phugoidens

Phugoid oscillations, sometimes referred to as phugoidens, are a class of longitudinal oscillations in aircraft dynamics. They occur when the aircraft exchanges kinetic and potential energy as flight path and speed vary around a trimmed condition, producing a long-period, gentle maneuver with modest changes in pitch.

In the standard longitudinal model, the phugoid represents a slow, almost sinusoidal motion in speed and altitude, with small deviations in angle of attack and pitch. The period is typically tens of seconds, and the damping is light compared with the short-period mode.

The phugoid is coupled to the shorter, faster short-period mode; together they define the overall longitudinal stability. The characteristics of a phugoid depend on lift and drag properties, thrust, weight, elevator effectiveness, tailplane stability derivatives, and energy management by the engine.

Gusts, throttle or pitch commands, or loss of altitude can excite phugoidens. In well-designed aircraft, aerodynamic damping and control systems suppress sustained phugoid motion, and pilots may dampen it with gentle pitch and throttle adjustments or autopilot trim.

In analysis, phugoidens are examined using linearized longitudinal equations of motion and eigenvalue analysis, to assess stability and damping margins. While they can be noticeable in flight tests, modern designs aim for adequate damping to avoid sustained oscillations while preserving controllability and ride quality.