As the thread is called a mostly outside of the canopy of the glider from the inside visible cantilevered short, usually red woolen thread in gliding. Even in modern high-performance gliders the thread is a cheap but important tool.
The thread is directed in flight like a flag along the slipstream and shows the pilot the exact direction of the air flow on the aircraft. In flight is controlled such that the thread is parallel to the longitudinal axis of the aircraft to fly with the least possible resistance. Namely, if the aircraft is not " inclined " moved laterally along its longitudinal axis, but by the surrounding air, the air resistance increases sharply, the plane loses unnecessary driving and / or declining. This condition is called sideslip.
Threads to avoid sideslip are common only to gliders. In the motor flying the ball dragonfly performs a similar task, but the thread on gliders is still much more sensitive.