Pratt & Whitney Canada PT6

In 1956, Pratt & Whitney Canada's (PWC) president, Ronald Riley, ordered engineering manager Dick Guthrie to hire a team of gas turbine specialists to design a small gas turbine engine. Demand for the Wasp radial engine was still strong and its production was profitable but the aim was to become Canada's prime engine company by focusing on a small gas turbine engine. Riley gave Guthrie a modest budget of C$100,000. Guthrie recruited twelve engineers with experience gained at various places including the National Research Council in Ottawa, Orenda Engines in Ontario, Bristol Aero Engines and Blackburn Aircraft.[3] They completed the detailed design of an engine for Canadair's small jet trainer, the CL-41. It was a 3,000 lb-thrust turbojet but the design was taken over by P&WA who developed it into the Pratt & Whitney JT12. The team had to wait for market assessments to define their next engine, a 450 shaft horsepower turboprop for twin-engined aircraft, the PT6.[4] The early development of the PT-6, which first ran in December 1963,[1][5] was beset with engineering problems, cost overruns and lack of sales. It was almost cancelled.[4] The team lacked the ability to deal with the technical difficulties, i.e. how to develop the engine, because, as one of the team Elvie Smith recalled, they came from research and design backgrounds. They learned how to run a development program, such as testing around the clock rather than on one shift, from a PWA team which directed the development for several months.[6]

The PT-6 first flew on 30 May 1961, mounted as a third engine in the nose of a Beech 18 aircraft which had been converted by de Havilland at its Downsview facility in North York, Ontario. Full-scale production started in 1963, with service entry the following year. The Beech 18 continued as a PT6 and propeller flying test-bed until it was replaced with a Beech King Air in 1980. The King Air test-engine or propeller replaced one of the standard ones. In 1974 the Beech 18 had been unable to fly fast enough and high enough to test the PT6A-50 for the de Havilland Canada Dash 7 so a Vickers Viscount was modified as a PT6 test-bed with a Dash-7 installation in the nose.[7]

The first production PT-6 model, the PT6A-6, was certificated in December 1963. The first application was the Beech Queen Air, enticing the U.S. Army to buy a fleet of the U-21 Ute variant. This helped launch the King Air with Beechcraft selling about 7,000 by 2012.[8] From 1963 to 2016 power-to-weight ratio was improved by 50%, brake specific fuel consumption by 20% and overall pressure ratio reached 14:1.[9] Its development continues and while today its basic configuration is the same as in 1964, updates have included a cooled first-stage turbine vane, additional compressor and turbine stages and single-crystal turbine blades in the early 1990s. Its pressure ratio is 13:1 in the AgustaWestland AW609 tiltrotor, the highest that can be used without cooled turbine blades.[10]

In response to the General Electric GE93, in 2017 Pratt & Whitney Canada started testing core technology and systems for a proposed 2,000 hp engine to replace the most powerful versions of the PT6.[11] It was considered likely to be a development of the PT6C core, and would fit between the 1,750 shp PT6C-67C/E and the 2,300 shp PW100 family. It was expected to be ready to launch by the end of 2017 for an initial helicopter platform with a 10-15% reduction in brake specific fuel consumption.[12] This 2,000 hp engine would target a possible new market such as a Super PC-12, a more powerful TBM, or a bigger King Air.[13]

Epicyclic reduction gears on Pratt & Whitney Canada PT6 gas turbine engine.

The rate at which parts deteriorate in a gas turbine is unbalanced insofar as the hottest parts need replacing or repairing more often than the cooler-running parts. If the hotter parts can be removed without disturbing the rest of the engine, for example without removing the complete engine from the aircraft, maintenance costs are reduced. It was achieved with the PT6 by having the hottest parts, the gas generator turbine and combustor, at the propeller end. They are removed without disturbing the rest of the engine with its connections to the aircraft. This arrangement was patented by designer Newland, one of the original PT6 team.[14] A similar general arrangement with a free-turbine power take-off at the exhaust end ( the 1,000 shp P.181 engine) had been shown by Armstrong Siddeley Motors at the Farnborough Airshow in 1957.[15]

An early design improvement, incorporated in the PT6A-20,[16] was the pipe diffuser patented by Vrana, another of the original PT6 team.[17] It replaced the vaned type diffuser used in centrifugal compressors. The pipe diffuser became standard design practice for P&WC.[18] Another design change improved the part-speed functioning of the compressor. It is common to bleed air from a compressor to make it work properly at low engine speeds. The PT6 has a bleed arrangement which reuses the bleed air by returning it in a tangential direction at the entry to the compressor, an idea patented by Schaum et al. and titled "Turbine Engine With Induced Pre-Swirl at Compressor Inlet".[19] It acts like a variable vane and is known as a "Jet-Flap".

