24/04/2018 - CENTRELINE – Propulsive Fuselage Conceptfuselage boundary layer in order to directly compensate the viscous drag effects in the fuselage wake flow field. The content

Ref. Ares(2018)2176664 - 24/04/2018

CENTRELINE D1.02 Specification of propulsive fuselage aircraft layout and design features Deliverable submission date: 24/04/2018

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Grant Agreement: 723242

Call identifier: H2020-MG-2016-Two-Stages

Project full title: CENTRELINE – ConcEpt validatioN sTudy foR fusElage wake-filLIng

propulsioN intEgration

Deliverable lead beneficiary: AGI

D1.02 SPECIFICATION OF PROPULSIVE FUSELAGE

AIRCRAFT LAYOUT AND DESIGN FEATURES

Authors: Frank Meller, Franz Kocvara

Internal Technical Auditor Name (Beneficiary short name) Date of approval

Task leader Franz Kocvara (AGI) 22.01.2018

WP leader Frank Meller (AGI) 23.03.2018

Coordinator Arne Seitz (BHL) 12.04.2018

Project Office Sophie Rau (ART) 23.04.2018

Abstract: This deliverable presents the activities performed in Task 1.2 “Concept design space exploration”.

State-of-the-art in fuselage BLI and wake filling has been reviewed and the most promising PFC aircraft

configuration down-selected and described as initial reference for CENTRELINE partners’ detailed studies.

Due date: 31.01.2018

Actual submission date: 24.04.2018

Publication date: 24.04.2018

Project start date: 01.06.2017

Project duration: 36 months


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Table of Contents

List of Figures ................................................................................................................................... 4

Glossary ........................................................................................................................................... 5

1. Executive Summary .................................................................................................................. 7

2. Introduction .............................................................................................................................. 8

3. State-of-the-art in Fuselage BLI and Wake Filling ...................................................................... 10

3.1 State-of-the-art Summary and Key Findings ................................................................................10 3.2 Propulsive Fuselage Configuration Studies .................................................................................. 13

3.2.1 DisPURSAL Propulsive Fuselage Concept ............................................................................ 13 3.2.2 NASA STARC-ABL Concept ................................................................................................ 15 3.2.3 Boeing SUGAR Freeze Concept ........................................................................................... 17

3.3 Hybrid Wing Body Concepts........................................................................................................ 19 3.3.1 DisPURSAL Hybrid Wing Body Concept ............................................................................. 19 3.3.2 NASA Hybrid Wing Body N3-X Study ..................................................................................21

3.4 Double-Bubble Fuselage Concepts ...............................................................................................23 3.4.1 MIT D8 Concept Study ........................................................................................................23

4. Concept Ideas and Assessment Candidates ............................................................................... 25

4.1 Cloud of Ideas and Concept Solutions ......................................................................................... 25

5. Aircraft Layout for Initial Propulsive Fuselage Concept .............................................................. 28

5.1 Basic Design Specification .......................................................................................................... 28 5.2 3-View-Drawing .......................................................................................................................... 29

6. Acknowledgement .................................................................................................................. 31

7. Appendix ................................................................................................................................ 32

8. Bibliography ........................................................................................................................... 33


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List of Figures

Figure 1: Illustration of initial PFC Reference definition process ..................................................................... 8

Figure 2: Illustration of power-saving benefit of (bottom) boundary layer ingestion and wake filling compared with (top) a conventional aircraft [Uranga, A. et al., 2017] ........................................................................... 10

Figure 3: Power saving potentials due to wake-filling analysed for various propulsion distribution concepts [Steiner et al., 2012]. A concentric fuselage propulsor (highlighted by red box) offers the highest power saving potentials, followed by a HWB configuration ................................................................................................ 11

Figure 4: Rendering of Propulsive Fuselage Concept of DisPURSAL [Isikveren, A.T. et al., 2015] ................. 12

Figure 5: Illustration of Hybrid Wing Body Configuration with distributed propulsion [Isikveren A.T. et al., 2014] ........................................................................................................................................................... 12

Figure 6: Isometric view and rendered picture of Propulsive Fuselage, cut-away view of the Fuselage Fan and corresponding friction lines, pressures and flow lines [Isikveren, A. T. et al., 2015] ........................................ 13

Figure 7: Boeing N+4 SUGAR Freeze Concept featuring Truss Braced Wing with LNG Fuel Cell Hybrid Gas Turbine and electric aft fuselage BLI propulsor [Bradley, M. K. et al., 2012] ................................................. 18

Figure 8: Isometric rendered image of DisPURSAL Distributed Multiple-Fans Concept [Isikveren, A. T. et al., 2015] ........................................................................................................................................................... 20

Figure 9: Schematic of power transmission layout of the Distributed Multiple-Fans Concept for a given side from aircraft centre-line [Isikveren, A. T. et al., 2015]................................................................................... 20

Figure 10: NASA H3.2 Hybrid Wing Body technology contributions to overall efficiency goal [Greitzer, E. M. et al., 2010] ..................................................................................................................................................... 23

Figure 11: Principal approach for Propulsive Fuselage Concept identification .............................................. 25

Figure 12: Rear Fuselage Propulsor with T-Tail proposal .............................................................................. 26

Figure 13: Rear Fuselage Propulsor with Conventional Tail proposal ............................................................ 26

Figure 14: Rear Fuselage Propulsor with V- Tail proposal ............................................................................. 27

Figure 15: Summary of initial PFC Concept Ideas: T-Tail, Conventional Tail and Canard Concept ................. 27

Figure 16: CENTRELINE – initial Propulsive Fuselage Configuration design process CENTRELINE – initial Propulsive Fuselage Configuration design process ...................................................................................... 28

Figure 17: Initial PFC aircraft 3-view-drawing ............................................................................................... 29


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Glossary

Abbreviation / acronym Description

AC Alternating Current

A/C Aircraft

ADHF Adaptive Dropped Hinge Flap

AEA All-Electric Aircraft

AR Aspect Ratio

APU Auxiliary Power Unit

BFL Balanced Field Length

BHL Bauhaus Luftfahrt e.V.

BPR By-Pass Ratio

CENTRELINE ConcEpt validatioN sTudy foR fusElage wake-filLIng propulsioN intEgration

CFRP Carbon Fiber Reinforced Polymer

DC Direct Current

DisPURSAL Distributed Propulsion and Ultra-high Bypass Rotor Study at A/C Level

ECS Environmental Control System

EHA Electro-Hydrostatic Actuators

EIS Entry Into Service

EMA Electromechanical Actuators

ETOPS Extended-range Twin-engine Operational Performance Standards

FAA Federal Aviation Administration

FBW Fly-By-Wire

FL Flight Level

GTF Geared Turbo Fan

HLFC Hybrid Laminar Flow Control

ECS Environmental Control System

HP High Pressure

ICA Initial Cruise Altitude

ICAO International Civil Aviation Organization

ISA International Standard Atmosphere

LFL Landing Field Length

LNG Liquid Natural Gas

LP Low Pressure


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LPT Low Pressure Turbine

LRC Long Range Cruise

MCL Maximum Climb

MDO Multi-disciplinary Design and Optimization

MEA More Electric Aircraft

MLW Maximum Landing Weight

MTOW Maximum Take-Off Weight

MUFW Maximum Usable Fuel Weight

NASA National Aeronautics and Space Administration

NLF Natural Laminar Flow

OEW Operational Empty Weight

OPR Overall Pressure Ratio

PAX Passengers

PFC Propulsive Fuselage Concept

PPS Power Plant Systems

R2000 SRIA Baseline Aircraft

R2035 CENTRELINE Reference Aircraft

RPK Revenue Passenger Kilometres

SLS Sea Level Static

SRIA Strategic Research and Innovation Agenda

TeDP Turbo-electric Distributed Propulsion

TOFL Take-Off Field Length

TOGW Take-Off Gross Weight

TLAR Top Level Aircraft Requirement

TO Take-Off

ToC Top of Climb

TR Taper Ratio

TSFC Thrust Specific Fuel Consumption

UHBR Ultra-High Bypass Ratio

ULD Underfloor Loading Device


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1. Executive Summary

The CENTRELINE project will demonstrate the proof of concept for a ground-breaking approach to propulsion airframe integration. The concept consists of a dedicated propulsive device that ingests and re-energizes the fuselage boundary layer in order to directly compensate the viscous drag effects in the fuselage wake flow field.

