Table Of ContentACOUSTIC ASPECTS OF ACTIVE-TWIST ROTOR CONTROL
Earl R. Booth, Jr. Matthew L. Wilbur
[email protected] [email protected]
Aeroacoustics Branch, AAA C Army Research Laboratory
NASA Langley Research Center
Hampton, VA, 23681
Abstract
The use of an Active Twist Rotor system to provide both vibration reduction and performance
enhancement has been explored in recent analytical and experimental studies. Effects of active-
twist control on rotor noise, however, had not been determined. During a recent wind tunnel
test of an active-twist rotor system, a set of acoustic measurements were obtained to assess the
effects of active-twist control on noise produced by the rotor, especially blade-vortex interaction
(BVI) noise. It was found that for rotor operating conditions where BVI noise is dominant,
active-twist control provided a reduction in BVI noise level. This BVI noise reduction was
almost, but not quite, as large as that obtained in asimilar test using HHC. However, vibration
levels were usually adversely affected at operating conditions favoring minimum BVI noise.
Conversely, operating conditions favoring minimum vibration levels affected BVI noise levels,
but not always adversely.
Notation and active means of reducing rotor noise. Most active
noise reduction technologies were originally intended as
BVISPL ban@ass acoustic level integrated from vibration reduction technologies and were subsequently
6.5 fop to 40.5 fop, dB shown to be effective innoise reduction as well. Three
blade passage frequency, Hz examples of such technology are higher harmonic
fbp
LFSPL ban@ass acoustic level integrated from control 1-3,individual blade control 4, and active rotor
0.5 fopto 6.5 fop, dB flaps5. In general, active noise control methodologies
NF fixed-system vertical shear force, aligned have exhibited loss of vibration control effectiveness for
with the rotor shaft, lb control inputs that reduce noise, and vice versa. Also,
nP ncycles per rotor revolution, i.e. 5P some of the noise reduction methods tend to increase
q tunnel dynamic pressure, lb/ft2 some types of rotor noise in order to decrease other
R rotor radius, 55 in types of noise. For example, it was found 1'2that HHC
SPL sound pressure level, dB reduced BVI noise, but increased low frequency noise.
V tunnel velocity, ft/s
w/q data scatter parameter, Pa Recent progress in active-twist technology for vibration
X distance downstream of hub center, in reduction and performance improvement made active-
Y distance to port side of hub center, in twist rotor control an attractive candidate to examine as
Z distance above hub center, in a potential noise reduction technology. This work isthe
(Z rotor shaft angle of attack, deg first exploration of active-twist rotor control to reduce
advance ratio, V/t_ helicopter BVI noise. The current results are from a
g2 rotor rotational speed, radians/s previously reported test6to explore vibration reduction
in which simultaneous acoustic and vibration
Introduction measurements were obtained for a large number of
operating conditions. The current work examines noise
Rotorcraft noise and vibration reduction are two and vibration results to determine the extent to which
primary thrusts of current rotorcraft research. Noise active-twist control meets the goal of comprehensive
rotor noise and vibration control.
reduction research has explored the use of both passive
Presented at the American Helicopter Society 58th Annual Forum, Montrdal, Canada, June 11-13, 2002.
Copyright ©2002 by the American Helicopter Society International, Inc. All rights' reserved.
Apparatus and Data Acquisition Figure 2 shows the Active Twist Rotor mounted on the
ARES helicopter testbed in the TDT. For this
Wind Tunnel configuration a four-bladed articulated hub with
coincident flap and lag hinges is used on the ARES.
The Langley Transonic Dynamics Tunnel is a
coupling is minimized. A more detailed discussion of
continuous-flow pressure tunnel capable of speeds up to
the ARES testbed can be found in reference 6.
Mach 1.2at stagnation pressures up to 1atm. The TDT
has a 16-ft square slotted test section that has cropped
ATR Blades. Active fiber composite (AFC) actuators
comers and a cross-sectional area of 248 ft2. Either air
are used to twist the ATR rotor blades. In figure 3,the
or R-134a, aheavy gas, may be used as the test medium.
