A Review of Recent Aircraft Concepts Employing Synergistic Propulsion

A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION Julian Bijewitz, Arne Seitz, Mirko Hornung Bauhaus Luftfahrt e.V., Taufkirchen, Germany

Keywords: distributed propulsion, propulsion system integration, boundary layer ingestion

Abstract e Specific energy [Wh/kg] Recognizing the attention currently devoted to Mcr Mach number at typical cruise [-] the exploration of a more synergistic propulsion m Mass [kg] system integration, this technical paper n Numerical quantity [-] provides a review of recently studied aircraft R Range [nm] concepts featuring tightly-coupled engine  Weighing factor [-] airframe integration solutions. Initially, a  Complexity index [-] comprehensive overview is given regarding the en Energy mass fraction (= men/MTOW) [-] various facets novel integration options such as oe Empty mass fraction (= OEW/MTOW) [-] distributed propulsion architectures may offer pl Payload mass fraction (= mpl/MTOW) [-] with regards to improvements in vehicular  System architecture descriptor [-] efficiency. Thereafter, the challenges associated with strongly integrated propulsion systems are Subscripts: discussed. An array of design solutions conc concept currently under investigation by industry and des design academia are screened, ranging from un- en energy manned to commercial long range applications. pl payload A classification scheme is proposed, based upon ref reference which an assessment process is described including the evaluation of the system Abbreviations: complexity. Several statistical analyses are A/C Aircraft presented aiming at the derivation of trends and AFP Aft-Fuselage installed Propulsor heuristics applicable to distributed propulsion BHL Bauhaus Luftfahrt vehicles: starting from the evaluation of the BLI Boundary Layer Ingestion relations between purpose, methods and BWB Blended Wing Body implementation approaches employed by CFF Cross Flow Fan distributed propulsion concepts, further studies CNB Commercial Narrowbody focus on the identification of important CWB Commercial Widebody characteristics relative to existing transport DSP Distributed/Synergistic aircraft. This includes key geometric Propulsion system integration parameters, system architecture characteristics DP Distributed Propulsion as well as an assessment of the integrated DiPs Distributed Propulsors performance. DEX Distributed Exhaust EPC Energy-to-Power Converter ESAR Energy Specific Air Range Nomenclature EST Energy Storage Type Symbols: FHV Fuel Heating Value E Energy [Wh] GA General Aviation

1 Julian Bijewitz, Arne Seitz, Mirko Hornung

HWB Hybrid Wing Body such as aerodynamics, propulsion, structural MIL Military Transport Aircraft design and flight controls. For example, the MTOW Maximum Takeoff Weight principles of boundary layer ingestion (BLI) and NASA National Aeronautics and Space wake filling (WF) have been identified as Administration promising ways of utilizing distributed OEI One Engine Inoperative propulsion systems. By entraining the boundary OEW Operating Empty Weight layer developing on wetted surfaces of the PAV Personal Air Vehicle airframe into a propulsion system appropriately PAX Passengers designed to compensate the airframe momentum PMAD Power Management and deficit through a momentum increment, a Distribution reduction in induced over-velocities and hence PPC Power-to-Power Converter propulsive power demand can be attained [5]. PPT Power-to-Power Transmitter Different implementation strategies of BLI have PREE Payload-Range-Energy been proposed for transport aircraft ranging Efficiency from aft-fuselage encircling installations [5-11] PTC Power-to-Thrust Converter and double-bubble configurations [12] to STOL Short Takeoff and Landing Blended Wing Bodies featuring ingestion of TaW Tube and Wing aircraft parts of the upper centerbody boundary layer UAV Unmanned Aerial Vehicle [13-16]. VTOL Vertical Takeoff and Landing By introducing novel propulsor arrangements, WF Wake Filling very low specific thrust levels without yielding excessive propulsor diameters may become practical, thereby paving the way towards 1 Introduction reduced noise emissions. Further potential to In an effort to narrow down the gap between the noise reductions may be offered through engine incremental enhancement of conventional shielding, which, depending on the aircraft technology, and the long-term environmental configuration, can often be realized more easily targets espoused by aviation worldwide [1-4], than with traditional tube and wing layouts. further significant efficiency improvements of Motivated by remarkable advancements in the propulsion and power system as well as the gravimetric specific energy and power of airframe are crucial. As engine installation electro-chemical on-board storage equipment issues tend to become more pronounced with and electric machinery, partially or fully ever decreasing specific thrust levels, and electrified power trains are currently under attainable system efficiency gains are expected extensive investigation, see e.g. References [17- to flatten out when retaining conventional 23]. Apart from the prospect of improved propulsion system integration, the exploration efficiencies of the energy and propulsion of novel aircraft morphologies is considered to system, this allows the decoupling of energy-to- be a key factor. In this respect, the perspective power conversion and power-to-thrust of introducing a more tightly coupled conversion. Such an approach opens up novel propulsion-airframe integration has generated ways to synergistically arrange the power and much attention throughout the aeronautical thrust generating devices, and allows for community. independent control of core engine and The benefits potentially attainable from aircraft propulsor, thereby possibly enhancing the designed according to a distributed and operational behavior of the power plant. As a synergistic propulsion system integration practical solution, multiple remotely placed paradigm are manifold. In particular, the propulsors may be arranged such that the exploitation of thus far untapped system level aerodynamic characteristics of the aircraft synergies is of interest when departing from the become improved. This involves enhanced classic separation of airframe and propulsion high-lift generation but may also refer to system. These effects cover various disciplines beneficial cascade effects due to the feasibility 2 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION of smaller sized lifting surfaces [24, 25]. In the associated with distributed and synergistic field of distributed propulsion, hybrid electric propulsion system integration. Moreover, the power trains have thus far been considered for implications regarding system complexity, small scale unmanned applications to geometry and integrated performance will be commercial widebody aircraft. discussed. Also, as distributed electric propulsion may be beneficially combined with vertical take-off and landing technology, new possibilities for 2 Overview of Distributed Propulsion personal transport and on-demand mobility may Aircraft become feasible [25-27]. In addition, the Sehra et al. [32] pointed out the benefits of arrangement of multiple propulsors may allow realizing a fully integrated airframe and for novel flight control options in terms of propulsion system and offered an initial vectored or differential thrust, thus giving scope compilation of distributed propulsion (DP) to decreasing or even eliminating control candidate solutions. Kim [33] presented a surface areas. The influence of propulsors may classification of DP types and discussed be used to actively adapt the aero-elastic selected examples. Gohardani et al. gave characteristics of airframe components [28]. comprehensive historic overviews of distributed Structural weight reductions are often associated propulsion configurations and related with arrangements allowing the omission of technologies [34]. Additional synopses of classic propulsion installation such as pylons possible design solutions and associated and parts of the nacelle structure. Concurrently technologies can be found in [35-38]. to (hybrid-) electric solutions, also power In previous reviews concerned with DP the term distribution through shaft and gear systems [14- “distributed propulsion” was occasionally tied 16] as well as pneumatic options [29-31] have to the number of propulsor units utilized [34]. In been explored. the present context, an extended scope is Apart from the discussed beneficial effects also covered. Similar to the characterization given in challenges arise from a more closely-couple [33], here, aircraft concepts are considered engine integration. For BLI applications this where in general propulsion-airframe typically refers to the distortion of the propulsor integration is configured such that synergistic inflow field with potentially detrimental effects benefits are maximized from the outset. Note on fan efficiency and stability, and typically that this does not necessarily require multiple degraded intake pressure ratios as a result of the propulsors but could in principle be afforded by low-momentum boundary layer flow [5, 6]. a single one. These concepts will be subsumed Architectures with decentralized or remotely under the term “Distributed / Synergistic arranged propulsors typically incur additional Propulsion System Integration” (DSP), where losses along the transmission chain. For hybrid DP configurations constitute an important and fully electric transport aircraft, still further subset. advances in the characteristics of energy storage, distribution and conversion devices are 2.1 Data Basis required. Multiple laterally arranged propulsors require careful positioning or dedicated design With the intent to consider a wide range of measures in order to avoid safety issues in case possible applications for DSP systems in the of uncontained engine failures. analyses following, an extensive survey on DSP The purpose of this paper is to provide a review design solutions was conducted and essential of aircraft concepts with strongly coupled information was derived primarily from propulsion airframe integration investigated in technical publications. In total, 40 concepts the recent timeframe. Based upon technical data were captured and considered to be a taken from literature sources, several analyses sufficiently representative set of sample points will be presented to explore the purpose, of recent DSP activities. The timeframe in mechanisms and implementation strategies which the screened concepts have been

