NAVWEPS 00-8OT-80  STABILITY AND CONTROL  speed of the airplane in the lightest practical  takeoff configuration. This will provide ade-  quate directional control for the remaining  conditions of flight.  high angles of attack is required to effect re-  covery during a spin. Since the effectiveness  of the vertical tail is reduced at large angles of  attack, the directional control power neces-  sary for spin recovery may produce a critical  requirement of rudder power.  SLIPSTREAM ROTATION.  A critical di-  rectional control requirement may exist when  the propeller powered airplane is at high  power and low airspeed. As shown in figure  4.26, the single rotation propeller induces  a slipstream swirl which causes a change in  flow direction at the vertical tail.  The rudder  must furnish sufficient control power to balance  this condition and achieve zero sideslip.  Once defined, the minimum directional con-  trol speed is not a function of weight, altitude,  etc., but is simply the equivalent airspeed (or  dynamic pressure).  to produce a required yaw-  ing moment with the maximum rudder deflec-  tion.  If the airplane is operated in the critical  unbalance of power below the minimum con  trol speed, the airplane will yaw uncontrolla-  bly into the inoperative engine. In order to  regain directional control below the minimum  speed certain alternatives exist: reduce power  on the operating engines or sacrifice altitude  for airspeed. Neither alternative is satisfac-  tory if the airplane is in a marginal condition  of powered flight so due respect must be given  to the minimum control speed.  CROSSWIND TAKEOFF AND LANDING.  Since the airplane must make a true path down  the runway, a crosswind during takeoff or  landing will require that the airplane be.con-  trolled in a sideslip. The rudder must have  sufficient control power to create the required  sideslip for the expected crosswinds.  Due to the side force on the vertical tail, a  slight bank is necessary to prevent turning  flight at zero sideslip. The inoperative engine  will be raised and the inclined wing lift will  provide a component of force to balance the 1  side force on the tail.  ASYMMETRICAL  POWER.  The design  of a multiengine airplane must account for the  possibility of an engine failure at low airspeed.  The unbalance of thrust from a condition of  unsymmetrical power produces a yawing mo-  ment dependent upon the thrust unbalance  and the lever arm of the force.  The deflection  of the rudder will create a side force on the tail  and contribute a yawing moment to balance  the yawing moment due to the unbalance of  thrust.  Since the yawing moment coefficient  from the unbalance of thrust will be greatest  at low speed, the critical requirement will be  at a low speed with the one critical engine  out and the remaining engines at maximum  power.  Figure 4.26 compares the yawing  moment coeflicient for maximum rudder deflec-  tion with the yawing moment coefficient for  the unbalance of thrust.  The intersection of  the two lines,determines the minimum speed  for directional control, i.e., the lowest speed  at which the rudder control moment can equal  the moment of unbalanced thrust,  It is usually  specified  that the minimum directional control  speed be no greater than 1.2 times the stall  In each of the critical conditions of required  directional control, high directional stability  is desirable as it will reduce the displacement  of the aircraft from any disturbing influence.  Of course, directional control must he sufficient  to attain zero sideslip. The critical control  requirement for the multiengine airplane is  the condition of asymmetrical power since  spinning is not common to this type of airplane.  The single engine propeller airplane may have  either the spin recovery or the slipstream rota-  tion as a critical design condition.  The single  engine jet airplane may have a variety of  critical items but the spin recovery require-  ment usually predominates.  The static lateral stability of an airplane  involves consideration of rolling moments due  LATERAL  STABILITY  AND  CONTROL  LATERAL STABILITY  294  Revised January 1965 