of blade pitch change. An adjustable friction control helps  prevent inadvertent collective pitch movement. The function of the throttle is to regulate engine rpm. If the  correlator or governor system does not maintain the desired  rpm when the collective is raised or lowered, or if those  systems are not installed, the throttle must be moved manually  with the twist grip to maintain rpm. The throttle control is  much like a motorcycle throttle, and works almost the same  way; twisting the throttle to the left increases rpm, twisting  the throttle to the right decreases rpm.  [Figure 2-46] A governor is a sensing device that senses rotor and engine rpm  and makes the necessary adjustments in order to keep rotor rpm  constant. Once the rotor rpm is set in normal operations, the  governor keeps the rpm constant, and there is no need to make  any throttle adjustments. Governors are common on all turbine  helicopters (as it is a function of the fuel control system of the  turbine engine), and used on some piston-powered helicopters. A correlator is a mechanical connection between the  collective lever and the engine throttle. When the collective  lever is raised, power is automatically increased and when  lowered, power is decreased. This system maintains rpm  close to the desired value, but still requires adjustment of  the throttle for fine tuning. Some helicopters do not have correlators or governors and  require coordination of all collective and throttle movements.  When the collective is raised, the throttle must be increased;  when the collective is lowered, the throttle must be decreased.  As with any aircraft control, large adjustments of either  collective pitch or throttle should be avoided. All corrections  should be made with smooth pressure. In piston helicopters, the collective pitch is the primary  control for manifold pressure, and the throttle is the primary  control for rpm. However, the collective pitch control also  influences rpm, and the throttle also influences manifold  pressure; therefore, each is considered to be a secondary  control of the other’s function. Both the tachometer (rpm  indicator) and the manifold pressure gauge must be analyzed  to determine which control to use.  Figure 2-47  illustrates  this relationship. The cyclic pitch control is mounted vertically from the  cockpit floor, between the pilot’s legs or, in some models,  between the two pilot seats.  [Figure 2-48]  This primary flight  control allows the pilot to fly the helicopter in any horizontal  direction; fore, aft, and sideways. The total lift force is always  perpendicular to the tip-path place of the main rotor. The  purpose of the cyclic pitch control is to tilt the tip-path plane  in the direction of the desired horizontal direction. The cyclic  control changes the direction of this force and controls the  attitude and airspeed of the helicopter.  The rotor disk tilts in the same direction the cyclic pitch  control is moved. If the cyclic is moved forward, the rotor  disk tilts forward; if the cyclic is moved aft, the disk tilts  aft, and so on. Because the rotor disk acts like a gyro, the  mechanical linkages for the cyclic control rods are rigged  in such a way that they decrease the pitch angle of the rotor  blade approximately 90° before it reaches the direction of  cyclic displacement, and increase the pitch angle of the  rotor blade approximately 90° after it passes the direction of  displacement. An increase in pitch angle increases AOA; a  decrease in pitch angle decreases AOA. For example, if the  cyclic is moved forward, the AOA decreases as the rotor blade  passes the right side of the helicopter and increases on the left  side. This results in maximum downward deflection of the  Governor/Correlator  Cyclic Pitch Control Throttle Control Figure 2-47. Relationship between manifold pressure, rpm,  collective, and throttle. Figure 2-46.  A twist grip throttle is usually mounted on the end  of the collective lever. The throttles on some turbine helicopters  are mounted on the overhead panel or on the floor in the cockpit. 2-29 Twist grip throttle Increasing the throttle increases  manifold pressure and rpm Lowering the collective pitch  decreases manifold pressure  and increases rpm Raising the collective pitch  increases manifold pressure and  decreases rpm Reducing the throttle decreases  manifold pressure and rpm Solution If manifold pressure is and rpm is HIGH LOW LOW LOW LOW HIGH HIGH HIGH