Turboelectric Distributed Propulsion Engine Cycle Analysis for Hybrid-Wing-Body Aircraft

TLDR: In this article, a propulsion system which transmits power from the turbine to the fan electrically rather than mechanically was presented, and the performance of the fan inlet was evaluated.

Abstract: Meeting NASA's N+3 goals requires a fundamental shift in approach to aircraft and engine design. Material and design improvements allow higher pressure and higher temperature core engines which improve the thermal efficiency. Propulsive efficiency, the other half of the overall efficiency equation, however, is largely determined by the fan pressure ratio (FPR). Lower FPR increases propulsive efficiency, but also dramatically reduces fan shaft speed through the combination of larger diameter fans and reduced fan tip speed limits. The result is that below an FPR of 1.5 the maximum fan shaft speed makes direct drive turbines problematic. However, it is the low pressure ratio fans that allow the improvement in propulsive efficiency which, along with improvements in thermal efficiency in the core, contributes strongly to meeting the N+3 goals for fuel burn reduction. The lower fan exhaust velocities resulting from lower FPRs are also key to meeting the aircraft noise goals. Adding a gear box to the standard turbofan engine allows acceptable turbine speeds to be maintained. However, development of a 50,000+ hp gearbox required by fans in a large twin engine transport aircraft presents an extreme technical challenge, therefore another approach is needed. This paper presents a propulsion system which transmits power from the turbine to the fan electrically rather than mechanically. Recent and anticipated advances in high temperature superconducting generators, motors, and power lines offer the possibility that such devices can be used to transmit turbine power in aircraft without an excessive weight penalty. Moving to such a power transmission system does more than provide better matching between fan and turbine shaft speeds. The relative ease with which electrical power can be distributed throughout the aircraft opens up numerous other possibilities for new aircraft and propulsion configurations and modes of operation. This paper discusses a number of these new possibilities. The Boeing N2 hybrid-wing-body (HWB) is used as a baseline aircraft for this study. The two pylon mounted conventional turbofans are replaced by two wing-tip mounted turboshaft engines, each driving a superconducting generator. Both generators feed a common electrical bus which distributes power to an array of superconducting motor-driven fans in a continuous nacelle centered along the trailing edge of the upper surface of the wing-body. A key finding was that traditional inlet performance methodology has to be modified when most of the air entering the inlet is boundary layer air. A very thorough and detailed propulsion/airframe integration (PAI) analysis is required at the very beginning of the design process since embedded engine inlet performance must be based on conditions at the inlet lip rather than freestream conditions. Examination of a range of fan pressure ratios yielded a minimum Thrust-specific-fuel-consumption (TSFC) at the aerodynamic design point of the vehicle (31,000 ft /Mach 0.8) between 1.3 and 1.35 FPR. We deduced that this was due to the higher pressure losses prior to the fan inlet as well as higher losses in the 2-D inlets and nozzles. This FPR is likely to be higher than the FPR that yields a minimum TSFC in a pylon mounted engine. 1

Figures

Figure 5. N3-X Concept

Figure 7. N3-X ADP TFSC Vs. FPR

Table 6. Estimated Electrical System Weights and Efficiencies

Figure 6. Estimated size and shape of the Turbogenerator

Figure 8. Assumed incoming boundary layer (B.L.) flow profile at inlet lip location with boundary layer height of δ ~ 18 inches and displacement thickness of δ* ~ 2.6 inches

