courtesy  :  science direct

green flying cars

Growing urban populations, land space limitations, and high infrastructure development costs present a challenge for the future of ground-based transportation. Flying cars, no longer confined to the realm of fiction, are in active development today and will see commercial deployment throughout the next decade. Recently in Nature Communications, Kasliwal et al. reported an analysis of the potential for flying cars to reduce both the travel time and carbon footprint of personal travel.

Motor vehicles powered by internal combustion engines emerged in the late 1800s, sparking a personal mobility revolution. Fast-forward to the early 21^st century and the entire globe is connected through vast route infrastructure traversed by a fleet of cars, trucks, trains, planes, and ships. Mass transportation enabled globalization of economies and brought indispensable quality-of-life benefits that support modern societal function. It was also proven to be a main driving force behind anthropogenic climate change and remains so today. Over 99% of all transportation in the U.S. is still powered by fossil fuels.¹ This sector alone accounts for over 28% of all greenhouse gas (GHG) emissions in the country, with similar trends found worldwide.²

Decarbonization of the transportation sector requires accelerated and incentivized adoption of low-carbon power sources and simultaneous technological progress in their efficiency-per-cost point. Emerging low-carbon distributed power sources include batteries,³ hydrogen fuel cells,⁴ and sustainable biofuels.⁵ Each technology has its own strengths, market opportunities, and deployment challenges.

The transportation sector is not only working to decarbonize its power sources; it is also evolving toward new organizational paradigms. Automobile and software stakeholders, such as Lyft, Tesla, Amazon, and many others, are making large investments in autonomous driving. The opportunity for reducing GHG emissions on the road from utilizing smart autonomous vehicles is significant, with projections of up to 60% fuel savings for a fully automated car fleet in a model urban city.⁶

A growing proportion of society is living in highly urbanized environments. Population models predict that 68% of humans on Earth will be living in cities by 2050.⁷ With growing urban populations, it will be insufficient to control congestion through intelligent automobile fleet management alone. A recent McKinsey report predicts that the world needs to spend $18.7 trillion dollars between 2016 and 2030 on roads, ports, airports, and railways to keep up with current projected growth.⁸ As cities run out of road space and cost-effective tunnel routes, at some point they will need to start utilizing aerospace for personal transport. Since it is not exactly practical to build airplane runways on every corner, realizing this would require vertical takeoff and landing aircrafts (VTOLs).

It is every car commuter’s dream to takeoff and soar to work in an aircraft on a direct path overlooking the underlying traffic jam during rush hour. Taking personal small VTOLs for short-to-medium distance travel may sound like a luxury, but, due to recent technological progress, there is a growing societal and economic argument for it. It is not the case that a fleet of extraordinarily loud helicopters will fly thousands of passengers around a city for personal travel. Electric distributed propulsion VTOL prototypes such as the NASA GL-10 are quieter and more fuel efficient than propeller-based helicopters.

With recent progress in VTOL design and performance, there is renewed interest in applications beyond existing niches such as specialized military use and firefighting. VTOLs envisioned for personal air transport are more commonly known by their romanticized moniker—flying cars. If there is some economic incentive to commercialize modern VTOLs, then market forces will do just that. If there are financial incentives and government directives (for example, to exploit secondary environmental benefits), then there is potential for real disruption in the transport sector. Before such a case can be made, it is imperative to project the environmental impact of electric VTOLs.

Depending on their application, there would be scenarios where flying cars would be less energy efficient than normal cars—for example, taking a VTOL from home to the nearby grocery store on a Sunday as a solo passenger. Yet, if a fleet of flying cars were shuttling a small number of commuters traveling non-trivial distances to work during highly congested rush hour, there could be a net reduction on carbon emissions. Recently in Nature Communications, Kasliwal and colleagues evaluated the sustainability potential of electric VTOLs in comparison with conventional ground-based cars using internal combustion or electric engines.⁹

Electric VTOLs have many obvious benefits over ground transportation. They enable near point-to-point travel at cruise speeds that far exceed highway speed limits. A prototypical VTOL operating within reasonable safety limits is capable of climb rates beyond 5 m/s, cruise altitudes of over 300 m, and cruise speeds exceeding 240 km/h (Figure 1). This could save a commuter over 83% of their travel time on a 100 km trip.⁹ To evaluate the carbon emissions of flying cars such as this, Kasliwal and co-authors take existing technology specifications and examine the GHG emissions for various trip lengths and number of aircraft occupants compared to reference cases for combustion and electric engine vehicles.

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Figure 1. Schematic Depicting Takeoff, Cruise, Hover, and Landing Modes in VTOLs

Approximate speed and altitude values for a prototypical VTOL for urban transport taken from Kasliwal et al.⁹

It was concluded that, in a highly conservative comparison of 1 occupant per vehicle on a 100 km point-to-point trip, a VTOL has a 35% reduction in GHG emissions compared to internal combustion engine vehicles (ICEVs) but a 28% increase compared to battery electric vehicles (BEVs). According to a recent U.S. National Transportation survey, the average occupancy of a vehicle on the road is 1.54 persons/car.¹⁰ The market entry point for VTOLs is unlikely to be for individual transportation; rather, companies like Uber are investigating its viability as a pooled taxi service. A more realistic base-case comparison, therefore, would be to evaluate the GHG emissions for a car occupancy of 1.54 and a VTOL occupancy of, for example, 4. In this case, VTOLs reduce GHG emissions by 52% compared to an ICEV and 6% relative to a BEV.