How to Fly Like a Bird

Slotted wingtips provide stability in the air

Pilots typically aim to keep a plane's nose facing front during flight. A vertical wing on the back of the fuselage prevents rotation from side to side, so the plane doesn't spin like a kayak entering a patch of white water. But birds don't have this luxury.

Unlike airplanes, which have a vertical tail for stability, birds adjust slotted wing tips.

Red-tailed Hawk by Cheryl Doherty

"The vertical tail can be found in any type of airplane, ranging from motor-less gliders and small aircraft to the largest transports or fastest supersonic vehicles and even aerospace craft," said Gottfried Sachs, professor for the Institute of Flight Mechanics and Flight Control at Technische Universitaet Muenchen in Germany. "There is no bird which possesses a vertical tail."

Instead, slotted wing tips in gliding kites, eagles, vultures, and storks may provide stability akin to a plane's backend helper, according to an article by Sachs in the Journal of Theoretical Biology (March 2006). The feathers of these and similar soaring birds separate, like fingers, both horizontally and vertically to form slots. The wing tips can be adjusted to reduce drag, increase lift, and prevent stalling.

The contour of the wing is crucial for bird flight. It reroutes air flow, creating high pressure right below the wing and low pressure right above. This leads to lift, which counteracts gravity. When birds flap they also create a thrust force, which counteracts drag. But in addition to forces, stability keeps birds in the air and on course.

By spreading slotted wing tips, birds can increase sweep, the degree to which the wings bend backward. The sweep angle, between the wing and a line perpendicular to the bird's body, can reach 40 to 45 degrees in birds with these slots.

Sachs found that birds with swept wings have greater stability than those without them. These birds are less likely to experience sideslip, meaning they are less likely to move sideways relative to oncoming wind.

When sideslip does occur, since the nose is not pointing directly into the wind, flight is less efficient. But yaw stability can restore symmetric flight.

After more than a century of modern aircraft development, planes still look like birds. And both provide models to help scientists study the other.

"For studying flight stability in birds, we apply techniques which we use in the flight mechanical investigations of aircraft," Sachs said.

Now, mini-aircrafts may further resemble birds. By increasing sweep through slotted tips, engineers have the option of eliminating the ever-present vertical tail.

—Elizabeth Quill, intern, science writing