ISABE-2019-24087 1

ISABE 2019

INTEGRATION ASPECTS FOR LARGE GENERATORS INTO TURBOFAN ENGINES FOR A TURBO-ELECTRIC PROPULSIVE FUSELAGE CONCEPT Rasmus Merkler Rasmus.merkler@mtu.de MTU Aero Engines AG Engineering Advanced Programs Munich Germany

Sebastian Samuelsson Chalmers University of Technology Division of Fluid Dynamics Gothenburg Sweden

Guido Wortmann Siemens AG Erlangen Germany

ABSTRACT

The present paper discusses some aspects of integrating large generators into turbofan engines for a turbo-electric propulsive fuselage concept (PFC) pursued within the European Commission funded collaborative research project CENTRELINE (“ConcEpt validatioN sTudy foR fusElage wakefiLIng propulsioN intEgration”). In this proof-of-concept project a rear-mounted electric fuselage fan ingests part of the fuselage boundary layer. The fuselage fan is powered by power offtakes from two under-wing podded geared turbofan engines. The enabler to generate the electrical power needed to drive the wake filling aft fuselage fan is a sound integration of a large generator (5MW class) into the under-wing podded turbofan engines. Different integration concepts were compared in a down selection process leading to the most promising concept. For the selected concept the most concept-critical topics are discussed.

Keywords: Large power offtake, power generation unit integration concept, thermal management concept, main geared turbofan engine considerations

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NOMENCLATURE

AC Alternating current ACOC Air cooled oil cooler APU Auxiliary power unit BPR Bypass ratio CENTRELINE ConcEpt validatioN sTudy foR

fusElage wakefiLIng propulsioN intEgration

DC Direct current EIS Entry into service FCOC Fuel cooled oil cooler FPR Fan pressure ratio GEN Generator GTF Geared Turbo Fan HP High pressure HPC High pressure compressor HPT High pressure turbine kW Kilowatt LP Low pressure LPT Low pressure turbine MW Megawatt PFC Propulsive fuselage concept PT Power turbine rpm Revolutions per minute TCF Turbine centre frame TEC Turbine exit case TO Take off ToC Top of climb TRL Technology readiness level UHBR Ultra high bypass ratio

1.0 INTRODUCTION

In the turbo-electric PFC arrangement pursued in CENTRELINE [1], the fuselage fan driving power is extracted from the under-wing podded main power plants via electrical generator offtakes, see Figure 1. Starting from a conventional reference aircraft with an entry into service (EIS) year of 2035 the under-wing podded main power plants are resized for the new boundary conditions as shown in [2]. Besides the new thrust requirements one or several electrical generators need to be integrated into the engine environment and the engine needs to be adapted accordingly.

Figure 1 Basic sketch of power plant and generator arrangement [1]

The extraction of power from advanced, ultra-high BPR turbofan engines sufficient to operate the wake-filling fuselage propulsor as shown in [3] poses significant challenges

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for the gas turbine design including cycle definition, flow path layout and turbo component aerodynamic design and performance. These aspects are covered in [2]. Moreover, a number of multi-disciplinary aspects need to be carefully evaluated with regards to the electric generator installation in the gas turbine environment. These include the design of the generator, the mechanical and spatial integration of the electric system components, thermal integration and management/insulation issues, power plant operability and operational flexibility under varying secondary load scenarios. The evolution towards more electric and all electric aircraft, even today, leads to the integration of larger electric machines into the engine. However, generating electric power for propulsion increases the amount of power offtake by more than one order of magnitude. Hence, a new integration concept needs to be developed, as the electric machine now takes a considerable share of the engine with respect to mass, volume and operational impact [1].

The main characteristics of the turbofan engine and the generator as derived in [1] are shown in Figure 2.

Figure 2 Visualization of main design features of the initial CENTRELINE propulsive fuselage configuration [1]

2.0 INTEGRATION CONCEPT SELECTION

A number of generator integration concepts have been evaluated during a preliminary analysis phase in the project and have been studied in a joint integration effort of engine and generator designers regarding their feasibility and impact on the main engine. Some examples are depicted in Figure 3.

