Control Technology Needs for Electrified Aircraft Propulsion Systems

NASA/TM—2019-220296 GT2019-91413

Control Technology Needs for Electri fi ed Aircraft

Propulsion Systems

Donald L. Simon, Joseph W. Connolly, and Dennis E. Culley Glenn Research Center, Cleveland, Ohio

July 2019

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NASA/TM—2019-220296 GT2019-91413

Control Technology Needs for Electri fi ed Aircraft

Propulsion Systems

Donald L. Simon, Joseph W. Connolly, and Dennis E. Culley Glenn Research Center, Cleveland, Ohio Prepared for the ASME Turbo Expo 2019 sponsored by American Society of Mechanical Engineers (ASME) Phoenix, Arizona, June 17–21, 2019 National Aeronautics and Space Administration Glenn Research Center Cleveland, Ohio 44135

July 2019

Acknowledgments This work was conducted under the NASA Advanced Air Vehicles Program, Advanced Air Transport Technology Project. The authors wish to thank members of the Commercial Aero-Propulsion Controls Working Group for their feedback on the Electri fi ed Aircraft Propulsion (EAP) control needs captured in this document.

This work was sponsored by the Advanced Air Vehicle Program at the NASA Glenn Research Center Level of Review : This material has been technically reviewed by technical management.

Available from NASA STI Program National Technical Information Service Mail Stop 148 5285 Port Royal Road NASA Langley Research Center Spring fi eld, VA 22161 Hampton, VA 23681-2199 703-605-6000 This report is available in electronic form at http://www.sti.nasa.gov/ and http://ntrs.nasa.gov/

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 systems take the form of several potential architectures as

Abstract

shown in Figure 1 (Refs. 3 and 4 ). These EAP architecture Electrified aircraft propulsion (EAP) systems hold potential options include: for the reduction of aircraft fuel burn, emissions, and noise.

Currently, NASA and other organizations are actively working • All electric: Batteries provide the sole source of to identify and mature technologies necessary to bring EAP propulsive power.

designs to reality. This paper specifically focuses on the • Hybrid electric: A combination of batteries and envisioned control technology challenges associated with EAP combustion engines provide propulsive power. In parallel designs that include gas turbine technology. Topics discussed hybrid designs, a battery-powered motor and a turbine include analytical tools for the dynamic modeling and analysis engine are both mounted on a shaft that drives a fan, so of EAP systems, and control design strategies at the propulsion that either or both can provide propulsion. In series hybrid and component levels. This includes integrated supervisory designs, only the electric motors are mechanically control facilitating the coordinated operation of turbine and connected to the fans; the gas turbine drives an electrical electrical components, control strategies that seek to minimize generator, which produces power to drive the motors fuel consumption and lessen the challenges associated with and/or charge batteries.

thermal management, and dynamic control to ensure engine • Turboelectric: Combustion engines provide propulsive operability during system transients. These dynamic control power with all (full turboelectric) or some (partial strategies include innovative control approaches that either turboelectric) of the engine power output converted to extract or supply power to engine shafts dependent upon electricity.

operating phase, which may improve performance and reduced • Series/parallel partial hybrid system: Has one or more gas turbine engine weight. Finally, a discussion of control fans that can be driven directly by a gas turbine as well as architecture design considerations to help alleviate the other fans that are driven exclusively by electrical motors.

propulsion/aircraft integration and certification challenges These motors can be powered by a battery or by a turbine- associated with EAP systems is provided.

driven generator.

The NASA Aeronautics Research Mission Directorate

Introduction

strategic implementation plan outlines a vision to transition to alternative propulsion and energy sources (Ref. 5 ). This Since the dawn of aviation, fossil fuel burning engines have served as the dominant source of aircraft propulsive thrust. includes a range of electrified propulsion solutions including all-electric, turboelectric, and hybrid electric designs. Several However, recent technological advances in batteries and electrified aircraft concept vehicles have been proposed by electrical systems have enabled the exploration of alternative designs that rely on the generation, storage, and transmission of NASA as shown in Figure 2 . This includes fixed-wing aircraft design concepts such as the all-electric X-57 Maxwell (Ref. 6 ), electrical power for aircraft propulsion. The motivation to the Single-aisle Turboelectric AiRCraft with Aft Boundary consider electrified aircraft propulsion (EAP) designs is being driven by aviation fuel burn, emission, noise, and cost reduction Layer propulsor (STARC-ABL) (Ref. 7 ), and the NX-3 blended wing body with distributed turboelectric propulsion (Ref. 8 ).

goals (Refs. 1 and 2 ). EAP offers flexibility in storing and Also shown are electrified rotorcraft vehicles proposed under transmitting electrical power, which enables aircraft designs that apply advanced propulsion concepts such as distributed NASA’s Revolutionary Vertical Lift Technology Project (Refs. 9 and 10 ).

electric propulsion and boundary layer ingestion fans. EAP NASA/TM—2019-220296 1 All Electric Parallel Hybrid Series Hybrid Battery Turbofan Turboshaft Electric Bus Motor(s) Electric Bus Motor Electric Bus Distributed Generator Fans Motor Battery Fuel 1 to Many Fan Fans Battery Motor Fuel Partial Turboelectric Series/Parallel Partial Hybrid Turboelectric Turbofan Turboshaft Turbofan Electric Bus Electric Bus Motor Motor Electric Bus Motor Distributed Distributed Distributed Fans Fans Generator Generator Generator Fans Fuel Fuel Fuel Fan Motor Motor Fan Battery Motor Figure 1.—Electrified aircraft propulsion architectures (Refs. 3 and 4 ).

