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.

Optimized power system architectures and lighter weight are enabling considerations for the successful development of all-electric aircraft (AEA). In this article, a cross-redundant connection architecture and weight reduction solutions are investigated for a 90-seater full battery-electric aircraft from the perspective of high-power aviation cable. Design criteria of the power system architecture are introduced. Material selection, sizing, and weight estimation methods of cable for AEA are discussed by combining ground cable standards with aviation requirements. The influence of the conductor materials, voltage level, current, battery pack quantity, and operating temperature on cable evaluation is thoroughly discussed and analyzed. Weight comparison under two controversial voltage level options (800V and 3kV) is conducted. Comparison results show that the utilization of an aluminum conductor, PTFE insulator, and a voltage level of 3kV proves to be a preferable selection for current AEA medium and high voltage cables. Increasing the rating operation temperature to 120°C is a conservative and secure option. The layout of battery packs consistent with the quantity of distributed electric motors is preferable to achieve the lightest cabling system. This study provides a guideline for the cable sizing methods of high-power aviation cables and an optimized design solution for the power system architecture of AEA from the perspective of cable layout and weight assessment.

The goal of this study is to determine the aero-propulsive performance of an over-the-wing distributed propulsion (OTWDP) system, and to understand how it depends on various operating conditions. For this, a windtunnel test is performed with a simplified OTWDP geometry consisting of three unducted propellers placed side-by-side above a rectangular wing. A numerical model combining 2D panel methods, a slipstream vortex model, and a lower-order method for propeller performance in non-uniform inflow is then used to analyze additional operating conditions. A comparison to experimental data shows that the numerical method captures the changes in wing and propeller performance due to aerodynamic interaction in cruise conditions, though it is inaccurate if flow separation occurs on the wing surface beneath the propeller. For a setup with propellers of diameter-to-chord ratio 0.6 placed above the wing at 80% chord, the sectional lift-to-drag ratio of the wing is found to increase by 40% – 70% for typical cruise lift and thrust coefficients, while the propeller efficiency is decreased by 10% – 15%, compared to the two components in isolation. Parameter sweeps demonstrate that the combined aero-propulsive performance improves with a variable-pitch propeller and at higher lift coefficients, thrust settings, or Reynolds numbers.

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.

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.

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.

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.

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.