Interior noise estimation of new aircraft concepts in the early design phase

Lionel ZOGHAIB1; Julien COUVRAT2; Alexander PEIFFER3, 1 Airbus Group Innovations, France

2 Assystem, France 3 Airbus Group Innovations, Germany

ABSTRACT The development of a new aircraft does sometimes require an overall estimation of the expected interior noise level at a very early stage of the design. It is particularly true for future aircraft platforms or novel concepts, for which severe loudness issues may ultimately hinder the project viability. The question of achieving approximate but reliable estimations despite the lack of information arises and may puzzle both the noise and vibration harshness team and the project manager. Airbus Group Innovations reports in this paper how these technical and organizational issues have been handled in two different projects. The first one concerns the promising “carry-through wing box” and “blended wing body” aircraft designs investigated in the ACFA European initiative (Active Control for Flexible 2020 Aircraft). The second one focuses on the breakthrough plane concept designed in the Zero Emission Hyper Sonic Transportation (ZEHST) project. After a discussion regarding the choice of a pertinent methodology and software strategy, intermediate noise estimations are presented, leading to general design recommendations to improve the acoustic performance. Keywords: Carry-through wing box, Blended wing body, Hypersonic I-INCE Classification of Subjects Number(s): 13, 76

1. INTRODUCTION Aircraft design may significantly change in the long run as the need for a drastic reduction of

fossil energies consumption becomes more and more crucial. Apart from the integration of new propulsion systems –distributed over the fuselage for instance–, the overall design may have to be reconsidered to improve the planes aerodynamic performances substantially. Blended Wing Body (BWB) and Carry-through Wing Box (CWB) are promising candidates in this respect, and have been analyzed in detail in the ACFA European initiative (Active Control for Flexible 2020 Aircraft).

Another need may also transform the future of aircraft: the need to transport people more quickly than ever before. The question of developing hypersonic planes has often been raised in the past years without any clear answer, the Concorde and its unsuccessful business model remaining in all spirits. Recently, however, the ZEHST project (Zero Emission Hypersonic Transportation) has attempted stepping forward a little bit by analyzing the technological feasibility of a carbon-free ultra fast aircraft. The ZEHST project predesign aircraft relies on a massive use of hydrogen and results in a design style as unusual as BWB or CWB platform aircraft.

Numerous issues are addressed when predesigning such innovative planes: propulsion, energy storage or aerodynamics among others. But the noise issue also has to be considered early enough to be able to anticipate problems, to propose design alternatives and by doing so, to secure the project. In this context, predicting interior noise levels is a real challenge, though: robust noise predictions normally require a good knowledge of the aircraft design from the engine or the wing profile until the cabin acoustic trim material; during the predesign phase, however, most of the information is 1 Lionel.zoghaib@airbus.com 3 Alexander.peiffer@airbus.com

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lacking, and even worse, is frequently changing. Airbus Group Innovations has been involved in the future aircraft projects mentioned above. The

acoustics team, in particular, has been in charge to set up a pertinent strategy for acoustic simulation considering this peculiar context. The strategy has mainly rested upon:

• The use of standard acoustic trim design rules • The use of predictive methods which can tolerate a large level of uncertainty, such as the

Statistical Energy Analysis • The use of preprocessing tools well suited to frequent design changes

The choice of using standard acoustic trim design rules has been made for two reasons: first, because the materials data collection has been validated throughout the years and reduces the overall level of uncertainty; second, because the comparison with conventional aircraft is made easier, since there is no bias regarding this specific point. More details about the chosen strategy and associated modeling options are presented throughout the paper, structured as following: the BWB and CWB platform aircraft are considered in a first part; the ZEHST hypersonic plane investigation is then depicted in a second one.

2. BWB and CWB aircraft

2.1 Introduction The high-lift Blended Wing Body (BWB) aircraft wings are smoothly blended into the body,

giving the aircraft an excellent lift-to-drag ratio. Carry-through Wing Box (CWB) aircraft have an ultra wide body fuselage and may also offer better aerodynamic properties than conventional aircraft. Both platforms have thus been investigated in the European ACFA 2020 project as potential solutions for the future of aeronautics (Active Control for Flexible 2020 Aircraft [1]).

One of the investigation topics concerned the impact of these new designs on the acoustic performances in typical cruise conditions. The focus has been put on the interior noise predictions due to the turbulent boundary layer. Other sources of interior noise have been neglected due to the early stage of this first prediction; jet noise, especially, has not been integrated to the calculation because of the lack of relevant information. Transmission loss calculations have also been performed on the most sensitive parts –and on some specific sidewalls in particular–.

