A Humpback Whale's Bumpy Flippers Taught Fan Blades a New Trick

A humpback whale is one of the largest animals on the planet, weighing something like 80,000 pounds, and yet it can turn in surprisingly tight circles. Divers and whale watchers see it bank and roll and loop through the water with a grace that seems impossible for something the size of a bus. Much of that agility comes from its flippers, and from something odd about them.

A humpback's flippers are enormously long and pale, so they flash white under the surface. Look at the front edge of one and you will see it is not smooth. It is lined with a row of large, irregular bumps, like knuckles. These bumps are called tubercles, and for a long time they looked like nothing more than a quirk.

Biologist Frank Fish, of West Chester University in Pennsylvania, thought they might be doing a job. With colleagues, he put model flippers in a wind tunnel, some with bumps along the front edge and some smooth, and measured how they behaved. The results, published in 2004, were striking. The bumpy flipper had about 32 per cent less drag and 8 per cent more lift, and it could be tilted much more steeply before it lost its grip on the flow: the point at which it stalled moved from about 12 degrees to about 18.

That last number mattered most. Anything that uses a blade or a wing to push against air or water, from a plane to a ceiling fan, works only as long as the flow clings smoothly to its surface. Tilt it too far and the flow breaks away, the blade stalls, and it suddenly stops working well. A blade that can tilt further without stalling is useful almost everywhere.

Fish co-founded a company in Toronto called WhalePower to put the bumps to work. The firm tested wind turbine blades with a humpback-style front edge, and reported that they kept producing power well in lighter winds than ordinary blades. It also fitted the bumps to big industrial ceiling fans, where tubercle-lined blades ran about 20 per cent more efficiently than conventional ones, and a Canadian ventilation fan maker took up the design.

Why do bumps help? On a smooth flipper or blade, when the angle gets steep the flow tends to peel away across the whole edge at once. The tubercles break the edge into sections. Water or air is steered into the channels between the bumps, where it swirls and stays attached to the surface for longer, so the stall is held back and when it comes, it comes gently rather than all at once.

For the whale, that means it can make sharp, slow turns without losing its grip on the water. For a fan or a turbine, it means a blade that keeps working in conditions that would make a smooth one falter. The whale has been using the idea for a very long time; it just took a wind tunnel for us to notice.