Within the work package radical new aircraft configuration of Cleansky2 Large Passenger Aircraft, a benefit of more than 20% in fuel consumption and CO2 emission (one of CS2 top level objectives) could be achieved by using various Distributed (hybrid) Electric Propulsion DEP architectures on different more or less radical aircraft configurations. It has therefore been identified as a disruptive technology which shall be de-risked in terms of achievable performance during wind tunnel tests and in terms of handling qualities during flight tests. The electric architecture with typical magnitudes shall also be studied in more detail. As already presented during AIAA SciTech Forum and Exhibition 2023, the D08 Distributed Electric Propulsion DEP version of the D03 Scaled Flight Demonstrator has been designed, manufactured and ground tested from 2020 to May 2023. An incident during the last ground test in May 2023 caused the total loss of this demonstrator. After its analysis, it was decided to robustify the electric architecture by improving the batteries, the wiring, the protections and the monitoring. These changes in the electric architectures lead to structural changes like the shift of the emergency parachute and bigger access hatches. The remanufacturing of the DEP-SFD2 has started in September 2023 for an exhaustive integration test campaign and taxi tests in January and February 2024. At the moment, the qualification flight tests will take place in April 2024 and the mission flight tests in May 2024.@en

Propeller-wing configurations are expected to return to the aviation industry due to their high propulsive efficiency and applicability in urban and regional air mobility. A knowledge gap exists around wing-propeller optimization because of the complexity of the propeller-wing system and the absence of a computationally efficient way to assess the coupled system. This paper addresses this gap by providing and validating a computationally efficient, mid-fidelity framework. The paper presents optimization results and recommendations for future iterations of the framework. The TU Delft PROWIM propeller is optimized with the framework, comparing sequential isolated optimization, trim optimization, and fully coupled optimization. The studies gives a conservative estimate of the efficiency gains that can be achieved by using coupled optimization, as compared to isolated optimization. Lastly, recommendations are given for future studies, such as including a battery weight model and including swirl velocities. It is expected that such model additions will affect the optimization results, and further emphasize the importance of coupled aerostructural optimization.@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 objective of the EU-funded research project CHYLA (Credible HYbrid eLectric Aircraft) was to identify opportunities or limitations/challenges for the applications of key radical hybrid-electric technologies and areas suitable for scaling them over different aircraft classes. This was done using a ombination of conceptual aircraft design supported by sensitivity studies, credibility-based MDO and assessment of a regional operative scenario. This article summarizes the key findings from the project and presents the landscape of technology application areas. Notably, the regional and commuter classes present the largest design space with significant fuel-saving potential depending on the mission.@en

Electrification of aviation is regarded as one of the means to make aircraft operations less polluting and to have lower climate impact. Yet, air transportation's environmental impact depends on power train technologies and novel designs and aircraft operations within airline networks. Fully- or hybrid-electric aircraft may enter existing air transport networks through fleet replacement yet require airlines to adapt in order to operate electrified aircraft strategically. This research studies how airlines can strategically adjust their network and fleet composition when considering electrified aircraft. The novelty of this approach is to provide a direct feedback coupling between fleet planning, conceptual hybrid-electric aircraft design and climate impact minimization. Therefore, a strategic airline planning model, consisting of fleet and network analysis, is coupled to a hybrid-electric aircraft design model. A case study on the sensitivity of a regional airline network is presented to demonstrate the framework and assess the impact of trying to design aircraft and fleets with minimal climate footprint. A decrease in emissions with respect to a kerosene fleet of 11% can be achieved when a hybrid-electric fleet is designed particularly for the specified network, at the penalty of a profit decrease of 13%. Limiting fleet diversity to three types results in only 7% emissions decrease. Increasing the battery-specific energy shows an expected beneficial effect on emissions.@en

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

This study aims at providing a landscape of opportunities and limitations for hybrid-electric aircraft (HEA) and hydrogen-powered aircraft by investigating several technological combinations applied to three aircraft classes: Regional (REG), Short-Medium Range (SMR) and Large Passenger Aircraft (LPA). The preliminary sizing of HEA using different hybrid-electric powertrain architectures, combined with various distributed propulsion layouts is conducted. The resulting HEA are then compared to a conventional design, on the basis of several performance metrics, for variations in harmonic range and passenger capacity. Throughout the design space considered, it is found that opportunities for radical aircraft design are scarce and offer limited prospective. @en

