ABSTRACT
Even as it flies along a straight line path, the airplane’s flight can be deflected due to disturbances, no matter what their source. We have seen that these may usually be resolved into two distinct modes-a faster short-period mode involving change in pitch angle and angle of attack that appears as a nose-bobbing motion, and a slower phugoid mode involving velocity and flight path angle, an exchange between the airplane’s kinetic and potential energies that appears as a lazy wave-like up-down motion. Of course, these motions are superposed over the mean forward motion of the airplane. As long as the short-period and phugoid modes are stable, the disturbances die out with time and the airplane recovers its original flight condition. How quickly the disturbances die out depends on the damping of the respective modes. We have derived expressions for the frequency and damping of these modes in terms of the flight condition, the aerodynamic coefficients and derivatives of the aerodynamic coefficients.