2.2 CWB and BWB model overview The numerical models have been created based upon the description given in the ACFA 2020

document specifying the design standards and requirements for CWB and BWB platforms. The models main characteristics are reported in Table 1.

Table 1 – Aircraft characteristics and cruise conditions

Aircraft platform type BWB CWB

Overall length [m] 43 63

Total height [m] 18 15

Wing span [m] 80 80

Wing area [m2] 1323 599

Max. Take off Weight [T] 373 377

Max. Landing Weight [T] 310 315

Operative Empty Weight [T] 207 212

Maximum Fuel [T] 117 117

Ground speed [Mach] 0.85 0.85

Flight level [feet] 33000 33000

Internal pressure [feet] 8000 8000

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The statistical Energy Analysis has been found particularly adapted to this predesign phase

project for three reasons: • New models can be built up with a few preprocessing steps only (compared to standard

deterministic methods such as the finite element method) • The method can cope with a high level of uncertainty, whichever its source (dimensions,

materials, excitation) • The team has a long experience with aircraft SEA modeling and has access to an extended

materials database, which can directly be exploited within the software VA One [2] It is worth pointing out that the SEA acoustic transmission model takes the flight and cabin

pressurization conditions into account.

2.3 Load case There are two relevant sources of excitation here: the turbulent boundary layer (TBL), which acts

directly on the fuselage skin panels, and the jet noise, which is neglected because of the lack of data regarding the engines.

Modeling the TBL excitation is still subject to an active ongoing research activity. Apart from purely numerical models, which are both computer-intensive and not entirely reliable, semi-empirical models are well accepted today. They are both straightforward to implement and yield results in a reduced amount of time. The drawback is their predictability level, which may be quite questionable, especially when flight conditions are changing. The identification of their parameters is thus made after a careful post-processing of the most relevant data collected during a wind channel test campaign.

The TBL loading semi-empirical models are described by both the autopower spectrum and the spatial correlation of the pressure fluctuations. In the software VA One, the autopower spectrum is based on Cockburn/Robertson model [3]. It can have two forms, depending on whether the boundary layer is attached or not to the structure, and varies with respect to Strouhal number and the boundary layer thickness (deduced from the ground speed). Corcos spatial correlation model completes the TBL field description with parameters controlling the magnitude decrease in the longitudinal and circumferential directions. This TBL loading model tends to be slightly over predictive and gives a conservative assessment of the sound level. The main issue consists in determining the three parameters of the semi-empirical model, so that they represent the flight conditions well enough. An overall discussion regarding their determination can be found in the paper by Bhat [4].

2.4 Geometry 2.4.1 BWB model

The BWB SEA model has been built from a finite element shell model displayed in Figure 1. The sequence of SEA subsystems in the vertical direction is also represented.

Figure 1 – Blended Wing Body half model (left) and investigated SEA model (right)

PAX cavity

Cargo cavity

PAX floor

Cargo floor

Double wall cavity

Double wall cavity

PAX ceiling

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VA One ribbed-plate model is exploited to model the fuselage; it combines flat skin panels with

stringers and frame directly. The stringers are located every 20 cm just as in an A320 aircraft. A flexural damping of 2% is assumed throughout the fuselage. A double wall construction separates the passenger and cargo cavities from the exterior. A double wall construction also pulls the cargo and passenger cavities apart. This type of construction ensures very good isolation properties but requires a specific SEA formulation to model the subsystems indirect coupling correctly.

Let us give a brief overview of the acoustic trim materials now. Cargo and passengers floor panels are modeled via a generic laminate material, with a honeycomb core. The cabin ceiling, for instance, contains two thin layers of fiberglass separated by a thick honeycomb layer, while the cargo ceiling combines aluminum and honeycomb layers. All interior panels for the cargo and passenger area have a specific absorption due to an acoustic treatment lay-up.

2.4.2 CWB model The CWB half section model is shown in Figure 2. This model includes all interior panels

surrounding the passenger and cargo cavities, hat racks, double walls and the cavities them-selves. Just as in the BWB model, a specific ribbed-plate model is used to model the fuselage; also, the main panels have been created using a generic laminate model. An acoustic treatment lay-up is applied to all inner panels. Most panel properties are identical to the BWB model. The passenger floor is modeled in the same way but also contains aluminum beams to increase its bending stiffness. The hat racks are made of flat plate subsystems with stiffer properties than trim panels for crash reasons.