The content of this deliverable presents the activities performed in Task 1.2 “Concept design space exploration” of work package one. State-of-the-art in fuselage BLI and wake filling has been reviewed and the most promising PFC aircraft configuration down-selected from a broad variety of conceptual candidates based on a robust and transparent down-selection procedure. The concept down-selection was based on a combined set of multi-disciplinary qualitative criteria and quantitative assessment metrics. The selected PFC configuration was derived on the basis of the R2035 Reference aircraft through primary component adaptations in rear fuselage shape and empennage arrangement, and by implementation of a rear fuselage propulsor, turbo-electric generators and a fuselage fan motor. Specific sizing and multi-disciplinary optimisation for the selected and pre-defined PFC aircraft configuration was performed in Task 2.5 “Aircraft integrated sizing and optimisation” of work package two.


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2. Introduction

Within the CENTRELINE project, the proof of concept for the Propulsive Fuselage Concept (PFC) as an innovative approach to propulsion airframe integration with an entry into service year 2035 will be demonstrated. The concept uses a conventional aircraft configuration but adds an electrically driven aft fan running on energy extracted from two conventional under-wing mounted turbofan engines. This separation of the power producing components (engines) and thrust producing components (propulsor) is a key factor in obtaining efficiency gains from effective propulsion-airframe integration. The largest unknown of the proposed turboelectric propulsion system is whether the performance increases that can be obtained from the BLI are sufficient to offset the additional electrical efficiency losses and system weight.

Fuselage boundary layer ingestion (BLI) and wake filling concepts have already been studied within various European and US research programmes. A review of state-of-the-art in Fuselage BLI and wake filling findings within theoretical and experimental results confirmed efficiency and energy saving benefits up to a lower two digit improvement. In order to progress and validate previous results achieved within the FP7 DisPURSAL project [Isikveren, A. T. et al., 2014], further detailed studies will be performed in CENTRELINE. Therefore the most promising PFC aircraft configuration has been down-selected from a broad variety of conceptual candidates based on a robust and transparent down-selection procedure. A comprehensive cloud of concept solutions including novel concept ideas was created by the project partners, under consideration of important design requirements, multi-disciplinary functionalities, and basic system architectural features. BHL prepared and established a combined set of decisive quantitative metrics and multi-disciplinary qualitative rating criteria enabling a well-structured concept down-selection process within a dedicated workshop involving all partners.

The selected PFC configuration represents a straight-forward approach on the basis of a conventional wing tube airframe with a high potential to meet technical maturity for the envisaged 2035 EIS time horizon. The concept consists of a twin engine aircraft with an under wing GTF UHBR propulsion system plus a dedicated additional turbo-electric propulsive device at the rear end of the fuselage that ingests and re-energizes the fuselage boundary layer flow in order to directly compensate the viscous drag effects in the fuselage wake flow field. The selected concept was adjusted to the 2035 technology level as identified within Task 1.1 and derived from the R2035 Reference aircraft through primary component adaptations in rear fuselage shape and empennage arrangement, and by implementation of a rear fuselage propulsor, turbo-electric generators and a fuselage fan motor (Figure 1). Specific sizing and multi-disciplinary optimisation for the selected and pre-defined PFC aircraft configuration was performed by BHL in Task 2.5 “Aircraft integrated sizing and optimisation” of work package two under consideration of relevant implications due to structural, aerodynamic, propulsion and turbo-electrical changes, resulting in a detailed definition of the initial PFC design at this early stage of the project.

Figure 1: Illustration of initial PFC Reference definition process


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This report comprises the description of the selection process and identification of the most promising PFC candidate. Within Appendix 1 it provides further details and an overview and basic design specifications of the cabin, fuselage and tail arrangements, basic configurations of the wing, landing gear and the propulsion and power system together with the electrically driven rear fuselage fan. This summary of the initial PFC design specification including representation in a CAD model will form the basis for further detailed PFC design elaboration in WP2 to WP4. Details of the initial PFC aircraft design definition on component level are presented in deliverable D2.10 “Report on initial PFC aircraft design definition” [Seitz, A. et al., 2018].


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3. State-of-the-art in Fuselage BLI and Wake Filling

This chapter summarizes the current results from most recent studies on a ground-breaking approach to synergistic propulsion-airframe integration within similar configurations of the so called Propulsive Fuselage Concept (PFC) and addresses also key findings on alternative promising configurations. It provides an overview of most advanced related studies in Europe and US.

3.1 State-of-the-art Summary and Key Findings

The overall objective of the ground-breaking approach is to reduce the drag generated by the aircraft fuselage boundary layer and resulting wake in the air stream behind the fuselage. The key principle is to position a propulsor in the central air stream at the rear fuselage as illustrated in Figure 2. The propulsor in this central aft fuselage position is able to provide a power-saving benefit by accelerating the ingested slower boundary layer. Resulting lower wake velocities of combined wake and jet result in a reduction in wasted kinetic energy.

Figure 2: Illustration of power-saving benefit of (bottom) boundary layer ingestion and wake filling compared with (top) a conventional aircraft [Uranga, A. et al., 2017]

The positive effect of wake-filling on propulsive power requirements has long been known from the field of marine propulsion, but is also applicable to airborne systems [Smith, 1993]. In Figure 3, the results of a simplified comparative analysis of the propulsive power saving potentials attainable from different conceptual approaches to wake-filling propulsion integration are presented [Steiner et al., 2012]. The power saving potential from wake-filling improves as the amount of momentum deficit in the airframe wake flow that is ingested by the propulsion system, i.e. the ingested drag ratio, is increased. For large commercial aircraft, the share of viscous and form drag typically ranges between 60–70% of the total drag in cruise. Approximately half of this share may be attributed to the fuselage body, making it the most interesting airframe component to be utilised for the purpose of wake-filling propulsion integration [Seitz, A., 2016]. Hybrid Wing Body configurations with rear fuselage distributed propulsion systems and boundary layer ingestion offer second largest efficiency potential.


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Figure 3: Power saving potentials due to wake-filling analysed for various propulsion distribution concepts [Steiner et al., 2012]. A concentric fuselage propulsor (highlighted by red box) offers the highest power

saving potentials, followed by a HWB configuration

Various studies in Europe and US have been conducted so far. Theoretical as well as first experimental results under representative conditions lead to the conclusion that drag reduction benefits resulting from boundary layer ingestion and wake filling can be confirmed.

