[American Institute of Aeronautics and Astronautics 11th AIAA/CEAS Aeroacoustics Conference -...

Vibration and Acoustic Correlation on Aircraft Fuselage Composite Stiffened Panels

Antonio Paonessa* and Massimiliano Di Giulio†

Alenia Aeronautica, Pomigliano d’Arco, Naples, Italy, 80038

Francesco Marulo‡ and Tiziano Polito§

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

The structural dynamics and the transmission loss behaviour of composite stiffened panels specifically designed for an aircraft fuselage structure application are an important issue with regard to the noise levels transmitted inside aircraft cabin. It is also becoming more significant since the increased use of composite materials for the next generation aircraft. The understanding of such acoustic behaviors, finalized to assess an optimized design process accounting for acoustic, structure and weight requirement at same time is the final scope of the current research work. The paper presents some of the basic results obtained during the development of a European funded research project: FACE (Friendly Aircraft Cabin Environment), which addressed, among the others, the vibroacoustic correlation for stiffened composite panels between numerical modeling and experimental measurements. Different panel configurations have been selected for studying the comparison between several parameters, essentially stringer shape, stringer pitch and skin panel lay-ups. Several numerical models have been prepared to study the different panels’ configurations, highlighting possible modeling indications with reference to the expected use of the model itself. In parallel, other techniques, like the Statistical Energy Analysis, have been used to extend the numerical simulation and prediction to frequency ranges where classical Finite Element Method is unlikely to be used. At those frequency ranges the structure modal damping measurements become critical too, but it is very important to set-up reliable structure acoustic models to develop design procedure. Therefore, a specific testing method and linked software tool, based on the Hilbert Transform approach has been developed to support the above needs. Additionally a numerical TL facility has been prepared to improve understanding of the correlation between vibro-acoustical measurements and numerical results. This model application, while validating, offers the possibility to minimize the laboratory measurements and helps to identify trends for TL behaviour of structural panel solutions.

Nomenclature TL = Transmission Loss FRF = Frequency Response Function NB = Narrow Band 1/3 OB = 1/3 Octave Band FEM = Finite Element Method SEA = Statistical Energy Analysis

* Manager, Acoustic & Environmental Control Dept., Viale dell’Aeronautica, ACS/CEAS Member † Engineer, Acoustic & Environmental Control Dept., Viale dell’Aeronautica ‡ Full Professor, Department of Aeronautical Engineering, Via Claudio 21, AIAA Senior Member § PhD Student, Department of Aeronautical Engineering, Via Claudio 21

American Institute of Aeronautics and Astronautics

1

11th AIAA/CEAS Aeroacoustics Conference (26th AIAA Aeroacoustics Conference)23 - 25 May 2005, Monterey, California

AIAA 2005-3035

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

I. Introduction HIS work has been supported by the European Commission in the FACE project (contract No. G4RD-CT2002-0076) conducted by 32 partners, all coordinated by Alenia Aeronautica. Major European Airframe industries,

Research Centers and some Universities active in the structure-acoustic design are involved in the project. Some small medium enterprises active in the acoustic measurement and treatment applied to the aircraft product are also involved. FACE project is a large Technology Platform started on 2002 and planned to have four year duration, working on aircraft cabin comfort subjects of turbofan powered aircraft including large aircraft, Regional jet and Business aircraft. Global cost of the Project is 35 Million of Euro 50% funded by the European Union.

T

A. FACE Project research approach The composite fuselage acoustic behaviour is one of the three main subject influencing the comfort treated,

while the others two are: air-quality and multimedia use on board. Further task is the development of a new Environmental Comfort Index (ECI) aimed to simulate the passenger comfort perception due to all the three above subject considered simultaneously1. The development of a passenger comfort perception model is also in progress linking the various system parameters (e.g. Noise levels, cabin temperature, pressure, etc.) with the subjective passenger perception, aimed to provide a project support tool providing estimation of complex design solutions efficiency against the comfort perception. Large aircraft facilities are properly designed and used to acquire data and subjective information to develop ECI by Building Research Establishment Ltd in England and by Fraunhofer-Institut für Bauphysik in Germany2,3.

