Why do maple seeds spin as they fall?
― Spinning turns a small wing into a "big circle"
In autumn, winged seeds spin as they drift down beneath maple trees. That spin is no decoration. By spinning, the seed grabs far more air than its wing alone would, which greatly cuts its falling speed. And every extra second aloft is another second the wind can carry it further.
You're walking through an autumn park, under a maple just starting to turn red. Suddenly something small spins past in front of you, falling.
You pick it up: a hard seed on one side, a thin wing on the other. It looks like a single wing torn off a dragonfly.
You lift it above your head and let go. For an instant it falls straight down, then it starts to spin. And like a tiny helicopter, it drifts gently to the ground.
Only two reasons it can fall so slowly
The wing is just one long thin blade, but once it spins, its tip traces out a whole circle that catches the air. That area is many times wider than the still wing, pushing air downward over a much bigger span as it falls.
A small swirl of air forms at the front edge of the spinning wing. Because this vortex stays put on top of the wing, the wing pulls in air far more strongly than an ordinary aircraft wing would.
Together, these two effects let the seed fall at a gentle speed of around 1 metre per second — slower than a person walking.
Spinning turns a small wing into a big circle
Anything falling feels air resistance. Resistance grows with the area pushing against the air. It also grows stronger the faster something falls.
Eventually, downward weight and upward resistance balance out, and the fall stops speeding up. This "settled speed" is slower the wider the area.
A maple seed's wing is a long thin blade, just a few cm long and about 1 cm wide. But once it starts spinning, its tip traces a circle. To the air, the whole circle the wing sweeps through acts like a single round disc pushing air aside. If the wing is 4 cm long, that circle's area is more than ten times the wing's own area.
Figure 1 shows how the falling speed changes with the size of that circle. Try changing the wing length with the slider. The longer the wing, the wider the circle, and the slower the fall. More time falling from a high branch means more time for the wind to carry it, so it lands further away.
A vortex clings to the wing's front edge
If catching air over a wide disc were the whole story, a paper disc would fall just as slowly. But maple seeds have one more trick.
A spinning wing travels through the air at a fairly steep angle. On an ordinary aircraft wing, too steep an angle causes the air to peel away from the top surface, and the lifting force is suddenly lost. This is called a "stall".
But on a maple seed's wing, the vortex that curls up at the front edge doesn't peel away — it stays riding on top of the wing as it keeps spinning. Air pressure is low inside the vortex, so the wing is pulled upward strongly. Compare the left and right sides of Figure 2.
This mechanism was confirmed in 2009 by a joint Dutch–American research team. They used an upward-blowing air rig to levitate real seeds, and also studied the airflow using a robotic model wing. They found that the vortex at the leading edge accounts for a large share of the force holding the seed up.
This is, in fact, the same kind of mechanism that small insects like bees and flies use when they beat their wings. Animals move their wings with muscle; maples spin their wing using only the momentum of falling. Two completely different kinds of living things have arrived at the same trick for using air.
A maple seed has its heavy part attached to one end, at the base of the wing. Because the weight is off to one side, as soon as it starts to fall the seed-end tips downward, and the wing meets the air at a slight tilt. That's why, no matter which way up you release it, it starts spinning on its own within a short distance.
If a helicopter's engine stops, it doesn't drop straight away. Pilots can keep the rotor turning using the air flowing up through it from below, and use that spin to slow the descent. This is called "autorotation". A maple seed does this every single day.
Summary
By spinning, a maple seed catches air over a circle many times wider than its wing. On top of that, the vortex clinging to the wing's front edge creates a strong force that resists stalling even at a steep angle. So the seed falls slowly, and while it falls the wind carries it far from the parent tree. Right beneath the parent tree is shade, a poor place for a young sapling to grow. Spinning as it falls is a trick for sending its offspring off on a journey.
A spinning wing is a tool for a small body to grab a wide span of air.
The longer it stays aloft, the further the wind can carry it.
The mechanism by which a falling object's speed settles to a constant value is the same one behind why raindrops don't hurt even though they fall from high in the sky. How leaves change colour in autumn is explained in our article on autumn leaves.
- In an autumn park, pick up two or three winged maple seeds (only pick up ones already on the ground — don't take them from the branch).
- Release one from about head height and count the seconds until it hits the ground. Then cut the same seed's wing in half with scissors and drop it again.
- Finally, drop a seed with the wing cut off entirely. Compare how each one spins and how long each takes to fall, across all three.
The shorter the wing, the faster it should fall, and a seed with no wing should simply drop straight down. If it's the wrong season to find seeds, you can mimic the shape by clipping a paperclip to one end of a thin strip of paper.
Want to know more? ― Terms, formulas, and links to textbooksWe mark which level each part belongs to, from middle-school science to university specialist courses
- M.S.Covered in middle-school science
- H.S.Covered in high-school "Physics Basics / Physics"
- H.S.+High-school advanced material, or textbook sidebar content
- Univ.Content from university specialist courses not covered in high school (fluid dynamics, aeronautical engineering)
- ResearchNot yet taught as settled fact even at university — something researchers are still investigating
M.S.Terms: this phenomenon has a name
- Samara: a fruit with a thin, wing-like part attached to the seed, as in maples or ashes. It gets carried and scattered by the wind.
- Terminal velocity: the speed at which the weight of a falling object balances air resistance, so it stops accelerating.
- Autorotation: a state where the rotor keeps turning purely from the force of air passing through it, with no engine power.
M.S.H.S.Check with a formula: how fast does the seed fall, and how far is it carried?
