Longitudinal Dynamics

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.