Editing Relaxed stability (section)

{{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}}