Research Activities on Electric Aircraft and Hybrid
Electric Propulsion System
Keiichi OKAI
Advanced Aeropropulsion Laboratory (AAL)
The University of Tokyo /
Japan Aerospace Exploration Agency
Europe-Japan Symposium
Electrical Technologies for the Aviation of the Future
March 26-27, 2015, EU Delegation, Tokyo, Japan
Long-term Research
(in “Sky Frontier Program”)
Emission-Free Aircraft Concept & Technology Study
(1) FEATHER project
(2) Conceptual study of hybrid/electric propulsion
(Flight-demonstration of Electric Aircraft
Technology for Harmonized Ecological
Revolution)
Maiden Flight （Nov. 2014）
Aircraft Image
Small airplane for
FEATHER project
Outline
JAXA’s unique electric propulsion
system
Validation of the new functions and
system performance
Flying laboratory toward electric
aircraft research
2
Propulsion System Schematic
Ultra Low Emission and Highly Efficient Propulsion System
-Distributed and Electric Propulsion System
Technological issues to be pursued
Light-weight and robust (distributed) electric fan
Electric Generator system (FC-GT hybrid )
Fuel and Power feed distribution control
Electric fan
2
JAXA conducts these research activities with University of Tokyo and Nihon University.
Contents
1.FEATHER Project
2.Hybrid/electric Propulsion system
for the aviation
3. Summary
FEATHER project
JAXA’s Electric Motor Glider
Wind tunnel testing
Electric motor
Li-ion battery
http://www.aero.jaxa.jp/publication/event/pdf/event140918/poster07.pdf
1. Japan’s first manned-electric aircraft demonstration
program
/ Through the program JAXA acquired electric flight and MEA related
systems research baseline.
1. Developed JAXA’s own electric motor for the test
2. Two characteristic features in utilization of the
propulsion motor – four-fold motor / regenerative function
4
FEATHER project
JAXA’s Electric Motor Glider
Wind tunnel testing
Electric motor
Li-ion battery
http://www.aero.jaxa.jp/publication/event/pdf/event140918/poster07.pdf
Specifications of the electric motor glider
Original motor glider
Diamond aircraft type HK36TTC-ECO
Width
16.33 m
Max. takeoff weight
850 kgf
Power source
Li-Ion battery
Engine
60kW electric motor
Crew member
1 person
5
System configuration
warning & caution
fourfold
electric
motor
inverters
measurement
system
indicators
display
Li-ion battery
reduction
gear(1:3.16)
fourfold electric motor
(4 elements of motor
connected in series)
JAXA’s Electric Motor Glider
6
Electric propulsion system for FEATHER
Li-Ion battery





Characteristics of electric
motor
 Permanent magnet type synchronous motor
 Fourfold motor (4 elements of motor connected
in series )
 Regenerative function
Electric motor
performance
 Maximum output: 60 kW(4min.)
 Power density: 2.1kW/kg（w/o reduction gear）
 Efficiency: 94% or higher
Inverter
 IGBT
 Four individual inverters for four motor elements
Cooling
 Water cooling(Motor and inverters)
Capacity:75 Ah
Total mass: 120kg
Open circuit voltage: 128 V
Maximum current: 750A(10C, 75s)
Configuration: 32cells in series
7
Characteristic features of the
electric propulsion system(1/2)
Thrust
failure
✓✓✓✓
✓×✓✓
fourfold motor
Time
Altitude
Safety altitude
Time
Avoidance of complete thrust loss
15
http://www.aero.jaxa.jp/publication/event/pdf/event140918/poster07.pdf
8
Characteristic features of the
electric propulsion system(2/2)
Drive
air drag
空力抵抗
Altitude
conventional air brake
air flow
風力
charge
充電
RGN
Powere lever
adjustable rage
（FEATHER）
large
small
electric power regeneration
Distance
Concept of regenerative air brake
The “Regenerative air brake system” is composed of a propeller, electric motor, inverter, battery and power lever.
The system enables us to not only charge the electric energy during descent but also control the angle of descent
without conventional air brake.
http://www.aero.jaxa.jp/publication/event/pdf/event140918/poster07.pdf
9
Results of FEAHER project
We have succeeded in flight demonstrating as follows:
i. Avoidance of complete thrust loss in engine failure
during climb by using the fourfold electric motor
ii. Regeneration of about 5kW electricity during descent
by the motor and propeller
iii. Control of descent rate by the regeneration without
conventional airbrake
iv. Continuous “regenerative soaring” free from
descent in thermal condition
10
Contents
1.FEATHER Project
2.Hybrid/electric Propulsion system
for the aviation
3.Summary
2. Hybrid/electric propulsion technology (1/2)
Motivation (Recognizing the potential of Liquid Hydrogen)
JAXA has been conducting R&D activities on LH2 fueled turbojet propulsion system for
Hypersonic flight.
LH2 is (1) zero CO2 emission fuel, (2) Coolant and (3) Cryogenic superconducting medium.
Okai, K., Long Term Potential of Hydrogen as Aviation Fuel, ICAO Environmental report 2010, pp. 164-167, 2010.
2. Hybrid/electric propulsion technology(2/2)
Target selection
NASA subsonic transport system level metrics[2]
IATA Technology Roadmap [1]
TECHNOLOGY
BENEFITS
Noise
(cum margin rel. to Stage 4)
LTO NOx Emissions
(rel. to CAEP 6)
Cruise NOx Emissions
(rel. to 2005 best in class)
Cruise
Fuel/Energy
Consumption
(rel. to 2005 best in class)
TECHNOLOGY
GENERATIONS
N+3 (2025)
-71dB
N+4
Better than -71dB
-80%
Better than -80%
-80%
Better than -80%
-60%
Better than -60%
Flightpath 2050 goal[3]
Flightpath 2050
target
CO2 emission per
passenger kilometer
(rel. to Year 2000
level)
NOx emission
(rel. to Year 2000
level)
Aircraft noise level
(rel. to Year 2000
level)
-75%
-90%
-65%
N+3 values are referenced to a 737-800
with CFM56-7B engines.
