Most have some degree of long-term
phugoid, plus they will simply wander after a half-dozen seconds no matter how well trimmed.
On this basis, the proportional-integral-derivative (PID) controller is used to achieve the desired flight performance under the assumption that the short-period dynamics are faster than the
phugoid mode [1].
On the other hand, parameters of the
phugoid mode change slowly enough for the pilot to modify the oscillation of the aircraft.
A
phugoid, while fun to say, is a large scale oscillation characterized by constant AOA but changing pitch and airspeed.
In reality, the resulting motion is a superposition of two fundamental and independent oscillations: the
phugoid (or long-period) oscillation and the short-period oscillation.
The T-50's triple-redundant, fly-by-wire control system feeds back a term of one minus the cosine of the pitch angle, essentially eliminating the natural long-period, longitudinal oscillation (known to engineers as the
phugoid) that is shared by all stable aircraft.
In slow flight, the Astore has a predictable
phugoid recovery when displaced from a trimmed airspeed, but we found this less so at higher speeds, where it was sluggish to recover from a hands-off pitch-down displacement.
The turns onto the localizer from the arc always looked like a shepherd's crook (the overshoot) followed by a decreasing
phugoid as you captured the beam.
When disturbed in pitch, it barely has a
phugoid at all, recovering to the trimmed airspeed in a cycle and a half.
In trimmed cruise, disturbing the pitch provokes a
phugoid that damps almost completely in two cycles, suggesting that the airplane will be a standout for instrument training and instrument flight.
Most have some degree of long-term
phugoid, plus they will simply wander after a half dozen seconds no matter how well trimmed.
An excited
phugoid seemed to damp back to the trimmed airspeed in less than a cycle, but given the turbulence, we couldn't judge it accurately.