Figure 2 – SEA model of the half section of the CWB

2.5 Lining and absorption A very short list of certified materials is available because of severe smoke and fire resistance

requirements. The standard glass fiber material belongs to this list. A variety of density and flow resistivity properties are available, however, as well as a number of possible arrangements; the typical acoustic blanket, for instance, is made of a light density glass wool covered by a thin septum cover. The blankets are fixed to the inner fuselage and to the interior trim (lower trim, trim, hat racks trim and ceiling, see Figure 2). The thickness of the blanket varies between 1 and 2 inches, depending on the area of application. Both BWB and CWB fuselage, for instance, receive a 2-inch thick treatment (~5 cm). No treatment is applied on the cargo fuselage because of humidity. Apart from specific restrictions like this one, the thickness is adjusted depending on the available space between the fuselage and the interior panels, the expected acoustic performances and the added-mass penalty.

Inside the cabin, the absorption is mainly driven by: • The seats • All other types of porous materials such as carpets, considered as a whole

Hat rack

Trim

Lower trim PAX floor

Cargo trim

Cargo floor

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The seats absorption is determined via a Kundt’s tube measurement and weighted by the effective seat area –taking into account the fact that the seats are not entirely covered by porous materials and that most passengers may also be sitting on them–. The seats absorption spectrum is completed by an overall absorption spectrum obtained by measurement in a standard single aisle aircraft. The spectrum is applied as a single, frequency-dependent coefficient to all cabin surfaces, assuming that the absorption is homogeneous throughout the cabin.

Finally, previous investigations conducted by Peiffer [5] have recommended inputting a damping loss factor deduced from an equivalent fluid model in the SEA model if the volume is filled with glass wool. It is the case of the thin cavity located between the CWB fuselage and the trim panels, and its damping loss factor has thus been altered accordingly.

2.6 Simulations results The results consist of third-octave spectra representing the pressure level in the cabin. Various

design variables have been tested to assess the noise sensitivity to the acoustic trim: treatments and skin thickness, blanket location, among others. We propose here to focus more specifically on the influence of the blanket introduction on the cabin noise. Three configurations of treatment highlight the general acoustic trend very well when adding more glass wool to the aircraft:

• Without any blanket • With blankets inside the thin side wall cavity, between the fuselage and the trim panel • With a full treatment (ceiling blanket and at a few other locations in addition to those

listed previously)

A summary of the results is displayed in Table 2. The application of glass wool clearly improves the insulation by decreasing the sound pressure level, especially in areas close to the fuselage outer skin (cabin side trim and ceiling). This synthetic table does not reveal the strong frequency-dependent influence of the treatment, which becomes more and more efficient as the frequency increases. But it clearly shows that the full NVH package can reduce the overall sound pressure level of the BWB and the CWB aircraft by 8 and 11 dBA, respectively. These figures have to be compared to the overall sound pressure level of a Generic Single Aisle aircraft (GSA), the value of which lies around 79 dB (A) –for a Mach number of 0.78–, that is to say 5 dBA above a BWB cabin. Slightly noisier estimations would be expected if the GSA noise computations had been performed at the same Mach number as the BWB and CWB aircraft. Both aircraft thus yield better acoustic performances than the conventional aircraft.

Table 2 – Overall sound pressure level in the passenger cavity

Treatment type BWB CWB

None [dBA] 82 88

Fuselage / inner trim panel [dBA] 78 78

Full treatment [dBA] 74 77

Complementary information can be found in Figure 3, which displays the cabin mean pressure

level spectrum for these three categories of aircraft –in case of a full NVH treatment–. The curves confirm the very good acoustic performance of the BWB structure on the whole frequency range. By comparison with the CWB, an improvement of 15 dB is obtained from 600 Hz and more than 20 dB after 2000 Hz. The large double-wall ceiling may explain the better acoustic performance of the Blended Wing Body platform.

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Figure 3 – Comparison of the overall sound pressure level in three types of aircraft

3. ZEHST HYPERSONIC AIRCRAFT

3.1 Introduction The Zero Emission Hypersonic Transport (ZEHST) project has started in 2010 and aimed at

designing a hypersonic civil aircraft that would respect the most stringent environmental requirements, in particular by emitting very low levels of carbon dioxide. Another fundamental idea of the project was to exploit “on shelf” technologies in order to target a certification by 2025. The experience of the Concorde aircraft made the need of considering noise issues –both internal and external noise– from the very beginning of the project in order to integrate these issues to the risk management and to develop solutions early enough to circumvent any problem efficiently.