Table 5. Comparison of N2B and N3-X Embedded Propulsion

Full text

American Institute of Aeronautics and Astronautics
092407
1
Turboelectric Distributed Propulsion Engine Cycle Analysis
for Hybrid-Wing-Body Aircraft
James L. Felder
1
, Hyun Dae Kim
2
and Gerald V. Brown
3
NASA Glenn Research Center, Cleveland, Ohio, USA
Meeting NASA's N+3 goals requires a fundamental shift in approach to aircraft and
engine design. Material and design improvements allow higher pressure and higher
temperature core engines which improve the thermal efficiency. Propulsive efficiency, the
other half of the overall efficiency equation, however, is largely determined by the fan
pressure ratio (FPR). Lower FPR increases propulsive efficiency, but also dramatically
reduces fan shaft speed through the combination of larger diameter fans and reduced fan tip
speed limits. The result is that below an FPR of 1.5 the maximum fan shaft speed makes
direct drive turbines problematic. However, it is the low pressure ratio fans that allow the
improvement in propulsive efficiency which, along with improvements in thermal efficiency
in the core, contributes strongly to meeting the N+3 goals for fuel burn reduction. The lower
fan exhaust velocities resulting from lower FPRs are also key to meeting the aircraft noise
goals. Adding a gear box to the standard turbofan engine allows acceptable turbine speeds to
be maintained. However, development of a 50,000+ hp gearbox required by fans in a large
twin engine transport aircraft presents an extreme technical challenge, therefore another
approach is needed. This paper presents a propulsion system which transmits power from
the turbine to the fan electrically rather than mechanically. Recent and anticipated advances
in high temperature superconducting generators, motors, and power lines offer the
possibility that such devices can be used to transmit turbine power in aircraft without an
excessive weight penalty. Moving to such a power transmission system does more than
provide better matching between fan and turbine shaft speeds. The relative ease with which
electrical power can be distributed throughout the aircraft opens up numerous other
possibilities for new aircraft and propulsion configurations and modes of operation. This
paper discusses a number of these new possibilities. The Boeing N2 hybrid-wing-body
(HWB) is used as a baseline aircraft for this study. The two pylon mounted conventional
turbofans are replaced by two wing-tip mounted turboshaft engines, each driving a
superconducting generator. Both generators feed a common electrical bus which distributes
power to an array of superconducting motor-driven fans in a continuous nacelle centered
along the trailing edge of the upper surface of the wing-body. A key finding was that
traditional inlet performance methodology has to be modified when most of the air entering
the inlet is boundary layer air. A very thorough and detailed propulsion/airframe
integration (PAI) analysis is required at the very beginning of the design process since
embedded engine inlet performance must be based on conditions at the inlet lip rather than
freestream conditions. Examination of a range of fan pressure ratios yielded a minimum
Thrust-specific-fuel-consumption (TSFC) at the aerodynamic design point of the vehicle
(31,000 ft /Mach 0.8) between 1.3 and 1.35 FPR. We deduced that this was due to the higher
pressure losses prior to the fan inlet as well as higher losses in the 2-D inlets and nozzles.
This FPR is likely to be higher than the FPR that yields a minimum TSFC in a pylon
mounted engine.
1
Aerospace Engineer, DSS, 21000 Brookpark Rd, Cleveland, OH 44145
2
Aerospace Engineer, RTM, 21000 Brookpark Rd, Cleveland, OH 44145
3
Aerospace Engineer, RXS, 21000 Brookpark Rd, Cleveland, OH 44145.

American Institute of Aeronautics and Astronautics
092407
2
Nomenclature
AC = alternating current
ADP = aerodynamic design point
BLI = boundary layer ingestion
BWB = blended-wing-body
CAEP = Committee on Aviation Environmental Protection
CESTOL = cruise efficient short take-off and landing
eBPR = effective bypass ratio (ratio of mass flow rate through all fans to rate through engine core)
EIS = entry into service
FAR = federal aviation regulations
FPR = Fan pressure ratio
HPC = high pressure compressor
HPT = high pressure turbine
HTS = high temperature superconducting
HWB = hybrid-wing-body
hp = horse power (1 hp ~ 0.7456 kilowatt)
IBF = internally blown flap
IOC = initial operating capability
ISA = international standard atmosphere
LCH4 = liquid (cryogenic) methane
LH
2
= liquid hydrogen
LPC = low pressure compressor
LPT = low pressure turbine
LTO = landing and take-off
MN = Mach number
MW = mega-watts
NOx = nitrogen oxides
OPR = overall pressure ratio (P3/P2)
PAI = propulsion airframe integration
PT = power turbine
SL = sea level
RTO = Rolling Take-off (SL/MN0.25/ISA+27)
SFW = subsonic fixed wing
STOL = short take-off and landing
TSFC = thrust specific fuel consumption
TOGW = take-off gross weight
η
poly
= polytropic efficiency

