This paper presents comprehensive guidelines for the design of alternative energy aircraft, with a focus on battery-electric and hydrogen fuel cell powertrains. Traditional first-order models like the Breguet Range Equation are found to be inadequate for predicting the performance of electric aircraft due to their inability to account for varying power requirements and thermal management complexities. To address these limitations, the study utilizes advanced aircraft sizing methods following the guidelines provided. The methodology incorporates conceptual design stage analyses of wing and powertrain sizing, energy source sizing, weight predictions, thermal management, and power off-takes. Practical examples of electric aircraft design are provided to demonstrate the application of these guidelines. The results, which are repeatable using the information and open-source software provided, highlight the potential for different assumptions to lead to more optimized solutions. This paper provides crucial metrics and insights beyond common specific energy or power-to-weight ratios, offering detailed information that both aircraft designers and component technologists can use to develop technology solutions and optimize aircraft designs for sustainable aviation by 2050.@en

Battery-electric aviation is commonly believed to be limited to small aircraft and is therefore expected have a negligible impact on the decarbonization of the aviation sector. In this paper we argue that, with the correct choice of design parameters and top-level aircraft requirements, the addressable market is actually substantial. To demonstrate this, the Class-II sizing of a battery-electric 90-seater is performed, and the environmental impact is assessed in terms of well-to-wake CO2-equivalent emissions per passenger-kilometer. The resulting 76-ton aircraft achieves a battery-powered useful range of 800 km for a pack-level energy density of 360 Wh/kg. For this range, it has an energy consumption of 167 Wh per passenger-kilometer and an environmental impact well below that of kerosene, eSAF, or hydrogen-based aircraft alternatives and comparable to land-based modes of transport. These results indicate that, to successfully reduce the climate impact of the aviation sector, battery-electric aircraft should not be designed as a niche product operating from small airfields but as commercial transport aircraft competing with fuel-based regional and narrowbody aircraft.@en

Thus far, battery-electric propulsion has not been considered a promising pathway to climate-neutral aviation. Given current and expected battery technology, in most literature battery electric aircraft are only considered feasible for short ranges (< 400 km) and small payloads (< 19 pax). As a result, battery-electric aircraft development focuses on new aviation segments such as regional and urban air mobility. However, little effort has been made to develop battery-electric aircraft that can replace existing larger aircraft. This paper re-examines the assumptions that lead to the conclusion of limited applicability of battery-electric aircraft. Starting from the range equation, this paper assesses the drivers of two key parameters: the ratio between energy mass and maximum take-off mass, and the maximum lift-to-drag ratio. This assessment, based on Class-I mass and aerodynamic-efficiency estimates, shows that there is a design space where these two parameters can reach significantly higher values than often assumed in the open literature. Based on this finding, several parametric aircraft designs are evaluated, relying on Class-II mass and aerodynamics methods. These parametric studies validate the conclusion from the Class-I assessment. This implies that battery-electric passenger aircraft can play a larger role in climate-neutral aviation than was previously envisioned.

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The impact of the ICAO code C gate span limit is assessed on the sizing of a serial Hybrid Electric Aircraft (HEA) of increasing Degree of Hybridization (DoH). For a set of Top Level Aircraft Requirements (TLARs) similar to the ATR-42, it is shown that the increase in MTOM due to the presence of the battery is such that only a maximum DoH under 30% can be achieved before the wing span of the serial HEA reaches the 36 meters gate size. The same aircraft is fitted with Leading Edge Distributed Propulsion (LEDP) to increase the wing loading and relieve the span constraint, though this introduces limitations regarding the available wing volume. It is shown that a combination of high wing loading and of low volumetric energy density for batteries compared to jet fuel can lead to the available wing volume being too small for the required volume of the energy carriers. Finally a value in wing loading is found which simultaneously meets the span and wing-volume constraints. The higher DoH enabled by LEDP leads results in a 33% reduction in fuel burn compared to the fuel-based reference aircraft, while the overall energy consumption is increased by 6%.@en

