DATA REPORT: EN ROUTE NOISE OF TWO TURBOPROP … · TWO TURBOPROP AIRCRAFT Werner Dobrzynski...

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DATA REPORT: "EN ROUTE" NOISE OF

TWO TURBOPROP AIRCRAFT

Werner Dobrzynski

Deutsche Forschungsanstalt f_r Luft- und Raumfahrt

Forschungsbereich Str_mungsmechanik

Institut fHr Entwurfsaerodynamik

Abteilung Technische Akustik

Flughafen

D-3300 Braunschweig

Braunschweig, im Juni 1989

Institutsleiter:

Dr.-Ing. H. KSrner

Verfasser:

Dr.-Ing. W. Dobrzynski

Abteilungsleiter:

Dr.-Ing. H. Heller To be published as DLR-Mitt. 89-18

81

https://ntrs.nasa.gov/search.jsp?R=19900015543 2020-03-24T11:58:09+00:00Z

Propel lerl2"rm, tPe-isefiD_[_rm

Datenbericht: Reiseflugl_rm yon zwei Turboprop-Flugzeugen

Ubersicht

Zur Beurteilung des Reiseflugi_rms kHnftiger Verkehrsflugzeuge

mit Propfanantrieben werden Vergleichsdaten von herk6mmlichen

Turboprop-Flugzeugen ben6tigt. Als Beitrag zu einer solchen Da-

tenbank wurden Reiseflugl_rmmessungen an zwei zweimotorigen Tur-

boprop-Flugzeugen in Flugh_hen zwischen 5182 m und 6401 m durch-

gefHhrt. Die Ger_uschpegel werden zusammen mit den Betriebsdaten

der Antriebspropeller und den meteoroi0gischen Umgebungsbedin-

gungen angegeben. Schmalband-Frequenzanalysen zeigen die beson-

deren Eigenschaften des gemessenen Propellerger_usches, n_mlich

die Dominanz des Pegeis der Propeiierdrehklangfundamentalen und

das Auftreten yon akustischen Schwebungen durch unterschiedliche

Drehzahlen der zwei Antriebspropeller.

Propeller Noise, En route Noise

Data Report: "En route" Noise of two Turboprop-Aircraft

.... Summary

In order £o Weigh en-route nolse immissions originating from

future propfan powered aircraft a data base of immission levels

from conventional turboprop aircraft is needed. For this reason

flyover noise measurements on two twin-engine turboprop aircraft

were conducted at flight heights between 17000 ft and 21000 ft.

Acoustic data are presented roger-her with propeller operational

parameters and environmental meteorological data. Narrowband

spectral analyses demonstrate the characteristic features of the

measurea propeiler noise s_g'natures: Noise Spectra are dominated

by the propeller rotational noise fundamental frequency and pro-

nounced noise beats occur as a consequence of differen£ rotatio-

nal speeds of the propellers.

82

Contents

Page

List of symbols ........................................ 7

i. Introduction ........................................... 9

2. Test aircraft .......................................... ii

3. Test matrix and measurement site ....................... 12

4. Environmental and operational data acquisition ......... 12

Meteorological data ............................... 12

Aircraft operational data ......................... 12

5. Acoustic data acquisition .............................. 17

6. Acoustic test results .................................. 19

6.1 Maximum linear- and A-weighted overall sound

pressure levels ................................... 19

Sound level time-histories ........................ 25

Narrowband spectra ................................ 29

7. Conclusions ............................................ 37

8. Summary ................................................ 37

9. Acknowledgement ........................................ 38

I0. References ................... _ ......................... 38

Appendix I ................................................. 39

Appendix II ................................................45

83

List of symbols

!I

BLN

BPF

f

H

HN

L

L A

- Number of propeller blades

Hz Blade passing frequency =(N/60) BLN

Hz Sound frequency

m Flight height

- Harmonic number

dB Overall sound pressure level

dB

M I

MHe 1

N i/min

p N/m _

r m

t sac

t sacs

T °C

V m/s

Overall A-weighted sound pressure level

(A-sound level)

Flight Mach number _ _

Helical propeller blade-tip Mach number

Propeller rotational speed

Sound pressure amplitude

Distance between sound source and observer _

Time

Cycle-time of sound beats

Temperature

Flight speed

Hz

8 deg

Circular frequency =2_ f :_

Elevation angle

Subscripts

o - Reference

max - Maximum value

Note: Sound pressure levels are referenced to Po=20_ Pa

84

i. Introduction

The significant and world wide increase in air-traffic during

the last decade has led to a noise nuisance caused by aircraft

in cruise, operating at high altitudes. Complaints are reported

both from resort areas with inherently low background noise and

from areas underneath crowded air-traffic junctions.

The issue of the so called "en route noise" has been raised re-

cently within the Working Groups of the ICAO-Committee on Air-

craft Environmental Protection (CAEP). A potential problem is

foreseen with the development and introduction of new propfan-

powered aircraft within the next few years. In fact, it is the

low-frequency harmonic noise signature of such propeller-type

propulsion systems which worries the acoustics engineers and ad-

ministrators alike, who expect an increase in en route noise re-

lated complaints.

In the United States the first flyover noise measurements on a

propfan powered research type aircraft were recently conducted.

In order to check measured noise characteristics in terms of

their "annoyance" potential they need to be compared against

some adequate reference. An appropriate reference could be the

noise characteristics of conventional turboprop-aircraft that

have been in operation for many years and are more or less ac-

cepted by the public.

