Document
THE APPLICATION OF MEASUREiHF.NT TECHNI(2UES TO TRACK FLUTTER TESTING H. R. Rog/in -- U. S. Na,'al Ordnance Te_t Station.
China Lake, California Abstract aircraft damage, ejectable components, and for the development and calibration of inertial guidance sys- This paper discusses the application of meas- tems and components. On the high-speed track, urement techniques to captive flight flutter tests at large test items can be brought up to supersonic the Supersonic Naval Ordnance Research Track velocities and sustained at these velocities long (SNORT), U. S. Naval Ordnance Test Station, China enough to make the observations and measurements Lake, California. required and stopped intact. One of the paramount virtues of the supersonic track lies in the relative The high-speed track, by its ability to prove the ease with which instrumentation, both photographic validity of design and to accurately determine the and electronic, can be precisely applied to the point actuai margin oI salety, otlers a umque memos oI ol action to insure optimum coverage.
flutter testing for the aircraft design engineer.
The problem of flutter has, in recent years, been given primary consideration in the design of IN TRODUCTION high-speed aircraft and missiles. The application of the supersonic track to flutter testing has been the In the few years that high-speed tracks have result of efforts to find more adequate means of been in existence, their usefulness has been demon- evaluating and testing new designs in their progress strated as a vital laboratory instrument in expanding to the flight test stage.
knowledge in many scientific fields. Capable of pro- viding high linear accelerations of relatively long duration with dependable recovery of the test item for SLED DESIGN examination and retesting, the supersonic track offers nearly all the advantages of laboratory testing, com- In supersonic track flutter tests, the test item bined with the advantages of free flight.
is mounted on a track vehicle properly designed to realize the required degree of simulation, and a The versatility and control of the test environ- series of runs are made, each at discrete incre- ment offered by the high-speed track provide an opti- ments of velocity until either flutter of the test item mum medium for experimental studies in the best of occurs or an adequate margin of safety has been analytical procedures. Tracks have been successlully demonstrated. A general-purpose sled is used where used for the captive flight testing of rockets, guided the flutter characteristics of these surfaces are not missiles, model or full-scale airplanes, or their com- unduly influenced by the aerodynamic effects of the ponents, under conditions approximating free flight vehicle itself.
into the supersonic range, including measurement of thrust, acceleration, velocity, lift, drag, vibration, Figure 1 is a view of a general-purpose track shockwave effects, flutter, and aerodynamic heating.
sled used for vertical stabilizer flutter tests. Fig- They have been used also for aeroballistic tests of ure 2 is a view of the same sled adapted for flutter high-velocity launching of rockets or projectiles, as tests of a horizontal stabilizer. It may be necessary well as tests of fire-control systems, fuze function, to incorporate an entire fuselage into the sled design to preserve the aerodynamic and structural effects
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on the stability of the tail structure, as shown in Figures 3 and 4. It is, of course, necessary that the complete control systems associated with the tail structures be incorporated into the design of the sled structure.
Figure 1 . General Purpose Flutter Test Vehicle with
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Vertical Stabilizer Figure 4. Navy Flutter Test Sled Utilizing Entire Fuselage of Plane CONTROL OF SLED VELOCITY The design of the sled vehicle and the propul- sion system to be used is mainly a problem in attain- ing the required velocities. It is convenient to con- sider the progress of the sled down the track as being in four distinct phases: the acceleration phase, the low-acceleration phase (or in other types of track tests, the sustain phase), the coast phase, and the braking phase.
Figure 2. General Purpose Flutter Test Vehicle Adapted for Horizontal Stabilizer Tests The acceleration phase is achieved by several rocket motors firing together o r in sequence, or by the use of one or more detachable booster sleds ac- celerating the main vehicle. When the thrust of the rocket motors is equal to the aerodynamic dragof the sled, a condition of zero acceleration is achieved, and the sled is sustained a t a constant velocity. In cer- tain flutter tests, it is required that the test item be
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accelerated to a velocity well below the expected critical velocity, and then accelerated more slowly to the critical velocity. For such tests, additional thrust is staged as required to bring about the low acceleration desired.
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:k In the coast phase, the test sled is decelerated
by the action of aerodynamic drag and track sliding friction. The braking phase adds the water-braking forces.
-- Accurate evaluation of all the acceleration and
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deceleration forces is necessary in designing the test vehicle to meet the test requirements. It is desirable, Figure 3. Track Flutter Test Vehicle Incorporating of course, to accelerate the sled as rapidly as possible Entire Fuselage in Sled Design to the required velocity so as to conserve range dis- allow the final velocity to be controlled within very tance and to permit adequate time for observation close limits. It can be seen, therefore, that by the and measurement of the behavior of the test item careful selection of available rocket motors and by before the braking phase must be started.
the use of such techniques as coast periods, sled velocity can be regulated in controlled increments The structural strength of the vehicle places for flutter tests.
limits on the acceleration that can be applied.
