Challenges of future aircraft propulsion: A review of distributed propulsion technology and its potential application for the all electric commercial aircraft

Progress in Aerospace Sciences 47 (2011) 369–391

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Progress in Aerospace Sciences journal homepage: www.elsevier.com/locate/paerosci

Challenges of future aircraft propulsion: A review of distributed propulsion technology and its potential application for the all electric commercial aircraft Amir S. Gohardani , Georgios Doulgeris, Riti Singh Department of Power and Propulsion, School of Engineering, Cranfield University, Bedfordshire MK43 0AL, United Kingdom

a r t i c l e in fo

abstract

Available online 13 October 2010

This paper highlights the role of distributed propulsion technology for future commercial aircraft. After an initial historical perspective on the conceptual aspects of distributed propulsion technology and a glimpse at numerous aircraft that have taken distributed propulsion technology to flight, the focal point of the review is shifted towards a potential role this technology may entail for future commercial aircraft. Technological limitations and challenges of this specific technology are also considered in combination with an all electric aircraft concept, as means of predicting the challenges associated with the design process of a next generation commercial aircraft. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Distributed propulsion All electric aircraft More electric aircraft Future commercial aircraft

Contents 1. 2.

3. 4.

5.

6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Historical review of distributed propulsion technology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. A few conceptual milestones of aircraft distributed propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. A few milestones of aircraft distributed propulsion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Historical trends of distributed propulsion for selected commercial aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Year of first flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Historical evolution of flight cruise speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Historical evolution of MTOW and OWE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Historical evolution of aircraft range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5. Historical evolution of propulsive power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6. Commercial aircraft payload and weight considerations for distributed propulsion technology . . . . . . . . . . . . . . . . . . . . . . . 2.4. Data reliability for commercial aircraft employing distributed propulsion technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A glimpse of past research endeavors in distributed propulsion technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The electric aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Selected milestones of the all electric aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. The more electric aircraft and related systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Electric motors for airborne applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A proposed framework for future commercial aircraft employing distributed propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Aircraft characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Aircraft propulsion system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Aircraft operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A few challenges for an all electric future commercial aircraft employing distributed propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

 Corresponding author. Tel.: +44 1234 754 666; fax: + 44 1234 751 232.

E-mail address: amir.gohardani@cranfield.ac.uk (A.S. Gohardani). 0376-0421/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.paerosci.2010.09.001

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Nomenclature E P R T V OWE PAY MTOW NoE

maximum cargo range (km) power output for each individual engine unit (kW) aircraft range (km) engine thrust (kN) aircraft speed (km/h) operating empty weight (kg) payload (kg) maximum take-off weight (kg) number of engines f piston/propeller unit engine power (kW) x number of engine units AC alternating current AEA all electric aircraft AEE all electric engine APU auxiliary power unit BLI boundary layer ingestion BWB blended wing body CESTOL cruise efficient short take-off and landing CMF common-core multi-fans CMP common-core multi-propulsors dB decibel DC direct current DEN distributed engines DEX distributed exhaust DFRC NASA Dryden Flight Research Center ECS environmental control system ERAST environmental research aircraft and sensor technology ESTOL extreme short take-off and landing

1. Introduction The intricate challenges of meeting future environmental goals in commercial aviation require a cross-disciplinary effort that focuses on: feasible propulsion systems, reduced fuel consumption, aviation safety and reliability, noise reduction, and optimized aircraft design to achieve desirable flight attributes. With a constant increase of air passengers, and the demands for technological innovation to reduce harmful emissions and jet noise, the impact of commercial propulsion systems becomes even more pronounced. Contemporary trends of intelligent engines raise a fundamental question that addresses the most promising propulsion system for commercial aviation and in retrospect, conceptual inventive engine systems are systematically investigated. The technical lessons learned from aviation history are important venues for future technical progress. One of the many intriguing subjects regarding future aircraft is the visions aviation enthusiasts anticipate for the future. Kuchemann’s early approach to recognize the need for additional efforts in the aerodynamics of propulsion is noteworthy as prior advances in propulsion technology were indeed extended far beyond the realm of airfoil theory [1]. Kuchemann and Weber’s comprehensive aircraft performance study at subsonic, supersonic and hypersonic speeds has further served as a gateway for improved understanding of aerodynamic shape and its evolution [2]. Challenges within the hypersonic flight regime are, however, particularly difficult to overcome, as strong shockwaves or disturbances are caused in response to lift generation and other means to provide volume and propulsion [3]. From a general perspective, it is possible to draw parallels between Kuchemann’s envisioned differences in the design procedures for various aircraft [4] and this study, as both seek to examine at least one

ETOPS FAA HALE HP HTS HALSOL HWB IDG LP MDO MEA MEE NASA PAI PFCC PM PPS PWM RAT RPM SFC SPS SR STOL TRU TV UHBR VF VTOL VSCF

extended range operation with two-engine airplanes Federal Aviation Administration high altitude long endurance horse power high temperature superconductive high-altitude solar energy hybrid wing body integrated drive generator low pressure multi-disciplinary optimization more electric aircraft more electric engine National Aeronautics and Space Administration propulsion-airframe-integration power factor correction number permanent magnet primary power systems pulse width modulator ram air turbine revolutions per minute specific fuel consumption secondary power systems switched reluctance short take-off and landing transformer rectifier unit thrust vectoring ultra high bypass ratio variable frequency vertical take-off and landing variable speed constant frequency

particular mode of propulsion in further detail. Air transport of the 21st century is no longer limited to technological constraints, but also to environmental restraints that in combination with increased flight safety, dictate the nature of future flight regimes and flight missions. Aircraft distributed propulsion is one of the promising propulsion systems currently considered for integration into a wide number of future air transport models. As with any promising system, the limitations and weak points of this technology are identified in light of its strengths and advantages. The aim of this paper is to make an assessment of aircraft distributed propulsion, with a mindset of environmental awareness. Throughout the scope of this study, an All Electric Aircraft concept is also considered in combination with the distributed propulsion technology, as the electric aircraft trend displays one of the environmental friendly propulsion options for future commercial aircraft. Distributed propulsion is based on dividing up the thrust for the beneficiary gain of noise reduction, shorter take-off and landing, enhanced specific fuel consumption and flight range. This is particularly true if the complete aircraft history is to be included in this definition, dating back to the early days of flight, where the means of propulsion were different from those of the jet engine era. Fig. 1 depicts a few historical milestones of aircraft distributed propulsion. The planes above and below the time axis categorize aircraft distributed propulsion into a conceptual domain and a manufactured domain, respectively. The conceptual domain revisits a few hypothetical ideas that have contributed to the implementation of aircraft distributed propulsion arrangements. Variation among these different configurations covers, however, a substantial portion of different propulsion system designs that have made it to the manufacturing phase. Many of the known aircraft incorporating distributed propulsion systems

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371

Fig. 1. A few conceptual milestones versus actual milestones of aircraft distributed propulsion technology.

are equipped with jet engines. On the other hand, if distributed propulsion is defined as an arrangement where the units of thrust are spread along the body of the aircraft, then the tracks of this arrangement are traced to a few years prior to the invention of the jet engine.

2. Historical review of distributed propulsion technology 2.1. A few conceptual milestones of aircraft distributed propulsion In 1924, Manzel (Fig. 1A) proposed multiple propeller units arranged in two rows or series as the propelling mechanism for airships, aircraft and the like [5]. The motivation behind this concept was the feasibility of ascent without a special landing field. Although the usage of wings, a major contributor to the aerodynamic lifting force, was negligible in this proposal, similar approaches stagnated, and Altieri’s invention (Fig. 1B) of introducing additional flight power in 1932, was based on using auxiliary propellers fore and aft of the aircraft wings [6]. Recognizing the small effect of supplemental propulsion assistance, using additional propellers, this concept was primarily aimed for proper and safe landings. In 1954, Griffith replaced the earlier propositions of propellers with gas turbines (Fig. 1C) and presented the concept of an aircraft with a master combustion engine unit in combination with a number of gas turbine ‘slave’ units that were spaced in the spanwise direction of the aircraft wing structure [7]. Providing the means for Thrust Vectoring (TV), short take-off and landing (STOL), and low fuel consumption, this invention combined many new technical features of significant potential. Reyle’s 1964 proposal (Fig. 1D) was related to an aircraft that could use gas turbine technology for the engines disposed between the ducting

surfaces and nuclear engines in the engine nacelles, if positioned at a distance from the fuselage [8]. Reyle envisioned that this concept would contribute to power-weight ratio enhancement, but did also recognize radiation concerns in the event of an aircraft crash. A novelty associated with this conceptual scheme was the coupling of two different means of propulsion systems. Because an additional propulsion unit could jeopardize the entire aircraft, careful attention to reliability was paid to the system. This cast light on one of the principal complexities of combining different propulsion systems. Even though the potential safety risk associated with nuclear power consistently has affected nuclear powered aircraft [9], future nuclear concepts have not entirely been abandoned [10]. Pursuing another research front, Malvestuto Jr. [11] took interest in an aircraft capable of carrying substantial payloads (Fig. 1E). Using a wing structure, divided into several wing portions equipped with rotors together with rotors in arrangement with lighter-than-air buoyancy units, this rotor-wing combination distributed the power over a much larger effective area to achieve considerably higher power loadings, in comparison to a conventional power loading of a helicopter. As a result, distributed propulsion was also considered and introduced for Vertical Take-Off and Landing (VTOL) aircraft. One could argue that this concept brought Manzel’s concept (Fig. 1A) to a new level, using a wealth of knowledge that was gained over almost 60 years. Referring back to an initial core idea, the new arrangement and position of propellers in another plane contributed to new features and an illustration of conceptual evolution is traceable in a new proposal presented by Phillips [12] in 1983. In this conceptual proposition (Fig. 1F) a solar powered aircraft was considered with a cruciform wing structure. Equipped with solar cells and multiple propellers positioned on the wingtips, details were provided on how to maintain surfaces

