Distributed Propulsion Vehicles

TH 27 INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES

DISTRIBUTED PROPULSION VEHICLES

Hyun Dae Kim NASA Glenn Research Center Cleveland, Ohio, USA Keywords : distributed propulsion, turboelectric, subsonic, superconducting Abstract CESTOL cruise-efficient short take-off and landing CFF cross flow fan Since the introduction of large jet-powered C lift coefficient L transport aircraft, the majority of these vehicles eBPR effective bypass ratio have been designed by placing thrust-generating FPR fan pressure ratio engines either under the wings or on the fuselage HWB hybrid-wing-body aircraft to minimize aerodynamic interactions on the MIT Massachusetts Institute of Technology vehicle operation. However, advances in NASA National Aeronautics and Space Admin- computational and experimental tools along with istration new technologies in materials, structures, and OPR overall pressure ratio aircraft controls, etc. are enabling a high degree RTO rolling take-off of integration of the airframe and propulsion SBIR Small Business Innovative Research system in aircraft design. The National SFW Subsonic Fixed Wing Project Aeronautics and Space Administration (NASA) SHP shaft horse power (1 SHP ~0.746 kW) has been investigating a number of revolutionary STOL short take-off and landing distributed propulsion vehicle concepts to TeDP turboelectric distributed propulsion increase aircraft performance. The concept of TRL Technology Readiness Level distributed propulsion is to fully integrate a TSFC thrust specific fuel consumption propulsion system within an airframe such that UAV unmanned aerial vehicle.

the aircraft takes full synergistic benefits of coupling of airframe aerodynamics and the propulsion thrust stream by distributing thrust 1 Introduction using many propulsors on the airframe. Some of NASA has been investigating distributed the concepts are based on the use of distributed propulsion concepts applied to future aircraft jet flaps, distributed small multiple engines, gas- under the Subsonic Fixed Wing (SFW) Project.

driven multi-fans, mechanically driven multi- In response to growing aviation demands and fans, cross-flow fans, and electric fans driven by concerns about the environment and energy turboelectric generators. This paper describes usage, the SFW Project identified four ‘corners’ some early concepts of the distributed propulsion (goals) of the technical trade spaces—fuel burn, vehicles and the current turboelectric distributed emissions, noise, and field length—for future propulsion (TeDP) vehicle concepts being aircraft design. Table 1 lists these technology studied under the NASA’s Subsonic Fixed Wing goals for three future time frames, where N+1, (SFW) Project to drastically reduce aircraft- N+2, and N+3 represent achieving a related fuel burn, emissions, and noise by the “Technology Readiness Level” (TRL) [1] of 4 to year 2030 to 2035.

6 by years 2015, 2020, and 2025 respectively.

Although it may not be feasible to meet all the goals for each time frame, the multi-objective Nomenclature studies will attempt to identify possible vehicle ADP aerodynamic design point concepts that have the best potential to meet CAEP Committee on Aviation Environmental the combined goals. To improve vehicle Protection HYUN DAE KIM performance in meeting NASA’s N+3 goals, Based on the above description, specific drastic changes in propulsion and airframe concepts qualified to be examined further for systems are required and proposed. One such application in distributed propulsion aircraft proposed concept is based on a distributed design are explained below.

propulsion system using advanced electric power generation and transfer of power to remotely 2.1 Jet Flaps located distributed electric fans. This concept is The jet flap is a concept where a high-velocity called turboelectric distributed propulsion thin jet sheet emanates from a tangential slot at (TeDP). Because this revolutionary propulsion or near the wing trailing edge and provides concept may provide such high performance and spanwise thrust for cruise and supercirculation the possibility of meeting the N+3 goals, some for high lift around the whole wing section previous and current research activities in the during take-off and landing. One such research area of distributed propulsion study are presented aircraft utilizing this concept was the Hunting as background for the TeDP concept.

