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FM TELEMETRY AID FREE -FLIGHT TECHNIQUES FOR AERODYNAMIC
MEASUREMENTS IN CONVENTIONAL WIND TUNNELS
by
Ronald J. Hruby and John B. McDevitt
National Aeronautics and Space Administration
Ames Research Center
Moffett Field, California
N65-35260
(ACCESSION NUMBER) (THRU)
j? s~
(PAGES) (CODE)
tZ/ASF
(NASA CR OR TMX OR AD NUMBER)
Presented at Twenty -First Semi-Annual Meeting
of the Supersonic Tunnel Association
Princeton, New Jersey
April 6-7, 1964
GPO PRICE $
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A 6b
SO
INTRODUCTION
Aerodynamic data obtained in conventional wind tunnels are
subject to possible interference effects from the model-support
system. Estimates of support -interference effects become uncertain
at hypersonic speeds, especially in studies of base flows or flows
over afterbodies on shapes such as the Apollo capsule. Dayman at
JPL (ref. 1) has successfully utilized a free-f light technique in
conventional wind tunnels to obtain force and moment data free of
support -interference effects. A natural extension of this technique
would be the incorporation of a telemetry system so that surface
pressures, convective heating, etc., could also be measured in the
absence of support -interference effects. The development of such a
telemetry system was successfully completed recently at the Ames
Research Center and descriptions of this sytems and the associated
free -flight technique are presented herein.
The free -flight motion of the test model is achieved by the use
of a simple pneumatic launching device placed in the tunnel down-
stream of the test section. The launch device propels the test model
upstream with sufficient velocity that its upstream motion terminates
near the upstream edge of the test -section viewing windows. High-
speed motion pictures of the model provide an extensive photographic
record which is evaluated in the same manner as in ballistic -range
testing. An important advantage of this technique, compared to the
usual ballistic -range method, is that the acceleration forces on the
models are relatively small. This, of course, greatly enhances the
possibility of designing accurate and reliable telemetering devices.
In this paper a description of the EM telemetry system is pre-
sented first, followed by a discussion of the free-flight technique.
Base -pres sure measurements for a 15° half -angle cone in free flight
at hypersonic speeds are then presented to illustrate the practical
application of these methods in aerodynamic testing.
FM TELEMETRY SYSTEM
The relatively small accelerations encountered during launch
and free-flight motion of the test model permit use of state-of-the-
art circuitry and components in the telemetry system. Nevertheless,
considerable care must be exercised in the choice of electrical com-
ponents to insure stability of the system, and the use of a wide FM
bandwidth is necessary to eliminate systematic errors as much as
possible .
An attempt has been made in the past (see ref. 2) to measure
stagnation pressures by radiotelemetry from a model magnetically
supported in a wind tunnel. It was found that large temperature
- 2 -
interactions with the telemetry center frequency could arise and
methods for reducing the temperature effects were suggested. In the
present telemetry system possible temperature interaction effects
were effectively eliminated by use of a temperature compensating
circuit in the telemeter and by mounting the telemeter in plastic
test models so as to thermally insulate the electronic components
from the surrounding hypersonic flow field during wind-tunnel testing.
The basic components of the telemetry system are shown in figure 1.
The 1. 5 -milliwatt VHF telemeter is implanted in the aerodynamic test
vehicle, as shown in figure 2. It consists of a miniature transistor-
ized oscillator with a capacitance -type pressure transducer incorpor-
ated in the circuit such that the oscillation frequency is controlled
by the differential pressure within the transducer. The receiving
system utilizes a VHF antenna (receiving coil) mounted directly on
the test-section viewing window. The antenna is coupled through a
VHF preamp to an FM receiver. The demodulated signal drives a record-
ing oscillograph and, if desired, an analog -to -digital converter and
digital data recorder. A description of the various components of the
system follows.
Telemeter
The telemeter is a VHF common -emitter oscillator (see discussion
in ref. 3)> frequency modulated by means of a variable -capacitance
pressure cell. The oscillator circuit and pressure transducer were
designed for use with an FM receiver having a tuning range of 105 -
140 me and a maximum I.F. bandwidth of ±0.8 me.
