Research on eVTOL aircraft takes flight

The Urban Air Mobility (UAM) aircraft stands to transform transportation in metropolitan areas, displacing helicopters and terrestrial vehicles. Investments in the UAM market are in the billions, with billions more anticipated as companies compete to publicize their services.

“The goal of bringing flying cars into operation has been an aspiration of humanity for at least a few decades,” said Shashank Sripad, a Ph.D. student working with Venkat Viswanathan, an associate professor, both of mechanical engineering. Their work, recently published in the Proceedings of the National Academy of Sciences (PNAS), explored the potential of rechargeable batteries in electric vertical take-off and landing (eVTOL) aircrafts. “Now with the advances in batteries and electric propulsion, we’re actually able to make these ‘flying cars’ operable. The key takeaway here is how they fit into the larger goals of decarbonization and sustainable mobility.”

Possibly achieving a higher energy efficiency in eVTOL aircraft in comparison to terrestrial alternatives, along with faster travel times, holds immense implications for emissions and environmental sustainability. In urban spaces, air pollution—cars contributing about one third—and traffic congestion are two of the biggest problems. Electrified UAM provides a plausible alleviation of these hardships.

The underlying motivation is decarbonization. The other target of this market is to alleviate congestion and lower urban pollution.

Shashank Sripad, Ph.D. student, Mechanical Engineering

“While decarbonizing transportation remains the underlying motivation,” said Sripad, “the other target of this market is to alleviate congestion and lower urban pollution.”

Prior to Viswanathan and Sripad’s research, it was largely believed that these aircrafts could not be electrified with current batteries. What they found is that the technology readiness level of batteries is actually sufficient for meeting the demands of UAM aircrafts.

“Generally, you expect that it would take a lot of energy to operate an aircraft that takes off vertically, flies through the air, and lands somewhere else,” Sripad explained. “However, if the aircraft is designed well—especially with fixed wings—the air will lift the craft, making it more efficient and less energy dependent.” The work also highlights the importance of power and energy available in the battery pack, while noting that battery lifetime performance and charging infrastructure remain open questions.

The world may start using these findings right away. Sripad emphasizes that their work was largely aimed at trying to inform the public and investing community on considering new variables that make electrified UAM a concept of tomorrow, rather than the distant future.

There is still work to be done. Sripad says that the next steps are to study new materials and systems that can be used to optimize batteries for eVTOL applications, as well as estimating the operational cost of such systems.

Since this paper was published, CMU researchers have worked with multiple companies to update their battery chart. Last year, Beta Technologies updated the Alia-250 aircraft to be a 6,999 pound bird instead of the previous 6,000 pound used in the study. They clarified that the max payload case (5+1 PAX) will be for a lower range mission while the max range of 250 nmi will be reserved for lighter missions. With these updated range numbers, Alia-250 battery packs are now within the prototype/novel region. Most recently, Lilium's battery system requirements for the Lilium Jet have been placed at under 230 Wh/kg (pack) and 2.3 kW/kg (pack_of specific energy and power). The operational range of the Lilium Jet was updated to 175 km/109 mi (previously 172 mi), including other considerations about the jet architecture.