City Creek Nature Notes – Salt Lake City

November 3, 2016

November 3rd

Helicopter Seeds

5:00 p.m. After a major storm on October 31st that involved wind gusts up to thirty-five miles per hour, today parts of the road are still covered in the lobe shaped seeds of Boxelder trees. The seeds are about three-sixteenths (4mm) in diameter, but are attached at the end of a half-inch “wing”. Overall, the seed and wing give the impression of a musical note. The seeds hang in symmetrical pairs on a catkin, a collection of about twenty seeds. Along the road in the first mile, there are many of Boxelders up to fifty feet in height that are covered in catkins. I walk up to one to pull a couple of seeds off, and each seed is surprisingly still strongly attached to its catkin, even though the tree is leafless. I can see why it took thirty-five mile per hour winds to dislodge the seeds that are on the road. I raise one above my head let it go. It starts to rotate quickly and like its cousin, the maple seed, it “helicopters” down to the ground. Testing several Boxelder seeds, they travel an average trajectory at about thirty degrees from vertical. As any middle or high school geometry student can tell you using a 30-60-90 degree right triangle, this means that a seed released from the top of a fifty foot tree might travel twenty-five feet horizontally from its parent tree. This is just enough to land outside the canopy of the mother tree.

During a wind gusts on October 27th, as I jogging past Boxelders near picnic site 6, a few of these “helicopters” would dislodge and float down will a light rain. One was freed from the highest branch by a first gust of wind, and as it floated to eye level, a second wind gust blew through. With that burst of wind, the helicoptering seeds stopped in mid-air and rose slightly, but as the gust diminished, it resumed its descent to the road.

Until relatively recently, the aerodynamics of these flying seeds was a mystery. Using the same principles of flight that govern birds and jetliners, the seeds should technically not float or “helicopter” slowly to the ground. The seeds should drop like a stone. Solving that mystery also explained other instances of creatures that should not fly and should not be able to hover, including several found in the canyon, i.e. – bees, dragonflies and hummingbirds.

In 1991, Lentink at Wageningen University of the Netherlands, Dickson and their colleagues determined that helicoptering maple seeds had a different mechanism of flight than that used by bird or man. As the seed helicopters, the leading edge of the seed’s wing generates a small, horizontal tubular vortex over the wing. This generates a low-pressure vacuum that lifts or sucks the seed upward. Unlike a bird, the wing has no familiar aerodynamic lifting shape. In normal flight like that of a bird or airplane, a smooth laminar flow over a wing’s special shape, similarly generates low-pressure above the wing, and the relatively higher pressure under the wing then lifts the wing and plane or bird into the sky. These horizontal vortices are called leading edge vortices or LEVs.

You may have seen analogous vortices when using a paddle in the water, when moving your arms while standing a pool, or when a plane lands through fog. Horizontal vortices form off the tips of paddles, your arms, or the tips of an airplane’s wing. In the case of the seed, a spinning vortex forms over the entire length of the wing’s flat surface.

In 1996, Ellington of the Vrije University in the Netherlands and his colleagues extended this concept to explain how many insects, like bees, moths and butterflies, can fly when aerodynamically, they should be unable to do so. They found the beating wings of moths generating the same leading edge vortices seen in helicoptering maple seeds. In 2000, Z. Jane Wang at New York University modelled flapping insects wing and noted that for some insects, two counter-rotating vortices are formed. One is a higher pressure vortex under the wing and it pushes up, and the second is a lower pressure vortex that “sucks” the insect up. In 2001, Lauder at the Harvard University built mechanical insect wings in order to better model the leading edge vortices. In 2004, Adrian Thomas at the Smithsonian Tropical Research Institute and colleagues studied dragonflies tethered inside wind tunnels, and they imaged the counter-rotating leading edge vortices (id. Fig. 11). In 2011, Liang and colleagues at Purdue University built mechanical wings with rigid veins similar to those seen in both Boxelder seeds and dragonfly wings, and they found that the ridge veins increased flight performance.

Wasps, bees and dragonflies have a different number of wings. Wasps only have two wings; dragonflies and bees have four. Bees have smaller fore-wings that lock into the larger back wings to form a single wing surface during normal flight. Only the dragonfly has two sets of independently moving wings, and only it moves the wings out-of-phase: while one wings goes up, the other flaps down. The dragonfly can rotate the angle of attack for each wing independently. In 2008, Z.J. Wang noted that the out-of-phase beating gives the dragonfly additional-enhanced lift. These results of Ellington, Lauder, Wang and Thomas give a clearer picture of how the dragonflies seen in the canyon hover and do their amazing acrobatic maneuvers (August 11th).

In 2005, Warrick at the University of Oregon and colleagues showed how hummingbirds also use leading edge vortices to feed while hovering in front of flowers.

In conclusion, the canyon currently hosts many examples of where nature has solved the problem of flight and hovering using leading edge vorticies instead of a bird’s flapping aerofoils or man’s propellers: Boxelder seeds, maple seeds, Variegated Meadowhawk dragonflies, red-rumped central bumble bees, Bald-faced hornets, Black-chinned hummingbirds, and several moths, butterflies and other flying insects. The first dragonflies, the massive Protodonata with 30 inch wingspans, appeared in the fossil record 325 million years ago. Flowering trees first began to dominate forests in the Cretaceous period beginning 145 million years ago, and they co-evolved with bees. Hummingbirds appeared 22 million years ago (McGuire et al. 2014).

Today in the canyon, even though the Boxelders where hammered by the strong winds, only a small fraction of their catkins were dislodged. Most Boxelders are still thick with seeds, and I can still look forward to more future showers of helicoptering seeds on windy days.

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