The acoustic studies linked to the composite fuselage structure represent the larger part of the project including activities targeted to explore optimized passive and active noise treatments focalized to composite structure application and meeting as well as possible weight constraints properly state to reduce the impact on the fuselage double wall design. Finally test activities on two large aircraft fuselage full scale barrels are planned: one on a wide body, provided by an other European Consortium (TANGO) and another on a business aircraft, provided by, to verify the developed modeling capability to simulate a complete fuselage case.

B. Specific paper discussion This paper is discussing a small part of the above project, the one concerning the approach used to analyze the

fuselage structure acoustic behaviors and the exploration of the main design parameters affecting the acoustic performance. The aim of this work is to derive concept for defining guideline to design structure optimized to account also for acoustic requirements while all other structural and environmental requirements are considered together with the weight constraints4,5. Therefore a multidisciplinary approach was followed to treat the subject, linking contribution from structure, interiors, acoustic and system design experts to achieve the goal.

Other works are presented within this conference by other FACE partners discussing on large panels Transmission Loss measurements6 and Active noise trim-panel suspensions7, also these works have been done within FACE Project activities.

II. Test panel configurations A set of 10 flat panels configurations has been designed, manufactured and tested in order to evaluate acoustic

behaviour of laminated composite structures and the influence that parameters like mass, stiffness and damping have on it. To evaluate this, panels configurations have been designed to provide variations of a reference one, by changing parameters having effects on mass and stiffness variation. An aluminum panel has been manufactured too, in order to have a comparison with an equivalent metallic structure. All the panels have been designed to comply with the same design loads being representative of the specific fuselage section of a wide body aircraft structure. Table 1 summarizes the considered configurations, providing also some of the main parameters. In details:

Panel P2 represent the reference configuration, it is a flat stiffened panel where skin is a laminated carbon fiber in epoxy resin composite and stringers are made up of two components, a braided T-section and a prepreg bulb element stitched to stringer web. Figures 1 and 2 show panel and stringer section details.

Panels P1 and P1bis have the same skin as P2, but without stringers, to evaluate only skin behaviour. The two panels differ each other for an added metallic net in P1bis, in order to simulate the lightning protection which is normally applied on the aircraft skin.

Panels P3 and P4 have the same dimensions than P2, but skin lay-up is different, in order to verify the effect of lamination order.

Panel P5 has 2 more plies in skin lay-up. Here skin thickness has been increased.


2

In panel P6 stringer pitch has been changed, while in P7 and P8 have been changed stringer sections, in web thickness and section shape.

Finally, the effect of material has been evaluated with panel P9, where both skin and stringers are metallic. For a deep investigation about damping performance and its influence on acoustic behaviour, further tests have

been pursued on four panels, P2, P3, P6 and P9, which some viscoelastic material have been attached to.

Figure 1. Panels scheme

Table 1. Small panels description

PANEL DESCRIPTION SKIN PANEL LAY-UP

STRINGER PITCH (mm)

P1 Baseline Skin [(90/0/45/-45)(45/0/-45)]s

P1b As P1 but with a

shielding metallic net

[(90/0/45/-45)(45/0/-45)]s

P2 Reference

configuration with 8 stringers

[(90/0/45/-45)(45/0/-45)]s 145.3

P3 As P2 but with

different lay-up for the skin

[(-45/45/0/90)(-45/0/45)]s 145.3

P4 As P2 but with

different skin lay-up

[(90/0/45/-45)(90/0/90)]s 145.3

P5 As P2 but with

different lay-up for the skin

[(90/45/0/-45/0)(45/0/-45)]s 145.3

P6 As P2 but with

reduced stringer spacing

[(90/0/45/-45)(45/0/-45)]s 127

P7 As P2 but with stiffer stringer [(90/0/45/-45)(45/0/-45)]s 145.3

P8 As P2 but

with different stringer

[(90/0/45/-45)(45/0/-45)]s 145.3

P9 Aluminium equivalent

with 8 stringers 150

60.0

8.0

1.7

8.0

35.0

1.7

27.0

Figure 2. P2 stringer shape

52

145.

137

63

III. Experimental activities Three types of test have been applied to all of the configurations above: (a) modal analysis, (b) structural

damping and (c) acoustic transmission analysis. All these testing activities have been carried on at the Alenia Aeronautica Acoustic Lab. Two types of modal analysis have been performed, a first one in “free-free” boundary conditions, where panel has been hanged up to springs, as shown in Fig.4; a second one where panels have been installed on Small Alenia Acoustic facility (Fig.7 and Fig.9) in order to set-up the installation effects too. The damping testing has been carried out using the same test set-up used for the modal analysis but, in order to measure damping characteristics at high frequency, also two different test were done: one up to 2KHz, and an other up to 10KHz.

