It’s called a yaw string – a 6” length of yarn taped to the center of the windshield at one end to indicate the direction of air flow past the nose. Wilbur Wright invented it. He tied a yarn to the front rail of the 1902 Wright glider.
As shown above for a glider, there are three axes: lateral (pitch), longitudinal (roll) and vertical (yaw). The FAA Glider Flying Handbook describes how a yaw string works on page 17 of Chapter 4:
“The most effective, yet least expensive slip/skid indicator is made from a piece of yarn mounted in the free airstream in a place easily visible to the pilot, as shown in Figure 4-30 [for a left turn]. The yaw string indicates whether the pilot is using the rudder [feet] and aileron [hand] inputs together in a coordinated fashion. When the controls are properly coordinated, the yarn points straight back, aligned with the longitudinal axis of the glider. During a slipping turn, the tail of the yaw string is offset toward the outside of the turn. In flight, the rule to remember is simple: step on the head of the yaw string. If the head of the yaw string is to the right of the tail, then the pilot needs to apply right pedal. If the head of the yaw string is to the left of the tail, then the pilot should apply left pedal.”
You won’t find a yaw string on the windshield of a single engine propellor airplane because the propellor is forcing the airflow. But you will find one on jet airplanes: the T-33 Shooting Star, the U-2, the F-86 and the F-4.
When I was a student glider pilot, I was told to pay attention to the yaw string to minimize drag by coordinating my turns. That’s also good advice for avoiding wasted motion in other situations. You should move ahead smoothly rather than sliding toward one side or the other (slipping or skidding).
Images of a glider and yaw string are from Figures 3-20 and 4-30 of the FAA Glider Flying Handbook.
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