Dynamic and Transient Performance of Turbofan/Turboshaft Convertible Engine with Variable Inlet Guide Vanes

NASA Technical Memorandum 4696

Dynamic and Transient Performance of Turbofan/Turboshaft Convertible Engine With Variable Inlet Guide Vanes

Jack G. McArdle, Richard L. Barth, Leon M. Wenzel, and Thomas J. Biesiadny

APRIL 1996

National Aeronautics and Space Administration NASA Technical Memorandum 4696

Dynamic and Transient Performance of Turbofan/Turboshaft Convertible Engine With Variable Inlet Guide Vanes

Jack G. McArdle, Richard L. Barth, Leon M. Wenzel, and Thomas J. Biesiadny Lewis Research Center Cleveland, Ohio

National Aeronautics and Space Administration Office of Management Scientific and Technical Information Program 1996 Dynamic and Transient Performance of Turbofan/Turboshaft Convertible Engine With Variable Inlet Guide Vanes

Jack G. McArdle, Richard L. Barth, Leon M. Wenzel, and Thomas J. Biesiadny National Aeronautics and Space Administration Lewis Research Center Cleveland, Ohio

Summary loads, so that any turbine power in excess of what is needed by the fan is available at the power output shaft for other A convertible engine called the CEST TF34, using the loads. Maximum turbine power is limited by cycle tempera- variable inlet guide vane method of power change, was tested ture (fuel flow) and speed. For high thrust the shaft load is on an outdoor stand at the NASA Lewis Research Center with reduced or decoupled, as by releasing a clutch. When shaft a waterbrake dynamometer for the shaft load. A new digital power is required, the fan is unloaded aerodynamically. Two electronic system, in conjunction with a modified standard general methods have been devised to unload the fan (ref. 3). TF34 hydromechanical fuel control, kept engine operation One method is based on variable-pitch fan blades—fan power stable and safely within limits. All planned testing was com- is reduced as the pitch is made "flatter." This method has pleted successfully. Steady-state performance and acoustic been demonstrated in tests of engines such as QCSEE (ref. 4), characteristics were reported previously and are referenced. Q-Fan (ref. 5), and Astafan (tested successfully as a convert- This report presents results of transient and dynamic tests. ible engine in a U.S. Army Research and Technology Labora- The transient tests measured engine response to several rapid tories test program). The other method is based on variable changes in thrust and torque commands at constant fan (shaft) inlet guide vanes (VIGV) that can be deflected to change fan speed. Limited results from dynamic tests using the pseudo- airflow and inflow swirl—fan power is reduced as the vanes random binary noise technique are also presented. Perfor- are closed. This method has been demonstrated in fan re- mance of the waterbrake dynamometer is discussed in an search tests (ref. 6) and in tests of a high-bypass-ratio turbo- appendix. fan with no output shaft power (ref. 7). Starting in 1981 the Convertible Engine Systems Technol- ogy (CEST) Program began to study the feasibility of the Introduction convertible engine concept. The program objectives included defining requirements for convertible engine systems and A convertible engine can produce turbofan thrust, turboshaft evaluating an engine using the VIGV method of power change. Power, or any combined thrust and shaft power continuously The system requirements published at that time are given in while operating up to full speed. Convertible engines could be references 8 to 11. The experimental work was done at the used to power verticaUshort-takeoff-and-landing (V/STOL) NASA Lewis Research Center using a TF34-4OOB (8000-lb- airplanes and advanced high-speed rotorcraft. For a jet- thrust class and high bypass ratio) turbofan specially modi- powered V/STOL airplane the convertible feature could be fied to a VIGV convertible engine called the CEST TF34. used to power a remote lifting device, such as a shaft-driven The tests were performed on an outdoor test stand with a lift fan, or to cross-couple the fans in a two-engine configura- waterbrake dynamometer for the shaft load. A new digital tion for safety in case one engine should fail. For a rotorcraft a electronic system was used to control engine speed and convertible engine would operate as a turboshaft engine to variable-geometry actuators. The digital system worked in drive a lifting rotor for takeoff and low-speed horizontal flight conjunction with a modified standard TF34 hydromechanical and as a turbofan engine to produce thrust for high-speed fuel control to keep the CEST engine operation stable and horizontal flight. Analytical studies of high-speed rotorcraft safely within design limits. The test objectives were to dem- have shown that using a convertible engine rather than sepa- onstrate the performance characteristics of the engine and rate engines for lift and cruise would give installation, cost, control system and to obtain data and experience applicable and performance benefits (refs. 1 and 2). to the design of future convertible propulsion systems. In a convertible engine the power turbine drives both the All planned testing was completed successfully. Results of fan and an output shaft connected to some other load. Total the steady-state engine performance tests are given in refer- turbine power is the sum of the power absorbed by all the ence 12, and results of fan acoustic tests, in reference 13. This report describes the results of transient and dynamic tests turbine. The core has a 14-stage axial compressor, an annular using the digital engine control system. The transient tests combustor, and a two-stage, high-pressure turbine. measured engine response to several rapid changes in thrust The engine modifications, shown in more detail in figure 2, and torque commands at constant fan speed. During these were made by using as many existing parts as feasible. The tests an enigmatic 80-msec dead time, between command resulting configuration, called the CEST TF34, was not meant start and the beginning of engine geometry or fuel flow change, to be a production engine. The fan was unloaded aerodynami- was discovered. Limited results from dynamic tests using the cally by deflecting part-span VIGV to change the rate and pseudorandom binary noise (PRBN) technique are also pre- swirl angle of the flow entering the fan tip. The vanes were sented. Some of the dynamic data were not evaluated in detail "part span" because they reached only from the outer wall to because of the possible effects of the 80-msec dead time. the core/bypass flow splitter and thus had little effect on core Typical test results are discussed to bring out important flow. When the vanes were deflected, the fan air load was engine and control system characteristics. The remaining data reduced and the power turbine could drive an external load are presented in appropriate graphical format without com- through the power output shaft. ment. The engine performed satisfactorily in all the tests. Each major modification is described in the following para- Performance of the waterbrake dynamometer, a useful shaft graphs. For more detail consult references 12 and 13. load for many applications, is discussed in appendix A. The Variable inlet guide vanes (VIGV). —A set of 30 vanes pseudorandom binary noise generator is described in appen- was installed just ahead of the fan rotor to unload the fan in dix B. Symbols are defined in appendix C. the turboshaft power mode. Each vane consisted of a fixed forward strut and a movable rear flap. The flaps were deflected together in the direction of fan rotation with hydraulic Apparatus actuators and a unison ring. The actuators were a General Electric (GE) J101 master/slave system with integral Engine servomechanisms and position sensors. VIGV position nor- mally ranged from 0° (fully open, straight ahead) to +840 The test engine was a previously used TF34-400B spe- (nearly closed). For the dynamic tests the flaps were moved cially modified by the manufacturer in the fan section, as beyond the fully open position (as far as -7) for VIGV sketched in figure 1. The standard TF34 is a 6:1-bypass-ratio perturbations around 0°. turbofan that can produce 9100 lb of sea-level-static thrust. Variable exit guide vanes (VEGV).—A set of 44 tandem The single-stage fan is driven by a four-stage, low-pressure airfoils replaced the same number of standard TF34 exit

r- Shrouded fan blade Variable inlet guide vane---\ Variable exit

Power output shaft -

Figure 1.—LEST TF34 convertible engine compared with standard TF34 engine.

