text: 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. 
  
  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. 
  
  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. 