3 Julian Bijewitz, Arne Seitz, Mirko Hornung

published ranges from 2003 to the first quarter multiple selections may be applicable. The of 2016. One historic concept (Breguet 941) criteria for each aspect are presented in Tab. 1. was included as a famous representative of early Implementation applications of DSP technology. For the sake of Purpose Mechanism comparison to the trends of a representative Approach array of contemporary in-service transport - Lift augmentation - Drag reduction aircraft, the corresponding key parameters were - BLI / WF taken from manufacturer information (primarily - Enable V/STOL - Distributed - Thrust propulsors b airport planning documents). Both turbofan and - Reduction of vectoring external noise - Distributed turboprop powered aircraft were included. - Engine exhaust Payload and range characteristics were - Improved system shielding redundancy - Cross-flow compared at design conditions assuming typical - Powered fan - Improved lift seating arrangements and a specific passenger propulsion - Aft-fuselage - Very low payload of 100 kg/PAX including luggage, if system installed specific c not otherwise specified in the literature. performance propulsor(s) thrust a The considered DSP concepts were grouped - Enhanced flight into several categories including: control systems  Unmanned Aerial Vehicle (UAV) a at acceptable fan diameters b distributed along lifting surfaces  Personal Air Vehicle (PAV) c includes e.g. (upper)-surface installed, (semi-) embedded  General Aviation (GA) and encircling fan arrangements  Military Transport Aircraft (MIL) Tab. 1: Distributed Propulsion classification scheme  Commercial Narrowbody (CNB)  Commercial Widebody (CWB) “Lift augmentation” includes the increase in For the classification of PAV concepts the maximum lift coefficient enabled e.g. by specification given by NASA [26, 27] was powered lift technologies such as externally adopted. Moreover, the concepts were clustered blown flaps. “Drag reduction” refers e.g. to according to the airframe morphology including vortex-induced drag via very high aspect ratio “tube and wing” layouts, “blended wing wing designs, but also to the elimination of bodies”, ”flying cars” offering multi-modal apparent airframe drag through boundary layer transport and “other”, which is applicable to ingestion and consecutive wake filling, leading unconventional layouts such as polyhedral to a reduction in motive power required. VTOL wings or three-surface designs not fitting into may be realized through thrust vectoring, i.e. the above given categories. tilting of either dedicated lift propulsors, all propulsors or even entire lifting surfaces. 2.2 Classification of Distributed Propulsion External noise reduction can be achieved by Concepts embedding engines into the airframe, or by In order to gain additional insight as to what introducing an arrangement of control surfaces topological options and propulsion system and power plants such that noise emissions are integration approaches have been pursued in the shielded to the exterior. “Improved system field of DSP, a scheme for the classification of redundancy” is measured relative to traditional DSP concepts is proposed. Targeting the twin-engine layouts. In particular, turbo-electric applicability to a broad range of aspects layouts allow for flexibility between power and pertinent to DSP, the scheme separately assesses thrust producing devices and hence enable the purpose a certain concept fulfills by using a strategies where a limited number of core DSP approach, the mechanism (i.e. critical engines drives a large number of propulsors. In technology aspects) that is used to address that case of a core engine failure this allows for purpose as well as the implementation multiple propulsors still being available and approach. Note that for the first two aspects thus ensures a uniform distribution of the remaining power. In addition, cases were

4 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION identified with more than two power generators advanced options including mechanical and still operating in the OEI case. pneumatic propulsion distribution as well as Additional synergy effects may be attainable hybrid and fully electric propulsion from “enhanced flight control systems” such as architectures. Thus, the setup is independent of differential thrust modulation for configurations the energy type(s) employed in the aircraft. The with multiple propulsors, and the avoidance of chosen approach is based upon a balance asymmetric thrust during OEI events, which between a sufficiently detailed resolution and could result in reduced control surface area. the intent to only rely on a relatively small Possible implementation solutions include amount of information. As a result, the model is distributed propulsors (DiPs) arranged along applicable to all considered concepts. The lifting surfaces. Note that this category includes chosen approach subdivides each architecture both ducted and unducted devices and is into a set of classes which are restricted to the applicable to cases of multiple discrete engines, major system component groups. Aircraft or common core/multiple propulsor systems requiring in-depth information such as arrangements with either mechanical or the power management and distribution electrical connection. A continuous blowing out (PMAD) system are not explicitly resolved and of the trailing edge of the wing (distributed instead represented by a single surrogate exhaust, DEX) to achieve wake filling, thrust element. The system boundaries and interfaces vectoring or flap blowing was identified as a between the classes were tailored to ensure further possible candidate. The cross-flow fan applicability to all energy types and power (CFF) constitutes a spanwise-installed 2D transmission paradigms considered. The propulsor with beneficial high-lift capability description of the classes is given below: [39-42], while previously mentioned aft-  Energy-to-Power Converter (EPC) fuselage installed propulsors (AFP) could be refers to the class which includes all fashioned as upper-surface installed systems between the energy storage and arrangements, (semi)-embedded in the airframe the conversion into power. The type of or encircling the fuselage. For transport aircraft power (e.g. shaft power, electrical this installation is often intended to employ BLI power,…) is not specified. It includes and wake filling. Each of the surveyed DSP batteries, heat-based engines such as gas concepts was subsequently assessed according turbines and piston engines, as well as to the criteria given in Tab. 1. The results along fuel cells, including the associated fuel with key information for each considered system. concept are included in Tab. 2 (overleaf).  Power-to-Power Converter (PPC) is a proxy for all components converting a power from one type into another. 3 The Assessment Approach Examples include electrical motors, or This section initially highlights the variety of the reverse principle, i.e. generators. system architectures applicable to DSP concepts  Power-to-Power Transmitter (PPT) is and proceeds to describe the assessment used to transmit a non-specified type of approach used to comparatively evaluate system power. A gearbox constitutes a complexity as well as the integrated mechanical PPT, while a PMAD is an performance. example for a PPT related to electric power. 3.1 Definition of System Architecture Classes  Power-to-Thrust Converter (PTC) comprises components generating net In order to consistently cluster the DSP concepts thrust such as propellers or ducted fan with respect to their system architectures, a and nozzle arrangements. classification scheme was established and Based upon the modular arrangement of the applied to the concepts. The scheme targets the classes defined above, a number of generic generalized treatment of conventional as well as system architecture types (indices A through K) 5 Julian Bijewitz, Arne Seitz, Mirko Hornung

were defined. The identified types range from architecture depends on whether the power node conventional mechanical layouts to fully electric between the system constituents is of electrical options covering several cases of electrified (serial hybrid) or mechanical (parallel hybrid) power trains. Following Reference [68], the nature. Obviously, mixed architecture types are distinction between a serial and parallel also possible.

Sys. A/C Mor- R n n n n n Concept Designation a des PAXc arch. EST EPC PPC PPT PTC Refs. type phologyb [nm] [-] [-] [-] [-] [-] indexd LEAPTech GA TaW 200 4 H 2 2 19 1 18 [24] Reynolds et al. (2014) CNB TaW 3980 200 D 1 2 10 1 8 [28] Joby S2 PAV TaW 174 2 K 1 1 12 1 12 [25] NASA GL-10 UAV TaW - e n/a D 1 1 11 1 10 [43] NASA/MIT D8.2 CNB TaW 3000 180 A 1 2 0 0 2 [13,44] ESAero f ECO-150 CNB TaW 5500 150 F 2 3 18 1 16 [18] e-volo VC200 PAV O ~54 2 H 2 2 19 1 18 [45] NASA N3-X CWB BWB 7500 300 D 1 2 16 1 14 [14,46] SOAR FanWing g CNB O - 65 A 1 2 0 0 2 [47] Joby Lotus h PAV TaW 700 2 G 2 2 3 1 3 [48] SAX-40 CWB BWB 5000 215 B 1 3 0 9 9 [14,49] NASA/MIT H3.2 CWB BWB 7600 354 B 1 2 0 6 4 [15] Quantum Systems VRT UAV TaW 270 n/a K 1 1 4 1 4 [50] OliverVTOL Hexplane CNB O 1300 ~100 A 1 6 0 0 6 [51] Boeing SUGAR Freeze i CNB TaW 3500 154 E 1 2 3 1 3 [11] Epstein (2007) MIL TaW 1000 33 A 1 30 0 0 30 [52] NACRE FW2 CWB BWB 7650 750 A 1 3 0 0 3 [53] Leifsson et al. (2005) CWB BWB 7750 478 C 1 8 0 0 1 [54] Ko et al. (2003) CWB BWB 7000 800 C 1 8 0 0 1 [31] Luongo et al. (2009) CNB TaW - 100 D 1 2 12 1 10 [17] Gologan et al. (2009) CNB TaW - 50 B 1 2 0 2 4 [42] BHL Claire Liner CWB O 2000 300 B 1 2 0 6 4 [55] BHL Propulsive Fuselage CWB TaW 4800 340 A 1 3 0 0 3 [8] XTI TriFan 600 PAV TaW 1500 6 D 1 2 5 1 3 [56] NASA HyperCommuter PAV O 174 2 K 1 1 12 1 12 [57,58] Aurora LightningStrike UAV O - n/a D 1 1 25 1 24 [59] NASA STARC-ABL CNB TaW 3500 154 E 1 2 3 1 3 [9] SOAR FanWing j CNB O - n/a A 1 2 0 0 2 [47] Joby Lotus k PAV TaW 140 2 K 1 1 3 1 3 [48] Joby Lotus UAV UAV TaW - n/a G 2 2 3 1 3 [60] NASA N2B CWB BWB 6000 0 B 1 3 0 9 9 [16] NASA CESTOL BWB CWB BWB 3000 170 A 1 12 0 0 12 [62] Univ. Virginia Sustinere CNB TaW 500 50 D 1 2 10 1 8 [63] ESAero f ECO-250 CWB TaW 5125 250 F 2 3 12 1 10 [18] TacticalRobotics Cormorant UAV FC 162 n/a B 1 1 0 4 4 [63] UrbanAeronautics X-Hawk MIL FC - 10 B 1 2 0 7 4 [64] Airbus Group VoltAir CNB TaW 900 65 K 1 1 1 1 1 [10] Pilczer (2003) CWB BWB - - C 1 9 0 0 1 [65] Airbus Quadcruiser Demo. UAV O - - K 1 1 5 1 5 [66] Breguet 941 CNB TaW 540 57 B 1 4 0 4 4 [67] a if the concept has not been assigned a dedicated designation, a most f Empirical Systems Aerospace recent reference is used as a descriptor g cargo version specified in Reference [46] b key: TaW = tube and wing, BWB = Blended Wing Body, h hybrid-electric version FC = Flying Car, O = other i “Hybrid BLI” version investigated in Reference [51] c or seating capacity of PAV and GA aircraft j passenger version specified in Reference [46] d System Architecture Index, see p. 6 for explanation of acronyms k fully electric version e indicates information not found in literature