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normal to the sun’s rays to utilize the direct solar energy. This concept, amongst others, served as a crucial step towards the development of solar airplanes, such as the first generation HighAltitude Long Endurance (HALE) vehicle, Pathfinder [13]. The discovery of the Antarctic Ozone Hole, earlier that decade, also boosted the need for further stratospheric research programs related to high-altitude aircraft, under the Environmental Research Aircraft and Sensor Technology (ERAST) [14] alliance, initiated by NASA and the industry. In 1988, NASA proposed a number of detailed concepts [15] for airframe and propulsion interactions and integrations. A commonality between these concepts (Fig. 1G–J) is the employment of different propulsion systems. SnAPII (Fig. 1G) featured twin fuselages separated by a circulation-control wing that contributed to high lift coefficients during takeoff and landing. Using two tail-mounted engines at the end of each fuselage with TV and reversing, fuselage Boundary Layer Ingestion (BLI), and smart inlet and nozzle technology, SnAPII also used a device to power flow control on the outer portions of the wing. Wing tip turbines could further reduce the wake hazard at takeoff and landing. This concept merged two individual fuselages with their propulsive units into one main body. A hypothetical scenario of total engine failure for either one of the combined fuselages was simplified in the subsequent proposal for a distributed engine (Fig. 1H) regional STOL aircraft. This airplane made use of an array of wing-integrated miniengines to provide lift augmentation and distribution with increased redundancy. Employing another array of mini-engines at the tail, integrated with inlet and nozzle, deflectors enabled the Coanda effect for TV. Using a similar circulation-control wing similar to SnAPII, a blended forward-swept wing body concept was envisioned (Fig. 1I). This aircraft used three aft-mounted high-bypass ratio turbofans with BLI, TV and reversing, smart inlet, nozzle technology and flow control systems. Trans-Oceanic Air-Train, (Fig. 1J) was characterized by two vehicles, the Lead and the Mule. These vehicles rendezvous to complete the cruise configuration of a long range transport of cargo. Although the design was aimed at freight flight in the low transonic regime, in favor of high aspect ratio wings and span loading for minimal fuel consumption, parts of this concept could potentially also be applied to commercial aviation. Equipped with TV-technology for optimal take-off performance, the Lead vehicle was designated as the primary fuel carrier and responsible for flight control activities of all Mule vehicles. All unmanned Mule vehicles incorporated pylon structures with morphing technology and powered by advanced ducted prop pylons, carrying enough fuel for takeoff, rendezvous, connection, abort and landing. Rendezvous between different aircraft that would transport future air travelers from point A to B, pose new unexplored propulsion challenges. Nonetheless, these concepts cannot be disregarded because of their levels of complexity. Most of the proposals, presented in this short journey throughout the conceptual milestones of aircraft distributed propulsion, have dealt with the subsonic flight regime. However, this does not imply that supersonic concepts were neglected or never proposed. In fact, the perspectives and demands for rapid air travel also point to the supersonic flight regime. 2008 marks the year when Lockheed Martin, in collaboration with other industrial partners and academic institutions, envisaged a future aviation concept, operational between the years of 2030 and 2035 [16]. Implementing synergistic combinations to tackle flight emissions, fuels and airport noise, the artist’s rendering of this concept shows a (Fig. 1K) distributed propulsion system and an environmentally friendly airframe system aimed for supersonic operation. However, it can also be argued that this supersonic concept only features four engines. Since a distinct definition regarding the distributed propulsion terminology is not readily available, a placement of an aircraft with four engines

within the distributed propulsion category would only hold if the definition of this technology indeed referred to an aircraft employing three engines or more. The short glimpses of implemented technologies in the conceptual milestones of aircraft distributed propulsion have revealed the use of hydrogen, piston engines, gas turbines, solar cells, electrical units and nuclear power, in various arrangements for aircraft propulsion. Despite the random chosen order of these concepts, these multifaceted propulsion tools exhibit many configurations that have been integrated into a variety of manufactured aircraft. Thus, it is important to revisit a few milestones of aircraft distributed propulsion that have partially been the fruit of thought from these referred concepts.

2.2. A few milestones of aircraft distributed propulsion A common theme instilling the conceptual time line of distributed propulsion marks the dawn of various aircraft that employed available propulsion units of their time for new technical arrangements. For the purpose of elucidating ideas that became reality, a short visit is made along the historical axis of time, to point out some aircraft that implemented three or more units of propulsion and were chosen for commercial, experimental, cargo, research and military applications. Unlike the early days of conceptual aviation where distributed propulsion was introduced in the airship industry, many promising proposals that would have progressed into production were never funded. One possible cause for this, at least in the latter part of the 20th century, emerged from the misconception that hydrogen was the primary cause of the Hindenburg catastrophe [17]. Doubtlessly, the term ‘Hindenburg syndrome’ [18] had a negative influence on the general public and the airship industry, but regardless of this significant impact, the aviation industry embraced many different designs featuring distributed propulsion. In 1929, Dornier Do X (Fig. 1M), the world’s largest aircraft at the time, flew for the first time [19]. Intended for transatlantic flights, this aircraft left Friedrichshafen, Germany, on 2 November 1930 with 17 passengers and crew for the USA. After eventful flights via a few European cities, Brazil, the West Indies, and Miami, the aircraft reached New York on 27 August 1931. Equipped with faired-in engine supports for its 12 engines, Dornier Do X also suffered many delays en route to New York and many of these were related to technical difficulties. Early long range flight attempts with distributed propulsion revealed many unforeseen parameters that could not be efficiently addressed or investigated during the conceptual design phase. Engine cooling was one of these problems. Using multiple engines without any cooling measures caused a thrust reduction for the rear engines. Conversely, the combination of distributed propulsion and commercial aviation appeared to have its own advantages. The same year the Dornier Do X aircraft left Friedrichshafen, Handley Page H.P.42 (Fig. 1L), made its first flight [20]. Intended for the purpose of linking various parts of the British Empire, this aircraft used two engines on each of the large unequal-span biplanes, leaving a brilliant record of safety with no fatal accidents after a decade of service. An innovative part of H.P.42’s design was to position the propulsion units on different wings. Seemingly a successful trend for long-range missions, multiple engine solutions were chosen more often and this involved also two historical flying boats. The first aircraft, Blohm und Voss BV 222 Wiking (Fig. 1N), the largest operational flying-boat during World War II, was specifically designed for long-range passenger transport in the late 1930s and was equipped with six vertically opposed engines distributed over the wing [21]. Following this success, a historical flight was made by Howard Hughes’ famous H-4

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Hercules (Fig. 1O) in 1947. H-4 Hercules was the largest flying boat ever built and consisted of a single hull and eight radial engines [22]. Taking into consideration the significant size of the aircraft, a substitution of wood for metal served as a new gateway for non-conventional approaches to aircraft design. The design practices of this aircraft revealed, however, many technical difficulties ranging from the integration of power systems to large control surfaces. These problems added a new dimension to the earlier observed difficulties with engine cooling procedures in aircraft distributed propulsion. During the transition to the jet engine era, the Avro Type 706 Ashton Mk 3 aircraft (Fig. 1P), equipped with either five or six turbojet engines, initially flew in 1951. It was principally used for research purposes. An interesting feature of the employed distributed propulsion system in Ashton Mk 3 was the wingembedded scheme. The Bell D-2127 aircraft (X-22) (Fig. 1Q) took the concept of distributed propulsion one step further with its tilting arrangement of ducted fans. 1966 was the first time this aircraft took to the skies and almost two decades later it had contributed significantly to the VTOL/STOL research through programs at NASA and Federal Aviation Administration (FAA) [19]. Various relations between the distributed technologies in the Bell D-2127 and Ashton Mk 3 aircraft could certainly be generalized to conceptual models. In the majority of all considered cases (along the time line in Fig. 1), ideas adopted on both planes, complemented each other regardless of sequential order. 1969 was the year when the Boeing 747 aircraft (Fig. 1R), perhaps one of the most commonly known historical airplanes in commercial aviation, had its first flight. The Boeing 747 used four turbofan engines in pods pylon-mounted on wing leadings edges. Equipped with air-cooled generators mounted on each wing for electrical supply, two additional generators could provide primary electrical power when the engine-mounted generators were not operational [19]. Technological advancement and the Boeing 747’s efficient propulsion system integration were evident in a blunt comparison to the Dornier Do X’s engine mishaps. The engine arrangement on the Boeing 747 has become a standard configuration for many commercial aircraft. Although the number of engines in some cases has been reduced to only two for other aircraft, this was not the case for the Antonov An225 MRIYA aircraft (Fig. 1S) which was not designed to transport air travelers, but rather to transport the Soviet space shuttle. In 1989, Antonov An-225 completed this task with its six engines fitted with thrust reversers and glass fiber engine cowlings [19]. Nine years later, two distributed propulsion systems were combined in a propulsion scheme with virtually no harmful emissions. Centurion (Fig. 1T), an unmanned solar-powered aircraft, first flown in 1997, with 61.8 meters wingspan and 14 brushless direct-electric motors, could reach altitudes of 30 km. Envisioned as the ‘Eternal Airplane’ with the objective to fly for months, solar arrays were used to power electrical motors [23]. The environmental impact of this aircraft has contributed to considerations for more environmentally friendly propulsion systems. The next section aims to identify a number of these trends for commercial aviation.

2.3. Historical trends of distributed propulsion for selected commercial aircraft Given the random nature of the chosen aircraft in the previous section, an interesting approach would be to consider a larger population of aircraft and derive a few historical trends of distributed propulsion technology. For this specific purpose, aircraft characteristics of 70 commercial aircraft employing at least three engines, as an indicator for the distributed propulsion

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Table 1 A comparison between different electric aircraft entering the manufacturing phase. Source: [99]. Name of electric aircraft

Number of seats

Engine power (kW)

Fuselage length (m)

MTOW (kg)

Antares DLR-H2 Electravia Electroclub Electravia Electrolight IFB Hydrogenius* Yuneec-Flightstar E-Spyder Yuneec 430

1 2 1 2 1 2

42 37 19 72 20 40

7.40 6.40 7.60 8.11 5.03 6.98

660 700 300 850 226 470

*—A combination of electric motor, batteries and hydrogen driven fuel cell systems with liquid hydrogen reservoir on board.

technology, are considered. The rationale behind this random selection is to consider a large variety of commercial aircraft. Evidently, the selected aircraft in Table 3 do not represent all commercial aircraft employing distributed propulsion, but only a selected population of aircraft with the mentioned number of propulsion units. Furthermore, the average number of engines on each aircraft considered in this study is 4 engines. The percentage distribution of the number of engines are as follows: 3 engines  19%, 4 engines  79%, 8 engines  1% and 12 engines  1%. Thus, one may argue that the derived historical trends of this study in fact are biased towards aircraft with 4 engines. Although the listed civil aircraft were only chosen based on the mentioned number of engines as a criterion, an early conclusion regarding the uneven plurality of engine distribution in this study serves only as a historical indicator for the manufactured aircraft employing this specific arrangement. Engine positions have not been explicitly used as a parameter of distributed propulsion arrangement for this study. A number of considered engine positions for this study include arrangements of engines: between the wings, spanwise beneath the wings, spanwise on the wings, at the root of each of the stub wings and on the ventral nacelle. In this study, the choice of employing 3 engines as a representative indicator for distributed propulsion is based on the actual meaning of the distributed term, or ‘to spread or diffuse over an area’ [24]. Hence, the considered aircraft in Table 3 employ a multiple of propulsion units rather than a single engine configuration with distributed thrust as in the case of Hunting H.126 [25] or variations of distributed thrust concepts [26]. Twinengine aircraft have been disregarded as aircraft with distributed propulsion technology because of the sheer possibility of in flight one-engine-out scenarios. These conditions would fail to convey the meaning of a distributed propulsion aircraft and even with recent changes in Extended Range Operation with Two-Engine Airplanes (ETOPS) regulations [27], which is now adapted to Extended Operations, this has still remained as the preferred approach. This study highlights not only the propulsion system units themselves, but also the number of employed propulsion units for a distributed propulsion arrangement. An even clearer image regarding the true historical assessment of the overall distributed technology can, however, only be provided, once a study of this nature is coupled and completed with an additional study in view of the historical trends of distributed propulsion technology for military aircraft. Bearing in mind the unique features of each and every aircraft and the significant variation of aircraft design procedures over time and in respect to the role of each specific aircraft, this section seeks to identify commonalities between the 70 commercial aircraft listed in Table 3, with emphasis on the following parameters: Year of maiden flight, number of employed engines, maximum take-off weight, empty operating weight, wing

