The Art of the Helicopter John Watkinson - Karatunov.net

6
Engines and transmissions
The power plant is an important component in all aircraft and no less so in helicopters.
However, helicopters necessarily have a more complicated power transmission system
than aeroplanes. In this chapter the choice of power plant is considered, along with
the operating principles of piston and turbine engines and associated transmissions.
Power plant control and instrumentation is also treated.
6.1 Introduction
The power source of a conventional helicopter must be able to drive a shaft, and
have reasonable weight and fuel consumption in relation to the power delivered. This
generally means an internal combustion engine: piston, rotary or turbine. Real engines
always turn too fast for real rotors and some reduction gearing will be needed to
transmit engine power to the various rotors and accessories. There have been some
exceptions to convention. Numerous efforts have been made to fit rockets, ramjets or
turbojets directly to the blades, or even to duct gases into the blades from the hull. The
goal is to eliminate torque reaction and reduce weight. These alternatives are considered
in section 6.28.
6.2 Choice of engine
All internal combustion engines work by burning fuel in air. This raises the temperature,
causing expansion which can do work. In practice more power can be obtained if
the air is compressed before the fuel burns. The turbine engine has dominated the helicopter
power plant market for some time. The main advantage of the turbine is a very
high power to weight ratio. The lightness comes from simplicity; the compression of
air and the extraction of shaft power are both done by rapidly spinning blades. There
is, however, a penalty for that simplicity which is that the fast moving blades suffer
from profile drag. Just keeping the blades moving consumes power. When used at high
power, the internal power loss is a small part of the overall power produced and the
turbine is efficient. When used at lower power, the power lost in the blades does not
reduce in proportion and so the efficiency falls. Small turbine engines tend to be inefficient
because the Reynolds numbers at which small blades must operate will be less
favourable. As a result there are very few small turbine engines and these tend to be used
as APUs (auxiliary power units) that are only used intermittently. Although simple in
concept, the turbine engine uses highly stressed parts and the initial cost is high.