The Art of the Helicopter John Watkinson - Karatunov.net

250 The Art of the Helicopter
the speeds of helicopter blades, as they cannot reach a high enough compression ratio
with only dynamic pressure at the intake. Their fuel consumption is about ten times that
of conventional engines, which outweighs the fact that they will burn almost anything.
Hiller’s powerblade was an unsolvable compromise because low drag required a slim
blade section too small to contain an efficient engine. The additional constraints of
the hot-cycle approach make blade design harder because the blades have to withstand
high temperatures as well as flight loads. Tip-mounted motors such as pulse and ramjets
create enormous drag when the motor stops. This makes the rate of descent in
autorotation rather too fast for comfort and as a result no machine of this type was
ever certified.
Pressure jet rotors don’t have a serious noise problem and their autorotation performance
isn’t impaired. However, the frictional losses of ducting the air to the blade
tip are serious, with the result that the overall gain when compared to a conventional
transmission is small. Finally, although there is no torque reaction, tip jet helicopters
may need a tail rotor in order to meet crosswind hover requirements.
Hiller has correctly pointed out that in order to construct a very large helicopter,
with a rotor diameter measured in hundreds of feet, the transmission weight would
be intolerable with a conventional design. Instead Hiller proposed the installation of
turbojet engines at the blade tips. With large, very slow turning rotors, the g force at
the blade tips would not be serious and a turbojet engine would work well there.
6.32 The electrical system
The electrical system is needed for engine starting, lighting, instrument power and
to operate the avionics. When the engine is running, electrical power comes from the
alternator. When the engine is stopped, the battery will be used. Most light helicopters
use a nominal 12 volt battery which is constructed from six 2 volt lead–acid cells in
series. Some machines have nickel–cadmium batteries and these will use ten 1.2 volt
cells in series. Late Enstroms and JetRangers have 24 volt systems.
A lead–acid battery only has a terminal voltage of 12 volts when it is supplying a
light load. In order to charge the battery, the terminal voltage must be raised to about
14.4 volts. Current will flow into the battery instead of out of it, and it will recharge.
Conveniently the battery voltage rises slightly as recharging is completed, reducing
the charging current automatically. If the applied voltage is maintained constant, the
battery will take current until it is recharged, when the current will fall to a trickle.
A lead–acid battery evolves hydrogen and oxygen when it is recharging, and provision
must be made to vent these gases to avoid explosion risk. In the Enstrom the battery
is beneath the starboard seat in the cockpit. Charging a flat battery in the machine
is forbidden since it could fill the cockpit with an explosive mixture. The level of the
electrolyte must be checked periodically.
A nickel–cadmium battery has different charging characteristics. When it is fully
charged, the voltage does not rise very much, and charging current continues to flow.
This current is converted to heat instead of charge. The battery temperature is sensed
and displayed on the instrument panel. If the battery temperature reaches an excessive
value, the alternator is switched off until some cooling takes place. An advantage of
Ni–Cd batteries is that they do not outgas or need topping up.
The regulator controls the system voltage when the engine is running and the alternator
is switched on. This may be a separate unit or integral with the alternator. An
alternator (Figure 6.40) consists of a rotor (3), driven by the engine, which can be
magnetized by direct current flowing through a coil. This field current is fed in through