Passage IV

NATURAL SCIENCE: This passage is from the book Feathers: The Evolution of a Natural Miracle by Thor Hanson.

In the passage, airfoil refers to the shape of a wing. Covert feathers serve as covers for other feathers.

Textbook diagrams of airfoils regularly leave out a critical detail: turbulence. Air passing around a wing never actually moves in the smooth lines of illustration—it swirls and eddies in complex patterns that change constantly with every subtlety of temperature, air pressure, wind speed, wing shape, and angle. There are layers of air dragged along with the wing, vortices tumbling above its surfaces, and spirals jetting off the tips. The process is far too complex for any drawing, but understanding it is critical to understanding drag, the natural resistance to a wing's forward motion. Any reduction in drag increases flight efficiency, offering immediate fuel savings to airlines. And no one reduces drag better than birds.

If you've ever flown in a window seat, you may have admired the silvery shine of an airplane wing and watched its several flaps raise and lower at various times during the flight. It's a precise and beautifully designed instrument, but must look terribly crude to a bird, whose own wings can flap and flex, extend and contract, spread, narrow, tuck, and twist, responding instantly to ever-changing conditions. Taken together, the overlapping flight feathers create a single dynamic airfoil. But they can also move independently and are themselves shaped like airfoils, acting as individual winglets within the greater whole. Vultures, eagles, and other soaring birds use small adjustments of their spread wing-tip “fingers” to manipulate air currents or change speed and orientation, and all birds utilize feather movements to instinctively alter the turbulence patterns around their wings. Slots can be opened or closed to direct air between primaries; covert feathers can be raised or lowered like tiny flags—the possibilities are endless.

Teasing apart these intricacies challenges even the most advanced computer models, but engineers have already learned that adding artificial “winglets” to the tips of airplane wings can mimic the efficiency of a soaring raptor. Passenger jets retrofitted with winglets have seen their fuel use drop by as much as 6 percent, a substantial savings considering that a fully loaded 747 can burn through a gallon or more every second. Now in common use, these small vertical fins have saved the airline industry billions of dollars in fuel costs. A potentially more lucrative lesson can be summarized in one unexpected word: fuzziness.

Photographs of birds in flight often show splayed and uneven flight feathers, or coverts lifted at sharp angles above the wing—like Ken Franklin's amazing pictures of a falcon catching shorebirds. Engineers now believe this intentional “roughening” of the wing surface may substantially reduce turbulence and drag. A fully feathered jetliner is probably out of the question, but simulations suggest that just covering the wings with simple bristles could improve flight efficiency by as much as 15 percent. Typically, air passing over the surface of a wing (or any airfoil) breaks apart into tiny eddies that pull away from the surface, a form of turbulence that results in additional drag and pockets of dead air directly behind the wing. When bicyclists tuck into the slipstream behind a lead rider, they're taking advantage of this principle—saving energy by riding in a low-pressure, low-turbulence position. It's counterintuitive, but rough surfaces can help reduce drag by manipulating the formation of eddies and keeping them close to the airfoil surface. Years from now, you may peer out an airplane window and see a fuzzy wing, each bristle the manufacturer's best approximation of a feather.

Managing the airflow around wings can have the added benefit of substantially reducing aircraft noise, an important consideration for anyone living on a busy flight path. When owls pass overhead, their eerily silent wing strokes seem otherworldly, and they've long been linked to mythologies of the spirit realm. But there is nothing supernatural about an owl's flight—their wings simply part the air in a different way. Owl feathers feature barb extensions on the leading and trailing edges that reduce turbulence over individual feathers as well as the entire wing, increasing efficiency but, most important, muffling the sound of their approach. This stealth gives them a key advantage over the wary and sharp-eared mammals and birds they prey upon. The promise of quieter airplanes makes owl feathers an intriguing model for commercial carriers, whose take-offs and landings at urban airports are dictated in part by sound levels.

Copyright © 20110101 Hanson, Thor. Reprinted by permission of Basic Books, a member of the Perseus Books Group.