Bringman, J. M.

Mccormick, B. W.

English

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UNDERWATER AND AIRBORNE NOISE
PRODUCED BY A GROUND-EFFECT
MACHINE HOVERING OVER WATER
by Barnes W, McCormick and Joseph M. Eringman
Prepared by
PENNSYLVANIA STATE UNIVERSITY
University Park, Pa.
for
N A T I O N A L A E R O N A U T I C S A N D S P A C E A D M I N I S T R A T I O N W A S H I N G T O N , D. C. M A R C H 1967
https://ntrs.nasa.gov/search.jsp?R=19720063724 2020-03-23T09:45:32+00:00Z
UNDERWATER AND AIRBORNE NOISE PRODUCED
BY A GROUND-EFFECT MACHINE
HOVERING OVER WATER
By Barnes W. McCormick and Joseph M. Bringman
Distribution of this report is provided in the interest of
information exchange. Responsibility for the contents
resides in the author or organization that prepared it.
Prepared under Grant No. NGR-39-009-033 by
PENNSYLVANIA STATE UNIVERSITY
University Park, Pa.
for
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
IUNDERWATER AND AIRBORNE NOISE PRODUCED
BY A GROUND-EFFECT MACHINE
HOVERING OVER WATER*
By Barnes W. McCormick and Joseph M. Bringman
SUMMARY
Airborne and underwater measurements have been made of the noise produced
by a peripheral-jet ground-effect machine (GEM) hovering over a water surface.
The machine, approximately 20 feet long, 10 feet wide and pointed at both ends,
was tested at gross weights of 2500 pounds and 2900 pounds at 80, 90, and
100 percent of rated fan speed. The data were analyzed over a frequency spec-
trum up to 30 kcps. The levels in a 1-cps band width decrease with increasing
frequency at approximately 6 dB per octave but are relatively insensitive to
loading or power within the limits tested. Based on these results and earlier
model tests, a tentative empirical equation is presented for predicting the
underwater noise level .of such machines.
INTRODUCTION
The National Aeronautics and Space Administration and other research groups
have accumulated a considerable amount of data on the fundamentals of ground-
effect machines (GEM). These data relate primarily to the performance, sta-
bility, and control of such craft. Since a GEM could be used for antisubmarine
warfare, the noise characteristics are also of interest, particularly when these
craft are operating over water. The amount of noise data that has been reported
to date is very limited. These data, presented in reference 1, were obtained
for a model with a 2^-inch diameter peripheral jet operating at heights from
1 to 2k inches above the water surface. Measurements of the underwater noise
spectrum are reported for frequencies from 5 "to 15 kcps.
The purpose of the present study was to extend the results of reference 1
to a full-scale GEM. This machine, designated GEM III, is described in detail
in references 2 and 3- Both underwater and airborne noise measurements were
made at frequencies up to 30 kcps on this machine operating over water. The
present tests were conducted jointly by the Ordnance Research Laboratory of
The Pennsylvania State University and the NASA Langley Research Center.
AQ thrust augmentation factor, weight of machine divided by thrust of
jet, W/T
dB noise level in one-cycle "band width relative to 0.0002 dynes/sq cm
D effective diameter of GEM (based on area of base of machine), ft
f frequency, kcps
h height of GEM base above surface, in.
q^ jet dynamic pressure, psfj
Q flux of air through machine, ft5/sec
T jet thrust, Ib
W weight of machine, Ib
TEST APPARATUS AND PROCEDURE
The ground-effect machine used in this investigation is shown in figure 1.
It is approximately 20 feet long and 10 feet wide and pointed at both ends. The
primary nozzle around the periphery of the machine is 3-1/2 inches wide and has
a total length of kQ feet and 'h inches. Secondary nozzles for stability aug-
mentation are 2 inches wide and have a total length? of 2h feet and ^ .5 inches.
The primary nozzle is directed inward at an angle of ^ 5°. For these tests, the
normal gross weight of the machine was 2500 pounds. An additional kOO pounds
were added to this weight for some of the tests.
' »'
At 100 percent of the rated turbine speed the two 12-blade fans rotate at
2020 rpm at a turbine speed of k-0,kOO rpm. In addition to the 12 fan blades,
each nacelle also has 9 stator blades.
The radiated-noise measurements of GEM III were made at a reservoir near
Hampton, Virginia. Two pickups were used; a Brush Type 4133 microphone with a
wind screen was used to pick up the airborne noise, and a Massa Type 115B
hydrophone was used to pick up the underwater noise. The microphone was mounted
on a tripod on the bank of the reservoir at a distance of about 150 feet from
the track of the GEM. The hydrophone was lowered to a depth of 7 feet and sus-
pended from a small buoy. The hydrophone cable was supported by small floats
about 5 feet apart. Figure 2 illustrates the relative position of the GEM and
the monitoring equipment.
The buoy outboard of the hydrophone buoy was
and the GEM was guided past it at ranges of 5 to 25
: : :• •**': :': •" • " : :•• •
•• •.. • • J.r • • • i. •
were amplified and recorded on a two-channel magnetic-tape redttrder* •«/? diagram I
of the recording system is shown in figure 3-
Calibration signals were recorded at the beginning and end of each test
period. Background levels in water and air were recorded at regular intervals
to determine the signal-to-noise ratio. In the results presented herein, all
of the signals were well above ambient noise.
