Control Technology Needs For Electrifi Ed Aircraft .
NASA/TM—2019-220296GT2019-91413Control Technology Needs for Electrified AircraftPropulsion SystemsDonald L. Simon, Joseph W. Connolly, and Dennis E. CulleyGlenn Research Center, Cleveland, OhioJuly 2019
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NASA/TM—2019-220296GT2019-91413Control Technology Needs for Electrified AircraftPropulsion SystemsDonald L. Simon, Joseph W. Connolly, and Dennis E. CulleyGlenn Research Center, Cleveland, OhioPrepared for theASME Turbo Expo 2019sponsored by American Society of Mechanical Engineers (ASME)Phoenix, Arizona, June 17–21, 2019National Aeronautics andSpace AdministrationGlenn Research CenterCleveland, Ohio 44135July 2019
AcknowledgmentsThis work was conducted under the NASA Advanced Air Vehicles Program, Advanced Air Transport Technology Project. Theauthors wish to thank members of the Commercial Aero-Propulsion Controls Working Group for their feedback on the ElectrifiedAircraft Propulsion (EAP) control needs captured in this document.This work was sponsored by the Advanced Air Vehicle Programat the NASA Glenn Research CenterLevel of Review: This material has been technically reviewed by technical management.Available fromNASA STI ProgramMail Stop 148NASA Langley Research CenterHampton, VA 23681-2199National Technical Information Service5285 Port Royal RoadSpringfield, VA 22161703-605-6000This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/
Control Technology Needs for Electrified Aircraft Propulsion SystemsDonald L. Simon, Joseph W. Connolly, and Dennis E. CulleyNational Aeronautics and Space AdministrationGlenn Research CenterCleveland, Ohio 44135AbstractElectrified aircraft propulsion (EAP) systems hold potentialfor the reduction of aircraft fuel burn, emissions, and noise.Currently, NASA and other organizations are actively workingto identify and mature technologies necessary to bring EAPdesigns to reality. This paper specifically focuses on theenvisioned control technology challenges associated with EAPdesigns that include gas turbine technology. Topics discussedinclude analytical tools for the dynamic modeling and analysisof EAP systems, and control design strategies at the propulsionand component levels. This includes integrated supervisorycontrol facilitating the coordinated operation of turbine andelectrical components, control strategies that seek to minimizefuel consumption and lessen the challenges associated withthermal management, and dynamic control to ensure engineoperability during system transients. These dynamic controlstrategies include innovative control approaches that eitherextract or supply power to engine shafts dependent uponoperating phase, which may improve performance and reducedgas turbine engine weight. Finally, a discussion of controlarchitecture design considerations to help alleviate thepropulsion/aircraft integration and certification challengesassociated with EAP systems is provided.IntroductionSince the dawn of aviation, fossil fuel burning engines haveserved as the dominant source of aircraft propulsive thrust.However, recent technological advances in batteries andelectrical systems have enabled the exploration of alternativedesigns that rely on the generation, storage, and transmission ofelectrical power for aircraft propulsion. The motivation toconsider electrified aircraft propulsion (EAP) designs is beingdriven by aviation fuel burn, emission, noise, and cost reductiongoals (Refs. 1 and 2). EAP offers flexibility in storing andtransmitting electrical power, which enables aircraft designsthat apply advanced propulsion concepts such as distributedelectric propulsion and boundary layer ingestion fans. EAPNASA/TM—2019-2202961systems take the form of several potential architectures asshown in Figure 1 (Refs. 3 and 4). These EAP architectureoptions include: All electric: Batteries provide the sole source ofpropulsive power. Hybrid electric: A combination of batteries andcombustion engines provide propulsive power. In parallelhybrid designs, a battery-powered motor and a turbineengine are both mounted on a shaft that drives a fan, sothat either or both can provide propulsion. In series hybriddesigns, only the electric motors are mechanicallyconnected to the fans; the gas turbine drives an electricalgenerator, which produces power to drive the motorsand/or charge batteries. Turboelectric: Combustion engines provide propulsivepower with all (full turboelectric) or some (partialturboelectric) of the engine power output converted toelectricity. Series/parallel partial hybrid system: Has one or morefans that can be driven directly by a gas turbine as well asother fans that are driven exclusively by electrical motors.These motors can be powered by a battery or by a turbinedriven generator.The NASA Aeronautics Research Mission Directoratestrategic implementation plan outlines a vision to transition toalternative propulsion and energy sources (Ref. 5). Thisincludes a range of electrified propulsion solutions includingall-electric, turboelectric, and hybrid electric designs. Severalelectrified aircraft concept vehicles have been proposed byNASA as shown in Figure 2. This includes fixed-wing aircraftdesign concepts such as the all-electric X-57 Maxwell (Ref. 6),the Single-aisle Turboelectric AiRCraft with Aft BoundaryLayer propulsor (STARC-ABL) (Ref. 7), and the NX-3 blendedwing body with distributed turboelectric propulsion (Ref. 8).Also shown are electrified rotorcraft vehicles proposed underNASA’s Revolutionary Vertical Lift Technology Project(Refs. 9 and 10).