Details of the oscillator circuit and its mechanical layout are
shown in figure 3. The complete telemetry unit is mounted on a 7/8-
inch-diameter printed-circuit wafer. The two -turn oscillator coil is
a printed-circuit element located on the outside perimeter of both
the front and back faces of the printed-circuit wafer. The feedback
element consists of a printed-circuit secondary coil located inside
the oscillator coil and provides a four-to-one reduction in signal.
The transistor used in the oscillator circuit (Fairchild FSP-293-1,
2D709, or equivalent) provides a gain bandwidth product of 4x10 s cps
in this application. A common -emitter circuit is used because it pro-
vides a more stable operation when the operating frequency is near
the l80° phase shift frequency of the transistor (see discussion in
ref. 3)* A transformer feedback circuit was utilized because capacitor
Cx corrects the feedback phase shift arising from the transistor VHF
a, characteristics. Temperature stability is provided by the therm-
istor T. Capacitor C2 provides a low impedance path across the
batteries to eliminate any reactivity they might introduce into the
circuit. Resistor R adjusts the quiescent operating point of the
- 3 -
circuit. The best operation was obtained with a quiescent collector
current Zc-q q of about 0.5 ma. A somewhat greater value of Ic-p ^
was used to minimize possible variations in circuit performance due to
transistor characteristics .
The nominal value of the transducer capacitance (no differential
pressure) is 7p.[if. With the coil and circuit of figure 3 > this gives
a nominal operating frequency of 117 me. However, manufacturing tol-
erances of the printed-circuit wafer and of the pressure transducer
cause the nominal center frequency to fall in the range 117 ±12 me.
The capacitance -versus -pressure slope of each transducer combined with
the available bandwidth of the FM receiver determines the usable pres-
sure range of each transducer. Generally, the pressure differential
which would give the maximum allowable frequency modulation is a very
small part of the linear mechanical range of the pressure cell. In
the present case the range of the transducer capacitance is ±0.096|apf
for the available bandwidth of ±0.8 me.
Antenna and VHF Preamplifier
The oscillator coil axis is oriented parallel to the center line
of the wind tunnel. The field pattern within the tunnel corresponds
to a cylindrical TE01 mode below cutoff (ref. 4 ). The antenna
(receiving coil) is oriented to provide maximum coupling to the tunnel-
wall-equivalent -surface current. Energy coupled to the receiving coil
develops a voltage across the 502 cable -matching resistor and drives a
preamplifier with high input impedance. The preamplifier output is
matched to a 502 coaxial cable which in turn drives the high impedance
RF input of the PM receiver.
The VHF preamplifier was used to allow flexibility in the RF
signal handling. A circuit diagram is shown in figure 4 . If two
receiving coils are used (e.g., one on each of the test section windows)
the preamplifier provides rejection of common-mode extraneous signals.
This differential connection of receiving coils and preamplifier pro-
vided a 6 db to 15 db improvement in signal -to -noise ratio. Inductance
L 0 provides the common -mode rejection at VHF frequencies. Transistor
Q 3 adjusts the sum of collector currents of transistors Q x and Q, 2 to
be 20 ma; hence, the output voltage has a source impedance given by
resistor R3, or 502 . The coupling capacitors were Corning Glass VHF
■units • The sib ility of this amplifier to operate satisfactorily in the
differential mode is dependent upon careful shielding between the dif-
ferent stages which is provided by the use of separate input and out-
put compartments. These compartments are shown in figure 5 (a). The
battery compartment and constant current stage are shown in figure 5(^)-
Two essential features of the preamplifier (ref. 5 ) are its high input
impedance (to prevent loading of the receiving coil circuit) and its
!
-In-
constant 50 ohm output impedance (to prevent signal distortion due
to output cable impedance) .
FM Receiver and Data Recorder
The FM receiver used in this system is the Defense Electronics,
Inc. , TMR-5A Receiver with a tuning range of 105-llk) me and an I.F.
bandwidth of 1.6 me. The dynamic range of the receiver is 2 microvolts
to 20,000 microvolts.