All versions of the engine consist of two sections that can be easily separated for maintenance: a gas generator supplies hot pressurized gas to a free power turbine.[20] The starter has to accelerate only the gas generator, making the engine easy to start, particularly in cold weather.[20] Air enters the gas-generator through an inlet screen into the low-pressure axial compressor. This has three stages on small and medium versions of the engine and four stages on large versions. The air then flows into a single-stage centrifugal compressor, through a folded annular combustion chamber, and finally through a single-stage turbine that powers the compressors at about 45,000 rpm. Hot gas from the gas generator flows into the power turbine, which turns at about 30,000 rpm. It has one stage on the small engines and two stages on the medium and large ones. For turboprop use, this powers a two-stage planetary output reduction gearbox, which turns the propeller at a speed of 1,900 to 2,200 rpm. The exhaust gas then escapes through two side-mounted ducts in the power turbine housing. The turbines are concentric with the combustion chamber, reducing overall length.

In most aircraft installations the PT6 is mounted so that the intake end of the engine is towards the rear of the aircraft, leading to it being known by many as the "back-to-front" engine.[4] This places the power section at the front of the nacelle, where it can drive the propeller directly without the need for a long shaft. Intake air is usually fed to the engine via an underside mounted duct, and the two exhaust outlets are directed rearward. This arrangement aids maintenance by allowing the entire power section to be removed along with the propeller, exposing the gas-generator section. To facilitate rough-field operations, foreign objects are diverted from the compressor intake by inertial separators in the inlet.[21] In some installation such as the PT6A-66B version in the Piaggio Avanti P180, the engine is reversed, with the propeller acting as a "pusher", the accessory gearbox facing the front of the aircraft.

From left to right: propeller mount, reduction gear, exhaust, 2-stage free power turbine, 1-stage gas generator turbine surrounded by the combustor, 1 centrifugal then 4 axial compressor stages, intake, and accessories

By the 40th anniversary of its maiden flight in 2001, over 36,000 PT6As had been delivered, not including the other versions.[22] Up to October 2003, 31,606 delivered engines have flown more than 252 million hours.[23] Till November 2015, 51,000 have been produced.[2] The family logged 400 million flight hours from 1963 to 2016.[9]

The PT6 family is known for its reliability with an in-flight shutdown rate of 1 per 333,333 hours up to October 2003,[23] 1 per 127,560 hours in 2005 in Canada,[24] 1 per 333000 hours from 1963 to 2016,[9] 1 per 651,126 hours over 12 months in 2016.[25] Time between overhauls is between 3600 and 9000 hours and hot-section inspections between 1800 and 2000 hrs.[26]

While lacking a FADEC, autothrottle can be installed as an aftermarket upgrade with an actuator, initially in single-engine aircraft like a PC-12 and possibly in twin-turboprop aircraft.[27]

A PT6A-67D engine on a Beechcraft 1900D. The turbine exhaust is prominent.

The main variant, the PT6A, is available in a wide variety of models, covering the power range between 580 and 920 shaft horsepower in the original series, and up to 1,940 shp (1,450 kW) in the "large" lines. The PT6B and PT6C are turboshaft variants for helicopters. In US military use, they are designated as T74 or T101.

