Editing Relaxed stability

{{Short description|Aircraft with low or negative stability}}
{{Refimprove|date=June 2009}}

In [[aviation]], an aircraft is said to have '''relaxed stability''' if it has low or negative [[Aircraft stability|stability]].<ref name="Nguyen">{{cite journal |last1=Nguyen |first1=L. T. |last2=Ogburn |first2=M. E. |last3=Gilbert |first3=W. P. |last4=Kibler |first4=K. S. |last5=Brown |first5=P. W. |last6=Deal |first6=P. L. |title=Simulator study of stall/post-stall characteristics of a fighter airplane with relaxed longitudinal static stability. NASA Technical Paper 1538 |journal=NASA Technical Publications |date=1 December 1979 |issue=19800005879 |page=1 |url=https://ntrs.nasa.gov/citations/19800005879 |access-date=6 July 2022 |publisher=NASA |language=en}}</ref><ref name="Wilhelm">{{cite journal |last1=Wilhelm |first1=Knut |last2=Schafranek |first2=Dieter |title=Landing approach handling qualities of transport aircraft with relaxed static stability |journal=Journal of Aircraft |date=October 1986 |volume=23 |issue=10 |pages=756–762 |doi=10.2514/3.45377 |url=https://arc.aiaa.org/doi/pdf/10.2514/3.45377 |access-date=6 July 2022 |language=en |issn=0021-8669|url-access=subscription }}</ref>

An aircraft with negative stability will have a tendency to change its [[Aircraft principal axes|pitch and bank]] angles spontaneously. An aircraft with negative stability cannot be [[trim (aircraft)|trimmed]] to maintain a certain [[Flight dynamics (fixed-wing aircraft)|attitude]], and will, when disturbed in pitch or roll, continue to pitch or roll in the direction of the disturbance at an ever-increasing rate.

This can be contrasted with the behaviour of an aircraft with positive stability, which can be trimmed to fly at a certain attitude, which it will continue to maintain in the absence of control input, and, if perturbed, will [[oscillate]] in [[simple harmonic motion]] on a decreasing scale around, and eventually return to, the trimmed attitude.{{citation needed|date=May 2011}} A positively stable aircraft will also resist any bank movement. A [[Cessna 152]] is an example of a stable aircraft. Similarly, an aircraft with neutral stability will not return to its original attitude without control input, but will continue to roll or pitch at a steady (neither increasing nor decreasing) rate.{{citation needed|date=December 2020}}

== Early aircraft ==
Early attempts at heavier-than-air flight were marked by a differing concept of stability from that used today. Most aeronautical investigators regarded flight as if it were not so different from surface locomotion, except the surface was elevated. They thought of changing direction in terms of a ship's [[rudder]], so the flying machine would remain essentially level in the air, as did an automobile or a ship at the surface. The idea of deliberately leaning, or rolling, to one side either seemed undesirable or did not enter their thinking.{{Sfn | Crouch | 2003 | pages = 167–168}}

Some of these early investigators, including [[Samuel Pierpont Langley| Langley]], [[Octave Chanute| Chanute]], and later [[Santos-Dumont]] and the [[Voisin brothers]], sought the ideal of "inherent stability" in a very strong sense, believing a flying machine should be built to automatically roll to a horizontal ([[Flight dynamics (aircraft)#Lateral modes |lateral]]) position after any disturbance. They achieved this with the help of [[Hargrave cellular wing]]s (wings with a [[box kite]] structure, including the vertical panels) and strongly [[Dihedral (aircraft) | dihedral]] wings. In most cases they did not include any means for a pilot to control the aircraft roll<ref name = c />{{Page needed |date=February 2014}}—they could control only the elevator and rudder. The unpredicted effect of this was that it was very hard to turn the aircraft without rolling.<ref name = c />{{Page needed |date=February 2014}}<ref name = l /> They were also strongly affected by side gusts and side winds upon landing.{{Citation needed |date=February 2014}}

The [[Wright brothers]] designed their [[1903 Wright Flyer|1903 first powered Flyer]] with [[Dihedral (aircraft)#Anhedral |anhedral]] (drooping) wings, which are inherently unstable. They showed that a pilot can maintain control of lateral roll and it was a good way for a flying machine to turn—to "[[banked turn|bank]]" or "lean" into the turn just like a bird or just like a person riding a bicycle.{{Sfn |Tobin |2004 |p = 70}}  Equally important, this method would enable recovery when the wind tilted the machine to one side. Although used in 1903, it would not become widely known in Europe until August 1908, when [[Wilbur Wright]] demonstrated to European aviators the importance of the [[Coordinated flight|coordinated use]] of elevator, rudder and roll control for making effective turns.{{Citation needed |date=February 2014}}

== Vertical wing position ==
The vertical positioning of the wing changes the roll stability of an aircraft.

