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Measurements and Predictions of Community Noise of a Pusher-Propeller General Aviation Aircraft

F. Marulo* University of Naples “Federico II”, Naples, Italy, 80125

A. Sollo‡ and M.Aversano§ Piaggio Aero Industries , Pozzuoli, Italy, 80078

U. Polimeno and F. Perna†† University of Naples “Federico II”, Naples, Italy, 80125

Community noise level should be considered as a performance parameter for commercial airplanes being their reached strong importance able to affect other aircraft parameters as speed, powerplant, geometrical layout and so on. Consequently it cannot be neglected since the initial phases of the design process of a new airplane. By virtue of these considerations Piaggio Aero Industries decided to launch a research program whose final objective is the integration of the acoustic requirements within the development activities of a new airplane. The aircraft community noise problem, at Piaggio Aero Industries, is an important issue. Their top product, the P180 aircraft, developed in the eighties, when the community noise requirements were a low priority compared with the interior noise levels, now deserves specific attention to such new environmental issues. A computer program for the prediction of the community noise has been developed within the research project with the aforementioned objectives. Parallely an experimental campaign has been carried out on the ground aimed to a validation of the computer program and to the definition of a database for further studies regarding the noise propeller efficiency and its exhaust flow interaction.

Nomenclature FFT = Fast Fourier Transform RPM = Rate Per Minute EPNL = Effective Perceived Noise Level FAR = Federal Aviation Regulation ICAO = International Civil Aviation Organization EASA = European Aviation Safety Agency

I. Introduction HE stringent requirements imposed by the airworthiness rules in terms of acceptable noise levels of aircraft require nowadays a particular attention to this aspect. It is well known, in fact, that in some cases these very

severe rules may reduce the operability of some airplanes, or, in extreme cases, may ground them. The pusher-propeller configuration is more prone to these regulations than classical tractor-propeller aircraft, due

to the added effect of the propeller-exhaust and wing-wake interaction, which generally increases up to critical values the noise levels transmitted to the community,1,2.

* Full 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 †† Engineers, Department of Aeronautical Engineering, Via Claudio 21

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

AIAA 2005-2984

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

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Based on these not neglectable considerations, an intense research activity has been promoted between Piaggio Aero Industries and University of Naples “Federico II” for improving the knowledge of the problem and for investigating for possible applicable solutions to existing and future airplanes.

The pusher propeller configuration reached a peak of interest during the end of the eighties when the oil crisis raised up the problem of fuel consumption reduction,3. At that time new powerplant concepts, like propfans, were studied for application to commercial airplanes. Similarly in the class of the general aviation airplanes Piaggio pursued the objective of a propeller airplane with jet performances. Considering, then, the reduced importance of the community noise compared to the cabin acoustic comfort, together with previous experience in the pusher propeller installation, this latter solution appeared as the most viable for the harmonization of the requirements. Nowadays the scenario is almost the opposite with regard to the noise issues. The environmental expectations are of primary importance and therefore they must be taken into account at the very early phases of the design process of new vehicles. These needs have driven the decision to improve the knowledge of the exterior noise prediction problem investing resources for creating internal capabilities in the complex problem of the aircraft noise prediction.

The developed computer program is addressed to the prediction of the noise perceived over the certification points,4-6, considering the different items of the airframe noise. The propeller noise is not computed using the theories of the computational aeroacoustics, whose results may be included, since they represent a completely different task,7.

This paper continues along a constant interest about the external noise problem,8 and collects the results obtained from both the numerical prediction and the ground measurements. Furthermore some parameters have been analysed for assessing possible improvement to the existing airplane. These studies, involving interaction between propellers and exhaust, are herein summarized from Ref. 9. The results of the research program have been also used for the initial steps of a new project and they show good reliability and possibility to effectively integrate the noise issues with the other requirements of the aircraft.