Several other versions of the PT6 have appeared over time :

• the Large PT6 added another power turbine stage and a deeper output reduction, producing almost twice the power output, between 1,090 and 1,920 shp (1,430 kW) ;
• the PT6B is a helicopter turboshaft model, featuring an offset reduction gearbox with a freewheeling clutch and power turbine governor, producing 1,000 hp (750 kW) at 4,500 rpm ;
• the PT6C is a helicopter model, with a single side-mounted exhaust, producing 2,000 hp (1,500 kW) at 30,000 rpm, which is stepped down in a user-supplied gearbox ;
• the PT6E is a large PT6A equipped with digital engine control ;
• the PT6T Twin-Pac consists of two PT6 engines driving a common-output reduction gearbox, producing almost 2,000 hp (1,500 kW) at 6,000 rpm ;
• the ST6 is a version intended for stationary applications, originally developed for the UAC TurboTrain, and now widely used as auxiliary power units on large aircraft, as well as many other roles.[28]

The PT6A family is a series of free-turbine turboprop engines providing 500 to 1,940 shp (433 to 1,447 kW)

T74-CP-700

(PT6A-20) United States military designation for the PT6A-20/27, used in the Beechcraft U-21 Ute.

T74-CP-702

(PT6A-29)

T101

United States military designation for the T101-CP-100 / PT6A-45R, used in the Shorts 330 and Shorts C-23 Sherpa.

PT6B-9

The PT6B-9 is a 550 hp (410.1 kW) turboshaft engine for use in helicopters. A later mark of PT6B is rated at 981 hp (731.5 kW).

PT6B-16

PT6C

The PT6C is a 1600 to 2300 horsepower (1190 to 1720 kW) engine for helicopters and tiltrotors.

PT6D-114A

The PT6D-114A is based on the PT6A-114A. The main difference is the deletion of the second-stage reduction gearing and output shaft, because the engine is intended for integration with a combining gearbox incorporating power turbine governors and a propeller output shaft.[33]

Soloy Dual Pac

2x PT6D-114A engines driving a single propeller through a combining gearbox, capable of independent operation.

PT6T

Twin PT6 power units combining outputs through a gearbox for use in helicopters.

ST6

The ST6 is a variant of the PT6 that was originally developed as a powerplant for the UAC TurboTrain power cars, but later developed as a stationary power generator and auxiliary power unit.

ST6B

The ST6B-62 was a 550 bhp (410 kW) version of the PT6 developed for use in the STP-Paxton Turbocar, raced in the 1967 Indianapolis 500.[34]

STN 6/76

The STN 6/76 was a 500 bhp (370 kW) version of the PT6 developed for use in the Lotus 56, raced in the 1968 Indianapolis 500 and later in Formula One races, in 1971.[35][36]

The engine is used in over 100 different applications.

• McGill University, Montreal (cutaway)
• New England Air Museum, Connecticut (cutaway)[38]
• Ontario Science Centre, Toronto, Ontario (cutaway)

Data from Jane's 62-63,[39]

General characteristics

• Type: Turboprop
• Length: 62 in (1,575 mm)
• Diameter: 19 in (483 mm)
• Dry weight: 270 lb (122.47 kg)

Components

• Compressor: 3-stage axial + 1-stage centrifugal flow compressor
• Combustors: annular reverse-flow with 14 Simplex burners
• Turbine: 1-stage gas generator power turbine + 1-stage free power turbine
• Fuel type: Aviation kerosene to MIL-F-5624E / JP-4 / JP-5
• Oil system: Split system with gear type pressure and scavenge pumps, with pressure to gearbox boosted by a second pump.

Performance

• Maximum power output: 578 hp (431 kW) equivalent power at 2,200 output rpm for take-off
• Overall pressure ratio: 6.3:1
• Air mass flow: 5.3 lb (2 kg)/second
• Specific fuel consumption: 0.67 lb/hph (0.408 kg/kWh)
• Power-to-weight ratio: 2.14 hp/lb (3.52 kW/kg)

Related development

• Pratt & Whitney Canada PT6T

Comparable engines

• Garrett TPE331
• Walter M601

Related lists

• List of aircraft engines

Wikimedia Commons has media related to Pratt & Whitney Canada PT6.

• "PT6A". Pratt & Whitney Canada.
• "PT6B". Pratt & Whitney Canada.
• "PT6C". Pratt & Whitney Canada.
• "Embraer EMB-110 type certificate" (PDF). National Civil Aviation Agency of Brazil. 20 December 1972.
• How PT-6 Turbines Are Overhauled. AVweb. 14 August 2015. Lay summary.