The winged seeds that whirl down from maples and many other trees don’t glide or float. They turn a fall into a spin, and the spinning wing works like a helicopter rotor. That spin generates lift of its own, slowing the seed’s drop well below that of a plain seed. The seed then hangs in the air long enough for even a light breeze to carry it out of the shade of its parent tree.
What Makes a Samara Shaped for Spinning
The botanical name for one of these seeds is a samara. It’s a dry fruit with a seed attached to a papery wing, and maples, ashes, elms, sycamores and some pines all make them. In maples the seed is a fat lump at one end, and the wing thins out toward the tip. Because the weight sits near one end, the whole thing turns around a point close to the seed. The wing is stiffened by veins that fan out from the seed side.
Maples grow their samaras joined in V-shaped pairs, yet the “helicopters” you see fly singly. The pair usually splits before or during the fall, and each half then spins on its own as a single-winged seed. A pair that stays joined tends to tumble rather than spin cleanly.
One wing turns out to beat two. Early conifers produced several designs of winged seed, including double-winged ones. Double-winged seeds, whether even or lopsided, fluttered down with barely any rotation.1In Defense of Plants. The Evolution of a Helicopter — In Defense of Plants Lopsided single-winged seeds stayed in the air twice as long as any other design.2In Defense of Plants. The Evolution of a Helicopter — In Defense of Plants Not every winged seed spins. Some tropical seeds, like the giant Javan cucumber, glide like a paper plane instead.
How the Spin Creates Lift
As the wing sweeps around, air curls into a small, stable whirlpool that rides along its front edge.3PubMed. Leading-edge vortices elevate lift of autorotating plant seeds Air in the whirlpool moves fast, and fast-moving air has lower pressure than the calmer air beneath the wing. The wing is pushed from the high-pressure underside toward the low-pressure top, and that pressure difference is the lift holding the seed up. The result is that maple seeds stay airborne more effectively than a range of seeds that don’t spin.4PubMed. Leading-edge vortices elevate lift of autorotating plant seeds
A still wing in falling air stalls, and the air flow breaks away from its surface. Spinning keeps fresh air moving over the wing every turn, and the swirl stays attached instead of shedding off. The seed is always falling, just slowly, because its lift never fully cancels its weight in still air.
Lift isn’t the same along the whole wing. The lifting and slowing forces change from the seed end to the tip.5Aerospace. Sectional Leading Edge Vortex Lift and Drag Coefficients of Autorotating Samaras The tip sweeps through a much larger circle each turn than the part near the seed, so it moves through the air faster.6Aerospace. Sectional Leading Edge Vortex Lift and Drag Coefficients of Autorotating Samaras Faster airflow means a stronger swirl and more lift per bit of wing, which is why the outer part matters so much. The part near the seed moves slowly and contributes little lift, which fits with it being the thick, heavy end that mostly acts as a counterweight. These forces also shift with wind speed and with each species’ wing shape.7Aerospace. Sectional Leading Edge Vortex Lift and Drag Coefficients of Autorotating Samaras
The wing flies almost flat, tilted only about 1 to 3 degrees.8PubMed Central. Experimental study into the effects of pitch and coning angles on the flight performance of the natural samara It also rides slightly upward in a shallow cone, and a cone of 5 to 15 degrees can raise its upward push by up to about 10%.9PubMed Central. Experimental study into the effects of pitch and coning angles on the flight performance of the natural samara
How a Falling Seed Starts to Spin and Stays Steady
Right after release, the seed first tumbles end over end along its wing.10Nonlinearity. The kinematics of falling maple seeds and the initial transition to a helical motion At that moment it isn’t yet moving fast through the air, so the wing has little airflow over it and can’t generate steady lift. The heavy seed end drops faster than the wing, so the seed pitches and flips as it falls.
Then it tilts toward upright and begins circling around a vertical line.11Nonlinearity. The kinematics of falling maple seeds and the initial transition to a helical motion Early in the fall the spin axis is tilted, since it started from a tumble in a random direction. The forces from the air gradually pull that axis toward vertical. As it picks up speed, airflow over the wing builds, and that growing flow lets the wing catch the air. The wing opens into a shallow cone and the motion settles into steady spinning.12Nonlinearity. The kinematics of falling maple seeds and the initial transition to a helical motion
Once spinning, the push of the air and the outward fling of the rotation balance each other.13PubMed Central. Maple samara flight is robust to morphological perturbation and united by a classic drag model So the wing’s tilt and cone angle hold steady through the fall.14PubMed Central. Maple samara flight is robust to morphological perturbation and united by a classic drag model Seed flight holds up even when the wing’s shape is damaged or altered.15PubMed Central. Maple samara flight is robust to morphological perturbation and united by a classic drag model The balance of forces adjusts itself to the new shape, so a chipped wing still flies. A smaller wing usually just spins faster and falls somewhat quicker, though badly torn wings can fail to spin at all.