Fixed-Wing Aircraft N3-X X-57 Maxwell STARC-ABL Partial Turboelectric Distributed Turboelectric All Electric Rotorcraft Quadrotor Side-by-Side Helicopter Tiltwing All Electric Hybrid Electric Turboelectric Figure 2.—NASA electrified aircraft concept vehicles (Refs. 6 to 10 ).

A multitude of EAP vehicle concepts are also being explored In addition to the technology challenges noted above, EAP in industry. Almost 100 electrically propelled aircraft are in also presents significant controls-related challenges. This development worldwide (Ref. 11 ). These are mostly all-electric includes development of the control design tools and strategies designs targeting the general aviation and urban air mobility to ensure reliable and efficient operation of EAP systems, both markets. EAP targeting larger commercial aircraft tend to be under normal and anomalous operating scenarios. This paper turboelectric or hybrid electric designs. Examples include the will specifically focus on the control technology challenges associated with the design and operation of EAP designs that E-Fan X series hybrid propulsion aircraft being developed by Airbus in partnership with Rolls-Royce and Siemens (Ref. 12 ) include gas turbine technology. Several of these challenges and Zunum Aero’s regional airliner with hybrid electric were identified by the Commercial Aero-Propulsion Controls propulsion (Ref. 13 ). Working Group (CAPCWG), a consortium of NASA and Multiple technology advances are required to enable EAP United States engine and aircraft manufacturers focused on implementation on next generation aircraft (Ref. 4 ). This identifying propulsion control and related technology includes improvements in electrical motors and generators to development needs that are aligned with NASA’s Aeronautics achieve higher efficiency and specific power, technology to Mission Directorate Programs and Projects. The EAP control enable increased battery specific energy, and power electronics technology needs identified by CAPCWG in Reference 14 are and power distribution system technology to enable operation further expanded upon and discussed in this document.

at higher voltage levels at altitude. Advances in gas turbine The remaining sections of this paper are organized as follows.

technology are needed to enable high levels of engine power First, a comparison between the control architectures required extraction or power addition. Another significant challenge is for conventional aircraft engines versus EAP designs is given.

thermal management of the EAP system. This is followed by a discussion of the modeling and control NASA/TM—2019-220296 2 design tools needed for developing EAP control systems. Next, between a conventional aircraft propulsion control architecture EAP control strategies are discussed. This includes a discussion and an EAP control architecture is shown in Figure 3 . These of the integrated control strategies required for coordinated two architectures will be further discussed in the paragraphs operation of turbine and electrical components, and the below.

potential control enhancements offered by the flexible nature of In the conventional aircraft engine control architecture shown EAP designs. The paper then provides a discussion of the test in Figure 3 (a), communication between the aircraft and each facilities required for EAP evaluation and maturation. The engine installed on the vehicle occurs through an Electronic paper concludes with a discussion of the control considerations Engine Control (EEC) computer. The EEC is a dual-channel related to the certification of EAP systems along with a computer that receives thrust demands along with power and summary. bleed offtake requests from the aircraft. These aircraft requests, along with engine sensed feedback measurements, are processed by control logic implemented within the EEC and

Nomenclature

used to calculate control commands sent to actuators installed on the engine. Fuel flow rate is the primary parameter adjusted AC Alternating current Commercial aero-propulsion controls to control engine thrust or torque output. Since engine thrust CAPCWG working group output cannot be sensed directly, a feedback measurement CP Contingency power correlated to thrust, such as fan speed or engine pressure ratio, DAL Development assurance level is used to establish a closed-loop fuel control design. Additional DC Direct current engine actuators such as variable guide vanes and bleed valves EAP Electrified aircraft propulsion are open-loop scheduled by the EEC to ensure engine ECS Environmental control system operability. The EEC supplies engine parameters back to the EEC Electronic engine control aircraft for cockpit gauge displays and health and status FHA Functional hazard assessment information purposes.

HEIST Hybrid electric integrated systems testbed Engine control systems must be robust to account for HIL Hardware-in-the-loop HPC High pressure compressor engine-to-engine performance variations that naturally exist.

HPT High pressure turbine Limit logic is applied to ensure that the engine does not IFPC Integrated flight and propulsion control encounter operability issues such as surge or combustor LPC Low pressure compressor blowout, and that structural and temperature limits are not LPT Low pressure turbine exceeded. Additionally, the engine control plays an important MCP Maximum continuous power function in engine fault detection, isolation, and MP Maximum power accommodation. This includes logic to diagnose and NEAT NASA electrified aircraft testbed accommodate faults. Accommodation actions may include NPSS Numerical propulsion system simulation switching to physically redundant hardware (e.g., computer MEE More electric engine channel, sensor, or actuator), commanding actuators to failsafe PLA Power lever angle Propulsion object-oriented simulation positions, or switching to revisionary control modes in the event PROOSIS software of a fault. The conventional engine control architecture tends to SFC Specific fuel consumption be centralized in its design, and the controller is certified along STARC- Single-aisle turboelectric aircraft with aft with the engine.

