Electric Vertical Takeoff Research Articles

Performance and cost of fuel cells for urban air mobility

Several companies are developing enabling elements of urban air mobility (UAM) for air taxis, including prototypes of electric vertical take-off and landing (eVTOL) vehicles. These prototypes incorporate electric and hybrid powertrains for multi-rotor and tilt-rotor crafts. Many eVTOLS are using batteries for propulsion and charging them rapidly between the flights or swapping them for slow charging overnight. Rapid charging degrades the battery cycle life while swapping requires multiple batteries and charging stations. This study has conducted a technoeconomic evaluation of the eVTOL air taxis with alternate powertrains using hydrogen fuel cell systems being developed for light-duty and heavy-duty vehicles. We consider performance metrics such as fuel cell engine power, weight, and durability; hydrogen consumption and weight of storage system; and maximum take-off weight. The metrics for economic evaluation are capital cost, operating and maintenance cost, fuel cost, and the total cost of ownership (TCO). We compare the performance and TCO of battery, fuel cell and fuel cell – battery hybrid powertrains for multi-rotor and tilt-rotor crafts. We show that fuel cells are the only viable concept for powering multi-rotor eVTOLs on an urban scenario that requires 60-mile range, and hybrid fuel cells are superior to batteries as powertrains for tiltrotor eVTOLs.

Evaluation and Comparison of Hybrid Wing VTOL UAV with Four Different Electric Propulsion Systems

Electric propulsion technology has attracted much attention in the aviation industry at present. It has the advantages of environmental protection, safety, low noise, and high design freedom. An important research branch of electric propulsion aircraft is electric vertical takeoff and landing (VTOL) aircraft, which is expected to play an important role in urban traffic in the future. Limited by battery energy density, all electric unmanned aerial vehicles (UAVs) are unable to meet the longer voyage. Series/parallel hybrid-electric propulsion and turboelectric propulsion are considered to be applied to VTOL UAVs to improve performances. In this paper, the potential of these four configurations of electric propulsion systems for small VTOL UAVs are evaluated and compared. The main purpose is to analyze the maximum takeoff mass and fuel consumption of VTOL UAVs with different propulsion systems that meet the same performance requirements and designed mission profiles. The differences and advantages of the four types propulsion VTOL UAV in the maximum takeoff mass and fuel consumption are obtained, which provides a basis for the design and configuration selection of VTOL UAV propulsion system.

Enhanced High-Order Scheme for High-Resolution Rotorcraft Flowfield Analysis

The recent growing interest in urban air mobility (UAM) worldwide has led to the demand for physical analyses of the aerodynamic performance and aeroacoustic characteristics of electric vertical takeoff and landing (eVTOL) rotorcraft. In a UAM eVTOL rotorcraft, the smaller vortices generated from multiple propulsors interact with lifting surfaces such as wings, fuselage, and propellers in a complex manner. Therefore, to accurately predict the performance and noise of UAM eVTOL rotorcraft, vortices should be preserved with little dissipation; and their interactions must be modeled precisely. These requirements need a numerical algorithm with a refined resolution than that allowed by conventional schemes. This paper proposes a newly modified enhanced multidimensional limiting process (eMLP) for vorticity conservation (eMLP-VC), which improves the original eMLP by accounting for the fact that most rotorcraft flowfields are vortex dominated and subsonic. For advanced capability in preserving vortices, the distinguishing criterion was modified through vortex profile analysis, and low-Mach-number adjustment was performed by reconstructing the interpolated primitive variables. The proposed scheme was applied to wave propagation, shock discontinuity, double Mach reflection, propeller–wing interaction, and Second Higher-Harmonic Control Aeroacoustic Rotor Test problems. The computed results confirmed that eMLP-VC exhibits superior resolution, numerical stability, and computational time efficiency as compared to the multidimensional limiting process, eMLP, and weighted essentially nonoscillatory methods.

도심항공 모빌리티(UAM)를 위한 틸트 덕티드 팬 형 eVTOL의 초기 사이징

A large amount of time and cost is consumed due to congestions caused by an increasing number of cars which results in a lot of emissions. To overcome these problems, a new electric vertical takeoff and landing (eVTOL) aircraft is being considered. Since vertical take off and landing without a separate runway is realized and electricity is used as a power source, it could solve the saturated ground traffic congestions without emissions. In this paper, the initial sizing was performed based on the Nexus 6HX of Belltextron which is a tilt-ducted fan type. In this study, the electric propulsion system that only uses battery was implemented instead of current Nexus 6HX hybrid electric propulsion. Aerodynamic analyses were performed using OpenVSP and XFLR5. Power-to-weight ratio, wing loading, estimated weight were calculated with these analyses.

