Reduction of rotor noise transmission at the Dornier 728 experimental aircraft

Abstract

In future aircraft well-established jet engines could be substituted by contra rotating open rotor (CROR) propulsion systems. Due to their fuel efficiency they are able to reduce costs and CO2 emissions significantly. But up to now the high noise emission of CROR engines in certain frequency bands impede dissemination on the market. This conflict can be solved by the implementation of modern noise reduction techniques like Active Structural Acoustic Control (ASAC) in aircraft. In this paper an approach for the reduction of noise transmission through a section of a fuselage is presented. The experimental aircraft Dornier 728 of the DLR is equipped with an ASAC system. The system is mounted on a side panel including two windows and covers an area of 1000 x 1500 mm^2. Sensors on the inner fuselage measure the normal acceleration in specified points of the structure. A narrow-band robust controller calculates the desired actuator signals to reduce the sound transmission at the specific CROR bands in the bandwidth from 1 to 500 Hz. Since the Dornier 728 aircraft has no CROR engines, their acoustic excitations have to be synthesized. A 112-channel loudspeaker array is placed close to the fuselage. The loudspeakers are driven with synthesized signals and produce a complex CROR sound pressure field on the outer side of the fuselage. The synthesized excitation gives the opportunity to test the ASAC system with realistic loads. Beside a detailed description of the ASAC realization and the algorithms, the results of the experi-ments and the effectiveness of the control concept are shown.