Electric Distributed Propulsion for Small Business Aircraft

15/03/2016
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Electric Distributed Propulsion for Small Business Aircraft

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DES AVIONS PLUS ÉLECTRIQUESMEA 2015 86 REE N°1/2016 Electric Distributed Propulsion for Small Business Aircraft Par Jean Hermetz, Michael Ridel Research Scientists, ONERA, Toulouse Depuis quelques années, le développement de la propulsion électrique est principalement observé dans le domaine de l'aviation légère pour la formation de pilote ou les vols locaux d’agrément. Sa mise en œuvre pour des applications telles que les voyages d'affaires, même de faible capacité en passagers, nécessite une montée en performance et en technologie. Grâce à son expertise dans l’ensemble des disciplines impliquées dans la conception d’aéronefs, l'Onera mène, depuis quelques années, des études exploratoires pour analyser les nouvelles technologies et les concepts qui pourraient participer à l’émergence de la propulsion électrique pour l’aviation commerciale, en répondant entre autres aux futurs besoins sociétaux de la « mobilité à la demande ». Les concepts d’appareil qui en résultent, illustrent l’utilisation de certaines de ces technologies-clés et les effets de synergie qui en accroissent la pertinence. Ces « concepts-planes » serviront de base à de futures recherches visant à démontrer à la fois le potentiel de per- formance et la faisabilité des technologies les plus pertinentes. Dans cet article, l'ONERA décrit les caractéristiques d’un de ces appareils en termes de conception et d’'architecture électrique embarquée. RÉSUMÉ Three-surface concept-plane with distributed EDF at the wing trailing edge. Introduction Environmental considerations, in terms of noise as well as pollutant emissions [1] [12], in addition to the potential reduction of carbon-based natural resources, lead to investi- gate the use of electric propulsion for transportation. Presently, electric propulsion for aircraft arises in the field of leisure aviation and promised performance of several pro- totypes suggest that some light two-seat electric-powered aircrafts could be in-service in the coming years for initial training (figure 1). Generally based on a pragmatic approach, several manufac- turers are investigating the possibility to substitute, on an existing airframe, piston engine(s) for a lithium-polymer or lithium-ion battery powered electric motor with a conventional propeller, connected to an electronic control unit. Performances, such as take-off distance, climb and cruise speed appear to be adequate for very light aircraft having a rather short range and low cruise speed, but a strong challenge appears when trying to extend the range beyond one hour of flight (excluding reserves): due to the energy density of such batteries, their mass increases rapidly REE N°1/2016 87 Electric Distributed Propulsion for Small Business Aircraft leading to redesign the aircraft, larger and heavier, reducing the expected economic viability, for a same payload. In order to contribute to the growth of this emerging mar- ket by investigating some potential solutions coping with the main issues of electric propulsion for aircraft, ONERA decided in 2011 to start an expert-based exploratory study, done in association with CEAtech. This expert-group concluded [3] that there is a potential for all-electric powered airplanes for civil transportation based on the association of several key techno- logies integrated in a new airframe configuration using distribu- ted propulsion, in addition to some changes in operational use. Breakthrough technologies concern: - - - bilities. Several relevant sets of those key technologies have been used in order to assess their performances and to show their pros and cons in an overall aircraft design approach. These assessments highlighted trends which led the expert-teams to conclude on the feasibility of electric powered aircraft using distributed propulsion. They also showed that the use - disciplinary coupling but also opens new degrees of freedom for the design of airplanes which lead to new compromises. Assessments were done using a simplified multidiscipli- nary approach based on a tool able to model several kinds of Propulsion & Power Systems (PPS) and to introduce them Figure 2: OAD approach and parameter list for key technologies. Figure 1: eFan – Source: Airbus Group Innovation & Al. DES AVIONS PLUS ÉLECTRIQUESMEA 2015 88 REE N°1/2016 at a conceptual design level, an aircraft for a given mission, as illustrated in figure 2. PPS