Page 23 - May 2015 Volume 9, Number 5

During World War II, Beech Aircraft Corporation built thousands of twin-engine Model 18 monoplanes, including the C45 illustrated here, that were used worldwide by United States and Allied air forces. In 1946, the company introduced a postwar upgrade of the Model 18 designated the D18S that set a new standard for business aircraft. (TEXTRON AVIATION)
In 1956, Pratt & Whitney Canada (PWC), based in Longueuil, Quebec Province, began to pull together a design team of specialists with the goal of developing a small, compact, lightweight and powerful gas turbine engine. To determine if demand for such an engine existed or could be created, teams were dispatched to the major manufacturers of general aviation aircraft in the United States, namely Beech Aircraft Corporation, Cessna Aircraft Company and Piper Aircraft Corporation. The chief question that these teams had to answer was simple: Was there a market, and more importantly, a sustainable market, for such an engine?
There was, however, some tough competition already hard at work, including General Motor’s Allison Division that was developing a gas turbine in the 250 shaft- horsepower (shp) range, and Great Britain’s Rolls-Royce with its 2,000-shp Dart. After assessing results of the marketing surveys, PWC officials Kenneth Sullivan and Elvie Smith recommended that the company proceed with development of a gas turbine generating 450 shp with growth potential to 500 shp. A critical goal of the program would be keeping operating costs on a level with piston engines of equivalent horsepower [it is interesting to note that the new engine would boast the same horsepower as the nine-cylinder R-985 radial that powered last-generation versions of the Beechcraft Model 18]. In addition, PWC’s engine would be ideally suited for small, single- and twin-engine airplanes such as the Model 18, de Havilland Beaver and Otter.
The next step was deciding what type of gas turbine PWC should build. Among the chief considerations were weight, overall dimensions, maintainability and specific fuel consumption. After weighing all the options available, the design team settled on a free turbine configuration. Their reasoning was as follows: “On the fixed-shaft engine, the gas generator and power turbine share a common shaft. On the free turbine, there are two units, one driving the compressor and one producing the power. The link between the two is not mechanical but is made by the flow of hot gases through the engine. The free turbine is more complex, hence costlier, but has such advantages as requiring less starting power
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and simpler fuel controls. The free turbine eliminates clutch requirements in a helicopter and makes easier the pairing of engines for more powerful installations. Fixed-wing aircraft could use an off-the-shelf propeller with a free turbine instead of a costly, tailor-made one required by a fixed-shaft engine.” 1
Although an engine design had been chosen, one major obstacle remained: selling the program to parent company Pratt & Whitney Aircraft in Hartford, Conn. A special team of engineers traveled there in December 1958 and presented their concept to the company’s chief engineer, Wright Parkins. He carefully examined PWC’s design as well as one proposed by a team from Hartford. In the end, he chose PWC.2
Flushed with success, high hopes and a lot of determination, the team returned to Longueuil and set to work. Although they had plenty of enthusiasm, members of the team lacked experience working together on a major project that could make or break the company’s future. One member recalled that, “We had no history, no experience as a team. This was a far cry from what would happen in a mature organization with a long history of design.” The lack of history and experience, however, proved to be highly advantageous because “we were uninhibited ... and had no past failures.”3
Unfortunately, as time went by the 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.
KING AIR MAGAZINE • 21