Figure 3 Basic sketches of three integration options for the turbo electric generators within the under-wing podded power plants [4]

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2.1 Concept down selection considerations Multiple integration concepts have been created and assessed with respect to generator sizing constraints, expected engine operability issues and engine weight and balance impacts. The main areas of interest during the evaluation process were:

Design of the power plant and the generator. Mechanical and spatial integration of the electric system components. Thermal integration and management. Power plant operability and operational flexibility under varying secondary load

scenarios. Potential electrical engine start-up capability. Potential electric taxi capability. Impact of failure cases.

Mechanical power can be drawn from the low-speed part of the LP shaft (fan speed), the high-speed part of the LP shaft (LPT speed), the HP shaft, or from multiple shafts. To identify the best configuration, the following points need to be taken into account:

The power offtake may be independent from the engine thrust setting. The power offtake configuration shall ensure a minimum impact on engine

component stability margins during part power operation. The electric machine shall be integrated in a way to exploit the maximum power

density (or minimum mass). Maintainability should be considered.

If some electromagnetic parameters of electric machines are assumed constant, for a given power, the power density of electric machines is proportional to the circumferential speed of the rotor. Hence, to minimize the machine mass, the circumferential rotor speed must be maximized. The circumferential speed is limited by the mechanical properties of the rotor shaft and the magnet sleeve. A circumferential speed of 180m/s is considered as the current limit of surface-mounted permanent magnet machines. To achieve the minimum mass from a machine point of view, the machine shall be sized for a circumferential speed of 180 m/s. Figure 4 shows the circumferential speeds throughout the engine at different radial positions for take-off conditions.

Figure 4 Circumferential speeds of the podded engine (take-off condition)

2.2 Evaluation of different generator location options 2.2.1 Fan section When looking at the fan section, a generator integrated in the hub and driven by the fan spool speed suffers from low circumferential speed and, hence, will have a low power density (Figure 5 bottom). If the generator is attached to the LP spool, the spool must be extended through the gear system or an additional gear system is necessary (Figure 5 top).

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Figure 5 Integration in the front end of the engine (fan hub)

The integration of an electric machine at the fan tip seems unfeasible for this machine type as it significantly exceeds the achievable circumferential speeds of this machine type (Figure 6 top). Another idea was to integrate the generator rotor in the fan blades (like snubbers on old fan blades) and to place the stator in the stator vanes (Figure 6 bottom). This configuration results in excessive air gaps, which will considerably decrease the performance of the machine.

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Figure 6 Integration in the front end of the engine (fan tip/splitter)

2.2.2 Mid-engine section The optimum radius for 180m/s may be achieved with a machine, which encircles the booster stages in the core cowl and runs at fan speed (Figure 7 top). This position enables air cooling in the bypass duct. However, in this configuration it is quite challenging to route the mechanical load paths from the fan to the external casing and the attachment to the fan spool is challenging as well.

If the generator is running at the fast-rotating LP shaft speed it may fit in the hub of the booster stages. As the booster disks are located in the hub, the machine may not be fitted inside the booster hub, but the section between booster and HPC may be extended to fit the electric machine in between (Figure 7 bottom). In this location, it may also be sized for the HP shaft speed and connected to the HPC. One issue which arises for this integration concept is that the compressed air at the booster outlet is already so hot that the magnets need to be actively cooled with cooler air.

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Figure 7 Integration in mid-engine section

2.2.3 Turbine section At the LPT stages, the circumferential speeds and the available space in the LPT hub look promising (Figure 8). Again, the machine cannot be integrated inside the LPT disks, but an integration behind the LPT in the hub section seems feasible. This position is also the best with respect to the lever arm of the electric machine to the wing, which minimizes the bending moment on the pylon. Additionally, this position allows to split the LPT in two separate turbines. The LPT, which is sized to drive the reference engine booster and fan, has four stages. Due to the lower thrust demand of the resized engines the LPT stage number is reduced to three, which allows to operate the booster and the fan. The mechanical power for the generator is generated by a single-stage free power turbine, which is mechanically decoupled from the LP shaft. The turbines are coupled aerodynamically, however, the mechanical decoupling allows to buffer torque oscillations and peaks which are generated by the electric machine during failure cases and abnormal operation. This integration concept is also the most promising solution to minimize the impact on engine stability limits during part power operation. Especially, as the power offtake may be independent from the engine thrust setting and the generator may run at a freely selected rotational speed. The big drawback of integrating the generator in the LPT hub is the hot environment, which requires extensive cooling of the rotor and the stator.