Bellmouth Strongback (mounted to thrust-measuring system)

Soft Full-chord Bleed seal— shroud—, valve — Variable exit Forward Variable inlet guide vane 3/8-in. cell spider by 2-in. thick guide vane Bay honeycomb Spider (not in line _ cooling with forward spider)— Core/bypass holes flows litter—/

Nonrotating fairing Rotating power output shaft Figure 2.—CEST TF34 design features.

stator vanes in the fan-tip (bypass) flow. The front foils were Test Stand deflected together with hydraulic actuators and a unison ring. The deflection angle was scheduled to the VIGV deflection The CEST TF34 installed on the outdoor static test stand at to prevent stall buffeting at high VIGV closure. The rear NASA Lewis is shown in figure 3. The engine was mounted VEGV foils were not movable. The actuators were a GE on a strongback and suspended from an overhead thrust sys- J 101 master/slave system with integral servomechanisms and tem capable of measuring up to 10 000-]b thrust. The engine position sensors. power output shaft was connected to a waterbrake dynamom- Power output shaft. —The power output shaft was attached eter by a driveline supported on a pedestal. The driveline to the fan disk and extended forward through the bellmouth contained several crowned splines to take up misalignment. air inlet. The extension included a flexible coupling and bear- Hydraulic power from a facility system having a 23-gal/ ings supported by spider struts built into the inlet ducting. min pump and a 5-gal accumulator provided actuator power Flow splitter.—The core/bypass flow splitter was extended for the engine variable geometry and the waterbrake flow forward to the VIGV to minimize core inlet flow distortion control valve. from the deflected VIGV. Shrouds on the fan blades were part of the splitter. Power Absorber System Fan.—The fan rotor had 28 blades. In the tip region the blade airfoils were the same shape as standard TF34 blades The power absorber system, shown schematically in fig- but included a part-span shroud (part of the modified flow ure 4, controlled power output shaft torque and provided the splitter). In the hub region a revised shape was used to im- shaft power load. The power-absorbing device was a prove core pumping. The fan was the same as that used for the waterbrake dynamometer operated up to 5300 hp as described steady-state tests reported in reference 12. in appendix A. The waterbrake was coupled directly to the Bleed valve (BV). —A core/bypass bleed valve was in- engine power output shaft. Shaft torque was varied by posi- stalled in the flow splitter behind the VEGV to improve the tioning the 4-in. exit water flow valve, which had a linear plug flow match between the fan hub and the core engine at low- (CV = 175) actuated at 2800 psig with a 2.5-in. cylinder, power operation. The valve was a sliding ring valve moved by 2.5-in. stroke, and 15-gai/min servovalve. Water throughflow three GE J 101 actuators with modified F404 servomechanisms was set by the bypass valve. The outflow was collected in an and position sensors. open sump and then pumped back to the cooling tower sup- Inlet and exhaust.—All testing was done with a bellmouth ply. The waterbrake capability, rated by the manufacturer, is and long inlet duct having a combination honeycomb flow shown in figure 5. straightener and protective screen (fig. 2). The engine had the Torque was controlled by a closed-loop system that posi- standard TF34 separate-flow, nonadjustable exhaust nozzles. tioned the exit water valve using feedback from a strain-gage Figure 3.—CEST TF34 engine and waterbrake power absorber installed at outdoor static test stand at NASA Lewis Research Center. load cell on the waterbrake torque arm. Input could be chosen (5) VIGV, VEGV, and water valve position: potentiometers from a manual potentiometer, a signal generator, or a pro- (6) BV position: linear variable differential transformers grammable function generator. (7) Speeds: magnetic pickups plus electronic counters Performance of the power absorber system is discussed in appendix A, which also describes an earlier waterbrake Steady-state data were recorded on the laboratory central configuration. digital data system. Transient and dynamic data were recorded on magnetic tape. In many cases the steady-state por- Instrumentation and Data System tions of transducer outputs were "bucked" with an analog computer to improve resolution of the time-varying portion of Transducers and measurement systems conventionally used the data. for engine performance had adequate frequency response, so that special instrumentation was not needed. For these tests Digital Electronic Engine Control the following types of instrumentation were used: A specially designed digital electronic engine control (1) Pressure: scanivalves (DEEC) system worked in conjunction with a modified stan- (2) Temperature: thermocouples dard TF34 hydromechanical fuel control to keep the CEST (3) Fuel flow: beam-type flowmeter engine operation stable and safely within design limits. (4) Forces: strain-gage load cells Operation was divided into two modes: "thrust" mode, for

4 Bypass Cooling valve tower water inlet Sensed torque

o Manual Fixed orifice Electro Control and hydraulic servo- o Random Preload noise servovalve amplifier 0 T(t) Bypass , ' t. watler Rot Inputs from engine 1 control room Load ceI l,Jl Waterbrake dynamometer

Exit flow valve

Pump Figure 4.—Power absorber (waterbrake dynamometer with control valve at exit) system.

turbofan-powered flight, and "shaft" mode, for rotary-wing or Inputs.—For these tests the inputs consisted of a manual twin-engine V/STOL flight. In either mode the engine could potentiometer setting for the reference shaft speed; a manual produce shaft power and thrust up to the limiting power potentiometer setting, signal generator, or programmable func- turbine inlet temperature. The control strategy for each mode tion generator signal for thrust command; and the standard is illustrated in figure 6. In the shaft mode both the engine TF34 power lever (which set a core-speed governor) for refer- thrust level and the shaft speed were commanded. The control ence core speed. After engine start the power lever was ad- system, using feedback data from the engine, set fuel flow and vanced to maximum so that performance was not limited by engine variable geometry to maintain shaft (fan) speed as the core speed. power output shaft load changed. In the thrust mode only the Mode switch. —The manually operated mode switch thrust level was commanded. The system locked the VIGV in changed control logic by some relatively simple circuit changes the fully open position and adjusted fuel flow to bring the fan in the digital control. This was an important feature of the to a preprogrammed speed to produce the command thrust. A CEST TF34 engine control system because the two control block diagram of the engine control system is presented in strategies could be implemented with the same hardware equip- figure 7. (For some tests the inputs and signal paths shown in ment. figure 7 were changed to measure specific data. The changes Reference shaft speed schedule.—The DEEC set fan speed are described along with the appropriate test results.) Impor- from a preprogrammed schedule to produce the desired thrust. tant features of the system are described in the following The actual thrust was not sensed by the control. The schedule paragraphs. Design philosophy, control schematic diagram, was set up for zero shaft power and was not adjusted for and predicted operating characteristics are discussed fully in variations in thrust caused by changes in core speed, such as reference 14. brought about by shaft load changes. 10x103

CL L 2 Ni 3 0 CI

Q) .n `o ^ 1 Q .8

2 1 2 4 6 8 10x103 Rotor speed, rpm Figure 5.—Rated waterbrake capability.

Performance feedback NF reference Hold reference NF Thrust Control Vary VIGV Engine command system Vary VEGV and BV

(a)

Performance feedback

Thrust Vary NF command _ Control Lock open VIGV Engine Vary VEGV and BV (b) E, Figure 6.—CEST TF34 control strategies. (a) Shaft mode. (b) Thrust mode.

6 Load I Sensed load demand Shaft load anticipation Sensed NF Reference NH (power lever set TF34 to maximum F_ - hydromechanical after startup) T4.5 I fuel control NF limit Reference NF + I I ("beeper") +I I WF Select 2Q + Engine I i minimum J I Sensed

I T I T4.5, NH, PS3 VIGV VEGV I BV Reference NF Power actuator actuator actuator schedule priority bias I WF PS3 I I limits limit , 4 Sensed Mode switch position 1:Shaft mode Thrust VEGV 2:Thrust mode -0 Thrust demand anticipation schedu (cockpit lever— or autopilot) 1 2^ Aircraft inputs or VIGV = 0° schedule test cell controllers Sensed NH NF ^J From engine Figure 7.—Block diagram of CEST TF34 engine control system.

