The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 233
and carbonize the oil. Synthetic oils have been developed which resist carbonization
better than conventional oil. Heat soak may be prevented with an electric pump to
keep the oil flowing after shutdown until the hottest engine parts have cooled.
The temperature at the bearings is such that conventional elastomeric oil seals would
have a very short life. Instead oil sealing is performed by the use of threaded sections
on the shafts that have the effect of screwing the oil back to where it should be. The
designer also uses the fact that much of the interior of the engine is at high pressure to
keep the oil in place.
6.23 Turbine fuel control
Turbine engine power is ultimately controlled by the amount of fuel injected. If the
fuel flow is increased gradually, the burner temperature rises and exhaust gases have an
increased velocity. This drives the turbine faster and in turn raises PC, the compressor
pressure. The greater the pressure between compressor delivery and atmospheric, the
more power the turbine can produce.
The air/fuel ratio cannot go outside the limits shown in Figure 6.27 or combustion
could cease. If the mixture is too weak the airflow blows the flame away, if it is too
rich the cool fuel quenches the combustion, resulting in a flameout. Neither of these
extremes is desirable, but can be brought about if the fuel flow fails to match the airflow.
This can happen if an attempt is made to change the engine power too rapidly. The
accurate fuelling need is provided by a constant-pressure fuel pump feeding a fuel
control unit. The fuel pumps are variable displacement devices. Figure 6.28 shows
that the pump pistons are driven by a swashplate. When the swashplate is square to
the shaft the pistons do not oscillate and there is no flow. As the swashplate is tilted, the
pistons create a flow proportional to the tilt. A spring tries to tilt the swashplate to the
maximum flow position, but the fuel delivery pressure is applied to a piston opposing
the spring. When the fuel flow is low the delivery pressure rises by the small amount
necessary to compress the spring and reduce the pump displacement.
The fuel flow control of a turbine engine has to ensure that the fuel flow is always
within the limits required for combustion. Power is controlled by slightly disturbing
the equilibrium toward one or other of the limits. If it is required to increase power, a
slight increase in fuel flow will begin to accelerate the spool, and as it turns faster the
Fig. 6.27 The range of air-to-fuel ratio that allows combustion is quite small for kerosene or AVTUR.