ABL boundary layer propulsor EAP control architectures are application dependent, but in TEEM Turbine electrified energy management general EAP control systems are expected to be more TLD Time-limited-dispatch distributed and more complex than their conventional engine Toolbox for the modeling and analysis of T-MATS control counterparts. A notional EAP control architecture for a thermodynamic systems hybrid electric propulsion system is shown in Figure 3 (b). Here, propulsive thrust is generated by gas turbine engines and an Comparison of Conventional Versus array of distributed electrically driven fans. Electrical components, including generators, batteries, power electronics,

EAP Control Architectures

electrical buses and motors, are included to enable the An aircraft engine’s control system plays a vital role in generation and delivery of electrical power to the distributed ensuring the safe, reliable, and efficient operation of the engine fans. EEC units control the operation of the gas turbines, while throughout the aircraft’s operating envelope, which includes an electronic component controller regulates the operation of controlling the engine during transient operation. A comparison the generators, battery, and distributed electrical motor driven NASA/TM—2019-220296 3 Aircraft Aircraft • Health and status information • Thrust demands • Cockpit gauge and indicator • Aircraft bleed and • Thrust demands • Engine health and information power offtake • Aircraft bleed status information demands Supervisory and power • Cockpit gauge and Controller offtake demands indicator information Electronic Engine Control Units Electrical Power Management Electronic Engine and Distribution Engine #1 Engine #2 Control Units EEC EEC Distributed Battery Management Engine #1 Engine #2 Electrically System EEC EEC Sensed Sensed Actuator Driven Fans Actuator Actuator Sensed feedback feedback feedback commands Motor commands commands measurements measurements measurements Control Unit Battery Generator Control Unit Pwr. Elec.

Control Unit Motor Power Motor Generator Engines Electronics Control Unit Control Unit Engine #2 Engine #1 Motor Generators a) Conventional aircraft propulsion b) Electrified aircraft propulsion control architecture control architecture Figure 3.—Comparison of conventional and electrified aircraft propulsion control architectures.

fans. A supervisory controller is included to control operation Control System Maturation of the turbine and electrical subsystems, and it also serves as the Certification Considerations communication interface between the aircraft and the Real-Time Engine System propulsion system. Given the coupling between turbine and Control Simulation Testing, Dynamic Design Design and HIL Flight Modeling electrical system operation, the supervisory controller plays a Concept Evaluation Testing vital role in coordinating the operation of both subsystems to optimize efficiency, reduce thermal management challenges, Development Iterations and maintain overall operating limits. As with the conventional Figure 4.—Aircraft engine control development process.

engine control architectures, the EAP design must be robust to This is typically obtained through system studies conducted to performance variations and system faults. Due to their diversity design and size the propulsion system to match its intended of components and coupled nature, EAP systems are expected aircraft mission. Given the propulsion system design concept, to present more failure modes and also enable new system the control development process includes the steps of dynamic reconfiguration options in response to faults. As such, fault modeling, control design, real-time simulation and hardware- detection and accommodation logic embedded within the in-the-loop (HIL) evaluation, engine testing, and flight testing.

control system is expected to play a vital role in supporting EAP Certification considerations are applied throughout this process system certification requirements.

to ensure that the design complies with the airworthiness standards set forth by regulatory agencies. The upcoming Aircraft Engine Controls Development sections will discuss the tools, control design strategies, facilities, and certification considerations related to EAP

Process and Applied Tools

control system development.

A high-level illustration of the aircraft engine controls development process and applied tools is shown in Figure 4 .

Modeling and Control Design Tools

Here, a series of maturation steps are shown, each of increasing cost and complexity. Often, development iterations are needed Dynamic system modeling and computational analysis tools to make control system updates. The process begins by are integral to the aircraft engine control development process.

receiving information on the propulsion system design concept.

During the development cycle of an engine, a non-linear NASA/TM—2019-220296 4 physics-based model of the engine is created and used to design of linear state-space models based on non-linear models are turbomachinery and evaluate system-level performance. Such needed. These linear state-space models should be extractable models are complex, capturing the behavior and coupling of all at multiple operating points spanning the EAP system’s engine components including the inlet, fan, compressors, operating envelope, allowing them to be coupled together in a combustor, turbine, and exhaust nozzle. Other design aspects of piecewise linear fashion (Ref. 19 ). Real-time code generation the engine such as bypass ducts, cooling flows, bleed and capabilities are also desired to support real-time simulation and mechanical power offtakes, and variable guide vanes are also hardware-in-the-loop evaluation of control systems (see represented in these models. Figure 4 ). Control design tools to coordinate operation of the The models may be either steady-state or dynamic, with turbine and electrical subsystems will also be beneficial.

steady-state models capturing the “on-design” performance of Challenges include modeling EAP systems to the proper the engine and dynamic models enabling simulation of the “off- level of fidelity. While power electronics and power design” performance encountered by the engine during management systems can have switching frequencies above the transients. Dynamic models are necessary for the design of 10 kHz range, there is a timescale tradeoff with model fidelity.

engine control systems, which are tasked with ensuring the safe The emphasis should be to develop tools that enable modeling and reliable transient operation of the engine in response to of the electrical system to the proper level of fidelity. This varying thrust requests and flight conditions. The dynamics includes modeling of control actuators and sensors plus captured by these models are typically in the 10’s of Hz range, capturing the system response to transient changes in set by conventional fuel and variable geometry actuation electrically driven propulsors, dynamic balancing of electrical systems and the dominant spool inertias transient response. loads, and the dynamic coupling between engines and the Higher frequency dynamics such as blade flutter, compressor electrical system. As with gas turbine models, these models stall, or combustion instabilities are typically not included in the should capture electrical system performance not only at the models used for control design, although the operating limits system design point, but at “off-design” transient operation as where these instabilities are expected to be encountered should well, spanning the operating envelope that the system will be be defined in the models. required to function within. The modeling of thermal loads over Various modeling tools are available for constructing gas a mission is also important, as such information can be used to turbine engine models, such as the Numerical Propulsion optimize the design of thermal management systems.

System Simulation (NPSS) (Ref. 15 ), GasTurb (Ref. 16 ), Functional operating limits of the system should also be Propulsion Object-Oriented Simulation Software (PROOSIS) included so that control protection logic can be incorporated to (Ref. 17 ), and the Toolbox for the Modeling and Analysis of restrict operation to appropriate regions.

