Turboelectric Aircraft Drive Key Performance Parameters and Functional Requirements

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Turboelectric Aircraft Drive Key

Performance Parameters and Functional

Requirements

Ralph H. Jansen, Dr. Gerald V. Brown and James L. Felder NASA Glenn Research Center, Cleveland, Ohio, 44135 and Dr. Kirsten P. Duffy University of Toledo, Toledo, Ohio, 43606 Advanced Air Vehicles Program Advanced Transport Technologies Project

Introduction

• There is substantial interest in the investigation of improvements to

aircraft by the introduction of electrical components into the propulsion

system.

• In the case of a turboelectric aircraft the electrical systems can provide

unmatched flexibility in coupling the power generation turbine(s) to the

fan propulsors.

• This flexibility can result in reduced noise, emissions, and fuel burn.

• However , the greatly expanded electrical system introduces weight and

efficiency burdens that oppose these benefits.

• A break - even analysis is presented here to determine the electrical

power system performance level necessary to achieve a net benefit at

the aircraft level.

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Approach

• In order to conduct the break - even analysis we will define the key

performance parameters, the key functional requirements, and the

electrical power system boundary.

• Then we will formulate range equations for a base aircraft and a

turboelectric version of that aircraft.

• Next we will find the range of possible benefits from a literature survey

and calculate the weight and fuel burn costs.

• Finally , we find the break - even point by setting the ranges of the two

aircraft types equal and using the same initial weight, operating empty

weight, and payload weight and implicitly solving for the electric drive

specific power and efficiency.

• The resulting parametric curves combined with the functional

requirements will be used as input requirements for the electrical power

system.

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Drive System Selected for Evaluation

• A wide electric drive configuration trade space exists. Selected differentiating factors are the power source, the distribution approach, the number of motor - driven propulsors, and the fraction of the total propulsive power that is provided electrically.

• This analysis will evaluate the performance parameters of a turboelectric

system where the system energy is stored as jet fuel. Therefore, the electrical drive considered here will be based on a turbine driving one or more electrical generators, motor driven propulsors, a power distribution system extending from the turbine to the propulsors, and a thermal management system. The power distribution includes power electronics, electrical cables, and protection devices.

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Electric Drive System Boundary

• The electric drive system boundary will include the electrical machines,

the power management and distribution system, and the thermal

• By this definition a representative turboelectric system would include

generator(s), rectifier(s), distribution wiring, inverter(s), motor(s), and

the thermal control for those components

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Key Performance Parameters

• Specific power ( Sp ) and efficiency (  ) are proposed as

ED ED

the two KPPs of the electric drive system in a turboelectric

aircraft.

• Specific power is the ratio of the rated power to the mass of

the power system .

• Efficiency is the ratio of the output power to the input power

of the power system .

• These quantities will be used to describe electrical power

system performance and establish levels of performance

necessary for successful aircraft.

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Key Functional Requirements

• Distinct from the KPPs are the functional requirements of the electric

drive system. Two of the crucial functional requirements for a

turboelectric aircraft power system are independent speed and power

control as well as redundancy and reliability levels .

• Independent speed and power control of individual fan propulsors is

required in most proposed electric aircraft drive configurations and may

enable configurations allowing

– fan and turbine speed decoupling allowing optimal operation throughout the flight regime – yaw control through differential thrust – the ability to provide high - velocity wing blowing with controlled thrust – noise reduction strategies.

• Redundancy and reliability requirements are not yet well defined for an

electric aircraft drive system; however, it is clear that the system must at

least meet the safety standards that current aircraft propulsion systems

meet.

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Modified Breguet Range Equation

• The basis of the analysis is an expansion of the traditional terms in the

Breguet range equation to include the efficiency and weight of the

turboelectric drive.

• As such, it applies for situations where overall efficiency  , lift - to -

overall drag ratio L / D , and flight velocity are constant over the flight.

• Given these constraints, the range R can be found if the intial ( W )

AC initial

and final weight ( W ) of the aircraft is known along with the fuel energy

final per unit mass h and the gravitational constant g .