II. The community noise (COMNOISE) prediction program The COMNOISE software, developed within the framework of the research project (Innovative Vectors for

Sustainable AeroTransports) VITAS, allows to extrapolate noise levels generated from the various aircraft sources to angles around a polar arc or at a selected location in the space during aircraft flyover. They represent the overall noise listened by the human ear. COMNOISE is built in two main parts: the first one, for the data pre and post-processing, oriented to a general user taking care of an easy-to-use human interface; the other one is the actual solver, which inaccessible to the common user, but it is open for the continuining improvements. It appears, therefore, easy to use and quite intuitive. The user interface has been developed in Visual Basic and designed to allow the end user to easily prepare and manage the input file. Data flows into the input file through the pre-processor routines. The Post-Processor, named "External noise" allows the interpretation of the results coming from the solver implementing several graphical representation as, for example, bi-dimensional and three-dimensional plots, colormap, contours, histograms, FFT, waterfall and other graphical representation.

COMNOISE computes the far field noise produced by an aircraft including different polar distances or static side distances, for single or multi engine aircraft while following a specified flight path (Fig.1); in a flyover situation, including airframe noise or along a straight trajectory.

Figure 1. Flight Path and geometrical definition for COMNOISE program

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In particular, noise generated by a single engine is predicted using technical parameters belonging to the propulsion group such as:

1. Fan RPM; 2. Pressure Ratio; 3. Jet Velocities; 4. Combustor exit temperature; 5. Ideally expanded one dimensional absolute jet velocity; 6. Mass flow for jet stream; 7. Turbine outlet total temperature; 8. Available measured noise levels can be input allowing the software to predict noise at several specific

positions around the polar arc, as shown in Fig.2.

Figure 2. Polar arc for the noise prediction

Further opportunities provided to the end user regards the installation of external effects or acoustic treatments as

deltas array applied to different directions and the extrapolation that can be calculated on more side stations. In COMNOISE the aerodynamic noise is modelled as due only to the singular individualized component of the

aircraft system, such as wings, flaps, slats, horizontal and vertical tail, landing gear, and so on, according to the basic theory reported in Ref.11.

The program predicts the one third octave band values of acoustic pressure on a polar arc of ray 150 feet. Subsequently, Doppler effect and extrapolation of the spectra are applied near to the observer taking into account spherical divergence, atmospheric and ground attenuation.

The final output file reports the maximum sound pressure level and effective perceived noise level (EPNL) in dB(A), together with the spatial behaviour and the distribution along the segmented straight trajectory. In addition, some of the COMNOISE output files may also be used as input file for the pre-processing software "External Noise" which, through interpolation algorithms (Kriging), allows to visualize a waterfall of the Sound Pressure Level, to plot colormap and contour map of the acoustic levels (Fig.3) and to properly correct the predicted values as prescribed by the certification rules (FAR 36, ICAO Annex 16, EASA CS-36).

Figure 3. Example of colored waterfall map output from the graphical post-processor

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III. On the ground measurements A ground test campaign has been undertaken in order to acquire experimental data to feed the program with

appropriate input for certification community noise prediction. Noise measurements have been performed on a polar arc 50m radius (see Fig.4) with one operative engine and

with different RPM. During the same test session different configuration of exhaust duct have been tested in order to evaluate the exhaust gas interference effect (see Fig.5)

Figure 4. Ground measured noise stations around the polar arc with one operative engine Microphones have been placed at 1.2 m from ground at 20° step along the polar arc. Then measured data have

been corrected for the reflection (ground reflection and wing reflection as well), while no attempt has been done to eliminate the fuselage reflection. Figure 6 summarizes the results of different measurements performed with different engine setting and different exhaust orientation effect.

Figure 5. Various exhaust configurations for ground noise measurements

Test 1 – 0° Test 2 – 90°

Test 3 – 30° Test 4 5 - ±30°

Test 6 – 60°

0°

20°

40°

60°

80° 90° 100°

180°

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R = 50mt

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Figure 6. Noise levels measured around the polar arc for different exhaust configurations

From this test campaign some important indications have been derived, for example the importance of the

exhaust directivity (Fig.5, Test 4-5 configuration) and shape optimization for minimizing propeller/exhaust interference (Fig.7). The measured reductions are well in agreement with predicted ones as already presented in Ref.8. The exhaust shape and orientation modification are today under evaluation for embodiment on production aircraft, as well as propeller RPM reduction is under evaluation.

