NAVWEPS 00-ROT-80  STABILITY AND CONTROL  Static directional stability must be in evi-  dence for all the critical conditions of flight.  Generally, good directional stability is a ftm-  damental quality directly affecting the pilots’  impression of an airplane.  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 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.  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.  The vertical tail is the primary source of  directional stability  for the airplane.  As  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  side force-on the vertical tail creates  a yawing moment about the center of gravity  which tends to yaw the airplane into the  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.  When the location of the vertical tail is set,,  the contribution of the surface to directional  stability depends on its ability  to produce  changes in lift-or  side force-with changes in  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.  The final chart of figure 4.22 illustrates the  fact that the instantaneous slope of the curve of  C,, versus @ will describe the static directional  stability of the airplane.  At small angles of  sideslip a strong positive slope depicts strong  directional stability.  Large angles of sideslip  produce zero slope and neutral stability.  At  very high sideslip the negative slope of the  curve indicates directional instability.  This  decay of directional stability with increased  sideslip is not an unusual condition.  However,  directional instability should not occur at the  angles of sideslip of ordinary flight conditions.  The lift curve slope of the vertical tail  relates how sensitive the surface is to changes  in angle of attack. While it is desirable to  have a high lift curve slope for the vertical  surface, a high aspect ratio surface is not  necessarily practical or desirable. The stall 