A. Modal Analysis A brief description of Test set up and instrumentations used for modal analysis are here given for the both test

types above. Instrumentation used were the same: the shaker used to excite the structure, a 64 channels Real Time Analyser (OROS) providing both NB and 1/3 OB analysis results (Fig.3), accelerometers of small size with measuring frequency range 2-10KHz.


3

A.1 “Free-Free” Modal Analysis Method/Procedure To measure modal shapes and characteristics of the panels the free field test configuration is used. This

configuration has been done by means of an hanging system, which have been hanged up the panel by means of two springs passing through two holes made on both corner of the panels. Springs for hanging up panels, see Fig.4, have provided 4 Hz cut-off frequency,.

Figure 3. OROS Real Time analyzer System: Front-end 64 acquisition channels & PC

Figure 4. Suspension system for free conditions

This suspension setup has been selected for avoiding any possible influence of the boundary conditions on the

measured modal parameters, thus allowing the best possible correlation with the numerical models. The frequency analysis range is 0-2 KHz for mode analysis and 0-10Kz for a Statistical base modal analysis. The vibration source has been provided by shaker (white noise 0-2 KHz) suspended with a tripod system as in Fig.5 and located on the panel at a position decided after a preliminary check on signals coherence for different shaker excitation places.

a)

b)

Figure 5. Setup for Free-Free modal analysis: hanged panels during test (a) - details of shaker excitation (b) One Hundred and Twelve measurement’s locations have been used for each panel, acquiring the corresponding

FRFs and identifying the mode shapes using a standard commercial code. The results of the first three measured mode-shapes are shown in Fig.6.

35.5 42.2 76.6 Figure 6. First mode shapes for panel P4 in the free boundary conditions. The value indicates the resonance frequency [Hz]


4

A2 “Installed” Modal Analysis Method/Procedure This type of modal analysis has been performed only for 2 panels (P2, P4 see Table 1) in order to verify the

constraints effect on modal behaviour. The installation of panels has been done by considering the actual constraints on a real fuselage panel, in particular:

The A/C panel-frame attachment is simulated by 2 iron L section frames fixed on the chamber window sides, and connected to the panel by means of edge bolts on both sides.

Panel stringer-frame connection is simulated by holes and a stringer clip connection between stringer web and window iron frames. The panel is fixed at the test windows, on its longest side (Fig.7).

Lateral sides are clamped as before stated, while the upper/lower sides of the panel are not blocked and shielded by the test windows frame, such simply supported contact areas have been treated with damping materials in order to limit the edge vibration reflection and to simulate as better as possible the infinite panel condition.

Details of panel installation - a

Details of Panel installation - b

Stringer clip installation

Figure 7. Set up for “installed” modal analysis Eighty measurement’s locations have been used for each panel, acquiring the corresponding FRFs and

identifying the mode shapes using a standard commercial code. The results of the first three measured mode-shapes are shown in Fig.8.

306.6 330.0 343.1 Figure 8. First mode shapes for panel P4 in the “installed” boundary conditions. The value indicates the resonance frequency [Hz]

The acoustic chamber will be kept open during the modal analysis while it will be hermetically closed for the TL

tests. A description is given in Fig.9. Frequency analysis range has been: 0-2000 Hz. Vibration source has been provided by shaker (white noise in the ranges 0-2 KHz or 0-10KHz) hanged up as in Fig.9 and positioned to excite the central bay of the panel (between two stringers) but laterally respect to the bay centre. This position of the shaker has been decided after a preliminary check on signals coherence for different shaker excitation places. Again the measurements locations have been 112 over the entire panel.


5

Figure 9. Small Acoustic Chamber set-up: speaker horn and shaker installation details.

B. Damping Analysis Damping values of all panels were measured using the modal analysis FRF and the half power method for mode

damping extraction up to 2KHz or up to the maximum mode extracted. Then a statistical base method was properly developed to analyze the FRF measured up to 10KHz using special low weight accelerometers and properly developed software implementing an analysis procedure based on the Hilbert transform approach. By merging the results of the two methods, the damping factor was achieved for the entire frequency range of interest.

