Recent National Transonic Facility Test Process Improvements (Invited)

16 Pages·2001·0.91 MB·English

by  Kilgore, W. A.

AIAA-2001-0756
Recent National Transonic Facility Test
Process Improvements (Invited)
W. A. Kilgore, S. Balakrishna, C. W. Bobbitt, Jr.
and J. B. Adcock
NASA Langley Research Center
Hampton, Virginia
39th AIAA Aerospace Sciences Meeting & Exhibit
8-11 January 2001
Reno, Nevada
For permission to copy or republish, contact the American Institute of Aeronautics and
Astronautics
1801 Alexander Bell Drive, Suite 500, Reston, VA 20191-4344

AIAA-2001-0756
RECENT NATIONAL TRANSONIC FACILITY TEST PROCESS
IMPROVEMENTS
W. A. Kilgore', S. Balakrishna', C. W. Bobbitt, Jr.t, and J. B. Adcock*
Aerodynamics, Aerothermodynamics, and Acoustics Competency
NASA Langley Research Center
Hampton, Virginia
ABSTRACT Improvements to the tunnel performance
This paper describes the results of two recent capabilities, reliability and data quality have also
process improvements; drag feed-forward Mach been implemented over the years. Still, despite its
number control and simultaneous force/moment extensive Reynolds numbers testing capability as
and pressure testing, at the National Transonic shown in figure 1, testing at the NTF is limited due
Facility. These improvements have reduced the to its expense when compared with conventional
tunnels.
duration and cost of testing. The drag feed-
forward Mach number control reduces the Mach
number settling time by using measured model Recently, several studies have been conducted to
drag in the Mach number control algorithm. investigate reducing the cost of testing at the NTF
Simultaneous force/moment and pressure testing through process improvements. The most recent
allows simultaneous collection of force/moment efforts (FY-2000) were focused on the
and pressure data without sacrificing data quality implementation of drag feed-forward Mach number
control and simultaneous force/moment and
thereby reducing the overall testing time. Both
improvements can be implemented at any wind pressure testing. Both of these improvements
tunnel. Additionally the NTF is working to develop reduce the cost of testing at the NTF by reducing
the test duration.
and implement continuous pitch as a testing option
as an additional method to reduce costs and
NOMENCLATURE
maintain data quality.
A area of duct
INTRODUCTION AWG American Wire Gauge
The U.S. National Transonic Facility (NTF) was afwa axial force, wind axis (i.e. drag)
the first large closed circuit fan drive cryogenic ESP electronic scanning pressure
pressure tunnel designed to provide flight FRS Flow Reference System
Reynolds number testing at transonic speeds in a igv inlet guide vane
ground based facility. Several problems have k_ circuit loss factor
plagued the NTF since it became operational in N fan speed
M Mach number
1984 and have limited its ability to consistently
provide cost effective high quality data at flight Pinlet fan inlet pressure
Reynolds numbers. Many of these problems have Ps tunnel static pressure
been solved over the years as lessons about large Pt tunnel total pressure
cryogenic tunnel operations and high Reynolds AP pressure loss
number testing techniques matured. q dynamic pressure
RCS Research Computer System
Dynamics and Controls Engineer, Research Facilities Branch, Re Reynolds number in million/chord
Senior Member, AIAA
t time
TSenior Research Scientist, ViGYAN Inc.
T tunnel flow total temperature
1Test Engineer, Research Facilities Branch, U velocity
:1:Consultant, ViGYAN Inc. x axial direction, tunnel flow axis
c( angle of attack, AoA
Copyright © 2001 by the American Institute ofAeronautics and
side slip angle
Astronautics, Inc. No copyright is asserted in the United States 1/1 test section mass flow
under Title 17, U. S. Code. The U. S. Government has a
royalty-free license to exercise all rights under the copyright 8,g_ change in inlet guide vane angle
claimed herein for Governmental Purposes. All other rights are
8rpm change in fan speed
reserved by the copyright owner.