Engine variable geometry.—The VIGV, VEGV, and BV Anticipation.—Anticipation circuits caused a transient varia- were driven by hydraulic actuation systems with proportional- tion in fuel flow for sudden changes in torque or thrust de- plus-integral position controls. The VIGV closure angle was mand in order to minimize speed droop or overshoot. The fuel set by the thrust command signal as modified by the power transients were proportional to the rate at which demand priority bias. The VEGV angle was set to a preprogrammed changed, but a deadband centered around zero rate prevented schedule based on experimental data (ref. 12). The BV open- anticipation for gradual demand adjustments. The thrust an- ing was vaned to a complex schedule by using VIGV, core ticipation was taken from the VIGV position signal. For the speed, and shaft speed data. This schedule was determined tests the load anticipation was taken from the waterbrake analytically and was not investigated further during the torque command; in an aircraft, anticipation could come from testing. any of several possible loads, such as rotor collective pitch or TF34 hydromechanical fuel control.—A standard TF34 power change in a lifting fan. fuel control was modified for the CEST TF34. The modifica- Power priority bias.—With large shaft load; commands for tions consisted of expanding the authority of a trim high thrust would cause shaft speed to droop if power-turbine torquemotor, which manipulated the fuel-metering valve. The inlet temperature or fuel flow reached a limit before the de- new authority limits extended from slightly above "idle" power sired thrust level was attained. This shaft speed droop might to maximum power. In this range the torquemotor accepted be unacceptable for rotorcraft applications because of rotor electrical signals from the digital control and positioned the lift loss. The power priority bias circuits overcame this droop metering valve in response. Also, the null bias on the by reducing the VIGV command when shaft speed fell torquemotor was changed to provide fail-safe action (decreas- 2 percent or more below the reference speed. ing fuel flow) if current were lost. Layout and support equipment.—The layout of the control The limits built into the control to ensure safe engine opera- system and its support equipment is shown in figure 8. The tion were not changed. control and its power supply were located in the test cell in a Control room Test cell Engine 115-V, 115-V, 60-Hz power 60-Hz power

Printer Cathode Floppy + ray tube disk

Digital 115-V, interface unit 60-Hz power r— Acoustic and environmental enclosure

Digital data link Engine Interface Digital connection unit 115 V, 60-Hz control panel power

115-V, DC signals Operator's Power input 60-Hz I panel [—supplyy 115 V, Analog computer Digital to and magnetic analog 60 Hz tape recorders power

Emergency and I 115-V, stall recovery 60-Hz overspeed test power and unlatch

Figure 8.—Layout of CEST TF34 engine control system. cabinet near the engine. The control contained the computer, video display was used to monitor system setup and operation the control algorithm on programmable read-only memories with a program that resided on a floppy disk. The video (PROM's), and all electronics needed for the digital func- display could be printed at the operator's request. The emer- tions. The control was connected to the engine and to equip- gency shutdown and overspeed test panel contained switches ment in the control room with shielded electrical cables. to accomplish those functions. In the control room the operator's panel contained 50 potentiometers and 16 switches that could be used to modify schedules, gains, and limits in the digital control. This panel Procedure received the thrust and reference speed commands and the torque command signal for the power anticipation circuit. A Transient Tests digital-to-analog converter unit provided up to 25 buffered control system outputs for recording or display. Recording Just before making a transient test the engine was set to was done on magnetic tape and strip charts. Many of the data each end point by using the DEEC operator's controls, and channels were routed through an analog computer to amplify the performance was recorded for calibration. The engine was the time-varying portion of the data for better resolution. The reset to the starting point by using the DEEC in the selected mode, and the test was performed by applying signals from a VIGV open programmable function generator to the thrust and torque --- VIGV partially closed command inputs. The data were recorded on magnetic tape.

Dynamic Tests CD En For open-loop frequency response tests the engine was set at U) the desired steady-state operating condition by using the DEEC and then perturbed a small amount on either side of this condi- a Operating \ 1 tion. The steady-state data were recorded for calibration. To load \ \ Fan perform the test, the PRBN generator signal (see appendix B) line 100 speed, was applied to the appropriate input, and dynamic data were 90 percent recorded on magnetic tape for 8 min. For the constant-fuel-flow tests the fan speed feedback signal to the hydromechanical fuel Airflow control was disconnected and replaced with an equivalent signal from a facility signal generator. This technique was used so that This characteristic is used to unload the fan. On a VIGV the fan speed could vary in response to its changing load and type of convertible engine turbine power is then available to without control reaction while keeping the limits and engine drive an output shaft load. On a V/STOL aircraft the VIGV safety features provided by the control. could be used to modulate fan thrust for hover flight. To provide a background for the discussion of the dynamic Data Processing test results, the sea-level-static operational characteristics of the CEST TF34 engine (taken from ref. 12) are illustrated in Digital.—The steady-state performance data were aver- figure 9. At each constant fan (shaft) speed (fig. 9(a)) opera- aged over several scans of the sampling pattern. Performance tion is bounded by performance with the VIGV open (high- parameters were computed from the averaged data on a main- thrust region), with the VIGV closed (low-thrust, high- frame computer. shaft-power region), and with limiting power-turbine inlet Transient.—The recorded data were played back on strip temperature, T4.5ma. Lines showing performance at constant charts calibrated from the end-point digital data. VIGV angle and constant T4.5 are called out in the figure. In Dynamic.ControI-room strip chart data were used for the high-thrust region closing the VIGV unloads the fan, and initial test evaluation. The taped dynamic data were digitized turbine power becomes available at the shaft. In the low- off-line at 50 samples per second for plotting or for analysis thrust region closing the VIGV continues to reduce fan flow, with existing computer programs. Frequency response data but the fan compression efficiency decreases significantly and were processed in the frequency domain with a fast Fourier shaft power levels off. The fan absorbs power by churning transfer (FFT) program. The transfer functions were calcu- and heating the relatively stagnant air in the fan duct, al- lated with the data-fit program described in reference 15. In though a small amount of air flows through for cooling. Zero this program the anticipated form of the transfer function was thrust is not attained mainly because of residual core thrust. input and then constants were manipulated to obtain the best At each speed maximum shaft power is produced with the simultaneous fit (least mathematical residue) between the FFT VIGV closed at T4.5m,,. results and the program's computed amplitude ratio and phase The operating envelope (fig. 9(b)) consists of many planes angle. of constant-fan-speed performance maps. With no shaft power and the VIGV open, the engine behaves like a conventional turbofan and produces maximum thrust at T4.5max and Results and Discussion 100-percent fan speed, NFm, Within the total operating envelope any combination of thrust and shaft power can be Operating Principles obtained by appropriately positioning the VIGV and adjusting T4.5 (fuel flow). Transient and frequency response tests were The CEST TF34 is a high-bypass-ratio turbofan engine. At made at several points within the operating envelope. The sea-level-static operation most of the thrust is produced by the engine ran smoothly and stably for all the tests. fan discharging through an unchoked, convergent exhaust nozzle of fixed area. As indicated on the generic fan map Transient Test Results sketched below, closing the VIGV has the effect of moving constant-speed lines toward lower flow and pressure rise. The Transient tests were performed to explore engine behavior load line is unchanged because the exhaust nozzle size is throughout the operating envelope at 95-percent referred fan fixed. speed NFR. The nominal transient test paths are shown in

9 T4.5ma, Constant T4.5 zl Constant ° VIGV angle 3 0 CL C? V- S rca

°c a 0

(a) Thrust

Shaft power

TA C ax

Thrust

Speed (b) Figure 9.—Operational characteristics of CEST TF34 engine. (a) Performance at constant fan (shaft) speed. (b) Operating envelope.

