The Art of the Helicopter John Watkinson - Karatunov.net

226 The Art of the Helicopter
in temperature so that mass flow is not reduced as seriously. At high altitude the
governor will attempt to maintain power by increasing fuelling. The reduced mass flow
reduces the dilution of the combustion products by excess air with the result that the
gas temperature entering the power turbine tends to rise. Consequently although the
engine could produce power at very high altitudes, in practice power will have to be
limited to protect the power turbine. It should be appreciated that the power needed
by the airframe may also fall with altitude so this effect is hardly a problem, especially
in helicopters.
Power is controlled through limiting fuel admission to the burners. As kerosene in not
very volatile it is either atomized at the burner nozzle or heated by passing it through
pipes exposed to the burning gases on the way to the burner nozzle. It then vaporizes
on leaving the nozzle and burns readily.
Starting a turbine requires an electric motor which will spin the gas generator spool
fast enough to make the compressor operate. Once a suitable compressor speed is
established (typically 15% of flight idle) the fuel is sprayed through the nozzle and the
igniter is operated. This results in hot gas generation that will increase the turbine
speed until engine power can take over from the starter motor. The starter motor
and igniter are typically turned off at 50% of flight idle. If insufficient compressor
speed is achieved before the ignition attempt, combustion pressure will overcome the
compressor pressure and the turbine equivalent of a backfire takes place. In some cases
the starter motor is permanently connected and becomes a generator when the engine
is running.
6.19 Compressors
The job of the compressor is to provide a steady flow of air under pressure to the
burners. Compressor design is fraught with compromise primarily because of the need
to deliver different flow rates depending on the power required. It is relatively easy to
design a compressor that is very efficient under one specific set of conditions, but it may
become very inefficient under other conditions. In practice it may be better to design
a compressor which is a little less efficient, but which maintains that efficiency over a
wide range of flow rates. The compressor can be centrifugal, axial or a combination of
the two.
The centrifugal compressor was used in early helicopter turbine engines as it allows
a shorter assembly and could be designed using experience from turbochargers.
Figure 6.19(a) shows a single-entry centrifugal compressor. The air enters axially near
the eye or centre of the impeller at a speed approaching the speed of sound. The blades
may be twisted at the eye to allow a smoother entry thus avoiding compressibility
effects. As the air moves away from the axis the impeller blades impart higher tangential
velocity. In practice the compressor impeller delivers air in a direction having both
radial and tangential components. This high velocity is then converted into pressure
using an assembly known as a diffuser: basically a divergent duct operating according
to Bernouilli’s theorem. The diffuser may at least double the pressure at the impeller
outlet and more in some designs.
It is possible to construct a double-entry centrifugal compressor as shown in
Figure 6.19(b). This allows twice the mass flow with the same diameter, but is less
efficient because the blades run hotter than in the single-entry design which obtains
some cooling at the rear of the impeller. A single centrifugal stage may produce a pressure
ratio of up to 4.5 : 1. Stages may be cascaded to produce higher pressure ratios, but
then the advantage of shortness is lost. The centrifugal compressor is relatively easy to