Flow-induced unwinding of a fiber from an air-launched bobbin

The blackout bomb, which is a weapon for neutralizing electric supply facilities, consists of a cylindrical bobbin that is wound with a long conductive fiber. While the blackout bomb is descending, the fiber unwinds from the bobbin and entangles with the wire of the targeted facilities, causing a short circuit. Although the fiber unwound by the flow passively, previous studies on unwinding fibers have been limited to situations where the fiber was forcibly unwound by a mechanical device such as a pulley, not by a surrounding flow. They have also used meter-sized bobbins, while the blackout bomb is about a few centimeters in size. The present study experimentally investigates the unwinding form and speed of a fiber, which is unwound by flow, when a centimeter-scale bobbin launches. The unwinding fiber shows a spiral shape because residual stress was formed inside the fiber as it wound around the bobbin. This spiral motion is more distinct as the bobbin velocity or radius decreases or the fiber diameter increases. Moreover, because the spiral shape induces a larger drag compared to the linear shape, the unwinding speed of the fiber becomes greater as the spiral tendency enhances. By contrast, when the spiral shape is not maintained due to negligible residual stress, the unwinding speed has an inverse relationship with the fiber diameter.