NAVWEPS 00-801-80  STABILITY AND CONTROL  retreating blade since the relative wind veloci-  ties are greater on the advancing blade. This  lateral dissymmetry of lift causes  the rotor to  tilt back due to the gyroscopic effect of the  rotor, further increasing the rotor angle of  attack.  Thus, the rotor is unstable with  changes in angle of attack at forward flight  speeds. Since the magnitude of the unstable  moment is affected by the magnitude of  the rotor thrust  as well  as the tilt of  the thrust force, a greater instability exists  for increases in angle of attack than for  decreases  in angle of attack. In addition, the  instability is greater for increases  in angle of  attack when the rotor thrust also increases.  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 the cg.,  the other rotor picks up  “translational lift”  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 the helicopter is  being rotated about.  Rotation about the  forward rotor, which is more common, re-  sults in pitch-down.  If the rotor angle of attack is held constant  and the rotor is given a translational velocity,  a dissymmetry of lift results since the velocity  of the advancing blade is increased while the  velocity of the retreating blade is decreased.  This dissymmetry of lift causes the rotor to  tilt  in a direction to oppose the change in  velocity due to the gyroscopic effect of the  rotor. Hence, the rotor has velocity stability.  A hovering helicopter exhibits some degree  of apparent stability by virtue of its velocity  stability  although it has neutral angle of  attack stability.  This type of hovering sta-  bility  is analogous to the apparent lateral-  directional stability an airplane exhibits due  to dihedral effect. Additional hovering sta-  bility can be obtained by the use of mechanical  stabilizers such as th,e Bell stabilizer bar, by  the use of offset flapping hinges, or by syn-  thetic or artificial stabilization devices.  The total static stability of a helicopter is  determined by combining the stability con-  tributions of all the components. The usual  result for a typical helicopter is instability  with angle of attack and a variable velocity  stability which becomes neutral or unstable  at high speeds. Of course, the helicopter  could be made stable with angle of attack by  providing a large enough horizontal stabilizer.  Unfortunately, adverse effects  at low speed or  Two types of stability are important in the  rotor: (1) angle of attack stability and (2)  velocity stability.  In hovering flight  the  relative wind velocity, angle of attack, and  lift on each blade of the rotor is the same. If  the rotor is displaced through some angle, no  changes in forces result. Therefore, the rotor  has neutral angle of attack stability when  hovering.  However, in forward flight,  an  increase in rotor angle of attack increases  the  lift on the advancing blade more than on the  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.  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:  323  (1) Rotor  (2) Fuselage  (3) Stabilizers  (4) Mechanical devices 