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Table 2 Summary of selected number of distributed propulsion studies in the following distributed propulsion categories: DEN (distributed engines), DEX (distributed exhaust) and CMF (common-core multi-fans). Research team

Publication year

Ko et al. [42]

2003

Category

DEN

DEN/DEX

Lundbladh and ¨ Gronstedt [165]

2005 DEN

DEN Ameyugo [48]

2007

CMF

Airframe: Conventional Blended Wing Body Design  Comparison between 4 and 8 engined aircraft featuring a conventional Blended Wing Body aircraft Airframe: Distributed Blended Wing Body Design  Comparison between 8 engined aircraft featuring different distributed propulsion effects

 In a comparison between a conventional and a distributed propulsion Blended Wing Body aircraft, the latter has a 5.4% lighter TGOW and uses 7.8% less fuel

Airframe: Conventional Aircraft

 4% gain in fuel consumption by wing embedding for a long range aircraft  High efficiency requires the pressure ratio to be over 40 and a bypass ratio of over 8 for smaller engines

 Comparison between 2 and 8 engined aircraft featuring different intake/installation configurations Airframe: Blended Wing Body Aircraft  Comparison between 4 and 16 engined aircraft featuring different intake/installation configurations

 Effects of distributed propulsion technology for a long-range subsonic airliner were analyzed

 Most of savings in TOGW is due to the effect of the the trailing edge jet on the induced drag and the increase in propulsive efficiency

 Distributed propulsion can avoid efficiency losses and become a competitive solution with other technologies  The feasibility of small gas turbines were found limited by their excessive fuel consumption  Distributed driven fan applications may improve in combination with superconductive elements as electrical power transmission seems promising

Airframe: Blended Wing Body Aircraft  Effects of distributed driven fans were analyzed

 Effects of presumably podded turbine engines studied Turboelectric propulsion was identified to come to fruition by

2009

the development of: Superconductive machines (25–40 kW/kg for motors), (40–80 kW/kg for high speed generators), low AC loss high temperature superconducting conductors ( o 10 W=Am at 500 Hz) and crycoolers capable of 30% Carnot efficiency and weighing 4 3 kg=kW DEN

CMF

Felder et al. [166]

Summary of key findings

Airframe: Conventional Aircraft DEN

Luongo et al. [51]

Distributed propulsion highlights

2009

 Comparison between two large non-distributed turbofans and 16 conventional small distributed turbofans Effects of two distributed fan configurations studied  Comparison between 2 engine cores, 2 electric generators, 16 motors (including refrigerators) and 2 engine cores, 2 electric generators, 16 motors (liquid hydrogen cooled)

 Refinement of physics-based models for superconducting machines and exploration of alternative aircraft concepts

 A minimum TSFC observed around a FPR  1:35

Airframe: Hybrid Wing Body Aircraft

 Effects of two distributed fan configurations studied  Standard approach to inlet performance calculation CMF

Gibson et al. [167]

2010

inappropriate for fan inlets of this study The hydrogen tank volume in a 2 fuel system is about 40% of inverters, 14 electric motors (all including refrigerators) the volume of the jet fuel tanks and 2 electric generators, 2 electric inverters, 14 electric motors (all liquid hydrogen cooled) in terms of both weight and efficiency  Total loss in superconducting devices as low as 0.03% and a 0.2% loss in the power inverter enabling a hydrogen/ jet fuel system to provide enough cooling using hydrogen

 Comparison between 2 electric generators, 2

 Great potential for improved aerodynamics, reduced drag, engine sizes and aircraft weight

Airframe: STOL Regional Airliner

CMF

 Effects of two distributed fan configurations studied  Comparison between 10 and 16 distributed in-wing  Further studies needed for motors distributed inside, across a fans using a turboshaft engine, for converting fuel energy to shaft power in combination with cryogenically cooled superconducting electric motors

wing or around a fuselage

 Studies needed which consider aerodynamics, structure weight and volume versus the number of motors for different size aircraft and various mission ranges

area, flight cruise speed, aircraft payload, total propulsion power output and aircraft range. These parameters are chosen to depict the representative characteristics of the selected aircraft considered in this study.

2.3.1. Year of first flight The listed aircraft in this study span over a period of 77 years, with the earliest flight in 1928 and the latest in the year 2005,

including aircraft from the following categories: Outsize cargocarrying transport, short/medium/long-range commercial transport, short/medium/long-range passenger and cargo transport, heavy freight transport, short/medium-range utility transport and flying boat transport aircraft. The indicated period of time also includes the sonic flight era, and enables a historical perspective on the evolution of cruise speed, aircraft weight, aircraft range and wing loading. In some cases, when the year of the maiden flight for a specific aircraft model has remained vague, it has been

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Table 3 Selected commercial aircraft employing distributed propulsion. Source: Compiled from sources: [19,171]. Name of the selected commercial aircraft

Year of maiden flight

Number of engines

MTOW (kg)

OWE (kg)

Wing Area (m2)

Cruise speed (km/h)

Payload (kg)

Propulsion Range power (kW) (km)

Aero Spacelines B377-PG Aero Spacelines Guppy 201 Aerospatiale/BAC Concorde Ahrens AR 404 Airbus A340-200 Airbus A340-300 Airbus A340-500 Airbus A340-600 Airbus A380 Airbus A380F Antonov An-10A Antonov An-12 Antonov An-22M Antonov An-124 Armstrong Whitworth 650 Argosy 100 Armstrong Whitworth 55 Apollo Avro Type 685 York Avro International RJ Avroliner Boeing 80A-1 Boeing 314 Boeing 377 Stratocruiser Boeing 707-300C Intercontinental Boeing 717 (KC-135 Stratotanker) Boeing 720B Boeing 727-200 Boeing 747-200B Boeing C-17 Globemaster III Boeing MD-11 Breguet Br.763 Breguet Br.892R Mercure Breguet Br.941S Bristol Brabazon I Bristol Type 175 Britannia 310 Canadair CL-44D4 Convair 3 (R3Y-1) Convair 30 (CV-990A) Dassault Falcon/Mystere 50 Dassault Falcon/Mystere 900 de Havilland DH.66 Hercules de Havilland DH.86B de Havilland DH.91 Albatross de Havilland DH.106 Comet 4 de Havilland DH.114 Heron de Havilland Canada DHC-7 Dash 7 Dewoitine D.332 Dornier Do X Douglas C-124C Globemaster II Douglas C-133 Cargomaster Douglas DC-4-1009 Douglas DC-6B Douglas DC-7C Douglas DC-8 63 Fiat G.12C Fokker F.IX Fokker F.XII Fokker F.XXII Fokker F.XXXVI Fokker (America) F.32 Ford 5-AT-D Tri Motor Handley Page H.P.42W Handley Page Hermes IV Hawker Siddeley (BAe) Trident 2E Ilyushin Il-18D Ilyushin Il-62M Ilyushin Il-96-300 Lockheed C-5 Galaxy Lockheed Martin C-130J Hercules Lockheed C-141B StarLifter Lockheed L-1011-500 TriStar Tupolev Tu-144

1962 1970 1969 1976 1991 1991 1991 1991 2005 2005 1957 1958 1965 1982 1959 1949 1942 1981 1928 1939 1947 1959 1961 1960 1963 1969 1991 1985 1951 1949 1967 1949 1956 1960 1950 1961 1976 1984 1926 1934 1937 1949 1959 1975 1933 1929 1949 1956 1938 1946 1953 1958 1940 1929 1930 1935 1934 1929 1928 1930 1948 1964 1957 1963 1988 1968 1954 1963 1970 1968

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 3 4 4 4 4 4 3 4 4 3 4 4 4 8 4 4 4 4 3 3 3 4 4 4 4 4 3 12 4 4 4 4 4 4 3 3 3 4 4 4 3 4 4 3 4 4 4 4 4 4 3 4

63,945 77,110 185,066 7938 275,000 275,750 376,000 374,000 560,000 590,000 55,100 61,000 250,000 405,000 39,916 20,412 31,115 38,102 7938 37,422 66,134 151,318 143,338 106,142 45,027 377,849 265,356 283,727 51,600 16,000 26,500 131,544 83,915 95,254 63,674 115,668 17,600 20,640 7076 4649 13,381 73,482 6123 19,958 9350 56,000 88,224 129,727 33,112 48,534 64,864 158,757 12,800 9000 7250 13,000 16,500 11,000 6123 12,701 39,009 65,318 64,000 165,000 216,000 379,657 79,380 155,580 224,982 180,000

41,275 45,359 78,689 4309 129,000 129,750 172,850 179,850 276,800 252,200 29,800 28,000 114,000 175,000 20,865 13,971 19,069 23,336 4800 22,801 37,875 66,406 48,220 51,204 46,675 170,177 125,647 131,036 32,535 8990 14,700 65,816 37,438 40,348 32,579 54,686 9150 10,545 4110 2943 9630 34,212 3697 12,247 5280 32,675 45,888 54,550 19,640 25,110 33,005 69,739 8890 5350 4350 8100 9900 6840 3556 8047 25,106 33,203 35,000 71,600 117,000 169,643 34,274 67,186 109,299 85,000

164.4 182.5 358.2 39.2 361.6 361.6 439.4 439.4 846.0 845.0 119.5 121.7 345.0 628.0 135.5 91.6 120.5 77.3 113.3 266.3 164.3 283.4 226.0 234.2 157.9 511.0 353.0 338.9 33.0 101.2 83.8 494.0 192.8 192.8 195.3 209.0 46.8 49.0 143.7 59.6 100.2 197.0 46.4 79.9 80.0 450.0 232.8 248.3 135.6 135.9 152.1 271.9 113.5 103.0 83.0 130.0 170.0 125.4 77.6 277.7 130.8 135.3 140.0 279.6 391.6 576.0 162.1 299.9 321.1 438.0

402 407 2179 314 869 869 907 907 945 1020 680 670 520 865 451 444 338 669 201 295 547 974 856 983 917 940 816 876 150 285 400 402 575 621 483 1006 800 927 177 229 338 809 295 428 250 175 370 520 365 507 571 966 308 175 205 215 240 198 196 161 435 974 675 900 875 898 644 910 974 2300

13,155 24,494 12,700 3992 30,800 42,250 43,300 55,600 66,400 153,400 14,500 20,000 80,000 150,000 12,701 3402 4536 7735 1847 5920 8960 42,229 37,648 18,371 18,598 68,720 76,658 51,059 10,800 3200 10,000 4000 10,640 28,725 21,773 11,920 2170 2185 1120 800 1760 6480 1120 5130 1784 13,600 33,566 49,896 6880 11,142 8400 21,520 1120 1440 1280 1760 2560 2554 1698 3040 5040 12,156 13,500 23,000 40,000 118,387 18,955 41,222 41,845 11,200

2089 2930 15,367 250 5250 5250 9165 9750 12,246 14,450 2384 2400 8948 8254 1205 602 966 864 313 894 2088 3430 2181 3279 2464 9253 6155 9985 1432 298 895 1492 2458 3418 3490 3077 549 816 250 119 313 1575 149 668 343 382 2267 4474 865 1491 2028 3402 459 298 254 298 447 343 250 331 1253 2159 2535 4045 5720 7157 2739 3542 9026 48,875

3219 831 6228 2234 14,800 13,700 16,100 14,360 15,200 10,400 1200 3600 5000 4500 3219 1513 4345 1631 740 5633 6759 9262 14,806 6687 4002 11,397 8704 12,569 2165 1000 1000 8046 6869 4627 4482 616 6480 7408 845 1287 1674 5190 1473 1279 2000 2200 6486 6437 4023 4836 7411 7242 1740 1140 1300 1350 1350 1191 885 805 3218 3965 3700 7800 7500 5526 5250 4725 9905 3500

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replaced by the indicated first flight year of the aircraft program. Limitations of the availability of accurate historical data reveal in an exemplary case for the Boeing 80A-1 aircraft that, as an ambiguous year of maiden flight for this specific model type was highlighted, this aircraft was listed under the first flight of the Boeing Model 80 aircraft instead.