BatteryAll ElectricElectric Bus1 to ManyFansFuelElectric BusGeneratorTurbofanElectric BusTurboshaftFanFuelPartial lectric ies HybridParallel HybridMotor(s)Electric BusGeneratorFuelFuelBatteryMotorSeries/Parallel Partial butedFansElectric rFigure 1.—Electrified aircraft propulsion architectures (Refs. 3 and 4).Fixed-WingAircraftX-57 MaxwellSTARC-ABLAll ElectricPartial TurboelectricQuadrotorSide-by-Side HelicopterN3-XDistributed TurboelectricRotorcraftAll ElectricHybrid ElectricTiltwingTurboelectricFigure 2.—NASA electrified aircraft concept vehicles (Refs. 6 to 10).A multitude of EAP vehicle concepts are also being exploredin industry. Almost 100 electrically propelled aircraft are indevelopment worldwide (Ref. 11). These are mostly all-electricdesigns targeting the general aviation and urban air mobilitymarkets. EAP targeting larger commercial aircraft tend to beturboelectric or hybrid electric designs. Examples include theE-Fan X series hybrid propulsion aircraft being developed byAirbus in partnership with Rolls-Royce and Siemens (Ref. 12)and Zunum Aero’s regional airliner with hybrid electricpropulsion (Ref. 13).Multiple technology advances are required to enable EAPimplementation on next generation aircraft (Ref. 4). Thisincludes improvements in electrical motors and generators toachieve higher efficiency and specific power, technology toenable increased battery specific energy, and power electronicsand power distribution system technology to enable operationat higher voltage levels at altitude. Advances in gas turbinetechnology are needed to enable high levels of engine powerextraction or power addition. Another significant challenge isthermal management of the EAP system.NASA/TM—2019-2202962In addition to the technology challenges noted above, EAPalso presents significant controls-related challenges. Thisincludes development of the control design tools and strategiesto ensure reliable and efficient operation of EAP systems, bothunder normal and anomalous operating scenarios. This paperwill specifically focus on the control technology challengesassociated with the design and operation of EAP designs thatinclude gas turbine technology. Several of these challengeswere identified by the Commercial Aero-Propulsion ControlsWorking Group (CAPCWG), a consortium of NASA andUnited States engine and aircraft manufacturers focused onidentifying propulsion control and related technologydevelopment needs that are aligned with NASA’s AeronauticsMission Directorate Programs and Projects. The EAP controltechnology needs identified by CAPCWG in Reference 14 arefurther expanded upon and discussed in this document.The remaining sections of this paper are organized as follows.First, a comparison between the control architectures requiredfor conventional aircraft engines versus EAP designs is given.This is followed by a discussion of the modeling and control
design tools needed for developing EAP control systems. Next,EAP control strategies are discussed. This includes a discussionof the integrated control strategies required for coordinatedoperation of turbine and electrical components, and thepotential control enhancements offered by the flexible nature ofEAP designs. The paper then provides a discussion of the testfacilities required for EAP evaluation and maturation. Thepaper concludes with a discussion of the control considerationsrelated to the certification of EAP systems along with CSTARCABLTEEMTLDT-MATSAlternating currentCommercial aero-propulsion controlsworking groupContingency powerDevelopment assurance levelDirect currentElectrified aircraft propulsionEnvironmental control systemElectronic engine controlFunctional hazard assessmentHybrid electric integrated systems testbedHardware-in-the-loopHigh pressure compressorHigh pressure turbineIntegrated flight and propulsion controlLow pressure compressorLow pressure turbineMaximum continuous powerMaximum powerNASA electrified aircraft testbedNumerical propulsion system simulationMore electric enginePower lever anglePropulsion object-oriented simulationsoftwareSpecific fuel consumptionSingle-aisle turboelectric aircraft with aftboundary layer propulsorTurbine electrified energy managementTime-limited-dispatchToolbox for the modeling and analysis ofthermodynamic systemsComparison of Conventional VersusEAP Control ArchitecturesAn aircraft engine’s control system plays a vital role inensuring the safe, reliable, and efficient operation of the enginethroughout the aircraft’s operating envelope, which includescontrolling the engine during transient operation. A comparisonNASA/TM—2019-2202963between a conventional aircraft propulsion control architectureand an EAP control architecture is shown in Figure 3. Thesetwo architectures will be further discussed in the paragraphsbelow.In the conventional aircraft engine control architecture shownin Figure 3(a), communication between the aircraft and eachengine installed on the vehicle occurs through an ElectronicEngine Control (EEC) computer. The EEC is a dual-channelcomputer that receives thrust demands along with power andbleed offtake requests from the aircraft. These aircraft requests,along with engine sensed feedback measurements, areprocessed by control logic implemented within the EEC andused to calculate control commands sent to actuators installedon the engine. Fuel flow rate is the primary parameter adjustedto control engine thrust or torque output. Since engine thrustoutput cannot be sensed directly, a feedback measurementcorrelated to thrust, such as fan speed or engine pressure ratio,is used to establish a closed-loop fuel control design. Additionalengine actuators such as variable guide vanes and bleed valvesare open-loop scheduled by the EEC to ensure engineoperability. The EEC supplies engine parameters back to theaircraft for cockpit gauge displays and health and statusinformation purposes.Engine control systems must be robust to account