NAVWEPS 00-SOT-80  STABIUTY AND CONTROL  have a stable rudder pedal feel through the  available range of sideslip.  DIRECTIONAL  CONTROL  REQUIRE-  MENTS.  The control power of the rudder  must be adequate to contend with the many  unsymmetrical conditions of flight.  Gener-  ally, there are five conditions of flight which  provide the most criticalrequirements of di-  ‘ rectional control power.  The type and mission  of the airplane will  decide which of these  conditions is most important.  capable of producing sufhcient yawing moment  for the critical conditions of flight.  The effect of rudder deflection is to produce  a yawing moment coefficient according to  control deflection and produce equilibrium at  some angle of sideslip. For small deflections  of the rudder, there is no change in stability  but a change in equilibrium.  Figure 4.25  shows the effect of rudder deflection on yawing  moment coefficient curves with the change in  equilibrium sideslip angle.  If the airplane exhibits static directional  stability with rudder lixed, each angle of side-  slip requires a particular deflection of the  rudder to achieve equilibrium.  Rudder-free  directional stability will exist when the float  angle of the rudder is less than the rudder  deflection required for equilibrium.  However  at high angles of sideslip, the floating tend-  ency of the rudder increases. This is illus-  trated by the second chart of figure 4.25 where  the line of rudder float angle shows a sharp  increase at large values of sideslip.  If the  floating angle of the rudder catches up with  the required rudder angle, the, rudder pedal  force will decrease  to zero and rudder lock will  occur. Sideslip angles beyond this point pro-  duce a floating angle greater than the required  rudder deflection and the rudder tends to float  to the limit of deflection.  ADVERSE YAW.  When an airplane is  rolled into a turn yawing moments are pro-  duced which require rudder deflection to main-  tain zero sideslip, i.e., coordinate the turn.  The usual source of adverse yawing moment is  illustrated in figure 4.26. When the airplane  shown is subject to a roll to the left, the down-  going port wing will experience  a new relative  wind and an increase in angle of attack.  The  inclination of the lift vector produces a com-  ponent force forward on the downgoing wing.  The upgoing starboard wing has its lift in-  clined with a component force aft.  The re-  sulting yawing moment due to rolling motion  is in a direction opposite to the roll and is  hence “adverse yaw.”  The yaw due to roll is  primarily a function of the wing lift coefficient  and is greatest at high C,.  In addition to the yaw due to rolling motion  there will be a yawing moment contribution  due to control surface  deflection.  Conventional  ailerons usually contribute an adverse yaw  while spoilers may contribute a favorable or  “proverse”  yaw.  The high  wing airplane  with a large vertical tail may encounter an  influence from inboard ailerons. Such a con-  figuration may induce flow directions at the  vertical tail to cause proverse yaw.  Rudder lock is accompanied by a reversal of  pedal force and rudder-free instability  will  exist. The dorsal fin is a useful addition in  this case since it will improve the directional  stability at high angles of sideslip. The re-  sulting increase in stability requires larger  deflections of the rudder to achieve equilibrium  at high sideslip and the tendency for rudder  lock is reduced.  Since adverse yaw will be greatest at high  C, and full deflection of the ailerons, coordi-  nating steep turns at low speed may produce  a critical requirement for rudder control power.  Rudder-free directional stability is appre-  ciated by the pilot as the rudder pedal force to  maintain a given sideslip.  If the rudder pedal  force gradient is too low near zero sideslip, it  will be difficult to maintain zero sideslip dur-  ing various maneuvers. The airplane should  SPIN RECOVERY. In the majority of air-  planes, the rudder is the principal control for  spin recovery. Powerful control of sideslip at  291 