The Art of the Helicopter John Watkinson - Karatunov.net

196 The Art of the Helicopter
setting to the load placed on the engine. In contrast the correlator is a device using
feedforward to predict the power setting necessary from the position of the collective
lever. This may be done mechanically with a shaped cam on the collective linkage. The
cam shape is obtained during tests of the prototype. Feedforward may also be applied
electrically, in which case the collective lever is fitted with a position sensor. A correlator
only gives an approximate control of RRPM since the position of the collective lever is
only one factor affecting the power required. There is, for example, no compensation
for air density or the degree of inflow through the rotor.
The gasoline engine is literally throttled to control its power. The induction passage is
restricted and this is a stable process. If the engine speeds up, the pressure drop across
the constriction will increase, dropping the induction pressure. If the engine slows
down, the pressure drop across the constriction will decrease, increasing the induction
pressure. Thus on a piston-engine helicopter governors are not essential and most
machines just have a correlator. One school of thought suggests that it is better to learn
to fly an uncorrelated machine because by manipulating the throttle a more thorough
understanding of the power needed in different conditions is obtained.
Neither the turbine nor the Diesel has a throttle because the induction system is fully
open at all times. As a result these types of engine will not hold their RPM unaided
and a governor is essential. The governor is a system that compares the desired speed
with the actual speed and uses the speed error to operate the power control to apply a
correction. This is an example of negative feedback as described in Chapter 2.
The principle of the governor goes back to the first steam engines. In the centrifugal
governor shown in Figure 6.3 a pair of weights are attached to a shaft driven by the
engine such that they will fly out as speed increases. This is arranged to reduce the
power so a set speed cannot be exceeded. The speed is set by a change to the spring
tension applied to the linkage. If the spring tension is increased this tends to increase
the power and the weights have to spin faster before they can produce enough force to
reduce power.
In practice the engine revs will not be held perfectly stable because if more power is
demanded the power control must be operated to provide it. The engine RPM must
reduce slightly in order to create an error. Figure 6.4(a) shows that this is known as
static droop. Static droop can be reduced by increasing the amount of power control
movement for a given speed error. This effectively changes the gain applied to the speed
(a) (b)
Fig. 6.3 The centrifugal governor is a very simple device as it relies on rotating weights being thrown outwards
as speed increases.