Tab. 2: Synopsis of important characteristics of selected DSP concepts

6 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

Below, the considered system architectures are  J: Serial hybrid, similar to type F, but listed in coarsely arranged order of increasing without relying on a combustion engine, electrification: but rather e.g. on a combination of  A: Fuel-based mechanic (state-of-the-art battery and a fuel cell. layout of in-service transport aircraft).  K: Fully electric, single type of electro-  B: Fuel-based mechanic with chemical energy supply (e.g. battery, mechanical distribution through shaft super-capacitor) and gear systems. Note that the given architectures do not  C: Fuel-based mechanic with pneumatic represent the complete combinatorial coverage distribution. This refers e.g. to concepts of theoretically possible scenarios, but rather where the exhaust gas is ejected through comprises an array of options that appear to be a distributed exhaust nozzle, or tip- practically relevant. turbine driven decentralized fan arrangements. 3.2 Definition of a System Complexity Index  D: Serial hybrid with a single type of The departure from the well-established energy supply. This includes “turbo- morphological setup of contemporary transport electric” cases where a fuel based power aircraft and the addition of advanced plant generates shaft power, which is technological features typically introduces extra subsequently converted to electrical system complexity that needs to be balanced power through a generator. Following against expected enhancements in vehicular distribution in a PMAD, the power is efficiency. As the variety of system used to drive electric motor(s) ultimately architectures considered for DSP concepts was being coupled to PTCs. This constitutes found to be large, a comparative assessment of a purely serial hybrid architecture. the complexity of the system architectures was Possible residual thrust of the turbo- deemed particularly important in the present generator(s) is considered negligible. context. Therefore, this section suggests a way Note that this architecture also captures to assess different system architectures by scenarios, where a fuel cell is used to generate power for electric motor(s). introducing a system complexity index () with intent to serve as a metric for initial comparison  E: Serial hybrid with a single type of between the different concepts. The definition energy supply, where turbo-generators are mechanically connected to a PTC of  is based upon the evaluation of the number providing additional net thrust. of instances the above given classes occur within the entire system architecture of a  F: Serial hybrid with multiple types of vehicle. In order to reflect the relative energy supply. This includes layouts, importance of the different classes, specific where electric motor power is drawn weighting factors from (turbo-) engine offtake, and e.g. from a battery. (1) i 1.0 , i 1.0  G: Serial hybrid, similar to option F, i with the addition of (turbo-) engines were applied. These may be tuned based upon being mechanically coupled to a PTC expert knowledge or the scope of the providing additional net thrust. investigation. Pursuing the purpose of an initial  H: Range extender architecture, where a comparative study, in the present context the fuel based engine is utilized to recharge factors were intuitively selected identically, in a battery. the first instance (Tab. 3).  I: Parallel hybrid, where the power shaft of the PTC is connected both to a fuel- based engine and an electric motor.

7 Julian Bijewitz, Arne Seitz, Mirko Hornung

Weighting factors Value to representative examples found in the EPC 0.20 literature.  0.20 PPC With the chosen values of i the turbo-electric 0.20 PPT layout was found to have a higher  than the  0.20 PTC other examples. Compared to cases B and K, the EST 0.20 Tab. 3: Applied weighting factors additional complexity is rooted in the requirement to firstly convert shaft power to As an initial plausibility check, the values of  electrical power, and afterwards transform it were compared to the mass breakdowns of a back to electrical power supplying the array of conventional and an electric power plant: for a electric motors, which eventually convert power generic high-bypass turbofan the fan including to net thrust. nacelle (corresponds to the PTC) accounts for System Architecture Index a approximately half of the total propulsion b system weight [69], which appears to be in A B D K n 2 3 2 1 agreement with the relative proportions between EPC nPPC 0 0 16 12 PTC and EPC. Similarly, for an electric short nPPT 0 9 1 1 range aircraft conceptually investigated in [23], nPTC 2 9 14 12 the mass share of the propulsors was found nEST 1 1 1 1 comparable to the share of the electric motors,  1.0 4.4 6.8 5.4 In-service Exemplary thus qualitatively verifying the proportions commercial [14] [13] [57] References between PTC and PPC. transports a see list on pp. 6 The mathematical representation of the b complexity index is given as Twin-engine reference aircraft Tab. 4: Exemplified evaluation of possible system N architectures and computed system complexity index    n , N EPC,...,EST (2)  i i i1 Pursuing the purpose of describing where ni represents the number of instances a class i occurs within a system architecture. For a commonalities and differences intrinsic to the conventionally powered, twin-engine reference considered system architecture options, relative configuration  results in a value of 1. descriptors are proposed herein. These include Note that depending on the available level of the ratio of the number of power-to-thrust detail or system knowledge further refinements converters to energy-to-power converters, nPTC to the model may be possible by increasing the 1  (3) resolution of explicitly evaluated components. nEPC While, for example, in the present scope and similarly, based upon nPPC: electrical and mechanical power transmission nPPC 2  (4) types were weighted identically, different sub- nPTC weighting factors could be applied to each Now, heuristics specific to each considered paradigm. architecture type may be constructed serving the For demonstration purposes of the established purpose of establishing relations between the evaluation method, 4 system architectures were different ni. For example, for type D the number selected. Types A (conventional) and K (fully of PPCs (generators and electric motors) will electric) were chosen to represent the extrema of always be equal to the sum of the number of the range of architectures, while B and D reflect EPCs (e.g. turboshaft engines) and PTCs (e.g. intermediate solutions. A schematic propellers), see also Tab. 4. Hence, for type D representation of the general architectural this boundary condition can be algebraically arrangements is presented in Fig. 1 (overleaf), formulated as: and Tab. 4 shows characteristics of the cases, n  n  n (5) the computed complexity index and references EPC PTC PPC

8 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

This relation can be conveniently expressed in For some of the remaining types similar terms of relative metrics using eq. (3) and (4) conditions as stated in eq. 6 can be fashioned

11 producing the expressions given in Tab. 5. 2,D  (6) 1 and thus gives an indication of the dependency between 2 and 1.