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American Institute of Aeronautics and Astronautics
AIAA-2001-0756
To implement this process, a relationship between
DRAG FEED-FORWARD MACH NUMBER the fan pressure control and afwa must be
CONTROL determined. For the NTF, this relationship is
It is well known that in all closed circuit wind between the drag force (afwa) and the 6,gvat a
tunnels a change in blockage, often related to a constant fan speed. Figure 2 shows this
change in model angle-of-attack (o0, causes a relationship for a range of tunnel conditions.
change in the test section flow momentum and
velocity and hence results in a disturbance to the The slope 6,gvlafwa has been analyzed over a wide
test section Mach number. Adjustments are made range of Mach numbers, Reynolds numbers and
to the fan pressure ratio via the Mach number tunnel temperatures, and a linear relation has
controller to recover and maintain the test section
been established between afwa and 6,gv. This
Mach number at the NTF. This recovery time is of relation is represented as:
the order of 7-10 seconds with the optimally tuned
Mach number controller. The recovery response
is related to Mach number measurement/process
0.02 (5--_
settling delays. 8igv= _ (afwa)
The NTF uses a combination of fan speed (N+6rprn)
Figure 3 shows a detailed graph of how well this
and fan inlet guide vane angle (ipv+5_gv) to achieve linear relation agrees with actual test data from
the required fan pressure ratio. This relationship
two different transport model tests, as compared
can be modeled as:
to a least-squares fit of the data.
This relation between afwa and 6,gvis unique and
therefore easily implemented as the drag feed-
- f(N+6 .....,i_v+ag_,)
forward component in the Mach number controller.
Figure 4 shows the schematic of NTF Mach
number control loop using this drag feed-forward
During the standard process for an angle-of-attack
approach.
sweep, N is held constant (6rpm=0) and _3_gvis
adjusted to account for changes in the circuit
The Mach number control process is as follows:
pressure losses. Therefore, to improve the
First, the test-section Mach number is calculated. 2
response of the Mach control, a faster system is
This Mach number is compared with the Mach
needed to sense the change in momentum in the
number set point to determine a Mach number
test section, and to translate this change into the
error. The Mach number error is passed through a
required adjustment in 6igv.
nonlinear gain schedule Proportional Integral (PI)
controller to determine the inlet guide vane angle.
The momentum change in the test section can be
represented by the momentum equation of duct
The implementation of drag feed-forward required
flow:
only minor software code changes that introduce a
AOP _ _(ril) _(filu) second summation where the afwa applies a small
k,#u_
_._ Ot Ox limited inlet guide vane angle bias to the igv
signal. This igv signal is then normalized with the
Where k_ =f(m [3, M, Re) represents the tunnel temperature and outputs the desired inlet
momentum loss coefficient due to blockage and guide vane angle.
other test section losses. By Newton's principles,
this loss in momentum can be represented as a The results of drag feed-forward Mach number
force (in this application, the axial force in the wind control are shown in Figure 5 for two identical
axis (afwa); i.e. drag). This force measurement, angle of attack changes (0k to -4°) of the same
made using the model force-balance, responds model at identical test conditions. The plot on the
more rapidly than the Mach number measurement left represents drag feed-forward "OFF" and the
system. Therefore this measured drag force can plot on the right represents drag feed-forward
be used for feed-forward to Mach controller, "ON". Comparing these two plots shows that the
thereby realizing faster Mach number recovery Mach number settles 4 seconds faster using drag
times. feed-forward. Additionally, the disturbance to the
Mach number appears significantly less
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AIAA-2001-0756
demonstrating that drag feed-forward also undertook the development of a simultaneous
maintains a tighter Mach number control during force/moment and pressure measurement
model attitude changes. This offers a significant process. The approach at the NTF similar to other
benefit in improved Mach number stability when facilities is to minimize the load carrying capability
performing continuous pitch testing and reducing of tubes or wires that bridge the balance and to
unsteady loads on the model. provide a simple method to verify that the
force/moment data is not being affected.
