Pratt & Whitney Canada - PT6A Turboprop Reverseflow Engine
Pilatus Aircraft are all behind this engine
Summary Description: The operation of the PT6 is very simple. Air enters the engine through the inlet screen, it is then compressed by a multistage compressor and fed to the combustion chamber, where it is mixed with fuel and ignited. The hot gas expands through two turbine stages. The first drives the compressor and the accessories, while the second (mechanically independent from the first) drives the propeller shaft by means of a reduction gearbox. Finally, the hot gas is discharged through the exhaust ducts.
The PT6A series engines for use in fixed wing aircraft are available in two configurations, a non-reversing propeller and a engine for reversing propeller applications.
All engines utilize two independent turbines, one driving a compressor in the gas generator section, the second driving reduction gearing for the propeller.
Inlet air enters the engine through and annular plenum chamber formed by the compressor inlet case. From there it is directed to the compressor. The compressor consists of there axial stages and one centrifugal stage assemble as an integral unit, which provides a compression ratio of 6.3:1.
A row of stator vanes, located between each stage of compression, diffuses the air, raises its static pressure and directs it to the next stage of compression. The compressed air is passed through diffuser pipes, turned ninety degrees in direction, then led through straightening vanes to the combustion chamber.
The combustion chamber liner located in the gas generator cases consists of an annular reverse flow weldment with varying size perforations which provide an entry for the compressed air. The flow of air changes direction to enter the combustion chamber liner where it reverses direction and mixes with fuel. The location of the combustion chamber liner eliminates the need for a long shaft between the compressor and the turbine, thus reducing the overall length and weight of the engine.
Fuel is injected into the combustion chamber liner by 14 simplex nozzles supplied by a common manifold. The fuel/air mixture is ignited by two spark igniters which protude into the combustion chamber liner. The resultant gases expand from the combustion chamber liner, reverse direction and pass through the turbine guide vanes to the compressor turbine. The turbine guide vanes ensure that the expanding gases impinge on the turbine blades at the correct angle, with minimum loss of energy. The still expanding gasses pass forward through a second set of stationary guide vanes to drive the power turbine.
The compressor and power turbines are located in the approximate center of the engine with their shafts extending in opposite directions. This provides for simplified installation and inspection procedures. The exhaust gas from the power turbine is directed through an exhaust plenum to atmosphere via dual exhaust ports provided in the duct.
An accessory gearcase is located at the rear of the engine with the relevant accessory drives and mounting pads. The accessories are driven from the compressor by means of a coupling shaft which extends the drive through a conical tube in the oil tank centre section.
A two stage planetary gearbox located in the front of the engine, provide the speed reduction between the power turbine and the propeller shaft. Output is accurately indicated by means of an integral torquemeter device located in the gearbox.
This information has been extracted from an original flight manual used during the "ferry flight" of one of Australian Army aircraft from Switzerland to Australia.
Description: The PT6A-20 engine has a three-stage axial, single-stage centrifugal compressor driven by a single-stage reaction turbine. Another single-stage turbine, counter-rotating with the first, drives the output shaft. Fuel is sprayed in the annular combustion chamber by fourteen individually removable fuel nozzles mounted around the gas generator case. An ignition unit and two coil igniter plugs are used to start combustion. A hydropneumatic fuel control schedules fuel flow to maintain the power set by the gas generator power lever. Propeller speed remains constant at any selected propeller control lever position through the action of a propeller governor, except in the Beta range where the maximum propeller speed is controlled by the Fuel Topping Governor. Immediately following touchdown, partial or full reverse thrust may be obtained by lifting and retarding the power lever aft of the detent. Varying amounts of reverse thrust are available, depending upon how much the power lever is retarted.
Engine Ratings: The engine ratings and power lever and propeller lever settings are as follows:
Take-off: This rating is the maximum power permissible and corresponds to 550 SHP at sea level up to 21 degrees C ambient temperature. The maximum allowable output torque must not be exceeded.
Maximum Continuous: This rating corresponds to 550 SHP up to 21 degrees C ambient temperature, sea level, static conditions. It is intended for emergency use only.
Maximum Climb: This rating corresponds to 538 SHP at sea level, standard day (15 degrees C ambient temperature) and is the maximum power approval for normal climb.
Maximum Cruise: This is the maximum approved power for cruising and is 495 SHP at sea level, standard day.
Reverse: Either full or partial reverse thrust is obtained by lifting and moving the gas generator power lever to any position aft of Idle. The maximum permissible power in the full reverse position is 500 SHP. In the extremely unlikely event of unwanted reverse pitch in flight, a very sharp drag rise, buffeting and a very noticeable steepening in rate of decent will be encountered. In addition, a red coloured low pitch warning light on the instrument panel will illuminate whenever the propeller blades move into the reverse pitch range. If such definite evidence appears, and is not correctable by a slight advance of the power lever, the propeller should be feathered. Reducing airspeed and/or shutting down the gas generator will facilitate feathering.
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