Acoustic Analysis and Sound Quality Assessment of a Quiet Helicopter for Air Taxi Operations

This paper investigates tonal and broadband noise for rotor designs used on urban air mobility vehicles. Quiet helicopter rotor designs with varying tip speeds and blade numbers are studied for the tonal and broadband noise at the same mission specification. The rotor aerodynamics in edge-wise forward flight are calculated using the blade element theory coupled with a dynamic inflow model and the moment-balance trim analysis. Loading noise and thickness noise are obtained using the lifting-line loading distribution and the dual-compact thickness noise model in PSU-WOPWOP. With the forward flight capability developed in UCD-QuietFly, broadband noise, including trailing-edge noise, trailing-edge bluntness noise, and airfoil stall noise, is predicted. Psychoacoustic metrics, such as fluctuation and roughness, are used to quantify the human subjective annoyance levels. The relative importance between tonal noise and broadband noise is investigated for various design cases and operating conditions. It is found that broadband noise is the dominant noise source for the rotor designs with low tip speeds and fewer blades, while tonal noise is dominant for the high-tip-speed designs. A low tip speed and more blades are found to be the preferable design features in terms of psychoacoustic metrics.