C. Acoustic Analysis With the aim to compare the acoustic performance due to the acoustic energy radiation coming from the panels it

was decided to use the small acoustic facility of the Alenia acoustic laboratory. This facility is not a standard TL facility, but it was used because of the low cost and because it has not time limited access as the large and standard facilities. The decision was also motivated by the scope of the test, focalized to compare different configuration and to assess differences linked to the configuration difference and not targeted to have correct evaluation of the TL value. Nevertheless, a complex procedure was also set-up to analyze the data in order to have a TL estimation. The following activity done in standard facility on the same structures have later confirmed that the TL evaluation here made is also sufficiently accurate for the scope of the work and therefore here shown too.

Acoustic analysis has been performed in the test chamber shown in Fig.9, made up of a treated acoustic chamber as a Receiving Room, with a side that could be fully open to attach testing panels. In place of using a reverberant Transmitting Room, acoustic source has been simulated by means of four speakers driving an acoustic wave by an horn, which is providing a flat wave excitation over more then 90% of the panel surface for frequencies over 400 Hz. Signal used for all tests was a white noise and the level the maximum one avoiding distortions (~120 dB, between 400-10000 Hz). Microphones measurements have been performed by using a movable antenna within the chamber (Fig.10).

Figure 10. TL Facility and Typical noise source spectrum measured


6

In particular, 5 microphones have been installed on this antenna and 1 microphone has been mounted outside (within the horn), close to the panel, in order to monitor the source level. Each panel configuration has been tested by considering four different positions of the antenna in order to provide a noise map of 20 measurements points inside the chamber. The panel attachment to the window frame is particularly accurate to simulate the panel attachment to the aircraft frame as well as possible. C1. TL Analysis Method/Procedure

Transmission Loss tests have been executed by considering 50-10000 Hz frequency analysis range. The Alenia acoustic facility used isn’t a real TL facility.

Then a specific analysis procedure was set-up in order to assure a correct comparison between measurements made on two different panels, while the absolute values of TL were only preliminary estimated by a special data analysis procedure not having the scope of defining a correct TL determination, even if late comparison with correct measurements done in a standard TL facilities at NLR and KTH facilities have shown that such evaluations give a sufficient correct estimation of the panel TL.

The connection between panels and the acoustic chamber has been chosen to correctly simulate the typical constraints configuration on fuselage. Therefore, the installation of the panel on the chamber is the same arranged for the “installed “ modal analysis but for the TL analysis, the panel has been blocked between chamber window frame and, externally, by speaker horn frame.

The measurement method is based on the Insertion Loss approach, in fact in principle it is comparing the Noise Levels within the chamber measured on two comparable set-up. This means that because the comparison between the two inner chamber noise field measurements provide to the insertion loss effects of the changed panels under measure, these value could be used to assess the due parameter effects. The difference between the two measurements is exactly the difference between the two unknown TL. All the eventual doubt on measurements due to the set-up can be suppressed assuring the complete control of the testing panels installation and the complete repeatability of the excitation loads.

Given the incompliance of the ALENIA acoustic facility with the standard set-ups for TL measurement, the following method has been used to define a preliminary estimation of the small panels TL by starting from the SPL measured within the anechoic chamber. The measured SPLs into the acoustic room, due to a simple and known aluminum septum, are averaged as well as the similar measure made with a compared panels of any structure but same dimension and installation. Then introducing the well known TL value of the aluminum septum it could be possible to estimate the TL of any panel under measurement. Furthermore, in order to correct for effects due to small instability of the noise loads during the test, a correction is also considered computing the difference between the external noise levels (excitation M6 SPLs) and adding the correction to the measurement of the compared test cases.

D. Test Results All panels have been tested according to the above procedure and within the relevant test facility, hereafter some

of the main are shown and discussed. Modal analysis results will be discussed later, in comparison with results achieved by numerical simulations. The

Transmission Loss data are hereafter discussed. The structural parameter analysis results are shown in Fig.11.a and 11.b, where the main parameters: Lay-up, stringer inertia, stringer pitch and added metallic net are given. All are compared to the reference configuration(panel P2).

