Propulsion controls

PROPULSION CONTROLS A LOOK INTO THE FUTURE Ronald D. Hackney Pratt & Whitney Aircraft Group Government Products Division SUMMARY Increased system requirernenh and functional integration wit.h t,he aircraft. have placed an increased demand on control system capability and reliability. To provide these at. an affordable cost and weight and because of the rapid advances in electronic technology, hydromechanical systems are being phased out in favor of digital electronic systems. The transition is expected to be orderly from electronic trimming of hydromechanical controls to full authority digital electronic control.

INTRODUCTION Alvin Toffler in his book Future Shock said - “We’re all aboard a train which is gathering speed, racing down a track on which there are an unknown number of switches leading to unknown destinations.

Most of us are in the caboose looking backward.” This can be expecially true for the propulsion control where there are at least two outside influences, engineers,directingthetrain.Thesearetheairframeandelectronicsindustries.Theairframeindustry provides requirements. The electronic industry provides t.echnology.

Future propulsion system controls will be highly reliable full authority digital electronic with selected component and circuit redundancy to provide the required safety and reliability. Redundancy may include a complete backup control of a different technology for single engine applications. The propulsion control will berequiredtocommunicaterapidlywiththevariousflight.andfirecontrolavionics as part of a n integrated control concept.

Development of the technology for advanced control systems will continue to evolve in the ongoing progression from hydromechanical controls to prime reliable digital electronic control systems for advanced aircraftinthelate 1980’s a n d 1990’s. Part of thist,echnologyprogressionhasalreadytakenplacewith programs supported by government and indust,ry. Two such programs have been the Full Authority Digital Electronic Control (FADEC) program and the Int,egrat.ed Propulsion Cont,rol System (IPCS) program. The FADEC program engine tested advanced technology control hardware. The IPCS program has developed and tested an integrated inlet/engine/nozzle integration concept in the F-111 aircraft,. A pianned NASA program,IntegratedAircraftControlTechnology, will developadedicated F-15 flighttestvehiclefor integrated aircraft/propulsion control research.

CURRENT TECHNOLOGY An e a r l y s t e p a t P r a t t & WhitneyAircraftwastheuse of alimit,edaut.horitysupervisorydigital electronic control and a full function hydromechanical control unit for t,he FlOO engine. This combination 4 9 allowed the realization of some of the benefits of digital electronic controls while maintaining the proven reliability of the hydromechanical control.

T h e FlOO afterburningturbofan,illustratedinFigure No. 1, is representative of current high technology engines. T h e FlOO is a low bypass ratio, twin-spool, axial flow, augmented turbofan engine. The control, basically hydromechanical with digital electronic trim, sets performance by cont.rolling the inlet guide vanes, compressor variable stators! compressor bleeds, main burner fuel, augmentor fuel and exhaust nozzle area. As the engine/control system is reaching maturity, the electronic t,rim control reliability and responsibility is increasing dramatically. In fact, current digital electronic reliability exceeds that of the hydromechanical. This same kind of supervisory system is currenbly being developed for advanced JT9D and JTlOD commercial engines. A full function hydromechanical unit is included in these control systems to provide the confidence necessary to introduce digital electronic controls into commercial service.

TECHNOLOGY EVOLUTION Hydromechanical systems are being phased out in favor of the more capable electronic systems. An orderlytransitionisexpectedoverthenexttenyearsasillustrated in FigureNo. 2. Firstgeneration electronics - ElectronicEngineControl(EEC) - act as a trim on the FlO0 hydromechanicalcontrol.

Second generation electronics - Digital Electronic Engine Control (DEEC) act as a full authority control, bututilizeahydromechanicalbackupcontrol.Thirdgenerationelectronics - FullAuthorityDigital Electronic Control (FADEC) - provide primary and backup control.

The major obstacle to universal acceptance of electronic systems is their relatively high failure rate while operating under severe environment.al stress. Simple engines can be controlled by hydromechanical devices thathavedemonstratedmuch higherreliabilitythancurrentelectroniccomputationdevices.

However, as computational complexity increases, the reliability of hydromechanical devices decreases more rapidly than that of the electronic devices. The electronic control syst.em is projected to be more reliable than hydromechanical systems for the engines of the 1980’s.

Several research and development programs are being conducted to evaluate the reliability o f full authority digital electronic systems when subjected to the environment of JT8D and.JT9D engines. For mid- termtransportapplications,adualchannelapproach is beingevaluatedtoprovideacceptablesystem failureaccommodation.Asinglechannel full authorityDigitalElectronicEngineControl(DEEC) in combination with a limited capability hydromechanical backup control is beingdeveloped for advanced FlOO engines. A full authority digital electronic control was also testedfor an integrated inlet/engine/nozzle system in F-111 aircraft under the Integrated Propulsion Control System (IPCS) program.

