text: The static directional stability of an airplane 
  can be illustrated by a graph of yawing moment 
  coe&cient, C., versus sideslip angle, 8, such as 
  shown in figure 4.22. When the airplane is 
  subject to a positive sideslip angle, static direc- 
  tional stability will be evident if a positive 
  yawing moment coefficient results. Thus, 
  when the relative wind comes from the right 
  (+p), a yawing moment to the right (+C.) 
  should be created which tends to weathercock 
  the airplane and return the nose into the wind. 
  Static directional stability will exist when the 
  curve of C,, versus fi has a positive slope and the 
  degree of stability will be a function of.the 
  slope of this curve. If the curve has zero slope, 
  there is no tendency to return to equilibrium 
  and neutral static directional stability exists. 
  When the curve of C. versus /3 has a negative 
  slope, the yawing moments developed by side- 
  slip tend to diverge rather than restore and 
  static directional instability exists. 
  
  When the location of the vertical tail is set,, 
  the contribution of the surface to directional 
  to produce 
  stability depends on its ability 
  changes in lift-or 
  changes in 
  side force-with 
  sideslip. The surface area of the vertical tail 
  is a powerful factor with 
  the contribution of 
  the vertical tail being a direct function of the 
  area. When all other possibilities are ex- 
  hausted, the required directional stability may 
  be obtained by increases in tail area. How- 
  ever, increased surface area has the obvious 
  disadvantage of increased drag. 