text: figure 4.36. The directional control require- 
  ments are easily met in a tandem-rotor heli- 
  copter because the engine torque from one 
  rotor 
  is opposed by the torque of the other 
  rotor thereby eliminating one directional mo- 
  ment. Of course, some net unbalance of torque 
  may have to be overcome if the engine torque 
  on the two rotors is different. 
  
  When a tandem-rotor helicopter is rotated 
  rapidly about one of the rotors rather than 
  about 
  rotor picks up 
  the other 
  “translational 
  as a result of the velocity 
  due to rotation and an increase in rotor thrust 
  results. This causes pitch-up or pitch-down 
  depending on which rotor 
  is 
  being rotated about. Rotation about 
  the 
  is more common, re- 
  forward rotor, which 
  sults in pitch-down. 
  
  The overall stability of a helicopter results 
  from the individual stability contributions of 
  the various components just as in the case of 
  the fixed-wing airplane. The stability con- 
  tributions can be divided as follows: 
  
  (1) Rotor 
  (2) Fuselage 
  (3) Stabilizers 
  (4) Mechanical devices 
  
  The destabilizing contribution of the fuselage 
  and the stabilizing contribution of a stabilizing 
  surface are similar in effect to an airplane and 
  will not be discussed here. The principal 
  stability characteristics that make the heli- 
  copter different from an airplane are those of 
  the rotor. 