Furtherdevelopment of electronic control technology is being conducted under the Navv Full AuthorityDigitalElectronicControl(FADEC)program.ThePratt & WhitneyAircraftFADECdesign features two processors in one box, selected redundancy, parameter synthesis, and built-in-test to provide a high degree of fault tolerance. Advanced component technology used in the Pratt & Whitney Aircraft FADEC design is based upon projections for production of a control system i n the mid-1980 time frame.For example, both central processors will be implemented with threevery large scale integration (VLSI), silicon- on-sapphire (SOS) complementary metal-oxide semiconductor (CMOS) devices. This degree will represent a significant technology improvement over an existing 11 chip LSI CMOS processor design, and indicates the rapid trend toward greater packaging density, higher reliability, and improved computational capability.

Anotherprogrambeingconductedaspart of NASA’s EnergyEfficientEngine(E3)program is identifying control technology areas requiring development. Programs like FADEC and E3 should continue because as automatic controls become more commonplace in the consumer market, industrial research will focus more on that need and less on the special needs of t.he aerospace industry.

Electroniccontrolsystemreliability will beenhanced by electroniccontrolswithinternalfault detection, parameter synthesis, and switching logic t h a t will transfer data and cont,rol functions for fail- operational performance. Electronic controls today are structured around a multi-chip processor. The cost of this processor will continue to drop as more complex architecture and instruct.ions are included on each chip. As illustrated in Figure No. 3, cost per calculation is decreasing a t a 50% per year rate. A significant improvementinreliability will also follow withdevelopment of asingle chip microprocessor and the associated reduction in external circuit connections.

As illustrated in Figure No. 4, a propulsion control system is not. just an electmnicbox, but consistsof many other varied components which are optimized as a system to meet. t,he system goals. I t is important to continue technology development for all components of the comp1et.e propulsion control syst.emto make possible theoptimization of performance,weight,cost,reliability,maint,ainabilit.yandotheroperating benefits. Important hardware considerations include the advanced out.put.int.erfaces,advancedsensors, control system environment, integration, and electronic and component. reliability.

Further research is required on advanced output interface deviceswhichcanbeincorporatedinto actuation systems to provide interfaces that are more compatible with digit.al comput.ers. An example of such an interface is the pulse-width modulated solenoid, developed for afuelmet,eringvalveunderthe NASADigital Output Interface (DOI) program. Newsensingdevices for propulsionsystemparameters should be developed that are compatible with digital controls and will reduce the input interface hardware requirements.

Optical communication has been proven feasible and costeffective for aircraft useby the ALOFT study and demonstration program. Presuming that immunity from electromagnetic interference is necessary, optical data links that are suitable for use in the engine environment must he developed. Figure No. 5 illustrates some potential advantages of optical communication. Also, alt.ernate interface configurations such as multiplexing of feedback signals to the control unit and locating power switching elements away from the computer control unit need t.o be pursued.

Electroniccomponentreliability is adverselyaffected by increasingtemperatures.Therefore, i t is necessary to provide cooling to the digital electronic control unit,. For engine mounted control systems, t,his cooling may be provided by flowing fuel through passageways in the control unit. This approach may not be adequate at the elevated ambient and fuel temperatures encountered during supersonic flight. Therefore, research into alternate cooling approaches should be conducted.

System integration of the propulsion and airframe would benefit. from cooperative programs in which (1) supplying data from t.he aircraft, central air data computer airframe and engine manufacturers consider: to the propulsion system controls; (2) supplying electrical and hydraulic power with acceptable characteristics from the aircraftpower systems to the propulsion system controls; (3) configuring the control system and intersystem communication links to accommodate such problems as lightning strikes, EMI, and common mode failures; and (4) design of the control system to minimize damage resulting from engine fires.

Asinglechanneldigitalcontrolwithselectivecomponentandcircuitredundancy will resultina system of minimum cost and complexity, but requires considerable substantiation to ensure that acceptable reliability levels will be obtained without the useof redundant channels or backup control configurations.

Technology advances are therefore required in thearea of digitalelectroniccomponentstoprovide continuing improvement in system reliability. Design studies are also required to determinehow to utilize advanced technology components and features such as selective redundancy and fault tolerance logic t o optimize the control system reliability. An Air Forcesponsored program, “Digital Electronic Control System Reliability,” has a goal to establish the definition of a Full Authority Fault-Tolerent Electronic Engine Control (FAFTEEC) system that has significantly better reliability than any of the electronic or hydromechanical alternatives and still maintains performance, cost and weight advantages. Figure No. 6 illustrates the goals of this program. Selected redundancy will be ut,ilized to minimize mission costleffectiveness.

