Rotorcraft Flying Handbook, FAA-H-8083-21

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.
Figure 4-2. A twist grip throttle is usually mounted on the end
of the collective lever. Some turbine helicopters have the
throttles mounted on the overhead panel or on the floor in
the cockpit.
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.
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.
CYCLIC PITCH CONTROL
The cyclic pitch control tilts the main rotor disc by
changing the pitch angle of the rotor blades in their
cycle of rotation. When the main rotor disc is tilted, the
horizontal component of lift moves the helicopter in
the direction of tilt. [Figure 4-4]
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
Low
and
R.P.M.
is
Low
Solution
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
High
High
Raising the collective pitch increases
manifold pressure and decreases r.p.m.
Reducing the throttle decreases manifold
pressure and r.p.m.
Figure 4-3. Relationship between manifold pressure, r.p.m.,
collective, and throttle.
Figure 4-4. The cyclic pitch control may be mounted vertically
between the pilot’s knees or on a teetering bar from a
single cyclic located in the center of the helicopter. The cyclic
can pivot in all directions.
4-2