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Martin
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Mimicking humpback whale flippers may improve airplane wing design

Public release date: 11-May-2004

Contact: Deborah Hill

919-401-0299
Duke University

Mimicking humpback whale flippers may improve airplane wing design
DURHAM, N.C. -- Wind tunnel tests of scale-model humpback whale
flippers have revealed that the scalloped, bumpy flipper is a more
efficient wing design than is currently used by the aeronautics
industry on airplanes. The tests show that bump-ridged flippers do not
stall as quickly and produce more lift and less drag than comparably
sized sleek flippers. The tests were reported by biomechanicist Frank
Fish of West Chester University, Penn., fluid dynamics engineer
Laurens Howle of the Pratt School of Engineering at Duke University
and David Miklosovic and Mark Murray at the U.S. Naval Academy. They
reported their findings in the May 2004 issue of Physics of Fluids ,
published in advance online on March 15, 2004.

In their study, the team first created two approximately 22-inch-tall
scale models of humpback pectoral flippers -- one with the
characteristic bumps, called tubercles, and one without. The models
were machined from thick, clear polycarbonate at Duke University.
Testing was conducted in a low speed closed-circuit wind tunnel at the
U.S. Naval Academy in Annapolis, Md.

The sleek flipper performance was similar to a typical airplane wing.
But the tubercle flipper exhibited nearly 8 percent better lift
properties, and withstood stall at a 40 percent steeper wind angle.
The team was particularly surprised to discover that the flipper with
tubercles produced as much as 32 percent lower drag than the sleek
flipper.

"The simultaneous achievement of increased lift and reduced drag
results in an increase in aerodynamic efficiency," Howle explains.

This new understanding of humpback whale flipper aerodynamics has
implications for airplane wing and underwater vehicle design.
Increased lift (the upward force on an airplane wing) at higher wind
angles affects how easily airplanes take off, and helps pilots slow
down during landing.

Improved resistance to stall would add a new margin of safety to
aircraft flight and also make planes more maneuverable. Drag -- the
rearward force on an airplane wing -- affects how much fuel the
airplane must consume during flight. Stall occurs when the air no
longer flows smoothly over the top of the wing but separates from the
top of the wing before reaching the trailing edge. When an airplane
wing stalls, it dramatically loses lift while incurring an increase in
drag.

As whales move through the water, the tubercles disrupt the line of
pressure against the leading edge of the flippers. The row of
tubercles sheers the flow of water and redirects it into the scalloped
valley between each tubercle, causing swirling vortices that roll up
and over the flipper to actually enhance lift properties.

"The swirling vortices inject momentum into the flow," said Howle.
"This injection of momentum keeps the flow attached to the upper
surface of the wing and delays stall to higher wind angles."

"This discovery has potential applications not only to airplane wings
but also on the tips of helicopter rotors, airplane propellers and
ship rudders," said Howle.

The purpose of the tubercles on the leading edge of humpback whale
flippers has been the source of speculation for some time, said Fish.
"The idea they improved flipper aerodynamics was so counter to our
current doctrine of fluid dynamics, no one had ever analyzed them," he
said.

Humpback whales maneuver in the water with surprising agility for
44-foot animals, particularly when they are hunting for food. By
exhaling air underwater as they turn in a circle, the whales create a
cylindrical wall of bubbles that herd small fish inside. Then they
barrel up through the middle of the "bubble net," mouth open wide, to
scoop up their prey.

According to Fish, the scalloped hammerhead shark is the only other
marine animal with a similar aerodynamic design. The expanded
hammerhead shark head may act like a wing.

The trick now is to figure out how to incorporate the advantage of the
tubercle flipper into manmade designs, said Fish.

The research team now plans to perform a systematic engineering
investigation of the role of scalloped leading edges on lift increase,
drag reduction and stall delay.



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martin@ : Martin Gregorie
gregorie : Harlow, UK
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