Pratt & Whitney Canada and the PT6 – Part ONE PT6 Engine

The revolutionary PT6 engine gas turbine nearly died during development, yet managed to survive its detractors, cost overruns and a dearth of orders to become one of the most successful turboprop engines in the history of business aviation.

Old pilots and antique airplane enthusiasts never tire of watching a “round engine” start up, belch a few clouds of smoke, spit out a momentary flash of flame, and then settle into a loping idle whose unique sound cannot be matched by any other piston powerplant. That type of engine (to be technically correct, the static, air-cooled radial engine as opposed to the dynamic radial or rotary), ruled the skies over America from the late 1920s until the early 1960s.

The nearly simultaneous development of gas turbine powerplants by Sir Frank Whittle in England and Hans von Ohain in Germany before the outbreak of World
War II, eventually resulted in the introduction of jet-powered fighters such as the Messerschmitt Me-262 and Gloster Meteor (the only Allied jet fighter to enter operational service in the war). By the late 1940s, the jet engine was the way of the future for the military, and to a lesser degree, the commercial aircraft industry.

It was, however, a different story for the postwar business aviation market in the United States, where the reciprocating piston powerplant (in both radial and opposed cylinder configurations) still reigned supreme. Beech Aircraft Corporation, which had staked its reputation on all-metal monoplanes powered by various static radial engines, developed an array of new airframe designs well into the 1950s that continued to rely on piston engines. During that time, however, the Wichita, Kansas-based company did briefly market the French-built, turbojet-powered MS760 business jet, but it was ahead of its time and the program was eventually terminated.

A chief obstacle to the introduction of early turbine engines into the business aviation segment was three-fold: The powerplants were expensive to manufacture, expensive to operate and expensive to maintain and repair. The military and airlines had the money to deal with these issues, but the general aviation industry did not.

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 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.

“We learned how to develop engines from Torrell,” a PWC colleague recalled, emphasizing 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.”⁴

In the wake of Torrell’s arrival, progress was being made but development work also faced serious opposition from, much to the team’s surprise and dismay, within PWC itself. Thor Stephenson, who served as president of the company from 1959-1975, said, “The early days of the PT6 program were not encouraging, technically or sales-wise.” He recalled that James Young, Pratt & Whitney Canada founder, accompanied by his friend on the board, Hubert Welsford, traveled to Hartford to see Jack Horner, chairman of Pratt & Whitney Aircraft Company, Limited. The two men argued not only for termination of the PT6 effort, but for Pratt & Whitney to transform PWC into a strictly sales and service organization, not an engine manufacturer. Horner rejected their pleas and development of the PT6 continued.⁵

By 1961, the engine was ready for flight testing. The first aircraft to fly solely under PT6 power was not an airplane but a helicopter, specifically the Hiller Ten99 that flew in July of that year. Meanwhile, back in Longueuil, PWC began searching for a twin-engine, flight test airplane that would be suitable for the PT6. The venerable Douglas DC-3 topped the list of potential candidates, but installing the powerplant in the nose section would require extensive structural modifications and the attendant stress analysis could prove difficult and expensive. The DC-3 was off the list.

Fortunately, PWC was able to obtain a Beechcraft C-45 “Expeditor” on loan from the Royal Canadian Air Force. The C-45 was flown to Downsview, Ontario, where de Havilland aircraft engineers and mechanics completed an extensive conversion of the nose section to accept installation of a pre-production prototype engine, which weighed only 270 pounds. Ground testing of the installation began early in 1961 and continued until May 30, when de Havilland test pilot Bob Fowler and PWC pilot John MacNeil took the C-45 aloft for its maiden flight. Although the Beechcraft flew well, it did exhibit minor instability that was solved by installing 23 pounds of ballast in the cabin.

After initial handling qualities and systems checks were deemed satisfactory, a rigorous test program began that focused on specific fuel consumption, propeller constant-speed operation and feathering; air starts along with noise and vibration surveys. During one flight, MacNeil climbed the C-45 up to 26,000 feet, much to the surprise of air traffic controllers unaccustomed to tracking a small, piston-powered airplane at that lofty altitude.

