Distributed Propulsion Research Articles

Toward more electric powertrains in aircraft: Technical challenges and advancements

The main purpose of this article is to provide an instructive review of the technological challenges hindering the road toward more electric powertrains in aircraft. Hybrid, all-electric, and turboelectric powertrain architectures are discussed as possible fuel consumption and weight reduction solutions. Among these architectures, the short-term implementation of hybrid and all-electric architectures is limited, particularly for large-capacity aircraft due to the low energy/power density levels achievable by state-of-the-art electrical energy storage systems. Conversely, turboelectric architectures with advanced distributed propulsion and boundary layer ingestion are set to lead the efforts toward more electric powertrains. At the center of this transition, power converters and high-power density electric machines, i.e., electric motors and generators, and their corresponding thermal management systems are analyzed as the key devices enabling the more electric powertrain. Moreover, to further increase the fuel efficiency and power density of the aircraft, the benefits and challenges of implementing higher voltage powertrains are described. Lastly, based on the findings collected in this article, the projected roadmap toward more electric aircraft powertrains is presented. Herein, the individual targets for each technology, i.e., batteries, electric machines, and power converters, and how they translate to future aircraft prototypes are illustrated.

Aerodynamic analysis and static stability analysis of Manned/unmanned distributed propulsion aircrafts using actuator methods

The development of personal aerial vehicles (PAVs) using electric power distributed propulsion methods is rapidly growing. As the aerodynamic performance and flight stability characteristics can be significantly affected by multiple-propeller operation, aerodynamic analysis reflecting the power-on effect is required for design/development. Simulating all propellers using the general computational fluid dynamics is inefficient in terms of computational time and cost. Therefore, this study evaluates the practicality of actuator methods in the aerodynamic analysis of two aircrafts with multiple propellers: a quad tilt propeller (QTP) unmanned aerial vehicle (UAV) and an optionally piloted personal air vehicle (OPPAV) of the Korea Aerospace Research Institute (KARI). The normal and side forces generated by the propellers under various angle of attack and sideslip angle conditions are found to be the most contributing factors to the deterioration in the aircraft longitudinal and directional static stabilities, respectively. The forces and moments generated by the propellers are found to make both longitudinal and directional stabilities of the QTP UAV unstable. However, the OPPAV appears to remain stable state even with the operating propellers. We conclude that actuator methods, being computationally efficient in terms of cost, time, and accuracy, possess great potential for aerodynamic analysis in various related industries.

Key performance indicators for turboelectric distributed propulsion

PurposeRecent advancements in electrified transportation have been necessitated by the need to reduce environmentally harmful emissions. Accordingly, several aviation organisations and governments have introduced stringent emission reduction targets for 2050. One of the most promising technologies proposed for achieving these targets is turboelectric distributed propulsion (TeDP). The objective of this study was to explore and identify key indicators for enhancing the applicability of TeDP in air transportation.Design/methodology/approachAn enhancement valuation method was proposed to overcome the challenges associated with TeDP in terms of technological, economic and environmental impacts. The result indicators (RIs) were determined; the associated performance indicators (PIs) were analysed and the key RIs and PIs for TeDP were identified. Quantitative measurements were acquired from a simulated TeDP case study model to estimate the established key PIs.FindingsIt was determined that real-world TeDP efficiency could be enhanced by up to 8% by optimising the identified key PIs.Originality/valueThis study is the first to identify the key PIs of TeDP and to include a techno-economic environmental risk analysis (TERA) based on the identified key PIs. The findings could guide developers and researchers towards potential focus areas to realise the adoption of TeDP.

Hybrid electric distributed propulsion overall aircraft design process and models for general aviation (FAST GA)

In the frame of the European objectives in terms of CO2 emissions, the aeronautics is looking for a technological rupture to achieve them, in particular, the aircraft design domain pursuits this through the research of innovative architectures. One of these innovative configurations currently being explored includes the hybrid electric energy source (thermal/electric) for Distributed Electric Propulsion (DEP) architecture. This Paper details a code developed to size a general aviation aircraft at concept level, by only defining its top level requirements and the main architecture parameters. The code can manage both conventional and hybrid power source as well as concentred or distributed propulsion architectures in order to allow the user to evaluate and compare the feasibility and benefits respectively of these innovative architectures. This code is a branch of the code “FAST-CS25” (Future Aircraft Sizing Tool for conventional CS-25 type) held by ONERA/ISAE-SUPAERO. The presented work aims at the expansion of the FAST code to CS-23 conventional type, hybrid electric energy source, and distributed propulsion system configurations. Through this paper, the models and the main sizing loops for the concept design are described, but putting special emphasis on the distributed propulsion aerodynamics and wing mass estimation. These detailed models where validated with the NASA X-57 DEP aircraft satisfactory. The whole concept design loop of a hybrid energy aircraft was validated with the eGenius hybrid energy aircraft.

