This paper describes the development of a new dual rotor test stand for the Open Jet Flow-Through Anechoic Chamber at the Penn State University. The Coaxial Acoustic Test System (CATS) can accommodate the testing of isolated, tandem, and coaxial rotor configurations operating in hover, climb, cruise, and descent flight conditions. The stand is equipped with a traversable microphone array that enables acoustic measurements to be acquired at a variety of azimuthal directivity angles. This paper highlights the design process as well as some of the encountered issues and their solutions. Sample results for two representative coaxial co-rotating rotor configurations are also presented. The first configuration consists of two vertically separated “stacked” rotors mounted on a single shaft. The rotors are mounted on separate shafts in the second configuration, so that the azimuthal position of the blades varies throughout the measurement. Close agreement was achieved in the average thrust and torque measurements for the two rotor configurations. The separated configuration had higher tonal noise levels than the stacked configuration, although broadband noise levels were similar.
Fixed-pitch coaxial, contrarotating rotors are often used in multirotor aircraft in applications ranging from payload delivery to urban air mobility. This paper examines minimizing net power at a given thrust for the coaxial pair using both an analytical approach based on blade element momentum theory and experimental results conducted in a hover test stand, as well as experimental results in hover for an X8 configuration multirotor. Appropriate distribution of thrust (realized via differential rotor speeds) to the upper and lower rotors results in approximately 5% savings in total power. Under this condition the thrust distribution is approximately 60% by the upper rotor and 40% by the lower rotor (which occurs when the lower rotor shaft speed is roughly 10% higher than the upper rotor shaft speed). Similar ratios were obtained in both the analytical solution and in experiment, at various total thrust levels. Hover flight tests of the X8 multirotor also showed approximately 5% reduction in hover power at this RPM setting.
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.
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