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F
AIAA-2001-1162
Reduction of Tunnel Dynamics at the NationalTransonic Facility (Invited)
W. A. Kilgore, S. Balakrishna, and D.NASA Langley Research CenterHampton, Virginia
H. Butler
39th AIAA Aerospace Sciences Meeting & Exhibit8-11 January 2001
Reno, Nevada
For permission to copy or republish, contact the American Institute of Aeronautics andAstronautics
1801 Alexander Bell Drive, Suite 500, Reston, VA 20191-4344
https://ntrs.nasa.gov/search.jsp?R=20010013155 2020-07-01T13:57:01+00:00Z
AIAA-2001-1162
REDUCTION OF TUNNEL DYNAMICS AT THE NATIONALTRANSONIC FACILITY
W. A. Kilgore', S. Balakrishna t, and D. H. Butler t
Aerodynamics, Aerothermodynamics, and Acoustics Competency
NASA Langley Research Center
Hampton, Virginia
ABSTRACT
This paper describes the results of recent effortsto reduce the tunnel dynamics at the National
Transonic Facility. The results presented describe
the findings of an extensive data analysis, theproposed solutions to reduce dynamics and the
results of implementing these solutions. Theseresults show a 90% reduction in the dynamics
around the model support structure and a smallimpact on reducing model dynamics. Also
presented are several continuing efforts to furtherreduce dynamics.
INTRODUCTION
The National Transonic Facility (NTF) is the firstlarge scale closed circuit fan drive cryogenic
pressure tunnel designed to provide flightReynolds number testing at transonic conditions in
a ground based facility. Throughout the NTF'shistory several testing programs were cut short, or
severely limited, because of dynamics. Theselimitations manifest themselves as excessive
balance dynamic loads, large modeldisplacements and increased data scatter.
Excessive model and/or tunnel movements (i.e.dynamics) can produce questionable results and
concern over the safety of the model,
instrumentation and the facility. Because the NTFis capable of generating large dynamic pressures(up to 7000psf) and high Reynolds numbers (up to
146million/ft) the dynamics are greatly amplified.Figure 1 shows the NTF operational envelope.
Dynamics and Control Engineer, Research Facilities Branch,Senior Member, AIAA
:l:SeniorResearch Scientist, ViGYAN Inc.
Tsenior DesignEngineer,ViGYAN Inc.
Copyright © 2001 by the American Instituteof Aeronautics andAstronautics, Inc. No copyright is asserted inthe United Statesunder Title 17, U. S. Code. The U. S. Government has aroyalty-free license to exercise all rights under the copyrightclaimed hereinfor Governmental Purposes. All other rights arereserved by the copyright owner.
Several efforts to understand and minimize the
dynamics were conducted and documented overthe history of the NTF. 1'2'3'4 These efforts had
limited opportunity to implement any proposedsolutions. In FY-2000 a concentrated effort was
established to "take action" and implement severalsolutions. The results of this effort are presented
in this paper.
A
M
PT
p'qRe
tT
NOMENCLATURE
Area
measured fluctuating acceleration, (RMS)Mach number
tunnel total pressure
measured fluctuating pressure, (RMS)dynamic pressureReynolds number in million/chordtime, s
tunnel flow total temperature
Subscripts and AbbreviationsX
Y
ZAFARC
DSSFFF
HSDMSFNF
PMRM
RMS
SFTSYM
axial direction, tunnel flow axis
lateral direction, orthogonal to X and Z
vertical direction, orthogonal to X and Ybalance axial forcearc sector
down stream support frame
fixed fairinghigh-speed diffusermodel support framebalance normal force
balance pitching momentbalance roll moment
root mean squarebalance side forcetest section
balance yaw moment
NTF STRUCTURE
The NTF dynamics can be easily divided in to
three separate areas (internal support structure,model support structure and model/balance/stingstructure). Each area has its own unique
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AIAA-2001-1162
dynamics, and when the dynamics in one areacoalesce with another area, excessive dynamicsOccur.
Internal Support Structure
Because the NTF is a cryogenic pressure tunnel,several unique features are inherent in its designto accommodate the large changes in
temperatures (150°F to -250°F) that preclude any
fixed mountings or supports. These uniquefeatures are most prevalent in the area of theinternal support structure for the test section.