10

15

20

25

30

35

40

45

100 1000 10000

1/3 O.B. frequency [Hz]

Tran

smis

sion

Los

s [d

B]

P2

P3

P4

P5

10

15

20

25

30

35

40

45

100 1000 100001/3 O.B. frequency [Hz]

Tran

smis

sion

Los

s [d

B]

P2 P7

P2:

P7:

skin lay up effect

stringer inertia effect

10

15

20

25

30

35

40

45

100 1000 10000


Tran

smis

sion

Los

s [d

B]

P2

P3

P4

P5

10

15

20

25

30

35

40

45

100 1000 100001/3 O.B. frequency [Hz]

Tran

smis

sion

Los

s [d

B]

P2 P7

P2:

P7:

10

15

20

25

30

35

40

45

100 1000 100001/3 O.B. frequency [Hz]

Tran

smis

sion

Los

s [d

B]

P2 P7

P2:

P7:

P2:

P7:

skin lay up effect

stringer inertia effect

Figure 11.a Structural parameters effects: skin lay up and stringer inertia


7

5

10

15

20

25

30

35

40

45

100 1000 100001/3 O.B. frequency [Hz]

Tran

smis

sio

n Lo

ss [

dB]

P2

P6

stringer pitch effect

10

15

20

25

30

35

40

45

100 1000 10000


Tran

smis

sion

Los

s [d

B]

P1

P1bis

P2

Metallic net effect

5

10

15

20

25

30

35

40

45

100 1000 100001/3 O.B. frequency [Hz]

Tran

smis

sio

n Lo

ss [

dB]

P2

P6

5

10

15

20

25

30

35

40

45

100 1000 100001/3 O.B. frequency [Hz]

Tran

smis

sio

n Lo

ss [

dB]

P2

P6

stringer pitch effect

10

15

20

25

30

35

40

45

100 1000 10000


Tran

smis

sion

Los

s [d

B]

P1

P1bis

P2

Metallic net effect

Figure 11.b Structural parameters effects: stringer pitch and metallic net

The comparison of metallic solution (P9) versus composite panels (P2) designed to comply with same fuselage

loads are given on Fig.12, where both bare structure and panels with damping skin treatments are compared. It is outlined the lower composite structure performance mainly in the medium-high frequency range. Furthermore the damping treatment effects is more efficient on metallic panels.

Figure 12. Aluminum versus Composite panel TL comparison

D1. Panel Damping Characteristics Measurements

One of the key point in the measurements done was the measure of the damping characteristics of those panels8. The measurement at low frequency, where the modal density is low and several mode shape can be isolate is relatively simple using standard half power approach by modal response. When the modal density is higher and it isn’t possible to isolate single modes, this technique cannot be applied. Because of this, a statistical based technique has been developed to extract the damping value relevant for the structure under measure. In Fig.11 the results of the measurement done on the P2 panels installed is shown. It is noted that the final estimation is done by mixing the two methods each one is more reliable within the proper frequency range, while at medium frequency it is noted the two methods gives similar values.

Figure 13. Damping factor measurements made by modal and statistical methods


8

IV. Modeling Many different numerical models have been prepared and analysed for understanding the effects of the various

geometrical, structural and material parameters9. In the framework of the FACE project this numerical activity was required for the modelling of large curved fuselage panels. In general a good practice for numerical modelling was considered an important tool for understanding the convenience and the effectiveness of a detailed approach compared with a parameter averaged approach. The MSC/Nastran has been used throughout the calculations of this article.

The basic approach has always been a sort of increasing complexity models, starting from structural models which have an exact theoretical solution (i.e. simply supported flat metallic rectangular panel) and moving to composite laminates, more complex boundary conditions (including elastic suspension) and modelling the stringers considering their geometrical layout.

Additionally other simple checks, as for example the total weight of the panel, or the thickness distribution, or some static displacement measurement, have been considered, specially when correlating the numerical and the experimental evidences. These simple checks may eventually result quite different from those expected, especially for composite panels when produced only for research purposes and not coming from a standard production process with all the quality indicators correctly monitored.

As examples of the modelling practice explored during this research program, the Fig.14 shows possible modelling of stiffeners, having detailed the geometry of the stiffener and analysing the effect of the offset. This analysis is relative to an aluminum alloy panel.

a)

b)

Figure 14. Stiffener modeling without offset (left) and including the offset (right); aluminum panel Similar analysis has been performed for the composite panel configuration presenting a different stiffener which

has the stiffening curvature substituted by a squared lobe and different junction with the panel, Fig.15. Both models have several thousands of degrees of freedom (from about 8000 to 10000), which appears to be not practical when dealing with a more complex structure. The results of such modeling show, in both cases, that considering the presence of the offset the natural frequencies increase from 5 to 10 percent for the configuration of Fig.14 and from 10 to 15 percent for the configuration of Fig.15.

a)

b)

Figure 15 Stiffener modeling without offset (a) and including the offset (b) for the composite panel configuration.


