Phased Array Measurements of a Full-Scale Helicopter

Single microphone measurements lack the ability to separate nondeterministic noise sources on multipropulsor vehicles, limiting their usefulness to understand the dominant noise generation mechanisms. To advance the state-of-the-art for measuring multipropulsor aircraft in support of future Urban Air Mobility (UAM) and Future Vertical Lift (FVL) testing, a 117-channel phased array was deployed during an Army/NASA acoustic flight test of an MD530F helicopter. A time-domain beamforming algorithm, namely, the ROtating Source Identifier (ROSI), was utilized to track the aircraft's forward motion and main rotor rotation. This process isolates nondeterministic sources of the main rotor, effectively filtering out contributions of the tail rotor and other nonrotating components. Source maps are provided for low-speed forward flight and illustrate aeroacoustic sources near the main rotor blade tips over a broad frequency range. Particular emphasis is given on the benefits of flying at a lower altitude relative to the array to enhance source separation through increased spatial resolution. A coherence-based microphone weighting scheme was utilized to support component-based integrated spectral assessments. Total integrated levels over the rotating scan grid are provided, representing the contribution of the main rotor to the total aircraft levels. Although additional analysis is needed to gain confidence in the beamforming methods applied, results herein minimally indicate this to be a valuable technique for qualitative source ranking.