Interior Noise Sources Identifications, in-Flight Measurements and Numerical Correlations of an Advanced

Business Aircraft

Francesco Marulo*

University of Naples “Federico II”, Naples, Italy, 80125

Antonio Sollo† and Marco Aversano‡

Piaggio Aero Industries, Pozzuoli, Italy, 80078

Giovanni Russo§ and Tiziano Polito**

University of Naples “Federico II”, Naples, Italy, 80125

One of the main appeal of a pusher-propeller configuration relies on the reduced levels of the cabin interior noise. Such reduced propeller noise may raise up other noise sources, specifically interior noise sources of the Environmental Control System (ECS) apparatus. An incorrect design of this system may reduce the benefit of the pusher-propeller configuration, in terms of passenger acoustic comfort. The problem has been addressed in this paper from both experimental and numerical point of view. Ground and flight measurements have been carried out with different interior furnishings. These tests have had the twofold objective of identification of interior noise sources on a specific airplane and to provide realistic input values for addressing a numerical correlation with a pre-design computer program. The satisfactory results obtained from this numerical-experimental correlation offer reliable confidence with regard to the computer code and give the opportunity to predict the noise effects of interior treatment modifications.

I. Introduction HE reduction of the interior noise levels of a business aircraft is an important requirement in order to facilitate a good acceptance of the airplane in its possible natural market. The achievement of such challenging objective

must be obtained without affecting the flight performance and limitating the number of flight hours for testing different configurations. In order to comply with these needs it is important to identify the most critical noise sources and to have the possibility to correlate the measurements with some numerical tool which may assist for the design and selection of the candidate interior treatments, [1]. For classical (tractor) propeller aircraft one of the main noise source comes from the pressure wake of the propellers with a periodic load on the fuselage propeller plane, [2]. In the pusher propeller configuration the pressure wake of the propellers leaves the airplane generating minimum interaction with the fuselage and, in any case, the propeller plane may result in the rear part of the fuselage itself. This last consideration is more evident for the P180 aircraft where the three surface configuration moves back the position of the wing and consequently the propeller plane, fig. 1. As result the interior noise level is considerably lower than similar aircraft with different propeller configuration. As generally happens when reducing the main origin of noise, the other sources becomes more evident and the expected benefits are reduced. Additionally the acoustic engineer should address his interests toward other different items, as, for example, the ECS. Contemporarily it is important to approach the problem from a numerical point of view in order to understand the relative importance of the various parameters.

T

* Professor, Department of Aeronautical Engineering, Via Claudio 21, AIAA Senior Member. † Head of Structure Dept., Engineering Department, Via Campi Flegrei 34. ‡ Acoustic Research Engineer, Engineering Department, Via Campi Flegrei 34. § Engineer, Department of Aeronautical Engineering, Via Claudio 21. ** PhD Student, Department of Aeronautical Engineering, Via Claudio 21.

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12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference)8 - 10 May 2006, Cambridge, Massachusetts

AIAA 2006-2491

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

The availability of a numerical tool, especially for business aircraft which are subject to very different customer requests, is a fundamental need together with the confidence of the users of such tools who should have the ability for a correct, realistic interpretation of the results produced numerically. The basic assumptions rely on the diffuse acoustic field inside each elemental volume and on the acoustic energy balance among the input source and the exchange output among the different volumes. Parallely the possibility of using some flight hours of a business aircraft, in different interior configuration has been the driving factor for approaching from different aspects the problem of the aircraft interior noise. The paper will present the results obtained during in-flight noise measurements and the successive numerical calculations coming from an in-house developed software based on the sound energy balance. The important link between the numerical and experimental values is found on a correct identification of the noise sources and their relative strenght which is an important parameter to be covered when dealing with such problems.

Fig. 1 – The Piaggio P-180 Avanti (three surfaces – pusher propeller configuration)

II. Test Description and Configurations One of the main objective of the experimental activities has been the measurement of the interior fuselage

acoustic field, over different configurations, for both ground and in-flight testing. The ground measurements have been essentially addressed to the identification of the contribution of each main noise sources. Actually the test matrix has been designed starting from the noisest configuration, ECS, engines and fan at their maximum level, to the quietest one with the only ECS on, powered externally. The in-flight measurements have been performed in cruise condition at 15000 and 22600 feet, with the systems operating normally. This test program has been established for obtaining the following results:

to setup a number of experimental tests for preparing a noise reference database to identify the main noise sources, their relative strenght and transmission path along the fuselage to evaluate the soundproofing efficiency of several interior furnishings to measure the noise produced by different ECS’s proposed for the installation inside the airplane to correlate the experimental measurements with the numerical predictions computed by an in-house developed

program, [3]. Three different furnished airplanes have been used. The figures 2-4 show the different configurations and the

relative identification:

Fig. 2 – Configuration 1 – Unfurnished (Green)

aircraft with ECS no. 2

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Fig. 3 – Configuration 2 – Layout and furnishings ver. 1 -

ECS no. 1

Fig. 4 – Configuration 3 – Layout and furnishings ver. 2 -

ECS no. 2

The microphone measurements have been selected for obtaining the most possible complete map of the acoustic

field. Their locations, as shown in fig. 7, is in correspondence of each window, with an height of 120 cm from the floor, and in the middle of each seat.

