View source for Yaw string

[[Image:FAA-8083-3A Fig 12-16.PNG|thumb|right|Diagram showing yaw string deflection on a multi-engine airplane flown incorrectly with wings level after an engine failure.]]

The '''yaw string''', also known as a '''slip string''', is a simple device for indicating a [[slip (aerodynamic)|slip]] or [[skid (aerodynamic)|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.<ref>{{cite book
 | last = Reichmann
 | first = Helmut
 | editor = Lert, Peter
 | title = Cross Country Soaring (Streckensegelflug)
 | year = 1988
 | orig-year = 1975
 | edition = English
 | publisher = Soaring Society of America
 | id = LCCCN 77-86598
 | page = 77
 }}</ref> Technically, it measures sideslip angle, not [[yaw angle]],<ref>{{cite web
| url = http://www.av8n.com/how/htm/multi.html#@default1498
| title = See How It Flies
| access-date = 2007-01-21
| author = Denker, John S.
| year = 2002
| quote = 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)....
}}</ref> but this indicates how the aircraft must be yawed to return the sideslip angle to zero.

It is typically constructed from a short piece or [[Tuft (aeronautics)|tuft]] of yarn placed in the free air stream where it is visible to the pilot.<ref>{{cite book
 | title = Glider Flying Handbook
 | year = 2003
 | publisher = U.S. Federal Aviation Administration
 | location = U.S. Government Printing Office, Washington D.C.
 | id = FAA-8083-13_GFH
 | pages = 4–14
 | url=http://www.faa.gov/library/manuals/aircraft/glider_handbook/
 }}</ref> In closed-cockpit aircraft, it is usually taped to the aircraft [[Canopy (aircraft)|canopy]]. 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.<ref>{{cite book
 | last = Selvidge
 | first = Harner, S.D.
 | editor = Licher, Rose Marie
 | title = American Soaring Handbook
 | orig-year = 1963
 | edition = 2nd
 | year = 1976
 | publisher = Soaring Society of America 
 | id = LCCCN 59-15668
 | pages = 30–31
 | chapter = Ch. 7 : Equipment I
 }}</ref> They are commonly used on gliders, but may also be found on [[jet aircraft]] (especially [[fighter aircraft|fighters]]), [[ultralight aircraft]], [[light-sport aircraft]], [[autogyro]]s,<ref>{{cite book
 | title = Rotorcraft Flying Handbook
 | year = 2000
 | publisher = U.S. Federal Aviation Administration
 | location = U.S. Government Printing Office, Washington D.C.
 | id = FAA-8083-21
 | pages = 18–4
 | url=http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf
 }}</ref> [[airplanes]] and [[helicopters]]. Its usefulness on airplanes with a [[tractor configuration]] (single [[propeller (aircraft)|propeller]] at the nose) is limited because the propeller creates turbulence and the [[spiral slipstream]] displaces the string to one side.<ref>{{cite web
| url = http://www.challengers101.com/YawString.html
| title = The Yaw String
| access-date = 2007-01-21
| author = Hurt, George
}}</ref>

The yaw string is considered a primary flight reference instrument on [[Glider (sailplane)|gliders]], which must be flown with near zero sideslip angle to reduce [[Drag (physics)|drag]] as much as possible. It is valued for its high sensitivity, and the fact that it is presented in a [[head-up display]]. Even the most sophisticated modern racing sailplanes are fitted with yaw strings by their pilots, who reference them constantly throughout the flight.

==History==
The yaw string dates from the earliest days of aviation, and actually was the first [[flight instruments|flight instrument]]. The [[Wright Brothers]] used a yaw string on their [[Wright Glider|1902 glider]] tied on their front mounted elevator.<ref>{{cite web
| url = http://www.first-to-fly.com/Adventure/Hangar/gliderrep.htm
| title = 1902 Glider Replica
| access-date = 2007-01-21
| author = Wright Brothers Aeroplane Company and Museum of Pioneer Aviation
| quote = 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....
}}</ref> Wilbur Wright is credited with its invention, having applied it concurrently with the movable rudder invented by his brother Orville in October 1902,<ref>{{cite journal
 |quote       = <nowiki>[Wilbur Wright]</nowiki> 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.
 |last        = Knauff
 |first       = Tom
 |date        = Feb–Mar 1995
 |title       = Inventing the Rudder
 |journal     = Free Flight
 |volume      = 1995
 |issue       = 1
 |page        = 26
 |id          = – 2557
 |url         = http://www.wgc.mb.ca/sac/freeflight/95_01.pdf
 |access-date  = 2006-01-21
 |issn        = 0827-2557
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20051031193648/http://www.wgc.mb.ca/sac/freeflight/95_01.pdf
 |archive-date = 2005-10-31
}}</ref> although others may have used it before. [[Glenn Curtiss]] also used it on his early airplanes.