A large variety of concepts for future aircraft have been proposed that use boundary layer ingestion (BLI) as a way to reach fuel efficiency levels not achievable with the conventional tube-and-wing design of current civil transport aircraft. From the basic architectural alternatives – with one or more (additional) propulsors at the rear end of the airframe – three main categories can be identified:

Propulsive Fuselage Concepts

Conventional Wing-Tube Airframes with a rear fuselage mounted propulsor to ingest the boundary layer of the entire cylindrical fuselage body represent the category of the so called Propulsive Fuselage Concepts (PFC) as illustrated in Figure 4. This concept represents a high potential to meet technical maturity level compared to other alternative concepts. In order to adequately address system redundancy stipulated by transport category certification requirements, the type of configuration additionally comprises two under-wing podded UHBR turbofans. The rear fuselage propulsor offers the opportunity to consider the installation of an electrically driven fan.

Recent studies on Propulsive Fuselage Configurations with turbo-electric fuselage fan indicate 15% design mission fuel burn reduction for a single aisle commercial transport concept (3500nm design range, 154 passengers at M 0.70 cruise speed) [Welstead et al. 2017] and 9-11% fuel burn reduction for a medium range twin aisle, gas-turbine only configuration (4800nm design range, 340 passengers at M 0.78 - 0.80 cruise speed) [Isikveren, A.T. et al. 2015] compared to a similar technology conventional configuration at entry into service 2035.


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Figure 4: Rendering of Propulsive Fuselage Concept of DisPURSAL [Isikveren, A.T. et al., 2015]

Hybrid Wing Body Airframes

Blended or Hybrid Wing Body Airframes (HWB) represent a second category of boundary layer ingestion concepts. This configuration offers the opportunity for a distributed propulsion system mounted on rear top of the main body, thereby ingesting large portions of the upper boundary layer. Efficiency gains from BLI effect are estimated between 4% and 6% [Steiner et al., 2012]. Overall efficiency gains of a HWB configuration go beyond BLI effects due to high level of structural, aerodynamic and propulsion integration of such airframe concept.

HWB concepts with a Distributed Multiple-Fans Concept (DMFC) driven by a limited number of gas-turbine engine cores were predicted at 8% design mission fuel burn reduction (4800nm design range, 340 passengers at M 0.80 cruise speed) [Isikveren, A. T. et al. 2015] compared to a similar technology conventional configuration at entry into service 2035. Investigations of HWB concepts with turbo-electric propulsion installation indicate BLI alone effects of 9.5% efficiency improvement [Greitzer, E. M. et al., 2010]. A typical HWB configuration is presented in Figure 5.

Figure 5: Illustration of Hybrid Wing Body Configuration with distributed propulsion [Isikveren A.T. et al., 2014]

Double-Bubble Fuselage Configurations

A Double-Bubble Fuselage as a modification of a Conventional Wing-Tube Airframe with conjunction of two cylindrical fuselages to create an unconventional lifting body is a third category of BLI concepts and can be considered as an intermediate step between PFC and HWB. Rear mounted propulsors ingest the boundary layer of the upper fuselage body or at least larger portions of it.

Experimental assessments carried out on a Double-Bubble Fuselage configuration, the D8 from 2010 to 2015 as part of a NASA N+3 Phase 2 Program in a back- to-back comparison of non-boundary layer ingesting and boundary layer ingesting versions of the D8 indicate 8.6% aerodynamic benefit [Uranga et al., 2017]. This work represents the first measurement of boundary layer ingestion performance.

Further efficiency gains of this configuration beyond BLI are resulting from an integrated design approach. Increased lift generated by the wide “double-bubble” fuselage means smaller wings are needed to carry the vehicle’s weight, resulting in less fuel to fly a given mission. When the engines are integrated into the back of the fuselage, thrust requirements are further reduced due to efficiencies from Boundary Layer Ingestion (BLI). This means that smaller engines can be used, which reduces weight and fuel even further.


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3.2 Propulsive Fuselage Configuration Studies

The focus of this chapter is on recent study results from configurations similar to H2020 CENTRELINE concept: Tube and wing baseline with rear fuselage propulsor, the so called Propulsive Fuselage.

3.2.1 DisPURSAL Propulsive Fuselage Concept

The EC had recognised the potential benefits afforded by distributed propulsion solutions by granting approval for a Level-0 Framework 7 project entitled Distributed Propulsion and Ultra-high By-Pass Rotor Study at Aircraft Level ( DisPURSAL), [Bijewitz, J. et al., 2014]. Coordinated by Bauhaus Luftfahrt e.V., this 2-year project, which commenced in February 2013, involved partners from the CIAM (Russia), ONERA (France) and Airbus Group Innovations (Germany).

Targeting an EIS of 2035 this project investigates aircraft concepts employing distributed propulsion with focus placed upon one novel solution that integrates the fuselage with a single propulsor (dubbed the Propulsive-Fuselage Concept). Aspects that were addressed included aircraft design and optimisation, airframe-propulsion integration, power-train system design and advanced flow field simulation as illustrated in Figure 5. The degree of conceptual design elaboration and technological maturity achieved in the DisPURSAL project represent the most current state-of-the-art in PFC-based fuselage wake-willing propulsion integration in the transport aircraft category.

DisPURSAL Propulsive Fuselage Conclusion

The reported work covers the conceptual design and assessment of a Propulsive Fuselage (PF) aircraft layout featuring a gas turbine based Fuselage Fan (FF) propulsion system in conjunction with two underwing podded advanced turbofans. Characteristic illustrations are given in Figure 6.

Figure 6: Isometric view and rendered picture of Propulsive Fuselage, cut-away view of the Fuselage Fan and corresponding friction lines, pressures and flow lines [Isikveren, A. T. et al., 2015]


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In order to evaluate the concept, a multidisciplinary workflow was established allowing for efficient design space exploration of the PF concept with regards to different geometric configurations. Moreover, a variety of FF power supply and transmission concepts for gas turbine based, and hybrid-electric options was conceptually investigated. The aerodynamic characteristics of the PF concept were assessed through 2D axisymmetric RANS simulations. Starting from an initial geometry, an optimization of the aft fuselage contour and the FF nacelle shape for minimum super-velocities and pressure gradients was conducted. Thereafter, a design space exploration involving a combination of different geometric settings was performed, which fed into the derivation of propulsion system sizing and performance models suitable for the integrated assessment. This allowed for the execution of integrated parametric design studies and the identification of a best and balanced design point. The final PF design exhibits a block fuel burn benefit relative to an advanced year of entry-into-service 2035 reference aircraft of 9.2%.

As a further approach regarding the exploration of the PF concept, a hybrid-electric PF concept was investigated in addition to the gas turbine based FF drive. Here, the required electrical power to drive the FF was assumed to be extracted from the underwing podded power plants, which therefore yielded an increased size relative to the turbo engine based PF concept. The estimated additional weight of the electrical power train propagated into a more severe OEW increase compared to the gas turbine based PF concept (+16.3%). The identified design features a block fuel reduction of 7.3% relative to the 2035 reference aircraft, while MTOW was predicted to increase by 8.0%.

The presented studies highlight the vehicular efficiency potential intrinsic to the PF concept.