Treating the circle traced by the spinning wing as a single disc pushing the air aside, we can estimate the settled falling speed (terminal velocity). Here's what each symbol means.
| Symbol | Meaning and unit |
| v | Falling speed (m/s) |
| m | Mass of the seed (kg) |
| g | Gravitational acceleration (m/s²) |
| ρ | Density of air (kg/m³) |
| C | Drag coefficient (no unit) |
| A | Area of the circle traced by the spinning wing (m²) |
| In symbols | v ＝ √( 2 × m × g ÷ ( ρ × C × A ) ) |
| In words | Falling speed ＝ the square root of ("2 × the seed's weight" divided by "air density × drag coefficient × circle area") |
| Where it comes from | Solving for v in the balance between downward gravity, m × g, and upward air resistance, ½ × ρ × C × A × v² |
| Mass of one seed (typical for a larger maple seed) | 0.1 g (0.0001 kg) |
| Area of the circle traced by the spinning wing (radius 4 cm) | About 0.005 m² |
| Density of air | 1.2 kg/m³ |
| Drag coefficient (treating it as a flat disc) | About 1 |
| Gravitational acceleration | 9.8 m/s² |
| 2 × mass | 2 × 0.0001 ＝ 0.0002 |
| Convert to weight (× gravitational acceleration) | 0.0002 × 9.8 ＝ 0.00196 |
| Air density × circle area (coefficient is 1) | 1.2 × 0.005 ＝ 0.006 |
| Divide | 0.00196 ÷ 0.006 ≒ 0.33 |
| Take the square root = falling speed | The square root of 0.33 is about 0.57, so about 0.57 m/s |
| Time to fall from a height of 10 m | 10 ÷ 0.57 ≒ 17.5 (seconds) |
| Distance carried by a 3 m/s wind | 17.5 × 3 ＝ 52.5 (m) |
By this estimate, the seed falls at just under 0.6 m/s, and from a 10 m branch it can be carried about 50 m. Measured falling speeds are actually around 1 m/s, a bit faster than this estimate of treating the whole swept circle as a disc, but the order of magnitude matches. Even at 1 m/s, it still takes 10 seconds to fall, carried about 30 m by the wind. By contrast, a heavy fruit with no wing falling 10 m with no air resistance takes only about 1.4 seconds, from the square root of 2 × 10 ÷ 9.8.
H.S.H.S.+Air resistance and the force on a spinning wing
H.S.In Physics Basics, you learn that air resistance grows with speed, and once it balances gravity, the object settles to a constant speed (terminal velocity). The formula above is the terminal velocity when resistance is proportional to the square of the speed. If the circle's area quadruples, the falling speed is halved.
H.S.+Each part of the spinning wing meets a "slanted wind" made up of the downward wind from falling and the sideways wind from rotation. The wing tip moves faster as it spins, so it meets a stronger wind. Because the seed's weight sits off to one side, the wing naturally settles into a stable state where spinning and falling balance out.
Univ.The leading-edge vortex and rotor theory
The vortex that forms at the front edge of the spinning wing is called a "leading-edge vortex". Because this vortex stays stable on top of the wing even at a steep angle of attack, the wing gets strong lift without stalling. In helicopter rotor theory, momentum theory treats the circle the rotor sweeps as an "actuator disc" to relate the descent speed to the circle's area. Proposed reasons the leading-edge vortex stays stable while spinning include an effect where flow moving from the wing's root toward its tip carries the vortex's momentum away outward.
📖 For the derivation of the formulas and further reading: Autorotation (Japanese Wikipedia) ／ Terminal velocity (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How far do seeds actually travel in a real forest? Wind inside a forest is turbulent, and some seeds caught in an updraft are said to be carried far further than the estimate. These rare "long-distance journeys" are hard to measure, and they're still not well understood.
- What effect do shape differences between species have? Even among maple species, wing length and the balance of weight differ. Which shape is advantageous in which environment is still being studied.
- Small flying devices that mimic the seed. Research is underway on small sensors, dropped from the air, that mimic the shape of a spinning falling seed. A detailed understanding of the forces on a small wing is still developing.
In other words, this article too describes things "as currently understood". Exactly why the leading-edge vortex is so stable is still being studied in detail.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| M.S. | Science Year 1 "Plant structure" / Science Year 3 "Balance of forces" | Samara, the balance of weight and air force |
| H.S. | Physics Basics "Air resistance and terminal velocity" | Check with a formula |
| H.S.+ | Physics, advanced "Circular motion" | The slanted wind the spinning wing meets |
| Univ. | Fluid dynamics / aeronautical engineering | Leading-edge vortex, actuator-disc momentum theory |
| Research | Seed-dispersal ecology / bio-inspired engineering | Long-distance dispersal, small flying devices mimicking seeds |
| ― | Everyday connections | Observing an autumn park, emergency helicopter landings |
- Lentink, D., Dickson, W. B., van Leeuwen, J. L., Dickinson, M. H. (2009) Leading-Edge Vortices Elevate Lift of Autorotating Plant Seeds. Science 324, 1438–1440.
- Azuma, A., Yasuda, K. (1989) Flight performance of rotary seeds. Journal of Theoretical Biology 138, 23–53.
- Terminal velocity - Wikipedia (Japanese) (終端速度)
- Autorotation - Wikipedia (Japanese) (オートローテーション)
※This article is a general-audience science explainer. The figures given are rough estimates meant to aid understanding of the mechanism, and vary greatly with the kind and size of seed and the wind conditions.