Energy-to-Power Power-to-Power Power-to-Power Power-to-Thrust Converter (EPC) Converter (PPC) Transmitter (PPT) Converter (PTC)

M GB Battery Capacitor Electric motor Gearbox

FC P Ducted fan G M A Electric generator D Power Management Fuel cell Gas turbine and Distribution System Propeller

Conventional (A) Mechanical distribution (B) Serial-hybrid (D) Fully electric (K)

G M P M

… … A … P M D M M G A

D … M …

Fig. 1: Overview of typical components used in system architecture classes (upper part), exemplified arrangements of possible system architectures (lower part)

Index Boundary Condition Transformed Boundary Condition 2 = f(1, nEPC) Equation

A, nPPC = 0 AA = 0 

B,C nPPC = 0 C = 0

11 D nEPC  nPTC  nPPC 2,D  (6) 1

11 nEPC 1 F nEPC 1 nPTC  nPPC 2,F  (7) 1 nEPC

a 11 nEPC H , nEPC  2 2,H  (8) 1 nEPC

E,G, I, J nEPC  nPTC 1 , nPPC = nPTC E,GJ 

K nPPC = nPTC   a this is considered the only practical case Tab. 5: Overview of system architecture-specific heuristics

9 Julian Bijewitz, Arne Seitz, Mirko Hornung

3.3 Metrics for Integrated Performance payload-to-energy mass fraction, pl/en. Assessment Recognizing m m MTOW In order to compare and contrast the pl  pl  (10) characteristics of the DSP concepts to the trends men MTOW men of contemporary transport aircraft, the typical eq. (9) can be rearranged as: weight fractions with respect to MTOW were m  pl PREE PL   e (11) derived from the data retrieved from the men en R literature. This includes the OEW fraction, oe, where e denotes the specific energy of the and the payload fraction, pl. For capturing also battery for fully electric aircraft, or the FHV for (hybrid-) electric architectures, the classic fuel conventional applications, respectively. fraction was adapted to yield the energy mass fraction, en, now comprising the combined mass of the energy constituents stored onboard 4 Analysis Results and Discussion the aircraft required to perform the design Following the discussion of the chronological mission including reserves. For traditional fuel evolution of DSP concepts, the relations based aircraft this refers to the design fuel mass between purpose, method and mechanism will including reserves, while for electric concepts be explored. Apart from the evaluation of the battery mass is considered instead. system complexity associated with DSP, this For the assessment of the integrated section is dedicated to the evaluation of performance of the DSP concepts, it is useful to important aircraft parameters and the integrated employ an appropriate efficiency metric performance. covering both the characteristics of conventionally powered and electric as well as hybrid energy concepts. In the present context, 4.1 Evolution of Distributed Propulsion the Payload Fuel Efficiency given by Torenbeek Fig. 2 (overleaf) presents the chronological [70] was chosen and suitably generalized, evolution of the concepts included in Tab. 2. On yielding the Payload Range Energy Efficiency, the abscissa the timeframe in which each m  R PREE  pl,des des (9) concept was or has been investigated is Edes displayed, while the ordinate shows the design in units of [kg∙nm/kWh], where mPL,des range. For simplification, the timeframe was represents the design payload carried over the aligned with the interval in which scientific design mission distance Rdes while consuming publications have been published. The marker the energy Edes. Given that in eq. (9) the symbols indicate the different aircraft types and nominator refers to the income potential, while the colors refer to the system architecture type. the denominator is correlated to a large For readability purposes the transmission proportion of the operating expenses, PREE can architectures were clustered into fuel-based, be interpreted as an efficiency of the transport mechanic transmissions (index A through C), system [70]. For classic fuel based aircraft, Edes hybrid-electric types (D through J) and fully and the fuel mass are coupled through the Fuel electric types (K). Heating Value (FHV ≈ 42.8 MJ/kg for Jet-A The increasing density of sample points towards fuel), while for battery-based concepts the the right end of the chart illustrates the growing knowledge of the gravimetric specific energy of interest devoted to DSP. With regards to the the battery is required to determine Edes from energy storage types employed, it is visible that men. In case of hybrid energy storage, the sum of electric DSP aircraft have only emerged in the fuel and battery based energy constituents needs recent timeframe and are currently primarily to be considered. For conventional or fully considered for GA, PAV and UAV applications electric aircraft, PREE can be correlated to the with limited design ranges. Only one projected concept was found showing a design range

10 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION compatible with the typical regional aircraft segment. In contrast to that, hybrid-electric concepts show a much larger bandwidth in DiPs 5 6 1 2 6 6 range up to 7500 nm, which was found comparable to the values immanent to mechanically powered concepts. Purely fuel- DEX 3 based concepts are distributed across the entire timeframe considered and were found (apart from one outlier) to be exclusively associated CFF 3 with CWB, CNB and MIL aircraft types. It can be concluded that for small aircraft types clearly Numbers and sizes of a trend to employ fully electric propulsion is AFP 5 3 circles reflects number of emerging, while in order to fulfill the mission instances, where an

Implementation Approach Implementation Approach of Distributed Propulsion requirements of large commercial aircraft these matches an Aircraft Type

DSP concepts primarily rely on a non-battery CNB CWB GA MIL PAV UAV based energy source as realized e.g. with turbo- Aircraft Type electric layouts. Here, a gradual departure from Fig. 3: Implementation approach and aircraft types the purely mechanical drive train is noticeable. In summary, the combination of DSP and It is apparent that DiPs are used for the entire electric drive technology seems to be a spectrum of aircraft types considered. AFPs are particularly attractive field for advanced studies. found to be primarily applied in the commercial aircraft segment. Moreover, GA, MIL, PAV and 4 10 UAV aircraft are found to be exclusively designed using a distribution of propulsors. While for PAV and UAV this is frequently motivated through VTOL requirements necessitating multiple propulsors, the 3 10 inexistence of combinations of GA and MIL with other than DiPs may be attributed to the PPS architecture: current limitation of the dataset. For CNB and mechanic transmission hybrid-electric transmission CWB aircraft, aft-installed propulsors are often fully electric transmission associated with BLI. This appears to be less

Design Range [nm] 2 10 Aircraft type: relevant for smaller applications such as GA, CWB CNB PAV and UAV due to much thinner boundary MIL layer flows. However, the fact that there are GA PAV applications with aft-fuselage propulsor existing UAV 1 in that segment [71-74] indicates other 10 2005 2010 2015 advantages stemming from such a propulsion Year Fig. 2: Evolution of DSP research activities over time integration, e.g. the fuselage being not immersed in the propeller wash. In a general sense it can be concluded that only a fraction of 4.2 Purpose, Mechanism and Implementation the theoretically possible combinations have of Distributed Propulsion been actively considered yet. The available data was analyzed with respect to Some combinations might lack the practical the DSP classification scheme given in Section feasibility such as the application of DEX to 2.1. Aspects of the results are elucidated in Fig. PAV and UAV as for these types most 3, which displays the number of instances in frequently electrical power trains are used. On which an implementation approach was the other hand, the installation of a CFF to GA, employed in one of the defined aircraft types. MIL, PAV or UAV types may constitute promising candidates for further studies, e.g.

11 Julian Bijewitz, Arne Seitz, Mirko Hornung

due to the potentially superior high-lift vehicles according to the DSP paradigm. For capability attainable from the CFF. example, boundary layer ingesting fans may be installed in an airframe-embedded fashion, thus, Numbers and sizes of circles reflects number of in addition to ultimately reducing the propulsive instances, where a Mechanism matches a Purpose of Concept power demand, also effectively shielding engine Low Spec. Thrust 5 10 7 11 9 6 noise to the external. In addition, provided the array of propulsors is arranged appropriately,

Powered Lift 8 5 4 7 8 1 5 novel ways of flight control augmentation may be possible, thereby decreasing the required control surface area and hence saving weight Eng. Shielding 2 12 11 10 6 5 and wetted area. To conclude, the figure reflects the extensive Thrust Vec. 1 4 14 12 3 9 research effort currently being expended to improve overall vehicular efficiency, but also to

Mechanism of Distributed Propulsion BLI/WF 2 14 10 11 6 4 decrease noise and realize new ways of personal transportation, for which STOL or even VTOL seems to be an important aspect. LA DR EV REN ISR IPP EFCS Purpose of Concept Abscissa labels: LA = lift augmentation, DR = drag reduction, EV = enable V/STOL, REN = Reduction of external noise, ISR 4.3 Distribution of the Number of Propulsors = improved system redundancy, IPP = improved propulsion system performance, EFCS = enhanced flight control systems After having examined the integration options Fig. 4: Mechanism / purpose of distributed propulsion available for DSP this study focusses on the evaluation of the numerical quantity of power- Additional insight is offered through Fig. 4, to-thrust converters (nPTC) used in DSP aircraft. which, similarly as Fig. 3, indicates the Fig. 5 shows the distribution of the number of instances in which a mechanism of DSP is concepts employing a specific nPTC. A color employed to address a certain purpose of DSP. code is used to visualize the share of propulsor Intuitively, large accumulations occur for thrust options that are employed with the analyzed vectoring being applied to realize VTOL. The concepts. In addition, the relative cumulative quantitative superiority of thrust vectoring distribution is given on the secondary ordinate. relative to powered lift technology with regards The spectrum of nPTC ranges from 1 to 30 and to the realization of V/STOL indicates the both odd and even numbers are used. The emphasis currently being placed on such highest number of concepts was found to be propulsion integration options. This may be associated with nPTC = 3. From the distribution attributed to the advent of hybrid electric of nConc it can be followed that a high share of distributed architectures, where a combination concepts features a rather low number of EPCs. with vertical takeoff capability seems to be Specifically, 55% of all concepts have an nPTC particularly beneficial. Also, much emphasis is of up to and including 4. All propulsor types are being placed on synergistic propulsion system represented within this bandwidth. Concepts integration with BLI and wake filling. with nPTC > 18 appear to be rare, only two Technically feasible arrangements with low samples were found. This is an indication that a specific thrust are associated with either very high number of EPCs is only justifiable if mechanical distribution or turbo-electric layouts certain unconventional aspects are addressed with remotely driven propulsors. As an implicit such as maximization of synergistic aero- result, the system redundancy often becomes propulsion interaction and very high redundancy improved. Judging from the considerable level requirements combined with VTOL capability. of employed combinations, the study In essence, synergistic propulsion airframe demonstrates the high degree of inter- integration does not necessarily rely on a high connections between the technologies, and the number of propulsors. synergistic potential offered by designing flight 12 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

10 1 30 Sample Points

8  = const. 1

[-] 8 0.8 25 4

Conc

) [-] 1 2

Conc 20 6 Propeller 0.6 Fan System Architecture Type:

A: FB [-] Cross-Flow Fan (CFF) B: FB with Mech. Distr.

