PdAVWEPS  OD-BOT-80  APPLICATION OF AERODYNAMICS  TO SPECIFIC PROBLEMS OF FLYING  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.  The term “power  settling” has been used to describe a variety of  flight  conditions of the helicopter.  True  “power settling”  occurs only when the heli-  copter rotor is operating in a rotary flow  condition called the “vortex  ring state.”  The flow through the rotor in the “vortex  ring state”  is upward near the center of the  disc and downward in the outer portion,  resulting in a condition of zero net thrust on  the rotor.  If the rotor thrust is zero, the  helicopter is effectively free-falling and ex-  tremely high rates of descent can result.  In the case of most small helicopters, at  least 300 feet of altitude is necessary  for an  average pilot to set up a steady autorotation  and land the helicopter safely without damage.  This minimum becomes  500 to 600 feet for the  larger helicopters, and will be even greater for  helicopters with increased  disc loading.  These  characteristics are usually presented in the  flight handbook in the form of a “dead man ’ s  curve” which shows the combinations of air-  speed and altitude above the terrain where a  successful  autorotative landing would be dish-  cult, if not impossible.  The downwash distribution  within  the  rotor is shown in figure 6.19 for the conditions  of normal hovering and power settling. Part  A of figure 6.19 illustrates the typical down-  wash distribution  for hovering flight.  If  sufficient power were not available to hover  at this condition, the helicopter would begin  to settle at some rate of descent depending on  the deficiency of power. This rate of descent  would  effectively decrease the downwash  throughout the rotor and result in a redistri-  bution of downwash similar to Part B of  figure 6.19. At the outer portion of the  rotor disc, the local induced downwash veloc-  ity is greater than the rate of descent and  downflow exists. At the center of the rotor  disc, the rate of descent is greater than the  local induced downwash velocity and the  resultant flow is upward.  This flow condition  results in the rotary “vortex ring”  state. By  reference  to the basic momentum theory it is  apparent that the rotor will produce no thrust  in this condition if the net mass flow of air  through the rotor is zero. It is important to  note that the main lifting part of the rotor is  not stalled. The rotor roughness and loss of  A typical “dead man ’ s curve” is shown in  figure 6.18.  The most critical combinations  are due to low altitude and low airspeed illus-  trated by area A of figure 6.18. Less critical  conditions exist at higher airspeeds  because  of  the greater energy available to set up a steady  autorotation.  The lower limit of area A is a  finite altitude because the helicopter can be  landed successfully  if collective pitch is held  rather than reduced. In this specific  case there  is not sufficient energy to reach a steady state  autorotation. The maximum altitude at which  this is possible is approximately ten feet on  most helicopters.  Area B on the “dead man ’ s curve” of figure  6.18 is critical because  of ground contact flight  speed or rate of descent, which is based on the  strength of the landing gear.  The average  pilot may have difhculty in successfully  flaring  the helicopter from a high speed flight con-  dition without allowing the tail rotor to strike  the ground or contacting the ground at an ex-  cessive  airspeed. A less critical zone is some-  times shown on this curve to indicate that  higher ground contact speeds  can be permitted 