On Swift Wings

What airplane designers could learn from the shape-changing wings of birds

Bird biomechanics

Wingtip bones (green) and forelimb bones (blue)— corresponding to human hand and forearm bones, respectively—provide the typical bird with two feathered airfoils, critical to controlling flight.

Jacqueline Mahannah (www.jacquelinestudios.com)
That fits well with what is known about swift “roosting” behavior: tracking the birds on radar as they sleep on the wing shows that they glide and periodically flap to maintain an average speed of twenty miles per hour, and to minimize altitude lost per minute. In other words, they glide at a speed that requires minimum energy during their snooze time.

At high speeds, however, the advantage swiftly shifts to swept-back wings. Swifts are called swifts, after all, because their peak flight speed is so high. They have been clocked at more than sixty miles an hour—no wonder they fall asleep at twenty! It turns out that sweeping the wings back becomes key to energy efficiency at fast cruising speeds of more than forty miles an hour. At those higher speeds, the swept wings, which minimize drag, become more efficient both in maintaining a shallow-angle glide and in maximizing the time spent aloft.

If straight-line flying benefits from spread wings at lower speeds and swept wings at higher speeds, what about turning? The F-14 Tomcat spread its wings wide for increased maneuverability during dogfights, which demand lots of fast turns. Keeping the spread-wing shape when turning is also a good strategy for the swift—at least at twenty miles an hour. At that velocity, a swift can turn at least twice as fast by spreading its wings as it can by bending them.

In higher-speed turns, however—as simulated in the wind tunnel—it becomes impossible to measure the relative efficiency of spread and swept wings, because beginning around thirty-four miles an hour, spread swift wings become vulnerable to damage from aerodynamic stress. At high speeds they naturally flex and twist slightly in the turbulent air, and so avoid being damaged by the high forces. So, when pursuing fast-flying and fast-turning insect lunches, a swift bends its wings to keep up; the calories from such a meal warrant the extra effort.

Human attempts at variable wing geometry have always been hampered by the complexity and weight associated with a system engineered from hinges. Students working with Lentink are now building flexible, lightweight aircraft the size of swifts, capable of wing morphing. If such devices can be scaled up, fighter pilots could have a fast, agile plane that could slow down and spread its wings to hold station and maximize its time in the air. Even then, though, the pilots probably won’t be dozing off.

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