AFC actuators are shown in conceptually and as the
Forward-flight testing of the ATR was conducted in the
actuator package used in the blade construction. Each
heavy gas test medium at a constant density of 0.0047
ATR blade uses 24 actuators, as shown in the figure,
sl/ft3.
embedded directly in the structure of each blade D-spar,
spanning from 0.30R (30% blade radius) to 0.98R. The
Model Description
actuators are placed in four layers through the thickness
Testbed. The Aeroelastic Rotor Experimental System of the blades and are oriented such that the active strain
(ARES) helicopter testbed, shown in figures 1 and 2, is applied at ±45° relative the blade spanwise axis to
was used for all forward-flight testing. The ARES is permit maximum torsional control of the blades.
powered by a variable-frequency synchronous motor Actuation of the AFCs is accomplished using separate
rated at 47-hp output at 12,000 rpm. The motor is high-voltage, low-current power channels for each
connected to the rotor shaft through a belt-driven, two- blade. The resulting torsion applied to the blade results
stage speed-reduction system. Rotor control is achieved in a controllable twist of the blade. The amplitude of
by a conventional hydraulically-actuated rise-and-fall this blade twist at the blade tip was measured in forward
swashplate using three independent actuators. flight conditions using projection moir6 interferometry 7
Similarly, rotor-shaft angle of attack is controlled by a (PMI) and was found 6 to be 1.1 degrees at 3P, 1.3
single hydraulic actuator. degrees at 4P and 1.4 degrees at 5P for an actuation
amplitude of 1000 volts. In addition, at a frequency of
A six-component strain-gage balance placed in the fixed 3P, blade tip twist amplitude was found to be 0.5 and
system 21.0 inches below the rotor hub measures rotor 0.8 degrees at 500 and 750 volts actuation amplitudes,
forces and moments. The strain-gage balance supports respectively.
the rotor pylon and drive system, pitches with the model
shaft, and measures all of the fixed-system forces and The ATR blades have a rectangular planform with a
moments generated by the rotor model. A streamlined chord of 4.24 inches, radius of 55.0 inches, and a
fuselage shape encloses the rotor controls and drive NACA-0012 airfoil section. Pretwist is linear with a
system; however, the fuselage shape is isolated from the twist of 10° from the center of rotation to the blade tip.
rotor system such that its forces and moments do not
contribute to the loads measured by the balance.
"l
8.37
1 "_--------- 4.99 _
/-- Model pivot I
point 1
L
7.78
i
Figure 1. Schematic of the Aeroelastic Rotor
Experimental System (ARES) helicopter testbed. Figure 2. ARES with ATR rotor hardware mounted
All dimensions are in feet. inthe TDT test section with microphones installed.
mode the ATR computer control system generates low-
MIT Active Fiber Composite
voltage, sinusoidal control signals for each blade that
are amplified by linear high-voltage amplifiers to a
voltage amplitude of 1000 volts, approximately one-half
of the AFC maximum design voltage.
Acoustic Instrumentation. Acoustic data were
Rotor Blade acquired using six B&K 4134 half-inch microphones
fitted with B&K UA 0386 streamlined nosecones. The
Spar ..._.._i_,_ ....
. ._,_:_ r_.. ........ microphones were mounted in fixed, minimally
Construction ._,:;_;:_;!_i::i__
intrusive, microphones holders. Three microphones
were mounted upstream of the model and three were
mounted downstream of the model. Location data for
the microphones is presented in table 1. Four of the
microphones are visible in figure 2,two mounted onthe
floor upstream of the model, one mounted on the floor
downstream of the model and one mounted on the right
side wall. A close up view of one of the microphones
installed inthe tunnel is shown in figure 4.