forengine-to-engine performance variations that naturally exist.Limit logic is applied to ensure that the engine does notencounter operability issues such as surge or combustorblowout, and that structural and temperature limits are notexceeded. Additionally, the engine control plays an importantfunction in engine fault detection, isolation, andaccommodation. This includes logic to diagnose andaccommodate faults. Accommodation actions may includeswitching to physically redundant hardware (e.g., computerchannel, sensor, or actuator), commanding actuators to failsafepositions, or switching to revisionary control modes in the eventof a fault. The conventional engine control architecture tends tobe centralized in its design, and the controller is certified alongwith the engine.EAP control architectures are application dependent, but ingeneral EAP control systems are expected to be moredistributed and more complex than their conventional enginecontrol counterparts. A notional EAP control architecture for ahybrid electric propulsion system is shown in Figure 3(b). Here,propulsive thrust is generated by gas turbine engines and anarray of distributed electrically driven fans. Electricalcomponents, including generators, batteries, power electronics,electrical buses and motors, are included to enable thegeneration and delivery of electrical power to the distributedfans. EEC units control the operation of the gas turbines, whilean electronic component controller regulates the operation ofthe generators, battery, and distributed electrical motor driven
AircraftAircraft Thrust demands Aircraft bleedand powerofftake demands Engine health andstatus information Cockpit gauge andindicator informationElectronic Engine Control UnitsEngine #1EECActuatorcommands Health and status information Thrust demands Cockpit gauge and indicator Aircraft bleed andinformationpower offtakedemandsSupervisoryEngine ine #1SensedfeedbackmeasurementsControllerElectrical Power Managementand DistributionElectronic EngineControl UnitsEngine #1 Engine #2EECEECActuatorcommandsSensed feedbackmeasurementsGeneratorControl UnitEnginesEngine #2DistributedElectricallyDriven FansBattery ManagementSystemGeneratorControl UnitMotorControl UnitBatteryPwr. Elec.Control UnitMotorPowerElectronicsMotorControl UnitMotorGeneratorsa) Conventional aircraft propulsionb) Electrified aircraft propulsioncontrol architecturecontrol architectureFigure 3.—Comparison of conventional and electrified aircraft propulsion control architectures.fans. A supervisory controller is included to control operationof the turbine and electrical subsystems, and it also serves as thecommunication interface between the aircraft and thepropulsion system. Given the coupling between turbine andelectrical system operation, the supervisory controller plays avital role in coordinating the operation of both subsystems tooptimize efficiency, reduce thermal management challenges,and maintain overall operating limits. As with the conventionalengine control architectures, the EAP design must be robust toperformance variations and system faults. Due to their diversityof components and coupled nature, EAP systems are expectedto present more failure modes and also enable new systemreconfiguration options in response to faults. As such, faultdetection and accommodation logic embedded within thecontrol system is expected to play a vital role in supporting EAPsystem certification requirements.Aircraft Engine Controls DevelopmentProcess and Applied ToolsA high-level illustration of the aircraft engine controlsdevelopment process and applied tools is shown in Figure 4.Here, a series of maturation steps are shown, each of increasingcost and complexity. Often, development iterations are neededto make control system updates. The process begins byreceiving information on the propulsion system design concept.NASA/TM—2019-2202964Control System MaturationCertification ntrolDesignReal-TimeSimulationand t IterationsFigure 4.—Aircraft engine control development process.This is typically obtained through system studies conducted todesign and size the propulsion system to match its intendedaircraft mission. Given the propulsion system design concept,the control development process includes the steps of dynamicmodeling, control design, real-time simulation and hardwarein-the-loop (HIL) evaluation, engine testing, and flight testing.Certification considerations are applied throughout this processto ensure that the design complies with the airworthinessstandards set forth by regulatory agencies. The upcomingsections will discuss the tools, control design strategies,facilities, and certification considerations related to EAPcontrol system development.Modeling and Control Design ToolsDynamic system modeling and computational analysis toolsare integral to the aircraft engine control development process.During the development cycle of an engine, a non-linear
physics-based model of the engine is created and used to designturbomachinery and evaluate system-level performance. Suchmodels are complex, capturing the behavior and coupling of allengine components including the inlet, fan, compressors,combustor, turbine, and exhaust nozzle. Other design aspects ofthe engine such as bypass ducts, cooling flows, bleed andmechanical power offtakes, and variable guide vanes are alsorepresented in these models.The models may be either steady-state or dynamic, withsteady-state models capturing the “on-design” performance ofthe engine and dynamic models enabling simulation of the “offdesign” performance encountered by the engine duringtransients. Dynam
Control Technology Needs for Electrified Aircraft Propulsion Systems . Donald L. Simon, Joseph W. Connolly, and Dennis E. Culley . National Aeronautics and Space Administration Glenn Research Center. Cleveland, Ohio 44135 . Abstract Electrified aircraft propulsion (EAP) systems hold potenti
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