Figure 2 shows the original build-up of the NTFtest section outside the plenum and indicates the
support points and the direction of constraint.
Unique to the NTF, the entire test section supportstructure is free to move; and constrained
vertically and laterally using pads and guides
between the test section and the plenum pressureshell as shown in figure 3. The upstream and
downstream portions of the test section areseparate and independently secured withaccommodations for movement via Bellevilles
spring washers at the front and back of the plenumbulkheads.
Model Support and Model/Balance/StinqsStructure
The model support system is mounted within the
test section support structure as shown in figure 4.The model is supported on the end of a 13ft stingand attaches to the roll drive that is installed in the
arc sector. The entire arc sector, roll drive, sting
and model (+350001bs) rotates to pitch the model
assembly.
The arc sector rides between four I-beams as
shown in figures 2 and 5 (one set of four on topand one set of four on the bottom). These I-beamsconnect between the down-stream support frame
(DSSF) and the model support frame (MSF)
providing stiffness to the model support structure.
The arc sector maintains its alignment and loadcarrying capability through four sets of lateral
bearings (two upper pairs and two lower pairs) andtwo thrust bearings (one upper unit and one lower
unit) as shown in figure 4. These bearings aremounted just below the innermost I-beams
adjacent to the arc sector. Each bearing providesa sliding surface on the arc sector that is springloaded to 20001bs with Bellevilles washers. All the
loads generated from testing are distributedthrough the bearings into the I-beams, DSSF andMSF. As shown in figure 2, the DSSF is
constrained vertically and laterally, while the MSFis cantilevered off the DSSF with no directconstraints.
Immediately behind the arc sector is a fixed fairing(does not move with the arc sector) as shown in
figure 4. The fixed fairing is a hollow aero-faringthat consists of horizontal flow liner panelsconnected to a solid vertical beam at the aft end of
the fixed fairing and extends forward to the aft
edge of the arc sector. This fixed fairing vertical
beam spans the test section, is secured to theceiling, and passes down through the floor for
several feet to where it is vertically pinned.
ANALYSIS
An extensive amount of dynamic data was
collected over several test programs covering theentire operational envelope of the tunnel. This
data included 128 channels (sampled at 320samples/second, simultaneous sample and hold)
of accelerations, positions, loads and fluctuatingpressures located throughout the high-speed legon the tunnel structure and model. 5
This current effort concentrated on reducing thetunnel structure dynamics in the lateral directionalong the high-speed leg as a first step in reducing
the dynamics.
Analysis of Tunnel Structural ResponseThe analysis of the fluctuating pressure data
shows that the all the fluctuating pressures arebroadband white noise without any specific
frequencies and the level varies with the tunneldynamic pressure. The model support area (arc
sector and fixed fairing) experienced the largestfluctuating pressures of the entire high-speed leg.
This relationship between the fluctuating pressureand dynamic pressure is shown in figure 6 for the
fixed fairing. The fixed fairing fluctuating pressures
(P'FF-Y) varies linearly with the tunnel dynamicpressure except for the exponential vertical spikes.These spikes occur when there is supersonic flow
in the test section, and an unsteady normal shockexists in the arc sector/fixed fairing area causing a
local increase in p'. Thus the tunnel can
experience high fluctuating pressures even at very
low dynamic pressures.
The analysis of the acceleration data indicated asimilar relation between the local RMS
accelerations (_.) of a structural component andthe dynamic pressure. Again, using the fixedfairing as an example, figure 7 shows the fixed
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AIAA-2001-1162
fairing RMS accelerations in the Y direction (_I.FF.Y)
along the centerlJne of the tunnel versus the
dynamic pressure. This relation varies linearlyexcept for the exponential vertical spikes
associated with supersonic conditions.
The data for the fluctuating pressures and RMS
accelerations were found to be mostlyindependent of temperature (and Reynolds No.).
There is some indication that the high-speeddiffuser response changes with pressure because
of the increase in diffuser gas mass.