Software development areas include propulsion and flight. controls int,egration and the applicationof advanced control methods. Because of the flexibility and logic programming capability of full aubhority digitalelectroniccontrols,anumber of sophisticatedcontrolfunct.ionscan be incorporatedwhich will promote efficient propulsion system operation, reduce pilot workload, improve safety of operation, potentiallyreducefuelconsumption,andmaket,hecontrolsystemlesssophisticated for theuser. For advanced supersonic transport and fighter aircraft applications, further technology development is required in the area of integrated aircraft/inlet./engine/nozzle control modes. Control algorithms should be investigated to improve the logic capability of the digital control instead of implementing hydromechanical control logic ir. electronic boxes. Technology development would also be desirable for performance seeking controls and integration with Engine Condit.ion Monit.oring functions. Performance seeking logic can be implementedon-linetoprovideimprovements in propulsionsystemandaircraftsystemperformance through optimization of control variable settings. The software capability of the propulsion control can be used to provide data to an engine condition monitor which analyzes the mechanical health and compnnent efficiency of theenginetoprovideearlyidentificationandprevention of problems,therebyreducing cperating and maintenance costs.

Closed loop test benches like the one illustrated in Figure No. 7 will be utilized to verifv hardware and software concepts even before engine definition. As illustrated, a hybrid computer can simulate the aircraft andtheenginecomponentsthat“turnandburn” - compressors,burners.turbines,augmentors,etc.

Control components are driven such that “real” engine operation is simulated.

INTEGRATION The design of propulsion systems has traditionally been basedon the primary objective of maximizing steady-state performance of the t.otal vehicle. New aircraft designs and technology advancements are giving designers a great range of aerodynamic and propulsive capabilit.ies for interactive/integrated force controls.

Thisrequiresthattheconfigurationbevisualized in terms of conceptssuch as force production,force distribution and force management. Force production incorporates aerodynamic propulsive interactive force systems such as in-flight vectored thrust, in-flight reversed thrust, jet flaps and external blown flaps. Force distribution includes advanced concepts such as relaxed static stability. canards and maneuver flaps. Force management includes features such as flight propulsion control, coupling svstems. maneuver load control, direct lift control, direct side force control, energy management and energy maneuverability.

Figure No. 8 illustrates a few potential next generation aircraft. These aircraft will dynamically blend the control functions of the weapon system. An example would couple flight control, propulsion control and laser tracker control to the weapon fire control with the object being to maximize aiming precision or target range. Performance seeking cont.ro1 actions could be supervised by the mission control system. Algorithms could be selected to maximize range, minimize time-to-target or maximize flight time. Contributing systems (flight,propulsion,navigation) could optimizeperformancewhilesimultaneouslyobservingsubsystem limits. Research to define these blended control modes will require cooperative “team” studies to assure that each subsystem is properly represented and modeled with adequate fidelity.

CLOSING THOUGHT The technologies supporting control system evolution draw from a wide variety of disciplines. While some of these disciplines are paced byprogress within the aerospace community, most of them are now heavily influenced by the demands of the consumer industries.

5 2 As automatic controls become more commonplace in the consumer market, industrial research will focus more on that need and will respond less to the special needs of aerospace products. Although some consumer products and techniques will be adaptable to our needs, the net effect will be a requirement to expend more research dollars for aerospace specialty items.

Research money alone will not, however, reverse the current trend of specialty industries to ignore or reject the aerospace market. Within these industries we see a rare consensus between the “managers” and the “innovators” that aerospace products are not worth the trouble. In addition to a low profit margin, the managers see a poor return on the investment of time and limited innovative talent. This reinforces their natural desire to constrain the innovations and react only to the consumer market. The innovators are not stimulated because long range military missions, plans and products are not visible to them. In addition, their novel or revolutionary ideas are frequently “stonewalled” by Milit.ary Specifications.

There are many other factors involved in this problem and a solut,ion is not obvious. Some research effortshouldbeexpendedtodefine new planning,budgetingandprocurement,proceduresplus new technology management methods that will encourage these specialty it,em subcont.ract,orsto participate in aerospace product development.

BIBLIOGRAPHY Preprint, 1979 Propulsion Controls Symposium, NASA Lewis Research Center, Cleveland, Ohio, May 17-19, 1979.