The task at hand, therefore, is to define a "level-number"

which, in combination with the particular propfan/propeller

noise characteristics, would be acceptable as not to further ag-

gravate the present en route noise problem. Since no extensive

data base exists for such a comparison, en route noise data from

turboprop aircraft must be collected to provide a reference as

an acceptable noise limit.

This report presents flyover noise data as measured from two

different turboprop aircraft at typical cruising altitudes along

with local meteorological and aircraft operational data. The

85

Fig. 2 Fokker 50 aircraft

88

measurement campaign was initiated and organized by the "Noise

Abatement Commissioner of the Hessian Minister for Economics and

Technology at Frankfurt Airport", Herr Held, and funded by the

"Flughafen Frankfurt Main AG".

2. Test aircraft

Two different types of aircraft were selected, the Fairchild

Metro III (Fi_. i) and the Fokker 50 (Fig. 2). Both aircraft are

powered by two turboprops each, the Metro III representing a

smaller but somewhat noisier aircraft compared to the larger

Fokker 50. Some overall design parameters are listed in Table I:

TABLE I: Test aircraft parameters

Wing span (m)

Max. T.O. Mass (kg)

Typical Cruising Speed

(ktslkm/h)

f

Power Plant:

Number of Engines

Engine Power (kW)

Propeller:

Number of Blades

Diameter (m)

Metro III SA 227

17.37

6577

2481459

Garret TPE 331-

IIU-612G

2

745.5

Dowty Rotol

4

2.69

Fokker 50

29.00

18990

2821522

Pratt & Whitney

PW 125 B

2

1864.0

Dowty Rotol

6

3.66

87

3. Test matrix and measurement site

k

Acoustic data were taken for three level flyover heights, i.e.

17000 ft (5182 m), 19000 ft (5791 m) and 21000 ft (6401 m) in

respectively two opposite flight directions with the engines

operating at cruise-power setting. Since relatively low flyover

noise levels were expected the measurements were taken at night

(between 0.00 am and 3.00 am) in a flat agricultural area loca-

ted south of Frankfurt airport. This site was selected to bene-

fit from existing navigational aids installed near airports and

to thus realize a precise and reproducible flight path over the

measurement station.

4. Environmental and operational data acquisition

In order to correctly evaluate acoustic test results, the local

meteorological conditions and pertinent aircraft operational da-

ta were recorded.

4.1 Meteorological data

Simultaneously with the acoustic flyover measurements, a weather-

balloon was ra{sed by the "DeutsCher Wetterdienst" near the test

site to obtain profiles of atmospheric pressure, temperature,

humidity and wind conditions versus height. Respective data _ re-

cords are_presented in Figs. 3 and 4 up to a height of 2000 m. A

complete data listing of wind conditions up to 6672 m and of

temperature and humidity up to 4245 m are presented in Appen-

dix I.

4.2 Aircraft operational data

|

iE

ZE

No external devices were used to determine the aircraft opera-

tional data, but the pilots were instructed to read and record

flight-height and -speed as well as air-temperature and power-

88

Fig. 3 Air-temperature (TT) and Relative Humidity (RF)

versus height (in meters) above ground

_GRIESSHEIM J_,._.F_

DATUM_30.O4.B9 RBDBRWIND [GEGLRETTET)

_02.10 MESZ

_go m INN1 IIli/HHH

0 III iO Io _Zl _Si ISil III 11,¢ l'#ll I_¢ lit ill"

Fig. 4 Wind-directlon (dd) and -magnitude (ff) versus

height (in meters) above ground

ORIGINAL PA(_ I_OF POOR QUALITY

89

Xnfllqht - Xnfo - Sheet

Type of aircraft: _£f_O_ 22

Re9istrati°n : _" CPE_

Type of engine : "T'P_ _1

I T.O.W. : _O_ k9I ATD = • Z _ _ ZI Pos. 7DM£ RID R 359 _Q._.. ZI .¥_.. Air.II

I 4DHE South I IA5/TAS /9_ "/ _]O kt$

I of RID FL 170 I Temp.: -./(_I northbound ___ I__ Z I Clouds: CLEQ_ ., _=I on R 359/179 RID I Power: Cc_ irGf _7_ ,

"I_ I anti ice'. 4,.,I I

-noM_ no,_h I XAS/TAS Z_" / -", kt, i_l of RID FL 170 I Temp.: _

l southbound

J on R ]59/179 RID 2 2 2 { Z I Clouds: C,E'ArZ .-,o/I Power: ,e_ _(_ I

I kt$ I_l 4DME South I IA$/TA5 /9_" / 2_O[ of RID FL 190 [ Temp.: -20 "I northbound Z 2. Z_ z I Clouds: cc_ .

[ Power:Ii °n R 359/179 RID I

!13DME north [ IAS/TAS /9Q / 130 kts I

x_/! of RID FL 190 ! Temp.: -ZO II southbound 22_7: Z I Clouds: C_.E__

R

I Power: m o_f)!

IuL- .... t.

I 4_...... t,, i Z^_/'T_S -2_/_" / 2.3Z kts- I[ of RID FL 210 [ Temp,:

[ Power :"_'_I on R 359/179 RID ! anti ice: ¢_¢_C6G7- _f_,_

I !