2.3.3. Historical evolution of MTOW and OWE Weight estimation of an aircraft with a distributed propulsion arrangement becomes even more intricate as the definition of the propulsion technology itself calls for a multiple of propulsion units. Fig. 3 depicts a historical view of the MTOW and the OEW for commercial aircraft employing distributed propulsion

Linear (MTOW)

Linear (OWE)

MTOW, OWE [kg x 103]

600 500 400 300 200 100

00

90

80

70

60

50

40

30

10 20

20

19

19

19

19

19

19

20

10

19

19

19

00

0

Year of First Flight Fig. 3. Historical MTOW and OWE trends for commercial aircraft employing distributed propulsion technology.

18.0 16.0 14.0 12.0 R [km x 103]

2.3.2. Historical evolution of flight cruise speed Cruise speed has always been treated as an important topic in civil aviation. Ever since the birth of aviation itself, overcoming the challenges of flying from point A to B, as quickly as possible, has been included in the visions of flight. Cruise speed defines a crucial aircraft characteristic, closely related to the aircraft propulsion system. A historical viewpoint incorporating cruise speed may contribute to the role this specific parameter has had for aircraft with distributed propulsion arrangements. Fig. 2 depicts a historical trend of commercial aircraft employing distributed propulsion technology. Interestingly enough, only two aircraft have cruised at supersonic speed in this study. Aerospatiale/BAC Concorde and Tupolev Tu-144’s presence in the supersonic domain is unlike the major population of aircraft, confined within the subsonic region. The fastest cruising aircraft for the selected aircraft is about 15 times faster than the slowest cruising aircraft. Moreover, it should also be noted that even though the indicated trend line suggests an increasing flight cruise speed, further progress to fly at transonic speed has stagnated for aircraft flying with distributed propulsion technology. Nevertheless, the average cruise speed has steadily increased from mid 1920s according to Fig. 2, and still the average flight cruise speed in this study corresponds to Mach  0:5. A more detailed discussion regarding different flight regimes and a few complexities associated with the transonic and supersonic regime will be discussed in a separate section about flight regime considerations for future commercial aircraft.

MTOW OWE 700

19

376

10.0 8.0 6.0 4.0

2500

2.0 0.0 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

2000

Vcruise [km/h]

Year of First Flight Fig. 4. Historical range trend for commercial aircraft employing distributed propulsion technology.

1500

technology. From the represented trend lines, it can readily be concluded that the average increase of MTOW has been more than twice as much as the average increase of OWE, for the studied commercial aircraft within the considered time frame of this study. The fact that both MTOW and OWE have increased for the aircraft in Table 3 imposes new restrictions on the weight penalty for future distributed propulsion concepts. Therefore, the weight impact for future propulsion systems and their restrictions gain momentum if the historical trend lines and the strive for larger MTOW and OWE are to be followed.

1000

500

10 20

90

00 20

80

19

70

19

60

19

50

40

19

19

30

19

19

10

20 19

19

19

00

0

Year of First Flight Fig. 2. Historical cruise speed trend for commercial aircraft employing distributed propulsion technology.

2.3.4. Historical evolution of aircraft range According to Fig. 4, the aircraft range has steadily increased over the period of time considered for the selected aircraft in this

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0.9 0.8 0.7 0.6

T/OWE [W/g]

study. The indicated trend line shows a clear shift from shortrange aircraft to medium/long-range aircraft for recent years. Further, the listed range for each and every aircraft is directly related to the considered payload for that specific aircraft. Hence, in some instances the aircraft is in fact capable of carrying a larger payload than listed in Table 3, as the listed payload is given in the context of the aircraft range carrying that payload. In the case of Tupolev Tu-144 aircraft, the maximum payload can be as large as 15 000 kg, but the listed payload is 11 200 kg for a range of 3500 km. Aircraft range considerations become crucial for regional operational air routes of the future, and will be further discussed in the section related to the framework for future commercial aircraft with distributed propulsion architecture.

377

0.5 0.4 0.3 0.2

2.3.5. Historical evolution of propulsive power Whilst the technological advancements of the latter part of the 20th century features jet engines, a distorted historical image would be provided if piston engines and turboprop engines were to be excluded from this study. On the propulsion system note, however, one should distinguish between the mixed terminology used to indicate propulsion power and thrust for the discussed aircraft in Table 3. In consideration of a fair propulsion comparison between different propulsion systems, the output from each and every engine is indicated as a power quantity, only. The following assumption is considered for all listed jet engines in this study:

ð2Þ

Further, the total propulsion power output for all considered propulsion systems at cruise condition is expressed as: PTotal ¼ x  P

1930

1940

1950

1960

1970

1980

1990

2000

2010

Year of First Flight Fig. 5. Historical thrust-to-weight ratio trend for commercial aircraft employing distributed propulsion technology.

0.60

0.50

ð1Þ

Further, the power output for each and every engine unit at a specific cruise speed is assumed to be 15% of the Take-Off power output. It is therefore assumed that the throttle setting at cruise speed is 15% of the Take-Off propulsive power. In the case of piston engines or turboprop engines, however, the power output is calculated with a maximum 80% propeller efficiency factor of the propeller/piston unit engine power: P ¼ 0:80  f

0.0 1920

0.40 PAY/MTOW

P ¼ 0:15  TTakeOff  VCruise

0.1

0.30

0.20

0.10

ð3Þ

Thrust-to-weight ratio and wing loading are frequently chosen as fundamental parameters for aircraft performance [28]. The thrust-to-weight ratio in this study is primarily based on the OWE as the weight parameter and is depicted in Fig. 5. Although the thrust-to-weight ratio is not increased substantially from a historical point of view, one can certainly interpret this gradual increase as a measure of enhanced engine performance. This conclusion is drawn against the background of increasing OWE shown in Fig. 3, and conveys a signal that as the number of propulsion units certainly have not increased historically, more efficient engines have been the major contributors to increased thrust-to-weight ratios. One exemplary case to verify this is provided by a comparison between the largest total engine power and smallest total engine power of this study, which shows the engine power of de Havilland DH.86B from 1934 to have an unbelievable 0.78% of the total engine power of 1969s Ae´rospatiale/BAC Concorde. In the same context, advanced and efficient propulsion units may also be identified as major obstacles for distributed propulsion arrangement as only a small number of propulsion units are needed for providing the required thrust for flight. Nonetheless, the motivation behind a distributed propulsion approach for future aircraft extends beyond the simple thrust requirement and will be discussed in subsequent sections.

0.00 0

100

200

300 400 MTOW [kg x 103]

500

600

700

Fig. 6. Historical PAY/MTOW vs. MTOW trend for commercial aircraft employing distributed propulsion technology.

2.3.6. Commercial aircraft payload and weight considerations for distributed propulsion technology In this study, all analyses regarding the performance of commercial aircraft are directed towards the payload rather than the number of passengers or passenger kilometers for a commercial airliner. In the strict cases where only passengers are transported without any additional cargo load, the weight of each individual passenger has been estimated to 80 kg. This was done, considering a rough estimate of FAA’s standard average passenger weight considerations [29]. Cleveland and Redelinghuys’ detailed discussions about sizing effects [30,31] raise a question regarding the validity of the square/cube law for commercial aircraft employing distributed propulsion. A further investigation of the relationship between PAY/MTOW and MTOW, in accordance with Fig. 6, suggests an opposite trend than that observed by Tennekes [32] and Filippone [33], and reveals further

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that the structural stress would rather decrease and not increase with the characteristic length. This fact is, however, brought to light against a background that the criterion of a proportional relation between the load and the structural weight is fulfilled according to Fig. 7. A more detailed investigation of the observed trends in Figs. 6 and 7 could potentially be completed with a similar study for military aircraft employing distributed propulsion technology. Using Filippone’s reasoning for the maximum cargo range [33], the following relation is used to include the aircraft range in the performance analysis: E¼

PAY R MTOW

4500 4000 3500 3000 E [km]

378

2500 2000

ð4Þ 1500

Fig. 8 indicates an increase of maximum cargo range for increasing MTOW, which in itself is also related to both larger

1000 500

1.00

PAY/OWE

.0

0 0.

00 10

0 0.

90

0 0.

80

0 0.

70

0 0.

60

0 0.

50

0 40

0

0. 30

0. 20

0.

0 10

0.

S [m2]

0.80 0.70

Fig. 9. Maximum cargo range vs. Wing Area for all commercial aircraft in Table 3.

0.60

wing areas according to Fig. 9, and more efficient propulsion systems. The maximum cargo range for this study varies between 63.5 km for Convair 30, and 3888.9 km for Boeing 717. Both aircraft made their maiden flight in 1961 and the increase of maximum cargo range is about 61 times between the mentioned aircraft. Interestingly enough, the largest wing area for the selected aircraft of this study, held by Airbus A380 with its first flight in 2005, corresponds to an almost 26 times larger wing area than 1951s Breguet Br.763, representing the smallest wing area. In order to reveal the mathematical relation between the minimum and maximum MTOW, OWE and PAY, respectively, it is fruitful to indicate the comparative relation between the magnitudes of the largest and smallest values included for all selected aircraft in Table 3. 1934s de Havilland DH.86B has a MTOW that corresponds to only 0.79% of the MTOW of Airbus 380F with its maiden flight in 2005. The OWE figurative number is 1.06%, and the PAY proportion, 0.52% when 2005s Airbus A380 is compared to de Havilland DH.86B from 1934. Raymer’s suggestion of choosing thrust-to-weight ratio and wing loading as indicators for aircraft performance [28,168] proves to highlight a gradual increase of the thrust-to-weight ratio for an increasing wing loading of the listed aircraft in Table 3. OWE is the considered reference weight for the mentioned aircraft performance parameters and the scatter of data points according to Fig. 10 suggests that values above 0.5 W/g and 600 kg/m2 are rare for the selected commercial aircraft of this study.