H.126 aircraft, shown in Fig. 1, that was built and flown in the 1960’s at lift coefficient C = 7.5 and maximum operationally usable 2 Types of Distributed Propulsion L C = 5.5. To enable such high lift, the engine L A number of fixed wing aircraft using diverted almost 60% of its thrust across its wing ‘distributed propulsion’ have been proposed and trailing edge to achieve very high lift capability.

flown before. Because what constitutes distributed propulsion for aircraft is not clearly defined—any aircraft with more than one propulsor could be classified as such—the following description is applied to further reduce the number of possible vehicle configurations: “Distributed propulsion in aircraft application is the spanwise distribution of the propulsive thrust stream such that overall vehicle benefits in terms of aerodynamic, propulsive, structural, and/or other efficiencies are mutually maximized to enhance the vehicle mission.” DISTRIBUTED PROPULSION VEHICLES 2.2 Cross-Flow Fan The cross-flow fan (CFF), or transverse fan, is a two-dimensional spanwise propulsor that is integrated within a wing structure to distribute the thrust along the wingspan. Fig. 2 shows one such configuration where the fan ingests the wing upper and lower surface boundary layer air and ejects the air at the wing trailing edge [2]. In this configuration, two gas generators mounted at the wing root and the wing tip transmit the power to the CFF rotors that are placed near the wing trailing edge and connected by flex- couplings or universal joints. However, because of low performance of the fan and difficulty of installation within an aircraft wing structure, this transport concept was never put into practice.

Recently the CFF concept has been studied in more detail [3,4], and fan efficiency has been improved using computational fluid dynamics 2.3 Multiple Discrete Engines (CFD) technique. Some of the current CFF Various types of aircraft using multiple technology applications are proposed in propulsors have been proposed and flown. For personal air vehicles (PAV) and unmanned these aircraft, propulsors such as propellers, aerial vehicles (UAV). Fig. 3 shows a UAV turbojets, or turbofans are mounted in front of flight demonstrator concept using the CFF as a the wing, at the back of wing, or within the propulsor [5].

thick section of wing. Although the number of propulsors required to be considered as distributed propulsion for a vehicle is not clearly defined, one can consider the 1940’s YB-49 flying aircraft as an example. It had four linearly arranged conventional turbojet engines in each side of wing with subsonic rectangular inlets at the leading edge and conventional circular nozzles at the trailing edge of the wing.

Recently, a cruise-efficient short take-off and landing (CESTOL) aircraft was proposed (Fig. 4) based on a high subsonic hybrid wing body (HWB), or blended wing body (BWB), transport configuration because of its high cruise efficiency, low noise characteristics, and a large internal volume for integrating embedded distributed propulsion system [6].

The propulsion system employed 12 small conventional engines partially embedded within the wing structure and mounted along the wing upper surface near the trailing edge to enable short take-off and landing (STOL) operation using low-pressure-fan diverted-bypass air. The vehicle concept uses distributed propulsion for HYUN DAE KIM quiet powered lift using an internally blown flap, with substantial engine noise shielding effect by the airframe, rapid climb out, and steep descent approach to provide a very low noise footprint on the ground. These characteristics of the aircraft may enable 24-hour use of the underutilized regional and city-center airports to increase the capacity of the overall airspace while still maintaining efficient high subsonic cruise flight capability.

2.4 Distributed Multi-Fans Driven by Few Engine Cores Distributed propulsion employing multiple that drove high-bypass-ratio turbofans. The propulsors driven by a few fuel-efficient engine turbofans and turbines were co-located in the cores has been studied and is being pursued wing section away from the gas generators. The under NASA’s SFW N+3 project. Under this hot gases from the gas generators were routed category, three types of propulsion system are through long ducts across the wing span to the identified and described below.

location where the turbines and fans were 2.4.1 Gas-Driven Multi-Fans installed. The inlets and nozzles for the turbofans and turbines were also all within the In the late 1960’s, a vertical/short take-off and landing (V/STOL) air-deflection and modulation wing structure away from the gas generators and (ADAM III) fighter concept shown in Fig. 5 was provided distributed thrust to the vehicle.

studied for various missions, but the design never Then in the 1970’s, a gas-driven multi-fan went into production possibly because of the transport aircraft was conceived, and a model was tested for STOL operation. The aircraft problem of ducting hot gas through the wing structure. [7] In this concept, the gas generators shown in Fig. 6 was based on a conventional and their inlets were installed near the fuselage to ‘tube and wing’ airframe configuration with 16 provide hot gas to the wing mounted turbines tip-driven fans spread along the top surface near DISTRIBUTED PROPULSION VEHICLES the wing trailing edge [8]. The tip-driven fans with fan pressure ratio of 1.25 were powered by high-pressure discharge air from the low- pressure compressor stages and mounted on a hinged flap to achieve high lift via supercirculation. In addition, the massive suction effect in front of inlets created additional lift on the airframe and delayed flow separation on the wing upper surface.

