A phugoid (pronounced /ˈfjuːˌgoɪ̯d/) is an aircraft motion where the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "uphill" and "downhill." This is one of the basic flight dynamics modes of an aircraft (others include short period, dutch roll, and spiral divergence).

Detailed description

The phugoid is a constant angle of attack (AoA - but varying pitch angle) exchange of airspeed and altitude. It can be excited by an elevator singlet (a short, sharp deflection followed by a return to the centered position) resulting in a pitch increase with no change in trim from the cruise condition. As speed decays, the nose will drop below the horizon. Speed will increase, and the nose will climb above the horizon. Periods can vary from under 30 seconds for light aircraft to minutes for larger aircraft. Microlight aircraft typically show a phugoid period of 15–25 seconds, and it has been suggested 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 × speed in knots) seconds, but this only really works for larger aircraft.

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 aeroplanes (typically showing a shorter period) it can be a cause of pilot-induced oscillation.

The phugoid, for moderate amplitude, occurs at an effectively constant angle of attack, although in practice AoA actually varies by a few tenths of a degree. This means that the stalling AoA 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.

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.

The name apparently is an example of poor Greek translation by Lanchester, a British aerodynamicist who first predicted it. The English word was mistakenly derived from the Greek word for "fleeing, flight" (phuge) instead of the Greek word for "flying, flight".

Phugoids in aviation incidents

Japan Airlines Flight 123 lost all hydraulic controls and its vertical stabiliser, and went into a phugoid before crashing into a mountain. With 520 deaths it remains the deadliest single-aircraft disaster in history.

United Airlines Flight 232 suffered an engine failure which caused total hydraulic system failure. The crew flew the aircraft with throttle only. Suppressing the phugoid tendency was particularly difficult. The pilots were able to reach Sioux City Gateway Airport but crashed during the landing attempt. The pilots and a majority of the passengers survived.

Another aircraft that lost all hydraulics was a DHL operated Airbus A300B4 that was hit by an surface-to-air missile fired by Iraqi insurgents in the 2003 Baghdad DHL attempted shootdown incident. This was the first time that a crew was able to land an air transport aircraft safely only adjusting engine thrust.

The crash of the Helios solar powered aircraft was precipitated by reacting to an inappropriately diagnosed phugoid oscillation the results of which ultimately resulted in the aircraft structure exceeding design loads.


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