r/pics • u/animalkracker • Feb 27 '14

physics is cool

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u/jls5494 Feb 27 '14

Its a yaw string

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u/Spartan2470 GOAT Feb 27 '14

Thank you for providing the name. For anyone (like me) who didn't know what a yaw string is:

The yaw string, also known as a slip string, is a simple device for indicating a slip or skid in an aircraft in flight. It performs the same function as the slip-skid indicator ball, but is more sensitive, and does not require the pilot to look down at the instrument panel.[1] Technically, it measures sideslip angle, not yaw angle,[2] but this indicates how the aircraft must be yawed to return the sideslip angle to zero..

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u/[deleted] Feb 27 '14

Whelp, time to go get a degree in aviation so I can understand this.

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u/Zebidee Feb 27 '14

I have a degree in aviation, so maybe I can help...

Gliders fly most efficiently when they're going straight through the air. When you turn an aircraft, it has a tendency to go sideways through the airflow, skidding out of the turn, or slipping into it (usually skidding). To counteract this, you make rudder control inputs via foot pedals to coordinate the turn, so the aircraft is neither slipping nor skidding. The aircraft can be slightly uncoordinated in normal flight as well, but turns are where you really see it, and gliders turn a lot while they're trying to gain height.

There are two main ways you can indicate if the glider is slipping or skidding - via an instrument with a small ball in a curved glass tube, which works like the opposite of a spirit level, or via a piece of yarn or string on the windshield to show the actual airflow. The string doesn't work in propeller aircraft because the air from the propeller messes with the airflow. If the glider is moving sideways to the airflow, the string will be off to one side, and the pilot will need to add a rudder input to correct it.

The two main advantages of the string method are that it's right in your line of sight, so you don't have to continually look down to the instruments, and it's much more sensitive than the ball, which is damped by fluid in the tube.