Jet Flap PTC 15 C: FB with Pneumatic Distr. Fan + CFF n D: Serial Hybrid (SH) 4 0.4 E: SH + Turbo Gens. prov. Thrust F: SH + Multi-Energy Sup. G: ´E´ + Turbo Gens. prov. Thrust 10 1 H: Range Extender Number of Concepts (n 0.5K: Elec., Single-Energy Supply 8 2 0.2 Aircraft Type: Commercial Narrowbody Relative Cumulative Distribution of n Commercial Widebody 4 5 2 General Aviation Military Personal Air Vehicle 0 0 0.5 Unmanned Aerial Vehicle 1 5 10 15 20 25 30 1 Number of Power-to-Thrust Converters (n ) [-] PTC 1 5 10 15 20 25 30 n [-] Fig. 5: Distribution of power-to-thrust converters EPC Abbreviations in legend: FB = fuel-based propulsion system, SH = serial hybrid 4.4 Evaluation of the System Complexity Fig. 6: Number of power-to-thrust converters vs energy-to-power converters Index Prior to discussing the evaluation of the DSP Fig. 7 (overleaf) provides additional insight with complexity index, Fig. 6 presents contour lines regards to the inter-play of 1 and 2 by showing of the relative descriptor 1 as a function of the the evaluation of the analytical relations given absolute numbers of nPTC and nEPC. The in Tab. 5. Apparently, the marker points considered concepts are represented through corresponding to the concepts are aligned with their respective data points. As can be gleaned the trending behavior of the identified heuristics from the chart and from inspection of Tab. 5, for pertinent to the system architecture employed in each of the concepts, thus verifying the derived classic fuel-based power plants 1 becomes unity reflecting the direct connection between relations. For the conventional power plant, the e.g. a turboshaft engine and the propulsor. The lines collapse into one single point (1 = 1, majority of concepts are found in the upper 2 = 0). Fully electric propulsion systems as well triangle (1 ≥ 1). This is conceivable since as types E, G, I and J feature parallelism to the although the installation of a higher number of abscissa (2 = 1,  > 0). For a given number of EPCs than PTCs might be feasible in special EPC and PTC (1 = const.) increasing values of cases, 1 < 1 generally appears to lack practical 2 indicate a growth in power conversion relevance. The two exceptions at nPTC = 1 refer required and therefore, the contour for e.g. 2,D to concepts featuring distributed exhausts, i.e. a features high values of 2. In addition, 1 may continuous jet out of the trailing edge. Apart be interpreted as a measure of the degree of from the outlier at nEPC = nPTC = 30, concepts of decoupling between EPC and PTC. While, as type D feature the highest numbers of PTCs. discussed above, conventional layouts adhere to The increased flexibility enabled through the 1 = 1, turbo-electric configurations feature a decoupling of EPC and PTC allows for high decoupling between the turbo-generator and the numbers of propulsors driven by a small number thrust producing devices (irrespective of the of core engines. In fact, for type D nEPC was energy types used along the power train) and found solely 1 or 2, while the corresponding hence yield 1 values greater than 1. For nPTC spans a range of 8 to 24 and accordingly, example, mechanically distributed options have large values of 1 are obtained. zero conversion and only moderate degrees of decoupling. On the other hand, architectures

13 Julian Bijewitz, Arne Seitz, Mirko Hornung

with electrification rely on conversion (2 ≥ 1) No relation was found between the (projected) and exhibit the full range of values for 1. Due entry-into-service year and the complexity to reasons of physical feasibility, no sample index. Interestingly, the highest value of points are found within the interval 2 = ]0; 1[.  (12.2) is associated with a concept featuring multiple conventional gas turbines. However, 1.8 this STOL military cargo transport with an array Marker Legend: see Fig. 6 of 30 turbofan engines generating powered lift 1.6 is considered to be an outlier, where the high 1.4 level of complexity seems to be accepted to realize very aggressive field performance 1.2 requirements. Under this premise a value of  = 1 11 appears to be a practical upper limit for

[-]

2 currently investigated concepts.  0.8 Sample Points Relation for type A ( = 1,  = 0) 1 2 Symbol (Aircraft Morphology): 0.6 Relation for types B, C ( = 0) Color (Aircraft Type): 2 BWB / HWB Flying Car CWB MIL GA Relation for type D (Eq. 6) 0.4 Relation for type F with n = 3 (Eq. 7) TaW Other CNB PAV UAV EPC Relation for type H (Eq. 8) 0.2 Relation for types E, G, I, J, K ( = 1) 2 12 0 11 0 5 10 15 20 25  [-] 10 1

[-] 9 Fig. 7: Heuristics for relative descriptors 1 and 2  8

In Fig. 8 the evaluation of the relative system 7 complexity index is presented for each system 6 architecture type. The color code refers to the aircraft type while the markers indicate the 5

morphology. Parity in  with the twin-engine 4 System Complexity Index reference aircraft is found only for a concept 3 employing a small number of turbofan engines, 2 two representatives of the CFF, and a concept 1 with a single electrically driven ducted fan. 0 A B C D E F G H I J K Hence, it can be argued that for most concepts System Architecture Type Index an increased complexity is traded against Fig. 8: Evaluation of System Complexity Index improvements in overall vehicular efficiency or other concept-intrinsic advantages. It is For mechanical distribution options (B), noticeable that no architecture is exclusively relatively moderate values of  are obtained used in a specific aircraft type, but different reflecting the lack of power conversions architectural options seem to be employed. required. In summary, there is an approximately Also, no dependency between aircraft size and  equal split between concepts with (55%) and was identified. For CNB transports the full without (45%) electrification. No range of degrees of hybridization is used, representatives of parallel-hybrid system however, the majority of concepts is found to architectures (I, J) were found during the rely on conventional or hybrid architectures. All literature research connected to this paper. of the analyzed PAVs feature elements of Apparently, this hybridization strategy, where electrification. It is recognized that for the vast extensive research effort has been expended majority of concepts (95%)  does not exceed 9, when it concerns conventional, non-distributive and 60% have  smaller than 3, signifying the configurations (see e.g. References [75-78]), is effort expended to minimize system complexity. currently not in the prime research focus in the context of DSP. 14 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

Concluding, it has to be noted that apart from its solid lines. Apparently, some concepts have system architecture the overall complexity of been sized to max out on the span constraint the vehicle is also strongly influenced by the applicable to their respective aerodrome airframe morphology itself. reference code. A few BWB concepts even feature wing spans in excess of all contemporary airport compatibility standards. 4.5 Analysis of Aircraft Key Parameters Fig. 10 displays the typical design cruise Mach Fig. 9 presents the trending behavior of the wing number, Mcr, used by the DSP concepts. span (b) as a function of MTOW. The color 0.9 code corresponds to the aircraft types. The well- Sample Points defined relation between b and MTOW across In-service Turbofan A/C several magnitudes of MTOWs and aircraft 0.8 In-service Turboprop A/C types is illustrated through the displayed non- 0.7 linear regression curve satisfying a power-based functional approach. Most sample points reside 0.6 within a +/-10% confidence interval. Two outliers towards larger values for b are 0.5 encountered in the CNB domain corresponding to strut-braced wing configurations indicating 0.4 the feasibility to apply larger wing spans than Typical Cruise Mach Number [-] 0.3 exhibited by the general trend.