Figure 7. Numerical prediction of the propeller/exhaust interference for different exhaust shape

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0°180°

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exhaust 0°exhaust 90°exhaust 30°exhaust +30/-30@2000rpmexhaust +30/-30@1800rpmexhaust 60°

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IV. Predictions and comparisons over the microphone measurement station Based on the ground measured data, the free-field propeller noise has been given as input to the COMNOISE sw

in order to make noise prediction over the certification point. The take-off profile of the P180 aircraft has been taken for noise propagation prediction (Fig.8), and engine and airframe noise prediction have been calculated by different modules of the sw. For this class of airplanes the measurement microphone is placed at 2500 m from brake release 1.2 m from ground, according to FAR 36 Airworthiness Requirements. According to the same requirements, the max dBA level has to be certified on the take-off trajectory.

Figure 8. Take-off and climb profile of the P180 aircraft for the community noise prediction/measurement Different take-off and approach profile have been simulated reproducing test profile, and table 1 report the

numerical-experimental comparison performed for the P180 aircraft. Table 1. Numerical-experimental comparison of the noise levels over the microphone measuring station

Amb. Temp [C°]

Umid. [%]

Wind Speed [m/s]

Amb. Press. [Pa]

Flyover Height

[ft]

Vy IAS [kts]

Prop. Rpm

∆ dBA Exp-Numerical

Take-off 26.5 62.9 0.09 101325 650 154 2000 2.6 Take-off 26.4 63.0 0.00 101325 780 154 2000 2.7 Take-off 26.5 63.7 0.13 101325 770 153 2000 4.0 Approach 26.5 63.5 0.25 101325 90 150 2000 0.2

As arises from table 1 comparison, the experimental data are underestimated by the prediction software. This is due to the fact that in the prediction software, the noise correction due to the wake-interference effect (ref. 9) have not yet been implemented.

V. COMNOISE application for the next generation business jet aircraft The aircraft noise prediction software presented here, validated through experimental data of P180 aircraft, has

then been widely used to make preliminary trade-off of the community noise of the new P1XX light-medium business jet aircraft, currently under development at Piaggio Aero Industries . The aircraft is a twin engine aircraft (fuselage mounted turbofan) ranging from 6 to 12 passenger accommodation for the different shortened-stretched version.

Vy=119 kts

400 ft50 ft

Microphone Measurement

position

850 shp @ 2000 rpm

Vy =154 kts

R/C=2950 ft/m

Max Torque 2230 ft*lbs

850 shp @ 2000 rpm

Vy =140kts

R/C=2550 ft/m

Flap MID

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Static engine noise measurements were made available by candidate engine supplier for different power setting, so the prediction methodology that has been employed was to project the measured static engine noise levels to acoustic certification flight conditions.

The COMNOISE methodology has been further updated and is consistent with the requirements that are described in the ICAO Environmental Technical Manual on the “Use of Procedures in the Noise Certification of Aircraft”. Noteworthy features of this prediction methodology include:

source separation of the measured noise spectra into broadband turbo-machinery, narrowband turbo-machinery, and broadband jet noise components to enable application of source-specific static-to-flight corrections

accounting for source noise changes due to motion induced effects, such as Doppler shifting, dynamic amplification, and jet noise modification

accounting for the attenuation of inlet-radiated and aft-radiated noise by acoustic treatment in the inlet and in the aft body of the nacelle

accounting for the aircraft flight profile at the noise certification conditions accounting for spherical spreading, atmospheric attenuation and ground reflection and attenuation

The predictions have been performed for treated and untreated nacelle, assuming that acoustic treatment has been incorporated into the inlet and the aftbody. The type of treatment has been assumed to be a single-degree-of-freedom locally-reacting linear liner, constructed of a bonded sandwich of wire screen, perforate plate, honeycomb, and an impervious backing plate. The computed attenuation have then been corrected by multiplying the peak attenuation by empirical directivity correlations made available by engine supplier. This is approximately the method used to determine the reduction in inlet-radiated or exhaust-radiated far-field sound pressure levels.

