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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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

This paper presents a preliminary study about a combined-cycle engine based on a turboshaft engine and an organic-Rankine-cycle (ORC) bottoming unit to be used onboard an aircraft with a turboelectric propulsion system. The aim is to analyse whether benefits with respect to mission fuel consumption can be derived by employing such a combined-cycle
engine when compared to a simple-cycle turboshaft engine. For this purpose, a multidisciplinary optimization framework is developed, incorporating models for the engine, ORC system, ORC turbine, heat exchangers, and mission analysis. This framework is coupled with an optimizer to identify the optimal combined-cycle engine design for minimum mission fuel consumption. The results suggest that fuel savings of around 1.5% are possible with the optimized system if compared to the aircraft employing turboshaft engines. Heat exchanger volume is identified as the most constraining parameter when it comes to combined-cycle performance. The analysis of the results suggests as aspects which might lead to further improvements the evaluation of other ORC architectures, working fluids and heat exchanger topologies.@en

The Flying V is a long-range, flying-wing aircraft where payload and fuel both reside in a V-shaped, crescent wing with large winglets that double as vertical tail planes. The objective of this study is to maximize the lift-to-drag (L/D) ratio of the Flying V in cruise conditions, i.e. CL= 0.26, M = 0.85 and to investigate its off-design performance in high-subsonic conditions. This is done by manually modifying the design parameters that describe the outer mold line of the Flying V and assessing the aerodynamic performance by means of computational fluid dynamics. A 15-million cell, third-order MUSCL, Reynolds-Averaged Navier Stokes solver with the Menter SST turbulence model is used to estimate the aerodynamic coefficients. This numerical model is validated using the experimental data of the ONERA M6 wing. A new, CATIA-based, parametrization of the Flying V is the starting point of the design. Three manual design phases improve the aerodynamic performance while satisfying all constraints. Design modifications include an increase in camber and aft-loading of the wing around 40% of the semispan and improved airfoil sections on the outboard wing generating the required lift coefficient towards an elliptical lift distribution. The twist distribution at the wing-winglet junction is optimized to reduce wave drag. This has resulted in an improvement of L/D from 20.3 from previous studies to 24.2 for the final version, while reducing the cruise angle of attack from 5.2 to 3.6 degrees. The drag divergence Mach number is estimated at 0.925.@en

This paper focuses on the conceptual design optimization of liquid hydrogen aircraft and their performance in terms of climate impact, cash operating cost, and energy consumption. An automated, multidisciplinary design framework for kerosene-powered aircraft is extended to design liquid hydrogen-powered aircraft at a conceptual level. A hydrogen tank is integrated into the aft section of the fuselage, increasing the operating empty mass and wetted area. Furthermore, the gas model of the engine is adapted to account for the hydrogen combustion products. It is concluded that for medium-range, narrow-body aircraft using hydrogen technology, the climate impact can be minimized by flying at an altitude of 6.0 km at which contrails are eliminated and the impact due to NO_x emissions is expected to be small. However, this leads to a deteriorated cruise performance in terms of energy and operating cost due to the lower lift-to-drag ratio (– 11%) and lower engine overall efficiency (– 10%) compared to the energy-optimal solutions. Compared to cost-optimal kerosene aircraft, the average temperature response can be reduced by 73–99% by employing liquid hydrogen, depending on the design objective. However, this reduction in climate impact leads to an increase in cash operating cost of 28–39% when considering 2030 hydrogen price estimates. Nevertheless, an analysis of future kerosene and hydrogen prices shows that this cost difference can be significantly decreased beyond 2030.

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This study evaluates the flight performance of a Flying-V aircraft designed for transonic passenger transport. The Flying-V is a disruptive aircraft configuration that has shown to possess promising aerodynamic performance during preliminary design. It is compared to a competitor aircraft reminiscent of the Airbus A350-1000, for the same thrust-to-weight ratio and a similar number of passengers. The most common performance metrics for the take-off, landing, climbing and cruise phases have been assessed using a modular flight mechanics model. Take-off and landing performance are evaluated through flight simulation using a simple Euler method, while climb and cruise performance are evaluated in trimmed, steady-state conditions. Only instantaneous performance is available for the latter two phases. The Flying-V outperforms its competitor for basically all investigated metrics. Take-off length is shorter, mainly due to a larger tail strike attitude that reduces the minimum unstick speed. Service and absolute ceiling are higher, and its superior lift-over-drag ratio results in a 21% increase in the cruise range parameter. Landing field lengths are similar for both aircraft, but the Flying-V has a significantly larger pitch angle during approach. This causes longer de-rotation length, and a large obscured segment of the pilot’s vision which could be problematic during operations.@en

