Propulsion Configuration Research Articles

Paving the Way for Sustainable UAVs Using Distributed Propulsion and Solar-Powered Systems

Hybrid systems offer optimal solutions for unmanned aerial platforms, showcasing their technological development in parallel and series configurations and providing alternatives for future aircraft concepts. However, the limited energetic benefit of these configurations is primarily due to their weight, constituting one of the main constraints. Solar PV technology can provide an interesting enhancement to the autonomy of these systems. However, to create efficient propulsion architectures tailored for specific missions, a flexible framework is required. This work presents a methodology to assess hybrid solar-powered UAVs in distributed propulsion configurations through a two-level modeling scheme. The first stage consists of determining operational and design constraints through parametric models that estimate the baseline energetic requirements of flight. The second phase executes a nonlinear optimization algorithm tuned to find optimal propulsion configurations in terms of the degree of hybridization, number of propellers, different wing loadings, and the setup of electric distributed propulsion (eDP) considering fuel consumption as a key metric. The results of the study indicate that solar-hybrid configurations can theoretically achieve fuel savings of up to 80% compared to conventional configurations. This leads to a significant reduction in emissions during long-endurance flights where current battery technology is not yet capable of providing sustained flight.

Multidisciplinary Optimization of Aircraft Aerodynamics for Distributed Propulsion Configurations

The combination of different aerodynamic configurations and propulsion systems, namely, aero-propulsion, affects flight performance differently. These effects are closely related to multidisciplinary collaborative aspects (aerodynamic configuration, propulsion, energy, control systems, etc.) and determine the overall energy consumption of an aircraft. The potential benefits of distributed propulsion (DP) involve propulsive efficiency, energy-saving, and emissions reduction. In particular, wake filling is maximized when the trailing edge of a blended wing body (BWB) is fully covered by propulsion systems that employ boundary layer ingestion (BLI). Nonetheless, the thrust–drag imbalance that frequently arises at the trailing edge, excessive energy consumption, and flow distortions during propulsion remain unsolved challenges. These after-effects imply the complexity of DP systems in multidisciplinary optimization (MDO). To coordinate the different functions of the aero-propulsive configuration, the application of MDO is essential for intellectualized modulate layout, thrust manipulation, and energy efficiency. This paper presents the research challenges of ultra-high-dimensional optimization objectives and design variables in the current literature in aerodynamic configuration integrated DP. The benefits and defects of various coupled conditions and feasible proposals have been listed. Contemporary advanced energy systems, propulsion control, and influential technologies that are energy-saving are discussed. Based on the proposed technical benchmarks and the algorithm of MDO, the propulsive configuration that might affect energy efficiency is summarized. Moreover, suggestions are drawn for forthcoming exploitation and studies.

Conceptual design and analysis of a box fan-in-split-wing tiltrotor eVTOL aircraft

PurposeWith the advancements in electric vertical take-off and landing (eVTOL) aircraft technology such as batteries, mechanisms, motors, configurations and so on, designers and engineers are encouraged to create unique and unconventional configurations of eVTOL aircraft to provide better capabilities and higher efficiencies to compete in the market. The box fan-in-split-wing tiltrotor eVTOL aircraft is an innovative design that aims to address the aerodynamic inefficiencies such as propeller effects in cruise and engine mounts drag that existed in traditional eVTOL aircraft designs such as vectored thrust, rotorcraft, lift + cruise and multi-copter configurations. This paper aims to propose a multi-disciplinary design process to conceptually design the box fan-in-split-wing Tiltrotor eVTOL aircraft.Design/methodology/approachAn unconventional methodology was used to design the UAM aircraft, and the following parameters are considered: capable of vertical take-off and landing, highly aerodynamic with a high lift-to-drag ratio, low Cd0 modern and appealing, rechargeable or battery swappable and feature to minimise or negate propeller drag. A heavy emphasis on improving performance and weight based on aerodynamics was enforced during the conceptual design phase. MAPLA and XFOIL were used to identify the aerodynamic properties of the aircraft.FindingsUpon determining the key parameters and the mission requirements and objectives, a list of possible VTOL configurations was derived from theoretical and existing designs. The fan in the wing/split wing was selected, as it could stow the propellers. A tiltrotor configuration was selected because of its ability to reduce the total number of lift props/motors, reducing powerplant weight and improving aerodynamic efficiency. For the propulsion configuration, a battery–motor configuration with a hexa-rotor layout was chosen because of its ability to complement the planform of the aircraft, providing redundant motors in case of failure and because of its reliability, efficiency and lack of emissions. Coupled with the fan-in-wing / split wing concept, the box wing seamlessly combines all chosen configurations.Originality/valueThe box fan-in-split-wing Tiltrotor eVTOL aircraft aims to address the aerodynamic inefficiencies of earlier designs such as propeller effects in cruise and engine mounts drag. The potential benefits of this aircraft, such as increased range, endurance and payload capacity, make it an exciting prospect in the field of Urban Air Mobility.