Based on these hypotheses different trade-off have been performed to determine the community noise levels for the three certification point (take-off, sideline and approach) to select the best compromise between:

- Engine setting - Flyover height - Cutback height - Nacelle treatment area

in order to have large margin with future noise certification level and to be capable to comply with local airport limits. Figure 9 summarizes some of the trade-off performed up to now.

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1600 1800 2000 2200 2400 2600 2800 3000 3200

Net Thrust per Engine [lbs]

EPN

L [d

B]

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Stage 3 Limit = 89

Stage 4 expected limit = 84 EPNdB

Figure 9. Predicted Noise Power Distance Curve for the P1XX Aircraft @ different engine settings and

different Flyover heights

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Conclusions The paper has collected the results obtained during a three-year research activity jointly developed by Piaggio

Aero Industries and University of Naples “Federico II” aimed to prepare and validate a numerical and experimental procedure for the predictions of the aircraft community noise. The need for these activities came by the importance for existing and new airplanes of the community noise emission. This problem is particularly important in Piaggio Aero Industries due to the pusher-propeller configuration of their aircraft (P-166 and mainly P-180). As result of such intense research activity, both numerical predictions and experimental measurements have been carried out. The numerical activities have involved the definition and the validation of a community noise prediction program and numerical trade-off studies performed using computational aeroacoustics for the sensitivity analysis of blade number, geometry and RPM, exhaust shape, and so on. The experimental activities has been addressed, mainly, to the validation of the prediction computer program through on ground measurements. The results of this research program have paved the way to possible modifications to be applied on existing aircraft and have offered the possibility to start the community noise evaluation of the next generation business jet airplane, allowing to take into account the noise related problems since the pre-design phase of such new project. The results of this activity have been very satisfactory and encouraging, such that new researches are continuing involving also aspects, as for example the wing-nacelle interaction, which were considered second priority only months ago.

References 1Neuwerth G., Lölgen Th., Staunfenbiel R., “Increased noise emission of propellers and propfans due to pusher installation”,

Proceedings of. ICAS 1990, 1990, pp. 127-138, 2Weir D.S., Marsan M., Lyon C., “Acoustical analysis capability for Pusher Propeller installations”, Proceedings. of 28th

Aerospace Sciences Meeting, 1990 3Soderman P.T., Clifton Horne W., “Acoustic and aerodynamic study of a Pusher-Propeller aircraft model”, NASA TP-3040,

1990 4"ICAO - International Standards and Recommended Practices - Environmental Protection Annex 16 to the Convention on

International Civil Aviation - Aircraft Noise". Vol. I, 2nd Edition, 1993 5"JAR-36 SUBPART C – Propeller Driven Airplanes Not Exceeding 9000 Kg. Application for Type Certification or

certification of Derived Version accepted on or after 1 July 1996". Issue 23 May 1997 6"Part 36 – Noise Standards : Aircraft type and airworthiness certification", FAA, DOT, Revised as of Jan 2003 7Mauk C., Farokhi S., “The effect of unsteady blade loading on the aeroacoustics of a pusher propeller”, Proceedings. of

AIAA/SAE/ASME/ASEE 29th Joint Propulsion Conference and Exhibit, 1993 8Massardo A., Dotta F., “Analisi sperimentale del rumore generato da eliche in configurazione spingente”, Atti del 51°

Congresso ATI, Udine, Italy, 1996, pp. 1385-1398 (in Italian) 9Sollo A., Aversano M., Di Mascio A., Ianniello S., Salvatore F.,. Gennaretti M “Evaluation of noise excess for pushing

propeller aircraft by CFD aeroacoustic calculation”, Proceedings of the 10th AIAA/CEAS Aeroacoustic Conference 2004, Manchester Town Hall, 2004

10ESDU, “The correction of measured noise spectra for the effects of ground reflection”, Item No.94035, ESDU International plc, London UK, November 1994.

11ESDU, “Airframe noise prediction“, Item No.90023, ESDU International plc, London UK, November 1990