If one assumes a white noise (broadband)
fluctuating pressure (p') is acting on an exposedarea of the structure (A), it will exert a force ontothe structure. It can be theorized that the structure
exposed to this force will then linearly respond at
its natural modes. This relation can be simplyexpressed as:
NTF Simplified Structural Dynamics Model
r
Area of - ,_exDosureJ Linear multimodal _ &FF
- '_,_1 multidegreefreedom _ _
Broadba_dd NTF model support _ aARC
excitation structure _ _'xx...... I
This simplified expression indicates that there is a
relation between p' and _. that is unique to each
specific structural component and this relationshipis characteristic of the component.
The data show a first order linear relation between
the local fluctuating pressure (p') and the localstructural acceleration (&) for all structuralcomponents. Again using the fixed fairing as an
example, figure 8 shows this relation.
From figure 8 the slope (A&/Ap') provides aparameter that is a measurement of the inversestiffness of the structure. This stiffness parameter
can then be determined and compared fordifferent structural components to identify areas of
relative weakness. Table 1 provides a summary ofthe stiffness parameter for structures of the NTFhigh-speed leg from measured data.
Structure, Direction
Downstream Support Frame, Y
Model Support Frame, YArc Sector Centerline, Y
Fixed Fairing Centerline, Y
High Speed Diffuser Lip, Z
StiffnessParameter
(in/s2/psi)
3,500
5,300
22,000
60,000
240,000Table 1 - NTF Structural Stiffness
This table shows that the most flexible and
responsive structure is the high speed diffuser
followed by the fixed fairing and arc sector.
Surprisingly, the MSF and DSSF are only about 3-4 times stiffer laterally than the arc sector, when
one can reasonably expect such support structureshould be significantly stiffer.
Analysis of Model Response
The RMS values of six components of the model
strain gauge balance (NF, SF, AF, PM, YM, RM)show a similar relation to the local fluctuating
pressure (P'Ts). The analysis shows that the NF,
SF, PM, and RM have a low response to P'TS, andthe YM and AF have a larger response. Figure 9
shows an example of this relation between the
RMS value of the YM to P'TS-
Frequency Domain AnalysisA frequency domain analysis was completed to
identify the modes, and their characteristics,associated with different structures. As expected
the amplitudes of each mode increased withincreasing dynamic pressure as shown in figures10 and 11.
Figure 10 shows the spectra of the arc sectorcenter Y direction. The dominant mode
frequencies are at 11-12Hz, 17-19Hz, 22-23Hz,34-42Hz and 70+Hz. Although not the largestmode the 34-42Hz mode shows the characteristics
of an aero-elastic mode where the frequency
decreases from 42Hz to 34Hz as the dynamicpressure increases. In a detailed analysis it wasdetermined that this mode changes frequency with
dynamic pressure but does not show evidence of
decreased damping. This mode behavior isexplained by aerodynamic loading of the elasticarc sector.
Figure 11 shows the spectra of the fixed fairingcenter Y direction. The dominant mode
frequencies are at 23Hz, 30-32Hz and at 70+Hz.
The 30-32Hz mode is by far the most dominantfrequency of the three modes. The smaller 23Hz
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mode is an arc sector mode transmitting from thearc sector due to contact between the fixed fairingand arc sector.
Summary
The analysis provides an understanding of the
dynamics problems at the NTF by first identifyingand understanding the behavior of the local
fluctuating pressures (p') as the excitation source
for the dynamics. The analysis also identified therelative stiffness of each structural component
along the high-speed leg and, based on spectraanalysis, identified the behavior and contributions
of each structural component to dynamics.
Based on this analysis, the reduction of lateral
dynamics response at the NTF can be
accomplished by reducing p' and/or increasing thestiffness of the model support structure in the load
path.
SOLUTIONS
It was established early that any proposed solution
could not change the tunnel flow lines; therefore
any efforts to minimize p' were not considered atthis time.
From the data analysis it was clear that changes
made in the model support structure (FF, ArcSector, MSF and DSSF) were the best way to
reduce the lateral dynamics. The solutions weredivided into two areas:
1) Increase support structure stiffness2) Change the structure boundary conditions
Increase Support Structure StiffnessTo increase the support structure stiffness (Arc
Sector, MSF and DSSF), large 0.75in aluminumshear plates were installed horizontally securing
together the MSF, outermost I-beams and DSSF.
Additionally, bulkheads with extending lowerbrackets were installed between the top I-beams. 6
Figures 12, 13, and 14 show simple graphical
representations of the shear plates (upper andlower), bulkheads and brackets.