The Flying V is a flying wing aircraft consisting of two pressurised passenger cabins placed in a V shape. Its longitudinal and lateral control is ensured via elevons and split flaps on the outboard wing, and rudders on the tip-mounted winglets. The goal of this study is to devise a design for the outboard wing of the Flying V through a constrained aerodynamic shape optimisation at cruise conditions. The design process is divided into a geometry preparation phase in which the existing parametrisation is adjusted, followed by a planform design optimisation guided by the Differential Evolution algorithm making use of a vortex-lattice method and an Euler flow analysis. The cross-sectional shape of the wing is subsequently optimised through a Free-Form Deformation (FFD) shape optimisation based on the Euler equations. Two FFD optimisations are conducted to evaluate the effect of the integration of the elevons. The highest lift-to-drag ratio is obtained by neglecting the control surface integration and amounts to 20.3. While the constraints related to this elevon integration reduce the efficiency to 19.4. The overall efficiency gain compared to the original aircraft design is equivalent to 13% and 8%, respectively. A further increase is expected once the inefficient outboard wing is optimised in more detail.

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Flying wings are known for their limited lateral-directional stability and handling qualities. This study aims at assessing the lateral-directional handling qualities of a conceptual flying wing aircraft currently in development at TU Delft, the Flying-V, in a moving-base flight simulator. It focuses on two aspects: First assess the lateral-directional handling qualities of the bare-airframe Flying-V, and the compliance to quantitative requirements. Second, improve these handling qualities through a prototype flight control system, and assess its effect on the handling qualities and the requirement compliance. These assessments were performed both analytically and with a pilot-in-the-loop simulator experiment, in order to experimentally validate analytical findings and obtain new pilot-subjective insights. The analytical and experimental assessment for lowspeed flight conditions both show the lateral-directional handling qualities of the Flying-V to be insufficient for requirement compliance, due to a lack of pitch, roll and yaw control authority and an insufficiently stable Dutch roll eigenmode. The prototype flight control system, consisting of an adapted control allocation and a stability augmentation system, showed both analytically and experimentally to improve the control authority, stability, and handling qualities of the Flying-V. While the effect on the lateral-directional stability was sufficient for stability requirement compliance, the control authority was not sufficiently increased for maneuverability requirement compliance at low speed. Thus, if the landing speed is not increased from the current baseline, a challenge remains to improve the handling qualities of the Flying-V. An approximation of the control authority required for full requirement compliance in the low-speed flight conditions tested showed a control authority increase of over a factor four to be required in that case.@en

As climate change aggravates, the aviation sector strives to minimize its climate footprint. To this end, international organizations, such as ICAO and ACARE, are promoting mitigation measures including novel technologies, operations, and energy carriers to reduce aircraft emissions significantly. Hydrogen (H2) as an alternative fuel has the advantage of eliminating CO2 and soot emissions and the potential to reduce NOx emission substantially. Nevertheless, burning H2 emits more H2O and increases the contrail formation probability. Therefore, the actual climate impact of hydrogen aircraft is still uncertain. This paper intents to evaluate the climate impact of a hydrogen powered aircraft considering the effects of H2O, NOx , and contrails . To frame the contribution of each individual climate agent, the research compares a hydrogen and a kerosene aircraft with similar mission capabilities. To assess the climate impact, a modeling chain was developed including network selection, flight routes calculation, aircraft and propulsion performance, emissions prediction, and climate impact assessment. In total, 2.24 million flights covering 1128 city pairs were analyzed. The energy consumption of hydrogen aircraft is about 10% higher than that of the kerosene aircraft due to the larger wetted area for hydrogen storage. However, the average atmospheric temperature response caused by the hydrogen aircraft is 67% lower compared to the kerosene aircraft due to the absence of CO2, the lower radiative forcing of hydrogen contrails, and the reduction in NOx emissions when assuming advanced hydrogen combustion technology. It was also observed that climate impact from hydrogen aircraft is more sensitive to flights over the tropics than to flights over the poles.@en

A novel aircraft configuration, the tailless Flying-V, is examined for its longitudinal handling qualities in cruise by means of piloted simulations. The Flying-V is controlled by two aileron/elevator (elevon) surfaces on each side, and rudders on each wingtip. Two control allocation schemes were created: a conventional one where both inboard and outboard elevons deflect in the same direction, and one where the change in lift the elevons generate is countered by deploying the inboard and outboard elevons in opposite directions, allowing more direct control of the resulting flight path. The longitudinal handling qualities in cruise conditions were investigated by pilot opinion in a moving base simulator. Three experiments were conducted: a traditional pitch tracking experiment with the conventional control allocation, and a new flight-path-angle tracking experiment, using both the conventional and the flight-path-oriented control allocation. The pilots indicated the conventional pitch attitude control to have Level 1 handling qualities for the pitch control task, and Level 2 for the flight path control task. The flight-path-oriented control allocation improved the performance of the pilots during the flight-part tracking experiment, but the perceived control authority was considered too small for most pilots to consistently rate it at Level 1.@en

