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From: james ssemakula <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED], [EMAIL PROTECTED], Buganda Discussion <[EMAIL PROTECTED]>
Subject: One number explains animal flight
Date: Mon, 20 Oct 2003 13:39:49 -0700 (PDT)
One number explains animal flight
Engineers and evolutionists take note: wings and tails beat to the same drum.
16 October 2003
JOHN WHITFIELD
Wings flap to minimize turbulence.
� alamy.com
One number describes the beating of animal wings and tails, researchers have found1. The simple rule of thumb for animal locomotion could help to design miniature flying machines.
"We've described the geometry of the wingbeat," says Graham Taylor of the University of Oxford, UK. Swimmers and fliers from insects to whales all cruise at the speed that lets them slip along most easily, he and his colleagues show.
Wings and tails create eddies as they move. These need to be left behind, because turbulent air or water is more difficult to travel through. So limbs shed vortices at the bottom of their downstrokes. Flap too quickly, and you have to fight this turbulence on the way up. Too slowly, and turbulence sticks.
A quantity called the Strouhal number measures how efficiently an animal cruises. The number describes how much up-and-down movement a wing or tail makes relative to a creature's forward speed. It is calculated as stroke speed multiplied by size, divided by forward speed.
Flying and swimming is most efficient at Strouhal numbers of 0.2-0.4. The cruising speeds of everything from bumble-bees to blue whales, via mackerel, locusts, pigeons and bats, fall in this range, Taylor's team finds. Bumble-bees go faster than whales, at about 30 kilometres per hour to whales' 20.
Physical laws shape the form that animals take, and the ways that they move, agrees biologist Ulla Lindhe Norberg of the University of Gothenburg in Sweden. "Flight is a very expensive mode of locomotion - every adaptation is going to increase its efficiency," she says.
Flying and swimming are most efficient at Strouhal numbers between
0.2 and 0.4.
� alamy.com
Working the other way round, you can use Strouhal numbers to calculate an animal's cruising speed. From a brief video clip of a flying bird, say, multiplying wingspan by beat frequency, then dividing by 0.3 gives an estimate. "This would be a very big help in looking at migration," says Taylor.
What's more, engineers trying to build small flying craft for the military and toy industries now know how fast their creations need to flap to travel at maximum efficiency.
Animal studies have already inspired prototype micro-air vehicles, agrees aeronautics expert Ismet Gursul of the University of Bath, UK. "We're trying to understand how the animal world works, and if anything useful from it can be applied," he says. But it is still unclear whether copying nature will be the best policy, he warns.
Very small insects are immune from the rule. For them, the air is so treacly that the normal rules of fluid dynamics do not apply.
References
Taylor, G. K., Nudds, R. L. & Thomas, L. R. Flying and swimming animals cruise at a Strouhal number tuned for high power efficiency. Nature, 425, 707 - 711, doi:10.1038/nature02000 (2003).
� Nature News Service / Macmillan Magazines Ltd 2003
How Birds Fly
http://www.amazon.com/exec/obidos/tg/detail/-/0963424408/qid=1066667851/sr=8-3/ref=sr_8_3/002-2657197-5264814?v=glance&n=507846
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