Recirculation-Delayed Anechoic Chamber (RDAC) for Propeller Acoustic Testing

This paper presents the characterization of a new 5.92 m × 8.51 m × 3.05 m recirculation delayed anechoic chamber (RDAC) according to the guidance of ISO 3745 free-field testing and an experimental study on the effects of RPM variation on rotorcraft unsteady aerodynamic loading and broadband noise. The chamber is shown to have a cutoff frequency of 250 Hz. UCD-Quietfly was used to predict the propeller noise of constant RPM. The loading and broadband noise of a 16 × 5.4 inches rotor were obtained while operating under constant RPM, impulse, step, linear, and sinusoidal RPM variation, as well as a simulated unmanned aerial vehicle (UAV) flight path and battery-powered operation. The results demonstrated a strong correlation between broadband noise and RPM across all cases, while abrupt or periodic RPM changes introduced larger unsteady aerodynamic loading and elevated broadband noise not only during the cycles but also after the rotor returned to a constant RPM. Linearly increasing and decreasing RPM were found to be ideal for smoother thrust transitions and lower overall sound pressure levels (OASPL). The battery-powered operation introduced additional RPM fluctuations that resulted in OASPL variations exceeding 6 dB. These findings highlight the importance of RPM control strategies in mitigating rotorcraft noise and provide the correlations of the varying rotational speed and the unsteady loading noise.