Chanqe the Boundary ConditionsThere were two solutions for changing the
structure boundary conditions. One solution was toinstall wedges between the fixed fairing and the
adjacent tunnel wall, and the second was theinstallation of new active bearings.
The fixed fairing wedges are designed to fill the
gap between the fixed fairing and the tunnel floor.
By filling this gap the wedges relocate the lateralsupport points of the fixed fairing from the vertical
pin up to the tunnel floor. This change reducesthe fixed fairing's effective length from 160in to
125in as shown in figure 15, thereby increasing itsnatural frequency and reducing its displacement.
The installation of new active bearings was based
on the data analysis that indicated the top of thearc sector experienced large lateral accelerations
and deflections. Additionally data from a previous
report indicated tightening the existing bearingsreduced the yaw dynamics of the model. 1Therefore two pairs of active bearings (a front pair
and an aft pair) were installed just above the
existing bearings as shown in figure 15. Thesenew active bearings can be remotely controlled to
set up to 100001bs load against the arc sector andstill allow the arc sector to freely move.
Operationally these new active bearings wereloaded prior to tunnel operations and remainedloaded at all times.
RESULTS
Installing the shear plates, brackets, bulkheads
and the fixed faring wedges greatly reduced thedynamics of the tunnel model support area (MSF,DSSF, arc sector and fixed faring).
Figures 16 and 17 show the significant reduction
in the stiffness parameter before and after thestructural modifications for the fixed fairing and thearc sector. These results are summarized in Table
2 showing the increase in the tunnel model
support structure stiffness.
Structure
DSSF, Y
BeforeStiffness
Parameter
(in/s2/psi)3500
Affer
Stiffness
Parameter
(in/s2/psi)200
MSF, Y 5300 470
ARC Centerline, Y 22,000 1300
FF Centerline, Y 60,000 3700
Table 2 -Structural Stiffness (Before and Affer
The increased structural stiffness has resulted in a
significant reduction of the amplitudes of thefrequency spectra for each model supportstructure component. Figures 18 and 19 show this
reduction for the fixed faring and arc sector.
For the fixed fairing spectra shown in figure 18 the
previously dominating 30-32Hz mode has beensignificantly reduced and shifted to 45Hz.
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AIAA-2001-1162
In the arc sector spectra shown in figure 19 there
remains four inherent modes at greatly reducedamplitudes. The first two modes around 8Hz and
12Hz are highly model/balance/sting dependent.The other two modes at 15-16Hz and 23Hz are
associated with the model support structure and
are inherent to the design of the arc sector.
With this significant decrease in the structuraldynamics, there is also a small decrease in the
model dynamics as shown in figure 20. This smallreduction indicates, as expected, that the model
dynamics are dominated by the
model/balance/sting modes. Future work inreducing the model dynamics will be to avoidmode coalescence between the
model/balance/sting and the modes inherent to the
model support structure.
Active Bearinqs
The use of active bearings showed that the Ydirection acceleration (and thereforedisplacement) at the top of arc sector was
considerably reduced. However the Y directionaccelerations of the centerline of the arc sector
showed no discernable change. This suggests
that the existing bearings are providing sufficientboundary conditions for the arc sector in their
current configuration. The active bearings dohowever, provide a limited ability to move the
frequencies of the arc sector modes. Furthertesting is required to fully document the results of
using the active bearings.
FUTURE IMPROVEMENTS
Several efforts are currently underway to furtherreduce model dynamics. The NTF has finishedfabrication of a new balance (NTF-116A) that is
4.3 times stiffer in pitch, 5.7 times stiffer in yawand 12 times stiffer in roll without sacrificing dataquality. 7 This balance should have significant
impact in reducing model dynamics by avoidingcoalescence with the modes inherent to the model
support structure.
The NTF is also working to develop new stings
using passive and active damping approaches toreduce the model dynamics and avoid any modalcoalescence.
CONCLUSIONS
Providing additional stiffness to the model supportstructure has significantly reduced the problems of
dynamics at the NTF. Efforts continue to furtherreduce the dynamics with future solutions focusing
on reducing the model dynamics.
There will always remain some fundamental
frequencies that are inherent to the supportstructure that cannot be eliminated. The design of
future model/balance/stings system must avoid the
frequencies of the support structure to preventcoalescence.