0.50 0.40 0.30 0.20 0.10 0.00 0

50

100

150 200 OWE [kg x 103]

250

300

Fig. 7. Historical PAY/OWE vs. OWE trend for commercial aircraft employing distributed propulsion technology.

4500 4000 3500 3000 E [km]

0

0

0.90

2500 2000

2.4. Data reliability for commercial aircraft employing distributed propulsion technology

1500 1000 500 0 0

100

200

300 400 MTOW [kg x 103]

500

600

700

Fig. 8. Maximum cargo range vs. MTOW for all commercial aircraft in Table 3.

All commercial aircraft in this study have deliberately been selected based on their employed number of propulsion units. Thus, the dominating propulsion aspect of this study shifts other aircraft features such as wing sweep etc., to secondary positions. A remarkable diversity in airframe geometries and the location, shape and operating system of the propulsion units makes this study a unique historical journey in distributed propulsion technology for commercial aircraft. Moreover, it should also be emphasized that the mentioned multi-lateral facets of the

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system solutions that address some of the published goals for future aviation [34]. These visions primarily target reduction of fuel consumption, aircraft emissions, aircraft noise [35,36] and may also stress the minimization of the industrial impact on the global environment [37,38]. In recent years, distributed propulsion has been suggested as a promising instrument to successfully address the likelihood of more stringent environmental regulations for commercial aviation. In this review, a summary of the major research efforts in distributed propulsion technology may provide researchers and scientists with a venue for a better understanding of this propulsion system. Elucidation of the in-depth details for this propulsion system is therefore beyond the scope of this study. Sehra and Whitlow Jr.’s review about power and propulsion for 21st century aviation provides a comprehensive insight into distributed propulsion technology [39]. In the broad aspect of engine configurations, one can divide distributed vectored propulsion into three main categories [40]:

0.9 0.8 0.7

T/OWE [W/g]

0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.00

200.00

400.00 600.00 800.00 OWE/s [kg/m2]

1000.00 1200.00

Fig. 10. Maximum cargo range vs. MTOW for all commercial aircraft in Table 3.

selected aircraft in this research work incur a number of factors that impact the reliability of the presented data and are as follows:

 Although many of the selected aircraft use the same number of









379

engines, influence and evolution of different engineering disciplines such as: material sciences, aerodynamics, flight control systems, propulsion systems and many other adjacent areas provide each and every aircraft with a unique characteristic, creating an intricate comparison process. Unavailability of aircraft data has introduced a larger level of uncertainty, if all the listed data were to be cross-verified with several different sources. This stems from different performance data for given aircraft, based on vague operating conditions. The listed data has therefore primarily been derived from one single source to minimize any inconsistencies [19]. Misinterpretation of the presented and the provided data is possible due to errors or data fabrication by aircraft operators or manufacturers, along with inconsistencies in parametric definitions that have evolved in aviation history, and assumptions (please see the following bullet point) that do not represent actual conditions. The presented data may also have been influenced by thrust assumptions, payload assumptions in accordance with range considerations, maiden flight assumptions on the basis of launched aircraft programs and the perspective of treating air passengers as payload with an average weight of 80 kg, in both calculations and analysis. Ambiguous definitions of a specific aircraft role may have caused misplacement of the aircraft categories as some aircraft have been used for combined commercial, transport and military applications.

 Distributed engines (DEN)  Common-core multi-fans/propulsors (CMF/CMP)  Distributed exhaust (DEX) Contrary to the great implications distributed propulsion may hold for the future, this technology has only been investigated by a limited number of institutions around the world. NASA’s research efforts in collaboration with a number of partner universities [42,43], companies [44,45] and research institutes [46] have successfully contributed to new findings regarding the many research aspects of distributed propulsion technology. In Europe, and the United Kingdom in particular, researchers at Cranfield University have also taken steps towards the exploration of distributed technology [47,48]. One of the adjacent research areas of distributed propulsion has been investigated through the silent aircraft initiative by Cambridge University and Massachussettes Institute of Technology [49] with some research contributions from Cranfield University [50]. Further research in this area has also led to suggestions to combine distributed and electric propulsion [51,52]. As a consequence of this quest for more electric aircraft, electric motors and high temperature superconductivity are proposed for future aviation. Fig. 12 illustrates a number of distributed propulsion concepts and components. Due to the large variety of research contributions associated with distributed propulsion, it is rather difficult to gain

3. A glimpse of past research endeavors in distributed propulsion technology In this section, a number of past research projects are discussed that have contributed to a better understanding of distributed propulsion arrangements. Only the basics of distributed propulsion will be described. The needs for aviation sustainability currently motivate the identification of propulsion

Fig. 11. A Hybrid Wing Body aircraft concept employing distributed propulsion technology. Source: [41].

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Fig. 12. A number of distributed propulsion concepts and components: (A) a side-by-side geared dual fan with BLI integrated on board a BWB aircraft, (B) single-fan podded engine integrated on board a BWB aircraft, (C) common-core multi-propulsor engines, (D) an electrically driven fan configuration considered for a BWB aircraft, (E) distributed engine arrangement for a conventional aircraft, (F) distributed driven fan configuration for a HWB aircraft, (G) fans considered as aircraft propulsors. Adapted from sources: [172,48,39,166].

an overview of these research efforts, unless different contributions are systematically listed. Table 2 presents some of the past research efforts in distributed propulsion technology. The selected papers include only research studies that target multiple challenges in commercial aviation. The successful silent aircraft research endeavors have not been included in this comparative selection. Instead, these are discussed in the following section on adjacent research areas. Whilst the number of highlighted research studies is limited, it can readily be established that all three distributed propulsion categories, mentioned earlier, are represented in Table 2. Furthermore, it is also evident that drawbacks of small gas turbine propulsion and suggestions for alternative technologies have identified the electrical pathway to optimization efforts with superconductivity. Distributed driven fans seem to become the obvious choice in regards to the latest development for this propulsion technology. One may, however, also argue that this

choice would prove to be of a different nature and highlight other technologies if considered beyond the N+3 advanced aircraft concepts or in other words, aircraft concepts three generations beyond the current commercial transport fleet (  Year 2030). Even though the mentioned statement might carry some weight, the aim of this section has merely been to observe possible historical trends and to minimize speculations regarding the outcome of possibilities. Future proposals are therefore based on historical trends that have shown potential for successful implementation of the environmental demands likely to be encountered in the future. Distributed propulsion has not always been regarded as an instrument for comparative aircraft propulsion analysis. Due to the multi-disciplinary trait of distributed propulsion and the many complexities associated with an overall assessment of this propulsion technology, significant research contributions have been made in adjacent research areas that impact the entire

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propulsion framework. Drela’s choice of presenting a control volume analysis of compressible viscous flow with focus on mechanical power and kinetic energy flow, rather than forces and momentum flow, shed some light on the benefits of a more comprehensive formulation for general wake ingestion and distributed propulsion in tightly integrated propulsion systems [53]. A mathematical formulation for the blowing out of the trailing edge of the wing was derived by Ko et al. [43]. Findings from this model did indeed verify an increase of propulsive efficiency for ducted aircraft engines, with part of the exhaust to exit along the trailing edge, in spanwise parts of the wing. Following these achievements, it was shown that jet-wing distributed propulsion could yield propulsive efficiencies on the order of turbofan engine aircraft [54]. Challenges of enclosing ducts and fans within the wings and other efficient ways to integrate the propulsion system with the airframe, soon became the research aim of Multi-Disciplinary Optimization (MDO) studies [169]. Initially combined with jet-wing distributed propulsion, MDO became a needed design step for distributed propulsion arrangements. MDO analyses have also been suggested for reduction of airframe noise [55] leading to new research areas in the design process of distributed propulsion aircraft [56]. Currently, distributed propulsion is commonly considered for both conventional [167] and unconventional airframes such as a catamaran aircraft [57], the Blended Wing Body (BWB) aircraft [58,59] and the Hybrid Wing Body (HWB) aircraft as shown in Fig. 11. This approach enables the implementation of emerging technologies for commercial aviation. A rather complete list of these design and technological concepts was assembled by Hill et al. [60] for noise reductions. These configurations included, amongst others: Embedded engines with boundary layer ingesting inlets, distributed exhaust nozzles installed on podded engines and distributed propulsion. Embedded distributed propulsion systems for high-lift generating airframes enabled Propulsion-Airframe-Integration (PAI) for which low-noise STOL operations and efficient high speed cruise could be achieved [61]. Moreover, BLI techniques have been proposed for a wide range of applications [1,62–66]. For PAI considerations, the benefits of BLI come into effect once the kinetic energy produced over and above the amount needed for propulsion is minimized and the aircraft wake is re-energized [67]. BLI can also improve the propulsive efficiency [68] or be combined with other technologies such as TV. Thrust generation and wake filling through trailing edge lifting surfaces contribute further to aerodynamic advantages of drag reduction and maximum lift increase. Pioneering work on crossflow propulsion systems considered embedded cross-flow fans in thick wings [69]. Nonetheless, detailed studies of thrust generation and wake fillings on thick airfoils revealed soon that the mentioned airfoils were not suitable for commercial aircraft cruising at high subsonic Mach number as geometrical constraints of the airfoil remain distinct for cross-flow fans within propulsive wing configurations. Today, the potential application of commercial aircraft employing Extreme Short Take-Off and Landing (ESTOL) capabilities are greatly dependent on understanding the flow physics of cross-flow fans and their innovative integration into the airfoil trailing edge [70]. Circulation control was proposed as highly synergistic with distributed propulsion systems [71]. Dygert and Dang demonstrated this feature through a research study that examined the feasibility and effectiveness of a crossflow fan embedded in an airfoil for simultaneous propulsion and circulation control [72]. Dang and Bushnell’s review of cross-flow fan propulsion and flow control concepts identified further future challenges in these applications [73]. Distributed propulsion has generated many innovative concepts towards aircraft noise reduction. One of these concepts is

381

Fig. 13. CFD calculations of predicted velocity contours for the reference nozzle and different DEX designs. Source: [45].

put into practice in the DEX nozzles that exhibited beneficial aeroacoustic properties applicable for noise reduction [74]. Fig. 13 illustrates predicted velocity contours for two DEX configurations. Research findings suggest that proper internal geometry design of DEX nozzles could reduce radiated noise even further [75]. Extensive noise reduction efforts have been made for a BWB by addressing the propulsion system noise and the airframe noise [76,77]. Distributed propulsion concepts are further also used for achieving Ultra-High Bypass Ratios (UHBR) engines. Current engine technologies with a bypass ratio of about 20 can facilitate a jet noise reduction of about 30 dB, according to Manneville et al. [78]. Further progress made by Hileman et al. [49] presents a conceptual design of an aircraft with a calculated noise level of 62 dBA at the airport perimeter. The opportunities for noise reduction offered by distributed propulsion technology involve many areas including aerodynamics, performance, materials, aircraft stability, dynamics and control, and mission operations.

