Pratt & Whitney Canada PT6 engines and the Saunders ST-27

The underlying premise for a De havilland DH114 Heron conversion to a Saunders Aircraft ST-27 was the use of aircraft engines superior to the Heron's Gypsy Queen engines. It just so happens that across the St. Lawrence River from Dorval airport was United Aircraft of Canada, also referred to as Pratt & Whitney Canada of Longueuil, Quebec and they were developing and producing a small turbo-prop turbine engine called the PT6 which developed 500 horsepower each as compare to 250 horsepower each for the Gypsy Queen.
Pratt & Whitney Canada was a subsidiary of the large Connecticut based Pratt & Whitney aircraft engine manufacturer. Before the development of the PT6, Pratt & Whitney Canada was only a Canadian based sales and service arm of the large parent firm.

ground running of a PT6-27 engine installed on a Saunders Aircraft ST-27 at Dorval airport Montreal around 1968 - photo from the Andre Belanger collection via JF.

A bit of PT6 history,in the late 1950's, Pratt & Whitney Canada’s CEO, Ronald Riley noticed a need in the aircraft market for a turbine engine to replace the traditional piston aircraft engines used to power small aircraft. Riley saw an opportunity for the Canada division to transition from its role as a service and support provider for the American division to begin producing engines of an original design. 
Under his direction, P&WC assembled a team of young, talented engineers with the goal of recycling spare parts from the company’s piston engines to develop a smaller, more lightweight turbine engine. At the time, no one could have known that the new engine design would make P&WC one of the world’s top engine manufacturers and change aviation forever.

It is interesting to note that during the development of the PT6, a Winnipeg native, Bruce Torrell was highly involved with the PT6 development.

from P&W Canada  "Unfortunately, as time went by the design team’s lack of gas turbine engine design experience began to taint the balance sheets a dark red. Costs were too high and a host of tough technical problems plagued development. Still, Wright Parkins, who was closely monitoring the work at Longueuil, believed in the engine and sent a group of engineers from Hartford to help resolve issues and put the program back on track. The six-man group arrived early in 1961 and was led by Bruce Torrell, a highly respected engineer who also happened to hail from Winnipeg, Manitoba. He had worked on engines with Canada’s National Research Council and spent time with Sir Frank Whittle’s Power Jets before joining Pratt & Whitney after the war. It would not be an exaggeration to state that without Bruce Torrell, the PT6 may have died an early death.
“We learned how to develop engines from Torrell,” a PWC colleague recalled, emphasising that Torrell kept a tight rein on the project. He quickly abandoned the “one-shift-a day” agenda and replaced it with a round-the-clock work schedule to accelerate testing. “When he was in town,” another engineer remembered, “he could be found in the plant at all hours. He was known to show up in the middle of the night wearing a raincoat over his pajamas."

Left-side view of the first PT6 to be tested in flight, as installed on a Beechcraft C45 borrowed from the Royal Canadian Air Force. The conversion was accomplished by De Havilland Canada at Downsview, Ontario
(De Havilland Canada, Pratt & Whitney Canada)

The PT-6 was finally ready for flight testing in 1961. A search began for a suitable twin engine airplane to test with the PT6, and the team chose Beechcraft C-45 “Expeditor”, borrowed from the RCAF.

Initial PT6A test flights on a Beech 18 in 1961. (P&WC)

In May 1963, Beechcraft began test flights of the proof-of-concept Model 87, a modified Queen Air with Pratt & Whitney Canada PT6A-6 engines. 
On July 14, 1963 Beech announced a new aircraft type, and a month later began accepting orders for the "King Air", with aircraft deliveries to commence in Autumn 1964.
The PT6 engine was certificated in 1963 and initial production engine deliveries started in December that year.  
Since then an astonishing tally of almost 51,000 PT6 engines have been delivered and PT6 versions continue to be produced at around 1,000 per year and the engine has achieved legendary status.

Saunders ST-27 with the PT6 installed inside the Saunders Gimli hangar - Ray Fread photo.

In most aircraft installations the PT6 is mounted backwards, so that the intake side of the engine is towards the rear of the aircraft. 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.

this under side photo of a PT6 installation in a DHC Twin Otter provides a good view in the engine installation.

Recognizing the engine's historical significance, P&WC bought back production engine No.1 in 1985. The engine was delivered to Beech in December 1963 to power the prototype King Air 90. (Guy Norris/AW&ST)

For an excellent backgrounder of the PT-6 development read How a 985 radial got the PT-6 in the air

and http://www.kingairmagazine.com/article/pratt-whitney-canada-and-the-pt6-part-one/

A 1.5 minute You tube video on how a PT-6 works at How a PT-6 works - You Tube 

Or a longer in-depth video at How a PT-6 work You Tube 5 minutes

This site for PT-6 operators pt-6 nation

More on Pratt & Whitney Canada at https://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada