The advancement of flying cars and eVTOL (electric vertical takeoff and landing) vehicles is reshaping the future of transportation. In addition to helping us reach our destinations more quickly and cut road traffic congestion, flying cars and eVTOL vehicles could help dramatically reduce the overall greenhouse gas (GHG) emissions from transportation globally.

Several companies call the vehicles they are developing “flying cars,” but these vehicles do not travel on roads like regular cars. In this article, the term “flying car” is used only for a vehicle that travels like a car on the road and flies like an airplane or helicopter in the sky. The Aeromobil, PAL-V (Personal Air and Land Vehicle) and Terrafugia Transition and TF-X concept vehicles are good examples of flying cars ^[1], though not all of them have VTOL capability at this time. In 2016, only about half a dozen companies were developing flying cars and eVTOL vehicles; now more than 250 companies worldwide are working on them.

Since many of the proposed eVTOL aircraft will be run purely on batteries, like electric cars, they will be cleaner and quieter. If they are charged using renewable energy sources, it will help humanity progress toward the goals of achieving zero emissions and keeping global temperature rise below 1.5 degrees Celsius by 2100. It is reported that comparing VTOLs that are fully loaded with three passengers to ground-based cars with an average occupancy of 1.54, VTOL GHG emissions per passenger-kilometer are 52 percent lower than internal combustion engine vehicles (ICEVs) and 6 percent lower than pure electric vehicles (EVs) ^[2].

Flying cars and eVTOL vehicles also could change the way cities are developed, with infrastructure development around building roads and bridges minimized and several trees saved from being cut. In these ways, ongoing development of flying cars and eVTOL could help to improve the environment ^[1]. Furthermore, fewer airports would need to be built, thus reducing air traffic-control problems. And new classes of industries for components would emerge — electrical, mechanical, electronics, signals, controls and communications — and many other related disciplines would take shape.

Defining the space

The difficulties of designing a flying car or eVTOL are more challenging than designing a small airplane or a regular car. Many of the eVTOL concepts are based on distributed electric propulsion (DEP) — using multiple electric motors, each spinning a simple propeller to generate thrust to achieve an efficient, quiet and safe system. The Vertical Flight Society defines five categories of eVTOLs ^[3]:

• Vectored thrust eVTOLs have a wing for an efficient cruise and use the same propulsion system for both hover and cruise. The Aurora LightningStrike, Joby Aviation S2 and S4 and Lilium Jet are in this category. The S4 consists of six five-bladed propellers powered by separate electric propulsion units, with four located on its wing and the other two on the tail. The four propellers tilt vertically, including its entire motor nacelle, and two of the propellers tilt vertically with a linkage mechanism. The new Lilium Jet uses the distributed propulsion strategy with 36 small electric ducted fans embedded in the wing and forward canard.
• Lift + cruise eVTOLs have a wing for an efficient cruise, like vectored thrust eVTOLs, but they use two different propulsion systems for hover and cruise flight. The Aurora Flight Sciences, Kitty Hawk Cora and Zee Aero Z-P2 eVTOL are in this category.
• Wingless eVTOLs have multirotors with large disk actuator surfaces, making them efficient in hover, but they do not have a wing for an efficient cruise. These vehicles are suited for short-range operations in cities, such as for air taxis, which could fly over traffic jams. Examples of this class are the EHang 184 and the Volocopter 2X.
• Hoverbikes are multirotors that can be flown like a motorbike with the pilot sitting on a saddle or standing.
• eHelos are electrical conventional helicopters.

Most of the recent developments in eVTOL are focused on the vectored thrust, lift + cruise and wingless types.

Unique design requirements

The design requirements of a ground vehicle are so different from those of an airplane that the task of trying to combine the two sets of requirements in a single flying-car system presents many challenges. For example, the transition of flying cars from airplane to ground mode and vice versa must be seamless for smooth operation. Also, to reduce the total weight of the system, combining the propulsion system for both road and air travel could be a major challenge because of the different requirements.

There are other major challenges:

• Sources of primary energy to achieve highest power and energy density, given the substantial power requirements during vertical takeoff, landing and cruise with opposing wind direction;
• Optimum propulsion architectures for a given vehicle considering the different load profile for the traction versus flight, as well as aerodynamic issues for flying cars with both road-drive and flight requirements;
• Controlling algorithms for stable operation throughout the flight, combined with propulsion controllers and fast response motor control systems to manage stability during flight;
• Determination of the right altitude level for a given flight profile under changing circumstances and weather conditions, as well as accounting for effect due to extreme weather conditions;
• Signal, communication, safety and reliability issues;
• Meeting all of the regulatory requirements, which can be quite distinct for road and air travel.