or Energetic chains modelled use three set of para- of the art, 2030 and 2050 & beyond). The list of parameters for key technologies is shown on figure 2. Several missions, e.g payload and range, were considered in order to scan the whole market of civil transport aircraft. In order to illustrate the results of such preliminary analy- regional aircraft with several possible energetic power chains. It focuses on a « regional » mission which consists in carrying 75 passengers on 1 000 km at a flight level (in cruise) of 5 000 m. Three technology-maturity levels are then consi- dered, as referenced before. On this picture, the red marks correspond to two reference aircrafts, respectively ATR 72 and A320 (this last one assessed on this mission, e.g with fuel suited for 1 000 km + reserves) [4]. Thus, if we consider for example a regional Aircraft using fuel cells as energy sources and distributed electric propul- sion, the state-of-the-art situation gives an aircraft weighting close to 57 tons and using about 100 GJ energy for doing its mission. Using 2030 assumptions, for the same mis- sion it requires only 50 GJ, close to an ATR 72, and weighs this Electric Regional Aircraft appears to be close to present ATR 72. Assessments done in that way have convinced both Onera and CEATech experts that All-Electric-Aircraft (AEA) will be feasible and potentially affordable in the future. Hence, the key technologies will have to be investigated in order to increase TRL and then to propose them to aircraft manufacturers. Concept-planes overview In 2014, a first pre-design exercise [5] has been done in order to illustrate the potential of such innovations in a first application-case based on the recent results of the UE funded PPlane project [6] dedicated to Personal Plane. Indeed, the need for a 4 to 6 seats small business aircraft, operated from downtown or close to urban area, able to cover ranges from 400 to 500 km in about two hours at low cruise altitude, up from this project (figure 4), with some requirements in terms of automation in order to be used by everybody without spe- cific skills and qualification. On the basis of this Top Level Aircraft Requirements (TLAR), an Onera’s engineer team has designed, at a conceptual le- vel, two concept-planes of AEA which illustrate the compro- mise resulting from the use of several of the breakthrough technologies mentioned previously (figure 5 & figure 6). Starting from main operational characteristics (TLAR) compliant with the CS23 regulations1 , an iterative multidis- ciplinary process (see figure 7) has been used to determine the main aircraft design characteristics: in order to increase aerodynamic lift (figure 8) in low speed 1 easa.europa.eu/system/files/dfu/CS-23%20Amdt%203.pdf Figure 3: OAD results for a regional mission. REE N°1/2016 89 Electric Distributed Propulsion for Small Business Aircraft Figure 4: Illustration of PPlane use-case. Figure 6: High-wing concept-plane with distributed EDF along the wing leading edge. Figure 7: Example of iterative multidisciplinary process. Figure 5 : Three-surface concept-plane with distributed EDF at the wing trailing edge. DES AVIONS PLUS ÉLECTRIQUESMEA 2015 90 REE N°1/2016 conditions, giving some STOL (Short Take-off and Landing aircraft) capabilities. Indeed, distributed propulsion pro- mises dramatic increases in aerodynamic and propulsive efficiency, and potentially noise reduction. - - formance analysis shows that the propulsive force must be controlled symmetrically at the same time on both wings particularly during the climb step in case of engine failure, flight performance analysis has shown that only 32 opera- to flight level (altitude < 3 000 m). defined as follow: - Starting from the greatest propulsive force required during the take-off and considering electric motor (97 %) and engine power unit (95 %) efficiencies, an electric power estimated with the following assumptions: 17 min for climb step (70 km), 78 min (330 km) for cruise and the descent steps and, roughly, 15 min for the safety reserve. To perform this flight, 400 kWh are necessary for the pro- pulsion which leads, considering the others consumers, to a total energy of 500 kWh. - ciplinary approach show that the Polymer Exchange the electric power (both for propulsion and systems) is the more promising solution for the TLAR considered, with the help of a battery pack for instantaneous power demands (figure 9). - Off Weight) of 2 400 kg is assessed. The mass breakdown is as follows: - tions. Overall characteristics, and mainly electric power designed for three-wing configuration. Propulsion