120 0.2 Sample Points ICAO Annex 14 Aerodrome Reference Code Marker legend: see Fig. 8

Regression Model 0.1 2 3 4 5 6 100 In-service Turbofan A/C 10 10 10 10 10 In-service Turboprop A/C Maximum Take-off Weight (MTOW) [kg] Fig. 10: Trend of typical cruise Mach number 80 F Again, for a set of in-service aircraft the E 60 corresponding values were derived from D manufacturer information and included in the figure. It is visible that for long range Wing Span (b) [m] 40 C applications Mcr is mostly within the domain of conventional aircraft. For CNBs, an B 20 agglomeration of concepts is found, which are A sized for significantly slower cruise speeds than Marker legend: see Fig. 8 typically used for conventional turbofan

0 1 2 3 4 5 6 10 10 10 10 10 10 powered aircraft. For some of the concepts this Maximum Take-off Weight (MTOW) [kg] is motivated by the fact that a slower cruise Fig. 9: Trends of wing span speed and the resulting smaller required wing For comparison, the figure is supplemented with sweep may be synergistically combined with the respective trends for conventional (turbofan other annexed technologies such as natural and turboprop powered) aircraft, which were laminar flow [15, 10]. This generates additional derived using the same functional approach as fuel efficiency advantages over the benefits for the DSP concepts. An offset between the attainable from the basic DSP aircraft trends for DSP and conventional turbofans is configuration itself. However, the visible reflecting the general trend of advanced corresponding penalty in aircraft productivity concepts to apply larger wing aspect ratios to needs to be recovered e.g. through dedicated generate additional benefits from reduced operational measures such as improved turn- induced drag. The figure also includes the span around times. Also, in addition to mitigated limitations according to ICAO Annex 14 [79] as compressibility drag resulting from slower 15 Julian Bijewitz, Arne Seitz, Mirko Hornung

cruise speeds, the feasibility of decreased wing corresponding trends of contemporary transport sweeps is typically associated with reductions in aircraft, for which regression curves are shown. wing structural weight, which constitutes an For conventional aircraft, the classic trends of important portion of the aircraft OEW. This also decreasing payload and empty weight fractions, applies to an electric aircraft, which is sized for as well as increasing energy weight fractions a Mach number at the lower end of typical against MTOW are visible [81-83], although for regional applications. As discussed in [80] this turboprop aircraft the trends of en and pl were is plausible since for electrically powered found less distinct than for turbofan powered aircraft the optimum energy-specific air range aircraft. Family members typically featuring (ESAR) is typically achieved with a cruise flight commonality in the design of lifting surfaces technique corresponding to lower Mach and systems are connected through dashed lines. numbers compared to similar conventionally For long range aircraft the energy fraction powered aircraft. Here, the key influence is becomes a significant weight share and related to the shortfall of core engine ram pre- approaches 50% of the MTOW. While for compression for the electric power plant conventional, in particular, turbofan aircraft compared to the gas turbine. For turbofan power most sample points reside within a relatively plants the resulting increase in core efficiency well defined corridor around the shown contributes to the improvement of overall regression functions, the DSP concepts tend to propulsion system efficiency with Mach deviate from the indicated trend curves. number, while for the electric aircraft the trades This may be rooted in the general characteristic between transonic drag rise and power plant that DSP concepts often employ differences in efficiency tend to shift the optimum of ESAR to the morphological setup, mission requirements lower Mach numbers. or energy type relative to traditional transports For UAV and PAV a large spread in Mcr is and hence increased scatter is induced to the apparent, reflecting the large variety of mission data. Inspection of Chart II in Fig. 11 reveals the roles and design paradigms associated with technological advances associated with DSP these aircraft types. The fact that Flying Car concepts. Hence, most CNB and CWB morphologies operate at low speeds is configurations feature en smaller than the understandable considering that the general trend of conventional aircraft, thereby configuration must allow for multi-modal partly triggering increases in oe.In particular, capability. BWB layouts consistently feature an offset to lower en values rooted primarily in the 4.6 Survey of Weight Fraction Trends enhanced aerodynamic efficiency expected from the BWB design. The high payload capacity of The weight fractions described in Section 2.1 BWBs is also visible in Chart III. are presented in Fig. 11 along with the

Chart I Chart II Chart III 0.5 0.5 0.7 Breguet 941 0.4 0.4

0.6 0.3 0.3

/ MTOW [-]

en pl 0.5 0.2 0.2

= m = m

= OEW / MTOW [-]



 Sample Points 0.1 0.1 0.4 In-service turbofan A/C In-service turboprop A/C Marker legend: see Fig. 8

3 4 5 6 0 3 4 5 6 0 3 4 5 6 10 10 10 10 10 10 10 10 10 10 10 10 Maximum Take-off Weight [kg] Maximum Take-off Weight [kg] Maximum Take-off Weight [kg] Fig. 11: Fractions of operating empty, energy and payload weight

16 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

On the other hand, no clear conclusion can be sample points is insufficient to derive similar established regarding the empty weight fraction trending behaviors. of BWB relative to those of tube and wing It is visible in Fig. 12 that technological designs [70]. Two outliers in en within the CNB advances are echoed in increased values of domain are encountered. These are related to a PREE. A distinct cluster of DSP concepts is historical STOL aircraft (Breguet 941) and a found for CNB tube and wing aircraft between fully electric regional aircraft concept with an 3000 and 4000 nm range. understandably large battery mass fraction. PAV and UAV concepts are found off the trend. 1

0.6 0.5 0.6 0.5 0.6 0.5 3 1.5 e = 3.5 2 Examination of Chart I shows that some DSP 0.9 batt Sample 2.5Points 500  / = const. (fuel-based, FHV = 42.8 MJ/kg) concepts feature significantly lower OEW 750 pl en 0.8 1000  / = const. (battery-based) fractions than typical for conventional designs. [Wh/kg] pl en For the two above highlighted concepts in the 0.7 1 CNB domain, this is due to unusually high 1.5 0.6 2 3 2.5 0.5 0.5 0.5 0.6 Marker legend: see energy mass fractions. For the Flying Car 0.6 0.6 3.5 0.5 Figs. 8 and 11 concepts, this may result from the combined 1 effects of a relatively low cruise efficiency (cf. 0.4 0.5 also Chart II) and/or extensive use of advanced 1.5 0.3 2 materials and manufacturing techniques. 1 0.5

0.2 3 2.5 3.5 0.5 0.1 1.5 4.7 Comparative Assessment of Integrated Payload-Range-Energy Efficiency (PREE) [t*nm/kWh] 0.1 1 0.1 Performance 0 0.5 0.1 0 2000 4000 6000 8000 10000 Design Range (R ) [nm] In Fig. 12, the payload-range-energy efficiency des Fig. 12: PREE as a function of design range PREE is presented as a function of design range.

The figure is supplemented with iso-contours of As noted before, the synergistic combination of the ratio of payload-to-energy mass fraction, aerodynamic improvements, weight reduction pl/en. For conventional turbofan powered technologies and power plant efficiency aircraft, a concave trend can be observed enhancements enabled through more tightly- resulting from counteracting effects [70]: long coupled propulsion airframe integration allows range aircraft tend to be penalized through the for substantial improvements in PREE and high fuel load necessary to fulfill the demanding accordingly increased values of  / . Some mission requirements leading to a high MTOW pl en BWB applications yield particularly high PREE and decreasing payload-to-energy mass values caused by the high transport capacity fractions. For short to medium range aircraft, enabled by the efficient utilization of volume. A the fuel fraction used during climb and descent number of concepts feature inferior PREE constitutes a substantial share of the block fuel, values compared to in-service aircraft. This may thus yielding decreased values of PREE. Based be partly attributed to the special purpose these upon the analyzed data for conventional concepts have been sized for such as superior turbofan powered aircraft, a weak maximum can robustness, extremely high thrust-to-weight be identified around 4000 nm range with ratios etc., thus tolerating deficits in PREE, and, corresponding values of pl/en ≈ 0.75. the historic technology level of the Breguet 941. For turboprop aircraft higher efficiencies are It can be argued that the high energy efficiency obtained at similar ranges compared to turbofan of the electric transport aircraft is propagated powered aircraft reflecting the superior into high PREE values, however at inferior propulsion system efficiency associated with payload to energy fractions compared to fuel- turbo-propeller engines operating at moderate based aircraft of similar mission role. airspeeds. For DSP concepts, the amount of

