Researchers at the University of Michigan in Ann Arbor and Ford Motor Co. in Dearborn have studied the environmental sustainability of flying cars, or electric vertical takeoff and landing aircraft (VTOLs), and determined they would not be suitable for very short commutes. They are more sustainable for longer trips of 50 to 300 miles.
VTOLs combine the convenience of vertical takeoff and landing like a helicopter, but have the efficient aerodynamic flight of a plane. Several companies around the world are producing prototypes, including ASX, a division of Detroit Aircraft Corp. located at Detroit City Airport.
The study, titled “Role of Flying Cars in Sustainable Mobility,” was published April 9 in Nature Communications and was completed by authors from U-M’s Center for Sustainable Systems and Ford Motor Co.’s research and advanced engineering team. It is the first comprehensive sustainability assessment of VTOLs and looked at energy use, greenhouse gas emissions, and time savings compared to ground-based passenger cars.
Flying cars, or VTOL aircraft, would be valuable in congested cities or places where there are geographical constraints as part of ride-share services, according to the study.
“To me, it was very surprising to see that VTOLs were competitive with regard to energy use and greenhouse gas emissions in certain scenarios,” says Gregory Keoleian, senior author of the study and director of the Center for Sustainable Systems at U-M’s School for Environment and Sustainability. “VTOLs with full occupancy could outperform ground-based cars for trips from San Francisco to San Jose or from Detroit to Cleveland, for example.”
VTOLs produce no emissions during flight, but their batteries require electricity generated at power plants. Researchers found that for trips of 100 km, or 62 miles, a fully loaded VTOL carrying a pilot and three passengers had lower greenhouse gas emissions that ground based-cars with an average vehicle occupancy of 1.54. Emissions tied to the VTOL were 52 percent lower than gasoline vehicles and 6 percent lower than battery-electric vehicles.
“With these VTOLs, there is an opportunity to mutually align the sustainability and business cases,” says Akshat Kasliwal, first author of the study and a graduate student at the U-M School for Environment and Sustainability. “Not only is high passenger occupancy better for emissions, it also favors the economics of flying cars. Further, consumers could be incentivized to share trips, given the significant time savings from flying versus driving.”
The global transportation sector is facing a growing demand for convenient passenger mobility while reducing congestion, improving safety, and mitigating climate change.
Electric and automated cars could contribute to some of these goals but are limited by congestion on existing roadways. VTOLs could overcome some of these limitations.
Airbus, Boeing, Joby Aviation, Lilium, and NASA have developed prototypes. One critical efficiency enabler for these aircraft is distributed electric propulsion, which involves the use of several small, electrically driven propulsors.
The researchers used publicly available information from these sources and others to create a physics-based model that computes energy use and greenhouse gas emissions for electric VTOLs.
“Our model represents general trends in the VTOL space and uses parameters from multiple studies and aircraft designs to specify weight, lift-to-drag ratio, and battery-specific energy,” says Noah Furbush, study co-author and a master’s student at the U-M College of Engineering. “In addition, we conducted sensitivity analyses to explore the bounds of these parameters, alongside other factors such as grid carbon intensity and wind speed.”
The researchers analyzed primary energy use and greenhouse gas emissions during the five phases of VTOL flight: takeoff hover, climb, cruise, descent, and landing hover. They use a lot of energy during the takeoff and climb but are relatively efficient during the cruise phase, traveling at 150 mph, making them most energy efficient on long trips when the cruise phase dominates the total flight miles.
For shorter trips, or anything less than 35 km (22 miles), single-occupant internal-combustion-engine vehicles used less energy and produced fewer greenhouse gas emissions than single-occupant VTOLs. The average ground-based vehicle commute is about 17 km, or 11 miles.
“As a result, the trips where VTOLs are more sustainable than gasoline cars only make up a small fraction of total annual vehicle-miles traveled on the ground,” says Jim Gawron, study co-author and a graduate student at the U-M School for Environment and Sustainability and the Ross School of Business. “Consequently, VTOLs will be limited in their contribution and role in a sustainable mobility system.”
The VTOL completed the base-case trip of 100 km much faster than ground-based vehicles. A point-to-point VTOL flight path, coupled with higher speeds, resulted in time savings of about 80 percent relative to ground-based vehicles.
“Electrification of aircraft, in general, is expected to fundamentally change the aerospace industry in the near future,” says Furbush.
The authors say other questions need to be addressed to assess the viability of VTOLs, including cost, noise, and societal and consumer acceptance.
Ford researchers include James McBride, Timothy Wallington, Robert De Kleine, and Hyung Chul Kim.
The study is available here.