Game Changer – Pratt & Whitney Canada’s PT6A, Part 1

The company’s small gas turbine proved to be the right engine at the right time for Beech Aircraft Corporation’s next generation of business aircraft

By the late 1950s, the military forces of the United States, Great Britain, Russia and other nations had been developing and flying jet-powered fighters and bombers for nearly 10 years. Lessons learned from the German Luftwaffe in World War II made it clear to the allies that the day of the piston-powered airplane was drawing to a close. The superior performance of the twin-engine Messerschmitt Me-262, in particular, caught the U.S. Army Air Forces by surprise with its 100-mph speed advantage over America’s premier fighter, the North American P-51 Mustang.

After the war, the commercial airlines were not only cautious about adopting jet engine technology, but deeply concerned about the costs associated with buying and operating such sophisticated powerplants. Instead, airlines clung to the proven, reliable, static, air-cooled radial engine that had reigned supreme since the 1920s. The advent of early jet-powered transports such as the revolutionary de Havilland Comet, the Avro Canada C102 (the first jet airline transport built by a company in North America) and later the benchmark Boeing 707 transformed airline flying and sealed the fate of the radial engine as a prime mover for long-distance airline service.

During the 1950s, Beech Aircraft Corporation had prospered under the able leadership of Olive Ann Beech, who assumed the reigns of power following Walter Beech’s death in 1950. Its chief products – the Model 35 Bonanza and the cabin-class Model 65 Queen Air – were selling well and framed an ever-expanding lineup of Beechcrafts to serve every mission. Always conservative but never afraid to look to the future, in 1955 Olive Ann Beech gave the green light for Beech Aircraft to act as the sole distributor in North America for the Morane-Saulnier MS 760 Paris Jet – a four-place, 410-mph, twin-engine, low-wing monoplane that seated four in pressurized comfort. The company’s brief foray into the “Jet Age” lasted less than one year, but gained the company a degree of prestige among business aircraft operators that would prove useful 10 years later.

Meanwhile, up north in Montreal, the Pratt & Whitney Canada division (PWC) of New England-based Pratt & Whitney Aircraft (PWA) was about to make a technical transition that would have a massive impact on its future business. In 1951 the Canadian company’s primary product remained the air-cooled radial engine, specifically the R-1340 Wasp. By 1954, however, when the long-running R-1340 program was nearing its end, the company began building Wright R-1820 radial engines under license to Wright Aeronautical. These powerplants were installed on the Royal Canadian Navy’s Grumman Tracker, an anti-submarine warfare aircraft.

By 1954, PWA officials had decided that the company’s future lie with turbines, not reciprocating engines, and announced that a new facility would be built in Longeuil, Montreal. Plans called for transferring to PWC all tooling for production of the R-985, R-1340, R-1830 and R-2000 radials, as well as spare parts, making the Canadian division the sole source for those components.

In 1956, however, the Canadian division was revamped under the direction of CEO Ron Riley. The reorganization included a plan to create a new group responsible for conducting design and development of gas turbine engines, and Riley was quick to authorize a search to find men who were well acquainted with design and development of such powerplants.

Although no one at the time could have known about the ramifications of Riley’s decision, it marked a critical first step toward uniting PWC and the Beech Aircraft Corporation. The chief question that emerged from a series of discussions centered on what type of engine would help make PWC one of Canada’s major engine manufacturers. The primary builders of gas turbine engines in the country at that time were Orenda and Rolls-Royce. Riley wanted to transition PWC from its long-standing function as a service and support provider for PWA, to designing and building engines of its own design.

“We were determined to reverse the picture as it existed in Canada with Orenda and Rolls-Royce as the big names. Riley and I looked at a variety of areas that could launch PWC into new product development. In the end, we decided to focus on a small gas turbine engine,” said engineer R.H “Dick” Guthrie.¹ Riley’s initiative was a bold one, but as Hugh Langshur, PWC’s chief engineer remembered, he was “surprised that we were allowed to enter the gas turbine business without being led by a ‘big name.’” He recalled that there were only two potential sources that employed men with the necessary experience – Canada’s National Research Council (NRC) and Orenda.²

By June, six engineers had been hired: Doug Millar and Elvie Smith were wooed away from the NRC, followed by another NRC engineer, John Vrana. The other three – Pete Petersen, Allan Newland and J.P. Beauregard – bade farewell to good jobs at Orenda for an uncertain future with PWC. “We were all excited about working at PWC, but aware that it was a gamble,” Peterson recalled. The last few engineers, Ken Elsworth, Gordon Hardy, Fred Glasspoole, Fernand Desrochers, Arthur Goss and Jim Rankin, joined the team in the summer of 1957.³