How the weight is spread decides the path you see from above.16Physics of Fluids. Mass distribution affects the circle or hypotrochoid trajectory of the maple seed A well-balanced seed traces a neat circle, while a slightly off-balance one swings in looping, flower-like patterns.17Physics of Fluids. Mass distribution affects the circle or hypotrochoid trajectory of the maple seed
Why Some Samaras Fall Faster Than Others
A samara’s falling speed tracks its weight divided by its wing area.18American Journal of Botany. THE TERMINAL VELOCITY AND DISPERSAL OF SPINNING SAMARAS Falling speed rises with the square root of that ratio, so doubling the weight raises speed by about 40%, not double.19American Journal of Botany. THE TERMINAL VELOCITY AND DISPERSAL OF SPINNING SAMARAS Lift grows with the square of the speed, so a heavier seed needs only a modest increase in speed to produce the extra lift that balances its weight.
Spinning samaras of all sizes, natural or artificial, follow the same single descent pattern once spinning steadily.20PubMed. Intrinsic equilibrium of stably autorotating samaras A better-lifting wing settles at a slower drop.21PubMed. Intrinsic equilibrium of stably autorotating samaras
You can test this in your yard. Drop several types of seed side by side from the same height. Big-winged, light seeds tend to land last, and a heavy seed on a small wing reaches the ground quickest. Wet or damaged seeds fall faster since they weigh more.
When Samaras Let Go and How Far They Travel
In silver maple, the attachment layer weakens fastest when air is dry, typically early afternoon.22Canadian Journal of Botany. Fruit abscission in <i>Acer saccharinum</i> with reference to seed dispersal Where the seed joins the branch, a thin layer of cells holds it on, and that layer shrinks as it dries. As it loosens its grip, even a modest tug from wind can snap the seed free. That’s why seeds tend to stay on in damp or humid weather and come off in a rush once the air dries out. Release rates also climb steeply as wind speed rises.23Canadian Journal of Botany. Fruit abscission in <i>Acer saccharinum</i> with reference to seed dispersal The wind threshold for release mainly affects short trips, not rare long-distance ones.24Journal of Ecology. Effects of canopy heterogeneity, seed abscission and inertia on wind‐driven dispersal kernels of tree seeds
Slow falling mostly lengthens trips over short and moderate distances. To go really far, seeds must be lifted above the treetops by updrafts.25Ecological Research. Long‐distance dispersal of tree seeds by wind Turbulent eddies up to a third of the canopy’s height can provide that lift.26Ecological Research. Long‐distance dispersal of tree seeds by wind Above the canopy, faster winds can carry them much farther.27Ecological Research. Long‐distance dispersal of tree seeds by wind
Most seeds land within a few tree heights, and breezy dry days release large bursts at once.
Landing isn’t the end of the trip. Many winged seeds keep moving afterward, blown along the ground or carried by animals, a stage still poorly studied.28PubMed Central. Secondary dispersal mechanisms of winged seeds: a review
Engineers Who Copy the Samara
Engineers at the University of Maryland built the world’s smallest controllable single-winged rotorcraft based on maple seeds.29University of Maryland. Spiraling Flight of Maple Tree Seeds Inspires New Surveillance Technology It can be dropped unpowered from a plane and steered remotely.30University of Maryland. Spiraling Flight of Maple Tree Seeds Inspires New Surveillance Technology That makes it useful for defense surveillance, fire monitoring and search and rescue.31University of Maryland. Spiraling Flight of Maple Tree Seeds Inspires New Surveillance Technology The whole body spins as the rotor, so no gearbox or second rotor is needed, and steering comes from small changes to the wing’s tilt.
You can try the idea at home with a paper samara. Cut a strip of paper with a paperclip at one end and drop it, and it spins like a maple seed. Shift the clip or trim the wing to see the falling speed change.