9

Similar results have been also obtained with a finite element model of the stiffeners. These models boost up the

number of dof’s without improving the results, but adding, on the other hand, further possible modeling mistakes and uncertainties.

The numerical modeling activity has been also addressed to the simulation of a typical TL test10. Fig.16 shows the model configuration of this virtual facility. The modeling of this system presents several interesting problems dealing with the very high number of dof’s11, with the double interaction between structure and fluid, with the correct simulation of the boundary conditions, and so on.

a)

b)

c) Anechoic Room Structure Reverberant Room

Figure 16 Model of the numerical TL facility In order to simulate acoustic characteristics of panels, a simplified numerical FEM model has been taken into

account, to reproduce Alenia acoustic analysis tests. At this purpose, only Anechoic Receiving Room has been modeled (Fig.16.a), where a fluid volume having the same dimensions and features of the one used for experimental activities (Fig.9) has been coupled to structural model of panel. A dynamic pressure load has been imposed to panel side opposite to fluid, that simulates a plane wave impinging the panel. This FEM model has a lower number of dof’s and gives a good result in 0-2000 Hz frequency range.

A Statistical Energy Analysis (SEA)12 has been performed to simulate acoustic behavior of panels in 1.5-10 KHz frequency range, because of very high modal density levels that make, within this range, only a statistical approach effective.

W12 W23

SourceCavity

Sub-system 1

ReceivingCavity

Sub-system 3

Dissipation

Panel

Sub-system 2

Dissipation

Loud speakerInput power

DissipationDissipation

Figure 17 SEA Configuration facility Figure 18 SEA model of the TL facility A SEA model has been built, using a specific SEA software, as shown in figures 17 and 18, having a

Transmitting reverberant room, the testing panel and a Receiving anechoic room. As a result, panel TL has been required.

V. Numerical and Experimental Correlation The numerical and experimental correlation has been performed using well-known procedures, as the MAC

(Modal Assurance Criterion), which allow an easier dissemination of the results. A typical problem which arises


10

when comparing numerical and experimental data is the consistent difference of values to deal with. Generally the number of dof’s of the numerical models is of order of magnitude greater than the experimental dof’s. With the help of dedicated programs this problem can be quite easily solved, finding the best combination of both results. In the case of flat panels correlation, Fig.19 shows the superposition of both experimental and numerical mesh and some examples of mode-shapes correlation.

Figure 19. Numerical and experimental dof’s superposition (left) and mode-shape correlation. The global behaviour of the results of the correlation are synthetically shown in the diagrams of Fig.20, where

the numerical values of the natural frequencies are plotted versus the experimental ones. Ideal correlation should have such values along a line with angular coefficient equal to unity. As shown in Fig.20, the quality of correlation, for several flat panels, is very good.

P5

R2 = 0.9932

0

50

100

150

200

250

300

350

400

450

500

0 100 200 300 400 500

FEA

EMA

P7R2 = 0.989

0

50

100

150

200

250

300

350

400

450

0 50 100 150 200 250 300 350 400 450

FEA

EMA

P8

R2 = 0.9593

0

100

200

300

400

500

600

0 100 200 300 400 500 600

FEA

EM

A

Figure 20 Numerical and experimental natural frequencies correlation for various panels

A correlation between experimental and numerical results has been made about acoustic analyses too. In this case, FEM, SEA and experimental TL results have been compared. Fig.21 shows an example of correlation for


11

panels P2 and P9 (see table 1 for reference). As expected, a quite good correlation is obtained with FEM simulation below 1500 Hz frequencies, while SEA simulation gives good results above 1500 Hz.

Figure 21 Experimental, FEM, SEA TL correlation

Conclusion The FACE project funded by the European Commission, has explored, among other aircraft cabin comfort

issues, the acoustic behaviors of a composite CFRP structure used to design the fuselage structure. The comparison with an equivalent metallic structure has been done showing the lower acoustic performance and worst efficiency at standard damping added treatments. These results leads to conclude that the composite structure applied to a fuselage design will need of a larger add-on noise treatment to assure similar noise levels in the cabin. The consequent weight increase could destroyed the gain achieved by the composite fuselage lower structure weight even if not larger weight could be necessary. Similar consideration could be done about cost involved.