Strengthening the carriage to withstand more acceler- INSTRUMENTATION FOR FLUTTER TESTS ation increases its weight. The loads imposed on the sled structure for any specified maximum velocity are in almost direct proportion to the total weight of Photographic and electronic instrumentation is the test vehicle. It is mandatory, therefore, that used to observe and measure the motions of the test weight be conserved not only to reduce these loads item throughout the entire high-velocity portions of a but also to reduce the amount of thrust required to flutter test. Measurements on these records are made achieve the desired velocity. It is perfectly possible to determine the velocity at which flutter occurred, that the addition of more thrust can result in a lower the frequency of the flutter, and the shape of the maximum velocity due to the weight of the additional flutter mode.
rocket motors.
Electronic Instrumentaton Figure 5 shows a typical velocity-distance pro- file of a flutter test in which a single staging of the The flutter frequency and the flutter mode propulsion rockets was used. Figure 6 shows a typical shape can be determined by the use of transducers three-stage velocity-distahce profile of a flutter test attached to a sufficient number of points on the test requiring a low-acceleration phase near the critical item to measure the deflections of the surface. The velocity of the :est item.
use of the accelerometer type of transducer, although offering the advantage of direct measurement, com- In this test, an additional coast phase was pro- plicates the instrumentation system and the assess- grammed between the first and second stages to ment of the data.
ACCELERATION BRAKING
.... PHASE_ COAST PHASE PHASE
IZUU - .....
J--M-AXe7.,'%% MAX. DECELERATION -6O's
IO00.VELOCIT_' [ "...%
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,.._ouu[ .. i °°'°°. DECELERATION
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600_ ""°..o. MAX.
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_.._ 400 k/ DECELERATION
200f i MIX.ACCELERATION 13 g_'s %%.
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l I I I I I = i _ _ I I I 0_
0 2 4 6 8 10 12 14 '21
RANGE DISTANCE (lO00sof FEET)
Figure 5. Typical Single-Stage Flutter Test Velocity-Distance Profile
HIGH LOW
ACCELERATION &ELERA& A COAST BRAKING i
PHASE I
I I PHASE I I PHASE ! PHASE
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MAX. VELOCITY I I
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1000 . DATUM VELOCITY -+*mmmommC.* MAX. ; I * . MAX. DE~ELERATION - 4 . 8 s ~ ~
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0 1 2 3 ' 11 12 13 14
RANGE DISTANCE (1000s of FEET)
Figure 6. Flutter Test Velocity-Distance Profile Achieved by Three Stages of Propulsion Since the output of the accelerometer i s pro- portional to the absolute acceleration, the acceleration of the sled is added to the output as "noise" and must be subtracted to determine the primary data. Accel- erometers sense, in addition, the random vertical, longitudinal, and transverse motions of the sled during its run. These motions must be measured by addi- tional transducers and instrumentation in order to obtain the relative deflections of the test item itself.
Accelerometers a r e expensive and must be mounted internally, and a r e lost if the test item is destroyed during the test. Strain gages, on the other hand, are inexpensive, can be mounted elther intern- ally or externally, and, by proper calibration, will indicate the direct structural deformatim cf the s'a- 1 . ...
uiiizer assembly. Conventional static load-deflection tests are made to convert strain gage readings to structural deflections in the calibration process.
Figure 7. Section of Telemetry Receiving-Recording FM/FM telemetry systems a r e normally used Station at SNORT to transmit the transducer outputs to the, ground- based recorders. In the FM/FM system, the outputs of the transducers modulate sub-carrier oscillators whose outputs are multiplexed into a composite sig- and received a t the ground station (Figure 7)) where nal. This signal is then used to modulate a c a r r i e r it i s demodulated and the multiplexed signal recorded frequency. The carrier is transmitted from the sled on magnetic tape. The magnetic tape or "master" is played back through bandpass filters, which separ- ate the frequency-modulated sub-carrier frequencies, to the various discriminators. One discriminator is used for each sub-carrier used in the sled-borne sys- tem. The output of each discriminator, which is a replica of the respective sled-borne transducer out- put, i s then recorded, along with other discriminator outputs, on a recording oscillograph for evaluation and assessment.
8 illustrates a typical sled-borne FM/ Figure FM telemetry system for flutter tests. Figure 9 is a typical FM/FM telemetric record obtained on a track flutter test. This record shows the initiation of flutter with build-up to destruction of the test item. The timing trace permits correlation with other recorded data while the track coil record indicates range distance.
