Method and apparatus for the engine control of output shaft speed

A system for determining an optimized output shaft speed in a gas turbine engine for a required thrust and setting an appropriate engine power in a gas turbine engine control system, the system comprising:

• a pilot command providing a required thrust value at a particular flight condition;
• a reduced power cruise condition determiner determining whether said required thrust is a low power condition;
• an output shaft speed determiner for determining a propeller speed value from a propeller performance map based on at least one of engine power, airspeed, and altitude, said required thrust value and a performance map for a free power turbine, when said required thrust is a low power condition;
• a fuel consumption determiner for at least one of increasing and decreasing said optimized output shaft speed using said propeller speed value.

A method for determining an optimized output shaft speed for a selected thrust setting in a gas turbine turbopropeller engine having at least one free turbine and an output shaft, the method comprising the steps of:

• acquiring a selected thrust setting;
• determining an engine power setting corresponding to the selected thrust setting;
• determining a current airspeed, altitude and air temperature;
• determining whether a low power condition exists, and
• if a low power condition exists then selecting an optimum shaft speed from a data store, the selection based on at least engine power setting, determined airspeed, altitude and temperature and data acquired from a performance map of a free power turbine.

A method of reducing fuel flow during low power operation of a turboprop engine having a free turbine, the method comprising the steps of:

• calculating an engine power required to achieve a pilot-selected thrust setting;
• providing a first fuel flow to the engine sufficient to achieve the calculated power;
• acquiring input data relating to airspeed, altitude, ambient air temperature and engine power;
• and
• determining if the selected thrust setting corresponds to a low power condition, and if so:
• using said input data and performance maps of a free power turbine to select an optimized propeller speed from a propeller performance map;
• and
• supplying a second fuel flow to the engine to achieve the optimized propeller speed.

A method of optimizing fuel consumption during low power operation of a turboprop engine having a free turbine, the method comprising the steps of:

• receiving a request for a desired thrust level;
• calculating an engine power setting based on the requested thrust level;
• and
• determining if the requested thrust level is within a low-power range, and if so then:
• determining an optimum propeller shaft speed from a data store, the steps of determining including using a stored map of free turbine speed and accounting for calculated power setting, current airspeed and current altitude;
• determining a corresponding fuel flow required to achieve the optimum propeller shaft speed;
• and
• sending a command to a fuel metering unit of the engine to achieve said corresponding fuel flow and thereby achieve the determined optimum propeller shaft speed.