Spinning in the rain: Maple samaras recover autorotation after drop impact

Samaras are known for their elegant and robust autorotation, a resilience that persists in the adverse conditions imposed by high-speed raindrops. Like flying insects, descending samaras are not unlikely to be struck by raindrops in an intense storm. In this study, we detail the impact dynamics for impact regions across the samara body and drop-shedding mechanisms that samaras exhibit to return to autorotation. The impact force in different impact locations can pitch the samara up to 60 degrees and, in some cases, induce spanwise roll. Raindrops may shatter or remain intact upon impact, pushing the undamaged samara downward 5-25 wingspan lengths before autorotation is recovered. Drops that strike near the wing tip elicit the greatest recovery distance, while impacts onto the nutlet mass are the least disruptive to the samara and most likely to cause the drop to induce splashing. Our results indicate that samaras are robust to raindrop impacts and, given sufficient height above the ground at impact, always recover autorotation. In so doing, a combination of water repellency, aerodynamic drag on the drop, and centrifugal forces ensure the entire drop is shed from the spinning seed.