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A diagrammatic representation of a fixed-wing airplane in phugoid. In aviation, a phugoid or fugoid (/ ˈ f juː ɡ ɔɪ d / ⓘ) is an aircraft motion in which the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "downhill" and "uphill".
The phugoid oscillation is a slow interchange of kinetic energy (velocity) and potential energy (height) about some equilibrium energy level as the aircraft attempts to re-establish the equilibrium level-flight condition from which it had been disturbed.
A short, sharp pull back on the control column may be used, and will generally lead to oscillations about the new trim condition. If the oscillations are poorly damped the aircraft will take a long period of time to settle at the new condition, potentially leading to Pilot-induced oscillation. If the short period mode is unstable it will ...
The aircraft began to exhibit phugoid oscillations, but the crew countered them and maintained a controlled descent at about 250 to 260 knots (460 to 480 km/h). They were able to bring the plane to 4,000 ft (1,220 m) and begin the approach to Tan Son Nhut's runway 25L.
Decreasing phugoid (long-period) oscillations can be achieved by building a smaller stabilizer on a longer tail, and by shifting the center of gravity to the rear. [citation needed] An aircraft that is not statically stable cannot be dynamically stable. [7]: 3
Pilot-induced oscillation as a result of an over-sensitive, yet slow-response flight control system was determined to be the cause. Subsequently, the flight control system was redesigned. Pilot-induced oscillation was blamed for the 1992 crash of the prototype Lockheed YF-22, landing at Edwards Air Force Base in California.
[3]: 1–6 With the total loss of hydraulic control and non-functional control surfaces, the aircraft entered phugoid oscillations lasting about 90 seconds, in which airspeed decreased as it climbed and increased as it fell. The rise in airspeed increased the lift over the wings, resulting in the aircraft climbing and slowing down, then ...
The longitudinal modes of a statically stable airplane following a disturbance were shown to consist of a long-period oscillation called the phugoid oscillation, usually with a period in seconds about one-quarter of the airspeed in miles per hour and a short-period oscillation with a period of only a few seconds. The lateral motion had three ...