Decades of improvements of engine efficiency on internal engine components through better materials, design methods and novel fabrication techniques have resulted in fuel consumption reductions. Another major factor for improving engine fuel consumption has been the increase of bypass ratio. However, this has a significant impact on engine dimensions and weight, and, consequently, the installation of the engine on the airframe. Evaluation of engine installation penalties is not a new topic; literature provides various studies on aerodynamic effects. These primarily studied the effects of drag increase and the impact on drag of engine location and nacelle shape. This article investigates the performance impact of installation penalties from an increase in bypass ratio on narrow body aircraft, specifically the fuel consumption, weight and stability. Additionally, an analysis is made comparing aircraft retrofit and redesign for increased bypass ratio engines. It can be concluded from retrofit analyses that engine size is more significant than its location. Changes in aerodynamic center, CLα , and CMAC cause stability/controllability criteria to shift to the left. Heavier engines at the same spanwise location cause a more forward CG location, which may become limiting. With the engine increasing in size (thus increasing the drag and increasing the weight), the overall increase in fuel burn is 5.9%. However, the decrease in fuel burn due when the SFC and engine effects are considered together, the fuel burn drops by 50%. The reduction in fuel burn thereby negating the increase in engine weight, drag, and integration issues. From BPR 10 onwards, the decreasing trend in tail size stagnates and actually reverses, indicating that larger tail sizes might be required for even larger BPR engines.@en

Differential thrust can be used for directional control on distributed electric propulsion aircraft. This paper presents an assessment of flight dynamics and control under engine inoperative conditions at minimum control speed for a typical distributed propulsion aircraft employing differential thrust. A methodology consisting of an aerodynamic data acquisition module and a non-linear six-degrees-of-freedom flight dynamics model is proposed. Directional control is achieved using a controller to generate a yaw command, which is distributed to the propulsors through a thrust mapping approach. A modified version of the NASA X-57 aircraft is selected for case studies, where the engine inoperative condition is considered to impact the three leftmost propulsors during climb at minimum control speed. The objective also includes the assessment of the impact of the aero-propulsive coupling for such an aircraft during a failure case. Results show that during the recovery manoeuvre, the aircraft experiences a 78% reduction in total thrust and 30% reduction in total lift caused by the aggressive yaw control effort required to control the heading of the aircraft. Consequently, the powered-stall speed is increased, and the aircraft temporarily loses altitude during the recovery manoeuvre. Differential thrust provides sufficient yaw authority during the engine inoperative condition, and is, therefore, seen to potentially replace the functionality of the rudder for the climb condition that was studied. Additionally, reduction of the vertical tail area was explored and seen to be possible if the response time of the system is low enough. For the studied configuration, this required a response within 400 ms for reduced vertical tail areas.

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The concept of an "Auxiliary Power and Propulsion Unit" (APPU) is introduced, which consists of a Boundarylayer ingesting (BLI) propulsor with an engine mounted at the rear of an passenger aircraft fuselage, replacing the Auxiliary Power Unit (APU) and contributing around 10% of total cruise thrust, as well as auxiliary power. This APPU unit is using hydrogen provided by an additional tank installed in the tailcone of the aircraft. The concept is aimed at lowering the threshold to installing both hydrogen-driven propulsion and BLI propulsors on aircraft in the short term, while minimizing resulting operational risk. The concept has been investigated using a preliminary aircraft synthesis tool and further component-level mass estimates. Operational aspects, sensitivities and limits to the design have been investigated. Estimates of mission fuel burn find that CO2 emissions emissions reduce roughly proportionally to the APPU thrust share, with additional savings due to improved overall efficiency. Further improvements are deemed feasible and are the topic of ongoing research.@en

Zero-carbon-dioxide-emitting hydrogen-powered aircraft have, in recent decades, come back on the stage as promising protagonists in the fight against global warming. The main cause for the reduced performance of liquid hydrogen aircraft lays in the fuel storage, which demands the use of voluminous and heavy tanks. Literature on the topic shows that the optimal fuel storage solution depends on the aircraft range category, but most studies disagree on which solution is optimal for each category. The objective of this research was to identify and compare possible solutions to the integration of the hydrogen fuel containment system on regional, short/medium- and large passenger aircraft, and to understand why and how the optimal tank integration strategy depends on the aircraft category. This objective was pursued by creating a design and analysis framework for CS-25 aircraft capable of appreciating the effects that different combinations of tank structure, fuselage diameter, tank layout, shape, venting pressure and pressure control generate at aircraft level. Despite that no large differences among categories were found, the following main observations were made: (1) using an integral tank structure was found to be increasingly more beneficial with increasing aircraft range/size. (2) The use of a forward tank in combination with the aft one appeared to be always beneficial in terms of energy consumption. (3) The increase in fuselage diameter is detrimental, especially when an extra aisle is not required and a double-deck cabin is not feasible. (4) Direct venting has, when done efficiently, a small positive effect. (5) The optimal venting pressure varies with the aircraft configuration, performance, and mission. The impact on performance from sizing the tank for missions longer than the harmonic one was also quantified.@en