A classical FM system would have utilized a bandwidth which was
minimum for the maximum information rate. For example, a signal having
an information content of 1 kc would be transmitted to a receiver
having an FM bandwidth of 0 to about 5 kc or 10 kc. However, the
present telemetry system was developed for a single enclosed channel
where possible errors (ref. 6) in signal might arise from oscillator
detuning, drift, frequency changes due to launch-flight stresses, and
the proximity of large metallic surfaces . These types of errors were
minimized by using a wide -band FM signal. For the deviation ratios
actually used, it was observed that the cumulative frequency errors
at the receiver output were not greater than 2 kc during any specific
test.
The direct -coupled output of the receiver detector drives a
recording oscillograph preamplifier (see fig. l) which provides both
the proper input impedance (for galvanometer damping) and voltage
range for the Consolidated Electrodynamic Recording Oscillograph. A
resistor -matching network was used to prevent overload of the Beckman
Digital data recorder and maintain proper voltage and source impedance
to the CEC oscillograph. This part of the system is also shown in fig-
ure 1. Although the pressure transducer with its inlet tube was found
to be capable of a frequency response dynamic range greater than 1 kc,
for practical reasons, a galvanometer with a flat response to 1 kc was
used. The data were simultaneously recorded on the Beckman Data
Recorder, which records the data in binary coded decimal form on mag-
netic tape, and the CEC oscillograph. The time required by the Beckman
to record one datum point was about O.k millisecond which was adequate
to provide data at frequencies to 1 kilocycle/sec. The recorded base-
pressure data were synchronized with the photographs of the model by
means of a timing pulse which was recorded simultaneously on the photo-
graphs, the magnetic tape, and the CEC oscillograph.
- 5 -
FREE -FLIGHT TESTING TECHNIQUE
The first attempts to obtain free -flight data in conventional
tunnels involved suspension of the test models on wires which were
broken at the appropriate time to release the model into a free -flight
trajectory through the test section. The use of an air gun on the
tunnel center line to propel the test models upstream into the viewing
area has been successfully used, as previously mentioned, by Dayman
(ref. 1). The latter method, when it can be used, has the advantage
of giving at least twice the amount of data of the wire supported system
since the model can be viewed in both upstream and downstream traverses
of the viewing area. The pneumatic launching technique was used in the
development of the present telemetry system and a brief description
follows.
A schematic of the Ames 14-Inch Helium Tunnel and pneumatic model
launcher are shown in figures 6 and 7 . The launch device consists of
a piston and rod on which the model is mounted (fig. 7). The piston
is contained in a tube and "firing" of the test model is accomplished
by release of the restraining pin. The upstream side of the piston is
vented to the tunnel static pressure (much less than p x ) while the
driving pressure, p 1 , is set at a predetermined value such that the
launch velocity, Vi, of the model results in an upstream trajectory
which terminates near the upstream edge of the viewing window (see
fig. 8 ).
The launch velocity V x (ft/sec) to give the trajectory indicated
in figure 8 and the time in seconds to travel the distance S 3 . (ft) is
calculated from the relationships
where
D aerodynamic drag of model, lb
1 % mass of test model, slugs
The time, t, is considered first and the maximum value is used which is
the time for a free-falling object to travel the vertical height h x
(see fig. 8 ). This defines the proper ratio between model mass and
aerodynamic drag. The required launch velocity V x can then be cal-
culated from equation (l).
- 6 -
The initial free -flight velocity, Vi, is relatively small
(usually less than 100 fps) and the required reservoir pressure p x
to achieve this velocity can readily he estimated by assuming that
the driving gas remains in equilibrium and that the expansion is
isentropic. It is also convenient to assume p 2 = 0 (see fig. 7)
since the upstream side of the launch tube is vented to tunnel static
pressure. The initial velocity at launch is then related to the
driving pressure p 1 by the approximate relationship
V
2
1
2^2
%1 + Iftp
ApP-jL^
(D + Df)
(3)
and by use of equation (l) a convenient expression for estimating the
reservoir pressure is obtained
_ D{l + ( s 1/^2 ),[ 1 + (®p/%) ]} + Dp
P x = —3 (U
Apf [7 y (WU)J
where (see figs. 7 and 8)
2 1 length of reservoir, ft
2 2 piston stroke, ft
p x reservoir pressure, lb/ft 2
Ap cross-sectional area of piston, ft 2
mp mass of piston ensemble, slugs
Dp friction drag of piston, lb
and the energy function is given by
f V’ W ’ (i 7fk - Ij
where 7 is the specific-heat ratio. The increase in energy available
by increasing the length of the reservoir, 2 X , for a given piston
stroke, I2, is indicated in figure 9. It is evident that little is to
be gained by using a large value of 2 X ._ Tn the present application a
piston diameter of 1 inch, a stroke of Z 2 = 6 in. , and reservoir length
of 2 X = 12 in. were used.