Toward inclusion of atmospheric effects in the aircraft community noise predictions.

This paper presents an atmospheric propagation model, based on ray acoustics, that accounts for realistic weather conditions in the evaluation of the noise footprint of an aircraft. Noise sources, obtained using the Ffowcs Williams and Hawkings acoustic analogy applied to scale-resolved flow simulation data, are stored on a hemisphere surrounding the vehicle. These noise sources are propagated using a propagation model that takes into account the vertical variability of air temperature and wind velocity. The electric vertical takeoff and landing aircraft, presented by Casalino, van der Velden, and Romani [(2019). in Proceedings of the AIAA Scitech 2019 Forum, January 7-11, San Diego, CA, pp. 1834-1851], is used as a case study; noise footprints, obtained considering various vertically varying temperature and wind velocity distributions, are compared. It is shown that weather conditions in the acoustic wave propagation can contribute to mismatch up to 4 dBA in the illuminated zone and a significant drop in the refractive shadow zone caused by the vertical air temperature and wind velocity gradients. This work constitutes the first accomplishment in including realistic atmospheric effects in aircraft community noise prediction based on scale-resolved flow simulations.

Temporal integration of partial loudness of helicopter-like sounds

When developing new vehicles that are to be operated in existing background noise, such as electric vertical take-off and landing aircraft (eVTOLs) in cities, a sound design goal should be to minimize the loudness in the given background noise. Rotorcraft sounds are characterised by their pulses, and the choice of rotor size and number allows to vary the temporal characteristics. We asked participants to compare the loudness of pulse trains with pulse durations of 1, 2, 5, 10 and 20 ms and a pulse rate of 20 Hz in a two-interval, two-alternatives forced choice task and a 1-up/1-down procedure. Street noise was presented simultaneously with the pulse trains, and had the same root-mean-square (RMS) level as the fixed reference pulse train of about 65 dB SPL. First results indicate that the sounds with a short pulse duration need considerably less RMS level to result in the same loudness as a long pulse duration, i.e. the partial loudness of shorter pulses is higher at the same equivalent sound pressure level.

Up, Up and Away! [Aviation - eVTOL

Hybrid-electric vertical take-off and landing (eVTOL) vehicles could transform the air traffic ecosystem. After more than a decade, the market for electric vertical take-off and landing aircraft is on the rise, and its goal of making urban air mobility (UAM) for everyone personal, on-demand and carbon-free looks within reach. Big money is backing air taxis. In 2021, at least four leading players will go public. All will reverse-merge into existing listed investment vehicles - or SPACs - which will receive funding from current and new investors. The various companies involved in developing eVTOL vehicles are discussed.

System Identification for Propellers at High Incidence Angles

Propellers used for electric vertical takeoff and landing (eVTOL) aircraft propulsion systems experience a wide range of aerodynamic conditions, including large incidence angles relative to oncoming airflow. In oblique flow, propellers exhibit deviations in thrust and torque oriented along the propeller axis of rotation, as well as significant off-axis forces and moments. Although important for modeling eVTOL aircraft aerodynamics, sparse experimental data or mathematical models exist for propellers at incidence. This paper describes a propulsion system modeling methodology for the LA-8 tandem tilt-wing, eVTOL aircraft. System identification methods are applied to isolated propeller wind-tunnel data gathered across the vehicle’s flight envelope to develop a mathematical model of the propulsion system, including a static motor model, dynamic motor model, and propeller aerodynamic model. Modeling results validated against data withheld from the modeling process indicate good predictive capability and agree with theoretical expectations. The results are followed by a discussion of model implementation strategies into high-fidelity eVTOL aircraft simulations.

