a2 structure of source models measurement methods r buetikofer

Ma te ria ls Sc ienc e &Te chno logy

Aircraft Noise: Source Model

R. BütikoferLaboratory of AcousticsEmpa, Switzerland

Empa, R. Bütikofer, October 2006 2

Aircraft Noise: Source Model

1. General remarks

2. Structure of the source model

3. Measurement methods


Acoustic engine of IMAGINE

Sound power leve ldirectivity,opera tion condition

Propagation Nois ereception

Lden

Distance , a ir absorption, groundeffect, curved pa ths , screening

Long te rmaverage

Sourceoutput

iteration


IMAGINE WP 4

Deliverable D10:Default source description andmethods to assess source data for aircraft

� WP 4 delivers a data structure, examples and a description on how to do it, not a complete data base.


From DOC.29 (2005) Vol. 1 Applications guide www.ecac-ceac.org

MODELINPUTS OUTPUT

Practitioner - Consultant - Adviser - Analyst - Technician

End-user - Policymaker - Administrator - Planner - Public

Modeller - Scientist - Programmer - Auditor

Scenario- Airport - Aircraft - Traffic - Routeings - Weights - Procedures - Population - Geophysical - Meteorology

Database- Acoustic - Performance

Contour - Area - Population - Other - Impact

Volume 2

Volume 1


Literature: Special edition of Acta Acustica spring 2007


The 4 components of a calculation

Input

Data from the real world

Scenario Aircraft sound calculation program

Aircraftacoustic data

Noise contour(Leq, Lden)

Aircraft per-formance data

Output


Structure of the Source Model


Characteristics of the source emission

� Free field conditions (no ground influence)� Spectral data (1/3 octaves): 50 Hz … 10 kHz� Describes lateral and longitudinal directivity� Doppler effect is not removed


Geometry

�

�

r

flight path

ground track�

height

sideline distance


Descriptor

Sound power, including direction dependent level adjustment: Lw,dir

Sound pressure level at receiver:

Lp (f,�,�,r) = Lw (f) + Dc(f,�,�) - C - A(f,r)

Lp (f,�,�,r) = Lw,dir (f, �, �) - 11 - A(f,r)

Sound pressure level at 1 m in free field (A � 0):Lp (f,�,�,1m) = Lw,dir (f, �, �) - 11


Parameters

aircraft type / engine

flaps / slats / gear

thrust

operation

speed

longitudinal angle

lateral angle

Lw,dir - Spectrumsource module


Lateral symmetry of sound emission

� = 90°

� = 0°

� = - 90°

� = + 180°/ - 180°

For fixed wing jet aircraft (but NOT for propeller or helicopters!):

Lateral symmetry (left / right)

� = ± �

Vertical symmetry (upper / lower hemisphere)


Sphere in Cartesian coordinates (I)


Sphere in Cartesian coordinates (II)

Example:

xx: fields use the interpolated (or constant) value of neighbouring cells


Data base structure

� Tables for specific aircraft and operation conditions may be arranged in a data base

� Not all combinations of operation conditions will have data � typical configurations with data, rest with default values

� Not all aircraft will have data � grouping of acoustically similar aircraft and substitution of acoustically less important aircraft


Measurement methods


Measurement methods

� Reverse engineering from NPD data� „In-flight“ measurements under

operational conditions� Arranged „In-flight“ measurements� Processed data from manufacturers


„In-flight“ measurements under operational conditions (I)

Set – up� Several microphones perpendicular to the axis

of the runway � at 3 to 5 km after brake release (departure) � at 1 to 3 km prior to touch down (landing)

� Measurement of the normal airport traffic during several days


„In-flight“ measurements under operational conditions (II)

Requirements� Precision tracking of aircraft (tracking radar or

optical)� Time synchronisation of position and of the

acoustic recordings� Exact aircraft identification (type, engine, carrier,

ATOW)� Information from carriers on flight procedures


„In-flight“ measurements under operational conditions (III)

Advantages� Measurement of „real-life“ situation� Average over many samples of the same aircraft

type � No additional costs for aircraft operation� Variety of different aircraft measured in the same

measurement period


„In-flight“ measurements under operational conditions (VI)

Disadvantages� Thrust is unknown (unless FDR recordings are

available)� Limited to specific parameter combinations

(speed, flaps, gear, thrust)� Measurements only close to the airport

(sufficient signal to noise ratio)


Arranged „In-flight“ measurements (I)

Set – up� Microphone array perpendicular to axis of flight

� May be single sided array for fixed wing aircraft� U – shaped with cranes

� One aircraft with test pilot operates at various levels and speeds

� Positional information (GPS + time) on board


Arranged „In-flight“ measurements (II)

Requirements� Airfield reserved for measurements� Aircraft available for test flights

Advantages� Well controlled measurements� Various lateral angles by varying the flight level


Arranged „In-flight“ measurements (III)

Disadvantages� Expensive� Only one sample measured. No information on

variations of sound power for a number of aircraft in normal operation.

ExampleWP 4 Flight tests (Nico van Oosten)

Example: Empa Measurement of Helicopters 1998

Flight paths

5m

50m

150 m

1

2

3

414

12

13

11

622201816

15 17 19 21 23 7 5

Array of 20 microphones

2 cranes left and right


Example: Wallops study of NASA (DC9, B767)

Report NASA/TM-2003-212433


dB TowerProject(USAF,Wyle,NLR)400 m high300 m fromrunway


Processing of measurements (I)

� 1/3 octave band processing, retaining exact time� For many discrete aircraft positions: combine

geometry (�,�,�,r) with measured spectrum.� „Depropagate“ the spectra for Lw,dir (�,�):

� Calculate propagation for the specific geometry, using temperature and humidity of the measurement day.

� Subtract propagation from measurement


Processing of measurements (II)

� For each 1/3 octave: Average and interpolate Lw,dir (�,�) in the spherical coordinates � and �

� Generate look-up tables Lw,dir (f,�,�)

� Validate results by reproducing measured data


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