17 Julian Bijewitz, Arne Seitz, Mirko Hornung

In general, the data required to derive PREE to in-service twin engine aircraft is traded proved difficult to find for electric concepts. In against higher overall vehicular efficiencies. In the present analysis only for one concept the terms of transport capability per unit energy required parameters were obtainable from the consumed, clear enhancements in efficiency public domain. However, judging from the were identified for most concepts. recent momentum associated with research In follow-on work the studies presented in this regarding electrified power trains for airborne paper will be complemented with additional applications, increased availability can be parameters not explicitly considered yet. expected in the near future. Moreover, taking into account additional concepts could increase the density of sample points and hence the statistical significance of 5 Conclusions the analyses. Owing to the high level of interest distributed propulsion currently attracts within the aeronautical community, this paper provided a References review of distributed propulsion aircraft [1] European Commission. “Flightpath 2050: Europe’s Vision for Aviation”, Report of the High Level Group on Aviation concepts being or having been investigated Research, Publications Office of the European Union, within the recent timeframe. The set of design Luxembourg, 2011 solutions identified includes configurations [2] Advisory Council for Aviation Research and Innovation in ranging from unmanned vehicles to long range Europe. “Strategic Research and Innovation Agenda”, blended wing body layouts. Considering the Brussels, 2012 advances currently experienced in the field of [3] National Aeronautics and Space Administration (NASA). “Advanced Concept Studies for Subsonic and Supersonic electro-chemical storage, distribution and power Commercial Transports Entering Service in the 2030-35 conversion technology, a strong focus on Period”, 2007 partially and fully electrified power trains was [4] International Air Transport Association. “IATA Calls for a recognized. While projected fully electric Zero Emissions Future”, Press Release No.21, 4 June 2007, published in http://www.iata.org, cited: 30 May aircraft associated with distributed propulsion 2011 are currently limited to the regional aircraft [5] Seitz, A., Gologan, C. “Parametric design studies for segment, hybrid electric configurations were propulsive fuselage aircraft concepts”, CEAS Aeronautical found up to design ranges corresponding to Journal, Vol. 6, Issue 1, pp. 69-82, 2015, DOI 10.1007/s13272-014-0130-3 contemporary long range aircraft. Here, the [6] Bijewitz, J., Seitz, A., Isikveren, A.T., Hornung M. “Multi- prospect of remotely arranged propulsors driven disciplinary design investigation of propulsive fuselage by a centralized power plant has been aircraft concepts”, Aircraft Engineering and Aerospace intensively studied in multiple projects. Apart Technology, Vol. 88, Issue 2, pp. 257-267, 2016, DOI: from significant enhancements in vehicular 10.1108/AEAT-02-2015-0053 [7] Bijewitz, J., Seitz, A., Isikveren, A.T., Hornung M. efficiency exhibited by more tightly-coupled „Progress in optimizing the propulsive fuselage aircraft engine-airframe integration, such configurations concept“, AIAA-2016-0767, 54th AIAA Aerospace also open up new perspectives for personal Sciences Meeting, San Diego, California, 2016 transport exploiting the principles of vertical [8] Isikveren, A.T., Seitz, A., Bijewitz, J., Mirzoyan, A., Isyanov, A., Grenon, R., Atinault, O., Godard, J.L., Stückl, takeoff and landing. Based upon an algebraic S. “Distributed propulsion and ultra-high by-pass rotor formulation for a complexity metric, the system study at aircraft level”, The Aeronautical Journal, Vol. complexity of various concepts was analyzed. It 119, No. 1221, pp. 1327-1376, 2015, DOI: should be noted that the presented analysis 10.1017/s0001924000011295 results rely on the screened data publically [9] Welstead, J., Felder, J. “Conceptual design of a single-aisle turboelectric commercial transport with fuselage boundary available. In future work, a systematic, scenario- layer ingestion”, AIAA 2016-1027, 54th AIAA Aerospace based variation of the weighting factors within Sciences Meeting, San Diego, California, 2016 the complexity metric could be used to assess [10] Stückl, S., van Toor, J., Lobentanzer, H. “VoltAir – the all the robustness of the introduced method. In electric propulsion concept platform – a vision for atmospheric friendly flight“, 28th International Congress summary, it was concluded that for most of the Aeronautical Sciences, Brisbane, Australia, 2012 concepts increased system complexity relative 18 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

[11] Bradley, M., Droney, C. “Subsonic ultra green aircraft [25] Stoll, A, Bevirt, J., Pei, P., Stilson, E. “Conceptual design research phase II: N+4 advanced concept development”, of the Joby S2 electric VTOL PAV”, AIAA-2014-2407, NASA CR-2012-217556, 2012 14th AIAA Aviation Technology, Integration, and [12] Drela, M. “Development of the D8 transport Operations Conference, Atlanta, Georgia, 2014 configuration”, AIAA-2011-3970, 29th AIAA Applied [26] Moore, M. “Personal air vehicles: a rural/regional and Aerodynamic Conference, Honolulu, Hawaii, 2011 intra-urban on-demand transportation system”, AIAA [13] Kim, H.D., Felder, J., Tong, M., Berton, J., Haller, B. 2003-2646, AIAA/ICAS International Air and Space “Turboelectric distributed propulsion benefits on the N3-X Symposium and Exposition, Dayton, Ohio, 2003 vehicle“, Aircraft Engineering and Aerospace Technology, [27] Moore, M. “NASA personal air transportation Vol. 86, Issue 06, pp. 558-561, 2014, DOI 10.1108/AEAT- technologies” SAE Technical Paper 2006-01-2413, 2006, 04-2014-0037 DOI: 10.4271/2006-01-2413 [14] Hileman, J.I., Spakovszky, Z., Drela, M., Sargeant, M., [28] Reynolds, K., Nguyen, N., Ting, E., Urnes Sr., J. “Wing Jones, A. “Airframe design for silent fuel-efficient shaping concepts using distributed propulsion”, Aircraft aircraft “, Journal of Aircraft, Vol. 47, No. 3, pp. 956-969, Engineering and Aerospace Technology, Vol. 86, Issue 06, DOI: 10.2514/1.46545, 2010 2014, DOI 10.1108/AEAT-04-2014-0050 [15] Greitzer, E., et al. “N+3 aircraft concept design and trade [29] Gerdes, R. “Lift-fan aircraft: lessons learned from XV-5 studies, Final Report Vol. 1”, NASA CR-2010- flight experience”, AIAA-93-4838-CP, AIAA International 216794/VOL1, 2010 Powered Lift Conference, Santa Clara, California, 1993 [16] Tong, M., Jones, S., Haller, W., Handschuh, R. “Engine [30] Winborn, B. “ADAM III V/STOL concept”, Journal of conceptual design studies for a hybrid wing body aircraft“, Aircraft, Vol. 7, No. 2, pp. 175-181, 1970, DOI: NASA TM-2009-215680, 2009 10.2514/3.44143 [17] Luongo, C., Masson, P., Nam, T., Mavris, D., Kim, H.D., [31] Ko, A., Leifsson, L, Mason, W., Schetz, J., Grossman, B., Brown, G., Waters, M., Hall, D. “Next generation more- Haftka, R. “MDO of a blended-wing-body transport electric aircraft: a potential application for HTS aircraft with distributed propulsion”, AIAA 2003-6732, superconductors”, IEEE Transactions on Applied AIAA's 3rd Annual Aviation Technology, Integration, and Superconductivity, Vol. 19, No. 3, Part 2, pp. 1055-1068, Operations (ATIO), 2003 2009 [32] Sehra, A., Whitlow, W. “Propulsion and power for 21st [18] Gibson, A., Hall, D., Waters, M., Schiltgen, B., Foster, T., century aviation”, Progress in Aerospace Sciences, Vol. Keith, J., Masson, P. “The potential and challenge of 40, pp. 199-235, 2004, DOI: 10.1016/ turboelectric propulsion for subsonic transport aircraft“, j.paerosci.2004.06.003 AIAA 2010-276, 48th AIAA Aerospace Sciences Meeting [33] Kim, H.D. “Distributed propulsion vehicles”, 27th Including the New Horizons Forum and Aerospace Congress of the International Council of the Aeronautical Exposition, Orlando, Florida, 2010 Sciences, Nice, France, 2010 [19] Brown, G. “Weights and efficiencies of electric [34] Gohardani, A., Doulgeris, G., Singh, R. “Challenges of components of a turboelectric aircraft propulsion system”, future aircraft propulsion: A review of distributed AIAA 2011-225, 49th AIAA Aerospace Sciences Meeting propulsion technology and its potential application for the including the New Horizons Forum and Aerospace all electric commercial aircraft”, Progress in Aerospace Exposition, Orlando, Florida, 2010 Sciences, Vol. 47, Issues 1-2, pp. 369-391, 2011, DOI: [20] Schiltgen, B., Gibson, A., Green, M., Freeman, J. “More 10.1016/j.paerosci.2010.09.001 electric aircraft: "tube and wing" hybrid electric distributed [35] Gohardani, A. “A synergistic glance at the prospects of propulsion with superconducting and conventional electric distributed propulsion technology and the electric aircraft machines”, SAE Technical Paper 2013-01-2306, 2013, concept for future unmanned air vehicles and DOI: 10.4271/2013-01-2306 commercial/military aviation”, Progress in Aerospace [21] Isikveren, A. T., Seitz, A., Vratny, P. C., Pornet, C., Sciences, Vol. 57, pp. 25-70, 2013, DOI: Plötner, K., Hornung, M. “Conceptual studies of 10.1016/j.paerosci.2012.08.001 universally-electric system architectures suitable for [36] Nalianda, D., Singh, R. “Turbo-electric distributed transport aircraft”, Deutscher Luft- und Raumfahrt- propulsion – opportunities, benefits and challenges”, kongress (DLRK), Berlin, 2012 Aircraft Engineering and Aerospace Technology, Vol. 86, [22] Vratny, P.C., Forsbach, P., Seitz, A., Hornung M. Issue 06, 2014, DOI: 10.1108/AEAT-03-2014-0035 “Investigation of universally electric propulsion systems [37] Laskaridis, P. “Application of distributed propulsion for transport aircraft”, 29th Congress of the International concept on different Aircraft configurations”, ISABE- Council of the Aeronautical Sciences, St. Petersburg, 2015-20284, 2015 Russia, 7-12 September, 2014 [38] Rolt, A., Whurr, J. “Distributed propulsion systems to [23] Seitz, A., Hornung, M. “Pre-concept investigation of maximize the benefits of boundary layer ingestion”, hybrid- and full-electric aero propulsion”, AVT-230/RSM- ISABE-2015-20288, 2015 033 Specialists Meeting on Advanced Aircraft Propulsion Systems, Rzeszów, Poland, 2015 [39] Hancock, J. “Test of a high efficiency transverse fan”, AIAA-80-1243, AIAA/SAE/ASME 16th Joint Propulsion [24] Stoll, A., Bevirt, J., Moore, M., Fredericks, W., Borer, N. Conference, Hartford, Connecticut, 1980 “Drag reduction through distributed electric propulsion”, AIAA-2014-2851, 14th AIAA Aviation Technology, [40] Kummer, J., Dang, T. “High-lift propulsive airfoil with Integration, and Operations Conference, Atlanta, Georgia, integrated crossflow fan”, Journal of Aircraft, Vol. 43, No. 2014 4, pp. 1059-1068, 2006, DOI: 10.2514/1.17610 19 Julian Bijewitz, Arne Seitz, Mirko Hornung