The paper provides a discussion on some of the preliminary results achieved during the development of the FACE research work, from which the following detailed conclusion are provided:

The achieved results have showed that it is possible to obtain a very good correlation degree when the experimental results are available

Panel structural parameters like skin lay-up, stringer shape and inertia, stringer spacing could play a significant role to design/optimize the Transmission Loss of the panels, but typically the best one imply higher weight.

Panels TL is normally lower then metallic solution designed for equivalent load, it is important to see that mass low region for composite shows also larger variations probably due to the lay-up design and basic material properties.

Damping characteristic of composite are typically higher then aluminum if sheet are compared, different is the conclusion if a complex structure is compared (stiffened panels). This could be explained by the fact that damping due to the riveted structure is not present. Therefore the composite structure will transmit better the acoustic energy that impinges on the fuselage.

A virtual TL facility may be used for trade-off studies, with the aim to select specific structural solutions and to assess the sensitivity of the radiated sound field to structural changes, application of damping treatments or acoustic insulating materials

Acknowledgments The work discussed in the present paper has been developed in the framework of the EU-FP5 project FACE

(contract No. G4RD-CT2002-0076). The authors thank the European Commission and all FACE Consortium to have accepted the publication of this paper and relevant results. Special acknowledge is going to the Alenia and University of Naples technicians having provided the test activities here presented.


12

References 1Paonessa A., “FACE Friendly Aircraft Cabin Environment”, Proceedings. of the Euronoise Conference, Naples, Italy, 2001 2Ross D., Hadi M., Palmer J., Perera E., Plathner P., Raw G., Upton S., “Cabin Air Quality Monitoring in Boeing 777s”, BRE

Client Report No.201543, Watford, UK, 2000 3Mayer E., “Objective Criteria for thermal comfort”, Building and Environment, Vol. 28, No. 4, pp. 399-403, 1993 4Paonessa A., Lecce L., Marulo F., “Vibrational and Acoustical Behaviour of Complex Structural Configurations using

standard Finite Element Program”, Proceedings. of the 6th IMAC Conference, Orlando, FL, pp. 827-833 5De Rosa S., Sollo A., Franco F., Cunefare K., “Structural-Acoustic Optimization of a Partial Fuselage with a Standard Finite

Element Code”, Proceedings. of the AIAA Aeroacoustic Conference, Maastricht, The Netherlands, 2001. 6Van del Wal H.M.M., Nilsson A. C., “Sound transmission measurements on composite and metallic fuselage panels for

different boundary conditions”, Proceedings. of the 11th AIAA/CEAS Aeroacoustics Conference, AIAA Paper 2005-3033, Monterey, CA, 2005

7Maier R., “Active Trim-Panel Suspension for Interior Noise Control”, Proceedings. of the 11th AIAA/CEAS Aeroacoustics Conference, AIAA Paper 2005-3036, Monterey, CA, 2005

8Ricci F., Monaco E., Polito T., Marulo F., “Influence of experimental testing set-up and geometric parameters on damping measurements”, Proceedings of SPIE 2004 Conference, Long Beach, CA, 2004

9Melluso D., Napolitano L., Marulo F., Monaco E., “Acoustic Testing and FEM modeling of carbon fiber flat stiffened Panels”, Proceedings of the EURONOISE Conference, Naples, Italy, 2001

10Marulo F., Polito T., “Comparison of panel solutions for noise and vibration reduction”, Proceedings of ISMA 2004 Conference, Leuven, Belgium, 2004, pp. 3653-3660

11Marulo F., Martini L., Perna F., “Application of an automated multi level substructuring method for the eigenvalues problem of a large degrees of freedom F.E. model”, Proceedings of ISMA 2004 Conference, Leuven, Belgium, 2004, pp. 2759-2770

12De Rosa S., Franco F., Ricci F., “Introduzione alla Tecnica Statistico-Energetica (S.E.A.)”, Liguori Ed., Napoli, Italy, 1999.


13

[American Institute of Aeronautics and Astronautics 11th AIAA/CEAS Aeroacoustics Conference -...

Documents

Transcript of [American Institute of Aeronautics and Astronautics 11th AIAA/CEAS Aeroacoustics Conference -...