Adaptive model predictive control of a six-rotor electric vertical take-off and landing urban air mobility aircraft subject to motor failure during hovering

In this article, motor failure control of a six-rotor electric vertical take-off and landing (eVTOL) urban air mobility aircraft is investigated using adaptive model predictive control (MPC) based on the linear parameter-varying (LPV) model developed using the nonlinear rigid-body aircraft model. For capturing the aircraft dynamics under motor failure conditions, a family of linearized models are obtained by trimming the nonlinear aircraft model at multiple equilibrium conditions and the LPV model is obtained by linking the linear models using the failed rotor speed, where the system transition from healthy to failure is modeled by a scheduling parameter calculated based on failed rotor speed caused by available motor peak power after failure. The proposed adaptive MPC is developed to optimize the system output performance, including the rigid-body aircraft velocity and altitude, by using quadratic programming optimization with reference compensation subject to a set of time-varying constraints representing the current available propeller acceleration calculated based on the motor power. Simulation study is conducted based on the developed LPV control design and original nonlinear rigid-body model, and the simulation results demonstrate that the designed adaptive MPC controller is able to recover and maintain the aircraft at desired stable condition after motor failure.

Challenges and key requirements of batteries for electric vertical takeoff and landing aircraft

<h2>Summary</h2> Electric vertical takeoff and landing (eVTOL) aircraft have attracted considerable interest as a disruptive technology to transform future transportation systems. Their unique operating profiles and requirements present grand challenges to batteries. This work identifies the primary battery requirements for eVTOL in terms of specific energy and power, fast charging, cycle life, and safety, revealing that eVTOL batteries have more stringent requirements than electric vehicle batteries in all aspects. Notably, we find that fast charging is essential for downsizing aircraft and batteries for low cost while achieving high vehicle utilization rates to maximize revenues. We experimentally demonstrate two energy-dense Li-ion battery designs that can recharge adequate energy for 80 km eVTOL trips in 5–10 min and sustain over 2,000 fast-charge cycles, laying a foundation for eVTOL batteries.

Trajectory optimization-based maneuverability assessment of eVTOL aircraft

The emerging electric vertical take-off and landing aircraft (eVTOL) aircraft promotes the convenient urban air mobility, but the maneuverability of the eVTOL has not gained equivalent attention as the endurance problem or control law design. The maneuverability is actually critical for hover and low-speed phases to perform efficient and safe flights. However, the existing assessment paradigms of the maneuverability are not applicable to eVTOL due to the diversity of the configurations, new features of the distributed electric propulsion, and insufficient empirical data. This paper proposes an optimization-based method for maneuverability assessment as an alternative to the piloted experiment approach. The methodology uses trajectory optimization to determine the achievable performance regarding specific tasks. The optimization results yield an attainable maneuverability zone (AMZ), which is essentially a four-dimensional space that reflects the maneuverability of the eVTOL. The AMZ of the exemplary eVTOL has an approximately ellipsoidal shape with the weak-spot maneuverability in the backward-upward direction. The quantitative maneuverability measure is further used in the design optimization of the propeller layout and determination of the worst-case performance by a bi-level optimization framework. The proposed method enables efficient maneuverability evaluation and provides a model-based analysis of aircraft design quality.

Capacitated Location-Allocation-Routing Problem with Time Windows for On-Demand Urban Air Taxi Operation

Urban air taxi (UAT) operation has gained traction with the advancements in distributed electric propulsion and the emergence of electric vertical take-off and landing aircraft. Start-up companies and aircraft manufacturers are pursuing the possibility of operating UAT at scale in urban and suburban areas and at an affordable price. However, considerable uncertainties remain about several strategic, tactical, and operational aspects that affect UAT adoption. We envision a mature state of UAT operation in which the UAT operator offers door-to-door, multimodal, on-demand, and per-seat service. We propose the concept of flexible meeting points for UAT operation where passengers are flexible about the location of the UAT pads for boarding and deboarding, and could therefore be pooled together to share an aircraft. Consequently, we model UAT fleet operation as a capacitated location-allocation-routing problem with time windows and present a mixed integer programming formulation. The formulation addresses decisions on request acceptance and rejection, allocation of requests to flights, and aircraft routing and scheduling. Additionally, it allows for consolidating the demand to increase the aircraft’s utilization and service rate. The numerical results indicate that the demand consolidation scheme could significantly decrease the number of rejected requests and the aerial mileage. Depending on the operator’s business model, the proposed formulation could be used offline in a static and deterministic setting when all requests are known in advance, or it could be employed online by sequentially solving the static and deterministic snapshot problems with no knowledge about future requests.