[41] Dang, T., Bushnell, P. “Aerodynamics of cross-flow fans [58] Antcliff, K. “Silicon valley early adopter CONOPs and and their application to aircraft propulsion and flow market study“, 2015 Transformative Vertical Flight control”, Progress in Aerospace Sciences, Vol. 45, Issues Workshop, Moffett Field, California, 2015 1-3, pp. 1-29, 2009, DOI: 10.1016/j.paerosci.2008.10.002 [59] Warwick, G. “Aurora wins Darpa VTOL X-Plane bid with [42] Gologan, C., Mores, S., Steiner, H.-J., Seitz, A. “Potential unique hybrid”, Aviation Week & Space Technology of the cross-flow fan for powered-lift regional aircraft [online], March 4, 2016 applications“, AIAA 2009-7098, 9th AIAA Aviation [60] Joby Aviation website, http://www.jobyaviation.com, Technology, Integration, and Operations Conference accessed 29.04.2015 (ATIO), Hilton Head, South Carolina, 2009 [61] Kim, H.D., Berton, J., Jones, S. “Low noise cruise efficient [43] Fredericks, W., Moore, M., Busan, R. “Benefits of hybrid- short take-off and landing transport vehicle study”, NASA electric propulsion to achieve 4x increase in cruise TM-2007-214659, 2007 efficiency for a VTOL aircraft”, AIAA 2013-4324, AIAA Aviation, 2013 [62] Ahmad, S. et al., “The Sustinere: a turboelectric distributed propulsion regional jet for 2025”, University of Virginia, [44] Uranga, A., Drela, M., Greitzer, E., Titchener, N., Lieu, 2013 M., Siu, N., Huang, A., Gatlin, G., Hannon, J. “Preliminary experimental assessment of the boundary layer ingestion [63] Tactical Robotics website, http://www.tactical- benefit for the D8 aircraft“, AIAA 2014-0906, AIAA 52nd robotics.com, accessed 08.03.2016 Aerospace Sciences Meeting, National Harbor, Maryland, [64] Urban Aeronautics website, http://www.urbanaero.com, 2014 accessed 08.03.2016 [45] e-volo website, http://www.e-volo.com, accessed [65] Pilczer, D. “Noise reduction assessment and preliminary 29.04.2015 design implications for a functionally-silent aircraft”, [46] Felder, J., Tong, M., Chu, J. “Sensitivity of mission energy Master Thesis, Massachusetts Institute of Technology, consumption to turboelectric distributed propulsion design 2003 assumptions of the N3-X hybrid wing body aircraft”, [66] Airbus Group. “Quadcruiser: an innovative hybrid aircraft AIAA-2012-3701, 48th AIAA/ASME/SAE/ASEE Joint concept”, product brochure, 2014 Propulsion Conference & Exhibit, Atlanta, Georgia, 2012 [67] Antoniou, M. “Flying qualities and performance evaluation [47] distributed Open-rotor AiRcraft (SOAR), of the Breguet 941 turbo-prop troop transport”, U.S. Army http://cordis.europa.eu/project/rcn/110975_en.html, Aviation Test Activity, Report ATA-TR-63-6, 1964 accessed 16.03.2015 [68] Lorenz, L., Seitz, A., Kuhn, H., Sizmann, A. “Hybrid [48] Stoll, A., Stilson, E., Bewirt, J., Sinha, P. “A power trains for future mobility”, Deutscher Luft- und multifunctional rotor concept for quiet and efficient VTOL Raumfahrtkongress (DLRK), Stuttgart, Germany, 2013 aircraft“, AIAA 2013-4374, AIAA Aviation Technology, [69] Donus, F., Bretschneider, S., Schaber, R., Staudacher S. Integration, and Operations Conference, Los Angeles, “The architecture and application of preliminary design California, 2013 systems”, GT2011-45614, Proceedings of ASME Turbo [49] Hall, C., Schwartz, E., Hileman, J. “Assessment of Expo 2011, Vancouver, Canada, 2011 technologies for the silent aircraft initiative”, Journal of [70] Torenbeek, E. Advanced Aircraft Design, John Wiley and Propulsion and Power, Vol. 25, No. 6, 2009, DOI: Sons, Ltd., West Sussex, 2013 10.2514/1.43079 [71] RFB Fantrainer. FanJet Aviation GmbH website. [50] Quantum Systems website. http://www.quantum- http://www.fanjetaviation.com, accessed 28.04.2016 systems.com, accessed 05.05.2015 [72] Douglas XB-42. Klassiker der Luftfahrt. [51] Paur, J. “Hexplane: the six-engine airliner-helicopter http://www.klassiker-der-luftfahrt.de, accessed 28.04.2016 hybrid”, http://www.wired.com/2012/01/hexplane-olivervtol/, accessed 21.05.2015 [73] Grob GF 200. Deutsches Museum. http://www.deutsches- museum.de, accessed 28.04.2016 [52] Epstein, A. “Distributed propulsion: new opportunities for an old concept”, Final Technical Report 11/1/06 – [74] LearAvia LearFan 2100. http://www.aviastar.org, accessed 10/31/07, Massachusetts Institute of Technology, 2007 28.04.2016 [53] Frota, J. et al. “NACRE new aircraft concepts research” [75] Schmitz, O., Hornung, M. “Unified applicable propulsion Final Activity Report 2005-2010 system performance metrics” Journal of Engineering for Gas Turbines and Power, Vol. 135, Issue 11, 2013, DOI: [54] Leifsson, L, Ko, A., Mason, W., Schetz, J., Haftka, R, 10.1115/1.4025066 Grossman, B. “Multidisciplinary design optimization for a blended wing body transport aircraft with distributed [76] Pornet, C., Kaiser, S., Isikveren, A.T., Hornung, M. propulsion“, MAD Center Report 2005-05-01, Virginia “Integrated fuel-battery hybrid for a narrow-body sized Polytechnic Institute & State University, 2005 transport aircraft”, Aircraft Engineering and Aerospace Technology, Vol. 86, Issue 6, 2014, DOI: 10.1108/AEAT- [55] Bauhaus Luftfahrt e.V. “The CLAIRE Liner”, Presentation 05-2014-0062 at ILA Berlin Air Show, 2008 [77] Pornet, C., Gologan, C., Vratny, P., Seitz, A., Schmitz, O., [56] Warwick, G. “Seeking a fan base”, Aviation Week & Isikveren, A., Hornung, M. “Methodology for sizing and Space Technology, August 31-September 13, 2015 performance assessment of hybrid energy aircraft”, [57] Warwick, G. “Vertical commute”, Aviation Week & Space Journal of Aircraft, Vol. 52, No. 1, 2015, DOI: Technology, July 20-August 2, 2015 10.2514/1.C032716

20 A REVIEW OF RECENT AIRCRAFT CONCEPTS EMPLOYING SYNERGISTIC PROPULSION-AIRFRAME INTEGRATION

[78] Vratny, P., Kaiser, S., Seitz, A., Donnerhack, S. “Performance investigation of cycle-integrated parallel- hybrid turboshafts“, GT2016-57539, ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition, Seoul, South Korea, 2016 [79] International Civil Aviation Organization. “Annex 14 to the Convention on International Civil Aviation – Aerodromes, Vol. I”, 6th ed., July 2013 [80] Seitz, A., Schmitz, O., Isikveren, A.T., Hornung, M. „Electrically powered propulsion: comparison and contrast to gas turbines“, Deutscher Luft- und Raumfahrtkongress (DLRK), Berlin, 2012 [81] Torenbeek, E. Synthesis of subsonic airplane design, Kluver Academic Publishers, Dordrecht/Boston/London, 1982 [82] Raymer, D. Aircraft design: a conceptual approach, 4th ed., AIAA Education Series, American Institute of Aeronautics and Astronautics, Inc., New York, New York, 2006 [83] Kundu, A. Aircraft Design, Cambridge University Press, Cambridge, 2010

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