NAVWRPS DD-807-80  STABILITY AND CONTROL  because of increase in the fuselage boundary  layer at the vertical tail location. The decay of  dir&ctional stability with angle of attack is  most significant for the low aspect ratjo air-  plane with sweepback since this configuration  requires such high angles of attack to achieve  high lifr coefficients.  Such decay in directional  stability can have a profound effect on the re-  sponse of the airplane to adverse  yaw and spin  characteristics.  destabilizing and the greatest contribution  will occur at high power and low dynamic  pressure as during a waveoff.  As in the case  of longitudinal static stability,  freeing the controls will reduce the effective-  ness of the tail and alter the stability.  While  the rudder must be balanced to reduce control  pedal forces, the rudder will tend to float or  streamline and reduce the contribution of the  vertical tail to static directional stability. The  floating tendency is greatest at large angles of  sideslip where large angles of attack for the  vertical tail tend to decrease  aerodynamic bal-  ante. Figure 4.24 illustrates the difference  be-  tween rudder-fixed and rudder-free static di-  rectional stability.  High Mach ntrmbers  of supersonic  flight reduce  the contribution of the vertical tail to direc-  tional stability because  of the reduction of lift  cnrve slope with Mach number. The third  chart of figure 4.24 illustrates the typical decay  of directional stability with Mach number.  To  produce the required directional stability at  high Mach numbers, a viziy large vertical tail  area may be necessary. Ventral fins may be  added as an additional contribution to direc-  tional stability but landing clearance  require-  ments may limir their size or require the fins to  be retractable.  CRITICAL CONDITIONS.  The most criti-  cal conditions of,staric directional stability are  usually the combination of several separate  effects. The combination which produces the  most critical condition is much dependent upon  the type and mission of the airplane.  In addi-  tion, there exists a coupling of lateral and di-  rectional effects  such that the required degree  of static directional stability may be deter-  mined by some of these coupled conditions.  Hence, the most critical demands of static  directional stability will  occur from some  combination of the following effects:  Center of gravity position has a relatively  negligible effect on static directional stability.  The usual range of c.g. position on any air-  plane is set by the Jinits of long&d&a/ stability  and control.  Within this limiting  range of  c.g. position, no significant changes take place  in the contribution of the vertical tail, fuselage,  nacelles, etc.  Hence, the static directional  stability is essentially unaffected by the varia-  tion of c.g. position within the longitudinal  limits.  The propeller powered airplane may have such  considerable power effects that the critical  conditions may occur at low speed while the  effect of high Mach numbers may produce the  critical conditions for the typical supersonic  airplane. In addition, the coupling of lateral  and directional effects may require prescribed  degrees of directional stability.  (1) high angle of sideslip  (2) high power at low airspeed  (3) high angle of attack  (4) high Mach number  In addition to directional stability, the air-  plane must have adequate directional control  to coordinate turns, balance power effects,  create sideslip, balance unsymmetrical power,  etc. The principal source of directional con-  trol is the rudder and the rudder must be  When the airplane is at a high angle of  a$tack  a decrease  in static directional stability can be  anticipated. As shown by the second chart of  figure 4.24, a high angle of attack reduces the  stable slope of the curve of C,, versus 8,  The  decrease  in static directional stability is due in  great part to the reduction in the contribution  of the vertica1 tail.  At high angles of attack,  the effectiveness  of the vertical tail is reduced  DIRECTIONAL CONTROL 