Modeling CO2 Emissions from Trips using Urban Air Mobility and Emerging Automobile Technologies

Urban air mobility (UAM) operations provide the potential for more, or more attractive, trips in a metropolitan area relative to wholly surface-based transportation. But the emissions produced by a UAM mode must be studied in relation to these benefits. In this paper, an emissions model for the UAM context using electric vertical takeoff and landing (eVTOL) aircraft is developed that incorporates CO2 gases emitted from the electricity production required to charge the vehicle batteries. The model quantifies trip emissions using UAM for part or all of the trip and compares these with automobile-based trips. The estimations consider using gasoline and electric automobiles, with the impact of autonomy and average ground speeds in traffic. Trip case studies in the Chicago and Dallas metropolitan areas showcase the regional differences when using UAM and different automobile technology scenarios. In particular, differences stemming from how electricity generation from power grids (i.e., grid emission index) contributes to CO2 emissions of eVTOL trips and electric automobile trips in the Chicago and Dallas metropolitan areas are computed. This paper introduces trip properties called the surface-to-air distance ratio and the detour ratio to understand how they influence the CO2 emissions of a trip. Results from the simulation on identified trip cases in Chicago and Dallas illustrate the significant impact of the grid emission index of a region’s power plant on the emissions of electric vehicles.

Real-time dispatching of air taxis in metropolitan cities using a hybrid simulation goal programming algorithm

Expected to begin its operations in the coming years, air taxi aims to provide everyday transportation services to customers in metropolitan cities. These vehicles offer urban air mobility (UAM) services using the electric vertical takeoff and landing (eVTOL) technology. This research is the first to present a hybrid simulation goal programming (HSGP) approach to dispatch vehicles in a centralized air taxi network. After each customer drop-off, the model makes real-time decisions on (i) whether the air taxi must become idle or pick up customers, and (ii) the station to which the air taxi should be dispatched (if the air taxi is operational). The feasibility of the HSGP approach is tested using potential air taxi demands in New York City (NYC) provided by a prior study. The results of the experimentation suggest that the minimum number of air taxis required for efficient operation in NYC is 84, functioning with an average utilization rate of 66%. In addition, the impacts of commuter’s “willingness to fly” rate, percentage of demand fulfillment, on-road travel limit, maximum customer wait time, and arrival distribution on the optimal number of air taxis, utilization rate, number of customers served, and cost incurred per customer are examined. Analyses show that the “willingness to fly” rate appears to have a linear influence on the number of air taxis and the efficiency, while on-road travel distance has an exponential impact on the performance measures. The HSGP algorithm developed in this paper can be used by any company that is interested in venturing into the air taxi market.

Aerodynamic Interaction Effects Between Propellers in Typical eVTOL Vehicle Configurations

Many electric vertical takeoff and landing concepts are characterized by nontraditional vehicle layouts with distributed propellers. Two propeller interaction types were distinguished in this Paper, which investigates how propeller interaction in side-by-side and one-after-another configuration affects performance, in terms of thrust, power, in-plane forces, and out-of-plane moments, and how those performance effects depend on axial and lateral propeller spacing. A wind-tunnel experiment was performed with two propeller units, one instrumented with a force/torque sensor and the other introducing the aerodynamic interaction. Total pressure and planar particle-image velocimetry measurements were taken to investigate slipstream characteristics. A strong dependency of interaction effects on the geometric layout was found. For side-by-side interaction characteristic of vertical takeoff and transition, interaction effects varied from weak at small angle of attack to strong at larger angles. A drop in rear propeller thrust of up to 30% was found at constant advance ratio. Keeping thrust constant resulted in power penalties up to 13% for the two propellers combined. For one-after-another interaction, characteristic of cruise, a maximum reduction of thrust of up to 80% was observed. Thrust compensation led to power penalties up to 30% for the rear propeller alone. An extended blade element momentum model captured most interaction effects with sufficient accuracy.