To reduce the climate impact of aviation, researchers are studying the replacement of fossil kerosene with liquid hydrogen and/or drop-in sustainable aviation fuel (SAF). These fuels can bring significant reductions in CO2 emissions and can offer savings in terms of non-CO2 climate effects. In addition, tube-and-wing aircraft can be optimized to decrease the global-warming impact by using a climate metric as a design objective rather than the operating costs. Previous research has shown that airplanes designed for minimal climate impact have a reduced cruise speed and fly at a lower altitude. This paper suggests a multidisciplinary, multi-level approach the evaluate the consequences of such design and fuels choices at the network level. Following the aircraft design step, a dynamic programming routine allocates the fleet and schedules the flights to maximize the network profit. We consider a hub-and-spoke network operating from Atlanta, with demand for domestic and international destinations. Compared to the reference cost-optimal kerosene fleet, a fleet consisting of climate-optimized kerosene aircraft can reduce the climate impact by 61% at a loss in network profit of approximately 21%. This design choice requires allocating an additional five aircraft. A fleet operating climate-optimal, hydrogen aircraft minimizes the climate impact. However, the high operating cost of long-range, hydrogen aircraft lowers the achievable profit. Aircraft powered by drop-in SAF provides Pareto-optimal solutions. These insights can be used to make decisions about the allocation of future aviation fuels in a network and the payload-range requirements of future aircraft.@en

This paper presents a preliminary study about a combined-cycle engine based on a turboshaft engine and an organic-Rankine-cycle (ORC) bottoming unit to be used onboard an aircraft with a turboelectric propulsion system. The aim is to analyse whether benefits with respect to mission fuel consumption can be derived by employing such a combined-cycle engine when compared to a simple-cycle turboshaft engine. For this purpose, a multidisciplinary optimization framework is developed, incorporating models for the engine, ORC system, ORC turbine, heat exchangers, and mission analysis. This framework is coupled with an optimizer to identify the optimal combined-cycle engine design for minimum mission fuel consumption. The results suggest that fuel savings of around 4% are possible with the optimized system if compared to the aircraft employing turboshaft engines. Heat exchanger volume is identified as the most constraining parameter when it comes to combined-cycle performance. The analysis of the results suggests as aspects which might lead to further improvements the evaluation of other ORC architectures, working fluids and heat exchanger topologies.@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 Flying-V novel aircraft design aims at reducing fuel consumption by an innovative low-drag, fuselage-free geometry. Possible issues related to certification requirements have been noted, however, regarding longitudinal handling qualities at low speed, the pull-up manoeuver, and the flight-path-angle response. This study aims at investigating these issues through a pilot-in-theloop experiment. Starting with a mathematical model of the Flying-V, based on the vortex lattice method, a preliminary off-line analysis of the handling qualities is conducted. A sensitivity analysis is considered over the proposed operational center-of-gravity range, approach speed (between 0.225 and 0.3 Mach, 149 and 198 knots indicated airspeed, respectively), maximum deflection of the control surfaces (between 20 and 30 degrees), and flight control system (Direct Law or Pitch-Rate Command). The pilot-in-the-loop experiment, its design guided by results from the analytical assessment, shows that the handling qualities provided by the current design of the Flying-V with Direct Law at 0.3 Mach are satisfactory with minor improvements related to aircraft responsiveness. For lower speeds (0.225 Mach), the handling qualities degrade due to a sluggish response, high compensation workload, insufficient control authority, insufficient sight angle, and tendency to pilot induced oscillations. Shifting the center of gravity away from the nose provides larger control authority at the expense of a minor reduction of responsiveness. Control augmentation proves to be very effective at improving the handling qualities. It is expected that the go-around certification standards will be satisfied, but approach speed will remain critical for controllability and safety.

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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 aircraft’s environmental performance on fleet level is so far completely decoupled from the design process. The climate impact from aviation arising from non-CO2 effects are largely independent from CO2 emissions, but rather depend on the atmospheric state. Previously complex climate-chemistry models were used to evaluate the non-CO2 emissions impact on climate. This is far too computationally demanding for a multidisciplinary design optimisation (MDO) process, requiring a multitude of climate impact evaluations. The question then is, how to efficiently design the next generation climate optimal aircraft? In this paper, a new concept for designing aircraft with minimum climate impact using Climate Functions for Aircraft Design (CFAD) is presented. The content of this paper provides an overview of the development of these innovative CFAD and demonstrates the ability to be integrated in an existing MDO framework. The mitigation potential by optimising aircraft design using CFAD is analysed with respect to different cruise conditions and by minimizing the overall climate impact. To validate the CFAD, a higher fidelity assessment is carried out. Finally, the key performance indicators, i.e. fuel consumption, flight time and operating cost, of the optimised aircraft design are compared to that of the reference aircraft.@en