The goal of this study is to analyze how the aeropropulsive benefits of an over-the-wing distributed-propulsion (OTWDP) system at the component level translate into an aeropropulsive benefit at the aircraft level, as well as to determine whether this enhancement is sufficient to lead to a reduction in overall energy consumption. For this, the preliminary sizing of a partial-turboelectric regional passenger aircraft is performed, and its performance metrics are compared to a conventional twin-turboprop reference for the 2035 timeframe. The changes in lift, drag, and propulsive efficiency due to the OTWDP system are estimated for a simplified unducted geometry using a lowerorder numerical method, which is validated with experimental data. For a typical cruise condition and the baseline geometry evaluated in the experiment, the numerical method estimates a 45% increase in the local sectional lift-todrag ratio of the wing, at the expense of a 12% reduction in propeller efficiency. For an aircraft with 53% of the wingspan covered by the OTWDP system, this aerodynamic coupling is found to increase the average aeropropulsive efficiency of the aircraft by 9% for a 1500 n mile mission. Approximately 4% of this benefit is required to offset the losses in the electrical drivetrain. The reduction in fuel weight compensates for the increase in powertrain weight, leading to a takeoff mass comparable to the reference aircraft. Overall, a 5% reduction in energy consumption is found, albeit with a 5% uncertainty due to uncertainty in the aerodynamic modeling alone.

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The number of studies on hybrid-electric aircraft is steadily increasing because these configurations can lead to lower operating costs and environmental impact than traditional aircraft. However, due to the lack of reference data of actual hybrid-electric aircraft, the design tools and results are difficult to validate. This paper analyzes the key points that must be validated when developing or implementing a hybrid-electric aircraft design tool by contrasting the assumptions and results of two independently developed sizing methods. An existing 19-seat commuter aircraft is selected as the baseline test case, and both design tools are used to size that aircraft. The aircraft is then resized under consideration of hybrid-electric propulsion technology. This is performed for parallel, serial, and fully electric powertrain architectures. Finally, sensitivity studies are conducted to assess the validity of the basic assumptions and approaches regarding the design of hybrid-electric aircraft. Both methods are found to predict the maximum takeoff mass (MTOM) of the reference aircraft with less than 4% error. The MTOM and payload-range energy efficiency of various (hybrid-) electric configurations are predicted with a maximum difference of approximately 2 and 5%, respectively. The results of this study confirm a correct formulation and implementation of the two methods and provide a reference data set that can be used to benchmark design tools.

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The goal of this study is to analyze how the aero-propulsive benefits of an over-the-wing distributed-propulsion (OTWDP) system at component level translate into an aero-propulsive benefit at aircraft level, and to determine whether this enhancement is sufficient to lead to a reduction in overall energy consumption. For this, the preliminary sizing of a partialturboelectric regional passenger aircraft is performed, and its performance metrics are compared to a conventional twin-turboprop reference for the 2035 timeframe. The changes in lift, drag, and propulsive efficiency due to the OTWDP system are estimated using a simplified numerical method, which is validated with experimental data. For a typical cruise condition and the baseline geometry evaluated in the experiment, the numerical method estimates a 45% increase in the local sectional lift-to-drag ratio of the wing, at the expense of a 12% reduction in propeller efficiency. For an aircraft with 53% of the wing span covered by the OTWDP system, this aerodynamic coupling is found to increase the average aero-propulsive efficiency of the aircraft by 9%, for a 1500 nmi mission. Approximately 4% of this benefit is required to offset the losses in the electrical drivetrain. The reduction in fuel weight compensates the increase in powertrain weight, leading to a take-off mass comparable to the reference aircraft. Overall, a 5% reduction in energy consumption is found, albeit with a ±5% uncertainty due to uncertainty in the aerodynamic modeling. @en

Recent developments in the field of hybrid-electric propulsion (HEP) have opened the door to a wide range of novel aircraft configurations with improved energy efficiency. These electrically-driven powertrains enable “distributed propulsion” configurations, in which the aerodynamic interaction between the propulsive devices and the airframe is exploited to enhance the aero-propulsive efficiency of the aircraft. In this context, the present research focuses specifically on over-the-wing distributed propulsion (OTWDP) for regional propeller aircraft. Over-the-wing (OTW) propellers are particularly promising because they can significantly enhance the lift-to-drag ratio of the wing, as well as reduce flyover noise due to shielding by the wing. The objective of this research is therefore to quantify the impact of OTWDP on the energy efficiency of hybrid-electric aircraft. For this, the research is divided into three main parts. First, a sizing method for hybrid-electric distributed-propulsion (HEDP) aircraft is developed, independently of where the propellers are positioned with respect to the airframe. Second, the aerodynamic interaction effects and performance characteristics of OTWDP systems are investigated, independently of the type of powertrain used to drive the propellers. And third, the sizing method and aerodynamic performance estimates of the previous two points are combined to assess the effect of hybrid-electric OTWDP on aircraft-level performance metrics. [...] @en