Aero-propulsion coupling effects and installation characteristics of a distributed propulsion system

The ducted fan, as a quintessential propulsion device utilized in Distributed Electric Propulsion (DEP) aircrafts, exhibits a significant influence on the aero-propulsion coupling effects due to its installation characteristics on the wing. In this paper, we employed steady-state numerical analysis techniques in conjunction with actuator disk models and overset mesh to investigate aero-propulsion coupling effects and installation characteristics of a DEP configuration with 5 ducted fans distributed along the wing's trailing edge. We compared various conditions to the wing baseline airfoil under hover condition and cruise condition of the fan tip speed ratio λ from 1.44 to 4.32 and fan installation angle δT from 0 to 90°. The results indicate that DEP system affects the flow over the wing downstream of the maximum thickness point of the baseline airfoil profile at an installation angle of 0. A higher tip speed ratio the flow is accelerated in the upper surface region near the wing's trailing edge, whereas the low energy fluid fails to be accelerated at lower λ, instead forming a local flow blocking. With the increase of the fan installation angle, the overall system lift increases, but the thrust decreases, and both trends increase with the increase of the tip speed ratio. Specifically, the thrust provided by the distributed fans is reduced, and the inlet becomes one of the drag sources when the installation angle is high.

Experimental Propeller Performance Analysis of Distributed, Single and Isolated configurations

Distributed propulsion (DP) configurations are a promising concept for future aircraft systems. The main objective of the presented experiment is to investigate aerodynamic interactions of such configurations in detail and compare DP results to a single propeller configuration. The experimental setup at the Propulsion Test Facility, TU Braunschweig, features three co-rotating propellers. These are not attached directly to the wing, but are mounted on a separate carrier. This decoupling allows the forces and moments acting on wing and propeller to be considered separately. Additionally, different relative propeller positions are set up easily. In order to eliminate side wall effects, only the centre propeller and the centre wing element are subject of investigation for the distributed configuration. The periodically repeating outboard propellers reduce the wind tunnel interference while providing a true DP setup for the instrumented centre. Additionally to the DP setup, tests for conventional propeller wing configurations (only centre propeller installed) as well as isolated propeller and clean wing tests were performed. Thus, the DP effects on wing and propeller and the effect of the downstream wing on the propeller are clearly identified. The comparison between single propeller and distributed propulsion configurations shows that with distributed propulsion the drag increase is reduced from 326% to 216% compared to a clean wing. This effect is intensified by a greater thrust level and higher angles of attack. In order to identify the distributed propulsion effects, the forces acting on the propeller as well as the resulting efficiency of the propeller are compared between the distributed and single propeller configuration for two different relative propeller positions. The efficiency of the centre propeller is increased due to the outer propellers by approx. 2% to 3%.

Prediction of environmental benefits introducing hybrid-electric propulsion on regional aircraft

The objective of this paper is to assess the environmental benefits arising from the introduction of hybrid-electric propulsion on regional and commuter turboprop aircraft. A great focus is put on the propulsion based on a turbine engine coupled with batteries, combining mature technologies. The introduction of novel propulsion architectures on aircraft deployed on regional and commuter networks allows a substantial reduction of the fuel needed by airlines. In this work, scenarios based on real airline networks are presented, in order to quantify the reduction of greenhouse gas emissions possible considering the expected technological advancement for 2035 and 2050. This analysis is conducted by retrofitting the hybrid-electric propulsive system onto existing aircraft that can carry 19 passengers (Dornier DO228) or 70 passengers (ATR72-600). For the 19-seat class, a clean sheet design is also considered to overcome some limitations of the reference aircraft. A precise assessment of the peculiarities linked to the chosen propulsive configuration shows that networks that have shorter flights are better suited to the introduction of this technology, as the restriction on the design payload is less stringent. The proposed propulsive architecture allows a reduction of the operators’ yearly fuel budget of up to 50% in 2035 and 80% in 2050. At last, the taxi phase is of particular importance for regional aircraft that perform several rotations a day, therefore a further analysis of this phase is carried out. The result shows that the considered aircraft are capable of completing a full Landing & Take-Off (LTO) cycle without resorting to the thermal part of the propulsive system, reducing considerably the impact of the aircraft on the airport area.

