Tribology and Lubrication Technology December 2012 : Page 6
FRoM THE EdiToR Evan Zabawski Friction-reducing feathers Swimming can be such a drag. Unless you’re a penguin. wE aLL Know THaT PEnGuinS can’T FLy. They can’t walk very well either, but they’re pretty good swimmers. Perhaps their best talent, which is a relatively unknown skill, is their ability to lubricate themselves in order to swim faster. A typical Emperor penguin can swim at speeds between three and six miles per hour (a medal-winning Olympian swims just over four miles per hour in short distances). When penguins leap from the water onto an ice ﬂ ow, they manage to get their speed up to more than 12 miles per hour before breaking the surface. This burst of speed is sometimes necessary to evade a predatory seal or to overcome several feet of ice, and only re-cently have we come to understand how this is possible. Competitive swimmers know that the greatest force working against them is fric-tion in the form of drag, since water is ap-proximately 800 times denser (and 55 times more viscous) than air. Various attempts have been made to reduce drag while swimming. The fi rst and foremost being body physics. When swimmers dive into the water, they as-sume a torpedo-like shape: one hand on top of the other on outstretched arms, elbows squeezed tight against the head, straight-body and legs terminating with pointed toes. Optimal stroke technique comes next. A swimmer must overcome the body’s imprac-6 tical design of rounded arms and legs to pro-duce the best stroke possible. The last aspect to consider is the body’s outer layer. Most swimmers will shave their bodies to remove their speed-robbing hair, and some may don a full-body swimsuit. In contrast, a penguin’s body is naturally shaped for optimal speed already, is equipped with effi cient ﬂ ippers and yet is covered in feathers. Feathers seem like an unlikely advantage, but recent studies have shown it to be true. Scientists Roger Hughes, John Daven-port, Marc Shorten and Poul Larsen have dis-covered the secret of the Emperor penguin feathers. Just before a penguin makes its leap, it surfaces and preens its feathers. Their feathers are very downy near the base and this preening action traps air. Then the penguin will dive to a depth of 40-50 feet, which allows the air trapped in their feathers to compress. Another feature of their feathers is that they terminate in a ﬂ exible membrane at-tached to muscles, which allow the penguin to alter the position of its feathers. So once the penguin begins its ascent, it ﬂ exes these muscles to pull the feathers closer to its body, which pushes the air through the feathers where the downy mesh creates <20 μ m pores that act like a diffuser. The escap-ing air forms a thin layer of microbubbles all over its body that greatly reduces the drag on the penguin without grossly affecting buoyancy, allowing it to accelerate more eas-ily. This low-friction coating is a temporary form of self-lubrication that gives a ﬂ ightless bird the ability to ﬂ y out of the water to safety. Though it’s a recent discovery, we have been aware of the concept for some time. One of the earliest examples can be found in a patent application by Sir John Isaac Thornycroft in 1877 for air-lubricated ship hulls. His design was never applied to a practical application, but developments over the next 75 years inspired the design for hovercrafts. During the Cold War, Russians were able to increase the speed of their torpedoes from 30-40 miles per hour to over 200 miles per hour by having a cavitator create a su-percavity of bubbles for the rocket-propelled torpedo to travel through. Some recent developments have been made regarding air lubrication for contain-er ships and supertankers, and perhaps one day soon we will see a signifi cant reduction in the energy requirements for ocean-borne transportation. Evan Zabawski, CLS, is a reliability specialist in Calgary, Alberta, Canada. You can reach him at evan. firstname.lastname@example.org. (W.C. Fields, comedian) Name the speaker: “Start every day off with a smile and get it over with”
From the Editor
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