Thermodynamic Systems (T-MATS) (Ref. 18 ). Enhancements to these tools are necessary to enable modeling of the electric

EAP Control Strategies

machines, energy storage devices, and power management and distribution hardware found in EAP concepts. These enhanced EAP systems will present a number of control design modeling tools should also enable modeling of the relevant challenges due to their complexity and integrated coupling.

dynamic interaction between electrical and mechanical However, they are also expected to enable exciting new components, as well as the effects of modulating available opportunities when it comes to controls. The need for integrated actuators. Additionally, these tools should model sources of control design approaches is anticipated, with an emphasis on heat generation and dissipation within the system for thermal coordinated turbine and electrical system operation to optimize management considerations.

efficiency and operability, while minimizing thermal Tools are also needed to develop models representative of management challenges. Potential EAP control strategies are off-nominal EAP system behavior. Simulating component further discussed in the subsections below.

performance variations due to variations in environmental conditions, manufacturing tolerances or normal deterioration Optimal Energy Management that components are expected to experience over their lifetime Focused on hybrid designs that combine output power from of use will allow control robustness to be assessed.

gas turbine engines and energy storage devices such as Additionally, the simulation of EAP system faults will allow batteries, energy management refers to the integrated control initial development and evaluation of fault detection, isolation, task of scheduling how power is drawn from all available and accommodation logic, including the logic required to sources to supply the demanded power. For optimal efficiency mitigate functional hazards and enable certification.

throughout a mission, the scheduling of this engine/energy Most aircraft engine control designs are based on linear storage device power split should be done to minimize fuel burn control approaches. As such, tools for the automated generation NASA/TM—2019-220296 5 while adhering to operating constraints. Optimal energy SFC vs. Power Output management approaches are applied within the automotive Idle Optimal SFC industry to define the power schedules implemented within the Max Continuous Power engine control systems of hybrid automobiles (Ref. 20). This is Max Power done by seeking to minimize a defined performance index, J , Contingency Power representative of the total fuel consumed during a reference mission of time T . Such a performance index can be calculated based on the integration of a defined cost function, L (·), as shown in the equation below (Ref. 20): Specific Fuel Consumption T , = J L t u t dt

( ) ( 1 )

( )

∫

0 Power Output Figure 5.—Notional turboshaft SFC vs. power output curve.

Here, u ( t ) is the control vector provided by the supervisory controller. The cost function L (·) reflects instantaneous fuel propulsion designs that include energy storage devices. In such flow rate plus any penalties incurred for violation of operating architectures, it might be possible to run the gas turbine at its constraints. These can be hard or soft constraints and can most efficient operating point for extended periods of time include a variety of factors such as engine and electrical system while using any excess engine power for battery charging and operating limits, battery charge/discharge rates and state-of- drawing power from the battery when the requested power charge limits, noise, emission, and thermal considerations. If exceeds that which can be supplied by the gas turbine alone.

desired, Equation (1) can be adapted to achieve goals beyond Considering a hybrid architecture that permits a battery to be minimizing fuel consumption. This may include minimizing both discharged and recharged during flight, there are four total energy consumption, mission cost, or other metrics of possible system operating modes: interest.

To illustrate the application of energy management strategies 1. Engine off, Battery discharging to aircraft hybrid electric propulsion designs, consider the 2. Engine on, Battery charging notional specific fuel consumption (SFC) versus power output 3. Engine on, Battery energy level is static (neither charging curve of a turboshaft engine at a given flight condition as shown or discharging) in Figure 5 . The green star denotes minimum or optimal SFC, 4. Engine on, Battery discharging which is the engine’s most efficient operating point. Typically, an engine is designed to operate close to this “design point” for Given these operating modes, a simplified hybrid EAP a significant portion of its intended mission. Additional power control schedule is shown in Figure 6 . Here, instantaneous settings of interest, which constrain available engine power power generated is shown versus instantaneous propulsive output, are also shown. This includes: power demanded. The dashed red vertical lines denote transition points in the control schedule where switching • Idle: Minimum permitted power setting. between the four operating modes described above occurs.

• Maximum Continuous Power (MCP): Maximum amount During regions of low power demand such as ground taxi, the engine is turned off and the battery supplies all demanded of power that the engine can continuously provide without any time restrictions. power (mode 1). When high levels of power are demanded, perhaps during takeoff or hover operations, both the engine and • Maximum Power (MP): The maximum power output that the battery are called upon to supply the required power the engine can provide for a finite amount of time (e.g., (mode 4). Those instances when the power demand is less than 5 min).

the engine’s optimal SFC power setting provide an opportunity • Contingency Power (CP): More common in two-engine to run the engine at its optimal SFC setting while using any helicopters, CP is a high power setting that an engine may excess engine generated power to recharge the battery provide for a short time (e.g., 2.5 min) during contingency (mode 2). As losses occur during the mechanical to electrical events such as the opposite engine becoming inoperative.

power conversion process, not all of the engine generated power can be converted into battery energy in mode 2. The From a fuel efficiency standpoint, the ideal case is to always range in power demand transitioning between the optimal SFC run the engine at its minimum SFC operating point. But this is power setting up to the power setting that marks the start of not possible in conventional propulsion designs as the requested propulsive power from the aircraft varies throughout a flight. mode 4 reflects a region of engine-only power generation with no battery charging or discharging occurring (mode 3).

However, a paradigm shift occurs when considering hybrid NASA/TM—2019-220296 6 Turboshaft generated power used directly for propulsion Battery discharge power used for propulsion Turboshaft generated power converted to battery energy (includes losses) Turbine Propulsive power demand Turboshaft generated power used for propulsion + battery charging temperature limit Maximum Thrust Transition between flat rated thrust and flat rated temperature Generated/Consumed Power Outside Air Temperature Figure 7.—Engine rated thrust schedule at a fixed flight condition.

