Theyaw string, also known as aslip string, is a simple device for indicating aslip orskid in anaircraft in flight. It performs the same function as theslip-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, notyaw angle,[2] but this indicates how the aircraft must be yawed to return the sideslip angle to zero.
It is typically constructed from a short piece ortuft of yarn placed in the free air stream where it is visible to the pilot.[3] In closed-cockpit aircraft, it is usually taped to the aircraftcanopy. 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.[4] They are commonly used on gliders, but may also be found onjet aircraft (especiallyfighters),ultralight aircraft,light-sport aircraft,autogyros,[5]airplanes andhelicopters. Its usefulness on airplanes with atractor configuration (singlepropeller at the nose) is limited because the propeller creates turbulence and thespiral slipstream displaces the string to one side.[6]
The yaw string is considered a primary flight reference instrument ongliders, which must be flown with near zero sideslip angle to reducedrag as much as possible. It is valued for its high sensitivity, and the fact that it is presented in ahead-up display. Even the most sophisticated modern racing sailplanes are fitted with yaw strings by their pilots, who reference them constantly throughout the flight.
The yaw string dates from the earliest days of aviation, and actually was the firstflight instrument. TheWright Brothers used a yaw string on their1902 glider tied on their front mounted elevator.[7] Wilbur Wright is credited with its invention, having applied it concurrently with the movable rudder invented by his brother Orville in October 1902,[8] although others may have used it before.Glenn Curtiss also used it on his early airplanes.
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In flight, pilots are instructed to step on the head of the yaw string; the head is the front of the string, where the string is attached to the aircraft. If the head of the yaw string is to the right of the yaw string tail, then the pilot should apply right rudder pressure. If the head of the yaw string is to the left of the yaw string tail, then the pilot should apply left rudder pressure. Or pilots may be instructed to view the attached point of the yaw string as an arrowhead pointing to the rudder pedal needing application of pressure. According toHelmut Reichmann, "...rudder against the string and/or aileron toward the string."
In aspin the yaw string always points in the direction of spin. Inclinometers do not work in spins because they follow the local centrifugal direction. In a spin, only turn needles and yaw strings are reliable. Turn coordinators work if erect but not when inverted.[9][10][11][12][13]
Yaw strings are also fitted to theLockheed U-2 high-altitudesurveillance aircraft[14] and variants of theGrumman F-14 Tomcat.[15] Aflat spin, caused by excessive sideslip even in level flight, happens much more easily at high altitudes. Some light twin-engine airplane pilots place yaw strings on their aircraft to help maintain control in the event of an engine failure, because theslip-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 zerosideslip due toasymmetrical thrust. The yaw string is the onlyflight instrument that will directly tell the pilot theflight conditions for zero sideslip.[17]
Yaw strings are also used on some (especially smaller) helicopters.
A variation of the yaw string is the side string, used in gliders for a determination of the angle of attack. In this way the speed for best glide angle, the best thermalling speed and the stall speed can be observed independently of other parameters like air speed, aircraft weight, acceleration due to turning, stick movements, and gusts.[18]
Investigations of the side string and on its use in glider flight were performed by theAkaflieg Köln.[19]
The slip string is commonly referred to as a "yaw string", even though it measures the slip angle, not the yaw angle (i.e. heading)....
{{cite book}}: CS1 maint: multiple names: authors list (link)For all the progress that has been made in a century of aviation, the Ventur 2 still has a "yaw string" attached where the pilot can see it. As does the Wright glider. This was the first flight instrument....
[Wilbur Wright] responded, "Yes...we will install a short piece of string out front where we can see it. This string will tell us all we need to know!" (paraphrased) Wilbur had just invented the yaw string — the first aircraft instrument.
A yaw string (like those used on sailplanes) above each canopy silently admonishes those who allow the aircraft to slip or skid when maneuvering.
You will see that with the wings level the yaw string does not lay along the centerline, yet the ball is in the center...
This article incorporatespublic domain material fromInstrument Flying Handbook.United States government.