On the noise generation mechanisms of overlapping propellers

Psychoacoustic modelling of rotor noise

“…These 7 master scaling stimuli covered approximately the whole range of LAeq of all the test stimuli used, which ranged from 39dBA to 68.9dBA. The sound used to synthesise the 7 master scaling stimuli was dominated by the present of potential field interaction tones, as the main sound generation mechanisms in contra-rotating systems with rotors closely spaced (Paruchuri et al, 2021;. A clearly dominant acoustic feature with sound levels varying widely, to cover the whole range of test sounds, allowed the derivation of a linear master scale as described in section III.F.…”

“…al. (Paruchuri et al, 2021) argues that for a single rotor, the radiated acoustic power varies as 𝑵𝑵 5.5 𝑫𝑫 3 , where 𝑵𝑵 is the rotational speed and 𝑫𝑫 is the diamater of the propeller. The total noise therefore follows a thrust scaling law of 𝑻𝑻 2.75 and velocity scaling law of 𝑼𝑼 5.5 , which is identical to the scaling law characteristics of aerofoil leading edge noise.…”

“…suggested an optimal rotor axial separation distance (relative to the blade diameter) between 0.2 and 0.4. Chaitanya et al (Paruchuri et al, 2021) discussed the reason behind this optimum and attributed it to an optimum balance between the various dominant sources. The potential field interactions were shown to dominate overall noise at separation distances smaller than the optimum distance, while the noise due to tip vortex interaction is dominant for distances greater than the optimum value.…”

“…The potential field interactions were shown to dominate overall noise at separation distances smaller than the optimum distance, while the noise due to tip vortex interaction is dominant for distances greater than the optimum value. Analytical predictions were also performed by Chaitanya et al (Paruchuri et al, 2021) to validate their hypothesis. McKay et al (McKay et al, 2019) carried out an experimental investigation on noise of contra-rotating systems with varying rotor axial spacing, blade diameter, and blade number.…”