American Institute of Aeronautics and Astronautics
092407
3
I. Introduction
n response to growing aviation demands and
concerns about the environment, NASA’s
Subsonic Fixed Wing (SFW) project identified four
“corners” of the technical trade space – noise,
emissions, aircraft fuel burn, and field length - for
aircraft design. Table 1 lists these technology goals
for three future time frames, where N+1, N+2, and
N+3 represent the years 2015, 2020, and 2030,
respectively. Although it may not be feasible to meet
all the goals for each time frame, the multi-objective
studies will attempt to identify possible vehicle
concepts that have the best potential to meet the
combined goals.
One of the vehicle and propulsion concepts that
NASA is exploring for N+2 is a synergistic
combination of a hybrid-wing-body airframe and a
distributed propulsion system. A number of fixed
wing aircraft using ‘distributed propulsion’ have
been proposed and flown before, although what
constitutes distributed propulsion is not clearly
defined. Examples include the 1940’s YB-49 flying
wing aircraft with 4 completely embedded engines in
each side of the wing and the 1960’s Hunting H.126
jet flap research aircraft which diverted almost 60%
of its thrust across its wing trailing edge to achieve
very high lift capability.
In 2006 NASA funded a one-year study that evaluated the synergistic benefits of distributed propulsion and
airframe integration with respect to cruise efficiency and quiet operation of aircraft from regional airports [1,2]. The
configuration for that study utilized 12 small conventional high-bypass-ratio turbofan engines, each with about
7,000 lbs (~ 31,000 N) of thrust at sea level, powering a hybrid-wing-body (HWB) vehicle (Figure 1). The HWB is
the main object of study to meet NASA’s N+2 goals.
For the Silent Aircraft Initiative the
Cambridge-MIT Institute (CMI) developed
the SAX-40 conceptual design [3,4]. Boeing
and NASA developed this concept further
with the N2 aircraft [6]. Figure 2 shows
these aircraft. The N2 is a conventional take-
off hybrid-wing-body aircraft with a 461,500
lbm MTOGW, 103,000 lbm payload and a
6,000 nm range. Two configurations were
examined. The N2A uses two pylon
mounted turbofan engines mounted on the
upper rear fuselage section. The N2B
replaces the two turbofan engines with three
propulsion modules. Each propulsion
module as shown in Figure 3 consists of
three side-by-side fans driven by a common turbine. The center fan is in line with the core and is directly driven by
the fan turbine. The other two fans are on each side and are driven by right-angle gearboxes powered by the fan
turbine.
I
Corners of the
trade space
N
a
+1 (2015 EIS)
Generation
Conventional Tube
and Wing (relative to
B737/CFM56)
N
a
+2 (2020
IOC)
Generation
Conventional
Hybrid-wing-
body
(relative to
B777/GE90)
N
a
+3 (2030–
2035 EIS)
Advanced
Aircraft
Concepts
Noise
(cumulative
below
Stage 4)
–32 dB –42 dB 55 LDN at
average
airport
boundary
LTO NOx
Emissions
(below CAEP/6)
–60% –75% Better than
–75%
Performance:
Aircraft Fuel
Burn
–33%
b
–40%
b
Better than
–70%
Performance:
Field Length
–33% –50% Exploit
metroplex
c
concepts
a
“N” represents current state-of-art aircraft as stated in parenthesis.
b
An additional reduction of 10% may be possible through improved
operational capability.
c
Concepts that enable optimal use of the airports (with shorter runway) within
the metropolitan areas.
Table 1 NASA's Technology Goals for Future Subsonic
Fixed Wing (SFW) Vehicles
Outboard No.1 engine installationOutboard No.1 engine installation
Figure 1 Cruise efficient short take-off and landing
(CESTOL) vehicle configuration using 12 small conventional
high bypass ratio turbofan engines

