Active control of counter-rotating open rotor interior noise in a Dornier 728 experimental aircraft

Counter-rotating open rotor, a lightweight design and new engine plays a vital role in the reduction of carbon dioxide for future civil aircraft. However, the poor transmission loss of lightweight design and emission of high sound pressure level remains a challenge in terms of aircraft interior noise.

Active concepts such as active noise control ANC, active vibration control AVC and active structural acoustic control ASAC are known to improve transmission loss at counter-rotating open rotor blade passing frequencies as lightweight-compliant solution toward passive methods.

The research conducted by Thomas Haase and colleagues focused is on an active vibration control system in order to improve low frequency transmission loss of an airframe excited by a synthetic counter-rotating open rotor sound pressure field. The work published in Journal of Sound and Vibration had three objectives; investigation of sound transmission of the counter-rotating open rotor sound pressure field through an airframe, experimental test of typical control actuators regarding their coupling capabilities and realization of active vibration control AVC system in order to improve the transmission loss at the counter-rotating open rotor frequencies.

The Dornier 728 experimental prototype aircraft used for this study has fully equipped airframe and cabin for   interior noise research. In order to excite the airframe with a counter-rotating open rotor pressure field, a 112-channel loud speaker placed closely in front of fuselage was used. The curvature of the speaker array could also be adjusted to that of curvature of the Dornier 728.

First seven frequencies, 119.6, 149.3, 268.5, 388.1, 417.8, 507.5 and 537.2Hz were superposed to investigate sound transmission and it was discovered that the noise was transmitted efficiently through the lower part of the test section. However, high sound intensity values were not observed in window area as it was expected from vibrational amplitude distributions which could be the frame which adds mass and stiffness in this area thereby increasing transmission loss.

When investigating actuator performance, frequency response functions FRFs from the actuators to the sensor grid of laser scanning vibrometer were measured sequentially one after another and it was discovered that the performance of actuators was mainly limited to the region between two vertical stiffeners which is caused by high structural damping induced by the rivets and stiffeners.

For system identification it was seen that the signal to noise ratio is not sufficient to identify a precise model in the frequency range of 0Hz to 70Hz due to reduced sensitivity of actuators and sensors in the lower range. In frequency greater than 540Hz, structural response of the control path was very low because excitation is low-pass filtered with a cutoff frequency F_C= 480Hz.

Feedforward control results show that the collocated system (sensors placed geometrically as close as possible to the actuators) generates a strong local vibration attenuation of over 30dB. This however leads to vibration restructuring instead of a reduction in cases of vibration reduction and sound radiation. With small local vibration reductions, the control system with 25 sensors produced a more homogenous reduction over the whole fuselage surface up to 5dB in the first and 3dB in the third counter-rotating open rotor frequency at 268.5Hz.

Both control systems achieved only vibration reductions that led to an amplification in radiated sound intensity when counter-rotating open rotor blade passing frequencies was above the third. Additionally, as global vibration amplitude increased in collocated system due to pinning effect, the reverse was the case for 25 sensor control system as global vibration is nearly unaffected.

In order to improve the effectiveness of active structures we have to switch from active solutions for passive designs to active by design, said Thomas Haase.

This study showed that active control system achieved a reduction up to 5dB at several counter-rotating open rotor frequencies. However, further investigations are needed in achieving better performance in order to address noise problems with an active vibration control system.