The flight characteristics of current-day large airplanes such as the Lockheed C-5A Galaxy can offer some clues as to what to expect from the really large machines of the future. For example, the C-5A’s longitudinal short-period oscillation approaches in frequency that of the phugoid oscillation (Mueller, 1970). Normally, the short-period oscillation is much higher in frequency than the phugoid oscillation.
This is not surprising since the phugoid oscillation frequency depends only on an airplane’s true airspeed and is invariant with airplane size. However, airplane short-period frequency is proportional to the square root of the quantity (linear dimension cubed divided by pitching moment of inertia). That combination of parameters varies roughly as
the reciprocal of a linear dimension, such as wing span or fuselage length, implying lower frequency short-period oscillations for large airplanes.
The practical consequence, according to Mueller, is that the C-5A is difficult to trim for a particular airspeed. Pilots report that the airplane wanders about a trim point.
Additional longitudinal dynamics problems could be encountered on very large airplanes as a result of low-frequency flexible modes.