American Institute of Aeronautics and Astronautics
092407
4
To improve vehicle performance enough to meet NASA’s N+3 goals, a drastic change in propulsion system is
required. In a paper presented at the International Powered Lift Conference [5] written by the authors, the 12
conventional turbofan engines of the cruise efficient short take-off and landing (CESTOL) aircraft shown in Figure
1 were replaced with 16 fans driven by superconducting electric motors. The fans were housed in a continuous
nacelle across the upper trailing edge of the HWB aircraft. Upper surface blowing (USB) by cool fan air of the outer
fans provide powered lift that dramatically enhances STOL performance, while the inner fans substantially improve
the effectiveness of the pitch effector. The power needed for these electric fans comes from two wing-tip mounted
gas-turbine-driven superconducting generators, with the power distributed through superconducting electrical lines.
This arrangement allows many small partially embedded fans operating in the boundary layer, while retaining the
superior efficiency of large core engines operating in undisturbed air. The resulting conceptual configuration is
shown in Figure 4.
Figure 2. The CMI SAX-40 and Boeing/NASA N2A and N2B Hybrid-wing-body Aircraft Concepts
Figure 3. Boeing/NASA N2B Three-fan/Single Core Engine

American Institute of Aeronautics and Astronautics
092407
5
I. N3-X Propulsion Cycle Design
A. Propulsion System Configuration
For this study of embedded superconducting turboelectric propulsion we used the N2A airframe as our baseline.
Two key N2A flight conditions were used as benchmarks in this study. They are the aerodynamic design point
(ADP) at 31,000 feet, MN 0.8, ISA where the vehicle thrust requirement is 30,000 lbf, and a rolling take-off (RTO)
condition at sea level, MN 0.25, and ISA+27
o
R where the thrust requirement is 108,000 lbf[6].
We modified this airframe by removing the pylon mounted turbofan engines and the vertical tails. In their place
is a nacelle on the upper surface at the trailing edge which contains the motor-driven fan modules. The fans are
driven by electrical power generated by the wing tip mounted turbogenerators. Figure 5 shows our conceptual design
We have dubbed this combination of airframe and superconducting electrical propulsion the N3-X
The propulsion system design point was set at the RTO condition, and the propulsion system was sized to
provide a total vehicle thrust of 108,000 lbf with two turbogenerators and a number of fans that depends on the ADP
FPR value being evaluated. Even though all power goes through a common power bus, symmetry was assumed and
the propulsion system simulation model consisted of one turbogenerator and half the fans. Further, the aggregate
performance of the fans was represented by a single fan module in the simulation. The simulation then produces
values for aggregate performance of all fan modules as well as the total inlet, fan and nozzle areas. Post convergence
calculations determined the flow areas for each individual fan module and the diameter of each fan. Future analysis
will split the fan into individual fan elements in the simulation, each with its own inlet and nozzle. This will allow
the span-wise difference in inlet conditions due to different fuselage length upstream of each inlet to be incorporated
into the propulsion system simulation.
The fan module design values at the RTO design condition were varied to yield the desired fan pressure ratio,
fan face MN, and maximum fan tip speed at the ADP.
The ADP thrust required by the N2A aircraft was specified as 30,000 lbf [6]. We reduced this value by 7% to
account for reduced system dissipations as a result of boundary layer ingestion. The 7% value was selected as a
reasonable value based on previous studies and papers. Each parametric engine design for the examined FPRs was
checked to make sure that it would meet the RTO thrust requirement at a maximum combustor exit temperature, T4,
Rear ViewRear View
Figure 4. A notional distributed turbo-electric propulsion CESTOL vehicle concept using 16
distributed electric fans driven by superconducting motors with power provided by two wing-tip
mounted turbo-electric generators

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Turboelectric distributed propulsion engine cycle analysis for hybrid-wing-body aircraft" ?

This paper presents a propulsion system which transmits power from the turbine to the fan electrically rather than mechanically. This paper discusses a number of these new possibilities. The Boeing N2 hybrid-wing-body ( HWB ) is used as a baseline aircraft for this study. 

Q2. What is the pitch effector for a turbine?

Pitch effector can be made of low temperature tolerance material: Because only cold fan air impinges on it, the pitch effector does not have to be made of high temperature tolerant materials.• 

Q3. What is the effect of a fan nacelle on the wing?