MacNeil later recalled that the modified C-45 was not the most pleasant airplane to fly. According to a report he filed in September 1961, the Expeditor “…is very unstable longitudinally, particularly at higher altitudes. It also has a rolling tendency about the longitudinal axis when high [power settings] are selected on all three engines…Care must exercised at all times to be mindful of its shortcomings.” MacNeil went on to state that he was “pleased with our engine operation,” and that “it starts quickly, both in the air and on the ground, and makes its thrust very obvious from the surface to 25,000 feet.”

One phase of the flight testing involved applying reverse thrust in flight, which MacNeil described as being “quite interesting” because the airplane “is rather unstable in that configuration and “suffers from elevator buffet.” Eventually, applying reverse thrust in flight was discontinued because of concerns about elevator buffeting and the potential for flutter of the control surfaces. Another important aspect of the test program was cold weather operation, and the icy winter of 1963 afforded PWC an excellent opportunity to test the PT6 under severe conditions. The airplane was flown from Montreal, where the OAT was +38 degrees Fahrenheit, to Knob Lake in northern Quebec province where the OAT had plunged to a frigid -21 degrees Fahrenheit.

Although the winter at Knob Lake promised to provide temperatures below -45 degrees Fahrenheit, that phase of the program was cut short when the C-45 was dispatched by PWC to the United States. The purpose of the diversion was to demonstrate the airplane and its PT6 to the military as a potential airborne counter-insurgency platform. MacNeil and flight test engineer John Hunt flew 45 hours of demonstration flights. One concern expressed by customers was whether the free-turbine design of the engine would provide adequate drag during steep descents to landing. Demonstrations allayed any concerns customers had, but MacNeil reported to PWC that he was not satisfied with a brief hesitation that persisted when propeller reversing was selected. ⁶

Investigation revealed the problem – the propeller reduction gearbox (RGB) was overheating and close to seizing entirely. Another RGB was sent from Canada, but again the same problem surfaced. Finally, disassembly and close examination of the unit showed that during reverse operation, when the gas generator was turning at a low RPM, the reversing system was demanding more oil than the engine’s lubrication pumps could provide. The problem was resolved by increasing the size of the pumps, relocating their position on the engine case, and installing a diverter valve.

Despite successful engine development and flight test programs, by early 1963 the future of the PT6 was still uncertain. The key issue was millions of dollars that had been spent to bring the engine to fruition, coupled with zero orders from any major airframe manufacturer. In addition, the competition was heating up. Garrett Air Research had a small gas turbine, dubbed the TPE 335, under development that packed a lot of punch for its size and weight. If PWC shelved the PT6, “that would be the end of it,” said one senior company official. Undaunted and determined to save the PT6, PWC president Thor Stephenson went to Hartford and strongly defended the program. Ultimately, PWC’s decision to keep or kill the PT6 lay with one company: Beech Aircraft Corporation.

Notes:

The author expresses his thanks to Kathy Roberge of Pratt & Whitney Canada, for her kind assistance in the preparation of this article.

1. Sullivan, Mark; “Dependable Engines – The Story of Pratt & Whitney;” American Institute of Aeronautics and Astronautics, Reston, Va.; 2008.

2. Ibid

3. Ibid

4. Ibid. In 1971, Bruce Torrell became president of Pratt & Whitney.

5. Ibid. In December 1962, Pratt & Whitney Aircraft Company, Limited, was renamed United Aircraft of Canada, Limited, to more clearly express the diverse interests of parent company,  United Aircraft Corporation.

6. According to Robert K. Parmerter, PWC bought the C-45 from the Royal Canadian Air Force in 1971, when it was refurbished and registered CF-ZWY-X. It was used to test many versions of the PT6 and last flew in June 1980 after flying 719 test flights that totaled more than 1,000 flight hours. The airplane was converted to its original twin-engine configuration and donated to the Ecole Nationale d’Aeronautique, an aviation trade school, located in St. Hubert, Quebec.