[Pollinator] Physics of flying keeps insects as busy as a bee while in the air

[Ladadams at aol.com]

Physics of flying keeps insects as busy as a bee while in the air
Keay Davidson, Chronicle Science Writer
Monday, November 28, 2005

  
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Insects were the world's first aviators, and to this day their evolutionary 
descendants perform aerial stunts more dashing than the Blue Angels: They zip 
past your eyes like meteors, then hover like helicopters over flowers, then 
vanish out of sight before you can swat them. 
Scientifically speaking, insect flight was shrouded in mystery for much of 
the 20th century and even now is haunted by enigmas. 
Studies have shown how insects fly by frantically flapping their wings and 
taking advantage of physical forces too microscopic to be exploited by 
airplanes. Now scientists are beginning to investigate how insects' brains, although 
extremely tiny, can manage the incredibly complex motions required for them to 
stay aloft. 
Traditionally, scientists assumed that the basic physics of insect flight 
resembled the basic physics of human aviation. 
For example, there's an urban legend that many decades ago, scientists 
analyzed the plump bodies and stubby wings of bumblebees and concluded they were too 
heavy to fly. Over the years, during repeated retellings of this story in 
schoolyards and barrooms, it acquired a punch line: "But bees don't know they 
can't fly, so they fly anyway." 
The urban legend is based on fact: A bumblebee study was conducted in 1934 by 
the European scientists Antoine Magnan and Andre Saint-Lague. They applied 
mathematical analysis and known principles of flight to calculate that bee 
flight was "impossible," say insect-flight researchers Douglas L. Altshuler, 
Michael Dickinson and three colleagues at Caltech and the University of Nevada, Las 
Vegas in an article for today's issue of the Proceedings of the National 
Academy of Sciences. 
"Since this time," the authors note, "bees have symbolized both the 
inadequacy of aerodynamic theory as applied to animals and the hubris with which 
theoreticians analyze the natural world." 
The mystery of bee flight is the tip of the iceberg, though. Researchers have 
long struggled to understand the flight of all types of insects, from teeny 
fruit flies to the satanic-looking dragonflies. That's partly because insect 
aviation and human aviation are very different feats; the physics of the latter 
can't explain the physics of the former, as scientists have long known. 
Because of their tiny size, flying creatures like bumblebees, dragonflies, fruit 
flies and other insects must take into account microscopic and incredibly complex 
physical forces and effects that have negligible impact on 747s. 
The latest example of such research is the study by Altshuler, Dickinson and 
their colleagues. As they report in their article, they used high-speed (6,000 
frames per second) digital cameras to image the wing-flapping of honeybees 
leaving a hive at the University of Nevada, Las Vegas. The scientists also 
analyzed bee motions inside transparent acrylic chambers, where the insects made a 
beeline to containers of sugary fluid and pollen grains. 
Their conclusions include that bees, while hovering, swing their wings over 
amplitudes of about 90 degrees, a narrower range than other insects. But they 
also beat their wings unusually quickly for insects their size. Insect-flight 
experts have long assumed that the smaller the insect, the faster it beats its 
wings; but in the case of honeybees, the creature -- technically known as Apis 
mellifera -- beats its 10mm-wide wings about 240 times per second, faster 
than the much smaller fruit fly, which manages only 200 beats per second. 
In addition, the researchers observed how the creatures flew under stressful, 
high-altitude conditions when they were flying inside chambers containing a 
low-density mixture of oxygen and helium gases. True to the saying "busy as a 
bee," the bees put in a hard day's work for the scientists, who, as Altshuler's 
article notes, continued analyzing the little creatures until they "exhibited 
lethargy or disinterest." 
Mathematician Laura A. Miller of the University of Utah, who works on 
mathematical models of insect flight, said the Altshuler team's article is "excellent 
... a significant contribution to the field of insect flight aerodynamics ... 
(It) should motivate many future studies on comparative insect flight." 
