Acoustic characteristics of counterrotating unducted fans from modelscale tests

Development of an acoustic technology that describes the noise characteristics of counterrotating fans is an important phase in the design of fuel-efficient, unducted fan engines. In order to obtain the needed data, measurements were made in an anechoic facility with approximately 1/5-scale counterrotating model fan blades. Tests were conducted over a range of power settings to determine the effects of variations in blade number, tip speeds, and rotor-to-rotor spacings on community noise. Selected results are presented and discussed in this paper. Significant overall acoustic benefits were measured with an increase in blade number, a reduction in tip speed for a given thrust, increased rotor-to-rotor spacing, and clipped aft blades.