The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 243
the AH-64 Apache. In the Chinook, shown in Figure 6.34(b), the front rotor thrust is
carried by the transmission, whereas the rear rotor has a thrust bearing at the top of
the fin. The drive shaft is fitted with a splined coupling so that hull flexing does not
stress the transmission.
6.28 Multi-engine transmissions
There are a number of reasons for installing more than one engine in a helicopter. One
obvious result is increased power, but clearly this is only obtained if all the engines are
working. Another reason may be to increase safety. If one engine fails, the other(s) will
continue to provide some power. Another possibility is to improve range or economy
by shutting down one engine in cruise.
In a multi-engine helicopter, each engine will have its own one-way clutch so that the
loss or seizure of one engine does not prevent the transmission turning. In most cases
twin engines are fitted for safety reasons. Single engine helicopters are not permitted
to fly over built-up areas. Ideally in the case of an engine failure the machine would
be able to continue normal flight. However, this would mean that each engine would
have to deliver the same power as a single engine. In the case of turbine engines this
is very inefficient because in normal flight both engines would be delivering only half
their rated power. However, the power needed to drive the compressors would be twice
the case for a single engine and this would impair the fuel consumption.
In practice a helicopter only needs full power for a short time, typically at take-off with
a full fuel load and landing at a hot/high destination. For the rest of the flight less power
would be acceptable after a failure by using lower speeds and reduced rates of climb.
Turbine engines are very reliable and failures are relatively uncommon. Consequently
instead of overengining a helicopter, it is more sensible to design engines that can be
overrated for short periods of time. Thus in addition to the continuous power rating
an engine would have a higher ‘contingency’ rating which it could only tolerate for a
few minutes. There may also be an even higher emergency rating that might only be
sustainable for half a minute. If an engine enters one of these conditions, an indicator
operates which can only be reset on the ground and a timer runs to measure the degree
of overload.
The use of these contingency power ratings in some cases may put the engine under
such stress that it will need immediate overhaul, but the frequency with which this
happens is so low that the saving in fuel when the engines are working normally is of
more consequence.
Figure 6.35 shows how a twin-engine helicopter might take-off from a rooftop helipad
in a built-up area. The take-off is conducted upwards and backwards at first so that the
helipad remains in the pilot’s view. If, during this initial climb, an engine fails the pilot
has the option of returning to the pad and the time for which contingency power is
needed would be quite short. The height reached in the initial climb has to be such that
if an engine failed just as the machine moved into forward flight it could use the power
of the remaining engine and the power obtained by losing height to reach minimum
power speed without falling below the height of the pad or nearby buildings.
If another engine of the same type is fitted to a single engine helicopter, the safety
aspect will be improved, but the fuel economy will suffer because the losses of two
compressors are being borne. At a sufficient height, one engine could be shut down so
that the other runs more efficiently. In the event that the running engine fails, provided
the second engine can be started promptly, flight could continue to a convenient point.