ACKNOWLEDGEMENT
The success of this work was made possiblethrough the efforts of several groups working
together for a common goal of improving the NTF.
The authors would like to especially recognize T.Arboneaux, P. Bauer, J. Barry, J. Bledsoe M.Chambers, M. Hilleren, L. Rash, and J. Zalarick fortheir hard work.
REFERENCES
1. Young, C.; Popernack, T.; and Gloss, B.:
National Transonic Facility Model and ModelSupport Vibration Problems, AIAA Paper No.90-1416, 1990
2. Buehrle, R. D.; Young, C. P., Jr.; Balakrishna,
S., and Kilgore, W. A.: ExperimentalInteractions Between Model Support Structure
and High Speed Research Model in theNational Transonic Facility, AIAA Paper No.
94-1623, 1994
3. Igoe, W. B.: Analysis of Fluctuating StaticPressure Measurements in the National
Transonic Facility, NASA TP 3475, March1996
4. Edwards, R. W.: National Transonic FacilityModel and Tunnel Vibrations, AIAA Paper No.97-0345, 1997
5. Balakrishna, S.: Data Analysis Report, NTF
Operational Data from Tests 100, 107 and111, Final Report - Part A, ViGYAN ReportR00-05, NASA Langley Research Center,
Contract NAS1-96014, Task RF01, February2000
6. Butler, D. H.: NTF Structural Modification
Proposal Based on NTF Operational Data
Analysis, Final Report - Part B, ViGYANReport R00-05, NASA Langley ResearchCenter, Contract NAS1-96014, Task RF01,
February 2000
7. Parker, P.: Cryogenic Balance Technology atthe National Transonic Facility, AIAA PaperNo. 2001-0758, 2001
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AIAA-2001-1162
7000...._,.140.0 million
" 6000 ,_D"_ ".,..
50001 /'+" ;'..100.0 million
+oot ...._ ::::i _60.Omillion5"_ 10 "
_>. 000i ,_c__ 20"0 milli°n
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Mach Number
Figure 1 - NTF Operational Envelope
I-Beams Arc Sector
MSFDSSF
Figure 5 - Model Support Structure
Figure 2 - Test Section Support Structure
Vertical Constraint Lateral Constraint
Figure 3 - Test Section Support Pads
Figure 4 - Model Support Structure
04
035
_ o3
n
9o2,5E
_ 02
o15
E
.-- OO5
- TI,_3-01
," TIOO-02 _ "
TI00 03 ..... _ ....' T100-04
TI_ 05
, T100-06
• T100-07
TI00-08
• T100+09
o T100+10
o II00q2
T101J-14
o T100-16
- T100 21
, T100-2rJ
5LIJ I000 1500 2000 2500 31_7(.i
Dynamic Pressure, psi
Figure 6 - Fixed Fairing Fluctuating
Pressure vs. Dynamic Pressure
35OO
._=E>
t-O
---_ 12OO
09
_; 08lie
##6
_ 04LL
"O_ O2
._xI.L
0
2 • Tl1_}-01 1 ; ' _'<' "5;uperson c + _i;_8 TmO-o.I ' -° _ . ,tit
• +mo-o+l • + + il .,_lll" T100-04I° + _ _../ _ "I,
1 4 , T100-07 _" I ' .: ........ :._illi_._
[1O009 <l _ 7_:!:7:"!_:_:" I_l[ ll"o T0040] Jl+<i.,_+':_ .... _iJ_ +_+ TI00+12I lli_ :++_"Ill .,+,ili_" ,ioo+i+1 : illl :_ ili .t/.;>Lt_t+ TI00-16i - _ ';+ilri -I._'_;
TIO0._CI " * ; +i ..... "*'"++,o+++•+4+
+ i
5oo _coo _oo 2ooo 2_ 3000 3_
Dynamic Pressure, psf
Figure 7 - Fixed Fairing RMS Accelerationvs, Dynamic Pressure
6American Institute of Aeronautics and Astronautics
AIAA-2001-1162
t--
O:>
==O
>-(/3
CE
16000
14000
12000
10000
8000
6000
4000
2000
00
--_:' Y' I1c, M=n_,7_, •
,m, ij., ! 1
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45p'Y-FF, psi
Figure 8 - Fixed Fairing RMS accelerationsvs. Fluctuating Pressure
0.9
0.8
0.7
0.6
0.5
0.4
0.3!