13DME north I IAS/TA5 I /_O/ _ ]__ kts i

_1 o_ RID FL 210 Tem.: -2Gsouthbound I 2 _'/ Z Clouds: C£E2A_on R 359/179 RID

Pos. 4DME southof RID, FL 210 2_ -_'v

End of testflightrequest clearanceto Frankfurt

m

Fig. 5

Power: _-o _6F f7 %anti Ice: e_

Data sheet as filled out by the Test-pilot of the

Metro III aircraft

b

90

ORIGINAL PAGE IS

OF POOR QUALITY

XnflJ_ht - l_fo - Sheet

Type o_ literary: _l_4tv 50

Type o! engine :_" dZG._

T.O.W. : Ig _gg kg

ATO , _.00 ZPos. 7O_Z R_D R 359 ... o.t,:

• .7.'.o._xlc.

4DF'_ sou_hof EIO rL 170nocthboundon R 359/179 RID

13DME northoE RZD TL 170southboundon R 359/179 RID

4DM£ southof RID FL 190northboundon R 359/17g RID

13DM£ noc=hof RID ?L 190louthboundon R 359/179 RIO

4DME southof RID FL 210northbcundon R 359/179 RID

13DME northOf RID FL 210southboundon R 359/179 RID

Pos. 40ME south

of RID, rL 210

End of testflightrequest clear,neeto Frankfurt

23i0z

ZsfT:

232 :

Zl_i z

IAS/TXS Z I, _s / 17Temp.: -t_"Clouds: c_vo_

inti ice: _ off

zxs/_xs 2_l_s / 285Temp.: - I&"Clouds: £_voll

an_i Lee'. _ off

iAS/TAS 20q V,_, /2_ ITemp.: -20"

Clouds: C_ v$_Power: "_8 ,_ T_._. _,._._•nti Lee: _ off

z_s/Txs 21Zk_ /286Temp. : _ _ q _Clouds: ca _V.

In_i ice: ¢._ o_f

zxs/:xs zo, /Temp. : - 2

anti ice: on" oEf .

ZAS/T_S ZoI_;_s /282Tern. : - 2c_ _

Clouds: C_v,_po_,r" -;,.,'_-_lOl _._anti ice: _n" off

k_s

kts

k_s

k_s

k=s

,J

Z

Fig. 6 Data sheet as filled out by the Test-pilot of the

Fokker 50 aircraft

OR|_INALPAGE IS

OF POOR QUALITY

91

setting from the cockpit instrumentation during each flyover.

"Inflight-Info-Sheets" are presented as Figs. 5 and 6.

Both aircraft are equipped with constant-speed propellers. Hence

power is adjusted automatically by blade-pitch setting to main-

tain a constant rotational speed corresponding to the following

values:

Propeller rotational speed (rpm)

Metro III

1543.

Fokker 50

1025.

92

5. Acoustic data acquisition

Two BrHel & Kjaer i/2"-Condenser Microphones (Type 4145) were

positioned (in close proximity) underneath the flight path. The

microphone signals were stored on an analog tape recorder. While

one of the microphones was mounted on a 1.2 m pole, according to

established noise certification regulations, the other micro-

phone was installed close (and inverted) to a 0.4 m diameter

ground board. This latter arrangement is frequently employed in

scientific measurements since it represents the best device

(other than a flush mounted microphone in a large concrete sur-

face) to avoid ground reflection interferences. Such ground re-

•flections tend to hdav_iy distort source noise spectra, depen-

ding on the particular relation between microphone height and

the fundamental frequency wavelength of the signature to be

measured.

Examples of such microphone arrangements are presented in

Fig. 7. However, for the tests described herein, the microphones

were located on a hard and flat "earthy" surface.

From basic priciples it is known that pressure doubling occurs

at an acoustically hard surface. Levels obtained by ground based

microphone arrangements are higher by up to 8 dB(!) compared to

those from pole-microphone installations. If however the micro-

phone height selected (accidentally) corresponds to multiplee of

!

I

I

z

Fig. 7 Illustrations of ground-board (top) and 1.2 m

pole microphone (bottom) arrangements

Original figures not available.

93

ORIGINAL PAGE IS

OF POOR qUALITY

the sound signature's wavelength both microphone arrangements

may give identical results.

A detailed discussion of ground reflection effects on propeller

aircraft flyover noise measurements is provided in [I].

6. Acoustic test results

Noise data will be presented as measured in terms of overall le-

vels, level time-histories, and narrowband sPectra. Since no

acoustical significant variations in flyover height could be

tested and ac0ustic signaturesturned _oJt t0_e dominated by the

low-frequency (about i00 Hz) fundamental of propeiier rotational

noise, no correction is applied to the data with respect to

flight height, air-temperature, atmospheric attenuation, etc.

Such corrections indeed should not be applied in an overall

manner, since the magnitude of respective level differences

would equal the observed data scatter caused by stochastic at-

mospheric disturbances. Application of such corrections should

therefore be iert to specialists who are then to apply sophis-

ticated computer codes for the calculation of the transmission

attenuation based on detailed mete0ro!ogical data.

6.1 Maximum linear- and A-weighted overall sound pressure

Tables II _ and Iii contain maximum linear (analyzed with time

constant "fast") and A-weighted (analyzed with time constant

"slow") overall levels numbered in the order of test flights

except the first flight of the Fokker 50 aircraft at 17000 ft

height which had been missed due tO communication problems. The

first measurement in that listing (Table II) does not pertain to

the en route noise test series, but represents the climb-out

signature of the test aircraft Metro Ill and has only been lis-

ted for completeness.

