text: in a single rotor heli- 
  copter is obtained by a tail rotor (antitorque 
  rotor) since a conventional aerodynamic sur- 
  face would not be effective at low speeds or 
  hovering. 
  require- 
  ments of the tail rotor on a typical shaft-driven 
  helicopter are quite demanding since it must 
  counteract the engine torque being supplied to 
  the main rotor as well as provide directional 
  control. Being a rotor in every respect, the 
  tail rotor requires some of the engine power to 
  generate its control forces. Unfortunately, the 
  maximum demands of the tail rotor occur at 
  conditions when engine power is also in great 
  demand. The most critical condition is while 
  hovering at maximum gross weight. The tail 
  rotor effectiveness is determined by the rotor 
  characteristics and the distance the tail rotor 
  is behind the c.g. The control specifications 
  require the helicopter to be able to turn in the 
  most critical direction at some specified rate 
  while hovering at maximum gross weight in a 
  specified wind condition. Also, it is required 
  that the helicopter have sufficient directional 
  to fly sideways up to 30 knots, an 
  control 
  important requirement for plane guard duties. 
  requirements are 
  easily met by a tip-driven helicopter since the 
  directional control does not have to counter 
  the engine torque. 