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Phugoid
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==Detailed description== The phugoid has a nearly constant [[angle of attack]] but varying [[pitch (flight)|pitch]], caused by a repeated exchange of [[airspeed]] and [[altitude]]. It can be excited by an [[elevator (aircraft)|elevator]] singlet (a short, sharp deflection followed by a return to the centered position) resulting in a [[pitch (flight)|pitch]] increase with no change in [[trim tab|trim]] from the [[cruise (flight)|cruise]] condition. As speed decays, the nose drops below the horizon. Speed increases, and the nose climbs above the horizon. Periods can vary from under 30 seconds for [[light-sport aircraft|light aircraft]] to minutes for [[airliner|larger aircraft]]. [[Ultralight aviation|Microlight aircraft]] typically show a phugoid period of 15–25 seconds, and it has been suggested{{by whom|date=March 2019}} that birds and model airplanes show convergence between the phugoid and short period modes. A classical model for the phugoid period can be simplified to about (0.85 times the speed in [[knot (unit)|knots]]) seconds, but this only really works for larger aircraft.{{Explain|reason=why larger aircraft? Is there an analogue for smaller ones? What counts as larger?|date=March 2019}} Phugoids are often demonstrated to student pilots as an example of the speed stability of the aircraft and the importance of proper trimming. When it occurs, it is considered a nuisance, and in lighter airplanes (typically showing a shorter period) it can be a cause of [[pilot-induced oscillation]]. The phugoid, for moderate amplitude,<ref>Charles Hampson Grant, ''Model Airplane Design and Theory of Flight'', Jay, New York, 1941</ref> occurs at an effectively constant angle of attack, although in practice the angle of attack actually varies by a few tenths of a degree. This means that the stalling angle of attack is never exceeded, and it is possible (in the <1g section of the cycle) to fly at speeds below the known stalling speed. Free flight models with badly unstable phugoid typically stall or loop, depending on thrust.<ref>Keith Laumer, ''How to Design and Build Flying Models'', Harper, New York, 1960</ref> An unstable or divergent phugoid is caused, mainly, by a large difference between the incidence angles of the wing and tail. A stable, decreasing phugoid can be attained by building a smaller stabilizer on a longer tail, or, at the expense of pitch and yaw "static" stability, by shifting the center of gravity to the rear.{{why|date=July 2022}}{{cn|date=July 2022}} Aerodynamically efficient aircraft typically have low phugoid damping.<ref name="Stengel">{{cite book |last1=Stengel |first1=Robert F. |title=Flight Dynamics |date=17 October 2004 |publisher=Princeton University Press |isbn=978-0-691-11407-1 |url=https://books.google.com/books?id=dWKYDwAAQBAJ |access-date=6 July 2022 |language=en}}</ref>{{rp|464}} The term "phugoid" was coined by [[Frederick W. Lanchester]], the British aerodynamicist who first characterized the phenomenon. He derived the word from the [[Greek (language)|Greek]] words {{lang|grc|φυγή}} and {{lang|grc|εἶδος}} to mean "flight-like" but recognized the diminished appropriateness of the derivation given that {{lang|grc|φυγή}} meant flight in the sense of "escape" (as in the word "fugitive") rather than vehicle flight.<ref>Frederick William Lanchester, ''Aerodonetics: Constituting the second volume of a complete work on aerial flight'', (London, England: Archibald Constant Co. Ltd., 1908), [https://archive.org/stream/aerodoneticscon02lancgoog#page/n13/mode/2up p. viii] and [https://archive.org/stream/aerodoneticscon02lancgoog#page/n371/mode/2up p. 348.]</ref>
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