94

Table II

En-Route Noise Measurement (Frankfurt/Griesheim; 30.4.89)

Aircraft Type: Metro III SA 227

Propeller Diameter = 2.692 m (4 Blades)

Operational Conditions: TAS = 230.0 kts

Propeller Rot. Speed = i543.3 rpm (BPF = 102.9 Hz)

I*

2

3

4

5

6

7

Flight

Height

ft

8000

17000

17000

19000

19000

21000

21000

-14

-14

-20

-20

-26

-26

MHel

LA,ma x (Slow) dB(A)

Ground Mic 1.2 m Mic

L (Fast) dBmax


61.7 56.9 78.4 74.2

0.7675 52.9 48.9 70.3

0.7675 54.1 50.5

0.7764 50.6

50.2

47.5

_4-_.50.7764

0.7858 52.1 48.2

0.7858 49.9 46.0

Level Averages (without No. i)

Level Differences (Ground -1.2 m)

Background Noise Levels:

67.2

72.0 ....... 68.-5--

68.0 65.7

68.1 65.9

68.7 65.9

68.0 -- - 64.§

51.6 I 48.1 _ 69.2 ] 66.4 J

z_: 3.5 I A= 2.8 j

I 39.0 I 37.9 I 54.0 I 53.0 I

* Take-off Power Setting

Listing of measured maximum overall noise levels from Metro III aircraft fly-

overs

Table III

En-Route Noise Measurement {Frankfurt/Griesheim; 30.4.89)

Aircraft Type: Fokker 50

Propeller Diameter = 3.66 m (6 Blades)

Operational Conditions: TAS (Average) = 280.5 kts

Propeller Rot. Speed = 1025.0 rpm (BPF = 102.5 Hz)

Flight l

Height!ft MHel

8 17000 -14 0.7554

9 19000 -20 0.7643

10 19000 -19 0.7628

II 21000 -24 0.7704

12 21000 -24 I 0.7704

Level Averages

Level Differences (Ground -1.2 m) I

Background Noise Levels: |

LA,ma x (Slow) dB(A)


51.0 48.5

53.9 51.1

46.9 43.5

45.9 43.9

46.6 44.0

Lma x (Fast) dB


67.5 64.4

70.9 68.8

63.7 ....

63.0 60.8

63.7 [ 60.1

48.9 1 46.2 I 65.8 I 63.5_j

: 2.7 I z_ : 2.3 I

39.0 1 37.9 I s4.o I 53.0 I

Listing of measured maximum overall noise levels from Fokker 50 aircraft fly-

overs

95

Together with the flyover noise levels these tables also contain

the calculated values of respective helical propeller blade-tip

Mach numbers, referenced to the air temperature at flight

height.

Calculated level averages (as determined from flyovers at diffe-

rent heights!) may be taken to correspond to the average flight

height of 19000 ft. From the measured and listed background

noise levels, on average a sufficiently large signal-to-noise ra-

tio of almost i0 dB is observed.

Levels on the ground turn out to be higher by some 3 dB compared

to those from the 1.2 m pole microphone. This level difference

can be taken as an order-of-magnitude value which may be consid-

ered as typical for conventional propeller-driven aircraft. Not

to hamper further data interpretation by accounting for ground

reflection effects, only ground microphone obtained noise signa-

tures will be discussed.

In Figs. 8 and 9 overall linear and A-weighted noise levels are

plotted versus flyover height for both aircraft As a simple re-

ference, the level attenuation for spherical spreading (i/r 2-

law) is indicated. Except For one data point (No. 9/Fokker 50)

noise levels are quite close to this reference. As will be shown

later there is no explanation for the noise level of flyover No.

9 to be almost 7 dB higher than expected. Effects of stochastic

atmospheric disturbances may have caused this discrepancy.

From an inspection and comparison of the data as presented in

Figs. 8 and 9 the Metro III aircraft seems 2.5 dB noisier com-

pared to the Fokker 50 aircraft. From the experience gained

within extensive wind tunnel propeller noise tests [ 2 ] such a

result may be assumed to originate from a slightly higher heli-

cal blade-tip Mach number as observed for the Metro III compared

to that of the Fokker 50.

_=

For both aircraft the differences between linear and A-weighted

noise levels range from 17 dB to 18 dB. This difference roughly

96

80D 75-

70-

s5_.I

0 60-Z:D 55-0

METROIll0

- -o..... _-- --/- -O--

/l/r 2- Dislance Law

0 .., I i

16000

FLIGHT

' I ' ' ' I ' ' '

18000 20000 22000

HEIGHT (FT)

80

75

_j 70

e5.._I

600ZZD0CO

FOKKER

. 0

/-0 -- --O--

1/r 2- Distance Law

55-

50 . . • , .16000 18000

FLIGHT

50

• l | v • I

20000 22000

HE IGHT (FT )

Fig. 8 As measured maximum overall flyover noise

levels versus flight height

97

65

60V

,-4,5550

_J

Z

o40O9

I"_ 35': ,

16000

METRO If !