Since the top of the fan nacelle is likely to have laminar flow over at least a portion of it, while the fuselage that is being covered is where the turbulent boundary layer is thickest, a increasing fan nacelle length might actually decrease drag. 

Q4. What is the effect of a low engine jet noise?

Minimal engine core jet noise: Designing to a low turbogenerator nozzle pressure ratio extracts the maximum amount of energy from the gas stream which has the effect of reducing the turbogenerator exhaust gastemperature and velocity and hence reduces noise. 

Q5. What are the possible non-linear aircraft control laws?

Possible non-linear aircraft control laws: Being embedded in the boundary layer may cause interactions between the external aerodynamics and the propulsion system.• 

Q6. What is the way to position the turbogenerators and fan modules?

The use of electrical power transmission allows a high degree of flexibility in positioning the turbogenerators and fan modules to best advantage. 

Q7. What is the effect of a variable ratio gearbox?

The speed of the power turbine shaft in the turbine engine is independent of the fan shaft speed - the electrical system functions as a gearbox with an arbitrary gear ratio. 

Q8. What is the role of power inverters in a turbine?

Power inverters are required to allow the speed of the fans to be reduced to match the available power while the power and speed of the remaining turbogenerator are brought to a maximum.• 

Q9. Why do the authors think superconducting motors and generators are needed in large transport aircraft?

The authors feel that for large transport aircraft it will be necessary to use superconducting motors and generators rather than conventional motors and generators in the aircraft propulsion system to reduce the weight fraction of the propulsion system. 

Q10. What is the advantage of a oversized turbogenerator?

The turbogenerators could be oversized with regard to the power needs of propulsion to provide significant amounts of electrical power for non-propulsion uses while in flight. 

Q11. What is the optimum FPR for the inlet ram?

Rerunning the propulsion system performance study with a lower 0.943 freestream to fan face pressure ratio may result in generally higher TSFC values and a higher optimum FPR. 

Q12. What is the effector's effect on the engine?

Greater pitch effector effectiveness: Blowing all the fan air over the pitch effector increases its effectiveness, allowing the pitch effector to be smaller and/or not located as far aft.• 

Q13. How much power loss is required for the entire superconducting portion of the electrical system?

For the purposes of calculating the cooling required, the authors have assumed a power loss of 0.03% total for the entire superconducting portion of the electrical system. 

Q14. What are the advantages of using a distributed turboelectric propulsion system with superconducting?

using a distributed turboelectric propulsion system with superconducting devices may present adverse effects in overall vehicle performance and operation. 

Q15. How did the inlet flow rate and ram drag at the inlet lip be calculated?

To match the inlet mass flow rate and inlet ram drag entering the fan face at the vehicle ADP condition and to account for total pressure loss due to boundary layer build-up along airframe surface at the inlet lip location, a 1st order approximationof the boundary layer profile was obtained using simple flat plate equivalent of upper airframe surface by matching assumed total pressure loss of N2A configuration and mass flow rate entering the current N3-X inlet. 

Q16. What are the advantages of a distributed turboelectric propulsion system?

The generators, motors, inverters, and the balance of the superconducting system may weigh more than a mechanical gearbox that accomplishes some of the same tasks as the turboelectric system.• 

Q17. What is the way to reduce turbogenerator noise?

If turbogenerator noise remains too high for an exposed mounting location, the turbogenerator can readily be moved elsewhere on the aircraft without disturbing the fan nacelle location or operation.• 

Q18. What is the TSFC curve for embedded engines?

the higher losses for the embedded installation compared to a pylon mount with a pitot inlet means that the bottom of the TSFC curve for embedded engines is likely to be at a higher FPR than for a podded, pylon mounted engine. 

Q19. How much methane flow is required for cooling?

Given this loss level, the methane flow rate required for cooling is approximately 17% of that required to fully power the aircraft with methane. 

Q20. What is the way to reduce the AC losses in HTS materials?

The several types of AC losses that occur in HTS materials can be reduced by reducing the size of HTS filaments in the composite conductor and twisting them.