text: The airplane in free flight has six degrees of 
  freedom: rotation in roll, pitch, and yaw and 
  in the horizontal, vertical, and 
  translation 
  lateral directions. 
  In the case of longitudinal 
  dynamic stability, 
  the degrees of freedom can 
  be limited 
  rotation, vertical and 
  translation. Since the airplane is 
  horizontal 
  usually symmetrical from port to starboard, 
  there will be no necessity for consideration of 
  coupling between longitudinal and lateral- 
  directional motions. Thus, the principal vari- 
  ables in the longitudinal motion of an airplane 
  will be: 
  
  to pitch 
  
  Due to the nature of the phugoid, it is not 
  necessary to make any specific aerodynamic 
  provisions to contend with 
  the oscillation. 
  The inherent long period of the oscillation al- 
  lows study to be directed to more important 
  oscillatory 
  the diffet- 
  tendencies. Similarly, 
  ences between the stick-fixed and stick-free 
  phugoid are not of great importance. 
  
  (1) The pitch attitude of the airplane. 
  (2) The angle of attack (which will differ 
  from the pitch attitude by the inclination of 
  the flight- path). 
  (3) The flight velocity. 