text: when the landing surface is smooth. 
  In ad- 
  dition, various stability and control character- 
  istics of a helicopter may produce critical con- 
  ditions in this area. The critical areas of the 
  “dead man’s curve” should be avoided unless 
  such operation is a specific mission require- 
  ment. 
  
  the low pitch limit, 
  the rotor RPM can be in- 
  creased only by a sacrifice in altitude or air- 
  speed. If insufficient altitude is available to 
  exchange for rotor speed, a hard landing is 
  inevitable. SufIicient rotor rotational energy 
  must be available to permit adding collective 
  pitch to reduce the helicopter’s rate of descent 
  before final ground contact. 
  
  “power 
  settling” has been used to describe a variety of 
  the helicopter. True 
  flight conditions of 
  “power settling” occurs only when the heli- 
  flow 
  copter rotor 
  ring state.” 
  condition called 
  The flow through 
  in the “vortex 
  is upward near the center of the 
  ring state” 
  disc and downward 
  the outer portion, 
  in 
  resulting in a condition of zero net thrust on 
  the rotor. 
  is zero, the 
  helicopter is effectively free-falling and ex- 
  tremely high rates of descent can result. 
  The downwash distribution within 
  
  POWER SETTLING.  The  term 