Experimental Acoustic Characterization of Anti-Phase Asymmetric Rotors

Abstract

The rapid growth of small-size rotorcraft such as Unmanned Aerial Vehicles (UAV's) creates new missions with a new range of issues. Rotor noise is an inevitable consequence of rotary wing flight and can lead to the annoyance or dissatisfaction of customers. This paper presents the experimental work to explore possible acoustic and aerodynamic performance benefits from a proposed anti-phase rotor technology developed previously by NASA Ames and team. The anti-phase alternating pattern from blade to blade aims to prevent harmonic reinforcement of the blade vortex structure that could theoretically lead to an acoustic reduction. A modified NACA-4412 rotor with a NACA-E63 root was used as the baseline rotor for acoustics and aerodynamic performance comparisons. Six 8inch rotors (two sets per design) were manufactured using 3D printing technology. Testing was conducted in the Open Jet Flow-through Anechoic Chamber on the UAV Rotor Test System at Penn State. A semi-circular array that has a radius of 104 cm and held 15 microphones was used to measure the far-field rotor noise. Three flight conditions, hovering and advancing side edgewise flight 9.7 m/s and retreating side edgewise flight at 9.7 m/s, were tested. A total of nine cases for Matching RPM (MR) and nine cases for Matching Thrust (MT) cases were conducted. Possible uncertainties in the study were identified. Recirculation effects of testing in a closed anechoic chamber was acknowledged. A single rotor hover test at Penn State determined that peaks of Sound Pressure Level (SPL) within 2,000-4,000 Hz showed similar values within 10% difference when the test was in the chamber and outside free from recirculation. This range was taken as the range of interest of this study. The repeatability of data between the three runs in each case, showed variations below 10 % in acoustic performance metrics and below 5 % in aerodynamic performance metrics deeming each case repeatable at the point of testing. Physical differences in the advancing and retreating side rotors of the same design caused by uncertainties in manufacturing were identified to have caused discrepancies in the OASPL readings at the compared microphones of up to 2.3 dB. These discrepancy values can be taken as the possible acoustic error value in this study. In hover the modified rotors showed decreased aerodynamic performance and no significant increase in acoustic performance compared to the baseline rotor. In MR cases, the asymmetric and symmetric design had 10 % and 11 % more thrust but required 12.8 % and 8.4 % more torque and had negative values for percentage 1 / Power Loading (1 / PL) respectively. Overall Sound Pressure Level (OASPL) acoustic delta values were up to +1.3 dB louder. For the advancing side of the edgewise flight cases, the asymmetric design had a -5.1% decrease in torque and 8.4 % 1 / PL value, making it a better design for aerodynamic performance as compared to the symmetric design and the baseline rotor. There were also no significant acoustic performance benefits from either modified rotor. The retreating side showed the most significant aerodynamic performance benefits for both modified rotors. In the MR cases, the asymmetric design had a -22.1 % reduction in torque and a percentage 1 / PL value of 27.1 %. At 3,000 Hz, both the symmetric and asymmetric designs demonstrated significant acoustic advantages over the baseline rotor in the MR case. The symmetric design was 4 to 5 dB quieter and the asymmetric design was 3 to 4 dB quieter. This initial experimental exploration of the anti-phase blade concepts showed promising aerodynamic performance and SPL Spectrum at 3,000 Hz acoustic benefits.