Development of a Series Hybrid Multirotor

Hybrid-electric power is an appealing technology for multirotor platforms due to its ability to enhance the range of the vehicle while providing low emissions and the precise thrust control required for vehicle stability. The development of a multirotor utilizing hybrid propulsion is an essential step for CfAR as it will be the basis of research on this technology in flight. The designed multirotor, MIMIQ (Modular Inertia Matching Quadcopter), is a 32kg quadcopter with a motor-to-motor diameter of 2.7m which will require a total of 3,700W of power to hover. This demand is primarily met by the series hybrid generator onboard which requires the battery to only supplement a portion of that power. At peak thrust however, the motors will demand 12,000W from the system. During these rapid bursts of energy the battery plays a critical roll in providing an immediate response to the power demand which the generator would not be able to provide otherwise.This multirotor is designed to accommodate a wide range of propulsion configurations to optimize performance. In addition, the mechanical characteristics are also intended to be easily adjustable. Parameters such as center of mass, inertia, and motor distance can be adjusted to mimic characteristics of future flight vehicles that implement hybrid systems. Before integration, ground tests were performed on various off-the-shelf hybrid generators in order to evaluate their performance and reliability to ensure that they are well suited for the MIMIQ. To characterize the different modes of operation an extensive performance map was pursued. A number of parameters were measured over time with the most important being load, power, and fuel consumption. It has been proven that at low loads the generator can provide power to the propulsion system while charging the onboard batteries. However, at higher loads both the generator and the battery pack are required to provide power. The tests demonstrated that the integration of a hybrid system into MIMIQ is possible. It was also verified that when the engine is integrated into MIMIQ, both the generator and batteries have to provide power simultaneously in all flight phases.

TJ-FlyingFish: An Unmanned Morphable Aerial–Aquatic Vehicle System

In this paper, we present in this work a fairly complete process for developing an unmanned aerial–aquatic vehicle system, TJ-FlyingFish, which includes an innovative design methodology of the aerial–aquatic platform and the cross-medium localization, dynamics modeling, and flight control systems. The development faces the challenge how to manipulate locomotion effectively in both water and air which presents substantial differences in fluid properties. Additionally, there are difficulties in perception and navigation because of the discontinuity of mediums. To cope with these challenges, we designed an innovative unmanned aerial–aquatic vehicle with an optimized dual-speed and tilting propulsion configuration. The rotors/propellers operate in different ranges of rotating speed in the two different mediums, providing sufficient thrust and ensuring output efficiency. Besides, thrust vectoring is achieved by rotating each propulsion unit around its mounted arm, facilitating agile underwater cruising. Another key component of our approach is a sophisticated multi-sensor-based cross-medium localization system that combines SLAM, sensor synchronization, and data capture mechanisms, enabling seamless transitions between aerial and aquatic environments, and supporting autonomous operations. The results are fully validated through actual flight experiments.

Improved PVTOL Test Bench for the Study of Over-Actuated Tilt-Rotor Propulsion Systems

In recent years, applications exploiting the advantages of tilt-rotors and other vectored thrust propulsion systems have become widespread, particularly in many novel Vertical Takeoff and Landing (VTOL) configurations. These propulsion systems can provide additional control authority, enabling more complex flight modes, but the resulting control systems can be challenging to design due to the mismatch between the vehicle degrees of freedom and physical input variables. These propulsion systems present both advantages and difficulties because they can exert the same overall forces and moments in many different propulsive configurations. This leads to the traditional non-uniqueness problem when using the inverse dynamics control allocation approach, which is the basis of many popular VTOL control algorithms. In this article, a modified Planar VTOL (PVTOL) test bench configuration, which considers an arbitrary number of co-linear tilting rotors, is introduced as a benchmark for the study of the control allocation problem. The resulting propulsion system is then modeled and linearized in a closed and compact form. This allows a simple and systematic derivation of many of the currently used control allocation approaches. According to the proposed PVTOL configuration, a two-rotor test bench is implemented experimentally and a decoupling control allocation strategy based on Singular Value Decomposition (SVD) analysis is developed. The proposed approach is compared with a traditional input mixer algorithm based on physical intuition. The results show that the SVD-based solution achieves better cross-coupling reduction and preserves the main properties of the physically derived approach. Finally, it is shown that the proposed PVTOL configuration is effective for studying the control allocation problem experimentally in a controlled environment and could serve as a benchmark for comparing different approaches.