2.4.2 Gear-Driven Multi-Fans A distributed propulsion concept employing a dual fan driven by one engine core on a HWB airframe was recently studied by NASA [9]. The study was to determine the effects of a dual-fan engine configuration on the vehicle-level performance (i.e., range) of a representative subsonic transport and to develop a preliminary understanding of the challenges associated with the implementation of distributed propulsion schemes. The Fig. 7 shows one such concept where an engine core drives two large-diameter similar gear-driven multi-fan propulsion fans via gears and shafts, providing a very high concept [10–12]. The purpose of this study was bypass ratio. In this configuration, the core to design an aircraft with noise being the engine is outside the airframe boundary layer primary design variable addressed, such that the flow with almost 100% inlet total pressure noise would be contained within the perimeter recovery, and the dual fan ingests full boundary of an urban airport. This aircraft, shown in layer flow approaching the inlet cowl lip.

Fig. 8, employs three engine nacelles where For the Silent Aircraft Initiative, the each nacelle houses three fans that are Cambridge-MIT Institute developed the connected to a single engine core through gears SAX-40 conceptual HWB aircraft using a and shafts. Similar to NASA’s study, this propulsion concept also has a very high bypass ratio and low engine noise. Also, it features inlets with a high amount of airframe upper surface boundary layer ingestion.

2.4.3 Electrically Driven Multi-Fans To improve performance and to reduce environmental impacts even further, a drastic change in the power transmission of distributed propulsion system for large transport aircraft was proposed and studied on HWB as well as tube and wing airframes [6, 13–19]. Using a new concept called ‘turboelectric distributed propulsion (TeDP)’, one of the vehicles adopts the previous 12-engine CESTOL-HWB airframe but employs two remotely located gas- turbine-driven superconducting generators to drive the distributed fans instead of using many small conventional engines. The power to drive HYUN DAE KIM these electric fans is generated by two remotely • For the multi-fan/single engine core concept, located gas-turbine-driven superconducting the propulsion configuration provides a very generators. This arrangement allows the use of high bypass ratio enabling low fuel burn, many small partially embedded fans while emissions, and noise to surrounding retaining the superior efficiency of large core communities.

engines, which are physically separated but connected to the fans through electric power 4 Turboelectric Distributed Propulsion lines. Since this concept is one of the several concepts pursued by NASA to meet N+3 goals, As seen in Table 1, NASA’s current SFW it has become a new area of research at NASA Project introduces four corners of design trade and will be further described in Section 4.

spaces—in terms of fuel burn, emissions, noise, and field length—to define vehicle concepts for different future time frames, where N+1, N+2, 3 Benefits of Distributed Propulsion and N+3 represent achieving a TRL of 4 to 6 by As suggested in the above sections, the benefits the years 2015, 2020, and 2025 respectively. In of using distributed propulsion for aircraft could particular, N+3 vehicle goals are defined as be found in improvement in aircraft reaching better than 70% fuel burn reduction performance, noise reduction to the surrounding and better than 75% landing and take-off community, and/or providing the capability of nitrogen oxides (NOx) reduction, achieving STOL. Specifically, the following possible –71dB cumulative below Stage 4 noise benefits of distributed propulsion concepts have regulation, and exploiting metroplex airport been identified through various studies operational concepts, compared to current state- mentioned in previous section: of-the-art aircraft. Although it may not be feasible to meet all the goals simultaneously, • Reduction in fuel consumption by ingesting multi-objective studies are being attempted to the thick boundary layer flow and filling in identify possible vehicle concepts that have the the wake generated by the airframe with the best potential to meet the combined goals. To distributed engine thrust stream.

meet these aggressive goals, drastic changes in • Spanwise high lift via high-aspect-ratio vehicle and propulsion system designs are trailing-edge nozzles for vectored thrust required and proposed. One of the proposed providing powered lift, boundary layer propulsion systems that may enable meeting the control, and/or supercirculation around the N+3 goals is called “Turboelectric Distributed wing, all of which enable short take-off Propulsion (TeDP)”. The concept employs a capability.