V /r m7-i
[1 + (22/21)]
(5)
-7-
TEST RESULTS
The first application of the present telemetry system involved
the measurement of base pressures on cones and afterbody pressures
on the current Apollo configuration. The test results for a 15° half-
angle cone are presented here as an illustrative example of the pres-
ent telemetry technique.
Model Details
A schematic drawing of the test model (15° half -angle cone) and
support system on which the model rests until launched is presented
in figure 10. The model and model holder were constructed entirely
of nonmetallic material (plastic) so that the telemeter center, frequency
would not be significantly affected by either the model or the holder.
The plastic model construction also served to thermally insulate the
telemeter electronic components from the surrounding hypersonic flow
field during wind-tunnel testing and thus minimize temperature inter-
action effects.
Since a differential -type pressure transducer was used, provision
had to be made for applying a known pressure level to the "reference"
side of the pressure cell. The inside cavity of the model, where the
telemeter and pressure transducer are located, serves as the reference-
pressure chamber. A hypodermic needle (see fig. 10), inserted through
a small rubber disk at the model base, is used to provide known changes
in the reference pressure during calibration of the system and to moni-
tor the reference pressure immediately prior to launching of the model.
The needle is pulled out of the model during the initial portion of the
launch .
Extensive tests were made in order to determine the accuracy and
response time of the system. By using hypodermic needles having inside
diameters greater than 0.03 inch and applying reference pressures greater
than 2000 microns (greater than about 0.0^ psi) the reference pressure
was subject to errors less than ±50 microns. For a step pressure impulse
applied at the base -pressure orifice the time required for the telemetry
signal to exceed 90 percent of the new steady-state value was approxi-
mately 0.6 millisecond.
For this particular test model, the differential pressure cell
was designed so that a 0.15-psi differential loading resulted in a
0.6 megacycle shift in center frequency of the telemeter. The calibra-
tion of the telemetry system is shown in figure 11, and it can be seen
that the system is linear for loadings up to about 0.05 psi. The battery
life of the telemeter oscillator was about 20 hours, which was more than
adequate for repeated preflight calibrations and tests of the model.
- 8 -
Base Pressure Measurements
The test of the cone model with base -pressure telemetry was made
for a free-stream Mach number of 10. 7 and free-stream static pressure
of 0.046 psi. For this tunnel the flow can be brought to the desired
test conditions in about 5 seconds. A timing device is then used to
initiate the test equipment in the proper sequence . The high-speed
camera that records the motion of the test model in the viewing area
is started first, then the data recording equipment, and, finally,
launching of the model is initiated. Reference time marks are used
to synchronize the motion pictures with the telemetry data recorder
in order that proper interpretation of the data can be made.
The possibility of changes in model temperature affecting the
telemeter frequency was carefully checked. A thermocouple was placed
in the reference pressure cavity of the model near the telemeter
package. The model was sting mounted (not launched) and the tempera-
ture monitored for a tunnel run of 20 seconds. In this time interval,
which is considerably longer than that required for a typical free-
f light test, the temperature at the telemeter did not change.
During launch, and during the free -flight trajectory in the
tunnel hypersonic flow, the test model is subject to appreciable, but
not excessive, acceleration forces. The test model, with the base-
pressure orifice closed, was launched into still air in order to
assess possible g effects. The recorded telemetry data during launch
indicated that the acceleration force did not noticeably change the
telemetry center frequency.