--e..... _o__- l/r 2- Distance Law/

' ' I ' ' ' I ' ' '

18000 20000

FL IGHT HEIGHT ( FT )

22000

65

60V

55

50.d

I35

16000

FOKKER58m

0

--0 ..... %-" __ -_

- l/r 2-Distance Law7 %---

• ' ' I ' ' ' ! ' ' '

18000 20000 22000

FL IGHT HEIGHT (FT )

Fig. 9 As measured maximum A-weighted overall fly-

over noise levels versus flight height

g8

corresponds to the A-weighting attenuation at a frequency of i00

Hz to 125 Hz which happens to coincide with the respective blade

passing frequencies of both aircraft. Already at this stage of

data analysis, one may safely conclude that flyover noise signa-

tures are entirely governed by the blade passing frequencies.

6.2 Sound level time-histories

In order to select appropriate instances in flyover time for la-

ter spectral analysis it is necessary to initially plot overall

level time histories. Such information is presented in Appendix

II both in terms of linear (time constant "fast") and A-weighted

(time constant "slow") overall level time-histories.

Typically all of these histories exhibit level fluctuations

which range up to 15 dB (!) for the representations of overall

linear levels. Two explanations may be offered: There are either

atmospheric effects during sound transmission over long distan-

ces, or sound beats due to the superposition of sound signatures

originating from two noise sources (propellers) radiating at

slightly different frequencies (rotational speeds).

To definitely prove that in fact sound beats are the reason for

these (periodic) level fluctuations, some more analysis is neces-

sary: If two pure-tone noise sources with identical pressure am-

plitudes Po are considered, one operating at a circular frequen-

cy of W 1 and the other at _2 ' the time history of the combined

pressure amplitude may be written as follows:

(I) p = 2 Po ' cos [ (ALJ/2)'t ]. cos (Wl't)

(with _ = _I- _2 )"

From this equation it is obvious that the pressure amplitude

may be doubled (a +6 dB) or tends to zero (_ minus _ dB) as a

periodic function of time corresponding to the cosine of the

beat frequency which is defined as

99

(2) 5os=ALd/2 -- 2 _/t s.

Now the effect of such beats on different source frequencies maybe determined as a function of propeller rotational speed from

the relation

(3)

and thus

(4)

= 2_ fHarm. = 2 _ (N/60).BLN.HN

_ = 2 _ (_N/60)'BLN.HN

From eqs. (2), (3) and (4) the time period of pressure fluctua-

tions may be calculated as

(5) t S = 2 _ /(_/2) = 2/[(_N/60). BLN.HN]

exhibiting faster repetitions in time of pressure minima and ma-

xima with increasing source frequency, i e. for higher harmonic

numbers HN. It is this particular feature of pressure level

fluctuations which allows the distinction between the stochastic

effects of long #ange Sound transmission through a turbulent at-

mosphere and the periodic effects of noise beats.

in 6rder £0 demonstrate £hat effect from the measured data, it

is necessary to compare time histories of different rotational

harmonic levels. Such analysis, however, is somewhat difficult

because of the Doppler-shift in frequency with flyover time. As

will be shown later, tracking filter techniques could not be ap-

plied Since - as a result of beats and the marginal signal-to-

noise ratio - harmonic levels frequently submerge into the back-

ground noise floor. Therefore flyover signatures were analysed

in terms of adjacent I/3-octave band level histories with the

fundamental frequency moving (continuously) from the 125 Hz band

(aircraft in approach) into the I00 Hz band and finally into the

80 Hz band for the aircraft receeding from the measuring sta-

tion. When combining such plots (synchronized in time) one may

obtain continuous level time traces at least for the first two

E

m

=

!

100

3.3 sec

1.65 sec

200 Hz --I-lOsec'l

FLYOVER -TIME

Fig. 10 I/3-octave band level time-histories of Metro III flyover No. 5

i_ !Fundamental Frequencyl --16 sec_J

OZ,<

OO!

_9 --160 Hz -,-- •.----- 125 Hz ---,-

[Fokker 50 / No.10 ]

•----- 100 Hz

i

---80 Hz

FLYOVER -TIME

Fig. 11 I/3-octave band level time-history of Fokker 50 flyover No. 10

101

rotational frequencies, which are apart by about i00 Hz and thus

never contribute to the same i/3-octave band level.

An example of such an analysis is presented in Fi 9. 10 for both

the fundamental frequency of the Metro III flyover noise signa-

ture and for the first harmonic level. From a comparison of le-

vel fluctuations in time for both frequencies, the first harmonic

(f _ 200 Hz) exhibits twice the beat frequency value (i.e. half

the corresponding time period) as is observed for the fundamen-

tal frequency, thus proving that level fluctuations are a result

of beats due to sl{ghtly different rotational speeds of both

propellers. From this example a difference in rotational speed

of 9 rpm can be calculated from eq. (5), to be responsible for

these rather significant levei_f!_ctuations.

Similar effects can be observed from the Fokker 50 flyovers. An

example is given in Fig. ii for the fundamental frequency only,

because no harmonic emerges from the background noise floor. In

this case a difference in rotational speed between both propel-

lers of 3 rpm is determined.