The analysis system is shown in figure U. Analyzed data were corrected
for the frequency response of the system, roll-off of the recorder, and for the
range to the GEM.
Several runs were made under each operating condition and from both port
and starboard aspects of the GEM. There was little difference between the noise
levels recorded from port and starboard aspects; therefore, the results were
averaged.
Runs were made at 80, 90, and 100 percent of rated fan speed and at gross
weights of 2500 and 2900 pounds. Some of the data was analyzed to 60 kcps, but
no spikes or line components existed above 30 kcps.
RESULTS AND DISCUSSION
Measured noise spectra, both underwater and airborne, are presented in
figures 5 through 10. Here the levels in an effective 1-cps band width relative
to an acoustic pressure of 0.0002 dynes per square centimeter are presented as a
function of frequency. The levels are shown corrected to a distance of one yard
from the center of the machine assuming a change of 6 dB per double distance.
The use of 0.0002 dynes/sq cm is standard procedure for underwater noise
analyses and is done here, also for the airborne noise for the purpose of
comparison.
Figure 11 presents the actual spectrum analyses of one run. The recordings
of all runs were analyzed and all spikes which appeared to be pure-tone were
read-out and a tabulation of the amplitude and frequency of the pure-tone spikes
are presented in table I. Very few pure-tones appeared in the lower frequencies.
The only ones noticeable in all analyses were spikes occurring at 60, 120,
and l80 cycles. These frequencies are in the output of the tape recorder and
are present in all of the recordings, including the ones made of the ambient
noise levels before and after the noise recordings. It is interesting to note
that most of the line components are found in both the air and water spectra;
however, the amplitude in air is about twice that measured in the water.
The results of reference 1 combined with the present data suggest that the
acoustic pressure received at the hydrophone is directly proportional to the
total flux of jet momentum and inversely proportional to the diameter of the
jet. The following relationship is offered as a tentative basis for predicting
the underwater noise levels of ground-effect machines.
*—:-^ _.-.-
dB = 20 log i - 16 k-
D
_ 20 log
10,000 39.5
In words, equation (l) states that the noise level increases by 6 dB per
double jet momentum, decreases by 6 dB per double diameter, decreases by l6 dB
per diameter increase in height and decreases by 6 dB per octave increase in
listening frequency. The constant of proportionality was actually obtained
from the model results of reference 1 and was found to apply closely to the
results obtained with the much larger scale GEM III.
Based on the results of reference 2, the estimated operating characteristics
were calculated for GEM III and are given below.
Percent
rated
fan
speed
100
90
80
psf
13.71
11.11
8.77
AC
W = 2500 Ib
5-7
7.0
8.9
W = 2900 Ib
6.6
8.2
10.3
h, in.
W = 2500 Ib
9.0
6.5
W = 2900 Ib
7.1
5.0
Using a pseudo-diameter of 1J.82 feet (based on the GEM III base area) and the
fact that the weight was equal to the product of T and AQ, predictions of
the noise levels for GEM III were made on the basis of equation (l). Typical
comparisons between measured and predicted noise levels are presented in fig-
ures 12 through 1*4-. From these figures it would appear that equation (l)
satisfactorily accounts for the effect of size and weight in the underwater
noise of a ground-effect machine. The complete calculations are indicated in
the appendix.
APPENDIX *.. ;.. ;•• *. *. ; ;•
** •• • • |
CALCULATIONS TO PREDICT THE NOISE LEVELS OF GEM III
The thrust augmentation curve for GEM III as obtained from reference 2 is
given in figure 15. The jet dynamic pressure must be proportional to the weight
of the machine divided by the augmentation factor. From an operating point
given in reference 2, h = -ik in., W = 1850 Ib, AQ = k.l and q^ = llj-.l psf.
Hence,
qj = 0.03125 1L psf (2)
For the same point, the total flux Q, through the machine was 1760 cfs. Hence,
» -yi i .
For 2500 pounds and full fan speed, the hovering height was measured at
9 in. From figure 15 and equations (2) and (3):
q. = 13.71 psf
d
Q = 1735 ft5/sec
The hovering heights were not measured for other gross weights and rota-
tional speeds. However, if q. is assumed proportional to the square of the
rpm, then one can write:
2
_ /Percent rated rpm\q^ — Ly. [11 J
J
 \ 100 /
Hence, by the use of equations (2) and (k) and figure 15, one can calculate the
hovering height h; for example, at 80 percent rated fan speed and a gross
weight of 2500 pounds.
From equation (4) q* = 8.77 psf. From equation (2), the augmentation fac-
tor must be equal to 8.9 which gives an operating height h, from figure 15, of
U.30 in. The pseudo-diameter, based on the base area for GEM III is 13.82 feet
so that, for this case, =r = 0.02.6. Thus, at a distance of one yard at a fre-
quency of 10 kcps, the predicted noise level would be (from equation (l)):
cffi = 20 log 2222 16 x 0.026 - 20 log 10'000 + 39-5
13.82 x 8.9 10,000
• •• APPENDIX
• •
or
dB = 65.2
By comparison with figure 5, this is seen to be two or three dB lower than
the measured value.
