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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?] [citation needed] Aerodynamically efficient aircraft typically have low phugoid damping. [3]: 464
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.
Dynamic stability is caused by damping. If damping is too great, the aircraft will be less responsive and less manoeuvrable. [3] [11]: 588 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]
Since the lift is very much greater than the drag, the phugoid is at best lightly damped. A propeller with fixed speed would help. Heavy damping of the pitch rotation or a large rotational inertia increase the coupling between short period and phugoid modes, so that these will modify the phugoid.
This yaw-roll coupling is one of the basic flight dynamic modes (others include phugoid, short period, and spiral divergence). This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping as airspeed decreases and altitude increases.
[3]: 319 This greatly excited the phugoid motion, [3]: 291 and the aircraft pitched up, before pitching back down after power was reduced. When power was added again, the aircraft rapidly pitched up to 40°, and the airspeed dropped down to 108 knots (200 km/h; 124 mph) at 6:49:30 p.m., [ 3 ] : 1–6, 291 briefly stalling at 9,000 feet (2,700 m).
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 ...
The logarithmic decrement can be obtained e.g. as ln(x 1 /x 3).Logarithmic decrement, , is used to find the damping ratio of an underdamped system in the time domain.. The method of logarithmic decrement becomes less and less precise as the damping ratio increases past about 0.5; it does not apply at all for a damping ratio greater than 1.0 because the system is overdamped.