A summary of outcomes [Isikveren et al., 2015] and insights for the PFC were documented as:

• For a design range of 4,800nm with 340 passengers at M0·80 cruise speed, block fuel burn reduction compared to an appropriately projected-technology, gas-turbine only aircraft utilising a conventional morphology 2035R was predicted to be 9% (nominal case)

• Assuming the same range, speed and passenger accommodation the block fuel difference to the SoAR was found to be nominally –38%

• Using methods in accordance with upcoming ICAO Annex 16 Environmental Protection Volume III, nominally 43% lower CO2-emissions versus the SoAR and 15% better than the 2035R were observed; this means the shortfall in CO2-emissions reduction with respect to SRIA 2035 is 8%

• There appears to be a good likelihood of meeting the SRIA 2035 external noise targets; however, realisation of the NOx-emissions target will be a challenge that requires close attention

• Assuming a nominal fuel price of USD 3·30 per US gallon up to 25% lower COC versus SoAR was predicted, equivalently this was found to be approximately 5% better than the 2035R

• If the PFC is sized according to an appropriate ‘optimal speed matching’ philosophy, i.e. Long Range Cruise speed of M0·78 instead of M0·80 (so-called PFC*) the reduction in block fuel and CO2-emissions was found to be at least 11% compared to the 2035R* (2035R resized for design cruise of M0·78). Whilst reducing design cruise speed only yields a modest level of incremental advantage, it does provide another avenue in realising emissions and external noise environmental targets.

Table 1 provides a summary and comparison of aircraft characteristics of the DisPURSAL Reference 2035 aircraft and related PFC [Bijewitz, J. et al., 2014].


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Table 1: DisPURSAL – Summary of aircraft characteristics, technology level for EIS 2035 [Bijewitz, J. et al., 2014]

Parameter Unit R2035 PFC Delta [%]

Design Range [nm] 4800 4800 0.0

PAX (2 class) [-] 340 340 0.0

Design Payload, max. PAX [kg] 34680 34680 0.0

MTOW [kg] 206270 208970 +1.3

OWE [kg] 123460 130585 +5.8

T.O. Thrust (SLS) [kN] 627.2 635.4 +1.3

T.O. Thrust to Weight (SLS) [-] 0.310 0.310 0.0

Wing Reference Area [m²] 335.4 339.8 +1.3

Wing Loading [kg/m²] 615 615 0.0

Wing Aspect Ratio [-] 12.6 12.4 -1.6

Wing Span [m] 65 65 0.0

Fuselage Length [m] 67 69 +2.9

Cruise Mach Number LRC [-] 0.80 0.80 0.0

Block Fuel @ Design Range [kg] 42257 38380 -9.2

3.2.2 NASA STARC-ABL Concept

NASA presented in 2016 a paper study of a Single-aisle Turboelectric AiRCraft with Aft Boundary Layer propulsion (STARC-ABL) [Welstead et al., 2016].

The study presented a single-aisle turboelectric aircraft with an aft boundary layer propulsor as a possible concept that takes advantage of a turboelectric architecture and the ability to distribute the power. The concept is a tube-and-wing configuration with two underwing mounted turbofans. Attached to each turbofan is a generator that extracts mechanical power from the fan shaft and converts it to electrical power to drive a rear mounted boundary layer ingesting, electrically powered fan. The STARC-ABL concept was developed as a first look at the turboelectric propulsion architecture for a single-aisle class commercial transport, and to determine if a benefit exists for a mostly conventional configuration with moderate distribution of propulsion power.

The STARC-ABL concept was sized for a mission similar to current day Boeing 737-800 or Airbus A320 configurations as indicated in Table 2 with related main aircraft characteristics. Two single-aisle commercial transport concepts were developed assuming an entry into service in the 2035 timeframe (N+3), a baseline N+3 conventional configuration (N3CC) and a turboelectric concept (STARC-ABL). Both concepts utilized technologies that were assumed to be at technology readiness level (TRL) 6 by the year 2025 and were similar in wing-body configuration with two underwing propulsors. Key differences between the concepts were the turboelectric architecture, the rear fuselage BLI fan employed by the STARC-ABL concept, and the T-tail empennage resulting from the rear fuselage fan placement. Greatest commonality between the concepts was determined to be the best way to evaluate the impact of the turboelectric propulsion system architecture with a rear fuselage BLI fan.


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STARC-ABL Propulsive Fuselage Conclusion

The single-aisle turboelectric aircraft with an aft boundary layer propulsor (STARC-ABL) represents one of the most basic uses of the turboelectric architecture, but was a solid starting point to gain understanding on how the turboelectric architecture changes the design space. For a point of comparison, an N+3 conventional configuration (N3CC) model was also developed to be representative of a 737-800like aircraft in the 2035 timeframe.

Initial 2016 results and revised 2017 STARC-ABL Rev.B results [Welstead et al., 2017] indicate that a significant fuel burn benefit can be obtained with the turboelectric architecture using conservative technology assumptions, especially conventional electrical components with moderate efficiencies. The STARC-ABL Rev. B has a 9% block fuel burn reduction for the economic mission, and 15% block fuel burn reduction for the design mission. Key sources of this benefit are the increase in propulsion system efficiency from the rear fuselage BLI fan that is assumed to ingest only a bottom half portion of the boundary layer, and the downsizing of the underwing turbofans, which helps offset the weight of the additional turboelectric system components.

It is reported that the propulsion system weights do contain some uncertainty due to the modelling methods used and require additional scrutiny, but the trend of the turbofan weight reduction offsetting some or all of the turboelectric components should remain. Overall, this research has shown that the STARC-ABL concept may be a good first candidate for a turboelectric propulsion system architecture that provides significant fuel burn benefits when compared to a similar technology conventional configuration.

Summary of STARC-ABL Rev. B Results [Welstead, J. et al., 2017]:

• Significant reductions in system fuel burn

- –12% reduction in start of cruise (SOC) TSFC

- –9% reduction in economic mission block fuel

- –15% reduction in design mission block fuel

• Fuselage propulsor details

- Only bottom half of boundary layer ingested

- BLI propulsor placed at most aft fuselage position

- Driven by an all-electric motor, nominally operating at 3500 HP (2.6 MW)

- Electrical system modelled assuming ~10% total system losses, 1000 Volt electrical system

• Partially turboelectric system is not a weight penalty

- Downsizing of underwing engines enabled by turboelectric offsets the weight addition of electrical components and tailcone propulsor

• Cable size/weight can become prohibitive if onboard voltage too low

• Electric system specific power based upon current AATT NRA efforts

Table 2: STARC-ABL (Rev.B) – Summary of aircraft characteristics, technology level for EIS 2035 [Welstead, J. et al., 2017]

Parameter Unit N3CC STARC Delta [%]


PAX (2 class) [-] 154 154 0.0

MTOW [lb] 137670 132480 -3.8

OWE [lb] 78540 77350 -1.5

T.O. Thrust (SLS) [lb] 43320 42940 -0.9

T.O. Thrust to Weight (SLS) [-] 0.335 0.322 -3.9


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Wing Reference Area [ft²] 1170 1140 -2.3

Wing Loading [lb/ft²] 118.1 116.3 -1.5

Wing Aspect Ratio [-] 11.9 12.2 2.3

Wing Span [ft] 118 118 0.0

Take-off Field Length [ft] 8200 8160 -0.5

Cruise Mach Number [-] 0.785 0.785 0.0

Block Fuel @ 900 nm [lb] 6910 6260 -9.4

Block Fuel @ Design Range [lb] 25170 21340 -15.2

3.2.3 Boeing SUGAR Freeze Concept

A further Propulsive Fuselage Concept dubbed “SUGAR Freeze” was studied by Boeing and research partner during a two day workshop held in June 2011 [Bradley, M. K. et al., 2012]. The concept was aiming at a corresponding operational date of 2040-2050 for N+4 technologies and therefore reached beyond the time horizon of DisPURSAL PFC and MIT STARC-ABL concepts as described before.

