal acoustic predictions of a fixed pitch rotor in edgewise flight are developed based on rotor inflow measurements. The induced velocity of a fixed-pitch rotor in hover and edgewise flight was measured with stereoscopic particle image velocimetry (sPIV) in an anechoic wind tunnel. The edgewise flight measurements covered advance ratios ranging from 0.065 to 0.268. The ensemble averaged sPIV measurements showed that an increase in advance ratio led to a skewing of the maximum induced velocity towards the advancing side of the rotor and an increased area of upwash velocity at the front of the rotor disk. The measured mean velocity field was combined with blade element theory and Sears Unsteady Airfoil Theory to estimate the unsteady blade forces, which were used in analytical tonal noise predictions. The predictions were on average within 5 dB of measured sound pressure levels at the rotor blade passage frequency.