number of superconducting electric motors to drive the distributed fans rather than using • Better integration of the propulsion system mechanical shafts and gears. The power to drive with the airframe for reduction in noise to the these electric fans is generated by remotely surrounding community through airframe located gas-turbine-driven superconducting shielding.

electric generators. This arrangement enables • Reduction in aircraft propulsion installation the use of many small distributed fans, allowing weight through inlet/nozzle/wing structure a very high effective bypass ratio (eBPR), while integration.

retaining the superior efficiency of large core engines, which are physically separated but • Elimination of aircraft control surfaces connected to the fans through superconducting through differential and vectoring thrust for electric power lines. Although various aircraft pitch, roll, and yaw moments.

configurations using TeDP are possible, three • High production rates and easy replacement studies have recently been undertaken; their of engines or propulsors that are small and features and important results are presented light.

here.

DISTRIBUTED PROPULSION VEHICLES 4.1 N3-X Vehicle Concept by NASA advantage. In the aircraft configuration examined the turbogenerators were located at the wing tips The HWB vehicle concept using the TeDP was where the turbogenerator would experience based on the earlier CESTOL airframe undisturbed free-stream conditions, while the configuration shown in Fig. 4 and modified with fan modules were positioned in a continuous fan a new TeDP system [15]. To simplify the nacelle across the rear fuselage where they ingest propulsion system effects on the vehicle, a more the thick boundary-layer flow, fill the wake of refined conventional take-off and landing aircraft with fan discharge air, and thereby (CTOL) ‘N3-X’ vehicle concept shown in Fig. 9 reduce the thrust required by the vehicle. The was proposed and is currently being analyzed in initial result for the propulsion system, using greater detail. The airframe is derived from N+2 engine component technologies, consists of Boeing’s N2A HWB configuration [20] with two turbogenerators, each producing 53,900 shaft similar mission characteristics of a 6,000-nmi horsepower (SHP) (40.2 MW), and 14 fans of (11,112-km) range, a 103,000-lb (46,720-kg) 50.6-in. (128.5-cm) diameter driven by 7,700-SHP payload capacity, and the ability to fly at the (5.74-MW) motors, both at the rolling-take-off aerodynamic design point (ADP) of Mach 0.8 at (RTO) design condition. The thrust specific fuel 31,000 ft (9445 m) altitude. The propulsion consumption (TSFC) is estimated to be 0.2781 at system utilizes superconducting electrically RTO and 0.4685 at the ADP. Table 2 shows the driven, distributed low-pressure-ratio (1.35) vehicle mission requirements and preliminary fans with power provided by two remote engine cycle analysis results using the same superconducting electric generators based on a vehicle size as the N2A airframe. This study, conventional turbofan core engine design [17].

including vehicle resizing, is ongoing at NASA The use of electrical power transmission allows to obtain more refined data in terms of vehicle a high degree of flexibility in positioning the fuel burn, emissions, and noise reduction for the turbogenerators and propulsor modules to best mission specified.

HYUN DAE KIM rib structure provide stress relief to the wing structure. In addition, a favorable aerodynamic advantage exists such that at low speed, thrust vectoring of a two-dimensional low temperature nozzle may provide supercirculation of airflow around the airfoil for a large improvement in lift coefficient. Another key feature of the concepts 4.2 “ECO-150/250” Configuration by is the use of liquid hydrogen both as a cooling Empirical Systems Aerospace fluid for the superconducting system and as fuel As a part of NASA’s Small Business Innovative for the turboelectric generator engine. Although Research (SBIR) phase 1 contract study, the study was very preliminary in nature, these Empirical Systems Aerospace, LLC, conducted propulsion system features along with the a system study of integrating an advanced vehicle configuration itself did certainly point cryogenic electric propulsion system onto a toward large reduction in fuel burn for both 150-passenger STOL regional airliner, the ECO-150 and ECO-250 configurations. The ECO-150, and a larger 250-passenger large detailed integration and the design of the split transport, the ECO-250 [18]. A key feature of wing with electric motors/fans are continuing as these two concepts, as shown in Fig. 10, is the a result of another SBIR Phase 1 award.