In addition, adequate air traffic control is necessary for handling hundreds or thousands of airborne vehicles. And, to keep these vehicles safe, they must be autonomous and offer many other advanced technologies, such as auto cruise control, pilot/passengers auto eject, extremely reliable 3D vision, etc.

Onboard power

The main considerations in the selection of a battery are power density, energy density, weight, volume, cycle life, operating temperature range, safety, material recycling, maintenance and cost. Satisfying both the power and energy requirements with a low battery weight is essential for eVTOL vehicles to be power and energy efficient. A battery-management system is necessary to balance the state of charge of individual cells and modules, and thermal management is important to achieve high cycle life and long life. Presently, the best lithium-ion batteries provide energy density of about 200 Wh/kg to 250 wh/kg. These batteries are sufficient for flying cars for road operation and short-range eVTOL operation with one or two passengers.

Several companies are working on other technologies that can provide higher energy density and power density than the present lithium-ion batteries. A few companies are working on lithium-sulfur (Li-S) battery technology. OXIS Energy, for example, has already achieved 450wh/kg at cell level and expects to achieve 600wh/kg by 2025 (www.oxisenergy.com).

Another emerging technology is the solid-state battery, in which the liquid electrolyte in lithium-ion batteries is replaced with a solid electrolyte. This type of battery is reported to be safer, offer long cycle life and have a faster recharge time. A glass solid-state battery can have three times higher energy density than lithium-ion. But these batteries are still in the research stage at cell level, and several issues related to technological challenges and manufacturing have to be resolved. QuantumScape, Solid Power and a few other companies are working on this type of technology.

An additional battery technology could be lithium air, which has theoretically five to 10 times the energy of lithium-ion batteries of the same weight. But the rechargeable lithium-air batteries would probably not be commercially available for several years because of technological challenges and recharging issues.

The proton exchange membrane (PEM) fuel cell has significant potential to provide the required power for both flying cars and also for eVTOL-only vehicles. The major issue is related to the supply of hydrogen and maintaining the entire system in a safe environment inside the vehicle. Intelligent Energy and several other fuel-cell manufacturers are working to exceed the U.S. Department of Energy (DoE) technical target of 2kW/kg for fuel cell stack. With the advancement of hydrogen storage and fuel cell stack technologies, hydrogen fuel cells would play a key role in the decarbonization strategy for aviation, as they can power aircraft efficiently.

Electric machines

The selection, technology, power density and thermal management of the electric motor are all very important for achieving high efficiency for eVTOL and flying cars. The number of motors being used depends on the aircraft design. For example, 18 brushless DC motors driving fixed-pitch propellers arrayed on a lattice ring powered the Volocopter VC200 air taxi during demonstrations. Thirty-six motors turning fans in a ducted wing and canard powered the electric Lilium Jet that was recently flown. But the flying cars with eVTOL capability have entirely different configurations, with one or two motors for ground propulsion and a separate tilt rotor or lift fan for takeoff and landing. If it is a tilt rotor system, the same also can be used for forward propulsion during the flight. The motors required for ground propulsion and eVTOL have different requirements in a few aspects. Although both need high-torque-density motors, an eVTOL system prefers a motor of high torque but relatively low revolutions per minute (RPM) in a direct drive to reduce propeller tip speed and mitigate noise without the need for a gearbox.

In eVTOL vehicles, permanent magnet (PM) machines are being increasingly used because of their high efficiency and power density. As an example, the lift fan in XV-24A LightningStrike uses PM motors with composite stators and embedded electromagnetic conductors to achieve the best power-to-weight ratio. The Lilium Jet also uses the PM synchronous motors with sinusoidal back electromotive force (EMF). Most of these PM motors are sinusoidal back EMF synchronous motors that would provide smooth torque operation.

Although the induction motor is rugged and well advanced in technology, it has very sluggish transient response. As a result, it takes longer for the motor to react, particularly for multi-rotor machines and when the rotor is used for controlling flight dynamics. Also, the efficiency and power density are not as high as that of PM machines.

The switched reluctance machines are extremely noisy during operation, have higher torque pulsations, lower efficiency, larger size and higher weight than PM machines. Synchronous reluctance motors have poor power factor and relatively poorer performance compared to PM machines.

The PM machine will continue to be the right choice for a long time to come, particularly for flying cars and for eVTOL-only vehicles. Monitoring the conditions of the electric machine is very important for detecting any impending failures such as bearing, rotor and stator faults, as well as to ensure that high reliability standards are met for the aircraft. 