Sustainable aviation fuel (SAF) and liquid hydrogen are currently being studied to replace kerosene in commercial aviation to reduce global warming. In this study, the question is how do the airplane design variables change when minimizing the global warming impact of aircraft powered by SAF or LH2? Secondly, how do these aircraft compare in terms of climate impact and operating costs, considering regional, medium-, and long-range categories? A multidisciplinary design optimization process varies airframe, turbofan engine and mission design variables to obtain the cost- and climate-optimal design solutions. A linearized temperature response model evaluates the average temperature response over 100 years considering both CO2 and non-CO2 effects. The trade-off between climate impact reduction on the one hand and operating cost, on the other hand, is studied for each fuel type and aircraft category. We conclude that LH2 can achieve the largest reduction in temperature response in all categories. The maximum reduction of 98% compared to the cost-optimal kerosene aircraft comes at an estimated increase of 30, 42, or 69% in operating costs for regional, medium-, and long-range missions. The SAF aircraft can reduce the climate impact by 86, 82, and 72% for regional, medium-range, and long-range aircraft. These savings lead to an 8, 14, and 26% increase in operating costs. The analysis shows that the SAF-powered aircraft are preferred over the cost-optimal hydrogen aircraft for the regional and medium-range categories. Hydrogen does provide a Pareto-optimal solution for long-range aircraft, albeit at a significant in-flight energy and cost penalty.@en

Ten years after the initial sketches of the Flying V were made, we intend to give an overview of the development of this concept until the present day. The Flying V is a new concept for an efficient aircraft. It is a pure flying wing which includes a V-shaped passenger cabin. In this work, we describe how the concept developed - from the first drawings, the reasoning behind the concept and the design of the planform geometry, to the introduction of a structural solution for the pressurized cabin, to the design of a family of aircraft, detailed investigations of handling qualities and the interior layout. As of today, the concept promises a 20% lower fuel burn than a conventional reference aircraft on the same mission with the same capacity and the same wingspan, assuming current manufacturing techniques and current engine technology. Research activities on the Flying V have been continuously increasing over the past decade. Projects are ongoing in fields such as aerodynamics, structures and manufacturing, flight dynamics and control, the environmental impact of the design, aircraft integration, noise, and airport operation.@en

This paper studies the climate impact of propeller aircraft which are optimized for either minimum direct operating costs, minimum fuel mass, or minimum average temperature response (ATR100). The latter parameter provides a measure of the global warming impact of the aircraft design, considering both CO2 and non-CO2 effects. We study turboprop-powered aircraft in particular because these offer higher propulsive efficiency than turbofan aircraft at low altitudes and low Mach numbers. The propeller aircraft are designed for medium-range top-level requirements, employing a multidisciplinary design optimization framework. This framework uses a combination of statistical, empirical, and physics-based methods, which are verified using existing engine and aircraft data. For this medium-range design case, a climate impact reduction of 16% can be realized when shifting from the cost design objective to the climate objective. The optimal solutions for the fuel mass and climate objectives are nearly identical as CO2 and other fuel proportional climate effects are the main contributors. The effects of NOx and contrails are lower than for the turbofan aircraft due to the lower cruise altitude of the propeller aircraft. Compared to turbofan data, propeller-powered aircraft can achieve a further 33% reduction in climate impact, comparing both climate-optimal designs. This reduction is lessened to 23% when the propeller aircraft is constrained to achieve the same mission block time as the turbofan aircraft. Note that these reductions in ATR100 require a propeller efficiency of 88%. Overall, the results show that the utilization of propeller-powered aircraft in the medium-range category can further reduce the climate impact compared to climate-optimal turbofan aircraft designs. @en

This paper presents a method to assess the key performance indicators of aircraft designed for minimum direct operating costs and aircraft designed for minimum global warming impact. The method comprises a multidisciplinary aircraft optimization algorithm capable of changing wing, engine, and mission design variables while including constraints on flight and field performance. The presented methodology uses traditional class-I methods augmented with dedicated class-II models to increase the sensitivity of the performance indicators to relevant design variables. The global warming impact is measured through the average temperature response caused by several emission species (including carbon dioxide, nitrogen oxides, and contrail formation) over a prolonged period of 100 years. The analysis routines are verified against experimental data or higher-order methods. The design algorithm is subsequently applied to a single-aisle medium-range aircraft, demonstrating that a 57% reduction in average temperature response can be achieved as compared to an aircraft optimized for minimal operating costs. This reduction is realized by flying at 7.6 km and Mach 0.60, and by lowering the engine overall pressure ratio to approximately 37. However, to compensate for the lower productivity, it is estimated that 13% more climate-optimized aircraft have to be operated for the hypothetical fleet under consideration.@en