• Although not true for the entire flight envelope, this description is a

reasonable approximation for cruise conditions

  W L h     initial W L h   initialEAC    R ln     R ln    overall AC   overallEAC EAC     W D g W D g   final   finalEAC     EAC Advanced Air Vehicles Program Advanced Transport Technologies Project

Modified Breguet Range Equation

• Breguet Range Equation   W L h   initial    R ln    overall AC   W D g   final   • First, we expand the terms in the overall efficiency to include an electrical efficiency (  ) in addition to the elec      propEAC elec thermEAC overallEAC thermal and propulsive efficiency • Next, we recognize the additional weight of the W W W W W     elec fuelEAC pay OEW initialEAC electrical drive impacts both the initial and final weight of the turboelectric aircraft and expand each to

W W W W   

elec pay OEW finalEAC explicitly account for the operating empty weight ( W ), payload weight ( W ), and fuel weight OEW pay ( W ).

fuelEAC   W L h   initialEAC • The turboelectric range equation is now stated,    R ln    overallEAC EAC   recognizing that the turboelectric system will have W D g   finalEAC EAC   different L / D , thermal efficiency, propulsive efficiency, initial weight, and final weight compared to the base aircraft.

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Fuel Burn Benefit Ranges from Literature

• Higher propulsive efficiency due to increased bypass ratio (BPR), higher propulsive efficiency due to boundary layer ingestion, and lift to drag ratio improvements have been frequently cited as enabled by turboelectric propulsion.

• Introduction of an electric drive system between the turbine and fan, allowing decoupling of their speeds and inlet - to - outlet area ratios. With this approach, high BPR can be achieved since any number and size of fans can be driven from a single turbine. Increasing BPR results in improved propulsive efficiency • Boundary layer ingestion (BLI) increases propulsive efficiency by ingesting lower velocity flow near the airframe into the propulsors, reenergizing the wake and thereby reducing drag. BLI can be implemented on both conventional tube and wing and HWB aircraft • Distributed propulsion is expected to improve both lift and L / D through wing flow circulation control. Improvements in L / D may result in smaller wing area and reduced drag and weight Propulsive L/D, BPR , BLI , percent percent percent 4 3 0 Minimum 6 5.5 4 Median Advanced Air Vehicles Program Advanced Transport Technologies Project Maximum 8 8 8

Weight Impacts as a function of KPPs

Electric Drive Efficiency

Electric Drive Specific

Impact

Power Impact

• The weight penalty of the additional

• Weight of electric drive is found by

fuel resulting from the electrical drive – Thrust at initial cruise is found by force balance losses is estimated by introducing the – Beginning of cruise power is product of additional electrical inefficiency term cruise thrust and cruise velocity into the overall efficiency, then holding – Takeoff power is estimated by ratios of all parameters on the base aircraft cruise and takoff parameters fixed.

v T     takeoff takeoff       • Using these assumptions we can find v T cruise cruise     the change in fuel weight from the

• Weight impact is a function of

difference of original fuel weight – initial aircraft weight divided by the electrical efficiency, less – Cruise velocity the original fuel weight – the electric drive specific power – propulsive efficiency – electrical efficiency .

      1 W W  fuel fuel   Sp v W  ED cruise initialEAC  elec   W  elec L

   

propEAC elec D EAC Advanced Air Vehicles Program Advanced Transport Technologies Project

Weight Impacts of Electric Drive System

95% eff weight breakout 90, 95, 99% weights

  Sp v W W    ED cruise fuel elec      1  AC   W  L   elec initial

   

propEAC elec Advanced Air Vehicles Program EAC D Advanced Transport Technologies Project

Fuel Burn Impact of Electric Drive

• The increased fuel burn is

estimated as the sum of the drive

efficiency cost and the normalized

weight change.

• The basic assumption is that a 1%

weight gain on the aircraft results

in a 1% increase in required fuel

as justified by the aircraft force

balance. A 1% increase in weight

results in a 1% increase in thrust

required. Holding the aircraft

parameters fixed also results in a

1% increase in fuel burn.

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Breakeven Analysis

• The break - even analysis determines the electric drive specific power and efficiency where the costs of adding the drive exactly equal the benefits .

• Base aircraft and turboelectric aircraft performance parameters are constant in this analysis.