Data Acquisition
ARES ATR Data Acquisition. Data acquisition
throughout the test was accomplished using multiple
separate data acquisition systems and the ATR
Figure 3.Active-twist concept. computer control system. The primary system used for
acquisition of the vibratory loads data was a Modcomp
ATR Computer Control System. Active-twist control computer with a Neff 600-series, 256-channel, 16-bit
of the ATR blades is achieved with a computer control analog-to-digital converter with sample-and-hold. Low-
system incorporating a digital signal processor board, 32 pass anti-aliasing filters were set to 200 Hz for each data
analog-to-digital channels, 6 digital-to-analog channels, channel and a sanapling rate of 1000 samples-per-
and 32 digital input-output channels. Available control second was used. Five seconds of data were typically
types include a collective twist mode where all four acquired on the Modcomp computer system for each
blade are twisted simultaneously, and an Individual data point. A subset of the channels processed by the
Blade Control (IBC) mode where each blade twists Modcomp system was also sampled by the ATR
according to a prescribed schedule associated with its computer control system. The sampling rate on this
position in the rotor azimuth. IBC mode control was system was 4000 samples-per-second, with data
used for the current investigation. For each operating acquired for 3 seconds for each data point. Generally,
the ATR computer control system was used to initiate
data acquisition streams on each of the systems
simultaneously.
Acoustic Data Acquisition. The acoustic data
acquisition system was triggered by the ATR control
system for each data point. The six microphone signals
Table 1.Microphone placement data.
Microphone X, in Y, in Z, in
1 -99.5 -58.8 -89.5
2 -113.8 59.0 -89.5
3 -113.8 59.0 89.5
4 174.8 -59.3 -89.5
5 183.5 -88.5 -47.5
6 205.0 -58.8 89.5
Figure 4. Close up view of fixed microphone holder.
wereeachsampleadt1024samplepserrotorrevolution
which microphone signals were good and which were
(nominaslamplreateofapproximate1l1y,70s0amplesbad?
perseconda),nti-aliafsilteredat5kHz,for60rotor
revolutionysi,eldinga samplteimeofslightlymore
In order to investigate this data quality issue, the
than5seconds.
acoustic data were post-test processed to obtain average
time history data. Data were averaged in blocks of 1024
Test points (one revolution) of data for 60 revolutions of
Procedures
data. In addition, the minimum and maximum values at
All testing was conducted inthe heavy gas test medium each time step were recorded. The resulting data
of the TDT at a nominal density of 0.0047 sl/ft3. The consisted of an average time history with a minimum
rotor rotational speed throughout the test was held at a and maximum boundary. As an example, time history
constant 688 rpm, resulting in anominal hover tip Mach data from a baseline condition (no active-twist control
number of 0.60. The bulk of the testing was conducted active) at g 0.20 and c_ 1.0 degree, are shown in
in forward flight with various steady-state trim figure 5. Microphone 1is one of the three microphones
conditions representative of sustained lglevel flight and located upstream, while microphone 6 is located
descending flight. A rotor lift coefficient of 0.0066 was downstream of the model on the tunnel ceiling. The
chosen for the nominal lifting task throughout the test, data scatter for microphone 1 is small, showing this
and the rotor-shaft angle of attack was chosen as a signal to be acceptably steady for an in-flow
function of flight speed to represent the various flight microphone, while data scatter (note the scale change)
conditions. for microphone 6 is not acceptable. The magnitude of
Data Analysis lOO
Microphone 1
Data Quality. The TDT is ahard wall wind tunnel, and
has only rarely been used to make acoustic 5o
measurements. In a previous study 1,it was shown that
sound power measurements can me made in this tunnel Q- o
to assess rotor noise reduction. Subsequent testing 2'3in d.
anechoic wind tunnels have borne out the results from
that test. -50
The hard wall tunnel test section can be regarded to be ,100
o 90 18o 270 360
reverberant except for two surfaces through which
sound power can escape the test section, namely the Azimuth, deg
tunnel cross sections upstream and downstream of the
model. If a control volume is drawn around the model (a) Time history data from microphone 1.
in the test section such that four faces of the box are at
the tunnel walls, then the remaining two faces become
200
the two control surfaces through which acoustic energy
can escape. Thus the sound power produced by the 150 Microphon¢e\ 6 T,-_/,,
model should be equal to the sound power flux across 100
these two surfaces. All that is required is to estimate the
¢_ 50
sound flux upstream and downstream of the model. g
Q- 0
_'-50
Accordingly, for this test, the microphones were
mounted in the test section, half upstream of the model -100
and half downstream of the model, using short low- -150 <' ! ',i\,/
intrusion microphone holders. These low-intrusion ti
-200
microphone holders were desired for this test to 0 90 180 270 360
minimize interference with the active-twist control
Azimuth, deg
vibration results. While the microphones upstream of
the model worked well, the downstream microphones (b) Time history data from microphone 6.