4. The electric aircraft Recent trends in commercial aviation point towards more advanced electrical systems on board an aircraft. Tracing the evolution of the electric aircraft is interesting from both a historical point of view, and for future considerations of civil aviation. One of the distinct characteristics of the electric aircraft is that it employs electric motors instead of internal combustion engines. For this purpose, the electricity can be supplied to the electric motors using different methods. In the past, fuel cells [79], batteries [80], solar cells [81], ultra capacitors [82] and other means have been considered for this purpose. The electric aircraft can broadly be divided into two main categories: The All Electric Aircraft (AEA) and the More Electric Aircraft (MEA). A deeper understanding of the Primary Power Systems (PPS) referring to the main propulsion power, and Secondary Power Systems (SPS)

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referring to the distributed power around the airframe and the engine systems can cast light on the AEA or MEA concepts [83,84].

4.1. Selected milestones of the all electric aircraft Early days of the electric aircraft included a minimal electric part, which primarily consisted of the electrical power dependency for ignition purposes for the very first powered flights in 1903. Growing dependency on electrical power was soon evident with more electrical subsystems, eg. radio communication [85]. Conversely to the fact that a number of different considerations have boosted the electric aircraft to the technological position it retains today, it can be argued that the majority of proposals for the electric aircraft came to light in the latter part of the twentieth century. In 1943 Kilgore et al. proposed the electrical airplane propulsion system shown in Fig. 14 to drive multiple rotating propellers [86]. Equipped with one or a small number of poly-phase synchronous generators in the speed range of 10,000 RPM to 20,000 RPM, a pole-number range of 4 and 8, and a number of propeller driving poly-phase motors energized from the generators, this power plant arrangement revealed a number of advantages. Additional power for take-off, reduction of runway length and propeller drag force, skid avoidance during the landing phase, wheel brakes, reduction of detachable conductors, elimination of sparks using induction-motors to drive the motors, and minimization of heavy concentrated weight burden on the wingstructure, were some of the significant benefits of this concept. Many complexities with electric aircraft propulsion have played a noteworthy role in the evolution of the AEA. Restrictions in a given technology have further motivated the exploration of alternative systems to be used in the electric aircraft. An important example for this is the introduction of fuel cells in aeronautics. Early fuel cells were associated with other technical objectives [87,88] rather than used as electrochemical devices to produce electricity [89]. Fuel cells provided an alternative technology for the electric aircraft. In 1974, an electro-motorically driven aircraft was suggested by Meier et al. [90]. This configuration employed fuel cells or batteries for driving the propellers. The perennial drawback of the weight to power ratio, along with

Fig. 14. Electrical airplane propulsion patent proposed to drive a plurality of rotating propellers in recognition of the need for additional power for take-off and reduction of runway length. Adapted from source: [86].

the excessive weights of fuel cells and batteries, constantly motivated researchers to restrict the usage of electric aircraft to unmanned, low speed aircraft with high aspect ratios wings. Many of these concepts employed a distributed propulsion arrangement. Even though substantial efforts were made to increase the power-to-weight ratios, many of the goals in favor of the electric aircraft could not be achieved. Suggestions made by the team of Meier, and other scientists around the world, considered a variety of possibilities for the electric aircraft. A true display of the electric aircraft technology came to reality through the solar-powered research programs initiated by NASA and AeroVironment, Inc., in the beginning of the 1970s. Similar research endeavors were also pursued around the globe by other scientists and research teams. The highlighted research programs at NASA represent a small portion of the technologies involved with the electric aircraft, and thus a few milestones of this specific era will be revisited. The concept of the Sunrise I airplane was born in the early 1970s and this aircraft made its first flight on November 4, 1974 as the world’s first solar-powered airplane [91]. Although the usage of solar power limited the aircraft to day flight and cloud avoidance, it served as a proof-of-concept to develop electric-powered fixed-wing aircraft. Even though Sunrise did not attain extended solar flights, it was able to provide the tools for an improved version of solarpowered aircraft, called the Sunrise II. Sunrise II displayed even more potential to reach high altitudes and could benefit from improved aerodynamics. In 1980, Gossamer Penguin used the removed solar panels from Sunrise II for its initial flights [92]. The aircraft had a 71-foot wingspan, and used 3,920 solar cells to produce 541 Watts of power. After flight tests with solar cells, batteries and an electric motor, it was proven that electric aircraft could also be manned. The first official manned flight of direct solar power was completed on 7 April 1980, and this concept was evolved into Solar Challenger that had a 46.5-foot wingspan and accommodated 16,128 solar cells. Solar Challenger was designed to withstand normal turbulence levels and was equipped with batteries, solar cells, an electric motor and a propeller. In late 1980, the initial flights were moved from California to Marana Airpark, northwest of Tucson, Arizona. By that time the aircraft had already moved from flights using batteries to solar-powered flights. Solar Challenger was able to complete a manned flight from Paris to London on 7 July 1981 in an attempt to show the feasibility of the aircraft’s efficiency [93]. The same year Solar Challenger took to the skies, the classified program High Altitude SOLar Energy (HALSOL) was launched by the U.S. Government to explore the feasibility of solar-electric flight above 65,000 feet. About a decade later some of the findings from the HALSOL program contributed to Pathfinder, an unmanned aircraft that was able to reach a record altitude of 50,500 feet for solar-powered aircraft. In 1997, Pathfinder was eventually transferred to Hawaii, due to the high levels of sunlight available in that location. Pathfinder was able to reach a world altitude record of 71,530 feet for solar-powered and propeller-driven aircraft [13]. Moreover, Pathfinder was upgraded to Pathfinder-Plus during 1988. This aircraft was able to reach even higher altitudes than the original Pathfinder by reaching an altitude of 80,210 feet and breaking the record altitude of propeller-driven aircraft. Some notable changes made to the Pathfinder Plus enabled it to reach higher altitudes than ever before and served as a framework for an even more improved solar aircraft called the Centurion [94]. Increased wingspan, additional motors, and more efficient silicon solar cells provided Pathfinder Plus with an additional 5000 Watts power in comparison to the 7500 Watts power used for the Pathfinder. An interesting observation regarding the engine power output is that the number of engines has steadily increased from the Solar Challenger to the Centurion aircraft. Centurion evolved the ideas of a solar-powered aircraft to higher levels and proved that it was possible for an

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aircraft to use telecommunications relay platforms and stay airborne for weeks and collect scientific sampling data and imaging data. Centurion’s flexible wing made of kevlar, carbon fiber and graphite epoxy composites was divided up into five sections and had no taper or sweep [92]. Solar cells were used to power the electric motors, communications, electronic systems and avionics. Centurion was further equipped with a backup lithium battery system that could allow additional two to five hours limited flight after dark. Extensive research progress made for the HALE aircraft category placed solarpowered aircraft concepts into practice. Many of these aircraft employed electric motors, driven by batteries and solar cells, as shown in Fig. 15. NASA’s solar-powered and electrical aircraft initiatives were only a fraction of the extensive research work, done in the direction of the electric aircraft. In many ways, the HALE aircraft are the true representatives of AEA. Further, an increasing number of electric aircraft have entered the manufacturing phase over the years. Table 1 lists a few electric aircraft in this category. IFB Hydrogenous stands out amongst the different electric aircraft in Table 1, as this particular aircraft also uses liquid hydrogen, batteries and a fuel cell on board. Fig. 16 depicts a plot of the engine power versus the MTOW for a number of electric aircraft. IFB Hydrogenius delivers the largest engine power through its combination of different power systems, which seems to be the most suitable option for larger MTOWs. This rather simplistic survey exhibits one of the distinguished traits of the electric aircraft which is the limited power densities for given airframe weights.

Fig. 15. An unmanned solar powered aircraft employing distributed propulsion. Source: [92].

100 90 80 IFB Hydrogenius*

Engine Power [kW]

70 60 50 Antares DLR-H2

Yuneec 430

40

Electravia Electroclub

30 Flightstar/Yuneec E-Spyder

20

Electravia Electrolight

10 0 0

100 200 300 400 500 600 700 800 900 1000 MTOW [kg]

Fig. 16. Engine power versus MTOW for a selected number of aircraft. Compiled from source: [99].

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Thus, a combination of different power systems is more likely to present a solution for larger engine powers and should therefore be considered in the future. NASA explored this direction through an analytical performance assessment of a fuel cell powered small electric airplane [95]. Similarly, researchers of the ENFICA-FC project have also looked into the feasibility of powering an all-electric propulsion aircraft with fuel cells [96,97]. For the sake of AEA discussions, it should be emphasized that fuel cells do not represent the only proposed complementary technology for AEA, but are still considered important components in electric aircraft schemes and as possible Auxiliary Power Units (APUs) [98].

4.2. The more electric aircraft and related systems An adjacent electric aircraft concept that represents the most electric aircraft in civil aviation today, is called a MEA. As the AEA concept is based on all electrical systems, a distinct conflict arises when this concept is applied to a larger airframe with increased gross weight. The challenges of combining the hydraulic and pneumatic power systems with the electrical system, while maintaining safe flight performance characteristics, results in an increasing role of electrical systems on board the aircraft. Consequently, the MEA terminology refers to the increasing impact of the electrical system among the other systems employed in the aircraft. NASA’s solar-powered aircraft and the electrical aircraft in Table 1 have clearly demonstrated the feasibility of the AEA concept. A transition from the AEA to a MEA with increased gross weight and larger number of air passengers imposes severe challenges in terms of increased need for power densities. Ironically, the aim of the MEA concept for commercial future aircraft with large number of air passengers is to revisit the AEA concept, while maintaining the same number of air passengers and an increased gross weight. The more electric aircraft concept can be regarded as an attempt to overcome the challenges associated with the AEA. A historical map of the MEAera could be simplified into two different terminologies: Fly-by-wire and power-by-wire, which in chronological order refer to the reinstatement of electronic linkages for hydraulic and mechanical linkages used for aircraft control, and the replacement of hydraulic/ mechanical actuators and drives on the primary and secondary flight surfaces [100]. Pursuing an electric aircraft has proved to include advantages that were not initially considered within the borders of such a technological achievement. It has been estimated that the more electric technology is capable of reducing the empty weight of a typical airliner by around 10% [84]. Doyle anticipated a comparable reduction in Specific Fuel Consumption (SFC) as well [101]. Reduction of moving parts has also boosted the perception that maintenance costs would be lowered and that the overall reliability would increase. Currently, most aircraft engines provide power using an external accessory gearbox, classically driven from the low pressure turbine via a mechanical drive shaft. More electric engines, or turbofan engines equipped with generators integrated into the engine, have been the response to this approach [105,106]. Supplying pneumatic power using air bleed from engine compressors is combined with the external gearbox that drives electrical generators and hydraulic pumps. Moreover, conventional electrical generators are both of constant and variable speed type. Typically, the electric power system used in most pre-MEA era airplanes was a combination of 28 V DC for avionics and batteryoperated services, and 115 V, 400 Hz for large loads [107,104]. Integrated Drive Generators (IDGs) on current engines are increasingly being proposed to be replaced by a proposal suggesting variable frequency AC systems and simpler generators. A single high-voltage DC output for transmission of electricity to the airframe is also a promising approach if the More Electric Engine (MEE) generated electricity is taken into consideration, at a range of

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frequencies. With losses proportional to the square of the current for a given cable, higher voltages are preferred in power transmission of aircraft electric power. Laskaridis investigated the performance and system architecture of MEA in an extensive research work [108]. One of the findings from this work was that the aircraft configuration could impact the AEA or MEA technologies. Parameters, such as mass, SFC, power requirements and overall fuel usage savings, were linked to the aircraft engine types, the number of engines, and design specifications. MEA and AEA designers have recognized the

Fig. 17. Modern electric aircraft subsystems. Adapted from: [102,103].