REFERENCES
• •
1. Carter, A. W.; McCormick, B. W.; and Bringman, J. M.: The Underwater Noise
Produced by a Model of a Ground-Effect Machine Hovering Over Water. NASA
TM X-836, 1963.
2. Johnson, A. F.; and Chaplin, H. R.: Results of GEM III Tethered Tests.
DTMB Rept. 15^ 6, Aero Rept. 1012, Aug. 1961.
3. Johnson, A. F.: Phase II Tethered Tests and Low-Speed Free Flight Tests of
GEM III. DTMB Rept. 1700, Aero Rept. 10^ 9, Dec. 1962.
_• g_
• ••
TABLE I
AMPLITUDE AND FREQUENCY OF PURE-TONE SPIKES FOR GEM III
Gross
weight,
lb
2500
2500
2900
Run
7
10
4
100 percent fan speed
Hydrophone
Freq,*
kcps
0.82
• 97
1.25
2.38
7.80
11.60
15.20
22.80
0.1)0
•52
.81
l.6k
ll.liO
15.00
25.80
0.84
1.26
2.40
Ik. 60
15.2023.30
Ampl,
dB
9.0
3.0
4.0
3-5
1.5
8.0
7.0
6.0
4.0
7.0
4.5
2.0
11.0
6.0
6.0
10.0
5.0
11.0
9.0
5.0
8.0
Microphone
Freq,*
kcps
1.65
2.80
7.80
11.40
15.20
7.80
ll.ltO
15.20
0.84
.96
1.21;
1.37
1.60
2.70
2.84
7-75
11.50
15.20
Ampl,
dB
5-5
5.0
8.0
8.0
20.0
6.0
20.0
14.0
7.0
10.0
16.0
7.0
5-56.0
6.0
5.0
19.0
18.0
Run
2
8
5
90 percent fan speed
Hydrophone
Freq,*
kcps
0.90
1.02
2.1)0
2.55
10.60
14.00
20.90
0.72
1.10
10.30
13-70
20. 40
1.17
2.38
2.50
10.30
13.80
20.40
27.60
Ampl,
dB
4.0
6.0
4.5
4.0
5.0
4.5
4.0
9.0
4.0
5.0
7.0
4.0
3-0
4.0
5-0
7-5
7.0
4.5
1.5
Microphone
Freq,*
kcps
0.75
1.03
1.50
10.20
14.00
20.80
27.50
0.74
1.10
1.23
2.55
10.30
13.80
20.40
27-30
0.86
1.12
2.55
7.10
10.40
13.80
20.50
27.60
Ampl,
dB
6.0
7-5
4.5
15.0
12.0
12.0
6.0
5-5
11.0
5-5
5-0
15.0
16.0
15-0
7.0
5-0
4.0
4.0
4.0
16.0
20.0
6.0
8.0
Run
3
9
6
80 percent fan speed
Hydrophone
Freq,*
kcps
2.40
2.50
9-20
12.30
14.80
19.00
9.30
12.20
14.60
18.40
24.50
2.38
2.50
2.72
9.40
12.20
14.60
18.60
24.30
Ampl,
dB
1.5
2.0
3.0
7-0
5.0
6.0
4.5
8.0
4.0
7.0
2.5
5-5
3-5
2-5
3-0
7.0
5-0
6.0
2.5
Microphone
Freq,*
kcps
0.65
1.02
1.30
9-30
12.40
18.30
24.30
1.12
1.35
2.20
9OO
12.40
18.30
24.60
0.68
1.32
2.28
7.10
12.20
12.40
18.20
24.30
24.80
Ampl,
dB
7-5
3.0
4.0
9-0
19.0
11.0
17.0
6.0
4.5
5.5
9.0
19.0
11.0
17.0
15.0
7.0
5-5
8.0
14.0
8.0
14.0
8.0
5.0
Frequency accuracy: percent.
8
• ': '." •" •' .— : :•• •
••• • • •• • j. •> • « . ••• ••• •
•• • . i • •••• •<
lO'O"
A-A
Main Nozzle
Secondary Nozzles
Spoiler Flap
Figure 1.- Principal dimensions and general arrangement of GEM III.
-9
.-.,.^.^:_:^ :^^:1:_!_L:_!U.!
• •:: •• • •'.. ••• ••"• -
• ••• *
5Q
' AMDI icviMr ' I ' ' GENERAL PATHAMPLIFYING ' • I ^.c r-c\» i
AND RECORDING I
 t OF GEM
.EQUIPMENT HYDROPHONE POSITION- (
' SUSPENSION ' MARKER
BUOY \ I BUOY FOR GEM
o o o o o o o o o o o o o o o o o ( ~ )
I . ' .,' I I\
Figiore 2.- Relative positions of microphone, hydrophone, and GEM.
10
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Underwater and airborne noise produced by a ground effect machine hovering over water

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