text: As in the airplane, there is one angle of 
  attack or blade pitch condition that will result 
  the most efficient operation. Unfortu- 
  in 
  nately, the typical helicopter rotor operates 
  at a near constant RPM and thus a constant 
  true airspeed and cannot operate at this most 
  eficient condition over a wide range of altitude 
  and gross weight as the fixed-wing airplane. 
  The airplane is able to maintain an efficient 
  angle of attack at various altitudes, and gross 
  weights by flying at various airspeeds but the 
  helicopter will operate with a near constant 
  rotor velocity and vary blade angle to contend 
  with variations in altitude and gross weight. 