Cohen et al. [6.9] and Dixon [6.10] cover the basic principles and main design features of aircraft propulsion units. Here the characteristics of propulsion units suitable for use as helicopter powerplants are considered in relation to the particular requirements of this type of aircraft. Power units for shaft driven rotors of conventional configuration are discussed. Engines are usually mounted in or on the fuselage driving the rotor system via some form of gearbox. Blade mounted propulsion systems (tip drive) are not discussed although their existence should be noted. Tip drive systems have been produced in the past (Djinn, Rotodyne) but have not been considered practical for many years.

Power units for helicopters may be grouped into three main categories:

(1) Reciprocating piston engine system. The piston engine is now in a highly developed state and is very attractive, especially to manufacturers of small helicopters, mainly because of its cheapness. However, one of the major penalties with the piston engine is the need for a clutch in the transmission chain, to enable the engine to be started without having to turn the transmission. The need for the clutch incurs a weight penalty. There is also a growing trend away from AVGAS as a fuel due to its cost and volatility.

(2) Fixed shaft turbine engine system. The fixed shaft turbine engine is used in a helicopter in exactly the same manner as a piston engine. Thus, it is connected to the rotor via a clutch. It may also suffer from significant performance penalties. These are discussed briefly below.

(3) Free shaft turbine engine system. In a free shaft turbine engine system, a separate power turbine is included. This turbine is completely divorced from the turbine that drives the compressor. Thus, as there is no mechanical link between the power output turbine and the rest of the engine, there is no requirement for a clutch in the transmission system. The free power turbine may be held by a brake if there is a requirement to maintain the rotor blades fixed during initial start-up.

All gas turbine engines, whether of fixed or free shaft design, have several advantages over reciprocating piston engines. These advantages include: [12]

the same work as larger piston engined machines, or alternatively, for the same sized helicopter to allow a better performance.

• Fuel consumption. The specific fuel consumption of turbine engines approaches that of piston engines, and when run at full power may be even better. Since the power requirements of a turbine powered helicopter will be less, due to the lower engine weight, the total fuel used will be approximately the same. However, since gas turbine fuel is cheaper than piston engine fuel, the gas turbine engined helicopter will be cheaper to run than an equivalent piston engined machine.

• Reduced vibration. Since the gas turbine engine is a continuous flow machine the output from it will be uniform and hence vibration levels will be lower. This compares with the piston engined machine where the drive shaft is loaded in sequence by each cylinder of the engine.

Fixed-shaft engines are constant speed powerplants since they are directly coupled to a rotor that must operate at a substantially constant RPM. Power changes (or changes in torque) are therefore obtained only by changes in combustion temperature, the mass flow rate remaining approximately constant. Such an engine is capable of rapid changes in power, which is desirable in a helicopter especially during take-off and landing. However, as the operating speed of the turbine has to be set at a level that can be sustained continuously, there is inherent in the fixed shaft engine a power limitation which prevents the achievement of high power even for short periods. The fixed shaft engine is simpler than the free shaft engine, but the weight saving obtained as a result of this simplicity can be negated by the requirement for a clutch.

The major advantages of the free shaft engine, over one with a fixed shaft, are the elimination of the requirement for a clutch and the freedom to select a wide range of output power. The ability in the free shaft engine to vary both the combustion temperature and the air mass flow allows a wide power range to be achieved whilst the free turbine runs at a sensibly constant speed and therefore the free turbine engine is well suited to multi-engined helicopter applications. There is, however, one main disadvantage of the free turbine engine. Changes in power involve changes in the gas generator speed, including, therefore, variations in compressor speed. The inertia of the gas generator and avoidance of compressor surge will thus prevent power changes from being made as rapidly as would be possible with a fixed shaft engine.

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