The vibroacoustic analysis completed in the ZEHST project was comparable to the analysis

performed on the BWB and CWB aircraft in many regards. The strategy outlined in the introduction of this paper was identically applied to both projects. There were still of few differences that are worth being pointed out. First, the investigated aircraft platforms are significantly different. Second, the acoustic activities have been conducted in parallel to other design activities in ZEHST. This second point had a major consequence on the modeling task since fundamental aspects of the aircraft did change quite frequently. This peculiarity forced the use of parametric pre-processing design tools to improve the work adaptability to the design variability. The results reported in this paper also correspond to a “frozen” configuration, fairly close to the ultimate version of the pre-design, completed by 2013.

Before delving any further into the description of the vibroacoustic investigation, it is worth

outlining some important characteristics of the aircraft design in the investigated configuration: • It is propelled by two conventional turbofan engine during takeoff (until Mach 0.6) • Vulcain engines are then started up for about 40 seconds to reach Mach 1.5 • A ramjet engine finally takes over the propelling and the speed of Mach 4 is ultimately

reached, at a flight level of 25 km to limit the drag A geometrical representation of the aircraft is given in Figure 4, where the so-called passenger

pods are visible. These pods, located on each wing, contain both crew members and passengers, since

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the whole central area is occupied by voluminous structural parts such as the oxygen and hydrogen tanks, as well as the ramjet and Vulcain engines.

Figure 4 – Geometrical model of the ZEHST A/C

3.2 Scope The overall context of the project has impacted the simulation scope, just as in the BWB/CWB

project depicted in the previous paragraph. Given the high level of uncertainty and the frequent design changes, it has been proposed to think of a flexible simulation strategy, which would help

• Integrating new elements of information over time • Carrying out frequent updates when data had changed

Apart from the changing design, the variable scope is also due to a series of undetermined sources of excitations. The indetermination stems from the difficult –if not impossible– access to the relevant data. Table 3 gives a summary of these sources, with the expected level of generated noise.

Table 3 – Analysis of the different sources of noise

Noise source Numerical assessment Expected level

Aeroacoustic Yes Variable

Turbofan airborne noise No Average

Ramjet airborne noise No Average

Vulcain airborne noise No Average

Turbofan structure-borne noise No Strong

Ramjet structure-borne noise No Strong

Vulcain structure-borne noise Yes Strong The noise prediction work has relied quite pragmatically on the sources of noise with any

available data to characterize them. The focus has finally been put on: • The Turbulent Boundary Layer (TBL) excitation • The structure-borne noise due to the Vulcain engine

The TBL loading is assessed using the same SEA approach as in the first part of this paper, with

the help of the VA One software. The main drawback is that the TBL model rests upon standard flight conditions. Cockburn/Robertson model should be updated to take new flight conditions into

Passengers pod

Turbo

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account (Mach 4, 82000 feet). The wind channel acquisition of such an unusual data fell however beyond the scope of this pre-design project and was thus not undertaken. The estimation of the structure-borne noise due to the Vulcain engine was made possible because of available acceleration measurement on the engine tailpipe.

3.3 Preprocessing The frequent design changes made the development of a parametric preprocessing tool necessary.

This preprocessing tool gathers geometrical information and is then able to create both pods and wings automatically, in the form of a shell finite element mesh, as shown in Figure 5. The created mesh had however to be completed by a manual pre-processing to create the central part of the aircraft.

Figure 5 – Aircraft shell mesh model The created mesh can be exploited to run both finite element and statistical energy analyses. The

SEA model, however, requires a few additional preprocessing steps to coarsen the mesh, in order to obtain subsystems of acceptable size –i.e. with a sufficient number of modes in the considered frequency band–. The work is made easier by focusing on the pod part only; it is assumed, indeed, that the TBL excitation of the rest of the structure has a limited impact on the pod interior noise only. An illustration of the final SEA model used is displayed in Figure 6. It can be seen that this model contains a double wall construction over its entire contour, just as in a standard aircraft, in order to reduce the TBL acoustic transmission.

Figure 6 – SEA pod model (hat rack, main cavity, and double wall cavity)

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3.4 Simulation results The results combine:

• SEA simulation results of the cabin noise due to the TBL excitation • FEM simulation results of the cabin noise due to the Vulcain engine vibrations

A series of design variants has also been investigated to reduce the overall level of noise and to

assess the sensitivity of the cabin acoustics to specific design variables. In the TBL case, for instance, two structural designs have been tested. The first one connects the pod outer skin to the interior trim panels via SEA structural subsystems, which model the fuselage stringers. The second uses the same type of modelling as with the BWB/CWB models presented in the first part of the paper; it is based on VA One special ribbed skin model, in which both stringers and frames are directly integrated to the skin. In this case, the structural coupling between the outer skin and the trim panels is assumed negligible.

