The flap won't die down

Science News, Feb 17, 2001 by P. Weiss

Your item headed "Silk and soap settle a century-old flap" (SN: 12/16/00, p. 390) prompts me to offer a less complicated answer to why flags flap. The key word is gravity. If you provide an air source blowing straight down and suspend a flag on a horizontal pole of any reasonable diameter, the flag will not flap.

So why all the motion when the pole is vertical? At the free vertical edge, the upper corner has nothing to retard a gravitational drop--so it drops. A fold of material (several, in fact) occurs at an angle downward and away from the pole. Assuming a horizontal wind flow, the folded material presents an angled and downward sloping surface that results in a vertical and horizontal reaction. Several such folds, of course, can occur at one time, resulting in the opposite motions called "flap." I hope this helps put an end to this current flap.

Richard B. Wallace

Bingham Farms, Mich.

One can't truly imitate the action of a flag in wind with a thread in soap film, which is nearly two dimensional.

Mary Hyde Berg

Gloucester, Va.

Lord Raleigh may have been wrong about the mechanism of asymmetry amplification that causes flags to flutter in the wind, but Jun Zhang, in his experiments described in the article, omits consideration of an obvious cause because of two oversimplifications: his "flag" is one-dimensional and he hangs it vertically.

An ordinary, two-dimensional flag flies horizontally in a strong wind, with two-point attachment to a vertical pole. When the wind calms, the flag droops under the force of gravity. As the wind picks up again, gravity's amplification of the inevitable small asymmetries in the system may be less obvious, but the forces are still the same and are quite sufficient to account for the flag's flap and flutter.

David Bortin

Whittier, Calif.

Zhang and his colleagues acknowledge that gravity tugs downward on a real, three-dimensional flag and may strongly affect how and whether the flag flaps.

Before trying to tackle the full, three-dimensional problem, they and other scientists have long been trying to fully explain flapping in a one-dimensional flag in a two-dimensional breeze.