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Impact of Propulsion Technology Levels on the Sizing and Energy Consumption for Serial Hybrid- Electric General Aviation Aircraft

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Serial hybrid-electric propulsion systems combine the advantages of combustion engines and electric motors. Additionally, they offer new design freedom. In the medium term, this technology might be the solution to more eco-friendly aviation. In this paper, the key technology parameters of general aviation aircraft with such powertrains are analyzed concerning their influence on maximum takeoff mass and primary energy consumption. Besides, technological thresholds will be identified. It is found that a power-to-weight ratio increase of electric motors does not yield as large improvements as expected and that fully electric powertrains are the best solution for aircraft designed to minimum primary energy usage once a certain battery energy density is exceeded.
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... Additionally, it was shown in multiple studies (Refs. [128], [129], and [130]) that the efficiency of the electric system has the lowest impact on the sized mass. Therefore, the simplified efficiency model is recommended for early conceptual design and used in the studies of chapter 7. ...
... Sensitivity studies by the author show that battery mass, motor mass, and engine mass are the key design drivers of hybrid-electric aircraft (Refs. [128], [129], [130]). Thus, the accurate prediction of those parameters should be the focus when modeling these systems. ...
Thesis
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Dr Finger researched design methodologies for aircraft that use hybrid-electric propulsion. He developed a new method to assess novel aircraft concepts. The findings indicate that hybrid-electric propulsion is not suited for any new aircraft, but for specific missions and applications, this technology can offer significant savings of energy and cost.
... In [23] a series hybrid-electric version of a Cessna 172 is investigated. Two versions are preliminary designed: a first version represents a plain replacement of the combustion engine with an electric motor. ...
Thesis
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Hybrid-electric aircraft possess the potential to reduce the CO2 emissions of general aviation aircraft. However, optimized propulsion systems are needed to leverage the advantages and lower the fuel consumption. In order to identify optimized designs, scaling characteristics of the individual propulsion elements are necessary. Furthermore, appropriate power and energy reserves in case of a propulsion component failure are required and the impact of an increased propulsion system mass on the aircraft mass and the aerodynamic drag needs to be considered. The derived trends are implemented into a sizing program and optimized hybrid-electric propulsion systems are determined for a 4-seat hybrid-electric aircraft with a cruise speed of 220 km/h as well as a 9-seat hybrid-electric aircraft with a cruise speed of 400 km/h.
... For each case study, a reference mission is defined, while different technology levels are applied to these missions. Similar mission requirements and technology levels are used by Ludowicy et al. [15]. The individual impact of several technological factors is evaluated for each mission and technology level by investigating their sensitivities. ...
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Hybrid-electric propulsion systems are able to combine the advantages of both fuel-based and battery-powered propulsion systems. Thus, they do not only promise a reduction in fuel consumption and direct emissions, but also a reduction in total primary energy consumption. This paper assesses the potential of parallel hybrid-electric powered general aviation aircraft by means of a sensitivity study. Therefore, an energy-based initial sizing methodology is applied to determine maximum takeoff mass and primary energy consumption for a 4-seat general aviation aircraft. The sizing process is repeated for variations of technological factors and mission performance requirements to identify those parameters that have a big impact on maximum takeoff mass and primary energy consumption. Results indicate battery technology as the most critical field in order to reduce both design objectives significantly. It is found that, in contrast to previous investigations, even comparatively high battery technology standards are able to further improve design objectives and therefore justify the additional development effort.
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The present work is a survey on aircraft hybrid electric propulsion (HEP) that aims to present state-of-the-art technologies and future tendencies in the following areas: air transport market, hybrid demonstrators, HEP topologies applications, aircraft design, electrical systems for aircraft, energy storage, aircraft internal combustion engines, and management and control strategies. Several changes on aircraft propulsion will occur in the next 30 years, following the aircraft market demand and environmental regulations. Two commercial areas are in evolution, electrical urban air mobility (UAM) and hybrid-electric regional aircraft. The first one is expected to come into service in the next 10 years with small devices. The last one will gradually come into service, starting with small aircraft according to developments in energy storage, fuel cells, aircraft design and hybrid architectures integration. All-electric architecture seems to be more adapted to UAM. Turbo-electric hybrid architecture combined with distributed propulsion and boundary layer ingestion seems to have more success for regional aircraft, attaining environmental goals for 2030 and 2050. Computational models supported by powerful simulation tools will be a key to support research and aircraft HEP design in the coming years. Brazilian research in these challenging areas is in the beginning, and a multidisciplinary collaboration will be critical for success in the next few years.