Summary
The use of a drag feed-forward Mach number Tubes and Wires
controller at the NTF has resulted in a faster In a typical test at the NTF there are several
recovery time for Mach number disturbances pneumatic tubes and electrical wires that support
caused by angle-of-attack variations. The actual the Electronic Scanning Pressures (ESP) and
savings from utilizing the drag feed-forward Mach Angle of Attack (AoA) systems, which cross the
number controller can only be estimated, as is will metric break of the force and moment balance.
vary for each tunnel condition and the step size (or The ESP system requires wiring for the data
change) in angle of attack. For typical high signals, environmental heaters, and temperature
Reynolds number (Re=40million), transonic measurements along with several pneumatic
(M=0.88) test conditions with a 15°point polar, the tubes. The AoA system also requires wires for the
estimated time saving is about 30 seconds. This data signals, environmental heaters and
corresponds to significant savings in LN2 cost temperature measurements.
alone (~$1000 per polar).
To support this effort, the following changes were
This new drag feed-forward Mach number made to the instrumentation tubing and wiring
controller responds immediately to blockage used in the model. The existing nylon pneumatic
changes caused by angle-of-attack changes tubing was replaced with identical size
instead of waiting for the response of the Mach (OD=0.060in) Teflon tubing, which is more flexible
number measurement system. The control law is and provides reduced friction. Teflon coated
simple and robust and the concept can be used in 24AWG and 26AWG wire (dia.=0.038in and
any closed circuit subsonic tunnel with Mach dia.=0.032in) was replaced with smaller, more
number capability via fan pressure ratio. flexible 34AWG Teflon coated wire
(dia.=0.0210in), requiring more wires but providing
SIMULTANEOUS FORCE AND PRESSURE more flexibility. The 34AWG wire tensile strength
In the past it has been common practice at the is about 85% of the 26AWG wire. Additionally, the
NTF to divide a test program into two parts. The type T thermocouples (solid 24AWG) used for
first part included the simultaneous measurement temperature measurements in the ESP system
of the balance forces/moments along with model were replaced with RTDs, which increased the
wing pressures and was followed by the second actual number of wires for temperature
part that included only balance forces/moments. measurements but permitted the use of 34AWG
It was a concern that the configuration in the first wire.
part compromised the balance force/moment data
because of the large number of pneumatic tubes Table 1 provides the tubing and wiring
and instrumentation wiring bridging the metric and requirements to each instrumentation system
non-metric portions of the balance. Therefore, before and after the process improvement. It
tubing and wiring would be removed from the should be noted that the actual number of wires
model and a reduced test matrix repeated as the bridging the balance increased from 58 to 115
second part of the test program to obtain the requiring an increase in the model build up time to
highest possible quality force and moment data. ensure proper configuration and quality are
This typically adds 25% to the duration of a test maintained (34AWG is very fragile and difficult to
and also increased total test cost. work with).
Several facilities throughout the world have been With these changes in place the next effort
successful in developing approaches and concentrated on crossing the balance and
methodologies that ensure high quality maintaining sufficient slack to avoid any loading
force/moment data whilst simultaneously making between the metric and non-metric ends. This
model pressure measurements. Because of this was accomplished by avoiding bundling of the wire
tremendous potential cost savings the NTF also and tubes together. Wires and tubes were
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American Institute of Aeronautics and Astronautics
AIAA-2001-0756
splayed and allowed to move freely in the Offsets other than the ideal as presented in figure
longitudinal direction. Additionally, because the 9 must be further examined. This figure shows an
NTF operates over a wide temperature range offset with a constant slope that is indicative of
(120°F to -250°F) sufficient slack was provided to inaccurate weight tare or weight tare correction.
allow for thermal contraction. These changes are Also shown is an offset that is not centered about
shown in figures 6 and 7. zero that indicates a poor WOZ. Sudden jumps or
hysteresis are indicative of cables/tubes/seals
Verification Method hanging or sticking and releasing.
A key to the successful implementation of
simultaneous force/moment and pressure testing The axial component offset is highly dependent on
is an easily performed and comprehensive the angle of attack rate sweep as shown in figure
verification method. At the NTF this requires a 9. This is attributable to lags between AoA and
verification method that can be performed with the balance gage signal processing and to inertial
model at both ambient and cryogenic conditions. loads.