6.3 Narrowband spectra

As is obvious from the level time-traces presented in the prece-

ding paragraph, the results of narrowband spectral analysis will

heavily depend on the instant in flyover time selected. To first

demonstrate the variety of spectral characteristics occurring du-

ring one flyover event, to further determine the relevant (D0p-

pler-shifted) values of the fundamental frequency and to thus

attempt a correlation of the fl_over signature_ wi_h noise-_mf_-

sion time (radiation angle ), narrowband spectra (bandwidth

Z_f = 3.125 Hz) were obtained at numerous instances in time for

eachoftheflyovers of the Metro III and the Fokker 50.

For this purpo£e it was feif_to be=_Suf_ficiehtly accura£e {o ob _

rain single sample spectra, manually released and correlated

with flyover time by eye-tracing of a simultaneously created

102

7O

dB ¸

j 6OLU>LU_J

w 5On-

tntn

_0

r_zD 300_n

2O

®

! - _

Ol

®/

__J

I HETRO m/No,51

0 O.t,FREOUENCY

&f = 3.12 Hz

!

0

®

Fig. 12 Narrowband frequency spectra at different

instances in time for Metro III flyover No. 5

wI I ! I ,,1 I

lMEtRom / No.s I10 dB

_t

I I-3o -2o -lo 0 lo 20 sec

Flyover Time

Fig. 13 Overall A-sound level time history of Metro III

flyover No. 5 indicating 11 instances in time

where narrowband spectral analysis was performed

103

plot of the respective overall level time history. Fi_. 12 pre-

sents examples of narrowband spectra as obtained in the course

of that procedure for the Metro III flyover No. 5, indicating a

seemingly chaotic variation of propeller harmonic levels for

different instances in time. Respective times - corresponding to

all samples taken - are indicated in the overall level trace as

presented in Fi_. 13 (in the spectra of Fig. 12 reference is

made to corresponding sample numbers of Fig. 13).

From every spectrum the instantaneous value of the fundamental

frequency is obtained. Its variation with time Can therefore be

checked against the calculated Doppler-shift in frequency. From

basic principles, a frequency shift with flyover time is due to

the relative motion of a source with respect to the observer and

is determined according to

(6) f(t) = fo/(l-M cos _ )

with the elevation angle

(7) = 180 deg - arcctg (v.t/H).

In eq. (7) "negative times" pertain to noise radiated from the

aircrar£ in approach, t_me t is zero for noise radia£ed from

overhead and "positive times" pertains to noise radiated during

departure.

Since the value of the Mach number M in eq. (6) is determined

from the relative speed of the aircraft with respect to the

measuring microphone, the effects of wind speed and idirection at

the flight height must be accounted for. From the meteorological

data-records the wind direction can be determined as near zero

degrees (i.e. from north) and its average magnitude to be appro-

ximately 4 m/s. Since flight No. 5 was conducted from north to

sbu£h, the aircraf£'s speed over ground is obtained by summing up

both IAS (see Fig. 3) and wind speed to end up with a value of

122.3 m/s. To finally determine the corresponding Mach number

the speed of sound needs to be approximated. In order to reduce

i

Z

=

104

T=-5,0C V:122.3M/S H:5791.2M F:182.9HZ

I I,-,160N

n- 150V

I'40)--0

z 130LU

o 120LLI

u_ 110

rv,w 100>

o 90)-_.!

u_ 80-50 0

FLYOVER-TIME

50

(SEC )

i

m

.-L.--

Fig. 14 Comparison of measured and calculated Doppler-

shift in blade passing frequency versus time

for the Metro III flyover No. 5

105

calculation efforts for the purpose of this rather qualitative

analysis an average speed of sound was determined to correspond

to an average (from ground level to flight height) air-tempera-

ture of -5°C.

Following this argumentation and using eqs. (6) and (7), the

calculated frequency variation is plotted in Fig. 14 versus

noise emission time. The correlation of that time-scale with

measured level time histories can now be obtained by time-shif-

ting the measured data points (frequency values) to yieid-a best_

fit between calculated _and-measured curves. From this _ procedure

(which however assumes flight- and propeller rotational speed to

be correctly measured) the absolute time scale had been deter"

mined as indicated on the abszissa of Fig. 13. That figure now

indicates that maximum noise levels are emitted for the aircraft

in approach.

The same type of analysis was conducted for the Fokker 50 fly _

over No. i0 (again with direction from North to South) _yielding

similar results, as presented in Figs. 15 and 16.

Finally some narrowband analyses were performed to check on the

reason of the overall level difference of about 7 dB for the two

Fokker 50 flyovers at 19000 ft height (No. 9 and No. 10).

Fig. 17 presen£s Spectra which per{ain to approximately corres-

ponding maxima and minima of level time-traces from flyovers No.

9 and No. i0. The observed difference in overall levels is domi-

nated by level differences at the fundamental frequency. Since

no contribution of extraneous noise sources can be detected from

the spectra, no explanation other than strong atmospheric effects

on noise transmission can be offered as a reason for this signi-

ficant level difference.

B

=

z

106

i

==

I I

I = = I VFOKKER NO. I

__dB i so/ _om

- "iJI I I I I I

-/,0 -30 -20 -10 0 10 $ec 20

Flyover Time

Fig. 1 5 Overall A-sound level time history of Fokker 50

flyover No. 10 indicating 10 instances in time

where narrowband spectral analysis was performed

N

-r

v

>-

LD

ZIll

LU

rY

U_

tYLU

ED

>-.J

U_

T=-5.6C

1E;8

158

148

138

128

118

188

98

88

Fig. 16

V:148.3M/S H:5791.2M F: 102.5HZ

I No !