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Figure 8 Integration in the back end of the engine

2.3 Evaluation of different power electronics location options The rectifier consists of multiple modules, which rectify the AC power from the generator to DC power. The core part of these modules are semiconductor switches, which represent a big share of the converter cost. For applications, which target the mass market, the semiconductor switches are stressed to their limit to use the smallest switch possible. The high loading leads to high losses and high operating temperatures of the chips. By reducing the loading of the chips and distributing the current to multiple chips, the losses and the operating temperature can be reduced. Through the reduction of the losses that each chip generates, air cooling of the semiconductor switches is considered possible.

Integrating the switches in the engine in such a way, that the air cooling works as soon as the engine is operating, would greatly increase the intrinsic safety of the rectifier. The only place where the air flow is cool enough to cool the power electronics is in the bypass flow. Two different integration options are shown in Figure 9. The semiconductor switches could be arranged directly at the duct surface to transfer the waste heat directly to the bypass air flow.

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Figure 9 Air cooled power electronics integration concept

2.4 Concept down selection The above detailed geometric assessment shows that there are only non-optimal solutions for the generator from an optimal power density point of view that operate at the fan speed. Electric machinery in the hub region of the fan suffer from too low circumferential speed and for the integration in the tip, the circumferential speed is too high. For the LP and the HP shaft, the optimum air gap diameters are well below the flow path radii, thus allowing for integration of a generator in the hub region of the core engine. As the disks occupy the space within the compressors, the generator can only be integrated between core engine components below a stretched flow path, adding length to the engine. For the integration in the back end of the engine the geometrical constrains are the most favorable. The straightforward approach to integrate the generator after the LPT directly driven by the LP shaft comes with some operational issues. The integration of a separated free power turbine only driving the generator offers the possibility to decouple the generator mechanically from the main engine. Hence, torque ripples and spikes are not transmitted to the engine shafts, which would increase the mechanical load on the blades. Both integrations in the back of the engine require active cooling, adding complexity and weight to the integration option.

For the option directly driven by the LP spool, potential electric engine start-up and taxi would be an option, whereas a shutdown generator at windmill would not allow the engine to spool up again. The opposite is true if the generator is driven by a free power turbine and is mechanically decoupled from the LP spool.

Figure 10 Down selection of integration concept

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Despite the higher cooling effort for integrating the generator in the hub downstream of the LPT, this concept was identified as the most promising solution (Figure 10). Taking the operational aspects and failure cases into account the option driven by a free power turbine was selected.

3.0 ELABORATION ON CHOSEN INTEGRATION CONCEPT

The evaluation of a number of different concepts shown in Figure 10 lead to one concept to be further pursued and two backup concepts in case the prime path does not work. The power generation unit consists of the electric machine, which converts mechanical to electrical power and the converter, which converts the AC power from the electric machine to DC power.

3.1 Power generation unit integration concept Figure 11 depicts the conceptual general arrangement of the podded engine with the integrated power generation unit. The generator is integrated in the hub region of a free power turbine (PT), which is installed on a separate shaft behind the fan-driving high-speed LPT. The free power turbine speed enables the generator to operate with favorable circumferential speeds and in the backend of the engine the generator geometrical design is not as restricted by tight keep out zones as would be the case with an installation in the front end of the engine. The decoupled arrangement allows reducing the impact on the engine during a generator failure. All other integration options resulted in worse circumferential speeds of the machine and, therefore, higher masses. Operating the generator from a free PT significantly alleviates turbo component instabilities encountered during engine part power and abnormal operation when the large-scale electrical machinery is mechanically coupled to one of the gas turbine shafts [5]. The generator integration behind the LPT has a positive effect on the power plant system’s center of gravity location, reducing the bending moment on the pylon. Moreover, the available space and the independent rotational speed allow exploiting the maximum circumferential speed of the machine to minimize the generator mass.

Figure 11 Chosen power generation unit integration concept

As the power electronics do not need to be located right beside the generator the rectifier is integrated on the surface of the bypass duct to allow optional passive air cooling. The outer bypass duct surface seemed to be better suited for the integration of the power electronics as the core engine section below the bypass duct inner surface is already densely packed with components.