The NASA report presents among other concept variants a paper study of a truss-brazed wing single-aisle aircraft concept (Figure 7) featuring an electric aft fuselage boundary layer ingestion propulsor utilizing electric power from a liquid natural gas (LNG) fuel cell. Additional 2040+ technologies are considered with a high aspect ratio strut braced wing with folding tips and an advanced LNG hybrid gas turbine propulsion system.

This final report documents the work of the Boeing Subsonic Ultra Green Aircraft Research (SUGAR) team on Task 1 of the Phase II effort. The team consisted of personnel from Boeing, FAA, General Electric, Georgia Tech, NASA and Virginia Tech and performed an MDO study using a quantitative workshop process combining technologies appropriate to aircraft operational in the N+4 2040-2050 timeframe, assuming TRL6 for 2030-2035. Technology development plans were developed and a series of concepts investigated with different combinations of some of these technologies. Weight, aerodynamic and propulsion data were generated for a series of configurations with different combinations of N+4 technologies. Performance and sizing has been conducted for these configurations to allow comparisons on a common basis. Main focus during these studies was on the Ultra Green Aircraft using Liquid Natural Gas: LNG fuelled aircraft have the potential for significant emissions advantages and LNG enhances the integration of fuel cells. Adding a fuel cell and electric motor into the main propulsion system (LNG Fuel Cell Hybrid Gas Turbine) also leads to improvements in emissions and fuel burn. An aft fuselage boundary layer propulsor also resulted in a fuel burn benefit.


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Figure 7: Boeing N+4 SUGAR Freeze Concept featuring Truss Braced Wing with LNG Fuel Cell Hybrid Gas Turbine and electric aft fuselage BLI propulsor [Bradley, M. K. et al., 2012]

Boeing SUGAR Freeze Concept Conclusion

Subsonic Ultra Green Aircraft Concepts SUGAR study results as provided in the NASA Report [Bradley, M. K. et al., 2012], based on N+4 technology level single-aisle aircraft with high aspect ratio strut braced wing with folding tips and LNG gas turbine propulsion system can be summarized as follows (details given in Table 3):

• For a single-aisle aircraft concept with 3,500nm design range and 154 passengers, an aft fuselage boundary layer propulsor driven by an electric motor leads to reductions in emissions and fuel burn at a typical 900 nm mission (-8.5%) compared to same projected-technology, LNG gas-turbine only aircraft.

• LNG fuelled aircraft require heavier aircraft systems and larger propellant tankage compared to conventionally fuelled aircraft. The higher heating value of LNG reduces the weight of fuel burned (-5.8%), but the heavier aircraft requires more total energy (+5.6%) for a given flight.

• LNG fuelled aircraft have the potential for significant emissions advantages over conventionally fuelled aircraft. LTO and cruise NOx are lower and less carbon dioxide is produced when it is burned.

• Adding a topping cycle fuel cell (LNG Fuel Cell Hybrid Gas Turbine) and an aft fuselage boundary layer propulsor driven by an electric motor leads to reductions in emissions and fuel burn (-8.5% at typical 900 nm mission) versus equal technology level LNG gas-turbine only aircraft (N+4 SUGAR Freeze).


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Table 3: Boeing SUGAR Freeze LNG Concept – Summary of aircraft characteristics, technology level for EIS 2040-2050 [Bradley, M. K. et al., 2012]

Parameter Unit N+4 SUGAR Freeze

SUGAR Freeze

Hybrid BLI

Delta [%]


PAX (2 class) [-] 154 154 0.0

MTOW [lb] 153300 158800 +3.6

OWE [lb] 99200 107300 +8.2

T.O. Thrust (SLS) [lb] 41200 41600 +1.0


Wing Reference Area [ft²] 1462 1624 +11.1

Wing Loading [lb/ft²] 104.9 97.8 -6.7

Wing Aspect Ratio [-] 19.56 19.56 0.0

Wing Span [ft] 169 178 +5.3

Take-off Field Length [ft] 8190 8190 0.0

Cruise Mach Number [-] 0.7 0.7 0.0

Block Fuel @ 900 nm [lb] 6038 5525 -8.5

3.3 Hybrid Wing Body Concepts

This chapter describes current outcomes of a second category of promising BLI configurations beyond tube and wing baseline: Hybrid Wing Body concepts studies conducted in European and US investigations suggest overall efficiency improvements out of a high level of aero, structure and distributed propulsion integration. In contrast to PFC concepts with BLI rear fuselage propulsor modifications of conventional current production tube and wing aircraft, these disruptive concepts comprise a significantly higher level of uncertainty and risk to achieve system maturity for EIS 2035.

3.3.1 DisPURSAL Hybrid Wing Body Concept

Technical work on Hybrid Wing Body concepts has been performed as part of the EC funded Level-0 Framework 7 project DisPURSAL as mentioned before in chapter 3.2.1.

Targeting an EIS of 2035 this project investigated aircraft concepts employing distributed propulsion with focus on one Propulsive-Fuselage Concept (PFC) as well as Distributed Multiple-Fans Concept (DMFC) driven by a limited number of engine cores. Aspects that are being addressed include aircraft design and optimisation, airframe-propulsion integration, power-train system design and advanced flow field simulation.


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Figure 8: Isometric rendered image of DisPURSAL Distributed Multiple-Fans Concept [Isikveren, A. T. et al., 2015]

As illustrated in Figure 8, the HWB has a flattened and reflexed airfoil shaped body. A low effective wing loading and beneficial trim effect means a complex high-lift system is not required. The distributed propulsion system is mounted atop of the main body, thereby ingesting large portions of the boundary layer [Isikveren, A. T. et al., 2015]. A schematic of a mechanical power transmission layout of the Distributed Multiple-Fans Concept is illustrated in Figure 9.

A primary goal of the DMFC is to make provision for BLI via a distributed propulsion system layout and provide significant improvements in propulsive efficiency even when the turbo-machinery operate in highly distorted flow. When BLI is introduced for a concept like a HWB, the airframe and propulsion system are much more closely coupled. Drag produced by the airframe is manifested in the form of lower momentum fluid in the boundary layer, which is ingested by the propulsion system. The propulsion system no longer takes clean and uniform free stream flow even at the design point, and as such the inlet flow distortion is at least an order-of-magnitude higher than what is typically the case at cruise operation for conventional propulsion system installations. Furthermore, with the configuration under investigation the engine exhaust mixes directly with the aircraft wake, whereas, in a conventional configuration the engine exhaust and the aircraft wake generally mix in a separate fashion. All of these effects increase the degree of coupling between the airframe and the fans/engines, requiring new approaches for the analysis and design of BLI propulsion systems.

Figure 9: Schematic of power transmission layout of the Distributed Multiple-Fans Concept for a given side from aircraft centre-line [Isikveren, A. T. et al., 2015]

DisPURSAL Hybrid Wing Body Conclusion

A summary of main results for the DMFC [Isikveren, A. T. et al., 2015] is provided in Table 4 and indicated as:

• For a design range of 4,800nm (8,890km) with 340 passengers at M0·80 cruise speed, block fuel burn reduction compared to 2035R (projected 2035 reference aircraft with a conventional configuration) was predicted to be 8% (nominal case – engineering target within worst-nominal-best interval)


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• The overall positive net outcome can be explained by the trade-off between L/D and overall power plant efficiency: when comparing the sizing outcome of almost parity in gross weight, compared to the 2035R, the 10% degradation in overall power plant efficiency is off-set by an almost 18% improvement in aircraft L/D. The complete L/D improvement is broken down as 7% attributable to morphological change from tube-and-wing to HWB with the remaining +11% being a product of ingested drag.