10 figure 10. Unless otherwise noted, the tests were performed in value was attained within 2 sec. Thrust also increased be- the "shaft" control mode with all the control loops active cause of greater core thrust, although no thrust change was except the power priority bias, and the anticipation circuits commanded. Anticipation circuits gave an initial fuel surge, were set at nominal gain. The thrust command and reference causing overtorque in the power turbine and increases in rotor fan speed were input to the DEEC (see fig. 6), and the torque speeds. Then the speed control reacted to reduce fuel flow, command was applied to the waterbrake control (see fig. 4). causing rotor speeds to fall back. Fan speed drooped as much The waterbrake provided the shaft load and had fast, but as 400 rpm (6 percent) from the reference speed. The over- sometimes nonlinear, response to the torque command as shoot could be reduced by optimizing the anticipation cir- discussed in appendix A. No additional rotary inertia or damp- cuitry gain, and the droop could be minimized by optimizing ing was added to the power output shaft. The waterbrake the fuel/speed loop gain. inertia was small relative to the combined inertia of the fan When torque decreased, the engine behavior was similar, and the power turbine, so that the results closely approximate except that torque and rotor speeds did not overshoot their the behavior of the engine alone. end values as much as when torque increased. The strip charts from each test show both increasing and Large thrust steps.—Large thrust changes were made in decreasing transients. An 80- to 120-msec delay (or dead both thrust mode (fig. 14) and shaft mode (fig. 15). In thrust time) occurred between the command signal and the begin- mode the VIGV was locked fully open and thrust was vaned ning of engine geometry or fuel flow changes, as illustrated in by fan speed. In this mode the CEST engine performed as a figure 11. The consistent occurrence of this dead time sug- conventional turbofan engine and changed thrust level stably gests that it is inherent in the engine control or facility appara- to the new end value. The anticipation loop in the DEEC was tus, but the cause is not explainable by the engine manufacturer inactive because the VIGV did not move (see fig. 7), and or in the use of the facility control or data acquisition equip- engine operation was safely limited by the hydromechanical ment. fuel control. Thrust changes were completed in about 2 sec Large torque steps.—The engine responded effectively to for both increasing and decreasing steps, and the waterbrake large torque changes commanded in 0.1 sec at low and high control system held torque reasonably steady. thrust levels, as shown in figures 12 and 13, respectively. The In shaft mode the reference fan speed was held constant steps were performed at fixed VIGV position. For increasing and thrust was vaned by VIGV deflection. The engine re- torque the transient behavior was the same for both cases. sponded to commands much more quickly in this mode be- Torque rose smoothly toward the new end value after an cause the fan did not have to accelerate. Thrust changes were initial delay but overshot by 500 ft-lb. The new steady-state essentially completed within 0.5 sec. The data suggest that

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m ^ ro 0 0 0 50 ' ' 0 50 0 0 100 ro Q o 100 3 0 1 2 3 4 5 3 0 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 12.—CEST TF34 performance during large torque steps at low thrust level. (a) Increasing torque. (b) Decreasing torque.

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ai ai C c a 40 40 O N tC c a) Q) 60 m` m o E o 60 CU Q U CU0 U U 3 80 3 80 0 1 2 3 4 5 0 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 14.—CEST TF34 performance during large thrust steps in thrust mode. (a) Increasing torque. (b) Decreasing torque.

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ro c -O ro c 'o

` 0 a^ `m —0 44 m n 0 44 @ a 0 0 1 2 3 4 5 3 0 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 15.--LEST TF34 performance during large thrust steps in shaft mode. (a) Increasing torque. (b) Decreasing torque.

16 the response time was limited by the VIGV slewing rate. The sidered important to the results because torque was nearly measured fan speed overshoot and droop (fig. 15) were about constant during these tests. 200 rpm (3 percent of reference fan speed), and the thrust VIGV perturbations. —The open-loop response of the en- ramps were smooth. The anticipation circuits worked effec- gine to small perturbations in VIGV position is shown in tively during fuel flow changes. Torque remained steady be- figures 20 and 21. For these tests the fan speed feedback cause the speed changes were small. signal to the DEEC was replaced with a dummy steady signal Combined transients.—A transient consisting of both thrust in order to eliminate the effects of fuel flow changes on and torque changes is shown in figure 16. For this case the engine variables. Waterbrake torque was held constant with anticipation circuits were disabled. The increasing transient fixed inlet and exit valve positions. began with the waterbrake exit valve operating in a region The steady-state response of engine variables to VIGV where torque did not change much with valve position (see position changes at the open (0°) position, a high-thrust, low- fig. 28). This initial insensitivity, possibly coupled with valve shaft-power region, is summarized in figure 20(a). As the starting stiction, caused torque to begin to rise slowly. Speed VIGV was closed, the fan-tip adiabatic compression effi- drooped as torque and thrust increased, and fuel flow in- ciency remained high, and fan speed increased to absorb the creased to bring speed back to the reference speed. During the available turbine power. When speed was higher, the fan-hub decreasing transient the sudden torque reduction allowed the discharge pressure increased and the bleed valve flow in- fan to overspeed. creased; both of these conditions reduced the core-inlet re- A slower combined transient with anticipation circuits ac- ferred flow and the core rotor speed. The net result of all these tive is shown in figure 17. Fan speed initially increased owing changes was to keep thrust relatively constant over this range to the anticipation fuel surge and the absence of torque change. of VIGV closure angles. Thrust increased owing to VIGV opening plus higher fan Important open-loop transfer functions are given in fig- speed. The fan speed control loop changed fuel flow to cor- ures 20(b) to (d). The fan speed transfer function has a single rect speed as necessary, but speed did not return to the refer- lag due to the rotary inertia of the fan system consisting of ence value for about 5 sec after the transient began. Behavior fan, power turbine, shafts, and shaft load. The core speed was similar for the decreasing transient, except that torque transfer function contains two lag terms numerically similar began to fall with no delay. to the fan system inertia term. The thrust transfer function Rotorcraft transients.—Transients of the types encoun- contains a derivative term and a lag term. The derivative term tered by advanced rotorcraft were suggested by requirements may be related to pressure in the fan discharge duct, which taken from reference 8. Figure 18 shows engine behavior varied in phase with fan speed at low frequencies but lagged during a collective pitch change with thrust for an antitorque speed changes as frequency increased. However, no pressure jet and directional control. For the increasing transient, torque measurements were made to study that argument. At still ramped smoothly with the torque command. Shaft speed ini- higher frequencies the fan speed response was attenuated and tially increased because of the anticipation fuel surge but thrust changes leveled off. Thrust changes probably would dropped when the VIGV opened. Thrust responded to VIGV have returned to zero at frequencies higher than plotted in change. Speed never quite returned to the reference value figure 20(d). because T4.5 reached its limit. The reverse transient showed Results of the same type of test, but with the VIGV closed no unusual behavior. down to 65°, are shown in figure 21. This is a low-thrust, Engine performance during a simulated unexpected gust high-shaft-power region where thrust, but not shaft power, encounter is shown in figure 19. Before the gust the engine was affected by VIGV position (see fig. 9). Over the range of was running near T4.5m,,,. The engine and control responded VIGV angles tested (fig. 21(a)) fan-tip efficiency and flow effectively to commands simulating expected load changes by rate both decreased as VIGV closure increased, and turbine holding shaft speed steady through the gust. power needed to drive the fan at steady speed was nearly constant. Thrust fell off because about half the thrust came from the fan. Rotor speed and core operation were not changed. Open-Loop Frequency Response Tests The fan speed transfer function (fig. 21(b)) shows that dynamic effects were much smaller than for the open-VIGV Open-loop frequency response tests (engine only) were case, but detailed interpretation of the data is difficult. Thrust made with the DEEC in the shaft control mode by using the response (fig. 21(c)) remained constant over the range of PRBN technique (see appendix B). As noted in the previous frequencies tested. section an unexplained 80- to 120-msec dead time was meas- Fuel flow perturbation.—Results of other open-loop tests ured in the transient tests. The same dead time may have been in which fuel flow rate was the driving function are shown in present in these tests; however, it is not accounted for in the figures 22 and 23. For these tests the fan-speed feedback data analysis or in the following discussion. Another dead signal to the DEEC was replaced with a driver signal from an time, in waterbrake response (see appendix A), was not con- external source, thereby effecting a fuel flow perturbation.