Fig. 18. Comparison of conventional and more electric engine/aircraft systems. Adapted from: [104].

increasing power level demands both in commercial and military aviation [109,110]. The United States Air Force and Navy have also conducted many research programs in pursuit of the AEA/MEA concepts [111,170]. To meet the Low Pressure (LP) shaft generator requirements of the MEA, a Permanent Magnet (PM) generator interfaced with a Pulse Width Modulator (PWM) voltage source converter was suggested by Mitcham and Cullen [112]. LP shaft related research for gas turbine aero-engines has focused on the power control of a variable speed permanent magnet and a fault tolerant generator for limitation purposes of the fault current, in pursuit of minimizing the current rating for converters and generators [113]. An initial comparison between a Switched Reluctance (SR) generator and a PM generator was considered in Mitcham and Grum’s work [114]. Both machine types may permit great fault tolerances. In the case of the PM generator a PWM active rectifier was suggested. Recent LP shaft research has also eliminated the problems that arise during an open and short-circuit fault condition when a substantial torque ripple and a destabilizing power swing occur [115]. Fig. 17 shows a typical scheme for a MEA. As increasing numbers of systems and subsystems in the aircraft are becoming electrically driven, a larger demand arises to convert current on board the aircraft. Transformer Rectifier Units (TRUs) convert AC into DC and are used to charge batteries from AC generators [116]. Cheng’s study [117] about AC and DC converters for the MEA has revealed that size and weight improvements of power electronics are confronted and limited to conventional design features in TRUs. Cheng has, however, predicted limited advancement for TRUs in the future, as they seem to stay within the existing power density figures. In contrast to TRUs, an active Power Factor Correction AC/DC Converter (PFCC) exhibits potential for further development and was predicted to increase the power density if high switching frequency was used. PFCC’s complex topology, design, and a number of key issues motivated more PFCC research efforts to enable this technology. The MEA architecture has also included the MEE. Provost’s comprehensive research work [104] explains the difference between a conventional and a MEE/aircraft system, as shown in Fig. 18.

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Generally, to implement a MEA architecture, one strives to replace the three individual electric, pneumatic and hydraulic secondary power systems with one globally optimized electrical system [118,119]. Furthermore, proper integration of secondary power and propulsion into the airframe via emerging power electronic technologies and electrical machines is also recognized. Two modern commercial aircraft that implement a MEA scheme are the Boeing 787 and Airbus A380. A number of smaller systems, horizontal stabilizer backup, thrust reverser actuation on the Airbus A380 function electrically. Some of the electrical system features on the Boeing 787 are brakes, ice protection, engine start, environmental control systems and electro-hydraulic pumps for actuation. Further, hydraulic devices will not pressurize subsystems such as landing gears throughout the entire flight, and only activate them as needed. Faleiro [120] points out that one MEA approach is to optimize individual systems on board an aircraft. The systems of interest can potentially consist of actuation systems, pneumatic systems, aircraft electrical systems, wing protection systems, Environmental Control System (ECS), engine start systems, and all subsystems involved. Another route is, however, to step away from an assembly of electrical systems and address the MEA at the aircraft level. According to Faleiro current trends toward this approach have been decreasing engine autonomy, increasing generation sources of electrical power, considerations for power-off takes at the engine, considerations for the effects of load distribution, and weight penalties associated with power electronics and motor drives. Composite materials are also suggested as a solution to address the weight penalties of the entire airframe structure. Fig. 19 illustrates that the material selection for the Boeing 787, one of the most recent MEA, is as follows: 50% composites, 20% alumnium, 15% titanium, 10% steel and 5% other materials [121]. 4.3. Electric motors for airborne applications The basic principles of electric motors are well established within the scientific community and electric motors step into new technological areas on a daily basis [122]. The aim of this section is to provide the reader with a brief and simplistic view of some general properties electric motors possess, and then shift the focus to a few airborne applications of electric motors. Hughes identifies the following seven common properties with electric motors [123]: 1. Speed is proportional to output power per unit volume. 2. Large motors have a higher specific torque and are therefore more efficient than small ones. 3. Motor efficiency improves with speed. 4. A motor can be modified for any voltage. 5. Most motors can stay overloaded for short periods without being damaged.

6. The output from any given motor is constrained by the cooling arrangement. 7. Motors with similar cooling systems have a rated torque almost proportional to the rotor volume (roughly the overall motor volume). Weight considerations are of paramount importance in aviation. Fig. 20 shows the weight percentage breakdown for a 300 seater aircraft. In consideration of an electrical aircraft, it is evident that the motors, generators and electrical cables dominate the electrical system weight. This imposes careful consideration for engine placement, number of motors and generators needed, and how a cooling system is more efficiently provided. Hence, it is not preferable to draw cables across the fuselage by placing the engine at the rear of the aircraft, or to use an excessive number of generators for distributed propulsion technology. Even though electrical cabling losses are much smaller than pneumatic and hydraulic losses, they still manifest limitations to the overall electrical system. In spite of the complex challenges associated with superconductivity [124], this concept is still worth considering in combination with distributed propulsion, as recommended by Ameyugo and others [47,51]. NASA has also looked into the possibility of replacing gas turbine engines with electric motors [125]. To enable a fair comparison, the propulsor weight was deducted from the total weight of the turbofan engine and the propulsor components were estimated to represent 30% of the total engine weight. NASA’s study showed that if gas turbines were to be replaced with electric motors over a range of power levels the following relationship would hold: Weight¼0.137(Power)0.9249. Furthermore, in a comparison of power density for different engines and motor designs, by Johnson and Brown, the power density of a cryogenic motor design is estimated to be 125 times larger than a small industrial motor, 40 times larger than a small aircraft reciprocating engine or 3.3 times larger than large turbine engines [125]. As a result, cryogenic motors display the largest power densities among various electric motors and engines. Given the complex nature of superconductivity as a whole separate technology, this review merely mentions only a few research efforts on airborne applications of High Temperature Superconductive (HTS) machines. A five year program launched in the United States to investigate the prospect of electric propulsion revisits a propulsion arrangement in which advanced superconducting, cryogenically cooled electric motors and generators drive a number of electric fans. Distributed propulsion is consequently brought to life as the fruit of this electrical arrangement. Given the large specific power range of 3–8 kW/kg for turbofans and the lower specific power of conventional motors

Carbon laminate Carbon sandwich Fiberglass Aluminum Aluminum/steel/titanium pylons Fig. 19. Material selections for the Boeing 787 Dreamliner. Adapted from source: [121].

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Fig. 20. Electrical system weight for a baseline 300-seater aircraft.

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(0.5 kW/kg), new solutions are considered. One of these suggest to modify the traditional turbine and fan coupling on the same shaft by a turbine and fan decoupling process through electrical converters. Mechanically linking each generator to a corresponding turboshaft aids the coupling of an electrically connected fan arrangement through an electrical gearbox. One of the beneficial traits of superconductors is that they enable light machines by carrying very high current with little resistance. Fig. 21 shows the rotating power for a number rotating machines of different configurations. Interestingly enough, the Helios motors are also included for comparison purposes with a cryogenerator. Industrial motors are too heavy for airborne applications, but recent findings indicate that all superconducting motors can become three times lighter, making them comparable with turbine engines in terms of power density [51].

5. A proposed framework for future commercial aircraft employing distributed propulsion Novel propulsion systems need to involve features beyond the current propulsion enhancement techniques of increasing the

turbine entry temperature [126,127], pressure [128] and the bypass ratios [129,130] of gas turbines. Biofuels [131,132], alternative propulsion systems based on hydrogen [133], and the all/more electric aircraft [120,134] are frequently proposed to meet future requirements of aviation sustainability and to overcome the conventional constraints of gas turbine technology. Seemingly, the only obstacle for new propulsion systems is the safety risk to air passengers, as perceived in the case of a nuclear powered aircraft [9]. Green predicts that reducing the environmental impact will become an increasing objective in the future and he emphasizes that all three aspects of environmental impact, i.e., noise, local air and climate change, are in need of constant attention [135]. In the next decades similar aviation regulations as those proposed in the context of Vision-2020 might be imposed. These aspects gain added weight with increasing global air transport growth as predicted by FAA [136]. In order to consider the concept of distributed propulsion systems for future applications, it is essential to address some of the parameters that are coupled with propulsion configurations. The propulsion system is obviously not an isolated component and interacts actively with other aircraft components. Fig. 22 illustrates a proposed commercial aircraft framework for employing a distributed propulsion scheme. This macroscopic scheme is primarily divided into three main sections: aircraft characteristics, aircraft propulsion system and aircraft operations. Each individual section involves additional degrees of freedom to assist in specifying the intended future aircraft. The macroscopic trait of the framework covers only a number of key questions and provides aircraft designers with a comprehensive overview of the aircraft system architecture. It is, however, essential to translate this aspect into a microscopic viewpoint, in order to assess the full potential of each individual component. All choices made on the left hand side of Fig. 22 contribute to the listed parameters on the right hand side, and consequently to the overall signature of the future aircraft. 5.1. Aircraft characteristics

Fig. 21. Specific power for a number of rotating machines compared to the turbine engine core. Source: [103].

One of the first items to consider is to decide whether the aircraft should be manned or unmanned. Although the idea of having unmanned commercial aircraft does not make practical sense, one should still recognize that the evolution of UAVs could bring unmanned aircraft probability into the possibility zone for

Commercial Aircraft Framework for Employing A DistribUted Propulsion Scheme (CAFEADUPS)

Aircraft Characteristics • Choice of manned/unmanned aircraft • Choice of airframe design • Choice of airframe materials • Choice of instruments for aerodynamic enhancement • Choice of numberof passengers and payload

Aircraft Propulsion System

Environmental Impact Aircraft Fuel Consumption Aircraft Noise

• Choice of one/more propulsion system(s) • Choice of distributed propulsion arrangement • Choice of the number of propulsion units • Choice of the propulsion unit integration with the airframe • Choice of one/more transmission methods for the chosen propulsion units

Aircraft Costs

Aircraft Operations

Overall Aircraft Impacton Civil Aviation

• Choice of Take Off and landing procedures • Choice of runway approach and leave • Choice of aircraft range • Choice of operating flight regime(s)

Fig. 22. Proposed framework for commercial aircraft employing distributed propulsion technology. Aircraft image adapted from source: [143].