• Assuming the same range, speed and passenger accommodation the block fuel difference to a year 2000 datum A330-300 aircraft (SoAR) was found to be nominally –37%

• There appears to be a good likelihood of meeting the SRIA 2035 NOx-emissions and external noise targets

Table 4: DisPURSAL Hybrid Wing Body – Summary of aircraft characteristics, technology level for EIS 2035 [Isikveren, A. T. et al., 2015]

Parameter Unit R2035 HWB Delta [%]


PAX (2 class) [-] 340 340 0.0

Design Payload, max. PAX [kg] 34680 34680 0.0

MTOW [kg] 206270 206540 +0.1

OWE [kg] 123460 127240 +3.1

T.O. Thrust (SLS) [kN] 627.2 603 -3.9


Wing Reference Area [m²] 335.4 614 +83.1

Wing Loading [kg/m²] 615 336 -45.4

Wing Aspect Ratio [-] 12.6 6.9 -45.2

Wing Span [m] 65 65 0.0

Fuselage Length [m] 67 37 -44.8


Block Fuel @ Design Range [kg] 42257 38960 -7.8

3.3.2 NASA Hybrid Wing Body N3-X Study

A further Hybrid Wing Body vehicle concept with boundary layer ingestion is currently being investigated by NASA, called the “N3-X” that uses a hybrid-wing-body for an airframe and superconducting generators, motors, and transmission lines for its propulsion system. The study is conducted in the N+3 framework of advanced concept studies for commercial subsonic transport aircraft and assumes an ambitious technology timeline for operational readiness in 2035.

A team at NASA proposed and examined a revolutionary aero-propulsion concept, a turboelectric distributed propulsion system, which employs multiple electric motor-driven propulsors that are distributed on a large transport vehicle. The power to drive these electric propulsors is generated by separately located gas-turbine-driven electric generators on the airframe. This arrangement enables the use of many small-distributed


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propulsors, allowing a very high effective bypass ratio, while retaining the superior efficiency of large core engines, which are physically separated but connected to the propulsors through electric power lines.

On the N3-X these new degrees of design freedom are used (1) to place two large turboshaft engines driving generators in freestream conditions to minimize total pressure losses and (2) to embed a broad continuous array of 14 motor-driven fans on the upper surface of the aircraft near the trailing edge of the Hybrid Wing Body airframe to maximize propulsive efficiency by ingesting thick airframe boundary layer flow.

NASA N3-X Hybrid Wing Body Conclusion

The concept employs a number of high-power electric motors to drive the distributed propulsors. The power to drive these electric propulsors is generated by separately located gas-turbine-driven electric generators on the airframe. This arrangement enables the use of many small distributed propulsors, allowing a very high effective bypass ratio (eBPR), while retaining the superior efficiency of large core engines, which are physically separated but connected to the propulsors through high power electric transmission lines. The following are some of the features and issues associated with TeDP system.

A summary of current N3-X results [Kim, H. D. et al., 2013] is given as follows:

• The N3-X would be able to achieve a reduction of 70% or 72% (depending on the cooling system) in energy usage relative to the current technology reference aircraft, a Boeing 777-200LR as indicated in Table 5

• The use of electrical power transmission allows a high degree of flexibility in positioning the turboelectric generators and propulsor modules to the best advantage.

• Large combined fan areas from multiple small fans provide very high effective BPR and low fan noise

• Because the majority of the power is extracted from the engine core to power the electric fans, the core jet noise is substantially reduced because of low core jet exhaust velocity

• In case of one-engine-inoperative (OEI) situation, the remaining operative turbogenerator still provides power to all operating electric fans for symmetric thrust. This feature greatly reduces or possibly eliminates the vertical tail, which is usually sized for an OEI situation on conventional aircraft sizing.

• Vehicle control could be achieved with a fast response electric fan module. Since the exhaust air from the fan is “cold,” and not “hot” combustion air from the core engine, thrust vectoring devices may employ conventional lightweight airframe materials.

• Electric components such as generators, motors, and transmission lines must be highly efficient and lightweight. Furthermore, the power distribution of multi-megawatts of electric power from the generators to the fan motors must be carefully considered and designed.

• Propulsion airframe integration (PAI) will play a greater role in achieving the N+3 goals because of the distributed thrust stream interacting with airframe.

Table 5: NASA N3-X Hybrid Wing Body – Summary of aircraft characteristics, technology level for EIS 2035 [Kim, H. D. et al., 2013]

Parameter Unit 777-200LR

GE90-110B

N3-X Delta [%]


PAX (2 class) [-] 300 300 0.0

Design Payload, max. PAX [lb] 118000 118000 0.0

MTOW [lb] 768000 515000 -32.9


Block Fuel @ Design Range [lb] 279800 84992 -69.6


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An earlier version of the N3-X, the H3.2 was presented in a final report for the NASA N+3 Phase 1 project “Aircraft and Technology Concepts for an N+3 Subsonic Transport” and represents the results of research carried out from 1 September 2008 to 31 March 2010 by a team from MIT, Aerodyne Research, Aurora Flight Sciences, and Pratt&Whitney.

This research results included effects of BLI on a Hybrid Wing Body configuration with an ultra-high bypass ratio distribute propulsion system (two turbo generators and four electrical propulsors). The BLI effect was assessed to contribute in the order of 9.5% to overall efficiency improvement as indicated in Figure 10.

Figure 10: NASA H3.2 Hybrid Wing Body technology contributions to overall efficiency goal [Greitzer, E. M. et al., 2010]

3.4 Double-Bubble Fuselage Concepts

A Double-Bubble Fuselage as a modification of a Conventional Wing-Tube Airframe with conjunction of two cylindrical fuselages to create an unconventional lifting body is a third category of BLI concepts and can be considered as an intermediate step between PFC and HWB. Rear mounted propulsors ingest the boundary layer of the upper fuselage body or at least larger portions of it. A typical concept is illustrated in Figure 14 and 15.

3.4.1 MIT D8 Concept Study

In 2008, the Massachusetts Institute of Technology (MIT), Pratt & Whitney and, Aurora Flight Sciences began an effort sponsored under NASA’s N+3 program to revolutionize how aircraft of the future will be designed. The result was a new aircraft configuration, known as the D8, which is capable of achieving significant reductions in community noise, emissions, and fuel burn. The D8 configuration has the potential of achieving a 71% reduction in fuel burn, a 60 EPNdB reduction in noise, and an 87% reduction in LTO NOx – all relative to current narrow-body aircraft in operation.

The aircraft configuration examined is the D8 double bubble, named for its characteristic fuselage cross section. A 180 passenger, 3000 nm range transport in the Boeing 737 or Airbus A320 aircraft class, this Massachusetts Institute of Technology (MIT)-designed configuration is characterized by a wide twin-aisle lifting fuselage which enables the use of smaller, lighter wings and a pi-tail empennage with the horizontal tail supported by twin vertical tails. The fuselage nose shape provides a nose-up pitching moment that reduces the trimming tail downforce in cruise and further shrinks both the wing and horizontal tail areas. A low-sweep wing that contributes to a lighter structure is made possible by a cruise speed of Mach 0.72, compared with around Mach 0.78 for a conventional tube-and-wing aircraft in the same class.