Propulsive Power Demand Figure 6.—Notional hybrid EAP power control schedule.

systems will play a key role in addressing thermal management There are several aspects of a hybrid EAP control schedule worth noting. First, having the mode 3 region as shown in issues. This includes scheduling engine and electrical system operation under varying levels of requested thrust and operating Figure 6 may only be practical for those engines where the conditions. In conventional aircraft engine control designs, optimal SFC power setting resides to the left of the MCP setting as for the example curve shown in Figure 5 . For engines where thermal challenges are partially addressed by applying a maximum rated thrust schedule as shown in Figure 7 . This optimal SFC resides at or above MCP, it may make sense to schedule reflects the rated or maximum thrust that the engine omit mode 3 entirely and simply transition directly from can produce as a function of outside air temperature at a given mode 2 to 4. Also, the transition between modes might be flight condition (Ref. 21 ). At this flight condition, maximum dependent on phase of flight or battery state of charge. While thrust is constant for outside air temperatures below the newer automobiles apply “start-stop” technology that shuts off temperature value where an engine’s turbine temperature limit their engine at stoplights to save gas, it is unclear whether is encountered. As outside air temperature increases beyond this similar technology could ever be certified for application to value, maximum available thrust is decreased to ensure that the aircraft engines in-flight. As, such it may only be practical to maximum turbine temperature limit is not exceeded. For EAP operate in all electric mode (mode 1) during taxi operations at designs, analogous thrust and power scheduling logic is needed the airport. Additionally, if a battery ever reaches its fully to provide temperature limit protection, although such logic is charged state, the control is forced to transition out of battery expected to be more complex and will likely involve the need charging (mode 2) and into one of the other modes, regardless to coordinate the modulation of available actuators (including of whether excess engine power is available. If such constraints are encountered, it will be necessary for the engine to operate engine bleed and mechanical power offtakes) to satisfy multiple temperature limits simultaneously.

off of its maximum efficiency point.

In addition to implementing limits to guard against temperature exceedances, EAP engine and energy storage

Thermal Management

devices will also need to provide the power to drive any thermal management active cooling systems. This task is closely related EAP system developers face significant thermal management to the optimal energy management task introduced in the design challenges (Ref. 4 ). The need for increased levels of previous subsection. In fact, thermal management constraints energy and power supplied by smaller and lighter components can be directly considered within Equation (1) giving rise to a are factors contributing to increased thermal loads. Advances in combined optimal energy and thermal management problem.

high temperature materials, the development of more efficient Here, the objective becomes scheduling the coordinated components, and innovations in passive and active thermal operation of engine and energy storage devices to ensure that management systems are needed to effectively withstand and all thermal limits are maintained while simultaneously dissipate thermal loads. EAP supervisory controllers tasked operating the system in the most fuel efficient manner.

with coordinating the operation of the engine and power NASA/TM—2019-220296 7 Also, it is likely that acceleration and deceleration schedules

Transient Control Schedules and

will be needed to restrict how rapidly the electrical system can

Limit Logic

respond during transient operation to ensure both electrical and turbomachinery operability limits. As previously shown in Aircraft engine control logic is responsible for ensuring the Figure 3 (b), a supervisory control to ensure proper integrated safe and reliable transient operation of the engine throughout its coordination between the engines and the electrical power operating envelope (Ref. 21 ). This includes protection logic to system is essential.

guard against exceedance of engine operational, structural or safety limits. The concept of transient control is illustrated in the compressor map shown in Figure 8 (Ref. 21 ). Here, the solid

Turbine Electrified Energy Management

black line represents the steady-state operating line that the

(TEEM)

engine will follow over the range of power settings. During Turbine Electrified Energy Management (TEEM) is a new throttle transients, engine operation will move off the steady- control technology that addresses the occurrence of off-design state operating line as denoted by the acceleration and engine operation that occurs during changes in engine power deceleration trajectories shown in the figure. Also shown are setting or other momentary disruptions (Ref. 22 ). These several engine operating limits. This includes the compressor operability issues represent potential risks to the engine that the surge line, the combustor blow-out limit, and the turbine control system traditionally mitigates by limiting the rates of temperature limit. During transient operation, the engine change of commanded variables, such as fuel flow rate.

controller regulates fuel flow to ensure that the engine does not Temporary off-design operation is an expected natural response exceed defined acceleration/deceleration rate schedules or of the turbine engine, but it is actually a symptom of an energy defined engine operating limits.

imbalance between rotating components of the engine that For conventional aircraft gas turbine engines, transient occurs during transient operation. Inertial energy stored in the control design accounts for approximately 75 percent of the rotating components and heat energy soaked into the mass of total control law design and development effort (Ref. 21).

the engine contribute to this energy imbalance.

Given their complexity and inherent coupling, EAP designs are The TEEM technology focuses on counteracting the energy expected to present similar transient control challenges.

imbalance inherent in gas turbine engines during transients Protection limits are expected to be necessary to ensure the through the use of electric machines applied to add/extract health and life of electrical components. This includes control mechanical power to/from the shafts of the engine. This enables limits placed on electric machine speed and torque levels, the engine to operate close to its on-design condition during battery charge/discharge rates, overall power levels, and transients. Applying TEEM, the rotational velocities of engine component operating temperatures. Additionally, consideration components are matched to the flow conditions in the gas path, needs to be given to the dynamic coupling between the as estimated by the commanded fuel flow rate. This allows the electrical system and the turbomachinery. This is especially true flow incidence angle to match the design point incidence at a given the fact that the electrical system dynamic response can particular power setting. In theory, by matching the rotational potentially be much more rapid than that of the turbomachinery.

speeds of the shafts with the instantaneous fuel flow rate that As such, it is likely that speed limits and over-speed protection defines an engine flow condition, it is possible to maintain the logic will be necessary for any electric motor driven propulsors.

steady-state operating line of the turbomachinery components, particularly the compressors. This is possible, even during Surge Line transient maneuvers such as a change in power setting. The Over-temp Zone steady-state operating line is generally the most efficient Surge Turbine Temp Limit Zone Acceleration operating condition for that component.

