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Pratt & Whitney PT6A-42 Turboprop
The Pratt & Whitney PT6A free shaft engine is the most widely used commercial turboprop engine in the world. It has been produced since the 1960's, and performance has grown from 500 horsepower, all the way up to 1,900 horsepower in the PT6A-68A large model. The PT6 is noted for its compact size and layout, which makes it very suitable for installation in an aircraft nacelle. The engine has a hot end drive with an integrated reduction gearbox to reduce output shaft rpm to a speed suitable for a propeller; between 1,900 and 2,200 rpm, depending upon the size of the propeller and the output rating of the engine.
The PT6 is a dual shaft free turbine turboprop engine, with a mixed flow compressor and a multi stage turbine. Mounted onto the very front of the engine is the accessory gearbox, which is driven by a shaft that protrudes out of the forward end of the compressor shaft. The accessory gearbox reduces the high speed of the gas producer spool and drives the accessory gear train. The engine accessories are mounted to the front of the accessory gearbox, and include the fuel pumps, the fuel controller, the oil pressure and scavenge pumps, and the electric starter/generator. Just behind the accessory gearbox, is a scroll type inlet screen, which admits air to the compressor. The compressor features 3 axial stages and a centrifugal compressor which increases air pressure to a level 7 times greater than atmospheric. The compressor features an air bleed for stall control, bearing seal pressurization, and turbine disk cooling. Compressor discharge air is fed to an annular reverse flow burner, where fuel is introduced through 14 fuel nozzles. The hot gas exits the burner towards the rear of the engine, where it drives a single axial turbine to drive the gas producer at approximately 45,000 rpm, and then a single counter rotating axial free power turbine at 30,000 rpm. The power turbine shaft continues rearward where it drives the compact rear mounted two stage planetary output reduction gearbox, which turns the propeller at a speed of 2,200 rpm. The exhaust gas escapes through two side mounted ducts in the power turbine housing, and is directed away from the engine in order to provide about 600 lbs. of supplemental thrust at full power. In most aircraft nacelles, the PT6 is mounted backwards, so that the intake side of the engine is facing the rear of the aircraft, and the exhaust side can directly drive the propeller. Intake air is usually fed to the engine via an underside mounted duct, and the two exhaust outlets are directed rearward.
There are a number of variants of the Pratt and Whitney PT6. The PT6A Small engine is described above. A PT6A Large version contains an additional axial power turbine stage and a deeper output reduction, and almost twice the power output. The PT6B is intended for commercial helicopters and features an offset reduction gearbox with an integral freewheeling clutch and power turbine governor, and a power output of 1,000 horsepower @ 4,500 rpm. The PT6C is intended as a military helicopter engine, and has a direct drive engine output and a single side mounted exhaust outlet. Power output is 2,000 horsepower @ 30,000 rpm. Finally, the PT6T Twin-Pac is actually two PT6C engines driving a common output reduction gearbox, for a power output of almost 4,000 horsepower @ 6,000 rpm.
Pratt & Whitney PT6A-42 Turboprop Engine
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