text: Static directional stability must be in evi- 
  dence for all the critical conditions of flight. 
  is a ftm- 
  Generally, good directional stability 
  damental quality directly affecting the pilots’ 
  impression of an airplane. 
  
  is the primary reference in lateral stability as 
  well as directional stability considerations. 
  The yaw angle, #, is a primary reference for 
  wind tunnel tests and time history motion of 
  an airplane. From the definitions there is no 
  direct relationship between @ and # for an 
  airplane in free flight, e.g., an airplane flown 
  through a 360° turn has yawed 360” but side- 
  slip may have been zero throughout the entire 
  turn. Since the airplane has no directional 
  sense, static directional stability of the air- 
  plane is appreciated by response to sideslip. 
  
  CONTRIBUTION OF THE AIRPLANE 
  COMPONENTS. The static directional sta- 
  bility of the airplane is a result of contribution 
  of each of the various airplane components. 
  While the contribution of each component is 
  somewhat dependent upon and related to other 
  components, it 
  is necessary to study each 
  component separately. 
  is the primary source of 
  The vertical tail 
  directional stability 
  the airplane. As 
  for 
  shown in figure 4.23, when the airplane is in 
  a sideslip the vertical tail will experience a 
  change in angle of attack. The change in 
  lift-or 
  the vertical tail creates 
  a yawing moment about the center of gravity 
  the 
  which 
  relative wind. The magnitude of the vertical 
  tail contribution 
  to static directional stability 
  then depends on the change in tail lift and the 
  tail moment arm. Obviously, the tail moment 
  arm is a powerful factor but essentially dic- 
  tated by the major configuration properties of 
  the airplane. 