Integrated Network Design and Demand Forecast for On-Demand Urban Air Mobility

Urban air mobility (UAM) is an emerging concept proposed in recent years that uses electric vertical take-off and landing vehicles (eVTOLs). UAM is expected to offer an alternative way of transporting passengers and goods in urban areas with significantly improved mobility by making use of low-altitude airspace. In addition to other essential elements, ground infrastructure of vertiports is needed to transition UAM from concept to operation. This study examines the network design of UAM on-demand service, with a particular focus on the use of integer programming and a solution algorithm to determine the optimal locations of vertiports, user allocation to vertiports, and vertiport access- and egress-mode choices while considering the interactions between vertiport locations and potential UAM travel demand. A case study based on simulated disaggregate travel demand data of the Tampa Bay area in Florida, USA was conducted to demonstrate the effectiveness of the proposed model. Candidate vertiport locations were obtained by analyzing a three-dimensional (3D) geographic information system (GIS) map developed from lidar data of Florida and physical and regulation constraints of eVTOL operations at vertiports. Optimal locations of vertiports were determined to achieve the minimal total generalized cost; however, the modeling structure allows each user to select a better mode between ground transportation and UAM in terms of generalized cost. The outcomes of the case study reveal that although the percentage of trips that switched from ground mode to multimodal UAM was small, users choosing the UAM service benefited from significant time saving. In addition, the impact of different parameter settings on the demand for UAM service was explored from the supply side, and different pricing strategies were tested that might influence potential demand and revenue generation for UAM operators. The combined effects of the number of vertiports and pricing strategies were also analyzed. The findings from this study offer in-depth planning and managerial insights for municipal decision-makers and UAM operators. The conclusion of this paper discusses caveats to the study, ongoing efforts by the authors, and future directions in UAM research.

Configuration Study of Electric Helicopters for Urban Air Mobility

There is currently interest in the design of small electric vertical take-off and landing aircraft to alleviate ground traffic and congestion in major urban areas. To support progress in this area, a conceptual design method for single-main-rotor and lift-augmented compound electric helicopters has been developed. The design method was used to investigate the feasible design space for electric helicopters based on varying mission profiles and technology assumptions. Within the feasible design space, it was found that a crossover boundary exists as a function of cruise distance and hover time where the most efficient configuration changes from a single-main-rotor helicopter to a lift-augmented compound helicopter. In general, for longer cruise distances and shorter hover times, the lift-augmented compound helicopter is the more efficient configuration. An additional study was conducted to investigate the potential benefits of decoupling the main rotor from the tail rotor. This study showed that decoupling the main rotor and tail rotor has the potential to reduce the total mission energy required in all cases, allowing for increases in mission distances and hover times on the order of 5% for a given battery size.

Distributed deep reinforcement learning for autonomous aerial eVTOL mobility in drone taxi applications

The urban aerial mobility (UAM) system, such as drone taxi or air taxi, is one of future on-demand transportation networks. Among them, electric vertical takeoff and landing (eVTOL) is one of UAM systems that is for identifying the locations of passengers, flying to the positions where the passengers are located, loading the passengers, and delivering the passengers to their destinations. In this paper, we propose a distributed deep reinforcement learning where the agents are formulated as eVTOL vehicles that can compute the optimal passenger transportation routes under the consideration of passenger behaviors, collisions among eVTOL, and eVTOL battery status.

Flight Test of L1 Adaptive Control on 120-kg-Class Electric Vertical Take-Off and Landing Vehicles

Flight Test of L₁ Adaptive Control on 120-kg-Class Electric Vertical Take-Off and Landing Vehicles

Impact of lift propeller drag on the performance of eVTOL lift+cruise aircraft

Electric vertical takeoff and landing (eVTOL) aircraft of the lift+cruise configuration are significantly impacted by the drag of the lift propellers even though they are only briefly used for takeoff and landing. This work aims to quantify the drag penalty of these high lift propellers, its impact on the performance of a lift+cruise eVTOL aircraft, and potential benefits from retracting the propellers during cruise. An eVTOL model is tested in the wind tunnel with propellers extended and retracted. Results show that retracting the propellers reduces parasitic drag by 38%. This measured drag reduction is used to estimate the performance of a passenger eVTOL and a surveillance drone employing a propeller retraction system. A passenger carrying eVTOL lift+cruise aircraft with a propeller retraction system could cruise 21% faster, at the same mission range, or increase range by 13% at the same flight speed. When endurance is the main mission objective, the benefits are smaller, in the order of 7%, and the potential gains are more sensitive to the weight of the retraction system.