Yaw string

It is typically constructed from a short piece or tuft of yarn placed in the free air stream where it is visible to the pilot.

It may also be mounted on the aircraft's nose, either directly on the skin, or elevated on a mast, in which case it may also be fitted with a small paper cone at the trailing end.

[6] The yaw string is considered a primary flight reference instrument on gliders, which must be flown with near zero sideslip angle to reduce drag as much as possible.

Even the most sophisticated modern racing sailplanes are fitted with yaw strings by their pilots, who reference them constantly throughout the flight.

The Wright Brothers used a yaw string on their 1902 glider tied on their front mounted elevator.

Some light twin-engine airplane pilots place yaw strings on their aircraft to help maintain control in the event of an engine failure, because the slip-skid indicator ball is not accurate in this case.

[16] In a multiengine airplane with an inoperative engine, the centered ball is no longer the indicator of zero sideslip due to asymmetrical thrust.