H. A. Rediess, Avionics,ControlsandHumanFactorsTechnologyPlan, NASADraft. Summary Report, June 1979.

R. K. Smyth, State of the Art Survey of Technologies Applicable to NASA’s Aeronautics, Aironics and Controls Programs, NASA Contractor Report 159050, Cont,ract. NASN-2961, May 1979.

J. R. Szuch, Control Technology, Presented to NASA Lewis Research Center Aeropropulsion Conference, Cleveland, Ohio, May 15-16, 1979.

C. H. Borgmeyer, A Streamlined Control System Development Process, Presented to AIAA/SAE/ASME 15th Joint Propulsion Specialist Conference, LasVegas, Nevada, June 18-20, 1979.

C. E. Bentz and J. R. Zeller, Integrated Propulsion Control System Program, SAE Paper 730359, April 1973.

W. L. Webb, R.J . Miller and R. D. Hackney, Overview of the Advanced Engine Control Challenge and Its Impact on Flight Controls, Presented to SAE Aerospace Cont,rol and Guidance Systems Committee, Cambridge, Mass., Sept. 28, 1978.

B. A. Barclay, T . G. Lenox and C. J. Bosco, Full Authority Digital Electronic Control - Highlights of Next Generation Propulsion Control Technology, ASME Paper 78-GT-165.

B. A. Barclay, FADEC - Digital Propulsion Control of the Future, AIAA Paper 76-652, July 1976.

T. G. Lenox, FullAuthorityDigitalElectronicControl, Phase I Final,Prepared for Naval Air Propulsion Center, Contractor Report N00019-76-6-0422, June 1978.

L. D. Emerson, FlightPropulsion Control System for 1990 Applications, Presented to Project Squid, Purdue University, May 12, 1977.

E . Rachovitsky, Opportunities in FlightPropulsion Control Coupling (FPCC), Presented at the SAE Air Transportation Meeting, Dallas, Texas, May 1974.

R. L. DeHoff, L. E. Baker and W. E. Hall, Impactof Automated Monitoring on Engine Operations and Support, Presented to AIAAISAEIASME 15th Joint Propulsion Conference, Las Vegas, Nevada, June 18-20, 1979.

J . Bayati and K. W. Williston, An Advanced Fighter Aircraft Propulsion Control Concept, Presented to AIAAISAE 13th Propulsion Conference, Orlando, Florida, July 11-13, 1977.

M. Athans, Industrial Liaison Symposium at MIT - Advances in Reliable Control Systems Design, October 19. 1977.

F. J. Hrach, et al., Design and Evaluation of a Sensor Fail-Operational Control System for a Digitally Controlled Turbofan Engine, NASA Report TMX-3260, December 1975.

AdvancedControlTechnology andIts Pot,ential for FutureTransport, Aircraft,, NASA Technical Memorandum, NASA TM-X-3409.

M. Endo, K. Nishio, N. Sugiyama, T. Koshinuma and Y. Matsuda, Resea.rch and Deue1opmen.t of Digital Jet-Engine Controls, Paper No. 60, Presented at ASMEIJSME1977 Tokyo Joint Gas Turbine Conference.

E. C. Simpson and R. J . Hill, The Answer to the “Engine” Deficiency Question, AIAA 1978.

R. A. Howlett and E. C. Beattie, Integrated Control Systems for Advanced Supersonic Engines.

D. Tesar, Mission Oriented Research for Light Machinery, Science, Vol. 201, 8 September 1978.

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F100 TURBOFAN WITH AUGMENTOR

Air-Cooled Compressor Turbines ,“-Afterburner Electronic Control

v

Variable Geometry Variable Convergent- Divergent Nozzle Figure No. 1

ORDERLY TRANSITION

TO ELECTRONIC CONTROL ~

3 FADEC

DEEC EEC Control Figure No. 2

ELECTRONIC COST PER CALCULATION

REDUCING 50% PER YEAR

C Calc Time Figure No. 3

CONTROL TECHNOLOGY/GOALS

Increased Mission High Durability Effectiveness Transmission Figure No. 4 5 6 FLY BY LIGHT Engine Cable Weight Reduced High Data Rate No Electromagnetic Interference Fail Gracefully

Security - No Leakage

Direct Communication With Computer Design by the Pilot’s Mother Lose Airplanes Redundancy Figure No. 6 5 7

CONTROL SYSTEM DEVELOPMENT/

INTEGRATION FACILITY

Airframe L - "

-

Hybrid Computing Equipment 1 .

Enaine I I I

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Nozzle Control Electronic Control q c t Load uation c

\

System

Pneumatic Pressure Generator A

Augmentor Environmental Chamber and Control Figure No. 7

c

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