N C ^ c = ro ^ ro Q E 6 E F- O F- 0 U 0

^ j D m.02500 L Q i 2500 r O' I 1500 U) 0 $ 1500 y 500 500

C + C N ro N CO ^ E 2 E L E L E o F- 0 0

5x103 5x103 N N LE — 3 3 F- 1 1

0 6500 6500 m CD a 6400 ii a 6400 u_ U) 6300 VOT ` 6300

17x103 m 17x103 W W a E aE N a N Q m 2 16 0 16 O U U 15 15

3 2500 3 2500 o 0 2000 2000 m ^ m_ ii 1500 LL 1500

1800 1800 Lq 'n 1700 1700 H 1600 H 1600 1500 1500

c c > O

> 50 > _o 50 °a 70 a 70

ai 0 m 0

c O CO c O > N ' N 50 ^, m 0 50 0 ^ o i^ 3 a 1000 1000 1 2 3 4 5 3 ¢ 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 16.--LEST TF34 performance during combined torque and thrust changes without anticipation circuits. (a) Increasing torque. (b) Decreasing torque.

=3 CO :3 ro ^E ETE E o U U

3x103 3x103 zr m L Q 7 i 2 U) V O UO zr 1

C ^, C ^ ro ^ ro E L E r E t E O O U U

3000 3000 2000 Ll 2000 1000 1000

E 6800 C 0 E 6800 LL a 6400 LL a a 6400 U) 6000 6000

17x103 E W 17x103 a a E m a 16 a 16 m 15 U`o U`0 15

2800 2800 3 _o r 2400 o s` 2400

2000 D 2000 LL LL 1600 1600

1800 1800 Lq 1700 1700 1600 1600

c c

_> :0 ai 60 ^_ :^ a^i 60 > o 70 > o ° 70 Q- n.

ai ai rop m p m 0 50 0 50 Caa 100 ro a100 3 ^ 0 1 2 3 4 5 0 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 17.—LEST TF34 performance during combined torque and thrust changes with anticipation circuits. (a) Increasing torque. (b) Decreasing torque.

a5 Ca Nc:3 ro =5 ro Q E F E o E E U U

4x103 4x103 3 .a Lro =p-7 3 srop -7'^ 3^ o U) `o 2 2 1 1 a c c N ro rn CO 2 E i E s E s E O O U U

6x103 6x103 N LL — t — 4 F- o E-

E 6500 E 6500 ii °'a a 6400 ii a^CL a 6400 N 6300 N 6300

16 800 Q) 16 800 a E 16 400 a E 16 400 E' 16 000 Q') 16 000 v 15 600 v 15 600

3200 o 3200 0 L 9 2800 z a 2800 LL LL LL 2400 2400

1900 1900 LS v 1850 1850 1800 H 1800 1750 1750

36 36 0 4p ^ 2 40 (D : a^i 44 0 a^i 44 > o II 48 > o^ 48 a 52 ° 52

7p m U ^ 70 U 0 80 a 5 90 a^ 90

0 1 2 3 4 5 0 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 18.—CEST TF34 performance during simulated collective pitch changes, with power priority bias loop active. (a) Increasing torque. (b) Decreasing torque.

20 a a^ D CO ^ m 2- E E oE E

U U

4x103 4: ro D ^ Q i r Q T o to o 3 2

c ,^ c N RS L7 E L7 E s E E ^- o F- o U U

N 5x103 in 5: a^ r - 4 4 F- F-

6500 a 6500 m ai a 6400 c^a a) a 6400 LL CL U- U) (n L 6300 L 6300

-o a: a E 17x103 a E 17: CL LCL L L L 0 16 16 U U

o L 3500 L 3500 wo r 3000 6 3000 n LL 2500 LL 2500

1900 L 1900 1800 1800 1700 1700

c c :o 30 30 > > .0 > o ° 40 > o° 40 a a

N N R c a c > 0 N No data > N No data a) o a`^_o m CL a e 0 1 2 3 4 5 0 1 2 3 4 5 (a) Time from start of transient, sec (b) Figure 19.—CEST TF34 performance during simulated unexpected gust encounter (anticipation circuits inactive). (a) Increasing torque. (b) Decreasing torque.

21 VEGV angle By position (open) T4.5

WF 0 ro TRQ c F m E N ro WA2.5R a) NH

^l (a) -8 -4 0 4 8 VIGV closure angle, deg

(D 10

Y_ 0)CO 2

1 1 i i i 0.01 0.1 1 10 Frequency, Hz

0 m

N -90 m t o_ -180 (b) 0.01 0.1 1 10 Frequency, Hz Figure 20.—Open-loop tests with small VIGV perturbations in shaft mode: VIGV open (0'); T4.5 =T4.5max; NF - 90 percent; PWSD = 640 hp; anticipation circuits and power priority bias off. (a) Steady-state calibration. (b) Fan speed response. (c) Core engine speed response. (d) Thrust response.

22 10

^^m 1 Ca 2

0.1 0.01 0.1 1 10 Frequency, Hz -90

-180

m -270 CO CO dL

-360

-450 0.01 0.1 1 10 Frequency, Hz

m 10 a

-c m CO 2

0.01 0.1 1 10 Frequency, Hz

0 F-

m m O 00 -90 _O OO 00 CO aL

-180 (c 0.01 0.1 1 10 Frequency, Hz Figure 20.—Concluded. (c) Core engine speed response. (d) Thrust response.

VEGV angle

BV position (closed) T4.5

WF m

ro TRQ

c^N E a^ U) F M ro 0 WA2.5R

(a) I I I 't 58 62 66 70 74 VIGV closure angle, deg

m 1

'E rn cC 2

0.1 0.01 0.1 1 10 Frequency, Hz

90 O O

O m 0 O0 0 O OO O ai O Lm ^0 0 6b O CL -90

_180 1 (b) I I I I I I I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I 0.01 0.1 1 10 Frequency, Hz Figure 21.—Open-loop tests with small VIGV perturbations in shaft mode: VIGV closure, 75 percent; T4.5 -T4.5max; NF = 90 percent; PWSD = 3700 hp; anticipation circuits and power priority bias off. (a) Steady-state calibration. (b) Fan speed response. (c) Thrust response.

OF _ -21.8 lb 40 OVIGV (1 + 0.011 S) Deg. deflection m 30 O O O - crn 20 ca 2i 10 0.01 0.1 1 10 Frequency, Hz

-90

0) (D

-180 CO s IL

-270 I (c) I I I I I I I I I I I I I I I I I I I I I I I I 0.01 0.1 1 10 Frequency, Hz

Figure 21.—Concluded. (c) Thrust response.