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In recent times many research efforts are aimed at the development of aircraft hybrid propulsion technology as a means to minimize the aircraft environmental impact. The objective is to provide an additional assessment of the energy efficiency of the hybrid powered light aircraft in comparison with current state of the art Diesel powered aircraft. Typical 4-seat propeller airplane is chosen as a prototype and its flight performance model is created using open source data. The comparison is performed for a realistic flight profile including the power demands for go-around and flight to alternate destination airport. Parallel hybrid architecture is considered. A parametric study with variation of the mass, efficiency and energy capacity parameters of the hybrid power train is performed in order to determine the parameter regions where the hybrid power train has advantage over the Diesel one. The present study is based on the previous experience of the authors in assessing the energy efficiency of alternative power sources for the aviation.
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Hybrid-electric propulsion systems are able to combine the advantages of both fuel-based and battery-powered propulsion systems. Thus, they do not only promise a reduction in fuel consumption and direct emissions, but also a reduction in total primary energy consumption. This paper assesses the potential of parallel hybrid-electric powered general aviation aircraft by means of a sensitivity study. Therefore, an energy-based initial sizing methodology is applied to determine maximum takeoff mass and primary energy consumption for a 4-seat general aviation aircraft. The sizing process is repeated for variations of technological factors and mission performance requirements to identify those parameters that have a big impact on maximum takeoff mass and primary energy consumption. Results indicate battery technology as the most critical field in order to reduce both design objectives significantly. It is found that, in contrast to previous investigations, even comparatively high battery technology standards are able to further improve design objectives and therefore justify the additional development effort.
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In this paper, the viability of general aviation aircraft with serial hybrid-electric powertrains is examined. They will be compared to conventionally powered aircraft with similar specifications, by the means of initial sizing results. Studies on battery specific energy and aerodynamic efficiency will be analyzed, the latter in the scope of a distributed electric propulsion concept, to see their influence on the viability of serial hybrid aircraft. Results show that a serial hybrid-electric aircraft with an optimized distributed electric propulsion concept is competitive to today's conventionally powered light general aviation aircraft, while enabling significant fuel savings. Additionally parameter variation studies are conducted to see the influence of takeoff distance, aircraft range and cruise speed on the optimal design point. The finding is that the conventional design rule is not valid anymore and for every new hybrid-electric aircraft a design space exploration should be conducted.
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In the past decades, interest in cleaner, alternative propulsion increased rapidly. Accordingly, novel approaches for aircraft propulsion are required to meet the spirit of the time. This paper aims to assess the viability of parallel hybrid-electric propulsion systems for General Aviation (GA) aircraft with state of the art technology. Therefore, a comparison to the conventionally powered competitors is performed by using initial sizing results. An assessment of clean sheet designs is performed, as well as an evaluation of retrofit designs. Additionally, parameter studies on battery specific energy and aerodynamic efficiency are conducted to analyze their potential in improving viability and advantages of such aircraft. The studies' results indicate that parallel hybrid-electric light aircraft are able to offer considerable benefits over conventional aircraft, even with today's technological standards. However, several design aspects have to be taken into account to make full use of the benefits arising from these concepts.
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This paper showcases the potential influence of hybrid-electric propulsion systems on the performance of general aviation aircraft. Using the Cirrus SR-22 as a baseline, parallel- and serial-hybrid propulsion configurations are explored. For these case studies a high level ap-proach is chosen, using an innovative initial sizing methodology to determine weight or energy consumption of new aircraft concepts. Mission parameters and aerodynamic performance are varied and the impact on aircraft fuel and energy consumption, as well as on take-off mass is studied. The studies’ results indicate that hybrid-electric aircraft consume less energy than conventionally propelled designs for certain missions. However, the aircraft’s design point, in terms of wing loading and installed power must be changed: Hybrid-electric aircraft should be sized with a higher power-to-weight ratio and a higher wing loading than their con-ventional counterparts.