This article describes an experimental investigation of the aerodynamic interaction that occurs between distributed propellers in forward flight. To this end, three propellers were installed in close proximity in a wind tunnel, and the changes in their performance, flow-field characteristics, and noise production were quantified using internal force sensors, total-pressure probes, particle-image velocimetry (PIV), and microphones recessed in the wind-tunnel wall. At the thrust setting corresponding to maximum efficiency, the efficiency of the middle propeller is found to drop by 1.5% due to the interaction with the adjacent propellers, for a tip clearance equal to 4% of the propeller radius. For a given blade-pitch angle, this performance penalty increases with angle of attack, decreasing thrust setting, or a more upstream propeller position, while being insensitive to the rotation direction and relative blade phase angle. Furthermore, the velocities induced by the adjacent propeller slipstreams lead to local loading variations on the propeller disk of 5% – 10% of the average disk loading. Exploratory noise measurements show that the interaction leads to different tonal noise waveforms of the system when compared to the superposition of isolated propellers. Moreover, the results confirm that an active control of the relative blade phase angles between propellers can effectively modify the directivity pattern of the system.

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In this study, unsteady RANS simulations are performed to investigate the effect of over-the-wing (OTW) propeller inclination on the aerodynamic interaction with a wing featuring a plain flap. A comparison to experimental data shows that the numerical approach is capable of modeling the wing and propeller separately and can capture the effect of the wing on the propeller in the OTW configuration, but under-predicts propeller-induced flow separation over the flap. The results show that, if the propeller is installed over the flap hinge and aligned with the freestream velocity (baseline configuration), the slipstream and blade tip-vortices generate additional adverse pressure gradients on the wing surface, leading to a local increase in flow separation downstream. However, if the propeller is tilted and aligned with the flap surface (inclined configuration), the slipstream increases the momentum in the boundary layer and the flow remains attached. The propeller alters the pressure distribution of the wing such that higher lift is generated in the baseline case, while a larger drag reduction is achieved in the inclined case. However, combined with the thrust vector of the propeller, the baseline configuration is found to have the largest combined axial force in thrust direction, while the inclined configuration presents the highest effective lift. These results indicate that inclining the propeller can enhance the low-speed performance of OTW distributed-propulsion systems.@en

In this combined experimental and numerical study, the propeller–airframe aerodynamic interaction is characterized for an aircraft configuration with propellers mounted to the horizontal tailplane. The contributions of the propeller and airframe to the overall loading are distinguished in the experimental analyses by using a combination of external balance and internal load cell data. Validated computational fluid dynamics simulations are then employed to quantify the interaction at a component level. The results show that the propeller installation shifts the neutral point aft with increasing propeller thrust. For the configuration considered herein, the yawing moment due to sideslip is increased by approximately 10%, independent of the propeller thrust coefficient. The changes in propeller loading due to the airframe-induced flowfield are the dominant factor to change the airframe stability and performance. The prominent installation effects occur at high angle of attack, because in that condition the propeller experiences a significant nonuniform inflow that affects the propeller and tailplane. The relatively large propeller diameter compared with tailplane span leads to a change of the tailplane root vortex that causes the tailplane effectiveness to reduce with an inboard-up rotating propeller.@en

This experimental study focuses on the aerodynamic interaction between an over-the-wing (OTW) propeller and a wing boundary layer. An OTW propeller is positioned above the hinge line of a wing with a trailing-edge flap. Measurements are carried out with and without axial pressure gradients by deflecting the flap and by extending the flat upper surface of the wing in the streamwise direction, respectively. Surface-pressure taps, microphones, and particle image velocimetry are combined to quantify both the time-averaged and unsteady interaction effects. Results show that the propeller generates an adverse pressure gradient on the wing surface that scales linearly with thrust and decreases with increasing blade-tip clearance. The pressure gradient is partially caused by slipstream contraction, which decelerates the flow near the wall. Additionally, the surface-pressure fluctuations generated beneath the propeller blades and slipstream are stronger than the time-averaged pressure increase due to flow deceleration. Consequently, the propeller triggers flow separation over the hinge line when the flap is deflected. A parametric study of different propeller locations indicates that increasing the tip clearance is not an effective way to mitigate flow separation. However, displacing the propeller half a radius upstream of the hinge line creates a Coandă effect, which allows the flow to remain attached.@en

