Phugoid
From Wikipedia, the free encyclopedia
A phugoid 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).
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[edit] Detailed description
The phugoid is a constant angle of attack (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 more than a few 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 x speed in knots) seconds, but this only really works for larger aircraft.
Colloquially known as porpoising, 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's a pure nuisance mode, and in lighter aeroplanes (typically showing a shorter period) it can be a cause of pilot-induced oscillation.
An interesting characteristic of the phugoid is that it occurs at effectively constant angle of attack (AoA), although in practice AoA actually varies by a few tenths of a degree. This means that the stalling AoA is never exceeded, and it's possible (in the <1g section of the cycle) to fly at speeds below the known stalling speed.
The name apparently is an example of poor Latin translation by Lanchester, a British aerodynamicist who first predicted it. The Latin verb for "to flee" (fugit) was used when what was desired was the Latin verb for "to fly" (volat).
[edit] Phugoids in aviation incidents
For a dramatic example of phugoids, read about the United Airlines Flight 232 incident, where an engine failure caused total hydraulic system failure. The crew flew the aircraft with throttle only. Suppressing the phugoid tendency was particularly difficult. In another incident, Japan Airlines Flight 123 also lost all hydraulic controls and went into a phugoid. Another aircraft that lost all hydraulics was a DHL operated Airbus A300B4 that was hit by an SA-7 'Grail' surface-to-air missile fired by Iraqi insurgents. This was the first time that a crew was able to land an aircraft safely with only adjusting engine thrust.[1]
