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Technologies for Active Vibration and Acoustic Comfort

Periodic Reporting for period 3 - TAVAC (Technologies for Active Vibration and Acoustic Comfort)

Reporting period: 2019-09-01 to 2020-02-29

There are two related important challenges faced in new aircraft development.
Challenge 1: The comfort of passengers should be continuously improved.
Challenge 2: Most importantly, as more powerful and efficient aircraft engines are introduced and new light-weight airframes are adopted the intensity of engine and airframe vibrations is increased while the damping capacity of the fuselage is reduced, setting a barrier in the improvement of airframe and engine efficiency.
To address both challenges, special noise cancellation techniques should be employed. The project aims to improve existing airframes and engine technology and efficiency, while sustaining the high level of the customer comfort through the introduction of novel integrated active technologies of noise and vibration reduction. This is accommodated in two discrete, but highly related directions: (1) Active vibration isolation and control; and (2) Active noise control. The project aims to technically improve current successful approaches and to adapt them for increased passenger comfort in business jets. The project objectives can be summarized as:
• Obj. 1: Develop an Active Vibration Control System to accommodate engine vibrations using active engine mounts.
• Obj. 2: Develop an Active Noise Cancelation System to reduce noise discomfort in the passenger area.
• Obj. 3: Develop an Active Vibration Control System to attenuate aerodynamic vibrations transferred to passenger area via the fuselage floor.
WP1 was dedicated to the development of an Active Vibration Control System (AVCS) at bespoke engine mounts, in order to reduce the amplitude of the resulting force on the aircraft fuselage. The AVCS should provide an engine vibration reduction of 15 dB at the neighbourhood of the typical N1, N2 engine rotational frequencies around 110Hz and 400Hz. The mass of the proposed AVC system, for the reduction of the engine noise should not exceed 15 kg.
The design and demonstration of an effective Active anti-Noise Control System (ANCS) able to address the above demanding issues was addressed in WP2. The primary objective of the ANCS was to generate a restricted quite zone (“sweet spot”) around the head of a passenger at the headrests of a salon seat configuration of two triple-seat sofas face-to-face, using local microphones and loudspeakers. The weight of the complete system should not exceed 9 kg. The usage of a modular and highly distributed controllers with various levels of optimization and adaptivity, combined with the quality and fidelity of the loudspeaker configuration, should ensure the target of noise reduction of 15 dB and would provide at least 10 dB attenuation at the typically N1, N2 engine rotational frequencies around 110Hz and 400Hz.
WP3 was dedicated to the improvement of the comfort level in a business jet aircraft, in terms of vibration cancellation. The developed system should be capable to operate in the frequency band of 5 to 15 Hz providing a reduction of 6dB on the fuselage floor. The system should be capable to adapt on typical in – flight aerodynamic perturbations and its operation should be based on one or more actuators located in the fuselage. For the needs of the current WP the aerodynamic perturbations must be applied on two different locations of the horizontal tail. The proposed system is subjected in a maximum weight limit of 30Kg total.
WP1 activities have simulated and virtually demonstrated the operational performance of a vibration control system to a full-scale aircraft, in order to achieve substantial reduction in its fuselage Acoustic Pressure. The target of reduction was set to 15 dB, the target frequency was the N1=110 Hz and the anti-vibration setup was a lab-scale demonstrator presented in D1.5 at the University of Patras. Concerning the installation parameters of the anti-vibration setup, a commercially available piezoelectric actuator was proposed (see D1.6) with minimal effect on the mount stiffness. The installation of each actuator was simulated to be parallel to the existing mount and the total weight of the anti-vibration system (actuators plus amplifiers) for each investigated case does not exceed 16 kg. Conclusively, the working procedure was presented to reach and overcome the target reduction of 15 dB in the fuselage Acoustic Pressure, by using a minimum number of actuators. Installation of the proposed system had slight effect on the existing mount stiffness, and operated with almost negligible power requirements.
In WP2, An ANC system has been designed and integrated inside each passenger seat, consisting of loudspeakers, microphones and control law device. The integrated ANC system has been implemented and experimentally tested in a two face-to-face three passenger seat configuration, inside a cabin mock-up. The ANC system is based on a conventional FxLMS algorithm for reliability and simplicity reasons and it has been tested through different noise signals, aiming to a local adaptive noise cancellation and can successfully:
• Reduce the overall noise level of noise up to 23 dB at the nominal ear position D.
• Reduce the spectrum peak amplitude of noise up to 20 dB at the nominal ear position D.
• The proposed spatial H/W arrangement (microphones and loudspeakers) of the ANCS achieves significant performance of ANC system building a quiet zone far away from the error microphone and close to a desired location (passenger’s ear). So, it is not important the error microphone to be placed closed to the ear in order the zone of quietness to be achieved in the vicinity of it.
• The random background noise does not show a remarkable change.
• The total weight of the H/W (FPGA controller, microphones, loudspeaker, amplifiers and power supply) is less than 9 Kg.
It is therefore concluded that the system can successfully meet the project requirements, i.e. to offer at least a 10 dB reduction at the engine rotation speed. Moreover, the system offers the potential for an overall 15 dB noise reduction under specific conditions. Finally, the overall weight of the system does not exceed 9Kg.
In WP3, the procedure followed was to model the full-scale aircraft in accordance with the input provided by DA, to validate that and to construct a coupled aircraft-SATMD model capable to tune at different frequencies. Finally, the proposed system performance was presented on the aircraft full scale model and multiple scenarios were presented. Two tuning cases were used and were also both commercial actuators and customized. Relative to the proposed SATMD scenarios the main conclusions are:
1. When using commercially available actuators, the required auxiliary mass exceeds the provided wight limitations for attempting a reduced behavior. This is due to the actuator’s stiffness restricting us from freely tuning the SATMD initially to the targeted mode. Also, the piezoelectric stack is not optimized and when tuned on lower modes the performance is not optimal, however with a higher auxiliary mass it achieves reductions up to 6 dB.
2. Custom actuators performance achieves higher reductions since we can tune the SATMD on the target mode using auxiliary mass within the weight limitation. Also, piezoelectric stack is optimized thus achieving higher performance when tuned on lower modes similar to the ones of the previous scenario decreasing also the mas
AVCS Prototype System (WP1)
AVCS Prototype System (WP1)
AVCS Prototype System (WP3)
The overall installation of the ANC system in an ‘aircraft cabin mock-up’.