Various research initiatives in hybrid-electric/sustainable aviation typically address only a single vehicle or single vehicle class. However, novel propulsion and energy solutions can be expected to be differently applied in different vehicle classes. The objective of the EU funded research project CHYLA (Credible HYbrid eLectric Aircraft) is to identify areas suitable for scaling, as well as limitations or challenges for development for the applications of key radical technologies on different classes of aircraft. This article provides an overview of the design approach followed for the CHYLA project, as well as initial radical designs and comparison to the CHYLA baselines. These provide the starting point for both the sensitivity study which will be presented in a later scalability assessment and economical assessments in the CHYLA project. A variety of regional, short medium range and large aircraft has been designed, all according to the same TLAR yet without detailed tuning of important power control variables. Results are distinguishable between concepts and provide sufficient detail to capture the necessary effects. The reduction of fuel consumption will require detailed assessment and fine tuning, though reductions may be achievable for regional and possibly SMR aircraft. @en

The current focus in the aviation industry for more sustainable designs, could mean the revive of propeller propulsion, due to their relative high propulsion efficiency compared to jets. In addition, wingtip-mounted propellers installed in tractor configuration can be used as tip-vortex attenuating devices, reducing the wing induced drag. So far, studies on wingtip-mounted propellers mainly concentrated on the aerodynamic interaction effects, disregarding the integration with the airframe and wing-structural mass. This paper presents a method to integrate into an aircraft sizing process the aerodynamic, aero-propulsive, and aero-structural effects of tip-mounted propellers, in the context of a typical turboprop featuring hybrid-electric propulsion. Subsequently, a number of case studies are performed to investigate the sensitivity to modifications of the propulsion system on wing and aircraft level. Results show that the performance benefit gained by the application of a wingtip-mounted propeller is easily overruled by the weight penalty that it introduces, an almost linear relationship between shaft power ratio and MTOM was observed. For variations of propeller diameter, it is seen possible to attain equal performance in terms of energy efficiency with a mass penalty. For phi=0.1, the reference performance is obtained for a tip-mounted propeller occupied span fraction of 0.175 with an increase in MTOM of 2.8%. For phi=0.2, this is obtained at a larger tip-mounted propeller (occupied span fraction = 0.275) and an increase in MTOM of 5% compared to the reference design. @en

Research initiatives in hybrid-electric/sustainable aviation typically address only a single vehicle or single vehicle class. However, it can be expected that for novel propulsion and energy solutions to reach a sufficient maturity level an approach not too different from the early days of aviation could be followed: starting small to prove a technology and gradually scaling it up to larger and larger classes of vehicles. Therefore, it is important to sketch a landscape of technologies and identify areas suitable for scaling, as well as limitations or challenges for development. The CHYLA project aims to contribute to the overall research and development in sustainable aviation by providing exactly this; an overview of scaling opportunities, challenges and limitations of key radical technologies where the credibility of underlying technology assumptions is an explicit factor in developing such a landscape. In CHYLA we introduce the concept of “credible aircraft design”, which refers to the influence of the uncertainty in the assumptions behind the design to quantify how realisable, or in other words, how credible, is the design. Since technical work on the project has just started, the article will present a general overview of the CHYLA scope, approach and objectives.@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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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

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

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

Four regional turboprop aircraft of different configuration are assessed on their key performance indicators by using an automated conceptual design method. The configurations include two wing-mounted propeller installations (high-wing and low-wing) as well as a fuselagemounted configuration and a tail-mounted configuration. The effect of the propeller and associated slipstream is included in the sizing of the horizontal tail plane of these aircraft. The results from the automated design method are compared to benchmarks from open literature showing a deviation in maximum take-off mass of around 10%. The four different aircraft configurations were applied to aircraft designs for a payload of 130 passengers and a harmonic range of 1500nm (2960km). Results from the configuration study show that the aircraft with propellers mounted on the tail or on the fuselage have about a 5% higher maximum take-off mass than the low-wing, wing-mounted configuration due to the absence of wing bending moment relief and an increase in trim drag resulting from a larger center-of-gravity excursion. The application of natural laminar flow on the clean wing as well as operational bounds on the center-of-gravity excursion were also investigated but did not show an improvement in aircraft performance indicators compared to the low-wing, wing-mounted designs. @en