Recent results [Uranga, A. et al., 2017] present the experimental assessment of the aerodynamic benefit of boundary layer ingestion carried out from 2010 to 2015 as part of a NASA N+3 Phase 2 Program.


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MIT D8 Double Bubble Conclusion

The aerodynamic benefit of boundary layer ingestion was quantified via a direct back-to-back comparison of non-BLI and BLI propulsor installations on the D8 aircraft in 1:11-scale, low-speed powered model wind-tunnel tests conducted in the NASA Langley 14- by 22-foot subsonic tunnel. Using the power balance framework and the mechanical flow power as the performance metric allows for the measurement of the BLI benefit as an aircraft configuration property independently of propulsor-specific characteristics.

The experimental measurements demonstrate an aerodynamic BLI benefit of 8.6% in mechanical flow power required at the simulated cruise condition when the same propulsors are used on both configurations. This benefit level is specific to the D8, for which roughly 13% of the total airframe kinetic energy defect (or surface viscous dissipation) is ingested by the integrated propulsors. Configurations with higher ingestion will have correspondingly larger benefits.

This work represents the first measurement of boundary layer ingestion performance improvements for a realistic civil aircraft configuration and provides a proof-of-concept for the use of boundary layer ingestion to improve fuel efficiency of subsonic transports.


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4. Concept Ideas and Assessment Candidates

This chapter describes concept ideas and potential candidates identified and taken into consideration for down-selection of the most promising concept as a starting point for the detailed work within CENTRELINE main work packages. The overall goal is to further analyse and optimize the T1.2 baseline Propulsive Fuselage Configuration within more detailed tasks, in particular for refinement of turbo-electric systems, structural and aerodynamic integration and experimental validation with the rear fuselage propulsor towards a further optimized and matured PFC aircraft concept.

4.1 Cloud of Ideas and Concept Solutions

The CENTRELINE technology concept aims to realise fuselage wake-filling in the most straightforward way on today’s tube wing configurations, i.e. through a single BLI propulsive device that concentrically encircles the very aft-section of the fuselage. The research and innovation actions in CENTRELINE are performed based on a twin-engine, tube and wing aircraft layout with cantilevered low-wing featuring a dedicated wake-filling propulsive device integrated at the aft-fuselage. The concept is turbo-electrically powered through generators driven by advanced geared turbofan (GTF) power plant systems podded under the wing. In this arrangement, the aft-fuselage propulsor is solely dedicated to the wake-filling purpose while all residual thrust required for the aircraft to operate is delivered from the wing-mounted GTF engines.

On the basis of the Reference Aircraft R2035 which was defined in Task 1.1 [Peter, F. et al., 2017] featuring technologies applicable for a 2035 EIS product a large variety of conceptual ideas and potential PFC candidates have been discussed in order to identify the most promising PFC aircraft configuration.

A prerequisite for an appropriate and successful assessment of overall benefit due to fuselage boundary layer ingestion by installation of a dedicated aft fuselage propulsor is a PFC concept selection with configuration adaptations strictly limited to Rear Fuselage Fan integration in direct comparison to the defined Reference Aircraft. An obvious straightforward approach to identify a potential concept solution is illustrated in Figure 11.

Figure 11: Principal approach for Propulsive Fuselage Concept identification

As an appropriate starting point for the PFC candidate identification, the forward part of the aircraft with its main components (fuselage, wing, GTF under wing, landing gear) are kept as far as possible unchanged while the rear fuselage is adequately adapted to the installation and most promising integration of an additional electrically driven Fuselage Fan propulsion system for boundary layer ingestion. Therefore, a comprehensive cloud of concept solutions including novel concept ideas were created and discussed by the project partners. Important design requirements, multi-disciplinary functionalities and basic system architectural features were considered. Three alternative rear fuselage configuration proposals specifically feasible for turbo-electric Fuselage Fan integration were discussed with pros and cons as follows:


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T-Tail concept

The T-Tail concept as illustrated in principal in Figure 12 was part of the previously conducted DisPURSAL study and an obvious conceptual approach with required specific turbo-electric adaptations for the CENTRELINE project. Initial justification and concerns can be summarized as:

Pros

• Horizontal tail smaller and lighter due to better aerodynamic efficiency and tail arm increase

• Only one structural support in the fuselage for the empennage attachment - more space for propulsor equipment istallation

• Propulsor ingestion flow with only minimized disturbance of root at vertical tail

Cons

• Vertical tail heavier due to loads from the horizontal tail

Figure 12: Rear Fuselage Propulsor with T-Tail proposal

Conventional Tail concept

The consideration of a Conventional Tail concept with integrated Rear Fuselage Fan is a second option (Figure 13). One major aspect to consider this approach is the minor deviation from the Reference Aircraft configuration.

Pros

• Empennage arrangement potentially the lightest

• Mature design and construction principle

• The closest comparison with R2035 aircraft configuration

Cons

• Disturbed PFC flow due to three surfaces in the air intake area of the propulsor

• Shading effects of HTP due to trim effects of stabilizer at angle of attack

• Limited space for propulsor systems and equipment installation and routing

• Structural integration of Fuselage Fan and APU challenging

Figure 13: Rear Fuselage Propulsor with Conventional Tail proposal


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V-Tail concept

The application of a V-Tail concept with integrated Rear Fuselage Fan is a further option (Figure 14). Compared to the other two options, the level of safety, maturity and demonstrated overall benefit in application for large commercial transport aircraft is uncertain. Therefore this proposal was not considered to be taken into the further down selection process. The main aspects are listed as follows:

Pros

• Potentially smallest empennage wetted area result in lowest drag

• Only two instead of at least three lifting surfaces, thus less structural weight

Cons

• Two surfaces against propulsor that could disturb flow

• Shading effects of V-Tail due to trim effects of stabilizer at angle of attack

• More complex flight control system, since two lifting surfaces used to pitch and yaw simultaneously

• Less control surface effectivity, because no control surface is orthogonal to the desirable force vector when pitching or yawing

• Less failure tolerance. If one lifting surface malfunctions, it will be difficult to do manoeuvres

Figure 14: Rear Fuselage Propulsor with V- Tail proposal

An additional candidate beyond the before mentioned proposals has been intensively discussed. A Canard concept featuring winglet yaw control and blown aft rudder represents an alternative configuration with larger deviation from the Reference Aircraft configuration. The case idea of a Canard Configuration as proposed by BHL and TU Delft is based on an obvious benefit of a clean aft propulsor inflow and less complexity for integration of the aft Fuselage Fan. The removal of control surfaces from the tail area is possibly feasible through installation of a forward fuselage canard surface plus integration of a blown rudder (as specifically proposed by TU Delft) at the end of the rear fuselage propulsor outflow.

The resulting three different concept ideas and novel solutions have been developed and discussed with the partners as summarized in Figure 15.

Figure 15: Summary of initial PFC Concept Ideas: T-Tail, Conventional Tail and Canard Concept


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5. Aircraft Layout for Initial Propulsive Fuselage Concept

This chapter presents the summary of the basic design specification of the most promising PFC configuration. The specification with details in Appendix 1 delivers the basis and initial starting point for further PFC design elaboration in WP2 to WP4.

5.1 Basic Design Specification

The initial PFC Aircraft Design characteristics are results of a preliminary multidisciplinary optimization loop [Seitz, A. et al., 2018] within Task 2.5 “Aircraft integrated sizing and optimization” conducted by BHL.