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3.2 Thermal management concept for power generation unit In order to generate the 4.6 MW at an efficiency of 95% the generator generates 230 kW waste heat. Particularly challenging for the chosen system arrangement are the ambient thermal conditions for the generator, which will increase the cooling effort, and the additional challenge of the power cables needing to be routed through the hot gas path of the power plant. Generally, the whole power generation unit can be located at the same position or apart. Taking the harsh thermal environment at the back end of the engine and the high heat generation of the generator it was decided to locate the power electronics in the cold part of the engine and explore an air-cooled concept. Generator and power electronics are connected via AC cables that have to be insulated to withstand the hot condition on its routing routes through the turbine exit case (TEC) and core compartment of the engine.

Heat losses within the generator which originate from the ohmic losses in the stator copper coils, hysteresis losses in the stator iron, rotor windage, losses of bearings and dynamic bearing chamber seals will require a heat management system. The high ambient temperature requires additional cooling to remove the heat from the generator. The flow rate of the cooling fluid depends on its inlet temperature, the permissible outlet temperature and its specific heat.

Different cooling options have been investigated:

Air cooling proved inefficient, considering the large quantity required and the available cross sections for air supply and cooling pipes inside and outside of the generator. Furthermore, there is no engine source where the air temperature is low enough and the pressure high enough to deliver the required flow rate. The high air flow would also have a negative effect on engine fuel burn.

Fuel cooling has not been explored as the prime concept for cooling, since any fuel leak at the hot engine exhaust could be a flight safety risk. Fuel coking in the pipes to and from the generator could be expected as well. Furthermore, fuel cannot be used for efficient lubrication of generator bearings and bearing chamber seals.

Oil cooling turned out to be the preferred option. It offers good heat transfer, is well suited for the temperatures encountered, oil leaks are less dangerous than fuel leaks, and it can also be used to lubricate the bearings and seals. This solution is in line with most large generators, which use their own oil cooling circuit (if engine mounted) or have an oil circuit integrated with the aircraft gearbox or APU.

Since the generator is installed at a position surrounded by hot exhaust gases, the ambient temperature is higher than that imposed on conventional externally mounted generators. The gas temperature, which surrounds the tail cone, has a temperature of up to 500 °C which drives the generator surrounding temperature to reach up to 400 °C. Figure 12 provides the typical maximum ambient temperature in the tail cone region of a running engine. Insulation measures will be required to reduce the heat flux to the generator.

Figure 12 Generator thermal environment

Temperature soak-back from hot components (mainly turbine disks) after engine shut-down is a more severe problem, since normally there is no longer active cooling available. This effect would be most severe in the case of an emergency shut-down from take-off power (see Figure 13). Soak-back temperatures have been estimated and are based on measurements on a comparable engine in service today. It has been assumed that a

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generator internal-temperature peak of 200 °C could be tolerated during an emergency shut-down starting at the conditions of Figure 12. With an estimated exhaust cone temperature profile that requires 40 minutes to cool down to 200 °C, this would require thermal insulation.

Figure 13 Typical temperature characteristic at emergency shut down during take-off

Shielding from the harsh thermal environment and active oil cooling of the generator is envisioned through jacket cooling using oil (see Figure 14).

Figure 14 Generator thermal management concept

Total mass of the thermal management system of the generator is summarized in Figure 15.

Figure 15 Generator cooling system mass

For the rectifier, a trade-off analysis has been carried out [6], which showed that an air-cooled rectifier with a low current loading on the semiconductor chips represents the most promising configuration compared to highly loaded chips, which require a heavy active cooling system. The air-cooled chips require a particular integration, because the thermal capacity is very low, and the chips may get damaged when they are operated without a cooling air flow. The rectifier is integrated into the tip surface of the bypass duct. This location ensures a cooling air flow as soon as the engine is running and is less packed with other externals as the inner bypass duct. The drawback of this location is the high sensitivity of pressure losses, which the cooling fins create in the bypass duct, because the engine specific-fuel consumption is very sensitive to pressure losses in this section. Figure 16 shows the integration concept of the rectifier in the outer surface of the bypass duct and Figure 17 shows a possible rectifier chip arrangement in detail.

