text: (4) The displacement or deflection of the 
  elevator when the stick-free condition 
  considered. 
  The longitudinal dynamic stability of an 
  airplane generally consists of three basic modes 
  (or manners) of oscillation. While the longi- 
  tudinal motion of the airplane may consist of a 
  combination of these modes, the characteristics 
  of each mode are sufficiently distinct that each 
  oscillatory tendency may be studied separately. 
  The first mode of dynamic longitudinal sta- 
  bility consists of a very long period oscillation 
  referred to as the phagoid. The phugoid or long 
  period oscillation 
  involves noticeable vatia- 
  tions in pitch attitude, altitude, and airspeed 
  but nearly constant angle of attack. Such an 
  oscillation of the airplane could be considered 
  as a gradual interchange of potential and 
  kinetic energy about some equilibrium airspeed 
  and altitude. Figure 4.20 illustrates the char- 
  acteristic motion of the phugoid. 
  
  is 
  
  Of course, the airplane must demonstrate 
  positive dynamic stability for the major longi- 
  the airplane 
  tudinal motions. 
  must demonstrate a certain degree of longitu- 
  dinal stability by reducing the amplitude of 
  motion at a certain rate. The requited degree 
  is usually specified by 
  of dynamic stability 
  the time necessary for the amplitude to reduce 
  to one-half the original value-the 
  time to 
  damp to half-amplitude. 
  
  The period of oscillation 