were adversely affected by test section wall boundary
layers and separated flow. Due to the importance of this Figure 5. Averaged time history for two microphone
data quality issue to the results of this study, it is worth signals showing minimum and maximum data
examination in detail. The fundamental question was scatter envelope for _t= 0.20 and c_= 1deg.
the scatter for microphone 6 is indicative of non- 0.25
acoustic pressure fluctuations resulting from unsteady
flow, possibly due to separation of flow from the tunnel
0.20
surface adjacent to the microphone location. The low
frequency energy from these large non-acoustic pressure
0.15
fluctuations tends to saturate the microphone signal. In
13"
addition, the unsteady flow results in a variable signal
path from the rotor to the microphone, which smears the 0.10
signal. These effects combine to render the
measurement unusable.
0.05
The first step to resolving a problem is to admit that the
0.00 ' ' , , | , o • , i , , , o |
problem exists. The next step is to determine the extent
0,10 0.20 0.30 0.40
of the problem. The means used in the current study to
determine the extent of this data quality problem was to Advance Ratio
define a data scatter parameter and then use that
parameter to separate the good data measurements from (a) Scatter data for microphone 1.
the unacceptable data measurements.
0.25
In figure 5, the difference between the maximum and
minimum values for each time step defines an envelope
0,20
width, w, which is in units of pressure.
w(t) = maximum(t)- minimum(t) 0.15
O"
The value of w(t) is averaged over the entire time period 0.10
to produce a single parameter, w, that characterizes data
scatter for that data measurement. The value of w was
0.05
then non-dimensionalized by the tunnel dynamic
pressure, q, and the resulting parameter was used to sort
the measurements based on signal quality. If data 0.00 ' , o o = o , , o = , , , |
scatter is primarily due to acoustic variability, as is 0.10 0.20 0.30 0.40
suggested to be the case for microphone 1 as shown in
Advance Ratio
figure 5(a), then the value of w/q should decrease with
increasing speed. If data scatter is due to flow-related, (b) Scatter data for microphone 4.
but non-acoustic pressure fluctuations, as appears to be
the case for microphone 6 in figure 5(b), then the value 0,50
of w/q will both be a larger value and will either
increase or tend to remain somewhat constant with
0.40
0.45 _ i
increasing flow velocity.
0,35
o
030 40
The scatter parameter w/q was calculated for each e o
microphone at each test point. The resulting data is 0.25
shown in figure 6(a) for microphone 1, which is also 0.20
representative of the results for the other two upstream
0.15
microphones. For microphone 1, w/q is shown to be
0.10
less than 0.10 at all conditions and the value decreases
as tunnel speed increases. This indicates that the 0.05
upstream microphone measurements are good 0.00 --" ......
throughout the test matrix. However, since acoustic 0.10 0,20 0,30 0.40
flux needs to be estimated on both the upstream and
Advance Ratio
downstream control surfaces, this result, while good
news, only solves half the problem. In figure 6(b), the (c) Scatter data for microphone 6.
scatter data for microphone 4 shows a marginally higher
value for w/q at g 0.14 than was shown for Figure 6 - Microphone scatter data as a function of
microphone 1 in figure 6(a), which indicates that this advance ratio for upstream microphones 1,4, and 6.
data is most likely good. The trend is for w/q to is more prevalent in this low speed operating region,
decrease with increasing tunnel speed at least between bt there should be no major impact from the advance ratio
0.14 and bt 0.17. Then w/q increases with tunnel limitation. Resolution of the data quality issue allowed
speed, although the value is less than approximately assessment of the noise characteristics of the ATR
0.15 for all conditions up to and including bt 0.27. system to proceed.