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Figure 16 Rectifier integration in bypass duct

Figure 17 Rectifier chip arrangement

The purple bars represent current bars, which are connected to the electric machine in the core engine. They may be solid bus bars or flexible cables. They may be guided through the bypass stator vanes or the struts. The semiconductor switches are located on the green circuit boards. The switches are cooled by aluminium heat spreaders (cyan), whose fin structure intrudes the bypass air flow. The red connector is the converter DC output. The grey box downstream of the circuit board contains a capacitor, which stabilizes the output DC voltage.

The air-cooling fins are adopted from existing engine surface oil coolers. The assumed geometry is shown in Figure 18. The fins have a thickness of 2 mm, a height of 35 mm and are spaced by 2 mm. The fins are mounted on a baseplate, which has a thickness of 2 mm. The rectifier of the generator requires a fin area of 1.6 m².

Figure 18 Cooling fin geometry [6]

Total mass of the power electronics and area occupied by the cooling fins in the bypass duct are summarized in Figure 19.

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Figure 19 Power electronics wetted area and mass

3.3 Main engine design considerations Several different options for the integration of the generator into the power plant have been considered. For the chosen integration option different bearing and turbine case arrangements are presented in this section. Furthermore failure cases and maintenance considerations have been evaluated.

The first possibility is to include a casing between the LPT and the PT and have two different bearing attachments, one through the casing between LPT and PT and one through the casing behind the PT. The advantage of this configuration is its relatively generous space for bearings. The drawback is the additional weight and length introduced through the additional casing, as well as the added pressure loss due to the casing. This arrangement is schematically represented in Figure 20. It should be noted that a final LPT design would have a less aggressive hade angle [2].

Figure 20 Bearing arrangement option with casing between LPT and PT.

The second option is to use an intermediate shaft bearing as shown in Figure 21. Here there is no casing between the LPT and PT. The bearing is attached to the TEC and an additional bearing is installed between the PT and LP shafts. This arrangement will have a lower weight than the first option, due to the removal of the turbine frame between LPT and PT. The intermediate shaft bearing is slightly more complex but also stabilizes the LP shaft, which is beneficial for the moments and tip clearances in the LPT.

Figure 21 Bearing arrangement option with an intermediate shaft bearing

The third integration option is to have an overhung LPT, i.e. to have the LP shaft bearing attached through the turbine center frame (TCF) that is located between the HPT and LPT (Figure 22). The PT shaft bearing is then attached to the turbine exhaust case (TEC). This concept might lead to larger tip clearances in the LPT, causing an efficiency penalty. Also, there is a risk of higher stress levels in the LPT due to the higher moments associated with this arrangement.

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Figure 22 Bearing arrangement option with overhung LPT

Since the configuration with a free power turbine is a new concept it carries an inherent risk. Therefore a backup solution without a free power turbine is kept in mind. Its bearing arrangement is shown in Figure 23.

Figure 23 Fall back option with generator on LP shaft

From a bearing arrangement perspective, it can be seen that two viable options exist for an integration configuration without an additional casing between the LPT and PT. Since this configuration is beneficial from a weight point of view, this is seen as the currently most promising option.

In case of a failure the generator shall remain intact to prevent the parts from exiting the casing (containment). The system design shall also prevent the occurrence of an electrical fault resulting in a fire.

The location at the rear end of the engine between turbine exit casing and tail cone enables a disassembly on wing. An infield removal within 15 or 45 minutes, depending on the application, is possible. Thus, the generator does not have to fulfil similar reliability rates as the turbomachinery.

3.4 Main engine performance and mass consideration The cooling oil of the generator needs an air-cooled oil cooler (ACOC) which is located at the inner part of the bypass duct. The power electronics are air-cooled and located at the outer part of the bypass duct. The additional pressure loss in the bypass duct of both components are 0,2 % and fall in line with losses in existing engine (Figure 24). They need to be taken into account in the engine design [2].