Development of a Turbo Electric Distribution System for Remotely Piloted Aircraft Systems

Turboelectric distributed propulsion systems are paving the way for more electric aircraft systems (TeDP). This type of system provides a solution for some of the drawbacks of current low-energy-density batteries, which limit the ability of long-endurance electric aircraft. However, turboelectric propulsion requires the use of advanced turboelectric motors, superconductive materials and cryogenic cooling technologies, which are still under development and may be in production in the near future. This paper investigates a turboelectric propulsion system that can be considered an initial step in the production of TeDP in a remotely piloted aircraft system with the use of existing technology. This is achieved by replacing the gear and the starter motor of a turboprop with a high-speed permanent magnet electric machine to generate electrical power and propelling the aircraft through a distributed electric propulsion system. In this theoretical study, an augmentation to Breguet’s range and endurance equation is developed. This study confirmed that the new system is 31% lighter than the turboprop engine. Then the effect of the weight savings is used in the distributed electric propulsion (DEP) aerodynamic studies and found that there is a drastic increase in the range for a TeDP developed with the high-speed machine.

Single Turboshaft Engine Failure Analysis of an Inner Bus Tie Architecture for Turboelectric Distributed Propulsion System

Single Turboshaft Engine Failure Analysis of an Inner Bus Tie Architecture for Turboelectric Distributed Propulsion System

Economic and environmental viability assessment of NASA’s turboelectric distribution propulsion

The concept of turboelectric-distributed propulsion (TeDP) has become integral to engineering because of its ability to generate electricity. However, social science compels careful evaluations of TeDP’s environmental and economic impacts—out of caution, such elements must be taken up before TeDP is put into practice. Responding to this call, this research investigates TeDP’s economic and environmental viability with a case study of the National Aeronautics and Space Administration’s (NASA) proposal for a TeDP aircraft, N3-X, using technical aspects and real data integration. The economic assessment measures NASA’s N3-X economic added value for aviation manufacturing, operations, and investors as well as net present value, internal rate of return, and payback period. Meanwhile, the environmental assessment looks at carbon monoxide and dioxide and oxides of nitrogen. The economic and environmental evaluation results establish the viability of TeDP.

Voltage-Based Current-Compensation Converter Control for Power Electronic Interfaced Distribution Networks in Future Aircraft

Superconductors have a potential application in future turboelectric distributed propulsion (TeDP) aircraft and present significant new challenges for protection system design. Electrical faults and cooling system failures can lead to temperature rises within a superconducting distribution network, which necessitates a reduction or temporary curtailment of current to loads to prevent thermal runaway occurring within the cables. This scenario is undesirable in TeDP aircraft applications where the loads may be flight-critical propulsion motors. This article proposes a power management and control method that exploits the fast-acting measurement and response capabilities of the power electronic interfaces within the distribution network to maximize current supply to critical loads, reducing the impact of a temperature rise event in the superconducting distribution network. This new algorithm uses the detection of a resistive voltage in combination with a model-based controller that estimates the operating temperature of the affected superconducting cable to adapt the output current limit of the associated power electronic converter. To demonstrate the effectiveness of this method and its impact on wider system stability, the algorithm is applied to a simulated voltage-source converter supplied aircraft dc superconducting distribution network with representative propulsion motor loads.