integration of the superconducting-electric- motor-driven fans with the wing such that the 4.3 “H3.1” Configuration by Massachusetts inboard wing is separated into top and bottom Institute of Technology (MIT) sections, and all electric-driven propulsors are completely embedded within the airfoil or wing Another TeDP vehicle concept named “H3.1” structure. This feature provides a benefit of was recently proposed and studied by MIT as a wing weight reduction through wing bending part of NASA’s SFW N+3 cooperative work moment relief because the distributed electric [19]. The vehicle shown in Fig. 11 is based on fans and the use of the common nacelle as wing the HWB configuration with a range of 7,600 nmi DISTRIBUTED PROPULSION VEHICLES (14,075 km), 354 passengers, and cruise Mach 0.8 at 35,000 ft (10,668 m) altitude. Similar to NASA’s N3-X vehicle, this vehicle also ingests upper airframe surface boundary-layer flow to significant contribution to the shock wave improve propulsive and hence the fuel signature is caused by the propulsion pods. By efficiency while minimizing noise impact to the carefully integrating the propulsion pods into surrounding community by shielding the the wing structure, a novel configuration might propulsion-related noise with the airframe.

be possible that will reduce wave drag and the Another key feature of this configuration is the intensity of the sonic boom and result in greater use of cryogenic methane as fuel because of its overall mission efficiency. Such a notional higher specific energy, which improve the fuel vehicle concept is shown in Fig. 12.

efficiency of the aircraft. In addition, the cryogenic fuel allows the use of superconducting materials to distribute the electric power from 6 Concluding Remarks three turboelectric generators to 23 electric fans that are semi-embedded in the upper surface of A main objective of the distributed propulsion the airframe.

concept is to achieve optimum vehicle benefits through integration of aerodynamic, propulsive, structural, and/or operational elements. The 5 Supersonic Distributed Propulsion Vehicle concept could be applied to various vehicle Concepts configurations such as traditional tube and wing, hybrid-wing-body aircraft (HWB), and Under the NASA’s Revolutionary System supersonic aircraft. However, in order to Concepts in Aeronautics project in 2004, a achieve maximum benefits, it will be necessary system analysis study conducted by the Georgia to design an aircraft with greater emphasis on Institute of Technology examined a 300- propulsion airframe integration right from the passenger supersonic distributed propulsion conceptual design stage. This paper described vehicle. Because this was supersonic flight, an some early distributed propulsion vehicle added drag term results from the shock waves concepts using jet flaps, cross-flow fan, gas- surrounding the vehicle. A goal of this study driven multi-fans, or multiple discrete engines.

was to minimize this ‘wave drag’ and possibly Recent interest in the HWB airframe has reduce the sonic boom on the ground.

prompted other advanced distributed propulsion Depending on the vehicle configuration, this concepts such as mechanically or electrically wave drag may constitute from 10% to 50% of driven multi-fans. Among these, the the overall vehicle drag. In addition to the wave turboelectric distributed propulsion concept drag, the shock waves coalesce to varying seemed to provide a revolutionary capability in degrees to cause a sonic boom on the ground. A HYUN DAE KIM [2] Hancock J. Test of a high efficiency transverse fan.

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7 Acknowledgements [15] Kim H, Brown G and Felder J. “Distributed turboelectric propulsion for hybrid wing body aircraft.

The author would like to acknowledge Gerald th 9 International Powered Lift Conference , United Brown, James Felder, and Michael Tong at Kingdom, July 2008.

NASA Glenn Research Center and Julio Chu at [16] Luongo C et al. Next generation more-electric aircraft: NASA Langley Research Center for their a potential application for HTS superconductor.

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principal investigator, Ruben Delrosario, and [18] Gibson A et al. The potential challenge of turboelectric project scientist, Richard Wahls, for their propulsion for subsonic transport aircraft. AIAA support on TeDP research activity. 2010-276, 2010.

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DISTRIBUTED PROPULSION VEHICLES 8 Contact Author Email Address Hyun.D.Kim@nasa.gov Copyright Statement The authors confirm that they, and/or their company or organization, hold copyright on all of the original material included in this paper. The authors also confirm that they have obtained permission, from the copyright holder of any third party material included in this paper, to publish it as part of their paper. The authors confirm that they give permission, or have obtained permission from the copyright holder of this paper, for the publication and distribution of this paper as part of the ICAS2010 proceedings or as individual off-prints from the proceedings.