25 VEGV angle By position (closed) VIGV angle T4.5 a) m C TRO m E F Cn ca WA2.5R m 2 NH NF

Tl (a) I I I I 2300 2400 2500 2600 2700 2800 Fuel flow rate, lb/hr

a) P 'E rn cC 2 0.1

0.01 0.01 0.1 1 10 Frequency, Hz

O —90

co a- -180

—270 " I i I i I i I i I I i I i I i I i I I i I i I i I\ 1 0.01 0.1 1 10 Frequency, Hz Figure 22.—Open-loop tests with small VIGV perturbations in shaft mode: VIGV open (0'); T4.5 =T4.5ma,; NF ~ 90 percent; PWSD = 400 hp; anticipation circuits and power priority bias off. (a) Steady-state calibration. (b) Fan speed response. (c) Core speed response. (d) T4.5 response. (e) Thrust response. 4

^ 1

'c m m 2

0.1

0.01 0.1 1 10 Frequency, Hz

0 °41 x1n0O(1-no rn ai -90 N dt -180 c

0.01 0.1 1 10 Frequency, Hz

C m 0.01

0.001

0.01 0.1 1 10 Frequency, Hz

-90

Nm -180 Lm d

-270

-360

0.01 0.1 1 10 Frequency, Hz Figure 22.—Continued. (c) Core speed response. (d) T4.5 response.

27 10

-E 1 m m 2

0.1 0.01 0.1 1 10 Frequency, Hz

0 rn o o° N -g0 cr--^ p p aL -180 (e) 0.01 0.1 1 10 Frequency, Hz Figure 22.—Concluded. (e) Thrust response.

28 VEGV angle BV position (closed) VIGV angle T4.5 W cc > TRQ C N E F m N WA2.5R cc NH NF

(a) T1 2300 2400 2500 2600 2700 Fuel flow rate, lb/hr

d .ER 0) 0.1

0.01 0.01 0.1 1 10 Frequency, Hz 0

-90

m ai -180 m aL -270

-360 0.01 0.1 1 10 Frequency, Hz Figure 23.—Open-loop tests with small WF perturbations in shaft mode: VIGV closure, 75 percent (65°); T4.5 =T4.5max; NF = 90 percent; PWSD = 2800 hp; anticipation circuits and power priority bias off. (a) Steady-state calibration. (b) Fan speed response. (c) Core speed response. (d) T4.5 response. (e) Thrust response.

m 1

R 'E m CU 2

0.1 0.01 0.1 1 10 Frequency, Hz

0 O r) O - ,.-, m

N -90 m L CL

-180 (c) 0.01 0.1 1 10 Frequency, Hz

0.2

0.1

tmC - 0.01

0.001 0.01 0.1 1 10 Frequency, Hz 0 O O O O -90 O ai Ca m L d -180

-270 (d) 0.01 0.1 1 10 Frequency, Hz Figure 23.—Continued. (c) Core speed response. (d) T4.5 response.

30 'E 0.1 0) M 2

0.01 0.01 0.1 10 Frequency, Hz

a -90 sro d

-180 (e) 0.01 0.1 10 Frequency, Hz Figure 23.--Concluded. (e) Thrust response.

Data interpretation can be made in a similar manner as de- the tests. A consistent but unexplained 80- to 120-msec dead tailed in the preceding discussion. time between the command signal and the beginning of engine geometry and fuel flow changes was observed during these tests. Concluding Remarks 3. Open-loop (engine alone) transfer functions were obtained by using the pseudorandom binary noise testing technique. The transfer functions describe engine dynamic behavior The CEST TF34 convertible engine, equipped with a spe- for variable inlet guide vane and fuel flow perturbations. The cially designed digital control system, successfully completed transfer functions can be useful for designing future convert- all planned testing on an outdoor static test stand at the NASA ible engines and their associated control systems. Lewis Research Center. Steady-state and acoustic test results 4. A power absorber system using a waterbrake dynamom- have been reported in previous NASA publications. Signifi- eter was a satisfactory engine power output shaft load for cant results of transient and dynamic performance tests re- these tests. The system was operated up to 5300 hp. The ported herein are as follows: closed-loop frequency response for torque-demand input was 1. The engine ran safely and stably through all the tests. flat to approximately 4 Hz but showed up to 50-msec dead 2. Transient tests were performed at 95-percent fan speed time. No explanation is known to account for the dead time, to explore engine behavior throughout the operating enve- but the dead time had little effect on these test results. lope. The tests included large torque and thrust steps in the shaft control mode, large thrust steps in the thrust control mode, and several combined thrust and torque changes. The steps were up to 4500 lb in thrust and 2500 ft-lb in torque. Lewis Research Center Features of the digital control system, such as the anticipation National Aeronautics and Space Administration circuits and speed control loops, were demonstrated during Cleveland, Ohio, June 10, 1995

31 Appendix A Power Absorber System Performance

Waterbrake functions are necessary for waterbrake operation, but torque can be varied by adjusting either valve. The waterbrake dynamometer, shown schematically in figure 24, consists of a rotor that spins between stator plates in a housing. The rotor is an assembly of disks mounted on a shaft. Torque Control With Inlet Water Valve Cold water enters the waterbrake around the shaft and is thrown radially outward by the rotor. Holes in the disks and stator plates An initial power absorber system was set up as sketched in generate high turbulence. The water forms an annular shape figure 25. In this configuration a three-way inlet water valve that rotates at about half the rotor speed and then flows out was used to keep constant water flow in the long line to the through exit ports in the housing. The rotor drag, which causes cooling tower water source. The intent was to minimize water torque on the shaft, increases as the depth of the annulus inertia effects on waterbrake operation. An orifice in the exit becomes greater. The power absorbed by the waterbrake is the line restricted water outlet flow. The steady-state performance product of torque and rotor speed. Power is converted to heat, of this system is shown in figure 26. For each exit orifice size, which is carried away by the water. both shaft torque and water flow rate increased as the valve The depth of the water annulus, called the water level, is was opened. When the waterbrake rotor became "fully wet- controlled by valves in the inlet or exit Iines. The inlet valve ted" (i.e., the water level extended from the housing all the controls the water throughflow and therefore controls the way to the shaft to completely submerge the rotor disks), the water level and the temperature rise for a given power absorp- torque limit was reached and the water flow rate became tion. The exit valve controls the water level by setting the constant. The steady-state performance was nearly indepen- pressure needed to pump water out of the system. Both valve dent of shaft speed. With this type of water level control the

Dal

h Shaft

Figure 24.—Schematic of Kahn Industries, Inc., model 108-130 waterbrake dynamometer.

32 Electro- Control o Manual hydraulic and o Random servovalve servoam noise 0 T(t) Cooling tower Inputs from water Sensed torque engine --p-inlet control room

Three-way inlet valve Preload r-- Fixed Bypass I /^^^ orifice water // Rotatio`

rump Figure 25.—Waterbrake setup with control valve at inlet. (This setup was not used for engine performance reported herein.)

