Aeroacoustics of a coaxial rotor in level flight

The aeroacoustic characteristics of a coaxial system with teetering rotors in level forward flight are compared to those of an equivalent articulated single rotor with the same solidity. A lifting line representation of the blade aerodynamics is coupled to Brown's Vorticity Transport Model to simulate the aerodynamics of the rotor systems. The acoustic field is determined using the Ffowcs Williams-Hawkings equation. Acoustic analysis shows that the principal contribution to noise radiated by both the coaxial and equivalent single rotor systems is at the fundamental blade passage frequency, but that the coaxial rotor generates higher sound pressure levels (by 10 dB for the evaluated configurations) than the equivalent single rotor at all flight speeds. The sources of blade vortex interaction (BVI) noise are investigated and the principal BVI events are identified. For the coaxial rotor, the most intense impulsive noise is seen to be generated by the inter-rotor BVI on the advancing side of the lower rotor. The impulsive noise that is generated by blade vortex interactions for the equivalent single rotor reduces in amplitude as the strength of BVI events on the rotor decreases with forward speed. Conversely, the BVI noise of the coaxial rotor intensifies with increasing flight speed due to the increasing strength of the interaction between the wake of the upper rotor and the blades of the lower rotor. The impulsive noise due to BVI for the coaxial rotor is found to be higher by 20-35 dB compared to the equivalent single rotor. The overall and impulsive noise characteristics of the coaxial system are found to be weakly sensitive to changes in rotor separation and the relative phasing of the rotors.