THROTTLE CONTROL
The function of the throttle is to regulate engine r.p.m. If the correlator or governor system does not maintain the desired r.p.m. when the collective is raised or lowered, or if those systems are not installed, the throttle has to be moved manually with the twist grip in order to maintain r.p.m. Twisting the throttle outboard increases r.p.m.; twisting it inboard decreases r.p.m. [Figure 4-2]
 

 
 
COLLECTIVE PITCH / THROTTLE COORDINATION
When the collective pitch is raised, the load on the engine is increased in order to maintain desired r.p.m. The load is measured by a manifold pressure gauge in piston helicopters or by a torque gauge in turbine helicopters.
In piston helicopters, the collective pitch is the primary control for manifold pressure, and the throttle is the primary control for r.p.m. However, the collective pitch control also influences r.p.m., and the throttle also influences manifold pressure; therefore, each is considered to be a secondary control of the other’s function. Both the tachometer (r.p.m. indicator) and the manifold pressure gauge must be analyzed to determine which control to use. Figure 4-3 illustrates this relationship.
If� Manifold� Pressure� is  and� R.P.M.� is  Solution
Low  Low  Increasing the throttle increases manifold� pressure and r.p.m.
High  Low  Lowering the collective pitch decreases� manifold pressure and increases r.p.m.
Low  High  Raising the collective pitch increases� manifold pressure and decreases r.p.m.
High  High  Reducing the throttle decreases manifold� pressure and r.p.m.

Figure 4-3. Relationship between manifold pressure, r.p.m., collective, and throttle.
CORRELATOR / GOVERNOR
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 r.p.m. close to the desired value, but still requires adjustment of the throttle for fine tuning.
A governor is a sensing device that senses rotor and engine r.p.m. and makes the necessary adjustments in order to keep rotor r.p.m. constant. In normal operations, once the rotor r.p.m. is set, the governor keeps the r.p.m. constant, and there is no need to make any throttle adjustments. Governors are common on all turbine helicopters and used on some piston powered helicopters.
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 through the use of smooth pressure.