Page 1
. :24856
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
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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.
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Page 3
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
Page 4
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
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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
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Page 6
Fig. 2 Fokker 50 aircraft
88
Page 7
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
Page 8
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
Page 9
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
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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$/TA5 /9_" / 2_O[ of RID FL 190 [ Temp.: -20 "I northbound Z 2. Z_ z I Clouds: cc_ .
[ Power:Ii °n R 359/179 RID I
!13DME north [ IAS/TAS /9Q / 130 kts I
x_/! of RID FL 190 ! Temp.: -ZO II southbound 22_7: Z I Clouds: C_.E__
R
I Power: m o_f)!
IuL- .... t.
I 4_...... t,, i Z^_/'T_S -2_/_" / 2.3Z kts- I[ of RID FL 210 [ Temp,:
[ Power :"_'_I on R 359/179 RID ! anti ice: ¢_¢_C6G7- _f_,_
I !
13DME north I IAS/TA5 I /_O/ _ ]__ kts i
_1 o_ RID FL 210 Tem.: -2Gsouthbound I 2 _'/ Z Clouds: C£E2A_on R 359/179 RID
Pos. 4DME southof RID, FL 210 2_ -_'v
End of testflightrequest clearanceto Frankfurt
m
Fig. 5
Power: _-o _6F f7 %anti Ice: e_
Data sheet as filled out by the Test-pilot of the
Metro III aircraft
b
90
ORIGINAL PAGE IS
OF POOR QUALITY
Page 11
XnflJ_ht - l_fo - Sheet
Type o_ literary: _l_4tv 50
Type o! engine :_" dZG._
T.O.W. : Ig _gg kg
ATO , _.00 ZPos. 7O_Z R_D R 359 ... o.t,:
• .7.'.o._xlc.
4DF'_ sou_hof EIO rL 170nocthboundon R 359/179 RID
13DME northoE RZD TL 170southboundon R 359/179 RID
4DM£ southof RID FL 190northboundon R 359/17g RID
13DM£ noc=hof RID ?L 190louthboundon R 359/179 RIO
4DME southof RID FL 210northbcundon R 359/179 RID
13DME northOf RID FL 210southboundon R 359/179 RID
Pos. 40ME south
of RID, rL 210
End of testflightrequest clear,neeto Frankfurt
23i0z
ZsfT:
232 :
Zl_i z
IAS/TXS Z I, _s / 17Temp.: -t_"Clouds: c_vo_
inti ice: _ off
zxs/_xs 2_l_s / 285Temp.: - I&"Clouds: £_voll
an_i Lee'. _ off
iAS/TAS 20q V,_, /2_ ITemp.: -20"
Clouds: C_ v$_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
Page 12
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
Page 13
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
Page 14
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.
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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
Ground Mic 1.2 m Mic
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)
Ground Mic 1.2 m Mic
51.0 48.5
53.9 51.1
46.9 43.5
45.9 43.9
46.6 44.0
Lma x (Fast) dB
Ground Mic 1.2 m Mic
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
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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
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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
Page 18
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
Page 19
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
Page 20
(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
Page 21
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
Page 22
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
Page 23
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
Page 24
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
Page 25
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
Page 26
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
Page 27
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
Page 28
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
Page 29
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
Page 31
APPENDIX I
Detailed listing of
meteorological data versus height
111
PRECEDING PAGE BLAr_K NOT FILMED &¥_E/,/C INi'[_TIONAIaLT BL_
Page 32
R, chtunq u. _..£hw.qeqlaettet
Pur,k t Zez t DDD/Gr. FF/m/s Hoer, e-m= Hoehe-aO$
• 0.00 20 1.0 • •
1 0.20 23 2.7 27 54
2 0.40 _L 5.0 75 96
!.00 34 6.4 119 143
4 1.20 _0 6.5 170 198
1.40 49 _.0 219 241
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
14 5.00 72 5.6 637 659
15 5.20 70 6.1 600 701
lb 5 40 70 6.2 722 744
17 6.00 69 6.0 761 779
IB 6.20 _O 6.1 800 021
19 6.40 68 6.1 843 866
20 7.00 69 6.0 884 902
2i 7.20 69 b.O 923 945
22 7.40 68 5.8 965 986
23 e.0o 66 5.5 1008 1030
24 8.20 63 5.1 1051 1073
25 8.40 62 4.6 1094 1115
26 9.00 60 4.3 1136 1158
27 9.20 57 4.0 1177 1197
28 9.40 56 3.9 1221 1246