X ,,,1 P

<CA'CULAT'E_N

-58 8 58

FLYOVER-TIME <SEC>

Comparison of measured and calculated Doppler-

shift in blade-passing frequency versus time

for the Fokker 50 flyover No. 10

[ FOKKER 50

Fig. 17 Comparison of narrowband |pectre at differ-

ent instances _n time for Fokker 50 flyover

NO. 10 (upper row} with spectra taken at

corresponding times but for flyover No. 9

(lower row) at the identical flyover height

ORIGINAL PAGE IS

OF POOR QUALITY

107

7. Conclusions

Since this report is thought as an initial contribution to a

reference data base which will allow judgment of the extent of

annoyance caused by propfan powered aircraft, no final conclu-

sions should yet be drawn from the results. However, two obser-

vations should be emphasized which are thought as typical for

propeller powered aircraft noise immission:

- First, the propeller rotational noise fundamental (at a fre-

quency of about 102 Hz) dominates the overall en-route noise

level and thus yields an "attenuation" of almost 18 dB due to

the A-weighting' This might be considered a problem since the

A-weighting function is suspected to not correctly simulate

the human noise perception at low frequencies.

- Second, noise beats were found to cause periodic A-sound level

fluctuations in the order of 5 dB, due to inadequate or alto-

gether missing synchronization of propeller rotational Speeds.

Such effects are felt to represent an additional annoyance

factor and efforts should therefore be undertaken to solve

this problem for future propfan powered aircraft.

Increasing complaints about aircraft en route noise shows the

necessity to judge en route noise characteristics of advanced

propfan powered aircraft. Such new type aircraft are expected to

bein service within the next few years. For this purpose a n exten-

sive _ata base on en r0ute noise levels of conventional turbo-

prop aircraft is _eeded. Respective measurements have been un-

dertaken on two twin-engine turboprop aircraft at different

flight heights. Noise data are presented together with operatio-

nal parameters and meteorological data. No noise level correc-

tion has been performed with respect to environmental parameters

influencing noise generation and transmission through the atmos-

phere. Data analysis is performed in terms of overall linear and

108

A-weighted noise level time histories. Corresponding level maxi-

ma are listed for two microphone arrangements, i.e. using a

ground board and a 1.2 m pole. Examples of narrowband spectral

analyses are presented to demonstrate the characteristic fea-

tures of noise signatures, namely the dominance of the low fre-

quency propeller rotational noise fundamental andtheloccurrence

of noise beats due to different rotational speeds of the two

propellers. This latter effect causes periodic A-sound level

fluctuations of up to 5 dB.

9. Acknowledgment

The measurement campaign was initiated by Herr Held, Noise

Abatement Commissioner of the Hessian Minister for Economics and

Technology at Frankfurt Airport , and funded by the Flughafen

Frankfurt Main AG . Herr Held perfectly organized the coopera-

tion between the Hessisches Landesamt f_r Umwelt , Deutscher

Wetterdienst and the aircraft flight crews. The permission gi-

ven to DLR to take noise data at the same time is highly appre-

ciated.

i0. References

[i] Dobrzynski, W. Interferenzwirkungen durch Bodenrefle-

xionseffekte bei Flugl_rmmessungen an

Propellerflugzeugen.

DFVLR-FB 81-28, 1981.

Ground Reflection Effects in Measuring

Propeller Aircraft Flyover Noise.

Techn. Transl. ESA-TT 742, 1982.

[2] Dobrzynski, W.

Heller, H.

Powers, J.

Densmore, J.

DFVLR/FAA Propeller Noise Tests in the

German-Dutch Wind Tunnel DNW.

(6 Appendices)

DFVLR-IB 129-86/3, 1986

FAA Report No. AEE 86-3, 1986.

I09

|

|

i

i

APPENDIX I

Detailed listing of

meteorological data versus height

111

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2.00 56 5.4 261 282

7 2.20 63 5.0 302 322

8 2.40 7_ _.1 344 _66

3.00 73 5.4 _86 407

10 3.20 74 5.5 428 450

11 4._0 75 5_3 490 531

12 4.20 7& 5.2 552 573

13 4.40 74 5 2 594 616

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56 19.00 358 ?.8 2538 25_3

57 19.20 _ 8.6 2586 2609

=

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=_

=

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112

ORIGINAL PAGE IS

OF POOR QUALITY

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ORIGINAL PAGE I$

OF POOR QUALITY

113

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ORIGINAL PAGE ISOF POOR QUALITY

115

E

LL

Z

L--Ez

z

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APPENDIX II

AS measured overall noise level time histories

PRECEDING PAGE BLANK NOT FILMED

B'IG[_l NTEN]'iONAi,'J-I' BLANK

117

Type of Aircroft: Metro 1Tr Flyover No. :

Microphone Position: Ground-board Microphone

1

>

,..I

r-D0

U3

IN,

C,w

.J

m

>

.J

DO

1"

,w

3I

<

80-

dB

70

6O

5O

60

5O

/.0

3O

Flyover-time

!-,--30 sec _-IFlyover-time

As measured overall level time-histories (Metro III

climb out)

118

=- .