Experimental Propeller Performance Analysis on a Large Scale Distributed Propulsion Configuration at High Lift

Experimental Propeller Performance Analysis on a Large Scale Distributed Propulsion Configuration at High Lift

Thermal management system design for a series hybrid-electric propulsion architecture

Abstract The current paper is focused on the conceptual design of a thermal management system with a liquid working medium for a commuter hybrid-electric aircraft, featuring a series propulsion configuration. Regarding the system’s architecture, parametric analyses are conducted, by altering the number of heat exchangers. To clarify, a centralised and a decentralised thermal management system architecture are examined. Furthermore, a computational model calculates the temperatures during the system’s operation and the required coolant mass flows to sufficiently cool all the compartments. Subsequently, the required heat exchanger surface is determined and the weight of each compartment that comprises the thermal management system can be calculated. It is worth noting, that the compartments’ cold plate weight is integrated. The results indicate that the decentralised configuration results in lower temperature fields for all components compared to the centralised configuration. However, the latter weighs 32.2% lower at 158.22kg while the decentralised configuration weighs 233.48kg.

Towards More Efficient Electric Propulsion UAV Systems Using Boundary Layer Ingestion

The implementation of distributed propulsion and boundary layer ingestion for unmanned aerial vehicles represents various challenges for the design of embedded ducts in blended wing body configurations. This work explores the conceptual design and evaluation of DP configurations with BLI. The aerodynamic integration of each configuration is evaluated following a proposed framework, including simulation analysis. Power saving coefficient and propulsive efficiency were compared against a baseline podded case. The results show the optimal propulsion configuration for the BWB UAV obtaining 3.95% of power benefit and propulsive efficiency (ηp>80%). Indeed, the aerodynamic integration effects for the proposed design maintain the BWB’s aerodynamic efficiency, which will contribute to longer endurance and better performance.

Mars mission capabilities enabled by nuclear thermal propulsion

As part of the Artemis era of space exploration, space agencies will be working together with their industry partners to establish systems and infrastructure that enable sustained Lunar missions and develop capabilities for Mars. Lockheed Martin has a longstanding history of designing, building, and operating systems for deep space applications, from Viking to Orion. Lockheed Martin develops concepts, trades, and early system designs for Moon and Mars exploration to enable future missions. In 2016, Lockheed Martin presented a vision for achieving crewed exploration of Mars. Known as Mars Base Camp (MBC), this design reference mission envisioned a crewed vehicle in Martian orbit from which astronauts could perform excursions to Phobos and Deimos and perform telerobotic exploration of the Martian surface, including sample return. This concept served as an “existence proof” for a novel, practical, and affordable path to enable human exploration of the Martian system. In 2017, an update was published that included the production of propellant from water and a single-stage fully reusable Mars lander concept. Then in 2021, Lockheed Martin introduced a nuclear thermal propulsion (NTP) configuration of Mars Base Camp, a technology that will be greatly enabling for crewed missions to Mars. This paper matures the design of the NTP configuration of Mars Base Camp and explores the Mars mission capabilities that the NTP technology will enable. These investigations include optimized trajectories for faster transit to Mars and an analysis of abort scenarios that are made possible by the high thrust and performance of NTP. The results provide insight into the performance and capabilities that are realized when NTP technology is matured and integrated into a Mars transit vehicle. The paper will also describe initial surface infrastructure capabilities that significantly enhance sortie science operations, including mobility and power. The resulting updated Mars Base Camp serves as an example of what is possible with current technologies and technologies under development. The MBC reference mission builds on the capabilities that will be developed and demonstrated through Artemis missions, enabling a viable, sustainable long-term Mars exploration program, with the first crewed Mars mission possible in the 2030s.