The verification method used at the NTF is to This process is repeated at every phase of the
graphically monitor the "balance residuals" during model build up to ensure the tubes and wires do
a wind-off angle of attack sweep of the model. not load the balance. If a problem is indicated the
The "balance residuals" are defined as the offset tube and wire layout is modified and the entire
in balance loads that remain after the weight tare, process is repeated. When the model is installed
wind-off-zero, balance interactions and in the tunnel, and throughout the test, (air and GN2
temperature corrections are applied. 2 In an ideal operations) the process is repeated to ensure data
case the balance residuals should be zero at all quality is maintained.
model angles during wind-off conditions. If the
wind-off balance residuals are not zero, then either Summary
the corrections have been misapplied or the The approach and methodologies presented for
balance is being fouled. using simultaneous force/moment and pressure
measurements result in significant savings (25%
Figure 8 shows the plot of Balance Residuals vs. time reduction and cost savings) for a test
Alpha used for the verification process. The x-axis program without compromising data quality. The
for each plot is the model angle of attack ((x) and process implemented at the NTF has shown the
the y-axes are the respective units for each ability to obtain high-quality force/moment data
balance component (Ibs, or in-lbs). while simultaneously making model pressure
measurements. Additional benefit is obtained from
In preparation for this verification process, several tracking balance residuals since it provides insight
events must occur to ensure accurate and proper to the quality of a weight tare, wind-off zero and
interpretation of the results. First, an accurate applied balance corrections.
weight tare must be obtained. The accuracy of
weight tare is a prime factor in the magnitude of FUTURE IMPROVEMENTS
the residual. Second, a wind-off-zero (WOZ) must Continuous Pitch
be taken with the model in the current position For the continuous pitch test technique, data are
(upright or inverted) for the angle of attack sweep. obtained continuously over a specified angle-of-
It is extremely important to have a good WOZ (true attack range at given test conditions. There are
(::]=0and 13=0). several possible benefits to this approach,
including a reduction in test time as compared to
Once these events are completed, the model is pitch-pause testing, and smoother testing through
moved at a constant pitch rate over the full range pitch buffet.
of test angles. This verification process is used for
both an upright and an inverted angle-of-attack To implement this technique at the NTF first
sweep. As the data is collected, the data trends on required the implementation of drag feed-forward.
the "balance residuals" display are monitored. Changes were then required to the model control
Ideally, when the model is moving all the data and data systems. Once these modifications were
should have a small, constant, and symmetric made, an initial checkout of the system was
offset from the zero load line for all model angles. performed. Figure 10 shows the pitching moment,
axial force, and normal force data for two different
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American Institute of Aeronautics and Astronautics
AIAA-2001-0756
pitch rates, as compared to traditional pitch-pause
data.
160.0
The results presented in figure 10 show that the
pitch pause polar takes 260s as compared to 40s
for a continuous pitch polar at 0.25°/s. This
savings corresponds to a potential 80% cost
savings for a test. The continuous pitch polar at
0.25°/s also obtained higher c_through pitch buffet.
Initial results are promising, but more work needs
0.40 060 080 100 1.20 140
to be done to make this a standard test-technique,
Mach Number
including addressing the current time lag in the
angle-of-attack measurement system, pressure
Figure 1- NTF Operational Envelope
measurement systems and addressing data
quality issues.
12 I
CONCLUSIONS
! I J"
10 ¸ j
The NTF is working to reduce the cost of testing /
/-
through several process improvements while still 8 /'J
maintaining data quality. The implementation of
drag feed-forward Mach number control and
O
simultaneous force and pressure testing are two "O
cost reduction examples presented in this paper
02 _
that can also be applied to other tunnels.
The NTF continues to develop continuous pitch as
a testing option as an additional method to reduce -2 - [ "
-4
costs and maintain data quality. The combined
0 200 400 600 800 1000 1200
use of continuous pitch and drag feed-forward will
also improve testing techniques near the stall afwa, Ibs
buffet boundary that may allow for testing safely Figure 2 - afwa vs inlet guide vane angle
through buffet.