==Operation==
[[File:US Navy 030421-N-0295M-002 A pilot looks through his Heads-Up Display (HUD) as he checks the systems of his F-14D Tomcat.jpg|thumb|Yaw string used in front of the cockpit of an F-14D Tomcat]]

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 to [[Helmut Reichmann]], "...rudder against the string and/or aileron toward the string." 

In a [[spin (aerodynamics)|spin]] 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. <ref>{{cite web |title=Glider Flying Handbook, FAA-H-8083-13A |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/glider_handbook/media/faa-h-8083-13a.pdf |publisher=U.S. Department of Transportation, FAA |access-date=13 December 2020 |pages=4-16,4-17 |date=2013}}</ref><ref>{{cite web |title=Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25B |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf |publisher=U.S. Department of Transportation |access-date=13 December 2020 |pages=8–18 |date=2016}}</ref><ref>{{cite web |last1=Benenson |first1=Tom |title=Thermals, Towlines & Tangents |url=https://books.google.com/books?id=50Hb0-4e5qUC&dq=yaw+string+tail+head+arrow&pg=PA103 |publisher=Flying Magazine |access-date=13 December 2020 |page=103 |date=2000}}</ref><ref>{{cite web |title=Helicopter Flying Handbook, FAA-H-8083-21B |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch09.pdf |publisher=U.S. Department of Transportation, FAA |access-date=13 December 2020 |pages=9-3,9-4,9-5 |date=2019}}</ref><ref name="hr">{{cite book |last1=Reichmann |first1=Helmut |title=Cross-Country Soaring, A Handbook for Performance and Competition Soaring |date=1993 |publisher=Soaring Society of America, Inc. |location=Iceland |isbn=1883813018 |page=147}}</ref>

==Use on powered aircraft==
Yaw strings are also fitted to the [[Lockheed U-2]] high-altitude [[surveillance aircraft]]<ref>{{cite web
| url = http://www.barryschiff.com/high_flight.htm
| title = High Flight
| access-date = 2007-01-21
| author = Schiff, Barry
| year = 2006
| quote = A yaw string (like those used on sailplanes) above each canopy silently admonishes those who allow the aircraft to slip or skid when maneuvering.
}}</ref> and variants of the [[Grumman F-14 Tomcat]].<ref>{{cite web | url=http://www.anft.net/f-14/f14-detail-sensorprobe.htm | title=HOME OF M.A.T.S. - the most comprehensive Grumman F-14 Reference Work - by Torsten Anft! }}</ref> A [[Flat spin (aviation)|flat 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 the [[slip-skid indicator]] ball is not accurate in this case.<ref>{{cite book
| author = Paul A. Craig
| title = Multiengine flying
| publisher = McGraw-Hill
| location = New York
| year = 2004
| isbn = 0-07-142139-4
| quote = You will see that with the wings level the yaw string does not lay along the centerline, yet the ball is in the center...
}}</ref>
In a multiengine airplane with an inoperative engine,
the centered ball is no longer the indicator of zero
sideslip due to [[asymmetrical thrust]]. The yaw string is the only
flight instrument that will directly tell the pilot the
flight conditions for zero sideslip.<ref>{{cite book
 | title = Airplane Flying Handbook
 | year = 2004
 | publisher = U.S. Federal Aviation Administration
 | location = U.S. Government Printing Office, Washington D.C.
 | id = FAA-8083-3A
 | pages = 12–23
 | url=http://www.faa.gov/library/manuals/aircraft/airplane_handbook/
 }}</ref>

Yaw strings are also used on some (especially smaller) helicopters.

== Side string ==
[[File:Seitenfaden.PNG|thumb|Side string]]
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.<ref>[http://www.dg-flugzeugbau.de/index.php?id=seitenfaden-e Use of the side string, DG Flugzeugbau]</ref>

Investigations of the side string and on its use in glider flight were performed by the [[Akaflieg Köln]].<ref>[http://www.akaflieg-koeln.de/news/newsdetails/alles-haengt-am-seit-den-enfaden-neue-version/ Use of the side string (German), Akaflieg Köln]</ref>

== See also ==
*[[Glider (sailplane)]]
*[[Gliding]]
*[[Sideslip angle]]
*[[Slip (aerodynamic)]]
*[[Tuft (aeronautics)|Tuft]]
*[[Tell-tale (sailing)]]
*[[Critical engine]]

==References==
{{reflist}}
{{USGovernment|url=http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/|title=Instrument Flying Handbook}}

{{Flight instruments}}

[[Category:Aircraft instruments]]
[[Category:Gliding technology]]