Line One potential implementation of TEEM is shown in the Max Power architecture given in Figure 9 . Here, electric machines are Pressure Cruise coupled to the rotating shafts of the turbine engine. Drawing Ratio Deceleration Line power from an energy storage device, the electric machines are Constant Combustor Idle used to implement shaft speed control during the momentary Speed Blow-Out Lines Combustor Limit periods where the shaft responses would naturally lag behind Steady-State Blow-Out Zone commanded fuel flow rate due to their high moments of inertia.

Operating Line In addition to supplying mechanical shaft power, the electric Corrected Mass Flow machines connected to the engine are also able to extract shaft Figure 8.—Compressor map indicating engine steady-state power that is converted to electricity and used to charge energy operation, transient operation, and operating limits.

NASA/TM—2019-220296 8 Fuel

Novel Cycle and More Electric Engine

Electric Electric LPC LPT HPC HPT

Machine Controls

Machine N2 N1 Compared to conventional aircraft engines in service today, the gas turbine engines included in EAP systems are expected Energy Electric to have fundamental differences, both in their design and Storage Machine control. In addition to providing propulsive thrust, today’s conventional aircraft gas turbine engines also supply bleed air Figure 9.—TEEM architecture.

for the aircraft’s environmental control system (ECS) and mechanical power offtake to generate electricity for the aircraft.

However, the engine power extracted for these functions is only a small fraction of the total power output of the engine.

Conversely, some EAP designs will extract substantially higher percentages of overall engine power. This is expected to necessitate the need for novel engine cycle designs, such as variable fan and variable core nozzles to provide stability margin when electric power is extracted or applied to engine shafts (Ref. 23 ). With these variable cycle engines will come Figure 10.—Schematic of the N1/N2/Wf relationship during steady-state and acceleration/deceleration transients. The the need to apply control strategies to schedule operation of the black arrows indicate the desire to modify the shaft speeds engine and its variable geometry in concert with the power during transients to operate closer to the steady-state extraction/addition demands placed upon it (Ref. 24 ).

operating line.

Within the aviation industry there is an ongoing trend to transition to more electric engine (MEE) designs (Refs. 25 and storage devices or drive other electric machines on the vehicle.

26 ). This replaces aircraft engine mechanical and pneumatic Figure 10 illustrates the typical dynamic behavior that is driven accessories with electrical-mechanical actuators. This observed for a dual spool engine during acceleration and includes apparatus such as accessory gear box-driven fuel and deceleration transients and the notional steady-state oil pumps, engine bleed off-takes for heat exchangers, ECS, and relationship that is to be maintained by TEEM for the low spool anti-ice systems. Replacing these accessories with electrically- speed (N1), high spool speed (N2), and fuel flow rate (Wf). The driven systems will help to reduce weight and improve overall objective is not necessarily to match the shaft speed to the engine efficiency. A primary advantage of electrically actuated design point but to maintain acceptable levels of stall margin systems is that their operation can be scheduled in accordance during the transient condition. Generally, this requires a high to the required demand as opposed to today’s gearbox driven impulsive power, but ideally it is not beyond the rating of the systems that must operate at the speed dictated by the rotation electric machine for its original design purpose. In terms of of the engine. This results in the need for bypass circuits to energy storage capacity to drive the machine, it is modest due absorb excess flow, which is inefficient. MEE designs also seek to the short duration of the transient.

to replace pneumatic or fluid driven actuators with electrical- The overall effect of TEEM is to reduce the amount of mechanical designs. The readily available source of electricity transient stall margin required in the compressor system.

offered by EAP systems is expected to further accelerate the Reducing the amount of margin implies that engine design can transition to MEE designs in the future. With this will come the be safely modified to achieve a number of benefits affecting associated control design needs to optimally schedule the performance and efficiency metrics. Those benefits generally operation of the electrical-mechanical actuators and systems appear in the form of weight and volume reduction such as the inherent in these designs.

elimination of compressor stages or elimination of stability bleed valves. It may also enable reduced off-incidence flow in

Integrated Flight and Propulsion Control

compressor blades leading to improvements in blade design for lower loss operation. Finally, optimizing the transient Unlike conventional flight control strategies, which primarily operability of the turbine engine may impact electrified view the engine as an actuator for adjusting thrust, Integrated propulsion system design by fully utilizing the engine as the Flight and Propulsion Control (IFPC) considers control of the most efficient means of converting fuel into power, thus vehicle and its propulsion system in a coordinated fashion. This minimizing the need for energy storage, which currently has a includes modulation of engine thrust output to perform vehicle high weight penalty.

flight control functions. IFPC has been applied in short takeoff NASA/TM—2019-220296 9 and vertical landing aircraft applications (Refs. 27 and 28 ). The feasibility of performing flight control of multi-engine fixed- wing aircraft through the modulation of engine throttles was also demonstrated under the Propulsion Controlled Aircraft project led by NASA during the 1990’s (Ref. 29 ). Given their distributed propulsion nature, EAP vehicle designs are well suited for IFPC. Coordinated modulation of the thrust output of Figure 11.—NASA Electric Aircraft Testbed (NEAT) facility.