Power electronics and motor control

Power electronics is an enabling technology for the development of flying cars and eVTOL systems, as it has been for electric vehicles. The selection of power semiconductor devices, converters/inverters, control and switching strategies, packaging of the individual units, thermal management and system integration are very important for the development of efficient and high-performance vehicles.

Wide band gap (WBG) devices such as silicon carbide (SiC) devices with inherent radiation resistance, high-temperature operating capacity, high voltage and power handling capability, high power efficiency and flexibility make them best suited for eVTOL systems. The wider band gap, larger critical electric field and higher thermal conductivity enable SiC devices to operate at higher temperatures and higher voltages. This offers higher power density and higher current density than pure silicon devices, enabling high-power density converters to be achieved.

Another WBG device, the gallium nitride (GaN) device, is projected to have significantly higher performance over silicon-based devices due to its excellent material properties, such as high electron mobility, high breakdown field and high electron velocity. However, GaN devices are still not ready for the high power level required in eVTOL systems.

In addition to power devices, high performance control methodologies such as direct torque and flux control or field orientation control are essential for controlling the motor using the inverter for stable and high-efficiency operation.

As in electric vehicles, the trend for eVTOL systems is to integrate the motor and inverter as one unit to achieve higher power density, ease of thermal management, reduction of cable lengths (and, in turn, reduction in susceptibility to electromagnetic interference) and ease of maintenance. For example, H3X (https://www.h3x.tech/) is developing an integrated, geared, high-speed electric motor with SiC inverter system-HPDM-250. H3X says it provides 13 kW/ kg continuous, exceeding the Advanced Research Projects Agency-Energy (ARPA-E) requirement of 12 kW/kg in the recently awarded ASCEND program for electric aircraft applications. Rolls-Royce has developed an integrated electric motor drive (RRP200D) with 200kW output with torque-to-weight ratio of about 30 Nm per kilogram (https://www.rolls-royce.com/innovation/propulsion/air-taxis.asp). NASA has also funded and demonstrated larger and higher power density motors.

Other issues

For practical and large-scale deployment, flying cars and eVTOL-only vehicles have to be intelligent, connected and autonomous. To achieve this, the communications and control aspects of these vehicles must be addressed. The most important consideration is safety; rapid-inflating parachutes, airbags, energy-absorbing seats and related features must be incorporated. Noise considerations also must be addressed for operation in urban areas. The altitude of operation also plays an important role, and the vehicles may have to be aligned in invisible lanes. Other issues such as stability, aerodynamic propulsion issues, airworthiness, flight testing, certification and economics also demand attention.

The future of transportation

eVTOL vehicles will revolutionize the future of transportation. These aircraft are emission-free, safe, fast, very low noise, and they could be designed for adaptability to different weather conditions. As battery technology improves, eVTOL vehicles such as air taxis will become more sustainable and fly for longer ranges and at higher speeds.

Although in recent years more emphasis has been applied to eVTOL-only vehicles, there is a parallel effort to develop flying cars that can go on the road and in the sky. As the technologies and infrastructure advance, there could be a possibility that flying cars — like Aeromobile, PAL-V, ASKA, Terrafugia TF-X — will be the future of all personal transportation. At present, these are purely IC engine-based or hybrid vehicles. In the future, these could also be converted to pure electric operation with eVTOL capability.

The charging infrastructure is also important for these vehicles to be realistic. Several companies are working to develop an extensive charging infrastructure based on renewable energy sources, which could lead to significant emission reductions from road and air transportation.

For long-distance operation, small airplanes that can carry about 10 to 15 passengers with hybrid propulsion and VTOL capability could be the future. These aircraft could use smaller airports that are closer to a set of residential neighborhoods for their operation.

Continuing research and development by industry, academia and government worldwide will further advance the technology of eVTOL vehicles, making them more viable with longer ranges and higher performance, safety, reliability and lower cost. The advancement of these vehicles is crucial for helping to keep global temperature rise within 1.5 degrees Celsius by 2100 and reducing the effects of global warming.

References

[1] K. Rajashekara, Q. Wang, and K. Matsuse, “Flying Cars — Challenges and Propulsion Strategies,” IEEE Electrification Magazine, Volume 4, March 2016, pp. 46 — 57.

[2] Akshat Kasliwal, Noah J. Furbush, James H. Gawron, et al., “Role of flying cars in sustainable mobility,” Nature Communications, Volume 10, 2019, Article 1555.

[3] Alessandro Bacchini and Enrico Cestino, “Electric VTOL Configurations Comparison,”MDPI Aerospace, 2019, 6, 26.