Assessment of the BWB aircraft for military transport

PurposeThe growth in air mobility, rising fuel prices and ambitious targets in emission reduction are some of the driving factors behind research towards more efficient aircraft. The purpose of this paper is to assess the application of a blended wing body (BWB) aircraft configuration with turbo-electric distributed propulsion in the military sector and to highlight the potential benefits that could be achieved for long-range and heavy payload applications.Design/methodology/approachMission performance has been simulated using a point-mass approach and an engine performance code (TURBOMATCH) for the propulsion system. Payload-range charts were created to compare the performance of a BWB aircraft with various different fuels against the existing Boeing 777-200LR as a baseline.FindingsWhen using kerosene, an increase in payload of 42 per cent was achieved but the use of liquefied natural gas enabled a 50 per cent payload increase over a design range of 7,500 NM. When liquid hydrogen (LH2) is used, the range may be limited to about 3,000 NM by the volume available for this low-density fuel, but the payload at this range could be increased by 137 per cent to 127,000 kg.Originality/valueThe results presented to estimate the extent to which the efficiency of military operations could be improved by making fewer trips to transport high-density and irregular cargo items and indicate how well the proposed alternatives would compare with present military aircraft. There are no existing NATO aircraft with such extended payload and range capacities. This paper, therefore, explores the potential of BWB aircraft with turbo-electric distributed propulsion as effective military transports.

Aerodynamic Characteristics of Different Airfoils under Varied Turbulence Intensities at Low Reynolds Numbers

A numerical study was conducted on the influence of turbulence intensity and Reynolds number on the mean topology and transition characteristics of flow separation to provide better understanding of the unsteady jet flow of turboelectric distributed propulsion (TeDP) aircraft. By solving unsteady Reynolds averaged Navier-Stokes (URANS) equation based on C-type structural mesh and γ - Re ˜ θ t transition model, the aerodynamic characteristics of the NACA0012 airfoil at different turbulence intensities was calculated and compared with the experimental results, which verifies the reliability of the numerical method. Then, the effects of varied low Reynolds numbers and turbulence intensities on the aerodynamic performance of NACA0012 and SD7037 were investigated. The results show that higher turbulence intensity or Reynolds number leads to more stable airfoil aerodynamic performance, larger stalling angle, and earlier transition with a different mechanism. The generation and evolution of the laminar separation bubble (LSB) are closely related to Reynolds number, and it would change the effective shape of the airfoil, having a big influence on the airfoil’s aerodynamic characteristics. Compared with the symmetrical airfoil, the low-Reynolds-number airfoil can delay the occurrence of flow separation and produce more lift in the same conditions, which provides guidance for further airfoil design under TeDP jet flow.

Novel fan configuration for distributed propulsion systems with boundary layer ingestion on an hybrid wing body airframe

The performance benefits of directly ingesting the boundary layer (BLI) on air vehicles with distributed propulsion (DP) systems has been documented and explored extensively. However, numerous investigations have demonstrated that the increase of the flow distortion in the inlets of conventional propulsors can dramatically reduce the expected benefits. Hence, this work presents an alternative fan configuration to re-energize the boundary layer, and at the same time, to perform properly in a distorted and non-uniform flow-field. This conceptual design utilizes a two-dimension idealized fan and replaces the rotational movement with linear displacement, avoiding the undesired effects of circumferential distortion on the propulsor operation. A quasi two-dimensional model based on the Discretized Miller approach has been used to compare the proposed configuration with a conventional axial fan. From the results obtained, it is observed that the thermal performance of the fan is less affected for the proposed configuration and furthermore, intake pressure losses are ameliorated by the use of a single mailbox shape inlet. The performance assessment of the proposed configuration coupled on the N3-X aircraft shows benefits of 4% in fuel savings compared with current BLI turbo-machinery configurations. The main contribution of this study lies on the definition of a preliminary design for an alternative propulsor configuration able to deal with circumferential distortion.

Optimization and Analysis of Hybrid Electric System for Distributed Propulsion Tilt-Wing UAV