The conducted steps towards the identification and down-selection of the initial PFC concept as performed and described in this report for Task 1.2 and the close link to the MDO sizing loop within Task 2.5 in work package two are the following:

• Down-selection of a most promising rear fuselage adaptation out of transparent conceptual decision making process

• First level integration of main components, e.g. re-shaped aft fuselage section with integrated electric propulsor, turbo-electric drive train, T-Tail etc.

• Initial weight, power and aerodynamic effects assessment with resulting input for aircraft sizing

• MDO Sizing loop for overall configuration characteristics and performance analysis

The resulting re-sized PFC configuration has still the same unchanged forward fuselage. However, other components are impacted and re-sized as illustrated in Figure 16:

• Primary weight changes as a result of component adaptations in rear fuselage and tail plus rear fuselage propulsor structure, rear fuselage propulsor, electrical components like GTF turbo-electric generators and Fuselage Fan motor and related power transmission

• Re-sizing effects due to aerodynamic and propulsion efficiency changes with impacts on overall aircraft flight performance efficiency (fuel consumption, drag), resulting in adaptations of wing area, vertical and horizontal tail area, GTF under wing propulsion characteristics, landing gear weight and height, secondary structural and aerodynamic impacts

Figure 16: CENTRELINE – initial Propulsive Fuselage Configuration design process CENTRELINE – initial Propulsive Fuselage Configuration design process


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5.2 3-View-Drawing

Figure 17: Initial PFC aircraft 3-view-drawing


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This deliverable D1.2 describes the activities performed in Task 1.2 “Concept design space exploration” of work package one of the CENTRELINE project.

A review of state-of-the-art in Fuselage BLI and wake filling findings within theoretical and experimental results was conducted. Fuselage boundary layer ingestion (BLI) and wake filling concepts have already been studied within various European and US research programmes, indicating efficiency and energy saving benefits up to a lower two digit improvement.

A comprehensive cloud of concept solutions including existing and novel concept ideas was created by the project partners, under consideration of important design requirements, multi-disciplinary functionalities, and basic system architectural features. BHL prepared and established a combined set of decisive quantitative metrics and multi-disciplinary qualitative rating criteria enabling a well-structured concept down-selection process within a dedicated workshop involving all partner. The most promising PFC aircraft configuration has been down-selected from a broad variety of conceptual candidates based on a robust and transparent down-selection procedure.

The selected PFC configuration represents a straight-forward approach on the basis of a conventional wing tube airframe and consists of a twin engine aircraft with an under wing GTF UHBR propulsion system plus a dedicated additional turbo-electric propulsive device at the rear end of the fuselage that ingests and re-energizes the fuselage boundary layer flow. This concept is considered with a high potential to meet technical maturity for the envisaged 2035 EIS time horizon. Specific sizing and multi-disciplinary optimisation for the selected and pre-defined PFC aircraft configuration was performed by BHL in Task 2.5 “Aircraft integrated sizing and optimisation” of work package two.

The initial PFC design specification as in detail described in Appendix 1 will form the basis for further PFC design studies and experimental work to be performed in CENTRELINE WP2 to WP4.

.


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6. Acknowledgement

The work presented was supported by expert guidance from Airbus Operations GmbH, Consultáero, ONERA, GKN Aerospace and DLR, which greatly assisted in the PFC definition process during the down-selection workshop. Arne Seitz, Fabian Peter and colleagues from Bauhaus Luftfahrt provided exceptional contributions for PFC concept proposals, workshop directions, related assessment and integrated sizing process. Mark Voskuijl, Arvind Gangoli Rao and Biagio Della Corte from Technical University Delft Delft provided valuable input for a canard concept candidate and for existing BLI studies. Furthermore, Bartłomiej Goliszek and the colleagues of the Politechnika Warszawska provided the structural concept of the PFC T-Tail configuration as well as structural considerations and expertise.


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7. Appendix

Appendix 1: D1.02 Specification of Propulsive Fuselage Aircraft Layout Design Features (Confidential)


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8. Bibliography

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Bradley, M. K., Droney, C. K., “Subsonic Ultra Green Aircraft Research, Phase II: N+4 Advanced Concept Development”, Boeing Research and Technology, Huntington Beach, CA, NASA/CR-2012-217556, May 2012.

Greitzer, E. M., Bonnefoy, P.A., De la Rosa Blanco, E., Dorbian, C.S., Drela, M., Hall, D.K., Hansman, R.J., Hileman, J.I., Liebeck, R.H., Lovegren, J. , Mody, P., Pertuze, J.A., Sato, S., Spakovszky, Z.S. and Tan, C.S. (Massachusetts Institute of Technology, Cambridge, Massachusetts), Hollman, J.S., Duda, J.E., Fitzgerald, N., Houghton, J., Kerrebrock, J.L., Kiwada, G.F., Kordonowy, D., Parrish, J.C., Tylko, J. and Wen, E.A. (Aurora Flight Sciences, Cambridge, Massachusetts), Lord, W.K. (Pratt & Whitney, East Hartford, Connecticut), “N+3 Aircraft Concept Designs and Trade Studies”, Final Report, Volume 1, NASA/CR-2010-216794/VOL1, 2010.

Isikveren, A. T., Seitz, A., Bijewitz, J., Hornung, M., Mirzoyan, A., Isyanov, A., Godard, J.-L., Stückl, S. and van Toor, J., “Recent Advances in Airftrame-Propulsion Concepts with Distributed Propulsion”, ICAS 29th Congress, St. Petersburg, Russia, September 7-12, 2014.

Isikveren, A.T., Seitz, A., Bijewitz, J., Mirzoyan, A., Isyanov, A., Grenon, R., Atinault, O., Godard, J.-L., Stückl, S., "Distributed Propulsion and Ultra-high By-Pass Rotor Study at Aircraft Level", The Aeronautical Journal, Vol. 119, No. 1221, pp. 1327–1376, DOI 10.1017/S0001924000011295., 2015.

Kenway G. K. and Kiris, C. C., "Aerodynamic Shape Optimization of the STARC-ABL Concept for Minimal Inlet Distortion", 2018 AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, AIAA SciTech Forum, (AIAA 2018-1912).

Kim, H. D., Felder, L. J., Tong, M. T. and Armstrong, M., “Revolutionary Aeropropulsion Concept for Sustainable Aviation”, XXI International Symposium on Air Breathing Engines (ISABE 2013), Busan, Korea, 9-13 September 2013.

Mistree, F., Lewis, K., Stonis, L., “Selection in the Conceptual Design of Aircraft”, 32nd AIAA Aerospace Sciences Meeting and Exhibit, January 10-13, 1994, Reno, NV, AIAA-94-4382-CP.

Peter, F., Bijewitz, J., Habermann, A., Plötner, K., Schmidt, M., Shamiyeh, M., Vratny, P., Seitz, A., „Specification of reference aircraft and power plant systems“, Grant Agreement No. 723242, CENTRELINE Project Deliverable D1.1, submitted 28.09.2017.

Peter, F., Seitz, A., Kling, U., Shamiyeh, M. (BHL), Voskuijl, M., Rao, A. G. (TU Delft), “Aircraft Configuration Sketches for the PFC Down-Selection Workshop on 16/11/2017”, Grant Agreement No. 723242, CENTRELINE internal report, 16 November 2017.

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24/04/2018 - CENTRELINE – Propulsive Fuselage Conceptfuselage boundary layer in order to directly compensate the viscous drag effects in the fuselage wake flow field. The content