Aircraft System Overhaul and Maintenance Aircraft Weight

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other applications. If commercial aircraft are used only for air cargo transportation, all systems related to cabin pressure and environmental control are drastically impacted. The choice of a conventional airframe or other unconventional airframes, such as a BWB/HWB aircraft, primarily affects the lift-to-drag ratio [137]. Aircraft materials are also constantly reviewed [138] with respect to their properties, preset airframe design requirements, and the flight conditions in which they will be operated. The aircraft designer should consider the impact distributed propulsion has on the airframe for successful airframe-propulsion integration. A combination of aircraft design/geometry and airframe materials will dictate the aircraft OWE. Weight will also need to be considered for the number of passengers or amount of payload carried, in addition to other weight parameters that contribute to aircraft MTOW. Many of the discussed parameters within the proposed framework are directly or indirectly interlinked. Aerodynamic enhancement could, for instance, involve active/passive flow control [139–141], TV [142] as well as all other engineering solutions that provide the means for high-lift devices to enable shorter take-off and landing. Further, an additional degree of freedom for aircraft operation from smaller airports is made possible through this enhancement. This reduces congestions at hub airports, but this detail is not part of the macroscopic viewpoint adopted in the present paper. 5.2. Aircraft propulsion system Decisions regarding one or a multiple of propulsion systems is an important topic for future aircraft propulsion. This new option adds another dimension to the flexibility of utilizing different propulsion systems for different flight segments. Opportunities do, however, also bring challenges, in which novel combined cycles must overcome the complex interaction and transition between the chosen propulsion systems in view of safe operational standards. In earlier sections we have analyzed different propulsion arrangements for distributed propulsion technology. Based on the number of propulsion units, and the integration of engines/motors/propulsors/fans/exhaust units, in/above/around/ across the wing/fuselage, transmission methods should be chosen in terms of pneumatic/mechanical/electrical or other alternative methods for optimal energy transmission and minimal energy loss. 5.3. Aircraft operations Using high-lift devices for STOL or Cruise Efficient Short TakeOff and Landing (CESTOL) aircraft, as shown in Fig. 22, does affect the aircraft operating possibilities and the noise footprint for runway approach and take-off. Dependent on the chosen aircraft design characteristics, propulsion power and aircraft weight, the aircraft range may vary, as shown from the historical aspects of distributed propulsion technology. The operating flight regime(s) will intentionally be discussed in a greater detail to highlight a number of adjacent technologies of interest. Flight regimes are chosen as the main fountain of different propulsion concepts, that in some respect could give birth to ideas that compete against distributed propulsion technology. Technical hurdles limited aircraft operations to subsonic speeds during the first half of the twentieth century. Breaking the sound barrier in 1947 opened the skies to subsonic, transonic and supersonic flight [144]. Nevertheless, commercial aviation has hitherto concentrated the majority of its activities in the subsonic and transonic regimes. Given that more than half a century has passed since the supersonic flight regime first was investigated, one might question why commercial aviation has

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not been exposed to a larger number of airplanes for supersonic air passenger transport. One of the drawbacks of air transport in the subsonic regime is said to be its speed limitation. Prior to a historical race for the first supersonic commercial airliner between Russia and the West, some researchers praised supersonic efforts, while pointing out the many challenges that lay ahead. Among these, a special need for further research to enhance lift to drag ratio at supersonic speeds, and specific propulsion problems such as matching engine flow to inlet flow were emphasized [145,146]. Davies [147] identified the severe challenges of supersonic flight early on, and emphasized that the considerable costs to ensure technical readiness in combination with economic and operational viability for a supersonic airliner should not be underestimated. In the aftermath of the oil crisis in 1973, many major economical issues associated with commercial supersonic aviation had still not been successfully addressed. In the case of the Concorde aircraft fleet the elusive profit was caused by a combination of too few passengers over too short routes. [155]. On the subsonic propulsion side, however, many fascinating steps were taken. A comprehensive review of the early prop-fan propulsion technology was presented by Gatzen and Adamson [149]. The prop-fan engine has many different designs. In a NASA study [148], the prop-fan was represented as a threespool shaft engine, where the high spool was an axial/centrifugal compression system driven by a single-stage high-pressure turbine. This specific design also used a pipe diffuser and mated a single-stage aerating burner with the centrifugal compressor. The low-pressure spool had a four-stage, low-pressure compressor driven by a single-stage low-pressure turbine. Moreover, the power turbine operated at high efficiencies and the propellers were driven by an in-line differential planetary gearbox with counter-rotating output shafts. During the small-scale models phase, prop-fan designs were considered for a cruising Mach number of 0.6–0.85. This specific NASA sponsored program demonstrated significant fuel savings potential, and verified different techniques that could influence the cabin noise comfort level, such as direction synchro-phasing [149]. Black and Murphy [150] addressed jet noise reduction for the first generation aircraft. Neitzel reviewed the potential and cost effectiveness of conventional turbo fans with optimized thermodynamic cycles [151]. Moreover, a merger between the refan and the re-engine approach contributed to the open rotor technology. Blythe [152] concluded that contra-rotating propellers provided one of the most efficient means of propulsion for Mach numbers between 0.6 and 0.8. Apart from the propulsion revolution of jet engines, a transition from the turboprop engine to a new open rotor technology placed advanced powerplant categories on the table. Early bypass engines led to improvements in propulsive efficiency, and were followed up by high bypass ratio turbofans. Borradaile [153] emphasized that the UHBR engine could be employed for a long range large aircraft because it had offered cruise SFC improvements and DOC reductions. However, there is still a need to understand better the associated flow physics and mechanical airframe-propulsion-integration of these configurations. These historical prop-fan milestones identify advanced prop-fan technology as a possible future aircraft propulsion competitor to distributed propulsion technology. Speed considerations for a commercial aircraft have always initiated discussions about the real importance of saving traveling time. Today, any airline passenger is aware of the fact that the elements of flight delay are usually dominated by a range of factors before take-off and after touchdown, and not necessarily during the actual flight time. This assumption is certainly only valid if severe weather conditions, aircraft malfunction circumstances, air traffic issues or other unforeseen events that may delay the flight journey do not occur. As mentioned, the push for higher flight Mach

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numbers is driven by the chase for faster air travel. As for the supersonic flight regime, the fundamental question remains whether the technical complexities, caused by sustained operation at high temperatures with high-pressure inlets and high-velocity exhaust systems [154], make supersonic commercial transport economically and environmentally viable [155].

6. A few challenges for an all electric future commercial aircraft employing distributed propulsion The number of challenges associated with an AEA featuring distributed propulsion is so great that a systematic approach is advisable. However, listing all possible challenges in such a multifaceted framework is beyond the aim of this section and thus only a selected number of challenges will be outlined. Apart from the technical challenges, attitudinal challenges also play an important role given the likelihood of stringent environmental policies. These two challenges make it doubtful that the industry will invest in the development and mass manufacturing of small gas turbines for distributed propulsion arrangements. One of the major challenges with a gradual transition from the MEA to AEA is the power output for airborne generators and actuators. Mature technology has been proven for 100–200 kW generators, but achieving 1 MW generators for airborne applications is a substantial challenge [156]. In order to maintain a lower weight for the electrical system, voltages higher than 115 V AC have to be used. Furthermore, conductor sizes could potentially be reduced if 270 V DC systems were used, but movement in this direction would also impose arcing electricity and disruptive electromagnetic effects associated with high-voltage DC. Moir identified the four options for MEA electrical power generation [134]:

scheme is that it increases the size and the number of generators. Other challenges arise from sizing electric components with efficient electric motors, generators, APUs and from implementing optimized electric transmission methods through system studies of novel airframes [163] and AEA propulsion architectures [164]. Detailed assessments of the development, direct operating and maintenance costs are required to choose between distributed propulsion and other proposed propulsion systems for future aircraft.

7. Conclusions In this paper the historical evolution of distributed propulsion technology for commercial aircraft and the challenges of implementing a distributed propulsion arrangement on future all electric commercial aircraft were discussed and the following key features were revealed through this study:

 Total aircraft propulsive power and weight are the two 

 

    

Variable speed constant frequency (VSCF)—cycloconverter. Variable frequency (VF). Variable speed constant frequency (VSCF)—DC link. Constant frequency IDG (IDG is a combined constant speed drive and generator unit).

These options add to the many choices available for the electric aircraft. Weight considerations would also enter the scene if redundant APUs are needed for reliability purposes and selected components are kept cooled and monitored to avoid electromagnetic interference [101]. These challenges are only a part of the many challenges MEA and AEA concepts are likely to face. The use of many electrical units and components, such as electric motors and cables, imposes new weight penalties. Therefore, for weight reduction the use of composite materials has been suggested. Yet, the impact this would have on the airframe in the event of lightning strikes needs to be recognized. Another vital challenge is the protection of the aircraft system/data network and the limitation of its usage to authorized personnel only. The need to investigate these issues has stimulated research areas of interest in the MEA/AEA architecture [157–159]. Proposals of implementing superconductivity as an enabler for distributed propulsion arrangements introduce new challenges as the superconductive technology has not yet reached its full potential [160,161]. Moreover, the use of cryogens for superconductivity contributes to additional volume, space and weight challenges [162]. Voltage drops, losses and cable weights are also influenced by the specific engine placement for a given aircraft. Novel airframes may further face the possibility of increased number of passengers, even more comfortable air cabin pressurization demands and electrical power needs for advanced on board entertainment systems. Another specific challenge with the distributed propulsion



 

dominant factors most likely to impact a future commercial aircraft employing distributed propulsion technology. The expected increase of future MTOW and OWE imposes weight limitations on future propulsive units because the number of engines employed for distributed propulsion is unlikely to increase proportionally. The majority of aircraft employing distributed propulsion is likely to be used mostly in subsonic aircraft. Employing three or more engines for a specific distributed propulsion arrangement is unlikely to be adopted by the industry since commercial aircraft usually employ only two engines due to enhanced performance of each propulsion unit. The average number of engines per aircraft considered in this study is four. Average values for investigated parameters in this study are as follows: MTOWAverage  116,609 kg, OWEAverage  55,726 kg, SAverage  231:6 m2 , average cruise speed  625:6 km=h, average pay load  20,371:8 kg, average total propulsive power  12,983:1 kW, ðOWE=SÞAverage  210:4 kg=m2 , ðT=OWEÞAverage  0:23 kW=kg, ðPAY=OWEÞAverage  0:37, ðPAY=MTOWÞAverage  0:19 and average range  5179:5 km. A more clearly defined distributed propulsion terminology is desirable to permit engineers, scientists and researchers to discuss this emerging technology more efficiently. Further studies are needed to identify an optimized framework for an AEA. These studies should focus on the weight and number of propulsion units, aircraft size, aircraft range and mission, as well as needed power densities to evolve a commercial AEA as a game changing alternative to a more sustainable civil aviation.

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