Figure 7 gives an illustration of the design variants investigation. In addition to the two structural variants, the impact of the glass wool –located on the fuselage and on the interior trim panels– is also represented. The conclusions of this investigation are rather straightforward: it is crucial to avoid any sort of coupling between the fuselage and the interior trim panels. If not, any absorbing treatment becomes absolutely useless and the level of noise overwhelming. Specific mounting devices have to be implemented to attach the trim panels to the fuselage while limiting the vibration transmission.

The suggested acoustic trim design shows good performances compared to alternative designs. The blanket arrangement reduces the overall noise level significantly, especially in the higher frequency range.

Figure 7 – Cabin noise level due to the TBL excitation (SEA)

The vibroacoustic FEM simulations showing the influence of the Vulcain vibrations are displayed

in Figure 8. A few variants have been constructed to observe the impact of the excitation area. In the so-called single point acceleration case, the vibration spectrum is applied as an enforced acceleration at a single point at the rear of the aircraft FE model. In the distributed acceleration case, several points associated to the plane tail are used to apply the enforced acceleration. Finally, two pod floor variants have been investigated: a first variant with a supporting frame made of titanium and a second one with an elastomer-based suspension. Figure 8 results show, as expected, that the more realistic distributed case of acceleration yields higher levels of noise in the pod cabin. This load

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distribution has to be modeled with great care to give reliable results. Also, it can be noted that the proposed suspension design, based upon elastomer, is very effective to reduce the largest noise peaks around 100 Hz.

Figure 8 – Cabin noise level due to the Vulcain engine structure-borne excitation (FEM)

The ultimate step has consisted in collecting all results into a sole figure (Figure 9) for a clearer

overview of the most problematic sources of noise. In the current state of the modelling, and to the best of our knowledge, it appears that the TBL is the prevailing one.

Figure 9 – Synthesis of the SEA and FEM results combining TBL and Vulcain structure-borne excitations

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4. CONCLUSIONS This paper has shown how a vibroacoustic analysis can be integrated at a very early stage of the

design, although a considerable amount of information may either be missing or rapidly evolving over time. The focus has been put on three specific aircraft platforms, which prefigure the future of aeronautics: the blended wing body (BWB), the carry-through wing box (CWB) and the carbon-free hypersonic aircraft developed in the ACFA and ZEHST projects, respectively.

The methodology has been adapted to the overall lack of data by setting up a strategy, based on

the use of standard trim design rules, the use of predictive methods that tolerate a large level of uncertainty (SEA) and finally based on the implementation of preprocessing tools suited to the frequent design changes. The vibroacoustic investigation, in such a context, is obviously not complete and thus not entirely satisfactory; but it is certainly sufficient to anticipate problems during the design phase and also sufficient to suggest design alternatives early enough. To sum up, it brings a useful support for managing the risk in future platforms projects, in which a high degree of uncertainty prevails.

The vibroacoustic simulations results contain cabin noise estimations, but also modeling and

design recommendations. In the case of the CWB and BWB aircraft, the first noise estimations are quite promising. The optimized acoustic trim design results in a sound pressure level of 77 and 74 dBA, respectively. These noise levels lie well below the one of a generic single aisle aircraft (estimated at 79 dBA). As far as the ZEHST hypersonic aircraft is concerned, the first estimations show a slightly higher sound level, probably related to the smaller volume of the cabin. These estimations –although incomplete– also show that the TBL source of noise seems stronger than the noise generated by the Vulcain engine vibrations. Finally, a few design options are discussed and some more pertinent suggested for the future versions of the aircraft.

REFERENCES 1. ACFA 2020, 2008; http://ec.europa.eu/research/transport/projects/items/acfa_2020_en.htm 2. VA One software, https://www.esi-group.com/software-solutions/virtual-performance/va-one 3. Cockburn J, Robertson J. Vibration response of spacecraft shrouds to inflight fluctuating pressures.

Journal of Sound and Vibration, 1974; 33(4):399-425. 4. Bhat W. Flight test measurement of exterior turbulent boundary layer fluctuations on Boeing model 737

airplane. Journal of Sound and Vibration, 1971; 14(4):439-457. 5. Peiffer A, Tewes S, and Brühl S, SEA Modellierung von Doppelwandstrukturen, Fortschritte der

Akustik, Stuttgart, 2007.

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