an array of distributed propulsors strategically positioned on the aircraft allows basic flight control maneuvers such as turns, climbs, and descents to be performed. This can eliminate or reduce the size of flight control surfaces reducing overall vehicle weight and cost. IFPC development for EAP vehicles will require a combined effort between flight and propulsion Turbofan 1 controls engineers. Key control issues are to ensure that the Command Fan Speed Control (NEAT – Motor) propulsors can efficiently provide the range and dynamic Torque Load on LP response in thrust needed for flight control, while adhering to Generator Inverter/ Rectifier Bus Motor 1 all operational limits and constraints. Control Tail Fan Command Pilot (NEAT – PLA Generators) Inverter/ Rectifier Bus Motor 2 Generator

Facilities to Enable EAP Test and Control

Torque Load on LP Corrected Fan Speed Command Turbofan 2

Maturation

Torque Command from Tail Fan Inverter/Control Fan Speed Control Command Torque on Tail Fan Spool (NEAT – Motor) DC Bus Current and Voltage The development of EAP systems and components will require corresponding facilities to test and mature the Figure 12.—STARC-ABL (top) and overview diagram of STARC-ABL turbomachinery and electrical system technologies. This includes facilities to perform testing of interconnections implemented at NEAT facility (bottom).

megawatt-class EAP designs (Ref. 30 ). To help address this need, NASA created testbeds to enable testing of EAP systems and their associated technologies. Examples include the Hybrid necessary torque at the tail fan motors to achieve the desired tail fan speed. The inverter/motor controllers also provide Electric Integrated Systems Testbed (HEIST) (Ref. 31 ) and the information to the generators so that the corresponding amount NASA Electric Aircraft Testbed (NEAT) (Ref. 32 ). The NEAT of torque load from each turbofan is commanded to extract the facility, shown in Figure 11 , is located at the NASA Glenn required power.

Research Center Plum Brook Station. NEAT is a reconfigurable In addition to serving as a testbed for powertrain technology, testbed developed to enable end-to-end development and NEAT also provides the capability to perform real-time testing of full-scale electric aircraft powertrains.

hardware-in-the-loop testing with emulated turbofan engines to The focus of testing to date at NEAT is subscale testing of enable initial evaluation of turbomachinery and electrical the powertrain of the STARC-ABL concept aircraft, a single- component integration challenges as well as control approaches aisle turboelectric aircraft with an aft boundary layer propulsor to coordinate their operation. For the STARC-ABL (Ref. 5 ). A depiction of the STARC-ABL aircraft along with an configuration, this is accomplished by developing nonlinear overview diagram showing the STARC-ABL turbomachinery dynamic real-time models of the turbofan engines and the tail and electrical system interconnections implemented at the fan. These models are developed using available turbomachin- NEAT facility is provided in Figure 12 . STARC-ABL consists ery modeling tools (Refs. 15 and 18 ). Prior to implementation of two wing-mounted turbofan engines and a tail fan propulsor of these real-time models at NEAT, a simple power flow model driven by electric motors. Pilot power lever angle (PLA) of the STARC-ABL electrical system was developed and commands specify the requested thrust output from the interfaced with the turbomachinery models to create a full- turbofans and the tail fan. In addition to producing thrust, the system simulation. The purpose was to facilitate control studies two turbofan engines also supply mechanical offtake power of the propulsion system with representative electrical delivered to generators to produce electricity. Alternating component models (Ref. 33 ).

current (AC) from the generators travels through rectifiers to After initial development and validation, the turbomachinery transport the power over direct current (DC) buses. Motor models were implemented in real-time computer systems and controllers command inverters to deliver the commanded interfaced with the STARC-ABL powertrain at NEAT. Model current at the appropriate voltage and frequency, to generate the NASA/TM—2019-220296 10 to aircraft systems, developing the system architecture, applying requirements, and system implementation. As these development steps are conducted, several additional processes integral to ensuring system safety, requirements validation, and process assurance are happening concurrently in a coordinated, iterative fashion. This includes a system safety assessment that consists of a functional hazard assessment (FHA) conducted to identify all potential failure conditions of each function, and classify those failures according to the severity of their effects on the aircraft or its occupants. The more severe a function’s Takeoff Begin Idle Cruise and Climb Descent failure condition classification, the greater the development assurance level (DAL) required for the function.

Typical engine functions considered during the system development and safety assessment process may include thrust Figure 13.—NEAT example flight profile tail fan spool modulation, thrust reverser control, communication of engine rotational speed results with real-time turbomachinery health and status information to the aircraft, and containment of simulation and 500 kW scaled powertrain hardware.

engine failures to ensure passenger safety. A combination of protective strategies are applied to ensure that engine functions outputs were used to drive electric motors included in the have safety levels in accordance with their DAL requirements.

NEAT facility to emulate the turbofan produced torque These strategies may include defined maintenance and overhaul supplied to the electric generators. This enabled real-time schedules, containment systems to prevent uncontained testing of the STARC-ABL propulsion system using emulated failures, over-speed protection logic, and fail-safe design turbomachinery and actual electrical system component concepts leveraging system redundancy. The engine control hardware. A short 15 min example flight profile consisting of a system plays a significant role in assuring engine fail-safe takeoff and climb phase, a cruise phase at 10,000 ft with a operation. Typically, the EEC is a redundant dual-channel generator transient, and a descent phase was run. While much design with built-in-test and monitoring capability for potential shorter than typical aircraft flight profiles, the example profile faults in processors, sensors, or actuators. In the event of a did allow evaluation of the system response throughout various system fault, logic within the EEC is designed to automatically phases of flight. The commanded and actual tail fan motor detect and mitigate the anomaly. Mitigation actions may speed for this example flight profile is shown in Figure 13 . The include reverting to physically redundant controls hardware, results indicate that the tail fan motor speed tracks the commanding actuators to failsafe positions, or reverting to commanded value well.

reversionary control modes that allow the engine to function NEAT and the STARC-ABL evaluation described above is safely, although perhaps at a reduced performance level.

