Identifying urban instability conditions for flying cars

Flying cars are a staple of many sci-fi visions of the future, but in the 21st century they are finally nearing reality. Advances in battery and electric propulsion technology have led to the creation of small, lightweight electric vertical take-off and landing (eVTOL) aircraft, many of which are in development, but questions remain about their performance in realistic urban conditions.

He et al. assessed the flyability and stability of eVTOL ducted fans — a common propulsion system for this type of aircraft — in urban environments.

Cities produce some of the most complex atmospheric conditions for small aircraft. The buildings form urban canyons that magnify and direct winds in unpredictable ways. The increased wind speed introduces additional turbulence, and the range of surfaces, from hot pavement to shaded parks and cool lakes and rivers, leads to challenging thermal environments.

Given these challenges, the authors set out to build a model that could predict the conditions that would lead eVTOL ducted fans — along with the resulting aircraft — to become unstable.

“By combining meteorology-informed computational fluid dynamics with nonlinear-dynamics diagnostics, we were able not only to characterize the aerodynamic response under realistic wind shear, but also to identify the route to instability and relate it to specific flow structures,” said author Yuhang He.

Their model revealed the evolution of the thrust response from steady to periodic to chaotic. Each transition is linked to concrete flow signatures — from tip-leakage impacts and windward separation to vortex breakdown, rotating stall, and duct reverse flow.

“Our next step is to extend these efforts with more realistic experiments and broader atmospheric conditions,” said Yuhang He. “Beyond that, we hope these results can support a more interdisciplinary framework involving boundary-layer meteorology, wind engineering, urban design, architecture, and flight control, ultimately contributing to multi-level forecasting, monitoring, and operational assessment strategies for urban eVTOL systems.”

Source: “Wind-induced period-doubling bifurcation and stability loss of a ducted fan in urban boundary layer,” by Yuhang He, Jingyuan Zheng, Dianqiao Lei, Yiwei Luo, Lijuan Wang, Yuping Qian, and Yangjun Zhang, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0311765 .