The high-speed data recording of the base -pressure telemetry
reading in free flight is reproduced in figure 12. With the model
near the launcher, point A of figure 12, the base-pressure reading is
quite high due to interference effects between the model wake and the
model-holder portion of the launcher. From point B to C, for which
the time interval is about 80 milliseconds, the telemetry signal is
constant and the pressure value is 0.017 psi. At point C, the model
has fallen into the boundary layer near the tunnel walls and the near
proximity of the telemeter oscillator to the tunnel walls results in
an abrupt shift in telemetry frequency. Shortly thereafter the signal
is lost.
The ratio of model base pressure to free-stream static pressure
is presented in figure 13 for this free -flight test and for prior
tests of the same model mounted on various sizes of support stings.
It is clearly evident that sting interference effects persist even
for very small sting sizes. For example, when the cross-sectional
area of the sting is only 4 percent of the model base area (Ds/Dm = 0.2),
the base pressure is about 50 percent higher than that measured in free
flight.
- 9 -
CONCLUDIWG REMAEECS
The present EM telemetry system Is currently being used at the
Ames Research Center to measure base pressures in free -flight on
conical test models and afterbody pressures on entry vehicles such
as the Apollo configuration. The response time of the pressure sens-
ing system has been found to be less than 1 millisecond in most
applications. Experience to date with this system has indicated that
the uncertainties in the pressure measurements are less than 2 per-
cent for pressure readings greater than about 0.1 psi and less than
100 microns (0.002 psi) for pressure readings less than 0.1 psi. The
pneumatic model launching method used provided relatively repeatable
free -flight trajectories in which the test model was free of support
interference effects for time intervals approaching 100 milliseconds.
The successful development of the present telemetry system was
due partly to the careful choice of components so as to insure sta-
bility in both the telemeter and the receiving equipment, and to the
use of a wide bandwidth EM system. In addition, possible temperature
interaction effects were effectively eliminated by use of a tempera-
ture compensating circuit in the telemeter and by mounting the tele-
meter in plastic test models so as to thermally insulate the electronic
components from the surrounding hypersonic flow field during wind-
tunnel testing.
Modifications to the present system are being made in order to
permit use of the free -flight telemetry technique in shock tunnels.
The development of a telemetry system for measuring convective heating
on models in free flight is also currently under way.
- 10 -
REFERENCES
1. Dayman, Bain, Jr.: Simplified Free-Flight Testing in a Conven-
tional Wind Tunnel. JPL Tech. Rep. 32-346 , 1962.
2. Clemens, P. L. : Radio Telemetry of Stagnation Pressure from a
Wind Tunnel Model Magnetically Supported in Supersonic Flow.
AEDC TDR -62-141, 1962.
3. Hunter, L. P. : Handbook of Semi-Conductor Electronics. Second
ed., McGraw-Hill Book Co . , Inc., 1962, pp. 14-1, 14-23.
4. Ramo-Whinnery : Fields and Waves in Modern Radio. Second ed. ,
John Wiley and Sons, Inc., 1956, pp. 177-^15*
5. Langford -Smith: Radiotron Designer's Handbook. Fourth ed. , RCA,
1952, pp. 450-453-
6. Terman, F. E. : Radio Engineer's Handbook. McGraw-Hill Book Co.,
Inc . , 1943 •
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NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
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Figure 2.- Photographs of typical model with telemeter.
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
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Figure 4.- VHF preamplifier circuit
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
AMES RESEARCH CENTER. MOFFETT FIELD, CALIFORNIA
(t>) Battery and constant -current stage compartment.
Figure 5 • - Concluded.
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AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
national aeronautics and space administration
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AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
_Ll
^■2
. - Variation of f[y, {lz/li)] with Ix/lz-
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
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Center frequency
Ny Telemeter frequency , m.c.
119.2 119.5 119.8
AP , inches of Hg ot 70° F
Figure ii.- Calibration of pressure transducer.
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AMES RESEARCH CENTER MOFFETT FIELD CALIFORNIA
Launcher
AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA
Figure 13*- Effect of sting size on model base pressure.
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AMES RESEARCH CENTER, MOFFETT FIELD, CALIFORNIA