Hybrid electric distributed propulsion tilt-wing UAVs show unlimited application prospects due to their high propulsion efficiency and vertical take-off and landing capabilities. Aiming at the power requirements of distributed propulsion tilt-wing UAVs, this article presents an optimization design method for series hybrid electric propulsion systems suitable for this type of aircraft. Based on the flight conditions and constraints of various modes, the power demand models for distributed propulsion tilt-wing UAVs are established for vertical flight mode, transition mode, and forward flight mode. The hybrid electric power solving models of each component of the series hybrid electric system are established by analyzing the series hybrid power architecture and power transmission paths. The hybrid electric system weight model is mainly decomposed into the weight model of each component and the fuel consumption model. Based on power demand and energy demand, a mass model of components such as batteries, engines, generators, power management and distribution systems are established. Besides, the fuel consumption characteristic curve of the scaled engine is established using the Willans line formulation, and the fuel consumption calculation in the full flight profile is completed. The optimization of the series hybrid electric system of a 200kg-class distributed propulsion tilting wing UAV under two mission profiles of urban freight and mountain freight and the sensitivity of the optimized results to performance are presented. The results show that the proposed method can sensitively capture the significant influence of the adjustment of the mission profile and the changes in the performance requirements of each flight phase, and adapt to various application scenarios.

Systems Evaluation of Subsonic Hybrid-Electric Propulsion Concepts for NASA N+3 Goals and Conceptual Aircraft Sizing

The air-travel demand is anticipated to grow in future and therefore the worldwide airtraffic is forecast to increase significantly. This growth in demand further increases the concerns pertaining to environmental and human health, which results in stringent aviation policies. Emission regulations have been set for the aviation sector to reduce its climate change impacts, and these support the efforts to meet the goals of the UN’s Paris treaty on climate change. The aviation sector is exploring sustainable and improved technologies to become more energy and cost efficient. Along these lines, NASA has developed the concept of ‘N+i’ goals to decrease fuel consumption, noise, and landing and take-off (LTO) oxides of nitrogen (NOx) emissions, and to enhance aircraft performance. The ‘N+3’ represents three technology generations into the future, where ‘N’ represents the current aircraft generation, with a forecasted technology readiness level 4-6, in year 2025 timeframe which will enable year 2035 service-entry. To meet NASA’s N+3 goals, significant improvements must be made in the air transportation system, airframe, mission design, and propulsion systems. A pivotal element to achieve these goals, is the propulsion system. This is because the role of propulsion system can be crucial in reducing emissions, noise, and fuel burn. This work evaluates the N+3 concepts in detail, based on the systems engineering approaches and selects the best of those concepts. A detailed analysis is presented for phase one of such a project using Georgia Institute of Technology’s Integrated Product-Process Development (IPPD) method. This work finds that the NASA N3-X turbo-electric distributed propulsion (TeDP) is the best concept for meeting the NASA N+3 goals, based on the systems engineering approach.

Preliminary Sizing Method for Hybrid-Electric Distributed-Propulsion Aircraft

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 hybrid-electric 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 HEP-compatible 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.

Model-based analysis of boundary layer ingestion effect on lateral-directional aerodynamics using differentiated boundary conditions

The noteworthy feature of aircraft with distributed propulsion configuration is the integration of a blended-wing-body type airframe and an embedded distributed propulsion system, thus inducing the specific boundary layer ingestion effect. Different boundary layer ingestion effects on the distributed engines may generate asymmetric flow fields on the airframe surface, and then lead to the unique lateral-directional aero-propulsive close coupling. To investigate the lateral-directional aerodynamics influenced by boundary layer ingestion, a new comprehensive computational method based on the differentiated boundary conditions is proposed. This method uses a synthetic three-dimensional computational model including the airframe and multi-engine to analyze the aerodynamic characteristics, and the essential boundary conditions can be extracted from the thermodynamic distributed propulsion system model to represent the different boundary layer ingestion intensities on the left and right engines. Subsequently, detailed model-based analyses of boundary layer ingestion influences on the lateral-directional aerodynamic characteristics are conducted, and the influence regularities under different flight states are revealed. All the results demonstrate that the differentiated boundary layer ingestion intensities on distributed engines can certainly affect the roll and yaw aerodynamic performance of the distributed propulsion configuration aircraft.