5x103

0 Q Y0 Y ro

0 20 40 60 80 100 Inlet water valve position, percent open

1200

1000

800 m cC 3 w° 600 L rn 0 y Engine Water exit 400 speed, orifice m percent diameter, in. 200 O 58 1.50 q 58 2.12 0 58 3.00 O 88 2.12 (b) 0z 1 0 20 40 60 80 100 Inlet water valve position, percent open Figure 26.—Waterbrake steady-state performance with control valve at inlet (see fig. 25). (a) Torque. (b) Water flow rate. orifice size must be chosen to ensure that the rotor does not response of this valve and actuator, shown in figure 29, was become fully wetted during dynamic testing. flat to approximately 10 Hz. The dynamic performance of The three-way inlet water valve was operated by a hydrau- this system configuration is shown in figure 30. This system lic actuator. No small-perturbation frequency response tests was faster than the system with the inlet water control valve, of this valve/actuator combination were made; however, from as shown by comparing figures 27(a) and 30(a). However, some step-input tests it was estimated that the valve/actuator this configuration also exhibited a significant dead time. No was critically damped and had approximately 6-Hz natural satisfactory explanation for the dead time is known. The frequency. The dynamic performance of this system configu- open-loop frequency response of the combined valve and ration is given in figure 27. Time responses to step inputs waterbrake system from PRBN tests is shown in figure 30(b). (fig. 27(a)) show that after approximately 50-msec dead time The valve response (fig. 29) was much faster than this com- the torque changed relatively smoothly to the new level. Ini- bined response; thus, the frequency response shown in tial torque response was faster for a valve-open than for a figure 30(b) closely approximated the response of the valve-closed step, indicating that the waterbrake filled more waterbrake alone. quickly than it emptied. The frequency response transfer func- A system performance comparison from these test results tion (fig. 27(b)) was deduced from numerical analysis of the is as follows: step-input response data. The function given includes the inlet water valve dynamics previously described. Other data obtained with this power absorber configuration Inlet water Exit water consistently showed about 50-msec dead time in torque re- valve valve sponse. Although no firm explanation could account for this, Water valve

it was believed to be related to the presence of an air space in Natural frequency, Hz 6 13.8 the vertical portion of the inlet line between the three-way Damping factor 1 0.62 valve and the waterbrake. Calculations showed that this line, made from large-diameter pipe, did not flow fully. In an effort Waterbrake to improve the waterbrake response by eliminating the dead Time constant, sec 0.43 0.145 time, the control valve was moved to the exit side. Dead time, sec 0.050 0.024

Torque Control With Exit Water Valve For the valves used, the waterbrake dynamics dominated The power absorber system using a control valve in a water performance. The results indicate that performance was bet- exit line is sketched in figure 4. This system was used for all ter with the control valve in the exit line. However, the dead the tests described in this report. The valve was installed in a time measured in these tests precluded simulation of dynamic trap in the water line to ensure that it was always submerged. inertia shaft loads using derivative torque feedback because The steady-state performance of this system is shown in fig- the waterbrake was unstable when operated with the engine ure 28. For each fixed setting of the inlet water valve the control system. The waterbrake dead time may also limit its water flow rate was constant. As the exit valve was closed, the use for other types of transient or dynamic testing. torque increased in a nonlinear manner and was not affected by engine speed. Note that low torque was obtained only with Closed-Loop Torque Response small inlet valve openings (as exemplified by the circle symbols in fig. 28). Thus, ramp changes in valve position, such as Using the power absorber configuration with the control were used for the transient test torque commands, produced valve in the water exit line (fig. 4), the control loop was very nonlinear demands on the engine when the torque com- closed with proportional-plus-integral electronics and adjusted mand changes were large. for best performance. The time response (fig. 31(a)) was The two-way valve in the exit water line was 4-in. size with better than 12 000 ft-lb/sec. Dead time was still present. The a linear plug. It was moved by a hydraulic actuator moving frequency response (fig. 31(b)) was flat to more than 3 Hz, a 2.5-in. cylinder and with a 2.5-in. stroke. The hydraulic reflecting the improved bandwidth expected from the prop- working pressure was 2800 psig. The open-loop frequency erly adjusted closed-loop control.

Valve-open step a 30 0 — — — Valve-closed step ^Y — CU c 3226 _m ^a C m 22 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Time, sec

4000 — — —

3500

F°- (a) I I I I I I I 1 1 3000 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Time, sec Dead time

1 ro c U)

OTorque _ (AValve position1 x e-0.05S d0.1 ODriver signal - ` ODriver l (1 +0.43S) m Cr where AValve position = 1 .ADriver 1 + 2x1 xS + S2 d (2ax6) (27rx6)2 0.01 0.1 1 10 Frequency, Hz

100

m -200 c^ ar 300

-400 I` I I I I I I 0.1 1 10 Frequency, Hz Figure 27.—Waterbrake dynamic performance with control valve at inlet. (a) Time responses. For each case, driver was a step input to valve at inlet. (b) Open-loop torque frequency response from numerical analysis of step-input response.

4x103

ff _o

CU LCU U)

V 0 20 40 60 80 100 Exit water valve position, percent closed

1000

Ec 800 m m Engine Water inlet m speed, m 600 valve, percent percent 03 L open 7 O 80 10 0 400 q 80 50 L 4 80 70 m O 91 50 CU 200

0 1 1 1 1 1 (b) 0 20 40 60 80 100 Exit water valve position, percent closed Figure 28.—Waterbrake steady-state performance with control valve at exit (see fig. 4). (a) Torque. (b) Water flow rate.

37 1

m c .Nrn d

4 0 0.1 N 0 CL m

0.01 0.01 0.1 1 10 100 Frequency, Hz

-90

N -180 m z CL

-270

-360 0.01 0.1 1 10 100 Frequency, Hz Figure 29.—Open-loop torque frequency response of control valve (two-way) at exit.

C 0 84 U) 0 CL 80 CD

> 76 m ro 72 0 0.5 1.0 1.5 2.0 2.5 3.0

4200

3800 i

ai 3400 6 3000

2600 0

FU 10 C Nm

4 m 6 0 d

0.01 0.1 1 10 Frequency, Hz

0 — 0

&- 0 -90 ai N fQ O -180

O 1" O -270 (b) 0.01 0.1 1 10 Frequency, Hz Figure 30.—Waterbrake dynamic performance with control valve at exit. (a) Time response. (Driver was a step input to valve at time = 0 sec.) (b) Open-loop frequency response from PRBN input to valve.

105 .NY0 a 95

> 85 m m 75 0 0.5 1.0 1.5 2.0 2.5 3.0 Time, sec 420C i 380C 340C Q 3000 H (a) 2600 0.5 1.0 1.5 2.0 2.5 3.0 r Dead time Time, sec

100 r-

ro C N

10 40 m Q `o d

0.01 0.1 1 10 Frequency, Hz 0 0 0

-90 O m ai -180 Ln sro a -270 O O -360 (b) ' 01 1 0.01 0.1 1 10 Frequency, Hz Figure 31.--Closed-loop response of power absorber system with control valve at waterbrake exit. Proportional- plus-integral control. (a) Time response. (Driver was a step input to valve at time = 0 sec.) (b) Frequency response.

40 Appendix B Pseudorandom Binary Noise Generator

The pseudorandom binary noise (PRBN) method was cho- generator signal consisted of random sharp-edge pulses. The sen for the small-perturbation frequency response tests. The vane motion resulting from this signal was slightly rounded PRBN method is quicker than the sine-sweep method, and because of the relatively high clock frequency. The frequency previous test experience has shown that results from the two spectrum of the vane position indicates the random changes methods compare satisfactorily (refs. 16 and 17). detected by the engine. The first cusp-shaped minimum oc- The PRBN generator block diagram is shown in fig- curred at the clock frequency, 5.5 Hz, as expected by theory ure 32(a). An electronic bit generator, using a 10-bit shift (ref. 17). Also, from theory, the minimum frequency present register running at a preset clock frequency, produced pulses in the perturbation was of random width and spacing. The pulse changes occurred at clock intervals. The pulses drove a dynamic system that per- Clock frequency turbed the plant. Use of the PRBN generator to perturb the —0.005 Hz 210 -1 VIGV system is illustrated in figures 32(b) to (d). The bit

41 PRBN VIGV CEST generator actuators Random engine vane Clock Driverposition signal excitation — 0- Dynamic system Plant Random bit generator

(a)

CD 0

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I Time -I 1 sec (b)

rn m

c O N O d

Time a --I I- 1 sec (c)

-20

CO -30

E L) L) -40 CL

m 3 a -50 m c^m m Q -60

-70 0 2 4 6 8 10 12 14 16 18 20 Frequency, Hz Figure 32.—VIGV driven by PRBN generator for frequency response tests. (a) Block diagram. (b) Driver signal. (c) Vane position. (d) Power spectrum of vane position.