However, the value of w/q increases further for bt 0.30
and above. Examination of spectral data for Data Processing. In the previous study 1, the
microphone 4 indicated that data up to and including bt microphone spectra were all averaged to produce the
sound pressure spectra that was then converted to sound
0.27 was acceptable, but data for bt 0.30 and above
power units by application of a transfer function. The
was not acceptable. In figure 7, spectra from
transfer function was applied to all spectra and band-
microphones 1, 4, and 6 are compared at bt 0.27.
pass acoustic metrics were calculated. The difference in
While the spectra from microphones 1and 4are similar,
the level of the metrics were taken as the noise
the spectra from microphone 6 displays a smoother
reduction numbers. A similar process was applied to
spectra at a higher amplitude because the level of the
the current data, except no effort was placed into
non-harmonic noise is greater than the harmonic noise
determining atransfer function to put the sound pressure
produced by the rotor. The upper bound for advance
spectra into sound power units. The noise reduction
ratio for acceptable data appears to be bt 0.27 for data
results from this study should be directly comparable to
measured with microphone 4.
the results from the previous study.
Scatter data for microphone 6 are shown to be of a
For the current study, the spectra were averaged on a
much higher level, even for low tunnel speeds and the
power basis for each frequency bin for all 60
trends remain approximately constant with increasing
revolutions of data. This produced an averaged spectra
tunnel velocity, as shown in figure 6(c). This appears to
for each microphone at each data point. The averaged
indicate that data measured with microphone 6 will be
spectra were then used to estimate the relative sound
unacceptable for all test conditions. The spectra shown
power levels for each rotor condition.
from microphone 6in figure 7confirms this indication.
Microphone 5 scatter data was similar to that for
Since only one of the microphones downstream of the
microphone 6. Perhaps in future tests, use of taller
model produced a usable signal, the averaging process
microphone stands at these locations will solve this
for the current study was weighted to give equal weight
problem, but the measurements obtained by
to the microphones upstream and the microphone down
microphones 5 and 6 are unusable for the current data
stream. The averaged spectra was then used to evaluate
set.
noise changes inthe rotor model.
So, based on these results, the portion of the data set
Acoustic metrics were computed for each operating
used for this study is limited to bt 0.27 and below and
condition using the acoustic energy in frequency bands
uses microphone 4 as the estimate for the acoustic flux
representing low frequency thickness and loading noise
over the downstream control surface. Since BVI noise
and blade-vortex interaction noise. The frequency band
chosen for the LFSPL, or low frequency sound pressure
130
level, are from 0.5 flop to 6.5 flop. The frequency band
Microphone 1 chosen for BVISPL, or blade-vortex interaction sound
120 .........................Microphone 4
......................... Microphone 6 pressure level, was from 6.5 flop to 40.5 flop. BVISPL
and LFSPL levels were then used, respectively, to
m110 determine reduction in the BVI noise levels and the
_1100 penalty incurred by any increases in the low frequency
noise, reducing all of the acoustic data to just two
9O numbers for each operating condition. The levels of
these metrics can be compared between active-twist
8O operating and not operating (baseline) conditions to
examine acoustic trends due to active-twist control.
700 500 1000 1500 2000 2500
Frequency, Hz Obviously, the use of a single microphone measurement
to estimate the downstream acoustic flux is not desirable
and can be a source of error in the acoustic trends. This
Figure 7. Spectra from microphones 1,4, and 6 for
effect was quantified by recalculating the acoustic
bt 0.27 andc_ 4deg.
metric deltas, which are used to assess acoustic trends,
usingonlya singleupstreammicrophone.This
control frequencies is most apparent, and even then the
calculatiownasperformefdor eachof thethree
4P control shows up to be nearly as effective as the 5P
upstreammicrophonfeosrtheentiredataset,andthe
control, except where the BVISPL reduction is greatest
resultindgatawerestatisticaallynalyzetodcalculatthee
at c_ 6 and 7 degrees, where 4P data were not
error.Thedifferencinecalculateddeltametriclevels
obtained. A maximum BVISPL reduction at c_ 6
usingallthreeupstreammicrophonaensdonlyone
degrees was found to equal 2.8 dB, followed by a 2.7
upstreammicrophonweasfoundtobeapproximately
dB reduction at c_ 7 degrees. At those conditions, 3P
0.5dBforBVISPLand0.7dBforLFSPLS. otheerror
active-twist control is not as effective as 5P control in
bandapplicabtloethenoiselevetlrendpsresenteddue
reduction of BVISPL. There are no corresponding 4P
to useof thesinglemicrophondeownstreafmor
data for c_ 6 and 7 degrees, conclusive statements
BVISPiLs+/-0.5dBandforLFSPiLs+/-0.7dB.
about the effectiveness of 5P vs. 4P cannot be made
with the current data set.
