Could flying cars be the answer to sustainable mobility?

The dream of many of us growing up was to one day fly to work just like George Jetson in the 1960s animated sitcom The Jetsons. Could a miraculous flying car, which transforms into a briefcase at the end of the trip, be close to reality?

Sorry to disappoint but a new study of the environmental sustainability impacts of flying cars finds that they wouldn't be suitable for a Jetsons-style short commute. However, they could play a niche role in sustainable mobility for longer trips and could be especially valuable in congested cities, or in places where there are geographical constraints, as part of a ride-share taxi service.

Electric vertical takeoff and landing aircraft or VTOLs combine the convenience of vertical takeoff and landing like a helicopter with the efficient aerodynamic flight of an airplane. The study from University of Michigan's Center for Sustainable Systems and Ford Motor Co. revealed that several companies around the world are developing VTOL prototypes, and they could be greener than you think.

"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 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," he said.

Published in Nature Communications, the U-M-Ford study is one of the first comprehensive sustainability assessments 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 km, 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% lower than petrol vehicles and 6% lower than battery-electric vehicles.

Akshat Kasliwal, first author of the study and a graduate student at the U-M School for Environment and Sustainability, said the findings can help guide the sustainable deployment of an emerging mobility system prior to its commercialisation.

"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 favours the economics of flying cars. Further, consumers could be incentivised 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.

Several aerospace corporations and start-up companies — Airbus, Boeing, Joby Aviation and Lilium, for example — and agencies such as NASA have developed VTOL prototypes. One critical efficiency enabler for these aircraft is distributed electric propulsion, or DEP, which involves the use of several small, electrically driven propulsors.

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.

"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," said 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," said Furbush, who is also a member of the U-M football team.

The researchers analysed 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, travelling at 240 km/h. As a result, VTOLs are most energy efficient on long trips, when the cruise phase dominates the total flight miles.

But for shorter trips — anything less than 35 km — single-occupant internal-combustion-engine vehicles used less energy and produced fewer greenhouse gas emissions than single-occupant VTOLs. That's an important consideration because the average ground-based vehicle commute is only about 17 km.

"As a result, the trips where VTOLs are more sustainable than gasoline cars only make up a small fraction of total annual vehicle-miles travelled on the ground," said study co-author Jim Gawron, 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."

Not surprisingly, 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% relative to ground-based vehicles.

"Electrification of aircraft, in general, is expected to fundamentally change the aerospace industry in the near future," Furbush said.

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.

Image caption: Artistic rendering of an electric vertical takeoff and landing taxi cruising through an urban centre. Credit: Dave Brenner/University of Michigan School for Environment and Sustainability.