The Art of the Helicopter John Watkinson - Karatunov.net

224 The Art of the Helicopter
Fig. 6.17 The turbine engine carries out the processes of the four-stroke engine continuously. Air is drawn
in and compressed, fuel is burned in it and power is extracted before exhausting the gases. The continuous
operation and purely rotating parts result in freedom from vibration. Air is drawn in by compressor blades (a)and
stationary blades (b). Compressed air is delivered to combustion chamber along with fuel (c) which burns at (d).
Hot gases drive the turbine (e)which has stationary blades (f). Turbine (e)drives the compressor by means of
shaft (g). Energy in hot gases exceeds power needed to run the compressor by a considerable amount, and also
turns power turbine (h)to produce shaft power at drive shaft (i). Concept is very simple being little more than a
turbocharged blowlamp. Practical engines require special materials which can withstand enormous rotational
forces and high temperatures.
Figure 6.17 shows that the turbine engine is effectively a turbocharged blowlamp;
in fact simple turbojet engines have been made using turbocharger components. Air
is drawn into the compressor: basically a very powerful fan. This compresses the air
continuously so that a steady pressure is maintained at the compressor outlet. Compressed
air passes to the flame cans or burners where fuel is sprayed in and burned
continuously. The charge temperature increases and the gases expand but they cannot
overcome the pressure of the compressor and must flow onwards. The expansion results
in the gases flowing much faster than the flow rate through the compressor. The hot gas
then encounters the power turbine. This converts the energy in the hot gases into shaft
power. Some of this is wasted driving the compressor, but the remainder is available to
drive an external load.
In the free turbine engine there are two power turbines. One drives the compressor
and the other drives the load. Effectively the first turbine and compressor together form
a gas generator that powers the free turbine. The gas generator and the free turbine do
not generally turn at the same speed. The free turbine should run at constant speed
because it is geared to the rotor, whereas the gas generator will turn faster if more drive
torque is required. As a result the free turbine engine has two RPMs and these are
called N1, the gas generator RPM, and N2, actually the rotor RPM but proportional
to the free turbine RPM. In practice, rotor RPM is controlled by adjustment of N1 in
order to stabilize N2.
The turbine engine works with steady pressures at each stage and develops continuous
power with only low octane fuel, unlike the gasoline engine which needs high octane
fuel to prevent detonation and which encounters a serious pressure increase during the
power stroke and needs to be strongly constructed to withstand it. The turbine can
be much lighter than the piston engine. In turbines, the equivalent of the compression
ratio is the pressure ratio: the ratio of compressor outlet pressure to inlet pressure.
Figure 6.18 shows a section through an Allison free turbine engine. Air enters through
a multi-stage axial compressor followed by a single-stage centrifugal compressor.