Fig. 7 – Microphone locations. Plane view (left), Lateral view (right) The acoustic measurements have been carried out with a multi-channel analyzer linked to a laptop computer, as

shown in fig. 8. At each microphone location an added measurement with a Type 1 Sound Level Meter has been performed, as reference.

Fig. 8 – Airplane internal microphone setup and data acquisition system

The data saved for each microphone location were referred to the Overall Sound Pressure Level, both linear and

A-weighted, the 1/3 Octave Band from 20 to 12500 Hz, the narrow band frequency spectrum from 0 to 6400 Hz.

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III. Theoretical Background and Numerical Model The numerical prediction of interior noise level has been computed through an in-house developed program

based on the simple equilibrium of acoustic energy. The interior volume is subdivided in a user selected number of elementary elements. In each volume the equilibrium between the entering, the dissipated and the transmitted acoustic power is computed, allowing the calculation of the sound pressure level. With regard to the typical acoustic sources of an aircraft cabin, such equilibrium equation may be written as, [3]:

VOLAdEEEEEE PROENBLoutass +++=+ where:

=assE

Acoustic Power dissipated inside each elemental volume

=outE

Outcoming Acoustic power from each elemental volume

=AEROE

Incoming Acoustic Power inside each elemental volume due to Turbulent Boundary Layer

=ENE

Incoming Acoustic Power inside each elemental volume due to propulsion noise

=PROE

Acoustic Power generated inside each elemental volume (internal sources i.e. air conditioning)

=.VolAdE

Incoming Acoustic power inside the elemental volume from each elemental volume

The input data are represented by the geometry of the volume under study and by the acoustical parameters

which may characterize the resulting interior noise level. The geometrical input is NASTRAN©-like and it can be generated by any commercial pre-processor able to

create grid points and solid elements. The acoustical input comes from the characteristics of noise sources (internal and external) and by the acoustic properties (transmission loss and acoustic absorption) of surfaces and volumes in octave or third octave band.

Writing the acoustic equilibrium equations for each acoustic power turns out in a system of non-linear algebraic equations, whose solution gives the sound pressure level distribution at each frequency band, in each volume:

⎪⎭

⎪⎬⎫

⎪⎩

⎪⎨⎧

⎥⎥⎦

⎤

⎢⎢⎣

⎡+−

⎪⎪⎭

⎪⎪⎬

⎫

⎪⎪⎩

⎪⎪⎨

⎧

⋅+⋅+⎥⎥⎦

⎤

⎢⎢⎣

⎡+=

−

−+

−−

∑

∑∑

10

10int

10

log10

1010int

10log10

101041010log10.

iwall

iwalliAd

i

ref

refp

iENiENiBLiBL

i

TL

ii

i

TLSPL

i

I

WPWL

TLSPLTLSPL

iext

S

SSSPL

α

The program has been validated by comparison with measurements obtained inside simple acoustic volumes

with different wall treatments and used in real application for different geometries. It may result very useful in preliminary design phase as well as in development phase of a new aircraft for evaluating the impact of different materials, different cabin layout, different location of systems and equipment on the cabin internal noise distribution. For a specific aircraft it can be tuned with measured data, allowing the possibility of evaluating specific solutions for particular customer needs.

The model used in this particular application for the numerical simulations is shown in fig. 9 with reference to the CATIA drawing of the same fuselage part.

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Fig. 9 – Fuselage section used for the interior acoustic simulations

It is the result of assembling 78 volumes, 26 pentahedrons and 52 hexaedrons, and 279 surfaces, 115 external

and 164 internal surfaces. The acoustic characteristics of these elements has been derived accordingly with the material data sheets and from the specific technical literature.

IV. In-Flight Measurements The in-flight measurements gave the opportunity to characterise the interior acoustic field of the P-180 aircraft,

to assess the efficiency of the furnishings and to compare the noise installation of two different ECS’s. Some examples of big amount od data collected during the flight tests are here reported. Fig. 10 shows the noise reduction obtained with the interior furnishings, at the first four Blade Passage Frequencies (BPF), including some asymmetry between left and right hand side.

SPL Furn. - SPL Unfurn.

-2

0

2

4

6

8

10

12

14

16

2 3 4 5 6

Window No.

dB

1st BPF 2nd BPF 3rd BPF 4th BPF

Left Windows

SPL Furn. - SPL Unfirn.