[2] Bradley, M. K., and Droney, C. K.: Subsonic Ultra Green Aircraft Research Phase II: N+4 Advanced
Concept Development, NASA/CR-2012-217556, 2012.
[3] Flightpath 2050 Europe’s Vision for Aviation, European Commission, 2011.
[1] ATAG(Air transport Action Group): Reducing emissions from aviation
through carbon-neutral growth from 2020, 2013
Low Carbon Fuels
Turbo/Hybrid-electric
Propulsion
Unconventional Airframe
Distributed Propulsion
Today’s high bypass DDTF and beyond
C
(1   )m
By-pass ratio:
m F

m C
m F
m C
C
T
Turbine
Combustor
Compressor
Small core engine generates large power to propel fan.
=> High bypass-ratio turbofan engines Limitation： α ~ 10
Direct-Drive Turbofan (DDTF)
Gear-Drive Turbofan(GDTF)
Open Rotor
14
?
Direct-Drive Turbofan (DDTF)
Gear-Drive Turbofan(GDTF)
(GDTF)
Open Rotor
? of fan from core engine
Separation
Multiple (electric-)fans powered by (one or small numbers of) core(s)
=> High (effective) bypass-ratio fan engine
C
T
Turbine
Combustor
Compressor
Distributed propulsion
Power to propel the fan(s):
 Mechanical (Gear Drive)
 Compressed Air
 Electrical
15
Advantages and Challenges
 Potential for synergy effects in design integration
Technologies related to the FEATHER
Distributed (semi-buried) distributed fans
program
⇒Less Power required for propulsion (BLI* fans)
⇒Extended and new flight control measure
(Relatively) large cores for effective thermal efficiency
Efficient Total Energy Management
Noise shielding capability
Enhanced reliability and redundancy with distributed propulsion
 Challenges are for:
Large-scale light weight fan module
Ultra-efficient core generator
Energy efficient EMS
Distributed propulsor (fans)
Loss-less energy transmission
Highly efficient core
Multi-fuels applicability
*BLI=Boundary Layer Ingestion
2-1 High-power density electric motor for propulsion(1/2)
-Tip-drive motor concepts
High specific power motor is essential:
𝑘𝑊
𝑃𝑜𝑤𝑒𝑟𝐷𝑒𝑛𝑠𝑖𝑡𝑦[
] ∝ 𝐻𝑡 × 𝐵𝑛 × 𝑁
𝑘𝑔
Luongo C.A., Masson, P. J., Nam, T., Marvis, D., Brown, G., Kim, H. D., Waters, M. and Hall, D.,
Next Generation More-Electric Aircraft: A Potential Application for HTS Superconductors,
Applied Superconductivity Conference, 2008.
High rotation speed is not applicable
for large scale motor
 Superconducting motor
Adaptation of MgB2
Tcr=39[K]
Cryogenic fuel has potential for
superconducting medium and coolant.
(Maintaining superconductivity is crucial and important )
 Tip drive motor
1.Driving coils (point of action) on the outer shell
2.No need of iron core
（Large current variation; Relax physical limits）
-> Small (relative) resistance loss
3.Energy recovery via LC circuit
US-Patent#7423405 Electromagnetic Rotating Machine
by Okai, K. et al.
Other tip-drive motor concepts for aviation
Magnetically Levitated Ducted Fan Being
Developed as a Propulsor Option for
Electric Flight (NASA)
NASA-TM-2006-214481
NASA Tip-Drive motor
(to fit around propulsive fan)
NASA-TM-2005-213800
2-1 High-power density electric motor for propulsion(2/2)
Experimental approach to grasp engineering physics behind
【Features in the (original) motor configuration】
1.Driving coils (point of action) on the outer shell; Infinity-shaped rotating coil
2.No need of iron core（Large current variation; Relax physical limits）
-> Small (relative) resistance loss
3.Energy recovery via LC circuit
Exp #1
Exp #2
Exp #3
2-2 Potential of Fuel-cell Hybrid Gas turbine core (1/2)
Various applications in aviation field as power source
０
HALE application
１
Passenger aircraft
Courtesy Dr. Kenya Harada (JAXA)
2-2 Potential of Fuel-cell Hybrid Gas turbine core (2/2)
Configuration definition and data acquisition for systems analysis
 Different power output ratio FC and GT
Ground power plant PFC > PGT
Propulsion system generator PFC ≤PGT
 Potential multi fuels
eg. H2 for Fuel cell and pre/post burner and
Bio-Jet fuel for primary gas turbine combustor
 Challenges
- Stable operation in the fuel cell part
- Realizing high specific power module
SOFC/Reactor module characteristics
High Temp and High Pressure experimental setup
High pressure operation and dynamic response data
Summary on Hybrid/electric propulsion system studies
 Hybrid/Turbo-electric propulsion system concepts are
promising but key technology are at this moment fairly
immature. Our focus is on motor and reactor parts in core
engine.
 SOFC/GT generator is applicable wide range of output power
in aeronautics field.
Contents
1.FEATHER Project
2.Hybrid/electric Propulsion system
for the aviation
3.Summary
3.Summary
1.
2.
3.
The presentation introduced the research
activities on the emission-free aircraft concepts in
the long-term perspective (≈20yrs).
Propulsion system will be highly integrated in the
airframe in design.
Hybrid/electric propulsion is promising regarding
the ‘Emission-free aircraft’ goal. Introduction of
innovative key technology and well-balanced
hybridization in total design are important.