Advances in aerodynamic and propulsive efficiency of future aircraft can be achieved by strategic installation of propellers near the airframe. This paper presents a robust and computationally efficient engineering method to estimate the load distribution of a propeller operating in arbitrary nonuniform flow that is induced by the airframe and by different flight conditions. The time-resolved loading distribution is computed by determining the local blade section advance ratio and using the sensitivity distribution along the blade, which is a property of the propeller in isolated conditions. The method is applied to four representative validation cases by comparing to full-blade computational fluid dynamics (CFD) simulations and experimental data. For the evaluated cases, it is shown that the changes in the propeller loads due to the nonuniform inflow are predicted with errors ranging from 0.5 up to 12% compared to the validation data. By extending the quasi-steady approach with a correction to account for unsteady effects, the time-resolved blade loading is also well approximated, without adding computational cost. The proposed method provided a time-resolved solution within several central processing unit seconds, which is seven orders of magnitude faster compared to full-blade CFD computations.

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In this paper a design-of-experiments is performed with the objective of determining the improvements in aeropropulsive efficiency required for a reduction in the energy consumption of turboelectric transport aircraft, when compared to conventional, gas-turbine based alternatives. Simplified representations of the powertrain and the aeropropulsive interaction effects are used, such that the results are independent of the design of the electrical system or the external layout of the propulsion-system. An evaluation of different mission requirements confirms that the turboelectric architecture presents the largest benefit for long ranges, and that the aeropropulsive benefit required for a predefined reduction in energy consumption increases with increasing cruise Mach number. Moreover, the impact of different technology maturity levels of the electrical drivetrain components is assessed. The results show that the shaft power ratio necessary to achieve a determined aeropropulsive benefit is a decisive factor, and that for a shaft power ratio of 20%, a 5% reduction in energy consumption is possible on the mid-term (circa 2035) if an 11% increase in aeropropulsive efficiency is achieved. A 15% reduction in energy consumption is only possible with extremely optimistic powertrain technology assumptions, and requires and increase in aeropropulsive efficiency of at least 14%, for the missions considered.@en

There has been a surge in research related to hybrid-/ electric propulsion (HEP) over the past decade, since this technology has the potential to reduce the energy consumption and in-flight emissions of commercial aircraft and, therefore, to bring the aviation sector closer to the sustainability targets established by the European Commission [1] and NASA [2]. Previous studies have shown that hybrid-electric [3,4] and fully-electric [5] general-aviation aircraft can lead to a reduction in both emissions and operating costs for short ranges, when compared with fuel-based alternatives. However, due to the enormous energy and power requirements of large passenger aircraft, fully battery-based propulsion is not a viable option to substantially reduce the climate impact of the aviation sector as a whole [6], unless the mission range is significantly reduced, or unrealistically high battery energy densities are assumed [7]. For this reason, hybrid architectures (especially parallel [8–10] and turboelectric [11–14] ones) are often investigated as a potential solution for large passenger aircraft.@en

This paper presents a synthesis of aero-propulsive interaction studies performed at Delft University of Technology, applied to conceptual aircraft designs with distributed hybrid-electric propulsion (DHEP). The studied aero-propulsive interactions include tip-mounted propulsion, wing leading-edge distributed propulsion and boundary-layer ingestion, combined with different primary propulsion-system arrangements. This paper starts with a description of the applied design framework and an overview of the aero-propulsive interactions. Subsequently, the different aircraft configurations are sized for a set of top-level requirements covering the range between regional turboprop to typical narrow-body turbofan aircraft. Results indicate that lower shaft power ratios show better performance, with the unoptimized DHEP concepts showing values of maximum take-off mass (MTOM) and payload-range energy efficiency (PREE) comparable to their reference aircraft. It was shown that beyond 20% shaft power ratio, the PREE decreases and MTOM increases much more than between 10% and 20%, indicating a possible local optimum between these values since even lower values did not yield any significant improvements. The benefits of tip-mounted propulsion are found to be constrained by the propeller blade tip Mach number in this particular analysis for the selected reference blade loading distribution. At the high range case for Mach 0.5, it can be seen that the distributed propulsion systems show the largest improvement.

