Gyroscopic stabilization minimizes drag on Ruellia ciliatiflora seeds

Fruits of Ruellia ciliatiflora (Acanthaceae) explosively launch small (2.5mm diameter x 0.46mm thick), disc-shaped seeds at velocities over 15 m s(-1), reaching distances of up to 7m. Through high-speed video analysis, we observe that seeds fly with extraordinary backspin of up to 1660Hz. By modelling the seeds as spinning discs, we show that flying with backspin is stable against gyroscopic precession. This stable backspin orientation minimizes the frontal area during flight, decreasing drag force on the seeds and thus increasing dispersal distance. From high-speed video of the seeds' flight, we experimentally determine drag forces that are 40% less than those calculated for a sphere of the same volume and density. This reduces the energy costs for seed dispersal by up to a factor of five.