It's called the Magnus-Robins effect. Other skybows have creased edges and always spin the same direction, these skybows must be set up properly in relation to the direction of the wind. The aerodynamic forces always act nearly at right angles to the ribbon face, but the ribbon is constantly turning. Since the ribbon has two faces, the frequency of the sound is twice the spin rate. A skybow flying well makes a howling sound reminiscent of the sound of high wind in a pine woods, a skybow that is not entirely steady sounds like a motorcyle race, with repeated accelerations and sudden downshifts of pitch.
Loudness increases drastically with wind speed.
The sound is very directional, with a quiet spot near the fliers and directly behind them. A skybow needs to spin faster where the wind is faster in order to fly high. Multiple sections that spin idependently can accomodate differences in windspeed along the skybow. Usually the fastest winds hit the high center of the arch. Tony Frame and Jim Mallos flew Mr. Big over the Washington Monument Grounds, Washington, DC on November 10, 1997.