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The number of case studies focusing on hybrid-electric aircraft is steadily increasing, since these configurations are thought to lead to lower operating costs and environmental impact than traditional aircraft. However, due to the lack of reference data of actual hybrid-electric aircraft, in most cases, the design tools and results are difficult to validate. In this paper, two independently developed approaches for hybrid-electric conceptual aircraft design are compared. An existing 19-seat commuter aircraft is selected as the conventional baseline, and both design tools are used to size that aircraft. The aircraft is then re-sized under consideration of hybrid-electric propulsion technology. This is performed for parallel, serial, and fully-electric powertrain architectures. Finally, sensitivity studies are conducted to assess the validity of the basic assumptions and approaches regarding the design of hybrid-electric aircraft. Both methods are found to predict the maximum take-off mass (MTOM) of the reference aircraft with less than 4% error. The MTOM and payload-range energy efficiency of various (hybrid-) electric configurations are predicted with a maximum difference of approximately 2% and 5%, respectively. The results of this study confirm a correct formulation and implementation of the two design methods, and the data obtained can be used by researchers to benchmark and validate their design tools.@en

In the Clean Sky 2 Large Passenger Aircraft (LPA) program the potential of innovative hybrid electric propulsion (HEP) air vehicle concepts has been investigated by the projects ADEC (Advanced Engine and Aircraft Configuration) and NOVAIR (Novel Aircraft Configurations and Scaled Flight Testing Instrumentation). In a combined effort the members of the two projects worked closely together to identify promising technologies for air vehicle concepts that utilize hybrid electric powertrains. The vehicle design studies were divided onto the three distinct teams of DLR, TU Delft/NLR (NOVAIR), and ONERA. This paper will summarize the design efforts and the results obtained from the HEP design space exploration. @en

Ducted propellers constitute an efficient propulsion-system alternative to reduce the environmental impact of aircraft. These systems are able to increase the thrust-to-power ratio of a propeller system by both producing thrust and by lowering tip losses of propellers. In this research, steady and unsteady RANS CFD simulations were used to analyze the possible impact of modifying a propeller duct shape from a circular to a square geometry. Initially, the two duct designs and the propeller were studied separately, in order to estimate the numerical errors and to compare with existing data. In the installed simulations, the propeller was first modelled as an actuator disk, and afterwards with a full blade model, in order to understand the time-averaged influence of the propeller on the duct before studying the complete unsteady propeller-duct interaction. In the current design, the square duct corners were found to be prone to separation, and to contribute towards the generation of strong vortices. Furthermore, due to the reduced leading-edge suction on the square duct, the square ducted system was found to be 4.5% less efficient than the circular one, for the conditions tested. By relating the aerodynamic interaction phenomena to the performance of the system, this study provides and important basis for the design of unconventional ducted systems. @en

The use of hybrid-electric propulsion (HEP) entails several potential benefits such as the distribution of power along the airframe, which enables synergistic configurations with improved aerodynamic and propulsive efficiency. This paper presents a comprehensive preliminary sizing method suitable for the conceptual design process of hybridelectric aircraft, taking into account the powertrain architecture and associated propulsion–airframe integration effects. To this end, the flight-performance equations are modified to account for aeropropulsive interaction. A series of component-oriented constraint diagrams are used to provide a visual representation of the design space. A HEPcompatible mission analysis and weight estimation are then carried out to compute the wing area, powerplant size, and takeoff weight. The resulting method is applicable to a wide range of electric and hybrid-electric aircraft configurations and can be used to estimate the optimal power-control profiles. For demonstration purposes, the method is applied to a regional HEP aircraft featuring leading-edge distributed propulsion (DP). Three powertrain architectures are compared, showing how the aeropropulsive effects are included in the model. Results indicate that DP significantly increases wing loading and improves the cruise lift-to-drag ratio by 6%, although the growth in aircraft weight leads to an energy consumption increase of 3% for the considered mission. @en