NAVWEPS DO-80T-80  STABILITY AND CONTROL  Adequate pitch and lateral control effective-  ness are easy to obtain in the typical helicopter  and usually present no problems. The more  usual problem is an excess  of control effective-  ness which results in an overly sensitive heli-  copter.  The helicopter control specifications  attempt to assure satisfactory control charac-  teristics by requiring adequate margins of con-  trol travel and effectiveness  without objection-  able sensitivity.  employ an offset flapping hinge to increase the  control effectiveness  by creating a centrifugal  force couple when the rotor is tilted.  This is  shown in figure 4.35.  The rotor is tilted by taking advantage of  the gyroscopic effect of the rotor system. This  effect  causes  a rotating mass which is disturbed  about one axis to respond about another axis,  as shown in figure 4.35.  A forward tilt to the  rotor is obtained by decreasing  the pitch of the  blade when at the starboard position and in-  creasing the pitch of the blade when at the  port position.  The lateral dissymmetry of  lift which results causes the rotor to tilt for-  ward by the gyroscopic effect.  Directional control 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.  The directional control 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  control to fly sideways up to 30 knots, an  important requirement for plane guard duties.  A differential blade pitch change like this  is called a cyclic.pitch change since each blade  goes thr0ugh.a complete cycle of varying pitch  angles as it completes one revolution of rota-  tion about the hub.  A cyclic pitch change is  accomplished by the pilot by the use of the  cyclic stick.  The control arrangement is such  that the rotor tilts in the same direction that  the cyclic stick is deflected.  A variation in rotor thrust is accomplished  by increasing>sthe  pitch of the blades simul-  taneously or collectively.  This type of control  action is called “collective pitch”  and is ac-  complished by the use of the collective pitch  stick.  In operation, the cyclic stick is an-  alogous to the control stick of an airplane,  and the collective stick is analogous to the  throttle ~of  an ,airplane.  The directional control requirements are  easily met by a tip-driven helicopter since the  directional control does not have to counter  the engine torque.  There are several possibilities for longi-  tudinal control of a tandem-rotor helicopter.  A pitching moment can be produced by tilting  both rotors by a cyclic pitch change in each  rotor, by a differential collective pitch change  that increases  the thrust on one rotor and de-  creases  it on the other, or by some combination  of these methods. The two basic methods are  illustrated in figure 4.36. Obviously, a change  in fuselage attitude must accompany the dif-  ferential collective method of longitudinal  control.  Directional control of a tandem-rotor heli-  copter is accomplished by differential cyclic  control of the main rotors. For a pedal turn  to the starboard, the forward rotor is tilted  to the starboard and the rear rotor is tilted to  port, creating a turning moment as shown in  321 