April 15, 2019
In the 1960s animated sitcom “The Jetsons,” George Jetson commutes to work in his family-size flying car, which miraculously transforms into a briefcase at the end of the trip.
A new study of the environmental sustainability impacts of flying cars, formally known as electric vertical takeoff and landing aircraft, or VTOLs, finds that they wouldn’t be suitable for a Jetsons-style short commute.
However, VTOLs, which combine the convenience of vertical takeoff and landing like a helicopter with the efficient aerodynamic flight of an airplane, could play a niche role in sustainable mobility for longer trips, according to the study published in Nature Communications.
Several companies around the world, including Airbus, Boeing, Joby Aviation and Lilium, are developing VTOL prototypes.
Flying cars would be especially valuable in congested cities, or in places where there are geographical constraints, as part of a ride-share taxi service, according to study authors from the School for Environment and Sustainability’s Center for Sustainable Systems and Ford Motor Co.
“To me, it was very surprising to see that VTOLs were competitive with regard to energy use and greenhouse gas emissions in certain scenarios,” said Gregory Keoleian, senior author of the study and director of the Center for Sustainable Systems.
“VTOLs with full occupancy could outperform ground-based cars for trips from San Francisco to San Jose or from Detroit to Cleveland, for example,” he said.
The study by Ford and U-M is the first comprehensive sustainability assessment of VTOLs. It looked at the energy use, greenhouse gas emissions and time savings of VTOLs compared to ground-based passenger cars. Although VTOLs produce zero emissions during flight, their batteries require electricity generated at power plants.
The researchers found that for trips of 100 kilometers, or 62 miles, a fully loaded VTOL carrying a pilot and three passengers had lower greenhouse gas emissions than 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.
Akshat Kasliwal, first author of the study and a graduate student at SEAS, said the findings can help guide the sustainable deployment of an emerging mobility system prior to its commercialization.
“With these VTOLs, there is an opportunity to mutually align the sustainability and business cases,” Kasliwal said. “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.”
In the coming decades, the global transportation sector faces the challenge of meeting the growing demand for convenient passenger mobility while reducing congestion, improving safety and mitigating climate change.
Electric vehicles and automated driving may contribute to some of those goals but are limited by congestion on existing roadways. VTOLs could potentially overcome some of those limitations by enabling piloted taxi services or other urban and regional aerial travel services.
The U-M and Ford 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.
They also analyzed primary energy use and greenhouse gas emissions during the five phases of VTOL flight: takeoff hover, climb, cruise, descent and landing hover. These aircraft use a lot of energy during takeoff and climb but are relatively efficient during cruise phase, traveling at 150 mph. As a result, VTOLs are most energy efficient on long trips, when the cruise phase dominates the total flight miles.
Not surprisingly, the VTOL completed the base-case trip of 100 kilometers 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.
The study’s authors note that many other questions need to be addressed to assess the viability of VTOLs, including cost, noise and societal and consumer acceptance.
Other authors of the Nature Communications paper from U-M are Noah Furbush, a master’s student at the College of Engineering, and Jim Gawron, a graduate student at SEAS and the Stephen M. Ross School of Business. Authors from Ford’s Research and Innovation Center in Dearborn are James McBride, Timothy Wallington, Robert De Kleine and Hyung Chul Kim.