0

2

4

6

8

10

12

14

16

18

2 3 4 5

Window No.

dB

1st BPF 2nd BPF

3rd BPF 4th BPF

Right Windows

Fig. 10 – Effect of furnishings on the interior noise levels along the fuselage

Similar results are presented in fig. 11, using a colour map representation with the frequency as x-axis and the

distance along the fuselage as y-axis. Finally the in-flight measurements offered the opportunity to include the noise characteristics for helping in the

selection of the environmental control system. As an example of the several tests conducted at different conditions, fig. 12 reports the noise comparison between the selected systems along the fuselage in cruise conditions @15000 ft.

The flight test activity has been particularly interesting and has offered the opportunity to train specific personnel

with regard to the comfort requirements of a business aircraft, both from the setup of the instrumentations, the definition of the tests and the analysis of the measurements.

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Fig. 11 – Colour map of the unfurnished (top) and the furnished (bottom) airplane

ECS Comparison

2 3 4 5 6

Window No.

SPL

[dB

(A)]

ECS 1 - Left Window ECS 2 - Left WindowECS 1 - Right Window ECS 2 - Right Window

1 dB

Fig. 12 – Comparison of the SPL measured in cruise conditions for the two selected ECS’s

V. Numerical and Experimental Correlations The numerical and experimental correlation have been performed initially for the unfurnished airplane, in order

to reduce the complexity of the numerical models which has not to take into account seats, tables, and part of the trim panels. For the same reason the selected test condition has been that without internal noise sources. The comparison has been carried out in two positions, where the recorded noise levels have been always resulted higher than the other locations, as shown in fig. 13.

Fig. 13 – Locations of the measurements for the num.-exp. comparison

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Fig. 14 shows the frequency behaviour of the SPL, predicted and measured, for the two selected locations. For both positions, the curves behaves similarly. For the position close to window #2 the comparison is very good for almost all the frequency range. Such result has not been reached for the position of window #1, in which a marked discrepancy is evident at high frequency. Both curves are able to identify the peak in correspondence of the BPF, and again for window #2 the prediction gives almost the same measured value.

Sound Pressure Level Numerical-Experimental ComparisonFilight Test Configuration 1 @15000 ft. - ECS OFF

10 100 1000 10000Frequency [Hz]

SPL

[dB

]

Exp - Win.#1 Exp. - Win.#2

Num. - Vol.#23 Num. - Vol.#9

Volume #9 compares with window #1 right (Yellow line)

Volume #23 compares with window #2 left (Green line)

10 dB

Fig. 14 – Numerical and experimental comparison of the internal SPL

As expected, the difference in terms of overall SPL is almost negligible, and it has computed equal to 1.8 dB. The in-flight measurements gave the opportunity to characterise the interior acoustic field of the P-180 aircraft,

VI. Conclusion The paper has summarised a long and dedicated effort produced by Piaggio Aeroindustries, with the

collaboration of University of Naples “Federico II”, devoted to the identification and characterisation of the interior noise level of the P-180 aircraft. Specific effort has been also addressed to the validation of a numerical code for the prediction of the interior noise of the fuselage. Highlights and limitations have been raised up, but many lessons have been learned, both from the experimental side and from the numerical point of view. The experimental measurements, both in ground and in flight, have allowed the identification of the noise sources, their relative importance and have indicated possible actions for the improvements of the cabin acoustic comfort. The numerical predictions, validated by the experimental comparison, have produced an high degree of confidence for the software, resulting in a tool for reliable calculations and verifying the validity of the input data. Others improvements may be indicated for the future developments of the program, as, for example, a better description of the acoustic load of the turbulent boundary layer or a more in-depth assessment of the partition transmission loss, but these eventual new features should maintain the friendliness and the efficiency of the code itself.

Acknowledgments The authors are indebted with the flight test department of Piaggio AeroIndustries for their professionalism and

technical support. This work has been partially supported by the Italian Ministry of Education, University and Research (MIUR).

References 1 Dandaroy I., Hartley D., Hund R., “Interior noise prediction of Hawker Horizon aircraft using statistical energy analysis”,

Noise-Con 2004, Baltimore, Md, July 12-14, 2004, pp. 333-346 2 Harvey Hubbard ed., Aeroacoustics of Flight Vehicles: Theory and Practice chapter 16, Interior Noise by J.S. Mixson and

J.F. Wilby, NASA TR 90-3052, August 1991 3 F. Marulo, P. Cascone, C. Ntounas, A. Sollo, A. Pezzolla “Un codice per il calcolo semplificato del rumore interno”, XV

Congresso Nazionale AIDAA, Torino, 15-19 Novembre 1999 (in Italian) 4 G. Russo, Interior Noise del P-180 Avanti, Master Thesis, April 2005 (in Italian) 5 ESDU, “Estimation of the Surface Pressure Fluctuations in the Turbulent Boundary Layer of a Flight Vehicle”, Item No.

75021, ESDU International plc, London, UK, November 1992

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