How does the number of wings on a seed affect spin as it falls?

Tristellateia australasiae, or “maiden’s jealousy,” is a species of flowering vine found in southeast Asia. The bright yellow flowers produce star-like seeds that rotate as they fall in the wind, similar to helicopter propellers.

The seeds can vary in number of wings, so Liu et al. set out to study the effect of wing number on rotation and speed.

“We wanted to investigate whether Tristellateia seeds could still rotate after some wings were clipped,” said author Lifang Zeng. “If so, how would the wing number affect the autorotation of the Tristellateia seeds when they fell?”

The researchers created a model to study these differences and found that three kinds of vortexes occurred while rotating, based on wing number.

“A stable separated vortex ring dominates the stable autorotation of Tristellateia seeds,” Zeng said. “However, an unstable separated vortex emerges on the seeds with fewer wings, which makes achieving stability with these seeds difficult.”

They also found that seeds with fewer wings have larger wing loadings, velocities and spin rates, meaning that they have more unstable characteristics.

Not all seeds have uniformly distributed wings, but when considering the engineering applications of the research, they used a symmetrical model.

However, numerical simulations of their model were consistent with free-fall experiments and the gravity of natural seeds.

Zeng hopes their work will inform aircraft design based on biological systems.

“We hope that scientists will understand the effect of the wing number on the flight performance of wind-borne seeds, which will help them in the design of biomimetic aircraft,” Zeng said.

Source: “Effects of wings number on the flight performance of autorotating seeds,” by Chenxi Liu, Lifang Zeng, Jun Li, and Xueming Shao, Physics of Fluids (2024). The article can be accessed at https://doi.org/10.1063/5.0244050 .