Acoustic characteristics of counterrotating fans from model scale tests

B.A. Janardan and P . R . Gliebe Acoustics Systems Technology GE Aircraft Engines Cincinnati, Ohio Development of an acoustic technology that describes the noise characteristics of counterrotating fans is an important phase in the design of the fuel efficient UnDucted Fan engines. In order to obtain the needed data, measurements were made in aanechoic facility with approximately !-scale counterrotating model fan bkiades. Tests were conducted at simulated takeoff and cutback conditions to determine the effects on community noise characteristics of variations in blade numbers, tip speeds, and rotor-to-rotor spacings. Selected results are presented and discussed in this paper. The data measured with various blade number combinations indicated significant overall acoustic benefit with increase in blade number. Tests with a given configuration indicated the steady-loading noise, as represented by the levels at the blade passing frequencies, to decrease with the reduction in tip speed for a given thrust. The amount of acoustic benefit due to increased rotor-to-rotor spacing was significantly different for configurations with standard diameter aft blades vs. clipped aft blades. For the clipped configuration, t h e forward rotor tip-vortices did not interact as strongly with the short aft blade, and hence the tip-vortex to aft-rotor interaction noise did not mask the f u l l acoustic benefit of increased decay of the forward blade wakes with increase in rotor-to-rotor spacing. BPF D dBA f EPNL