June 16, 2009 | David F. Coppedge

Plants Use the Perfect Propeller

What kid hasn’t played with maple seeds to watch them spin in the air like helicopters?  Scientists watch them, too.  A team from the Netherlands and California found out how they stay in the air for so long without engines to drive them.  One would think in an era of advanced aeronautical engineering the physics would all be worked out, but the abstract explains that the seeds know more than the engineers do:

As they descend, the autorotating seeds of maples and some other trees generate unexpectedly high lift, but how they attain this elevated performance is unknown.  To elucidate the mechanisms responsible, we measured the three-dimensional flow around dynamically scaled models of maple and hornbeam seeds.  Our results indicate that these seeds attain high lift by generating a stable leading-edge vortex (LEV) as they descend.  The compact LEV, which we verified on real specimens, allows maple seeds to remain in the air more effectively than do a variety of nonautorotating seeds. 

The trees involved often grow in nutrient-poor environments, so their seeds rely on wind, updrafts and turbulent gusts to spread out.  They can sometimes fly several kilometers away from the parent tree.
    The team studied three species of maple and one species of hornbeam, because “The seeds from all four of these species are known to generate high lift.”  The seeds start rotating within a meter of detachment.  “They autorotate because the heavy nut, and hence the center of gravity, is located at the base of the wing-shaped seed, they said, emphasizing again how this is partly mysterious: “The stable autorotation of maple and other rotary seeds depends on an interplay between their three-dimensional (3D) inertial and aerodynamic properties, which is not fully understood.”
    Measurements of the seeds in wind tunnels showed that the leading-edge vortex (LEV) is attached at the 25% span location where the sectional lift coefficient is the greatest.  “The stable attachment of the LEV is noteworthy, given that the local angles of attack are well beyond the stall point for conventional aircraft wings and helicopter blades.”  They explain how the stalling that endangers pilots is overcome by the seeds:

Like autorotating seeds, insect wings generate very high lift despite operating at angles of attack well above those that will stall conventional aircraft wings and helicopter blades (11) (Fig. 1D).  Instead of stalling, insect wings generate a prominent leading-edge vortex (LEV), which is known to be responsible for elevating both lift and drag.  Building upon these observations, we hypothesized that autorotating seeds create a LEV that enables them to generate high lift at high angles of attack during their descent.

The authors believe aircraft engineers should go back to the drawing board.  “The enhanced aerodynamic performance of autorotating seeds could inspire the design of more effective autorotating vehicles.”
    As to how this phenomenon arose, the scientists noted that a similar LEV lift-generating principle is employed in the wings of bats, birds, insects and seeds.  To them, this suggested “that the use of LEVs represents a convergent aerodynamic solution in the evolution of flight performance in both animals and plants.”
    For more on Dickinson’s fascinating research on biological flight design, see the 11/20/2006 entry, and especially the 12/08/2003 entry.

1.  Lentink, Dickson, Leeuwen, and Dickinson, “Leading-Edge Vortices Elevate Lift of Autorotating Plant Seeds,” Science, 12 June 2009: Vol. 324. no. 5933, pp. 1438-1440, DOI: 10.1126/science.1174196.

Everything about this article was wonderful except the evolutionary storytelling, which incited wonder of a different kind (see Brett Miller’s treatment of the Convergence Concoction).  Convergence is not an answer; it’s a sidestep.  What should have falsified evolution is turned into a support for it by the hocus-pocus of employing a miracle word.  Aircraft engineers don’t need miracles, they need real-world physics, inspired by nature’s designs.  Keep the materialistic metaphysics out of it.
    A great film on seed dispersal, including stunning images of autorotating seeds, is the Moody Science production Journey of Life.  Take your kids on a walk in the woods where maples grow and play with the seeds.  Make it a teachable moment.  Older kids might want to do a school science project on autorotating seeds.

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