• The breakeven equation is found by – First, the range expressions of the base aircraft and the turboelectric aircraft are equated – Then the common terms are canceled and the efficiency terms expanded – Next, the terms are arranged so the benefits are on left and costs are on the right with expanded weight terms – Finally, the electrical drive weight as function of specific power, and the aircraft parameters are included • Breakeven lines are found be implicitly solving equation balancing the costs and benefits across a range of specific powers at a expected benefit level 𝑊 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 ln 𝑊 𝑣 α 1 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑐𝑟𝑢𝑖𝑠𝑒 𝑊 + 𝑊 + 𝑂𝐸𝑊 𝑝𝑎𝑦 𝐿 𝜂 𝑆𝑝 𝑒𝑙𝑒𝑐 𝐸𝐷 𝐿 𝜂 𝑝𝑟𝑜𝑝𝐸𝐴𝐶 𝜂 𝐷 𝜂 𝐸𝐴𝐶 𝑡ℎ𝑒𝑟𝑚 𝑝𝑟𝑜𝑝 𝐷 = 𝜂 𝑒𝑙𝑒𝑐 𝐿 𝑊 𝜂 𝜂 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑡ℎ𝑒𝑟𝑚𝐸𝐴𝐶 𝑝𝑟𝑜𝑝𝐸𝐴𝐶 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 ln 𝐷 𝑊 + 𝑊 𝐸𝐴𝐶 𝑂𝐸𝑊 𝑝𝑎𝑦 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 𝑝𝑟𝑜𝑝𝑢𝑙𝑠𝑖𝑣𝑒 𝑏𝑒𝑛𝑒𝑓𝑖𝑡𝑠 𝑎𝑒𝑟𝑜 𝑐𝑜𝑠𝑡 𝑤𝑒𝑖𝑔ℎ𝑡 𝑐𝑜𝑠𝑡 𝑓𝑟𝑜𝑚 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑝𝑜𝑤𝑒𝑟 𝑎𝑛𝑑 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 𝑏𝑒𝑛𝑒𝑓𝑖𝑡𝑠 Advanced Air Vehicles Program Advanced Transport Technologies Project

Breakeven Results

• Along the break - even line, the fuel weight reduction is equal to the additional electric drive weight. The fuel burn along this line is less than that of the base aircraft.

• If the system has KPP parameters in the region above the curve, the overall system will close with a reduction in the combined fuel and drive weight, which can be taken as payload or some alternate benefit.

• The figure shows the specific power and efficiency relationship using the median - level benefit estimates, cruise velocity of 0.8M and 0.27 base aircraft fuel fraction • With these assumptions, the minimum required drive specific power must be approximately 9 kW/kg if the system is 100% efficient and the minimum required efficiency is 92% at a specific power of 20 kW/kg.

Propulsive  , Benefit prop 𝐿 L / D L/D, BPR , BLI , Product  propEAC 𝜂 𝜂 𝑡ℎ𝑒𝑟𝑚 𝑝𝑟𝑜𝑝 𝐷 percent percent percent Base Aircraft 17 0.6 𝐿 𝜂 𝜂 𝑡ℎ𝑒𝑟𝑚𝐸𝐴𝐶 𝑝𝑟𝑜𝑝𝐸𝐴𝐶 4 3 0 Minimum Min. Turboelectric 17 0.64 0.93 𝐷 𝐸𝐴𝐶 𝑝𝑟𝑜𝑝𝑢𝑙𝑠𝑖𝑣𝑒 𝑏𝑒𝑛𝑒𝑓𝑖𝑡𝑠 6 5.5 4 Median 17.7 0.67 0.86 Med. Turboelectric 𝑎𝑒𝑟𝑜 Maximum 8 8 8 𝑏𝑒𝑛𝑒𝑓𝑖𝑡𝑠 Max. Turboelectric 18.4 0.70 0.79 Advanced Air Vehicles Program Advanced Transport Technologies Project

Breakeven Results

• The figure is a plot of the break - even curves for the three levels of benefit assumptions.

• Not surprisingly, if the benefits are large, the KPPs of the power system do not need to be as aggressive.

• If the benefits are small, the KPP requirements become substantially more difficult.

• The minimum required specific power is reduced 6kW/kg and the minimum efficiency to around 85% at 20kW/kg when using the most favorable benefit assumptions.

Conclusions

• Specific power and efficiency are proposed as two key performance parameters for the electric drive system of a turboelectric aircraft.

• The costs were associated with the proposed KPPs. Analysis of the costs leads to the conclusion that below a specific power of approximately 5 kW/kg, the specific power is the dominant cost, whereas above that level the efficiency becomes dominant. Additionally it is noted that the fuel burn cost can never be less than the inefficiency of the electric drive system .

• A breakeven equation was developed by using range equations for a base air craft and a turboelectric aircraft. It was developed in a form which separated the costs and benefits of the system.

• KPP break - even weight curves were found for the minimum, median, and maximum turboelectric benefit cases and the region of power system performance that will result in a net weight benefit is shown.

• Further work will need to be done to define the net fuel burn benefit region and consider hybrid or all electric configurations.

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