Математична модель розрахунку аеродинамічних сил, утворюваних роторним рушієм для пропульсивних крил

The subject of the study is the modeling and obtaining of aerodynamic characteristics and determining the aerodynamic forces acting on an aircraft with a propulsive wing using a mathematical model. The research is based on mathematical methods and proposes a comprehensive model that accurately describes the interaction of airflow with wing and rotor propulsion. The aim of this study is to find solutions for faster calculations and approximate analysis of wings and aircraft that use them, compared to CFD methods. The task is to find methods that can perform calculations based only on geometric shapes and a minimal set of data that can be obtained by the researcher. The article begins with a review of previous research in the field of aerodynamics, demonstrating the need for the development of new models to better understand the aerodynamic forces generated by wing and rotor propulsion. A new mathematical model is considered, using methods based on Bernoulli's equation and taking into account parameters such as wing shape, angle of attack, airflow velocity, and rotor propulsion characteristics. This article describes the mathematical equations and approaches used to model aerodynamic forces. They include physical laws such as Newton's laws, the conservation of mass and momentum, and basic aerodynamic equations. Validation of the model was conducted by comparing the obtained results with experimental data. To verify the correctness of the presented claims, a series of computational experiments using numerical methods are performed to calculate the dynamic characteristics for different wing and rotor propulsion configurations, and the obtained data are compared. Through careful experiments and data analysis, the research results are expected to provide valuable insights into the practical implementation of an integrated tangential fan system in a wing by evaluating its efficiency, limitations, and potential advantages. This work can help engineers determine optimal wing and rotor propulsion configurations to achieve better aerodynamic efficiency and ensure the desired flight characteristics. This research can make an important contribution to the development of aviation technology and to the improvement of aircraft with rotor propulsion. Conclusions: the article proposes a new mathematical model for calculating the aerodynamic forces generated by wing and rotor propulsion and demonstrates its speed and efficiency by comparing it with experimental data. This research is just the first step in creating a mathematical framework. To improve calculation results and enable automation for greater variability in shape, the panel-vortex method with the consideration of flow acceleration over panel surfaces is more suitable. However, even at this stage, the research results can contribute to the development and optimization of aircraft structures with propulsive devices embedded in the wing design.

Experimental Characterization of Composite-Printed Materials for the Production of Multirotor UAV Airframe Parts.

In this paper, the characterization of 3D-printed materials that are considered in the design of multirotor unmanned aerial vehicles (UAVs) for specialized purposes was carried out. The multirotor UAV system is briefly described, primarily from the aspect of system dynamics, considering that the airframe parts connect the UAV components, including the propulsion configuration, into a functional assembly. Three additive manufacturing (AM) technologies were discussed, and a brief overview was provided of selective laser sintering (SLS), fused deposition modeling (FDM), and continuous fiber fabrication (CFF). Using hardware and related software, 12 series of specimens were produced, which were experimentally tested utilizing a quasi-static uniaxial tensile test. The results of the experimental tests are provided graphically with stress-strain diagrams. In this work, the focus is on CFF technology and the testing of materials that will be used in the production of mechanically loaded airframe parts of multirotor UAVs. The experimentally obtained values of the maximum stresses were compared for different technologies. For the considered specimens manufactured using FDM and SLS technology, the values are up to 40 MPa, while for the considered CFF materials and range of investigated specimens, it is shown that it can be at least four times higher. By increasing the proportion of fibers, these differences increase. To be able to provide a wider comparison of CFF technology and investigated materials with aluminum alloys, the following three-point flexural and Charpy impact tests were selected that fit within this framework for experimental characterization.

Operating and Environmental Cost Considerations Comparison for Hybrid-Electric Propulsion Architecture Variants

A promising solution for sustainable aviation is hybrid-electric propulsion, which is gaining popularity in recent years. However, hybrid-electric propulsion variants are affected by the technological maturity of electric components, which have a direct impact on the electrification benefit, compared to conventional propulsive configurations. This work aims in investigating three different Entry Into Service dates, i.e., 2027, 2030, and 2040, for two hybrid-electric propulsive configurations, namely the Series and the Parallel. Six hybrid variants are sized, one for each date and configuration, using an in-house tool, and their mission performance metrics, operating costs, and environmental impact are compared to a conventional aircraft with technological assumptions of 2014. Results indicate that the Parallel variants promise a block fuel and energy reduction of up to 33 % and 28 % respectively for 2040, whereas the Series variants show a respective reduction of 66 % and 58 % accordingly. Moreover, the 2027 Parallel aircraft has reduced annual operating costs compared to the Series 2027, whereas a turnaround point exists in which the Series becomes more economical to operate, compared to both Parallel and the Reference aircraft after 2030. Finally, the Life Cycle Assessment suggests that the Series variants have a lower environmental impact compared to the respective Parallel variants and that both configurations have better scores than the Reference aircraft. The Parallel and Series configurations of 2030 promise an environmental impact reduction of up to 22.8 % and 59.8 % respectively, compared to the reference aircraft.