Instead of a telemetry system, sled-borne re- Figure 8. Typical Sled-Borne Telemetering System corders, either the magnetic tape or recording os- for Flutter Tests cillograph type, can also be used to record trans- ducer outputs. However the rather rugged environ- ment of the sled or the need for many channels of information may preclude their use. There is also a problem of time-correlation of data if sled recorders are used. Means must be provided to correlate the range master timing system. If a timing oscillator sled-recorded information, either by sled-borne os- is used it must be very stable, o r its output must be cillator or by a sled-borne timing receiver, to the telemetered for comparison with the master system.
TINIWG T R A C E 1
Figure 9. Typical FM/FM Telemetric Record of a Flutter Test Photographic Instrumentation L ? flutter iesling high-speed photography i s invaluable in determining the nature of the lifting surface motions. Photographic coverage can be either by sled-borne cameras to view the test surface in its own frame of reference, o r by ground-based equipment, either fixed in place so as to cover the significant portions of the test run o r installed on tracking mounts.
Sled-Borne Cameras Due to the rather extreme physical environment of the test sled, special photographic recorders a r e used. Some instruments normally used for ground installations, such as the Fastax, have been modified to withstand this environment.
The newer prism-type cameras, such as the Wollensak Fastair and the Fairchild HS100, have been used very successfully for on-board recording, and offer sampling rates up to 5,000 per second. Varisus lenses a r e available for use with these cameras, the choice dependingupon the configuration of the sled and the test item. These cameras offer certain weight and power advantages over the Fastax camera, although the Fastax is still Figure 10. M-45 Tracking Camera Mount used for used for on-board recording. In addition to these Tracking Studies of Sleds cameras, two pin-registered cameras for sled use have been developed; oce has a 35mm half-frame format, the other a 16mm full-frame, offering frame length lenses, for tracking studies of high-speed test rates at 200 and 300, respectively. These cameras vehicles. These units a r e mobile, self-powered, and will operate at better than 50 g ' s in any axis.
provide tracking rates up to 60" per second. These mounts a r e normally used on 25-foot highdirt mounds Ground-Based Cameras located 3,000 feet off-track at various distances down range. Placing the mounts above the desert terrain tends to minimize image degradation due to heat waves while their 3,000-foot off-track position not Ground-based photographic instruments a r e lo- only protects the operator but gives him some advan- cated either off the track o r on track overheads.
tage in tracking fast-moving sleds.
Their down-range location and field of view a r e pre- set on the basis of the best available prediction of the position of the test vehicle during flutter of the test item. Several cameras can be set up a t different 10- INSTRUMENTATION CONTROL cations to provide over-lapping coverage if required.
The Eastman High Speed camera, offering 1 6 n m Photographic ground instrumentation i s usually black and white o r color recording at frame rates up contrclled on a time-basis by an adtomatic sequencer.
to 3,000 per second, and the 16mm and 35niin Fastax, Figure 11 i s a view of the SNORT programmer which for black and white recording a : up io 5,OOC frames supplies control signals a t the proper time andduration p e r second, are used for high-speed recording from to start and stop instrunientation equipment. !t -!so ground .locations. Various lenses, up to 48" in focal prs~icies iiie p u k e at "zero" time which actuates the length, a r e available for these cameras. The 1 6 ~ ~ firing contactors in the blockhouse. At each instru- zsd t h ~ 35mm Piitchell cameras a r e used for medium ment locaticn down range, a control box receives the speed recording (up to 120 frames per second) xith signal and in turn controls power to the camera.
lenses to 96" in focal length available.
The control of ground-based cameras operating at high-frame rates becomes critical since such Tracking Mount cameras may provide only fractions of a second re- cording time. Since it i s necessary that such cameras be properly sequenced with the event, carbon rods o r The "M-45" tracking-camera mount (Figure 10) micro switches, which a r e broken o r actuated by the is a. basic tracking unit capable of supporting both passage of the sled, a r e used to effect camera control Mitchell and high-speed cameras with long focal the photograph.
Figure 13 is a rear view of a general purpose flutter test sled showing the knife bladesused for control of rocket staging and for instrumentation equipment. This view also illustrates the water- brake probe extending below the sled.
Figure 11. SNORT Programmer for Control of Instrumentation During Test Firings on a sled-position basis instead of the time-basis control afforded by the programmer.
Control of sled-borne photographic and elec- tronic equipment is accomplished by either the range programmer (with pull-away plugs) o r by the use of a sled-borne pistol switch actuated when knife blades on the sled cut charged screens mounted on the track beam. Squibs in the switch a r e fired in this manner to either open o r close contacts for the control of the on-board equipment. Knife blades a r e also used to effect rocket staging.
