The electric, battery-powered vertical takeoff and landing aircraft, also known as VTOLs, or, if you’re trying to have a little fun with it — flying taxis — may use less energy than conventional electric passenger cars, according to new research. In one of the first efforts to assess so-called flying taxis and their potential impact on global emissions, the new research also suggests that this technology will only be optimal for certain kinds of routes.

Those routes likely won’t include your morning commute, Gregory Keoleian, a professor of chemical engineering and director of the Center for Sustainable Systems at the University of Michigan, tells Inverse.

While VTOLs — which hover like helicopters but have wings like airplanes — will probably take flight within a decade, they will likely be too energy-intensive during takeoff and liftoff to be flown over short distances.

“The hover and climb are very energy intensive,” Keoleian says. “But the cruise is very efficient, so the longer you go, the overall energy per passenger mile goes down.”

A flying car concept from the Russian startup Bartini. Bartini

In other words, while VTOLs are most likely never going to be the best way to get to and from the office — nationwide, the average commute distance is between five and 12 miles, according to data from the Brookings Institution — they might some day be a fixture of weekend and business travel.

Keoleian and his team, which included a number of researchers from Ford Motor Company, estimate that the break-even point between VTOLs and ground-based electric cars will be about 60 miles. Their findings were published Tuesday in the journal Nature Communications.

There’s another caveat, too: VTOLs have to be “fully loaded,” that is, with at least three passengers and a pilot to become more energy efficient than ground transport. In their comparison models, researchers used the national average US occupancy for a passenger car, about 1.5 occupants per car.

“The niche for deployment is where you have a fully loaded VTOL, and then going greater than 60 miles,” Keoleian says. “They’re advantageous where you have congestion. So if you’re in a heavily congested area, where an electric car would just sit there idling, [a VTOL] is going to just cruise right over that.

“The other situation where a VTOL outperforms is where you’re geographically constrained, so, for example, if you have a lake.”

Uber has big plans for vertiports and less plans for a flying car. Uber

It’s an important observation, because future transportation technologies will have to be designed with sustainability in mind. Some researchers fear that if mobility technology improves too much without the proper protocols, the benefits will be eaten up by people using said technology to transport themselves significantly farther than they would have otherwise. (Transportation researchers called this the “Rebound Effect.”)

Some research has already shown self-driving cars able to one day reduce global emissions because they could enable ride-sharing along much more efficient, crash-less routes. In practice, these technologies could contribute to city-wide gridlock. That’s because without the proper regulations, self-driving cars would be incentivized to elude parking fees and restrictions, and wind up roaming around the streets indefinitely.

“If you make travel too convenient, people are going to live farther from work, and that would be a disaster,” Keoleian says. “We do not want to increase the vehicle miles traveled.”

The main hope for this research, then, is that it will influence the design of future prototypes, which, right now, run the gamut. Boeing recently conducted a successful test of a flying taxi prototype, which aligns closely with the VTOL paradigm that Keoleian and his colleagues examined. (You can check it out in the video below.)

Other prototypes are radically different. For example, the recently unveiled PAL-V Liberty Pioneer, a gyroplane that can drive along roads but only seats two, or the concept developed by Audi and Airbus, which would hook into a hyperloop system like those envisioned by Elon Musk’s Boring Company or Virgin Hyperloop One.

Finally, the findings call to question one of the main ways that flying taxis have taken life in the public imagination; a way to get above increasingly crowded cities with skylines that stretch ever higher.

In these densely packed future megalopolises, Keoleian thinks people will likely be better off getting around using subterranean transit lines and conventional rail.

Abstract

Interest and investment in electric vertical takeoff and landing aircraft (VTOLs), commonly known as flying cars, have grown significantly. However, their sustainability implications are unclear. We report a physics-based analysis of primary energy and greenhouse gas (GHG) emissions of VTOLs vs. ground-based cars. Tilt-rotor/duct/wing VTOLs are efficient when cruising but consume substantial energy for takeoff and climb; hence, their burdens depend critically on trip distance. For our base case, traveling 100 km (point-to-point) with one pilot in a VTOL results in well-to-wing/wheel GHG emissions that are 35% lower but 28% higher than a one-occupant internal combustion engine vehicle (ICEV) and battery electric vehicle (BEV), respectively. Comparing fully loaded VTOLs (three passengers) with ground-based cars with an average occupancy of 1.54, VTOL GHG emissions per passenger-kilometer are 52% lower than ICEVs and 6% lower than BEVs. VTOLs offer fast, predictable transportation and could have a niche role in sustainable mobility.