Precession dynamics and morphology of rolling samaras.

Abstract

Samaras of species such as <i>Fraxinus americana</i>, <i>Fraxinus excelsior</i> and <i>Liriodendron tulipifera</i> exhibit a unique dual-axis descent characterized by spanwise rolling superimposed on vertical helical precession. While prior work has established the aerodynamic mechanisms of non-rolling samaras, <i>Acer</i> spp. serving as the classic example,, the aerodynamic role of rolling motion remains largely uncharacterized. Here, we present a comparative kinematic and morphological analysis of 30 rolling samaras across three species using high-speed multi-camera imaging and digital tracking. Our results reveal that all species maintain stable autorotation by rolling approximately seven cycles for each precession cycle, inducing periodic modulation of angle of attack and sinusoidal lift generation without observable wingtip wobble. Species-specific variation in wingspan, mass and thickness generates distinct descent velocities, but across all groups wing thickness emerges as the single most predictive morphological trait. We demonstrate that descent velocity scales inversely with thickness, with additional inverse power-law relationships linking thickness to both rolling and precessional angular velocities. Analytical modelling of lift-induced torque predicts negligible vertical oscillation, confirming observations. Samaras released into a vertical wind tunnel from a static position show that the time to stable autorotation strongly depends on the initial release orientation. In our observations, rolling begins before precession, and the stochastic direction of rolling sets the precessional direction.