Figure 24 Performance impact of power generation unit

To integrate the results of this study into the engine and overall aircraft design process, a mass estimation of all components of the power generation unit was conducted. For the reference parameters, which are given in [6], the component masses shown in Figure 25 have been derived as follows:

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The mass of the generator is obtained by multiplying the mass of the active parts, which is calculated from the electro-magnetic design by the software SPEED [6], with a factor that reflects the mass share of the passive parts, like winding heads, the casing, bearings and the rotor shaft. The mass of the AC cables is obtained from the power cable model, which was described in [6] and the according current levels. The mass of the cooling system is estimated from the maximum machine losses and an empiric factor, which reflects the mass of the cooling system designed for typical aerospace environment conditions. The results were crosschecked with masses in existing applications and they fall into the same ballpark. For the given TRL of the CENTRELINE project and, taking the fidelity of the other models into account, the mass of the thermal management system seems feasible. The masses of the power electronic devices and the cooling fin structure are estimated from the pre-design tool and the surface specific mass described in [6].

Figure 25 Weight impact of the power generation unit

The thickness of the power electronic devices is roughly 5-6 cm and depends on the size of the integrated semiconductor case. The required area is determined from the maximum power level that the rectifier and the inverter need to convert. The rectifier of the generator requires a fin surface area of 1.6 m². The axial length is 17 cm, when the rectifier encircles the entire duct at the outer diameter.

4.0 CONCLUSIONS AND FUTURE WORK

The integration concept of the electric power generation unit components into a turbofan engine along with some preliminary sizing results have been presented, which are investigated in the frame of the CENTRELINE project.

First the integration concept selection is shown. Different generator and power electronics locations are discussed. The result is a generator that is integrated together with a free power turbine downstream of the LPT and runs on a separate shaft driven by the free power turbine at constant speed. This integration concept comes with the maximum power density (or minimum mass) of all concepts studied. The power electronics are located in the bypass duct enabling air-cooling.

For the chosen integration concept a thermal management strategy was developed: The power electronics are passively air cooled in the bypass duct. The generator is oil cooled and shielded from the harsh thermal environment by a jacket. The oil is cooled using an ACOC located in the bypass duct. This oil cooling and the air cooling of the power electronics generate an additional pressure loss that has to be taken into account in the performance calculation [2].

The performance impact of the ACOC in the bypass duct is one of the main negative impacts of this cooling system. Future work includes using an fuel cooled oil cooler (FCOC) and recirculating the fuel back to the fuel tank. Another study looks into cooling the magnets of the generator using air taken from the spinner region of the engine and routed through the low pressure spool shaft (Figure 26).

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Figure 26 Option for air cooled magnets

For the geometrical integration of the generator a bearing and turbine case arrangement has been proposed: There is no turbine case between the LPT and the free power turbine. A TEC is located behind the free power turbine. Through the struts of the TEC the oil for the generator cooling, the AC cables from the generator to the power electronics and the structure supporting the bearing can be routed. Between the shaft of the free power turbine and the LPT an inter-shaft bearing is envisioned. This arrangement has been fed back into the design process of the turbomachinery [2].

A weight estimate of the components for the whole power generation unit was presented. Together with the weight reduction of the geared turbofan engine shown in [2] this is needed on overall aircraft design level to evaluate the overall impact.

ACKNOWLEDGMENTS

This paper is based on the work performed by the CENTRELINE project consortium. The CENTRELINE project has received funding from the European Union’s Horizon 2020 research and innovation program under the Grant Agreement No. 723242.

REFERENCES

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[2] S. Samuelsson, R. Merkler and G. Wortmann, “Adaption of a turbofan engine for high power offtakes for a turbo-electric propulsive fuselage concept,” in ISABE, Canberra, Australia, 2019.

[3] F. Troeltsch, J. Bijewitz and A. Seitz, “Design Trade Studies for Turboelectric Propulsive Fuselage Integration,” in ISABE, Canberra, Australia, 2019.

[4] A. Seitz, “H2020 CENTRELINE – Project Preview,” in Presentation at 7th EASN International Conference, Warsaw, Poland , 2017.

[5] A. Seitz, M. Nickl, A. Stroh and P. C. Vratny, “Conceptual Study of a Mechanically Integrated Parallel-Hybrid Electric Turbofan Design,” in EASN, Warsaw, Poland, 2017.

[6] G. Wortmann, “Electric Machinery Preliminary Design Report (D4.04),” in CENTRELINE public deliverable, 2018.