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This paper describes the methodology and the benefits of an initial sizing algorithm that is able to consider aircraft with hybrid-electric propulsion systems by using an innovative point- and mission performance analysis. A central element is the use of the matching diagram (Power-to-weight ratio P/W vs. wing-loading W/S). Conventionally, aircraft are designed for minimal installed power, while still satisfying all performance constraints. This assumption is not applicable to hybrid-electric designs. The combination of P/W and W/S that results in minimal gross weight needs to be determined in conjunction with the optimal degree of hybridization. A sizing study is shown that indicates that the result of such an optimization can be a reduction in sized weight and increased aircraft performance. Hybrid aircraft should be sized with a higher installed P/W and a higher W/S than their conventional counterparts.
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One of the biggest challenges in aviation is the design of transitioning vertical takeoff and landing (VTOL) aircraft. Thrust-borne flight implies a higher mass fraction of the propulsion system, as well as much increased energy consumption in the takeoff and landing phases. A good VTOL design will offset this disadvantage by transitioning to conventional forward flight, thus travelling at much higher efficiency than a comparable rotorcraft, for an overall improvement in mission performance. This paper intents to support the configuration designer of VTOL aircraft by giving a review of some of the available configuration possibilities, considering the latest advancements in technology. While VTOL aircraft can use the conventional wing-fuselage-stabilizer configuration, much of new development efforts involve unconventional planforms. The advent of distributed propulsion and electric-or hybrid-electric propulsion systems offers additional opportunities to optimize the vehicle layout and improve flight performance. This review considers propeller driven designs, lift fans and ducted fans, as well as jet lift and hybrid configurations that use a mix of propulsion methods.
Thesis
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Boundary Layer Ingestion is an integrated propulsion concept in which a propulsor operates in boundary layer flow instead of the free streamflow with the goal to reduce the fuel flow for a given operating condition. The objective of this thesis is to obtain a better understanding of the power benefit of an installed pusher propeller at the aft fuselage by designing the aerodynamic shape of the propeller and validate the design by means of CFD simulations. A propeller analysis tool for uniform inflow (UI) and non-uniform inflow (NUI) named N-XROTOR is developed using the lifting line code XROTOR in combination with XFOIL to calculate airfoil properties. The tool is validated using experimental results and results from CFD simulations of uniform inflow propellers. N-XROTOR shows good agreement of the trend of the CT-J and CP-Jcurves but a constant over prediction of both thrust and power with respect to experimental data is observed and several deviations are explained. A series of CFD simulations in ANSYS Fluent™ using a reduced wedge shaped domain of one blade of the N250 propeller are performed for several advance ratios including a grid refinement study. Minor deviations between a transient and a steady simulation are found and the steady method is chosen based on computational cost. The trends of N-XROTOR in terms of CT-J and CP-J compare well with the CFD simulation with a constant over prediction of the performance quantities by N-XROTOR. These over predictions are also noticeable in the radial distributions of thrust and torque with slight over predictions in the high loaded region on the blade. For a moderate advance ratio of J = 0.79 the thrust and power are over predicted by 5.25% and 3.67% respectively. A comparison with the standard kapppa-omega SST turbulence model and the SST model with low Reynolds number correction is made. The radial flow on the propeller blade is shown to be quite significant and varies along the blade and shows good agreement with the distribution of bound circulation and the resulting trailing vorticity. A design procedure is developed in which the propeller shape is optimised using shape functions to describe the pitch and chord distribution and a NACA four series airfoil is used to limit the number of design variables for a gradient based optimisation algorithm in Matlab environment. The interaction effects are assumed to be determined a-priori and a tapered aft fuselage and the pressure field induced by the fuselage are neglected. Input quantities for the design routine include an inflow field from CFD analysis, the design advance ratio and a thrust requirement. The design objective of all optimisations is minimum power. For the reference design case an axisymmetric body from ESDU is subject to CFD simulations to obtain the inflow profile and fuselage drag for the isolated and installed configuration. Interference