021
0.1
0
":' r: P
ElalanceNTF113, S1S. ' , _', I_.'bo'l.l ,_*°
M=05-093. q=tS0O-2cjo0 p_ - =_ ',_IP_ 11_16_> ,.'0 !,
•++':, +',i+,+++__ gP ++';
//:J_;°*P_r _+"_:/'._. , " _ II . T1TT11111-[34Q3
/ ++ - ' z ; I " Tt + 1-06
• "< ...... I T111 07
"_ ' ' I ' T111-0B
"#"' '" ' T! 11-09
_° -" I Tl11-10
0 0.05 0.1 0.15 0.2 0.25
P'Y-TS,psi
Figure 9 - RMS Yaw Moment vs. FluctuatingPressure
.E 20C
_1800U-.£1600_1400
12008 1000< 800E
"E
•_ 4001200:
0 0 10 20 30 40 50 60 70 80 90
Frequency Hz
Figure 11 - Fixed Fairing Spectra forHigh and Low Dynamic Pressures
100
Bulkheads
Arc Sector
Upper Shear Plates
Brackets
DSSF
Figure 12 -Arc Sector/Support Structure
(Top)
Bulkhead Sector
c-O
.<sO
O3
<
8O
70
60
5O
40
3O
2O
10
10 20 30 40 50 60 70 80 90
Frequency Hz
Figure 10 - Arc Sector Spectra for High
and Low Dynamic Pressures
Shear PlateBrackets
Additional
Bearings
ExistingBearings
Figure 13 - Arc Sector/Support Structure
(Top)
100
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American Institute of Aeronautics and Astronautics
AIAA-2001-1162
Lower Shear
Figure 14- Arc Sector/Support Structure
(Bottom)
MSF DSSF
ActiveBearings
60in
Wedges
Figure 15 - Arc Sector/Support Structure
(Sideview)
r-
u.
r
18000 a TIO0Z-14, BEFORE " M:0 2t , T=-25OF'_-20_80 _s,a
{ T131-000, ,a.FTEF_
16000 * TI00-20, BEFORE - • " .",''.,. " "
- T131-005, AFTER _'._:_",_'.-_-_** -" ,,',
14000 • T100-08 BEFORE I * ", "=_ l[_e_-'_ &
100oo __._" -."Z_[lli_ Befor ^....... _,_. C'_]mll '
8000 _'_" _!_ slope --E]0O0 ,r,sls,'ps,
"_ p'_ .... ..... After2000 '= ' : i i
005 01 015 02 0_ 03
p'Y-FF,psi
Figure 16- Fixed Fairing Results (Before
and After)
t-
5OOO
45O0
4O00
3500
3OO0
2500
2O0O
1500
1000
5O0
° _,_I_,BEFO_ _ ' ' /: TI31-O06, AFTER =: _=/-
- T100-/{} BEFORE _ " " - "
- T131.O05 AFTER _/*''* _ _,'" ._ J
< T131-O04 AFTER _,_ _'° &'L'-_._-._ " : t
@_t" o • "'_ - . " "
. tt'_"_'_' =;_m.t_'/_i_" Before
005 01 015 0"-' ON 03
p'Y-FF, psi
Figure 17 - Arc Sector Results (Before
and After)
140
120 - -
20
Before
After
i00 20 40 60 80 100 120 140 160
Frequency, HzFigure 18- Fixed Fairing Results (Before
and After)
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AIAA-2001-1162
%d_
¢-.m
<,
600[ --I
i!
500 -
4OO
20O
100
Before !
!i
00 20 40 60 80 100 120 140 160
Frequency, Hz
Figure 19 - Arc Sector Results (Before and
After)
Pathfinder Model, 113C Balance
j
'°°°t- !_1400_ - - _ L_ Before
E 1200 _ - - _- ,lit
__looo_o 800
rr 600After
400 :I
2ool , l ;00 - 500 1000 1500 2000 2500
Dynamic Pressure, psf
Figure 20 - Model Yaw Moment Results
(Before and After)
30OO
9
American Institute of Aeronautics and Astronautics