Nuclear fusion powered Titan aircraft

This paper discusses a system for Titan exploration enabled by nuclear fusion power. Titan is one of the most interesting locations in the solar system with a thick atmosphere, surface oceans, under-ice oceans and complex terrain. This paper provides a conceptual design of a fusion-powered system to explore many parts of Titan and enable the use of high-power instruments. The design includes a fusion-powered orbital transfer vehicle and an electric Titan science aircraft. A Direct Fusion Drive (DFD) propulsive engine could bring a sizable spacecraft to Titan orbit in less than two years. A second fusion reactor, configured as a closed-loop power generator, would be used for an electric Titan science aircraft. Both reactors are based on the Princeton Field-Reversed Configuration (PFRC) concept which combines an FRC with a magnetic mirror. PFRC uses a novel radio-frequency plasma heating system and deuterium-helium-3 fuel. A lower temperature plasma flows around the closed-field FRC region removing the fusion products. In the DFD propulsive configuration, this secondary flow permits direct and variable thrust and exhaust velocity. The science aircraft would do a powered entry to Titan and then have the capability to fly anywhere on the moon at subsonic speeds. The DFD-powered transfer vehicle would allow the in-orbit transfer stage to change inclination as needed to cover different areas of the surface.

Prediction noise of propeller on wing for distributed electric propulsion configuration

Prediction noise of propeller on wing for distributed electric propulsion configuration

Aerodynamic optimization of the exhaust system of an aft-mounted boundary layer ingestion propulsor

PurposeNovel aircraft propulsion configurations require a greater integration of the propulsive system with the airframe. As a consequence of the closer integration of the propulsive system, higher levels of flow distortion at the fan face are expected. This distortion will propagate through the fan and penalize the system performance. This will also modify the exhaust design requirements. This paper aims to propose a methodology for the aerodynamic optimization of the exhaust for novel embedded propulsive systems. To model the distortion transfer, a low order throughflow fan model is included.Design/methodology/approachAs the case study a 2D axisymmetric aft-mounted annular boundary layer ingestion (BLI) propulsor is used. An automated computational fluid dynamics approach is applied with a parametric definition of the design space. A throughflow body force model for the fan is implemented and validated for 2D axisymmetric and 3D flows. A multi-objective optimization based on evolutionary algorithms is used for the exhaust design.FindingsBy the application of the optimization methodology, a maximum benefit of approximately 0.32% of the total aircraft required thrust was observed by the application of compact exhaust designs. Furthermore, for the embedded system, it is observed that the design of the compact exhaust and the nacelle afterbody have a considerable impact on the aerodynamic performance.Originality/valueThis paper presents a novel approach for the exhaust design of embedded propulsive systems in novel aircraft configurations. To the best of the authors’ knowledge, this is the first detailed optimization of the exhaust system on an annular aft-mounted BLI propulsor.

Historical development of the coaxial contra-rotating propeller

AbstractWe review the development of the contra-rotating propellers from the origins to the present. Initially, these systems were proposed to increase speed, then to increase propulsive efficiency, and thus reduce fuel burn. Ultimately, they hit another environmental limit: too much noise. Acoustics has been in fact the main focus of the development in the past 30 years. Pioneering work done across countries demonstrated several unique features of this propulsor. Various embodiments are available, namely contra-rotating, counter-rotating, co-axial, tandem, open rotor and prop-fans, collectively named contra-rotating propellers. This review only considers concepts that have been applied to real aircraft, prototypes that are known to have been flight tested (about 70 vehicles), or representative laboratory models. Five classifications are proposed: pioneers (before 1940), golden years (1940–1950), Western airplanes (1950s onwards), Soviet-Russian airplanes (1950s onwards) and modern developments (1980s onwards). Selected experimental aircraft and laboratory concepts are mentioned, where these appear to advance the state-of-the-art. Power plants evolved from internal combustion engines to the modern gas turbine engines requiring new solutions. Engine layouts and propulsion configurations are analysed where appropriate. It is concluded that propulsive efficiency can only be achieved at a cost of multiple engineering problems, some of which remain unsolved.