Type of Aircraft: Metro TT[ Flyover No :


2

i

>

_.1

c":DOL/')

I--

O

Co--

_.J

80-

d8i

7O

in Jl • •• nmt • il J t,

_1 17 II1| • 11J • II1_ 1 I1 IIkF N. 111 l]

II il INlil II ii r

I1_ lilrll i,i i il |l i • •II IJIIU • l UP ILl !I "Jl_ •

III Ill " • • • • ! il ]'rll ill 1 • r ! Ji IF l_

II1 • _ ! " ! • I 2-

r'l,II I I I II I [

5O

Flyover-lime30 sec

w

>

.J

"Ui'-:3OV")

.C::

,m

G;

I

70-

riB(A)

60-

5O

_0

3O

Flyover- timeI"-30 sec

As measured overall level time-histories (Metro III/

No. 2, flight height: 17000 ft)

119

Type of Aircraft: Metro 1T[ Flyover No. :


3

!

>ea

._1

C

0

L

0

E

QJ>

0O_

qJ

r-

qJ

I

80

dB

70

6O

SO

Flyover-time30 sec

70-

dB{A}

60 • i

5O

_0

3O

Flyover- time30 sec

=

t



120

Type of Aircraft: Metro M Flyover No. : &.._.___

Microphone Position: Ground-board Hicrophone

>

...I

"IOr-:3ou3

=L_

0

C

._l

80

dB

70

6O

5O

Flyover-timeI-.- 30 sec _t

u

>

.._t-:30

U3

r"

.m

!

.<

7O

dB(AI

6O

5O

z.0

3O

Flyover- timet--- 30 sec ---1



121

Type of Aircraft: Metro TTT

Microphone Position: Ground-board

Flyover No. :

Microphone

m

_.I

"0

0O0

i_

C3

t-

..J

m

_J

"0C

0

"00j

r-C_

!

80

dB

70

60

50

70

dBIA)

60

50

Z.0

30

Flyover-time30 sec

__- ..... -T-__ '-"o.-,-1_-

_'11 - q I

, ,,,,, , ,

l i rl --_- _, ..............

t_- 30 sec --4

Flyover- lime

=

E

=



122

Type of Aircraft: Metro M Flyover No. : 6


m

>_U

.J

C:30t;3

L-

0Q;C

.-I

80-

dB

70

60

5O

Flyover-time30 sec --I

m

_.1

C

0

ii

!

'7O

dB(A)

6O

5O

/.0

3O

Flyover- timeI--- 30 sec -_



123

Type of Aircraft: Metro Tl1'


Flyover No. :

Microphone

7

m

>

.J

C

0U3

L_

0

C

_J

i

aJ>

_J

r

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.C

,m

I

.<

80

d8

70

6O

5O

70

dB(A)

60

5O

Z,0

3O

Flyover-limeI-.- 30 sec -.-I

!--- 30 sec ---IFlyover- lime

2

m



124

Type of Aircraft: Fokker 50 Flyover No. :


8

m

>

...I

t-

OU3

O(IJE:

,m

_..I

80 _-

dB

70

60

5O

Flyover-timeI-,-30 sec --_

m

¢J

C

OU3

(IJ

I,<

?0

dB{A)

60

5O

_0

30 _ 30 sec --_

Flyover- time

As measured overall level time-histories (Fokker 50/


125

Type of Aircraft: .Fokker 50


Flyover No. :

Microphone

g

m

>_J.J

D0

U_

L

0

c"

_J

!

_J

0tO

r

I

dB

7O

60 Ill _" "ldL 11 !1| • _1. JIJ[_l • _

• r_ll U if rhll,ad! 111Ik mll._ m -- " • I I I-- 1_i

i i U _ I I i I ",i_l I.

5O

Flyover-time1-.- 30 sec ----[

7O

dB(A}

60

5O

/,0

Flyover- time

i|==

mm

__=

E



126

Type of Aircraft: ,Fokker 50


Flyover No. : I__._0_.0

Microphone

i

>

_.I

C

0U_

I--

C

_J

80

dB

70

6O

5O

Flyover-timeI-.- 30 sec --I

m

>

_J

C

0U_

r"

.i

I

70-

dB(A)

60

50

/.0

3O

Flyover- time

I----30 sec --_



127

Type of Aircraft: F_okker 50


Flyover No. :

Microphone

11

80-

i

¢' dB>

70

0or)

I--

0

CmI

-J

• i _! • = .......

6O

Flyover-lime!...- 30 sec ---I

i

q,I>

_1

'10

OU3

70

dB(A)

6O

50

"10

t-

ei

3=I

_0

3Ot-.--30 sec -.-I

Flyover- time

=

r

L

As measured overall level time-histories (Fokker 501


128

Type of Aircraft: Fokker50

Microphone Position: (3round-board

Flyover No. : I.__2._2

Microphone

m

>_J..I

CDOU_

6

O

_2

80-

dB

70 ;

60LL

5O

Flyover-timeI---30 sec -_1

w

>

_2

CDO

_D

4=

w_

!<

70-

dB(A}

60-

50

40

30

Flyover- timeI-,-30 sec---I

As measured overall level tlme-histories (Fokker 50/


129

2=d

= 7= _ ÷ :

m_

--L

r_

z

DATA REPORT: EN ROUTE NOISE OF TWO TURBOPROP … · TWO TURBOPROP AIRCRAFT Werner Dobrzynski...

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Transcript of DATA REPORT: EN ROUTE NOISE OF TWO TURBOPROP … · TWO TURBOPROP AIRCRAFT Werner Dobrzynski...