42 Appendix C Symbols

F engine gross thrust,' lb TRQ output shaft torque, ft-lb

FR referred engine gross thrust, ] F/S T(t) torque control system input signal from programmable signal generator NF fan, power-output-shaft, and power-turbine speed, percent of 6890 rpm T4.5 power-turbine inlet total temperature, °R

NFmax maximum fan operating speed, rpm T4.5 ma,, limiting power turbine inlet temperature, °R

NFR referred fan, power-output-shaft, and power- WA2.5 core airflow, lb/sec turbine speed, NF/ -\J—O , percent of 6890 rpm WA2.5R referred core airflow, WA2.5 V-0 /6 NH core-engine speed, rpm WF fuel flow rate, lb/hr PS3 core compressor discharge pressure, psia WFR referred fuel flow rate, WF/60 PWSD output shaft power, hp S ratio of pressure to 14.696 psi PWSDR referred output shaft power, PWSD/6 0 ratio of temperature to 518.7 °R S LaPlacian operator TI fan tip adiabatic compression efficiency T temperature, °R

`Called "gross thrust" because, by convention with the TF34 engine, the reported thrust is net (measured in test stand) thrust plus calculated core cowling scrubbing drag.

43 References

1. Eisenberg, J.D.: Rotorcraft Convertible Engines for the 1980's. NASA 9. Biggers, J.C.; and Linden, A.W.: X-Wing-A Low-Disc-Loading TM-83008,1982. V/STOL for the Navy. SAE Paper 851772, Oct. 1985. 2. Goldstein, D.N.; Hirschkron, R.; and Smith, C.E.: Rotorcraft Convertible 10. Gill, J.C.; Earle, R.V.; and Mar, H.M.: Rotorcraft Convertible Engine Engine Study. (R83AEB047, General Electric Co.; NASA Contract Study. (DDA-EDR-10978, Detroit Diesel Allison; NASA Contract NAS3-22743.) NASA CR-] 68241, 1983. NAS3-22742.) NASA CR-168161, 1982. 3. Abdalla, K.L.; and Brooks, A.: TF34 Convertible Engine System Tech- 11. Eisenberg, J.D.; and Bowles, J.V.: Folding Tilt Rotor Demonstrator nology Program. Proceeding of the 38th Annual Forum, American Feasibility Study. Proceedings of the 42nd Annual Forum, American Helicopter Society, 1982, pp. 163-169. Helicopter Society, Vol. 2, 1986. 4. Quiet Clean Short-Haul Experimental Engine (QCSEE) Under-the-Wing 12. McArdle, J.G.; Barth, R.L.; and Burkardt, L.A.: Steady-State Perfor- (UTW) Engine Composite Nacelle Test Report. (R78AEG573-Vol. 1, mance of a Turbofan/Turboshaft Convertible Engine With Variable General Electric Co.; NASA Contract NAS3-18021.) NASA Inlet Guide Vanes. NASA TP-1673, 1987. CR-159471, 1979. 13. McArdle, J.G.: Fan Acoustic Characteristics of a Turbofan/Turboshaft 5. Schaefer, J.W.; Sagerser, D.R.; and Stakolich, E.G.: Dynamics of High- Convertible Engine With Variable Inlet Guide Vanes. NASA Bypass-Engine Thrust Reversal Using a Variable-Pitch Fan. NASA TM-100207,1987. TM X-3524,1977. 14. Gilmore, D.R., Jr.: TF34 Convertible Engine Control System Design. 6. Jones, B.A.; and Wright, D.L.: Convertible Fan/Shaft Engine Variable Proceedings of the 40th Annual Forum, American Helicopter Society, Fan Geometry Investigation. USAAVLABS-TR-70-28, PWA-FR- 1984, pp. 609-620. 3567, Pratt & Whitney Aircraft, 1970. 15. Seidel, R.C.: Transfer-Function-Parameter Estimation From Frequency 7. Bobula, G.A.; Soeder, R.H.; and Burkardt, L.A.: Effect of a Part-Span Response Data-A FORTRAN Program. NASA TM X-3286,1975. Variable Inlet Guide Vane on the Performance of a High-Bypass 16. de los Reyes, G.; and Gouchoe, D.R.: The Design of a Turboshaft Speed Turbofan Engine. AIAA Paper 81-1362, July 1981. Governor Using Modem Control Techniques. (General Electric Co.; 8. Williams, R.M.; and Boyd, T.H.; Preliminary X-Wing Characteristics NASA Contract NAS3-22763.) NASA CR-175046, 1986. and Interface Document. DTNSRDC/TM-16-81/06, David W. Taylor 17. Cottington, R.V.; and Pease, C.B.: Dynamic Response Testing of Gas Naval Ship Research and Development Center, May 1981. Turbines. J. Eng. Power, Jan. 1979.

44 Form Approved REPORT DOCUMENTATION PAGE OMB No. 0704-0188 Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503. 1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED April 1996 Technical Memorandum 4. TITLE AND SUBTITLE 5. FUNDING NUMBERS Dynamic and Transient Performance of Turbofan/Turboshaft Convertible Engine With Variable Inlet Guide Vanes

6. AUTHOR(S) WU-505-68-30

Jack G. McArdle, Richard L. Barth, Leon M. Wenzel, and Thomas J. Biesiadny

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER National Aeronautics and Space Administration Lewis Research Center E-9637 Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING AGENCY REPORT NUMBER National Aeronautics and Space Administration Washington, D.C. 20546-0001 NASA TM-4696

11. SUPPLEMENTARY NOTES

Responsible person, Thomas J. Biesiadny, organization code 2780, (216) 433-3967.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - Unlimited Subject Category 07

This publication is available from the NASA Center for Aerospace Information, (301) 621-0390. 13. ABSTRACT (Maximum 200 words) A convertible engine called the CEST TF34, using the variable inlet guide vane method of power change, was tested on an outdoor stand at the NASA Lewis Research Center with a waterbrake dynamometer for the shaft load. A new digital electronic system, in conjunction with a modified standard TF34 hydromechanical fuel control, kept engine operation stable and safely within limits. All planned testing was completed successfully. Steady-state performance and acoustic characteristics were reported previously and are referenced. This report presents results of transient and dynamic tests. The transient tests measured engine response to several rapid changes in thrust and torque commands at constant fan (shaft) speed. Limited results from dynamic tests using the pseudorandom binary noise technique are also presented. Performance of the waterbrake dynamometer is discussed in an appendix.

14. SUBJECT TERMS 15. NUMBER OF PAGES 47 V/STOL; Controls; Turbofan engines; Turboshaft engines; Propulsion; Convertible 16. PRICE CODE engines; Variable inlet guide vanes; VIGV; Waterbrake A03 17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT OF REPORT OF THIS PAGE OF ABSTRACT Unclassified Unclassified Unclassified NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89) Prescribed by ANSI Std. Z39-18 298-102