Figure 13. Rear View of General Purpose Flutter Figure 12 shows a typical instrumentation con- Test Sled Showing Knife Blades and Water trol panel mounted in the sled. The battery pack for Brake Probe photographic cameras is located on the left, and the is shown on the lower right of pistol switch assembly The frame rate of the high-speed cameras used in flutter tests must be sufficiently high to permit detailed examination of the test item motion on an extended time basis. At least 20 frames of recording is required per cycle of flutter motion, and s o the minimum frame rate must be at least 20 times the expected flutter frequency. The film capacity of the particular camera and the required recording time set limits on the maximum frame rate that can be used.
Sled-borne cameras a r e usually started before the sled rockets a r e fired to eliminate their starting under high linear accelerations. In determining the maximum frame rate of these cameras, adequate con- sideration must be given to the times involved in the acceleration, and high-velocity phases of the test as well as the required coverage during the coast phase.
Timing Systems Time correlation of photographic and other recorded data i s obtained by the use of master range Figure 12. Typical Sled-Borne Instrumentation Con- timing systems. Timing pulses at various rates are trol Equipment transmitted by radio links to the instruments down Instantaneous velocity determinations can be range requiring time-correlation. Timing signals made by the use of two magnets mounted aknown are provided to sled-borne instrumentation by either distance apart on the test s l e d o r by the use of track- a sled-borne fixed-frequency oscillator, o r by a sled- mounted current-conducting glass rods connected to borne receiver for reception of the range time sig- the track coil system. Measurement of the time nals. When required, the fixed-frequency oscillator interval between the magnet pulses or pulses gener- signals can be telemetered and recorded for compar- ated by the breaking of the rods, yields velocity ison with the range master system. The rocket- determinations at specific points down range.
firing pulse is used as a reference o r staring point in Figure time, which is usually considered as "zero time". 14 i s a section of a sample record of the track coil are used: (1) a nine- system using two sled-mounted "U" magnets and the At SNORT, two radio links glass-rod break system.
channel pulse coded modulated c a r r i e r of 505mc, and (2) a single channel pulse amplitude modulated c a r r i e r of 360mc. The single channel equipment is used to More precise velocity data can be obtained at SNORT by means of a precision velocity measure- c a r r y the lOOBCT signals. The 9-channel PCM ment system, more commonly known as "VMS". The equipment transmits the 100BCT, and 8 other signals instrumentation of this system consists of two data as required between d.c. and 10KC.
sources: sled-position vs. time i s measured with the magnetic track coil system, and sled-acceleration Acceleration-Velocity Data Systems a sled-borne accelerometer vs. time i s measured by Sled position as a functionof time i s the primary and a PDM telemeter system. The tape recorded data data requirement of every flutter test conducted on the i s converted to digital form and entered into the IBM supersonic tracks since it yields, by calculation, 701 computer by automatic assessment equipment.
information on velocity and acceleration. The posi- The two different s e t s of data are combined by a tion-time measuring system at SNORT i s a track coil near-optimum digital-filtering technique to provide a o r magnetic-pickup system. s e t of hybrid wide-bandwidth data.
It consists of a permanent magnet, either of the This system is capable of measuring the vel- "U" or "E" configuration, mounted on the test vehicle, ocity of a test vehicle over a range of 200 feet p e r pickup coils mounted every 100 feet for the entire second to 2,000 feet p e r second, to an accuracy of 21,500-foot length of track, and transmission lines 0.1 feet per second o r better, and with a bandwidth connecting the coils to the terminal equipment in the of 50 cycles. Figure 15 i s asection of a typical track Test Control Building. When the magnet passes over coil record using a sled-mounted "E" magnet. The the coils, pulses are generated which are recorded V M S precision pulse, shown on the record, accurately by the terminal equipment. The time between succes- indicates the cross-over o r "zero" point of the magnet sive pulses determines the average velocity and pulse.
average acceleration of the test vehicle between coils.
BINARY DECIMAL
100- PPS BINARY CODED TIMING\
CODED TIMING.
1000- PPS
REFERENCE
LINES
Figure 14. Section of Sample Record of Time-Position Data Using Sled-Mounted "U" Magnets and Glass-Rod Break Circuits
BINARY DECIMAL
CODED TIMING.
100-PPS BINARY CODED TIMING \
'HS
Figure 15. Section of Sample Record of Time-Position Data Using Sled-Mounted "E" Magnet
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CONCLUSIONS trolled velocities, the high-speed track offers a unique method of flutter testing. By its ability to
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prove the validity of design and to accurately deter- l mine the actual margin of safety, the high-speed With the development of measurement techniques track has become a much needed test facility for the and testing procedures, coupled with the ability to aircraft design engineer. reproduce realistic free-flight environments at con-