eVTOL Design Complexities and Opportunities
Electric Vertical Takeoff and Landing (eVTOL) aircraft represent a promising leap in urban air mobility, offering solutions to congestion and transportation inefficiencies. However, their development presents significant engineering challenges.
These challenges include designing efficient propulsion systems, ensuring battery reliability and longevity, managing weight constraints while maintaining structural integrity, and addressing noise reduction for urban environments. Furthermore, integrating advanced avionics for autonomous operation and securing regulatory approval add layers of complexity.
We will investigate the tradeoffs companies are making to balance their operational performances with design limitations by focusing on three key areas:
- Operational Requirements – What is needed to integrate eVTOLs into urban mobility
- Aircraft Architecture – How have operators looked to solve the operational requirements through the design of their aircraft
- Forces of Change – What external factors may influence how OEMs and operators approach eVTOL design
Operational Requirements
Operating Strategy Considerations
Balancing range and payload is a critical challenge for vehicle designers, especially for eVTOLs where technological immaturity exacerbates this trade-off. Increasing payload capacity typically necessitates a larger and heavier aircraft, demanding more energy to achieve the same range. Conversely, extending range often means reducing payload to conserve energy.
Designers must optimize this balance to meet specific operational requirements. Urban air taxis may prioritize higher payloads for short trips, while inter-city eVTOLs might focus on extending range at the expense of carrying fewer passengers or lighter loads.
Operational strategies play a significant role in balancing range and payload. For instance, eVTOL operators may implement flexible payload limits based on the required range for specific missions. Optimizing flight paths and schedules can maximize efficiency, ensuring that eVTOLs operate within their optimal performance envelopes.
Battery and Charging Consideration
Fast charging is crucial for eVTOLs, enabling operators to maximize vehicle uptime and revenue generation. Industry estimates suggest that 95% of eVTOL charging will be fast charging, compared with only 10% for electric cars, to maintain tight schedules and effectively serve the short-distance taxi market.
The cornerstone of fast charging for eVTOLs lies in advanced battery technology.
These batteries must handle high power inputs without degradation. While lithium-ion batteries are currently the most common choice, their limitations in charging speed and thermal management pose significant challenges. Solid-state batteries, offering higher energy densities and better thermal stability, are being researched as potential alternatives.
Urban areas, where eVTOLs are expected to operate frequently, require strategically-placed charging stations capable of delivering high power levels, often several hundred kilowatts, to quickly recharge eVTOL batteries. This infrastructure challenge is immense, requiring substantial investment in grid upgrades and renewable energy integration to support sustainable and reliable charging.
Like conventional aircraft, eVTOLs experience peak power output during takeoff and landing, necessitating propulsion systems and batteries that can handle these demands without overheating or suffering rapid wear. eVTOLs will have short cruising times, leading to frequent and intense power and thermal cycles. Designing systems that can deliver this peak power while managing heat is a significant engineering challenge.
Cruise power, the sustained power needed for level flight at a constant speed and altitude, is significantly lower than maximum power but must be highly efficient to maximize the range and endurance of the eVTOL. Efficient cruise power management involves optimizing aerodynamics, including the design of wings and rotors, to reduce drag and improve lift-to-drag ratios. Designers must ensure that propulsion systems and batteries handle peak loads without compromising long-term efficiency and battery life.
Noise and Regulatory Considerations
As eVTOL technology progresses, regulatory hurdles must also be addressed. Given the urban nature of eVTOL operations, noise will be a critical concern. The European Union Aviation Safety Agency (EASA) has taken the lead in creating noise standards for eVTOLs.
Urban air mobility will require the creation of specific air corridors and vertiports, focusing on minimizing the impact on surrounding areas. Just as inner-city airports face strict noise restrictions, eVTOLs will need to comply with regulatory frameworks, potentially leading to less optimal flight paths and operational limitations during certain hours.
Noise reduction technologies, such as advanced rotor designs, sound-dampening materials, and active noise cancellation systems, will be essential. Integrating these technologies into eVTOL designs will be necessary to meet regulatory standards and ensure community acceptance.
Aircraft Architecture
Three design directions are taken forward for analysis, representing the more prevalent architectural designs shown by OEMs. Due to the range of applications that AAM manufacturers are attempting to address, there are almost infinite design directions that can be explored. Since this is a new area for designs there is yet to be design convergence, which often occurs.
Exhibit 1. Design Directions by Various OEMs
Multicopter
OEMs such as Volocopter and EHang utilize multicopter designs, characterized by multiple rotors providing vertical lift. The typical rotor configuration employs four or more rotors, arranged symmetrically around the airframe, enhancing stability and control. This configuration ensures that the loss of one rotor does not critically impact flight stability, providing essential redundancy and safety.
VTOL capabilities make multicopter designs suitable for urban air taxi services and emergency medical transport, offering quick, point-to-point travel. Most current multicopter eVTOLs are designed for short-range urban missions, typically covering distances of 20 to 50 miles per charge. This range is suitable for intra-city travel but limits inter-city operations.