* An aircraft with a "high" wing position (i.e., set on top of the fuselage) has a higher roll stability. For example, the [[Cessna 152]].
* An aircraft with a "low" wing (i.e., underneath the fuselage) has less roll stability. The [[Piper PA-25 Pawnee|Piper Pawnee]] uses a "low" wing.

== Unstable aircraft ==
[[Image:Usaf.f117.750pix.jpg|thumb|The [[Lockheed F-117 Nighthawk]] is not an inherently stable design.]]

Modern military aircraft, particularly low observable ("[[stealth aircraft|stealth]]") designs, often exhibit instability as a result of their shape. The [[Lockheed Martin|Lockheed]] [[F-117]] Nighthawk, for instance, employs a highly non-traditional fuselage and wing shape in order to reduce its [[radar cross section]] and enable it to penetrate air defenses with relative impunity. However, the flat facets of the design reduce its stability to the point where a computerized [[fly-by-wire]] system is required for it to fly.<ref name=asc>{{cite book |title=Airplane stability and control: a history of the technologies that made aviation possible |year=2002 |publisher=Cambridge Univ. Press |location=Cambridge [u.a.] |isbn=978-0-521-80992-4 |pages=335–337 |url=https://books.google.com/books?id=D-ctX2Q-CSIC |first1=Malcolm |last1=Abzug |edition=2 |first2=E. Eugene |last2=Larrabee}}</ref>

Relaxed stability designs are not limited to military jets. The [[McDonnell Douglas MD-11]] has a neutral stability design which was implemented to save fuel. To ensure stability for safe flight, an LSAS (Longitudinal [[Stability Augmentation System]]) was introduced to compensate for the MD-11's rather short horizontal stabilizer and ensure that the aircraft would remain stable.<ref name="Boeing">{{cite journal |title=The Effect of High Altitude and Center of Gravity on The Handling Characteristics of Swept-wing Commercial Airplanes |journal=Aero Magazine |volume=1 |issue=2 |url=https://www.boeing.com/commercial/aeromagazine/aero_02/textonly/fo01txt.html |access-date=29 June 2022 |publisher=Boeing}}</ref> However, there have been incidents in which the MD-11's relaxed stability caused an "inflight upset".<ref>{{cite news |last1=Pasztor |first1=Andy |title=FedEx Jet Has Control Issues |url=https://www.wsj.com/articles/SB123776874104009733 |access-date=1 October 2015 |agency=WSJ |date=March 24, 2009}}</ref>

== Intentional instability ==
[[Image:F-16 June 2008.jpg|thumb|The [[F-16 Fighting Falcon]] is an intentionally unstable design.]]

Many modern fighter aircraft often employ design elements that reduce stability to [[supermaneuverability|increase maneuverability]].  Greater stability leads to lesser control surface authority; therefore, a less stable design will have a faster response to control inputs. This is highly sought after in fighter aircraft design.

A less stable aircraft requires smaller control deflections to initiate maneuvering; consequently, drag and control surface imposed stresses will be reduced and aircraft responsiveness will be enhanced. Since these characteristics will typically make control by the pilot difficult or impossible, artificial stability will typically be imposed using computers, servos, and sensors as parts of a [[fly-by-wire]] control system.{{Citation needed |date=February 2014}}

== See also ==
* [[Index of aviation articles]]
* [[Dual control (aviation)]]
* [[Trim drag]]

== Citations ==
{{reflist |
<ref name= c>{{Cite book|last= Villard|first= Henry Serrano|title= Contact!: the story of the early aviators|year = 2002 | publisher =Dover Publications|location= Mineola, NY |isbn= 978-0-486-42327-2|pages= 39–53|url = https://books.google.com/books?id=tDmR7DhM_uEC&pg=PA40}}</ref>
<ref name= l>{{Cite book|last= Letcher|first= Piers|title= Eccentric France: the Bradt guide to mad, magical and marvellous France|year=2003|publisher=Bradt Travel Guides|location=Chalfont St. Peter, England|isbn = 978-1-84162-068-8 |pages=[https://archive.org/details/isbn_9781841620688/page/38 38]–39|url= https://archive.org/details/isbn_9781841620688|url-access= registration}}</ref>}}

== General and cited references ==
* {{Cite book | last = Crouch | first = Tom D | title = The Bishop's Boys: A Life of Wilbur and Orville Wright | place = New York | publisher = WW Norton & Co | year = 2003 | isbn = 978-0-393-30695-8 | url = https://archive.org/details/bishopsboyslifeo00crou_0 }}
* {{Cite book | last = Tobin | first = James | title = To Conquer The Air: The Wright Brothers and the Great Race for Flight | place = New York | publisher = Simon & Schuster | year = 2004 | isbn = 978-0-7432-5536-3}}

{{DEFAULTSORT:Relaxed Stability}}
[[Category:Aerodynamics]]