REFERENCES
1. Balakrishna, S.; Kilgore, W. Allen; and
0.02 1--
Thibodeaux, J. J.: Control of Large Cryogenic
Tunnels, AIAA 92-3930, 17th Aerospace 0.015
Ground Testing Conference, July 6-8, 1992,
Nashville. TN
2. Foster, J. M.; and Adcock, J. B.: User's Guide _ 0.01
afwa
for the National Transonic Facility Research
Data System, NASA Technical Memorandum
110242, April 1996 0.005
_0 10 20 30 40 50 60 70
Reynolds number/chord, million
Figure 3 - Reynolds No. vs 5_g_Jafwa
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American Institute of Aeronautics and Astronautics
AIAA-2001-0756
PT N
Mach Set Pt
afwa
i rill
Mach No.
Schedule igv-afwa Inlet Guide Vane
PI Control Component Normalization
Law
N
Flow _1_ PT
afwa
Figure 4 - Schematic of Mach No. Controller
Drag FF "OFF" Drag FF "ON"
Ii
II J I
0.824 !f' 0.824 i !
I'1
0.822 0.822 ! 1_,,1
0.82
'' "'_ _f_ll I_, 0.82, I'I/_F,p_r!_II', r' l Tol.
l!I' i
0.818 0.818 m
i
I i
0.816 0.816 f
!
0 I 10 20 30 40 0 1Q 20 30 40
I I Time,s I _ Time,s
!1
! |
3000 3ooo .j,
I
2000 ""1 2OOOl II! +
i,
1000 1000! il i
.o
-- 0 ij r ! ! 0 I
u_" u_" I
z _1000 z _1000 I
, --r'--
-2000
-2000 ,
? i
-3000 -3000
0 j 10 20 30 40 0 l(J 20 30 40
I ! Time,s I m Time,s
!1
II
(z=0 ! I(x=-4
c_=01 Io_=-4
Figure 5- Mach No. Performance with Drag Feed-forward
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American Institute of Aeronautics and Astronautics
AIAA-2001-0756
Before After
Number Size Number Size
30 26/24 57 34 AWG
AWG
5 24 AWG 19 34 AWG
8T/Cs 24 AWG 5RTDs 34 AWG
(2wireseach) (solid) (4 wires each)
6 0.060" 6 0.060"
Nylon Teflon
12 26 AWG 12 34 AWG
2 24 AWG 4 34 AWG
1 RTD (3wire) 24 AWG 1RTD (3wires) 34 AWG
Table 1 - Before and After Changes to Wires and Tubes
Wires/tubes cradles
Not tied or laced together
Slack to allow for thermal changes
Figure 6 - Instrumentation in balance
adaptor area
installed with slack and "s" bends to
Figure 7 - Model cavity area for ESP and
AoA System
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American Institute of Aeronautics and Astronautics
AIAA-2001-0756
18,e 3°.0
- 'q _i !! !
- ,_._.
__ ......_..;..... ""---__ SF
.-iC! ...................
-_. 2di ........
i i al ! T i
12e.e
-:[
?_: _;I#:i,::,;N! 5:NS??N _'_;i;2_;;: . ;i: : i
.L_ -
-,,,.-1:_?:'_ }_ _q22f_!_ i_1
I) i 3 ! i )1 i i 1
-5 5 -5 5 -5
_i_ii_ _i_ ¸
Figure 8 - Balance Residuals vs Alpha
Bad weight tare Bad WOZ =.,
2.° 2,O
" _[I][ ...............
4_1 AF
......................
1.°
_ _,_-_ _,_,_._.!:_,,_
o., 1%,,,!7! ..:-._, - o.0
-I,
-2,
:' -" i i ,'
5
or
2.1!t
-_1 AF
I.J :_ NTF 113 Series
Quoted
Accuracy
0.0 Axial: _+1Ib
i" t i
Figure 9 - Balance Residual vs Alpha for Axial
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American Institute of Aeronautics and Astronautics