and Power System Architecture An aircraft results from compromise between several viewpoints in order to fulfil some requirements, e.g mis- sion objectives and safety constraints. Conventionally speaking, this compromise concerns aerodynamics, struc- ture, propulsion, performances and handling qualities, those considerations, several new coupling aspects appear with also new degrees of freedom in the design space. The required compromise comes from an iterative and multi- disciplinary approach which considers all of these aspects. The following explanations regarding the main characteris- tics of the designed Propulsion and Power System (PPS) architecture are the result of this iterative multidisciplinary approach. Figure 8 : Distributed propulsion based on co-located EDF. Figure 9: Schematic internal arrangement of fuel cells, hydrogen tanks and battery packs. Figure 10: Estimated MTOW for the three-surface concept-plane. REE N°1/2016 91 Electric Distributed Propulsion for Small Business Aircraft Distributed Propulsion Architecture The first step is to define the optimal distributed propul- sion architecture which would be compliant with the CS23 certification rules but also optimised to participate in the motion control of the airplane during flight. As explained previously, the propulsive force must be controlled symmetrically at the same time on both wings and also on each wing. Consequently, starting from these considerations, an advanced propulsive architecture has controlled individually which allows reconfiguration of the overall electric propulsion system. - rall thrust, and in that failure-case, the bi-symmetrical alloca- which is critical for safety issues. Power Architecture hydrogen tank), battery stack and its own electrical wiring inter- connection systems (EWIS). To reduce the power architecture chosen between fuel cell (available output level)/battery stack power cables (two phases instead of three phases). As a result, an electrical system based on 10 primary power distributions has been selected, in which each pri- mary distribution is composed of Busbar, Solid State Power Controller (SSPC) modules allowing electrical load control remote and cable protection, cable harnesses and inverter To supply the power to the other systems, two secondary distributions (called essential and non essential) are desig- Figure 11: Example of advanced propulsive architecture by EDF clusters allocation. Figure 12: Example of advanced power architecture. DES AVIONS PLUS ÉLECTRIQUESMEA 2015 92 REE N°1/2016 distributions (essential one) is also connected to a dedicated Li-ion battery system (autonomy 20 min) allowing to supply the power to the main bus if the two primary distributions are out of order. The two secondary distributions are linked by a Bus Tie Breaker (BTB) to transfer the electrical power between the non-essential and the essential buses. To evaluate the electric power required by all others consumers, the embedded power of the Cirrus SR22 (2,4 kW) has been used as a reference to which additional power, roughly 1,6 kW, has been added for the new on-board System). Taking into account the redundancy of the power system, an additional capacity of 8 kW has been taken into account for the hydrogen fuel cells. Component evaluation The next step is now to evaluate the main characteristics of each component of our proposed advanced power archi- tecture. fuel cell systems (44 kW) to supply in addition on-board and preserve the power capacity at different flight levels (from ground up to 3 000 m), the inlet pressure has been set to 2 bars. Increasing the fuel cell pressure allows to increase its rated power. This 2-bar pressure rate results from the use of two air compressors. In these conditions and with a fuel cell efficiency of 0,6, a power density of 1,8 kW/kg is assumed. Thus, the gross hydrogen tanks, associated with air compressors) is about 226 kg and its volume reaches 1 m3 energy of 500 kWh, it needs to store 30 kg of hydrogen (at 25,5 kg and 0,051 m3 each (with a diameter and length of 0,3 m and 1,63 m respectively). The instantaneous power demands will be assumed by battery packs which are linked in parallel to each fuel cell systems. In our case, we have estimated it to be 10 % of total propulsive energy of our flight configuration, that is 40 kWh. Considering an energy density of 300 Wh/kg, we obtain a battery packs weight of about 133 kg. on the assumption that commercial SSPC modules will be used, a power density of 15 kW/kg has been estimated from data sheets, leading to a SSPC weight of about 27 kg. harnesses length of the