Comparison of Candidate Architectures for Future Distributed Propulsion Aircraft

Turbine-engine-driven distributed electrical aircraft power systems [also referred to as turboelectric distributed propulsion (TeDP)] are proposed for providing thrust for future aircraft with superconducting components operating at 77 K, in order for performance and emission targets to be met. The proposal of such systems presents a radical change from current state-of-the-art aeroelectrical power systems. Central to the development of such power systems are architecture design trades which must consider system functionality and performance, system robustness, and fault ridethrough capability, in addition to the balance between mass and efficiency. This paper presents a quantitative comparison of the three potential candidate architectures for TeDP electrical networks. This analysis provides the foundations for establishing the feasibility of these different architectures subject to design and operational constraints. The findings of this paper conclude that a purely ac synchronous network performs best in terms of mass and efficiency, but similar levels of functionality and controllability to an architecture with electrical decoupling via dc cannot readily be achieved. If power electronic converters with cryocoolers are found to be necessary for functionality and controllability purposes, then studies show that a significant increase in the efficiency of solid-state switching components is necessary to achieve specified aircraft performance targets.

An assessment of distributed propulsion: Part B – Advanced propulsion system architectures for blended wing body aircraft configurations

Studies such as those by NASA predict large performance benefits when integrating Distributed Propulsion (DP) with the Blended Wing Body (BWB) aircraft configuration. This is because the BWB planform geometry is particularly suited to the ingestion of boundary layer air. The present study evaluates the relative benefit of DP through a comparison with an advanced turbofan reference system of the same technology level as in Part A of this two part paper. Fuel savings of 5.3% relative to the reference aircraft have been shown to be achievable, although it was found that the system is particularly sensitive to duct losses.

Design point analysis of the turbofan-driven turboelectric distributed propulsion system with boundary layer ingestion

The performance benefits of boundary layer ingestion in the case of air vehicles powered by distributed propulsors have been documented and explored extensively by previous study. However, the increased inlet flow distortion and the system total weight would dramatically reduce the expected benefits. In this paper, a novel turboelectric distributed propulsion module on the N3-X airframe has been developed. It includes two direct-shaft-driven turbofans (each drives a free power turbine) and a propulsors array, and therefore both the direct-shaft-driven turbofan and the propulsors unit produce thrust. Secondly, a parametric and quasi two-dimensional study at component and at system level is carried out so that the effects of these two aerodynamic issues over the system performance can be assessed. The figure of merit in this study is the thrust-specific fuel consumption and the system parameters examined are: the thrust split between propulsors and engines, the capture height and the direct-shaft-driven turbofan parameters. Preliminary results found optimum configurations at around 60–90% thrust split ratio, when installation effects were neglected. The final value depends on the propulsor intake pressure loss and the bypass ratio of the propulsion system.

Integrated flight/propulsion optimal control for DPC aircraft based on the GA-RPS algorithm

The new distributed propulsion configuration (DPC) can effectively improve fuel efficiency and reduce pollution, but also brings in special cross-coupling effects between the flight and propulsion systems. To tackle this problem, integrated flight/propulsion modeling and optimal control of DPC with boundary layer ingestion (BLI) and supercirculation features are systematically investigated. As the basis, by taking the inherent BLI and supercirculation features into consideration, an integrated flight/propulsion model of SAX-40 is built to reflect the actual complex engine-aircraft coupling effects. Then an integrated flight/propulsion optimal control scheme is proposed to deal with the strong coupling effects and to implement the comprehensive control of redundant control surfaces as well as the distributed engines. Under this scheme, a detailed description of the optimal control problem is presented, and a novel two-stage optimization algorithm named genetic algorithm-random pattern search (GA-RPS) is proposed to solve this complex optimal problem. By the new strategies of “parallel genetic algorithm computing in sub-regions” and “random pattern search” in the GA-RPS algorithm, the optimization accuracy and convergence speed are effectively improved. Simulation results demonstrate the effectiveness of the GA-RPS algorithm and the significant performance improvement to DPC by optimization.

Development challenges of distributive propulsion systems for advanced aeroplanes

Purpose – This paper aims to consider main challenges of development of advanced architectures of propulsion systems, i.e. distributed propulsion systems (DPS). Design/methodology/approach – This paper is a comparative analysis of different types of DPS. Findings – Mechanical driving DPS seems as more feasible in near-term outlook, and turboelectric and full electric DPS are imagined feasible in mid- and far-term outlook. Research limitations/implications – Additional comprehensive numerical and experimental researches are needed to approve the efficiency of DPS. Practical implications – Possible impact of installation of DPS on aeroplane fuel efficiency are shown. Originality/value – Application of DPS on long-range aeroplanes is new a engineering solution, which may allow to meet future advanced efficiency goals.