Today's paper is the latest in a series of studies on insect flight over the 
last decade. A key finding has been that there's a big difference between the 
flight of insects and the flight of airplanes. 
An airplane flies because the upper part of its wing is a fixed, curved 
structure. That way, air flowing over the top of the wing has to travel faster, and 
a greater distance, in the same amount of time as air flowing under the wing. 
This causes the upper wing's air pressure to drop, so that the higher 
pressure beneath the wing forces the wing upwards -- and the plane with it. That's 
the basic principle behind airplane flight. 
For insects, flight is much more complicated. In insects, "the morphology 
(shape) of the wing has almost no role," Dickinson, a professor of bioengineering 
at the California Institute of Technology, said in an interview. "What 
matters is not the shape of the wing but how the insect moves it. That's very 
different from conventional (airplane) aerodynamics, where the shape of the wing is 
everything." 
Insect wings are constantly in motion, he said, so they're more like 
propellers than fixed aircraft wings. 
In the 1990s, crucial work in the field of insect-flight research was 
conducted by Charles Ellington of Cambridge University in England. He and other 
scientists, including Dickinson, built big "robotic" models of insects. With these 
mechanical critters -- "Robofly," Dickinson named one of them -- they measured 
the forces on different parts of the robots' wings as they flapped back and 
forth. Also, improved observational techniques (using miniature wind tunnels) 
and high-speed computers made it possible to model the dynamics of air around 
the flapping wings. Also in the 1990s, experimenters using sensitive 
observational equipment and high-speed cameras discovered that a beating insect wing 
forms a swirling funnel of air -- technically known as the leading-edge vortex, a 
kind of micro-tornado -- just above, and clinging to, the upper part of the 
wing. Air pressure inside the vortex is lower than surrounding air, just as air 
pressure inside a tornado is lower than in surrounding air. Thus 
higher-pressure air beneath the bug wing pushes it upward, providing lift to the insect. 
But such things alone don't explain how insects stay aloft once they're 
airborne. 
Bugs' wings also flap backward and curl while flapping. This rotational 
motion creates additional uplift for the same basic reason that the backspin on a 
soaring baseball keeps it aloft longer than it would in the absence of 
backspin. To be specific: Because the ball's top turns back toward the pitcher while 
the bottom turns away from him, air flows faster over the top than the bottom. 
Faster-flowing air has lower pressure. Therefore, the air pressure is lower on 
top of the ball, hence the higher pressure underneath the ball pushes it 
upward. This gives the ball "lift," which keeps it from falling back to Earth as 
fast as it would in the absence of backspin. 
Scientists still have only scratched the surface of the puzzle of insect 
flight. An insect must continually flap its wings to stay aloft, and must 
continually alter its wing and body orientations to counteract the numerous forces 
that are dragging it downward. This requires more than wing agility; it also 
requires a sharp little brain. 
"What would you need to know if you really wanted to build a fly?" Dickinson 
asks. "Understanding what the wings do is just a tiny part of it." If you 
built a robotic fly and its wing simply flapped back and forth, "the thing very 
rapidly crashes like a brick." 
To fly, "every moment (the insect) has to be constantly figuring out: 'Am I 
yawing? Am I pitching? Am I rolling? Am I drifting backward? Am I falling? Am I 
rising? And all that information is constantly streaming into a brain the 
size of about a poppy seed. Understanding insect flight requires understanding 
how that little 'computer' works -- and that's just as essential as 
understanding how the wings work. 
"There's still a lot of stuff to be excited about -- we're not going to solve 
it all in my lifetime." 
E-mail Keay Davidson at kdavidson at sfchronicle.com. 

Laurie Davies Adams
Executive Director
Coevolution Institute
423 Washington St. 5th
San Francisco, CA 94111
415 362 1137
www.coevolution.org
www.nappc.org

Our future flies on the wings of pollinators.
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