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 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.

Vertical takeoff and landing like a helicopter

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 recently in 'Nature Communications'. Several companies around the world 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 lift-sharing taxi service, according to study authors from the University of Michigan's Center for Sustainable Systems and from Ford Motor Company. "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," he says.

Energy use, greenhouse gas emissions and time savings

The U-M-Ford study, the first comprehensive sustainability assessment of VTOLs, 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 100km, 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 per cent lower than gasoline vehicles and six per cent 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, says the findings can help guide the sustainable deployment of an emerging mobility system prior to its commercialisation.

'Incentivised to share trips'

"With these VTOLs, there is an opportunity to mutually align the sustainability and business cases," says Kasliwal. "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 startup 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.

Lift-to-drag ratio and battery-specific energy

"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," says Furbush, who is also a member of the U-M football team. The study began while Kasliwal and Furbush were summer interns at Ford. The work continued when the students returned to Ann Arbor, with the help of a Ford-University of Michigan Alliance grant. 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 150mph (240km/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 35km (22 miles) - 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 17km (11 miles).

'More sustainable than gasoline cars'

"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," says 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 100km much faster than ground-based vehicles. A point-to-point VTOL flight path, coupled with higher speeds, resulted in time savings of about 80 per cent 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 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.