Experimental Study of Noise and Performance on Counterrotating Propellers

Abstract

For the purpose of observing the acoustic and aerodynamic phenomena of counter rotating propellers under take off and landing conditions, a 400 mm diameter scale model was tested. The test was made in an anechoic chamber with 68 m/s incoming flow, keeping the front blades at 9000 rpm. The rotational ratio, the axial space between two rotors, front and aft blade angles were chosen as parameters. From noise data analysis, reinforced high harmonics were recognized, which haven't been observed in single rotating propellers. While large spacing yielded good results on noise level reduction, unfavorable effects were recognized on the aerodynamic performance, due to increasing axial velocity acceleration in two rotor blades. When there were nonsynchronous rotational speeds between two rotors, spinning sound waves occurred with a beat frequency of three times the rps difference.