Vibratiolneveilsrepresenbteydtheamplitudoefthe4P
componeonfttheoscillatofriyxed-systevemrticaslhear
force(4PNF).Althougahnynumbeorfmetriccsanbe The magnitude of BVI noise reduction achieved by
usedtocharacterviziberatiolnevelsit,wasfeltthatthis active-twist control is a bit disappointing, but is not as
parametwearsprobabtlhyemosrtelevant. insignificant as it may appear at first. In the previous
study 1,the maximum noise reduction achieved was 5.6
dB and that was achieved at g 0.11. The current study
Results
was limited to _ 0.14 and above, due to tunnel speed
limits that currently exist. The maximum control
A subset of operating conditions that allows evaluation
amplitude for the current and previous studies are both
of active-twist control effectiveness as a comprehensive
1.5 degrees. If the results from the previous study from
noise and vibration reduction method can be obtained
0.14 and above are considered, then the maximum
by considering only two conditions from each active-
noise reduction is between 3 and 4 dB, which is only a
twist phase sweep. In order to determine the magnitude
dB or sogreater than the reduction shown by the current
of BVI noise reduction, and assess any acoustic or
data set. So active-twist control isnearly, but not quite,
vibration penalties, the active-twist control phase
as effective as HHC in reduction of BVISPL. This
conditions for which BVISPL was minimized were
statement is further supported by examination of the
chosen. Similarly, in order to assess any acoustic
averaged spectra for the baseline condition and 5P
penalties incurred by maximum vibration reduction, the
active-twist control 1000 vamplitude for g 0.14, c_
active-twist control phase conditions for which 4P NF
was minimized were chosen. 6 deg as shown in figure 9. The reduction in the
frequency range from 300 Hz to 900 Hz accounts for
most of the BVISPL noise reduction shown in figure
Accordingly, the results section is divided into two
8(a). The trend shown in figure 9 isthe same shown in
parts, first an examination the BVI noise reduction
figures 6 and 8(b) of the previous study 1. So the active-
potential of the ATR system, and second, an
twist control shows similar, if smaller, magnitude
examination acoustic consequences of best vibration
reduction of BVISPL and a similar character in the
reduction using active-twist. The results are plotted
spectral data comparison.
with the values for BVISPL, LFSPL, and 4P NF plotted
as a function of c_for each g. In all cases, the baseline
case, which is the rotor operated without active-twist In the HHC studies _-3,it was found that increasing the
control, is plotted with both symbols and connecting amplitude of the control deflection increased the noise
line for clarity, while the values achieved by 3P, 4P, and reduction. In figure 10, the BVISPL increase relative to
5P active-twist control inputs at 1000v actuation voltage the baseline case is plotted as a function of control input
amplitude are plotted as symbols only. phase for 5P active-twist at two actuation voltage
amplitudes. One thing to notice is that for this case,
BVI noise reduction active-twist control is equally adept at increasing and
decreasing BVISPL level. Also, the change in BVISPL
In figure 8, BVISPL level as a function of c_ is for the 1000 v case at 320 degrees corresponds to the
examined for the advance ratio matrix. Two points are point plotted in figure 8(b). It is interesting that the
immediately apparent: in general, 5P active-twist trends in BVISPL change are similar between the two
control achieved lower BVISPL levels than 3P and 4P, actuation voltage cases, and it isapparent that the higher
and although most test conditions show some decrease actuation amplitude provides the greater level of
in BVISPL, the magnitude of that reduction is rather BVISPL reduction. Larger amplitudes of active-twist
small, usually on the order of 2 dB or less. At _ 0.14, control blade tip deflection would probably provide
the difference between the effectiveness of the various more BVI noise reduction.
110
110
105
105 rn
m
"O J
J a. 1oc
O. 100
_ _O'_ 3P1000V rn o 3P 1000 v
95 _.:_ 4P 1000 v
_::_ 4P 1000 v <_ 5P1000 v
_ 5P 1000 V Baseline
Baseline
9% ; ....