propulsive part is close to 114 m. The environmental flight conditions (altitude) and installation cha- racteristics analysis, give a wire gauge of “8” with an average linear weight of 0,130 kg/m. The corresponding cable weight is about 39 kg including their installation penalties (estimated at 30 % of the cable weight). The total weight of advanced power architecture about Conclusion Electric propulsion for aircraft is a promising solution for reducing environmental impact of aviation together with a decrease of fossil energy dependency, considering also that passengers will accept in the future to fly differently: lower, slower and probably with more bounds than to-day for a gi- ven range. Based on recent Onera’s investigation, this propul- sive innovation requires to address several key technologies Propulsion, using distributed hybrid energy sources based on couple of fuel cells and batteries, will be the basis for the emergence of All Electric Aircraft, first for small business ope- rations but potentially for more demanding ones. Key-technologies are the following: 2. Command and control through the association of multi- Figure 13: Weight distribution of the advanced power architecture. REE N°1/2016 93 Electric Distributed Propulsion for Small Business Aircraft - In order to start investing some of these technologies, Onera recently decided to focus effort on the two first above- mentioned items, aiming at the use-case of concept-planes described in this paper: distributed propulsion and its effects on aerodynamics and control, and controllability of such planes using unconventional, distributed and heterogeneous actuators. Through this Carnot funded 3-year project, Onera intends to go further in the knowledge of such technologies by combining computational and experimental approaches. increased thanks to the key technologies modelling and the increased understanding of their coupling effects. In that way, Onera intends to participate in the future of aviation and is ready to go with research and industrial par- tners to promote, develop and make effective All Electric Aircraft. Acknowledgment Authors would like to thank colleagues from Onera who Philippe Choy, Claude Le Tallec, Thierry Lefebvre and Peter for his participation in first steps of this series of studies. References [1] Advisory Council for Aviation Research and Innovation in Europe (ACARE), “Realising Europe’s Vision for Aviation: Strategic Research and Innovation Agenda, Volume 1”, September 2012. [2] European Commission (EC), “Flightpath 2050: Europe’s Vision for Aviation, Report of the High Level Group on Aviation Research”, Publications Office of the European Union, Luxembourg, 2011. [3] Rapport de synthèse du Groupe de Travail Études Prospectives “GTEP12”, Onera – CEAtech, Septembre 2013. [4] “Conceptual Feasibility Study for A Fully Electrically Powered Regional Transport Aircraft”, C. Döll¹, B. Paluch¹, A. Guigon¹, D. Fraboulet; CEA Tech, France; ¹ONERA – LES AUTEURS Jean Hermetz is graduated from « Ecole nationale supé- - rieure des techniques aérospatiales » (ESTA 1990). He has as deputy director. His main research interests are in the multidisciplinary design and optimisation of aeronautical systems. In that frame, he has been involved in numerous projects on conceptual design of future fixed-wing vehicles, and is leading the development of a flying scale demon- strator. He recently decided to investigate the electric pro- pulsion for airplane and is now leading an internal project dedicated to the maturation of several EP key-technologies. Michael Ridel ONERA. His main research interests are the integration of new numerical techniques and process to take into account the Electrical Wiring Interconnection System at aircraft level. DES AVIONS PLUS ÉLECTRIQUESMEA 2015 94 REE N°1/2016 The French Aerospace Lab, France, 29th Congress of the International Council of the Aeronautical Sciences ICAS, Saint Petersburg, Russia, 7-12 September, 2014. [5] L’avion d’affaire personnel à propulsion électrique, un « Concept Plane » ? Aero-Club de France, Symposium sur la Propulsion Electrique des Aéronefs (symposium on Electric Propulsion for Light Aircraft), 07th April, 2014. [6] PPlane projet - http://www.pplane-project.org and http:// www.onera.fr/en/zoominthelab/after-velib-planelib-pplane Acronyms AC Alternative Current AEA All-Electric-Aircraft BTB Bus Tie Breaker EWIS Electrical Wiring Interconnection Systems OEI One Engine Inoperative PPS Propulsion & Power Systems SSPC Solid State Power Controller STOL Short Take-off and Landing aircraft TLAR Top Level Aircraft Requirements