TURBOPROP ENGINES AND TURBOPROP FUEL CONTROL SYSTEMS
TYPES OF TURBOPROP ENGINES
Turboprop engines can be categorised according to the method employed to provided propeller drive. These are as follows:
COUPLED POWER TURBINES
This is the most simple adaptation of the turbojet engine in that the propeller drive is provided by means of an extension of the drive shaft of a single spool engine. Operating the propeller at constant speed means operating the engine at constant speed.
COMPOUNDED ENGINES
The compounded engine is similar to the coupled turbine but employs two independent gas generator spools. It therefore exhibits the benefits of flexibility and efficiency provided by multi-spool turbojet engines. Propeller drive is provided by an extension of the low pressure spool shaft, through a reduction gearbox. Operating the propeller at constant speed means operating the low pressure compressor at constant speed. The high pressure compressor RPM varies to match the power selection.
FREE POWER TURBINES
In this form of engine the gas turbine engine serves the function of a gas generator providing a high energy gas stream to a separate free power turbine. The free power turbine is not connected to the compressor in any way, but drives the propeller through a reduction gearbox. Power to drive propeller is extracted from the hot gasses produced by the gas generator. Because the compressor is not connected to the free turbine each can assume its optimum speed independently of the other. This ensures maximum operating flexibility and efficiency. Furthermore because of the absence of propeller drag on the compressor, the free turbine engine is easier to start than a coupled or compounded engine.
TURBOPROP ENGINE FUEL CONTROL SYSTEMS
Three forms of turboprop engine fuel control systems are currently in use. These are:
INTEGRATED RPM AND FUEL CONTROL
This system is suitable for coupled turbines and compounded engines. This system typically employs an engine condition lever to switch fuel on and off, and a power control lever. Propeller RPM and power output are selected by a power control lever which simultaneously adjusts the fuel flow to match the selected RPM. Up to maximum RPM increases in power demand increase both RPM and fuel flow. Power demands above maximum RPM are achieved through increases in fuel flow, with the constant speed unit increasing blade angle to maintain constant RPM
DIRECT FUEL CONTROL
This method is suitable for use in free power turbine engines. This system typically has one lever to control propeller RPM and another to control fuel flow and hence power output. The RPM lever controls the propeller constant speed unit and is typically set in the flight condition and left there throughout flight. The power output is controlled by a power lever which varies fuel flow to match power demand. The fuel flow and gas generator RPM vary in response to changes in power lever setting. As power selections change, the speed of the propeller / free power turbine is maintained by the constant speed unit, which alters blade angle to match increases or decreases in power output.
DIRECT CONTROL OF BLADE ANGLE(BETA CONTROL)
This method can be employed for the control of any type of turboprop. An engine condition lever or HP cock is used to switch fuel on and off. This is typically set in the flight condition and left there throughout flight. The power control lever selects blade angle directly, while the fuel system automatically adjusts fuel flow to maintain propeller RPM.
CONTROL OUTSIDE FLIGHT RANGE
When outside the normal flight range and particularly in the reverse thrust range, the engine/propeller combination is usually controlled by the beta system. The transition between this and the normal control system is usually indicated by a stop or detent in the throttle lever quadrant.
The majority of gas turbine engines do not employ manual mixture control. There are however exceptions to every rule. A few use it to aid starting.