902' ' '_' ' ':;' ' ' _' ' ';' ' '_ Alpha, deg
Alpha, deg
(d) g 0.23
(a) g 0.14 110
110
105
105 m
"o
m
"{3 J
O. 100
_. 100
0_ o a3 o 3P 1000 v
>
a3 o 3p 1000 v 95 E_._ 4P 1000 v
95 _ 4P 1000 v 5P 1000 v
5P 1000 v A Baseline
Baseline
90.]_' ' '(_' ' '_' ' ' _' ' ';' ' '_
90'..' ' '(_' ' '_' ' '_' ' ';' ' '_ Alpha, deg
Alpha, deg
(e) g 0.27
(b) g = 0.17 Figure 8. BVISPL as a function of c_ for each
advance ratio for minimum BVISPL conditions.
110 -
120
-- Baseline
5P 1000 v
110
105
m _100
-a.90
_J
0. 100 O ft.
0q m 80
o 3P 1000 v
m 70
95 _.:;_ 4P 1000 v
_ 5P 1000 v 60 .... 50|0.... 100|0.... 150|0.... 200|0.... 250|0
Baseline Frequency, Hz
9o. ; ; Figure 9. Spectral comparison between baseline and
maximum BVI noise reduction case for g 0.14 and
Alpha, deg
c_ 6 deg.
(c) _ 0.20
2¸ 115
t2') 1 ?/ ,_ _.: \
110
m
_. 105
_( ...._..+...-...5..Pi000v _+_'-° i,
,-I O 3P 1000 v
-2 ........ | ....... |
0 180 360 100 _:._ 4P 1000 v
Control Phase ,_ 5P 1000 v
Figure 10. Change in BVISPL level as function of Baseline
control phase for two amplitudes of 5P active-twist
control for g 0.17 and cz 4 deg. 95_)' ' ';' ' '_' ' '_' ' ';' ' '_
Alpha, deg
(c) _t 0.20
120
115
m
"0
13.11
(/)
--I o 3P 1000 v
105 _:::_ 4P 1000 v
5P 1000 v
--_-_ Baseline
lO%.
Alpha, deg
(d) _ 0.23
120
115_
m
"O
_.110
iJ.
--I {3 3P 1000 v
105 _::_ 4P 1000 v
0 5P 1000 v
--_ Baseline
100_' ' ';' ' '_' ' '_' ' '_' ' '_
Alpha, deg
(e) _t= 0.27
Figure 11. LFSPL as a function of c_ for the
minimum BVISPL case.
3O o 3P 1000 v 30 O 3P 1000v
_.:;_ 4P 1000 v _,::,_ 4P 1000 v
25 <> 5P 1000 v 25 _ 5P 1000 v
{ Baseline Baseline
0 20
20_ .O
u."
Z 15 Z 15
Q. 0.
10 1(
5
%...;..._..._...;..._ %..,_..._..,_..,;..._
Alpha, deg Alpha, deg
(d) g= 0.23
(a) 1_= 0.14
30 - 0 3P1000 v
30 O 3p 1000 V
_i:::i_4P 1000 v
O _:::_:_ 4P 1000 v
25 _ 5P 1000 v
25 <_ 5P 1000 v
Baseline
Baseline
20
.o 20 .mO
la."
15
Z 15 Z o
O.
13.
1o,[
10
%'";'"_'"4"'_'"_
oz';'_'_'; _
Alpha, deg
Alpha, deg
(e) g = 0.27
(b) _ 0.17
Figure 12. 4P NF amplitude as a function of c_for
30
o 3P 1000 v the minimum BVISPL case.
o
25 o E_::_ 4P 1000v ? °
0 5P 1000 v
z_ Baseline
20 15( _:iii <>
(_12o
Z 15
Q.
10
N 60 :iiii 0
o o
5 30 _ _
% ;..._..._ Ei::i_.... ;,.,_ 0 ' ' , .;:.:i:..... ,_::;., ._.. ,
0.1 0.15 0.2 0.25 0.3
Advance Ratio
Alpha, deg
Figure 13. Azimuth angle corresponding to
(c) _ = 0.20
maximum negative blade twist for maximum
BVISPL reduction. Legend same as for figure 12.
