text: pitch 
  frequency. Gen- 
  erally, the greater the static longitudinal and 
  the higher will be the 
  directional stability, 
  coupled pitch-yaw frequency. When the air- 
  plane is subject to roiling motion, the inertia 
  couple disturbs the airplane in pitch and yaw 
  with each roll revolution and provides a dis- 
  turbing 
  the airplane is 
  rolled at a rate equal to the coupled pitch-yaw 
  frequency, the oscillatory motion will either 
  diverge or stabilize at some maximum ampli- 
  tude depending on the airplane characteristics. 
  The longitudinal stability of the typical high 
  speed configuration is much greater than the 
  directional stability and results in a pitch fre- 
  quency higher than the yaw frequency. 
  In- 
  creasing the directional stability by increasing 
  the vertical tail area, addition of ventral hns, 
  or use of stabilization systems will increase the 
  coupled pitch-yaw frequency and raise the roll 
  rate at which a possible divergent condition 
  could exist. 
  Increasing directional stability 
  by the addition of ventral fins rather than by 
  addition to the vertical tail has an advantage 
  to the positive dihedral 
  of not contributing 
  effect at low or negative angles of attack. 
  High dihedral effect makes higher roll rates 
  more easily attainable in roll motion where 
  proverse yaw occurs. 