just one example of the required EAP test facilities and the type Today, aircraft engines and their control systems receive type of controls development testing that can occur in these certificate approval as a stand-alone system to signify their facilities. Test facilities are also needed to test EAP electrical airworthiness. However, the complex coupling and distributed components and full-scale EAP designs, including the control nature of EAP designs are expected to place added challenges and operation of these systems in flight test environments.

on the certification of these systems. FHA’s are needed to identify and assign DAL’s to all propulsion system functions. It is expected that redundancy within the EAP architecture will be

Certification Considerations Regarding

required to assure that the propulsion system can still deliver

EAP Control Design

propulsive thrust or torque in the event of a failure. As with conventional engine designs, the EAP control system is Established aerospace practices define guidelines for the expected to play a significant role in assuring that EAP systems development of civil aircraft and systems (Ref. 34 ) and for conducting safety assessments on these systems (Ref. 35 ). This comply with the airworthiness standards set forth by regulatory agencies. This includes fault detection and mitigation logic, includes guidelines for onboard electronic hardware and reversionary control modes, and contingency control modes to software, such as that included in control systems (Refs. 36 , 37 , respond to EAP system faults. The reconfiguration flexibility of and 38 ). Starting with an initial concept, the development EAP architectures may allow multiple acceptable control process of an aircraft/system readies the concept for mitigation responses for an individual fault, thus enabling implementation (Ref. 34 ). The aircraft development process optimal control reconfiguration based on current mission includes defining aircraft functions, allocating those functions NASA/TM—2019-220296 11 objectives. Additionally, as with conventional engine EEC 3. Felder, J.L., (2015), “NASA Electric Propulsion System designs, the application of Time-Limited-Dispatch (TLD) Studies,” presentation, 5th EnergyTech 2015, Cleveland, concepts for EAP control systems is anticipated. TLD is a OH, Nov. 30–Dec 2.

concept where a redundant system is allowed to operate for a 4. National Academies of Sciences, Engineering, and predetermined length of time with faults present in the elements Medicine. 2016. Commercial Aircraft Propulsion and of a redundant companion system, before repairs are required Energy Systems Research: Reducing Global Carbon (Ref. 39 ). This requires appropriate fault detection and fail-safe Emissions. Washington, DC: The National Academies mitigation logic to be included in the EAP control system. Press. https://doi.org/10.17226/23490 .

Other propulsion-related functions that must be considered 5. “NASA Aeronautics Strategic Implementation Plan 2017 during the development of aircraft equipped with EAP are the Update,” National Aeronautics and Space Administration, propulsion flight deck controls and displays. In conventional (2017), designs, individual throttle levers and cockpit gauges are https://www.nasa.gov/sites/default/files/atoms/files/ sip- available for each engine. For EAP designs with multiple 2017-03-23-17-high.pdf [retrieved 19 September 2018].

distributed propulsors, flight crew control and monitoring of 6. Borer, N.K., Derlaga, J.M., Deere, K.A., Carter, M.B., each individual propulsor may be untenable and increase the Viken, S.A., Patterson, M.D., Litherland, B.L., Stoll, A.M., likelihood of human error. Therefore, considerations must be (2017), “Comparison of Aero-Propulsive Performance given to the format in which thrust commands are delivered to Predictions for Distributed Propulsion Configurations,” th the EAP system and then distributed to multiple propulsors AIAA–2017–0209, AIAA SciTech Forum, 55 AIAA installed on the aircraft as well as how EAP health and status Aerospace Sciences Meeting, Grapevine, TX, Jan 9–13.

information is conveyed back to the flight crew. 7. Welstead, J.R., Felder, J.L., (2016), “Conceptual Design of a Single-Aisle Turboelectric Commercial Transport with Fuselage Boundary Layer Ingestion,” AIAA–2016–1027,

Summary

AIAA SciTech Forum, 54th AIAA Aerospace Sciences Electrified aircraft propulsion systems hold potential for Meeting, San Diego, CA, January 4–8.

reducing aircraft emissions, noise, and fuel burn. Several 8. Kim, H.D., Felder, J.L., Tong, M.T., Armstrong, M., technology barriers must be addressed to bring these designs to (2013), “Revolutionary Aeropropulsion Concept for fruition, including development of the controls technology Sustainable Aviation: Turboelectric Distributed required for EAP. Given their complexity, distributed nature, Propulsion,” ISABE-2013-1719, 21st International Society and the inherent coupling between turbomachinery and for Air Breathing Engines, Busan, Korea, September 9–13.

electrical systems, EAP system control designs are expected to 9. Johnson, W., Silva, C., Solis, E., (2018), “Concept provide new challenges. This paper discussed several control Vehicles for VTOL Air Taxi Operations,” AHS Technical technology needs to enable EAP. These include modeling tools Conference on Aeromechanics Design for Transformative for creating integrated turbine and electrical system models for Vertical Flight, San Francisco, CA, January 16–19.

control design and evaluation, control architectures and control 10. Silva, C., Johnson, W., Antcliff, K.R., Patterson, M.D., strategies for EAP systems, test facilities for the development (2018), “VTOL Urban Air Mobility Concept Vehicles for of EAP systems and controls, and fault detection and mitigation Technology Development,” AIAA–2018–3847, 2018 functions included within control logic to enable EAP Aviation Technology, Integration, and Operations Conf., certification. Development of the control technologies AIAA Aviation Forum, Atlanta, GA, June 25–29.

necessary for EAP systems will require a concerted effort by 11. Bruno, M., “Aerospace Sector Could See Overhaul From NASA and the aerospace community.

Electric Propulsion,” Aviation Week & Space Technology , p. 28, August 24, 2018.

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