Long and short period oscillations
Oscillating motions can be described by two parameters, the period of time required for one complete oscillation, and the time required to damp to half-amplitude, or the time to double the
amplitude for a dynamically unstable motion. The
longitudinal motion consists of two distinct oscillations, a long-period oscillation called a
phugoid mode and a short-period oscillation referred to as the short-period mode.
The phugoid or long-period mode is the one in which there is a large-amplitude variation of air-speed, pitch and flight path angle, and altitude, but almost no angle-of-attack variation. The phugoid oscillation is really 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. The motion is so slow that the effects of
inertia forces and damping forces are very low. Although the damping is very weak, the period is so long that the pilot usually corrects for this motion without being aware that the oscillation even exists. Typically the period is 20-60 seconds.
The short-period mode is a very fast, usually heavily damped, oscillation with a period of a few seconds. The motion is a rapid pitching of the aircraft about the center of gravity. The period is so short that the speed does not have time to change, so the oscillation is essentially an angle-of-attack variation. The time to damp the amplitude to one-half of its value is usually on the order of 1 second.
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22-11-2007, 12:30
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