29 10.00 54 3._ 1266 1287
30 10.2_ 52 3._ 1308 1329
31 10.40 52 3.8 1352 1375
32 11.00 51 3.6 1396 1417
33 11.20 48 3.5 1441 1466
34 11.40 40 3.6 1488 1510
35 i2.00 33 3.q 15_5 1561
36 I_.20 33 4.2 1582 1603
37 12.40 34 4.3 1627 1652
]8 13.00 32 4.6 1671 1691
39 13.20 31 5.1 1714 17_7
40 13.40 31 5 6 17_2 1788
,11 14.00 29 5.7 1e11 1835
42 14.20 2_ 5.7 1857 1880
4_ 14.4@ 24 5.9 1904 1928
44 15.00 23 6.2 1952 1977
45 15.20 19 &.5 2002 2027
4_ 15.40 13 _.8 2049 2072
47 I£.00 9 7.0 2096 2121
_8 16.20 5 ?.I 2145 2169
49 16.40 2 7.2 _193 2217
_0 17.00 _57 7.4 2242 22_8
51 17.20 354 7.6 2291 2315
52 17.40 352 7.5 2342 2370
53 1_.00 3_0 7.7 2394 2418_
54 1_.20 350 7.8 2442 2467
55 18.40 353 7.6 2490 2513
56 19.00 358 ?.8 2538 25_3
57 19.20 _ 8.6 2586 2609
=
=
=_=
=_
=
z_
112
ORIGINAL PAGE IS
OF POOR QUALITY
Page 33
Punkt
58
59
6061
626364
65
6667
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697071727]7475767778798_81
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848_86
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COD/Or
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7.9
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5.7
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5.6
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7.47.9
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ORIGINAL PAGE I$
OF POOR QUALITY
113
Page 34
121122
123124
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135136137
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61726214 +626663106554
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=
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114
Page 35
gr_es$_e2m _0.04.8q 02.10 me_= nr. 2
p_t ..... ppp ..... tt---t#t4--r_ ..... td--tOOt_f'--h,ue.gr--Ot/100m--Oemerk.
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3 996.0 7.1 4.2 b3 0,5
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5 975.0 7.3 4.0 59 -O.Z
6 9a0.0 _.5 3.5 61 -0.4
7 947.0 5,7 3.0 64 -0.5
B 917.0 3.8 I. I 62 -2.8
9 881,0 0.7 -0._ 76 -_.0
10 828.0 -3.8 -4,3 91 -5.1
11 601.0 -&.O -6.5 90 -7.4
I2 780.0 -7.3 -7.9 87 -q.l
13 753,0 -9,9 -9.9 100 -9.9
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9.1 3412 1.24 e|S
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ORIGINAL PAGE ISOF POOR QUALITY
115
Page 37
APPENDIX II
AS measured overall noise level time histories
PRECEDING PAGE BLANK NOT FILMED
B'IG[_l NTEN]'iONAi,'J-I' BLANK
117
Page 38
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
=- .
Page 39
Type of Aircraft: Metro TT[ Flyover No :
Microphone Position: Ground-board Microphone
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
Page 40
Type of Aircraft: Metro 1T[ Flyover No. :
Microphone Position: Ground-board Microphone
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
As measured overall level time-histories (Metro III/
No. 3, flight height: 17000 ft)
120
Page 41
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
As measured overall level time-histories (Metro III/
No. 4, flight height: 19000 ft)
121
Page 42
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
=
As measured overall level time-histories (Metro III/
No. 5, flight height: 19000 ft)
122
Page 43
Type of Aircraft: Metro M Flyover No. : 6
Microphone Position: Ground-board Microphone
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 -_
As measured overall level time-histories (Metro III/
No. 6, flight height: 21000 ft)
123
Page 44
Type of Aircraft: Metro Tl1'
Microphone Position: Ground-board
Flyover No. :
Microphone
7
m
>
.J
C
0U3
L_
0
C
_J
i
aJ>
_J
r
0U')
.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
As measured overall level time-histories (Metro III/
No. 7, flight height: 21000 ft)
124
Page 45
Type of Aircraft: Fokker 50 Flyover No. :
Microphone Position: Ground-board Microphone
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/
No. 8, flight height: 17000 ft)
125
Page 46
Type of Aircraft: .Fokker 50
Microphone Position: Ground-board
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
As measured overall level time-histories (Fokker 50/
No. 9, flight height: 19000 ft)
126
Page 47
Type of Aircraft: ,Fokker 50
Microphone Position: Ground-board
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 --_
As measured overall level time-histories (Fokker 50/
No. 10, flight height: 19000 ft)
127
Page 48
Type of Aircraft: F_okker 50
Microphone Position: Ground-board
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
No. 11, flight height: 21000 ft)
128
Page 49
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/
No. 12, flight height: 21000 ft)
129
Page 50
2=d
= 7= _ ÷ :
m_
--L
r_
z