effects are approximated using an Actuator Disk (AD) model at the predefined location of the propeller with a pressure jump equal to the defect in total pressure in the boundary layer based on findings from previous research. An 11% increase of drag is found for the equilibrium condition which is primarily due to increased pressure drag. Larger pressure jumps show only a marginal increase in drag. In a comparison study, the number of blades is set to four, an advance ratio of J = 1.50 is chosen and in combination with a radius equal to 99% of the total gage pressure of the undisturbed air yields a tip Mach number of around 0.50. The optimisation results show that the NUI propeller requires 6.93% less power compared with the UI propeller despite the 11% higher thrust. The thrust distribution of the NUI propeller shows a significant increase in thrust in the lowaxial velocity region towards the root and the maximum thrust is shifted inboard. The ratio of thrust to power dT/(dQ Omega)­ along the propeller blade shows a constant distribution for the UI propeller, while the NUI propeller has a smooth increasing distribution towards the root. This distribution shows that thrust requires a relatively low power when the local axial velocity is relatively low. It is found that this is the main benefit of positioning a propeller in the boundary layer. The bound circulation distribution shows a shift towards the root compared with the distribution of the uniform inflow propeller which is the result of the optimised propeller shape which benefits from the favourable thrust to power ratio in the inner radii. The NUI propeller has a significant increased chord compared with the optimal UI and also a higher lift coefficient distribution. The local efficiency defined as eta = dTVa/(dQ Omega) with Va as the local inflow velocity. Optimal UI propellers have a constant efficiency distribution, but the NUI propeller shows a decreasing trend towards the root which is also found in literature. The trend of lower local efficiency is also found when an optimisation for minimum power is performed using a radially varying actuator disk with the same inflow and thrust requirement as for the full blade propeller. Additional analysis on the NUI propeller include a comparison of off design conditions and additional optimisations are performed to quantify the effect of the number of blades, radius and advance ratio. The optimised NUI propeller in the installed configuration is simulated using CFD. N-XROTOR over predicts the thrust and power by 4.15% and 4.71% respectively compared with the CFD simulation, which are deviations of the same order as the N250 simulation. The thrust to power distribution shows good correspondence. In the root region this ratio is under predicted by N-XROTOR which is expected to be the result of a large pressure and velocity gradient at the junction of the spinner and propeller surface resulting in a region of recirculation. Also the blockage effect of the tapered spinner results in larger angles of attack in the root region. The outer region shows trailing edge stall which is found to be primarily due to the coarse mesh in that region. Improved results are obtained when N-XROTOR uses airfoil data obtained from two-dimensional CFD analysis of a particular airfoil section. The kappa-omega SST model with low Reynolds number correction shows almost exact agreement with XFOIL. The standard turbulence model shows a decambering effect and an earlier stall behaviour. The remaining deviations between N-XROTOR with approximated CFD airfoil properties are expected to originate from the radial flow on the blade and the variation in circulation in chordwise directions which are not simulated in N-XROTOR. Both the externally induced radial flow by the tapered aft fuselage and the self induced radial flow are expected to result in a decambering of the airfoil due to the influence on boundary layer growth as well as a reduced chordwise velocity resulting in a locally lower dynamic pressure experienced by the airfoil contour. The interference effects of the propeller onto the fuselage are compared with the Actuator Disk (AD) approximation. An over prediction of 0.74% of the drag by the AD model of the fuselage excluding spinner is observed. Downstream of the full blade simulation the pressure is rapidly decreased to a low finite value at the aft end of the spinner. This is the result of the finite bound circulation at the propeller root which releases a strong trailing vortex from each blade. These vortices combine into a strong axial vortex which induces a strong tangential velocity and therefore in a low pressure acting on the spinner. A slipstream analysis is performed of circumferentially averaged flow quantities in radial direction at a plane behind the propeller and the axial development of several averaged flow quantities is shown. Several recommendations for future work are formulated to improve the propeller design, improve the design procedure, reduce the interference effects and increase the power benefit of the non-uniform inflow propeller.
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