The ability to hover and perform precise vertical maneuvers is a key advantage, enabling operations in areas where traditional aircraft cannot go. Current designs utilize many smaller rotors, improving maneuverability at the expense of complexity.
Lift-Plus-Cruise
A more complex design is used by Eve and Archer, by combining lift and forward cruise capabilities. These aircraft typically feature separate propulsion systems for vertical lift and horizontal cruise, optimizing performance for each flight phase.
Vertical lift is usually achieved with multiple rotors, similar to multicopters, providing stability and control during takeoff and landing. Once airborne, dedicated forward-thrust propellers or fixed wings take over for efficient horizontal flight.
Operational strategies for lift-plus-cruise eVTOLs focus on maximizing efficiency and flexibility. These aircraft are well-suited for both short urban hops and longer regional flights, offering a broader range of applications compared to multicopters. The ability to switch between vertical and horizontal flight modes allows operators to optimize routes and flight profiles based on mission requirements. For instance, during longer inter-city flights, the aircraft can cruise at higher altitudes and speeds, conserving energy and extending range.
Vector Thrust
The most complex design offering the greatest operational variability is vector thrust. Supernal and Lilium are examples in this category. This innovative design offers a versatile approach to urban air mobility, combining the efficiency of traditional fixed-wing aircraft with the vertical takeoff and landing capabilities of helicopters. Vector thrust eVTOLs typically feature tilting rotors or ducted fans that can pivot between vertical and horizontal orientations, optimizing performance across different flight phases.
During takeoff and landing, the rotors or fans are oriented vertically to provide lift, similar to a helicopter. Once airborne, these propulsion systems pivot to a horizontal position, allowing the aircraft to cruise like a conventional airplane. This dual-mode operation enhances energy efficiency and extends the range compared to multicopters and lift-plus-cruise designs.
Despite their advantages, vector thrust eVTOLs face several limitations. The complexity of the tilting propulsion systems adds weight and maintenance requirements, impacting the overall payload capacity and operational costs.
Forces of Change
Although the design direction of eVTOLs may eventually converge, this convergence is likely to be driven as much by external forces as by novel materials and architectural innovations from engineers. Various factors, including environmental conditions, infrastructure development, and strategic partnerships, play significant roles in shaping the evolution of eVTOL designs.
A notable trend is the signing of the majority of eVTOL Memorandums of Understanding (MOUs) with cities in hot climates, such as Dubai and Abu Dhabi. High ambient temperatures in these regions accelerate battery degradation, requiring more frequent charging and reducing operational range. Additionally, the higher temperatures increase the strain on propulsion systems during takeoff and landing. Engines must operate at higher RPMs to generate the required lift, further stressing the aircraft’s components and potentially shortening their lifespan.
Infrastructural developments are another critical factor to enable the uptake of eVTOLs. Despite substantial investment in eVTOL development, funding often prioritizes OEMs over the necessary supporting infrastructure. As a result, the technical capabilities of eVTOLs may be hindered by an inadequate operating environment. For instance, if energy supplies are insufficient or charging facilities are lacking, the operational capabilities of eVTOLs could be severely limited.
Strategic partnerships play a crucial role in shaping the design and direction of eVTOLs with collaborations between manufacturers and leading engineering firms, e.g., Avincis and Airbus, bringing valuable expertise and resources. However, these partnerships can influence smaller OEMs to align their designs with the broader goals of their larger partners, rather than their original/initial mission statement. Additionally, the operational profiles dictated by external partners, such as military versus commercial applications, further drive design variations, highlighting the need for tailored solutions to meet diverse use cases.
Furthermore, regulatory and economic factors are likely to shape the eVTOL landscape. Regulatory bodies are developing standards and guidelines for eVTOL operations, particularly concerning safety, noise pollution, and environmental impact. Compliance with these regulations will be crucial for market entry but may necessitate design changes or adaptations. Economic considerations, such as the cost of advanced materials and technologies, will also impact the feasibility and scalability of different eVTOL designs.
Conclusion
As the eVTOL industry continues to evolve, balancing internal and external influences will be essential for developing viable, efficient, and widely accepted urban air mobility solutions. While we may see convergence of design directions or separate markets suited to differing designs, the final decision will ultimately reflect a complex interplay of technological innovation, environmental adaptation, infrastructural support, strategic collaboration, and regulatory compliance.
Sources
IEEE Spectrum. (2021). “Advances in Solid-State Battery Technology.”
National Renewable Energy Laboratory. (2021). “Optimizing Power Management for Electric Aircraft.”
Journal of Aerospace Engineering. (2020). “Lightweight Materials in Aerospace Design.”
Urban Air Mobility News. (2021). “Distributed Electric Propulsion and its Benefits.”
Journal of Energy Storage. (2020). “Thermal Management in High-Power Lithium Batteries.”
European Union Aviation Safety Agency (EASA). (2022). “Certification Specifications for eVTOL Noise Levels.”