Part of the team were transferred to PWA’s headquarters in Hartford, Connecticut, to begin design studies on a small, lightweight engine. That same year the RCAF released specifications for a new jet trainer, and Canadair offered the CL-41 Tutor – a single-engine design featuring side-by-side seating for student pilot and instructor. Officials at PWC immediately recognized the opportunity to supply an engine. The team worked feverishly on a configuration that featured an axial compressor section and would produce 3,000 pounds static thrust. Initially designated the DS-3J, the engine was redesignated as the FDS-4J and eventually the JT-12. It is important to note that in addition to military applications, the DS-4J also showed promise as an engine for business aircraft.⁴

Throughout the second half of 1957, the Canadian engineering team in Hartford continued to work on the jet until early in 1958 when PWA assumed responsibility for the project. The primary reason for the shift was simple: PWC lacked the money, manpower and facilities to complete the job. That decision, however, would prove to be providential for Canada and Beech Aircraft Corporation.

Free to begin work on another small gas turbine, the team reassembled in Longueuil began a number of preliminary design studies, but further progress was slowed until PWC conducted a survey to determine if a market existed for such an engine, and if so, what power range was required. Finally, in July of 1958 the decision was made to focus on a series of turboprop powerplants in the 200-2,200 shp class, including an emphasis on engines rated at 250-500 shp.

These engines seemed best suited to small, single- and twin-engine private and business aircraft built by airframe manufacturers Piper Aircraft Corporation, Cessna Aircraft Company and the Beech Aircraft Corporation. A series of meetings were held between PWC and all three companies. Cessna, of course, already had extensive experience with gas turbines in the T-37 jet trainer, of which hundreds had been delivered to the U.S. Air Force since 1955. Piper officials were not keen on turbine engines, but Beech Aircraft management was interested in “turning to turbines with all possible speed,” according to a PWC official. Both engineering and marketing at Walter H. Beech’s company were certain that the “future of the light aircraft lay with turbines, especially turboprops, and was ready to install an engine as soon as it was available.”⁵

Although CEO Olive Ann Beech believed in a “go slow” approach to technical innovation and new aircraft designs, she realized that the company could not afford to rest on its past successes and remain on the leading edge of development in business aircraft. Engineering already had made preliminary plans to mate a turboprop in the 450-shp class to a next-generation Beechcraft based largely on the successful Model 50 Twin Bonanza (that “next-gen” airframe would become the Model 65 Queen Air). In addition, turboprop engines were being tested in France on the venerable Model 18, and those experiments were being closely monitored by the company.

As 1958 came to an end, the chief issue facing Dick Guthrie’s design team was choosing a configuration for the new engine. Key factors affecting that choice included reliability, cost, specific fuel consumption, weight and maintainability. Two concepts finally emerged – free turbine and fixed-shaft. Members of the team freely debated the merits of both – a fixed-shaft would cost less to build, but the free turbine had a few distinct advantages every King Air pilot should be thankful for: less power required to start the engine, less complex fuel controls, and in the case of one engine becoming inoperative, only a part of the engine would freewheel with the propeller feathered, creating less drag.

In addition, the engine could use existing propellers, obviating costly development of a propeller designed specifically for a fixed-shaft turbine. One other advantage of the free turbine design is that the engine’s gas generator typically operates at about 35,000 rpm and the propeller rotates at about 2,000 rpm, resulting in a reduction gearbox ratio of 15:1. The compressor section can be operating at a high rpm with the propeller idling, resulting in much less noise. By contrast, a fixed shaft turbine and its propeller rotate at the same speed and create a much higher noise level (compare a PT6-powered King Air to a Mitsubishi MU-2 powered by the AiResearch TPE331).

The free turbine was selected, but the next question centered on location of the air inlet. Earlier free turbines featured an inlet at the front of the engine with a long, concentric shaft running through the center of the engine to transfer power from the rear section to the front. Because the new design (now designated DS-10) would be small and lightweight, size constraints were an important factor. The team elected to use a reverse-flow configuration whereby air entered at the rear of the engine and passed through the compressor section on its way to the power turbine. By placing the air inlet at the rear, components could be mounted on the reduction gearbox (RGB), which could be removed and replaced without demounting the entire engine from the aircraft.

A basic description of the DS-10 was provided to airframe manufacturers including Beech Aircraft and included a number of key points:

• A 450-shp, free-turbine turboprop and turboshaft engine suitable for fixed-wing, helicopter or VTIOL aircraft.
• Size based on airplane design studies and surveys of light aircraft manufacturers.
• A pressure ratio of 6:1 should be attainable.
• Turbine inlet temperature (ITT) set at a level consistent with turbine disks weight, but higher temperatures will be possible if integrally cast turbine wheels prove feasible.

Armed with a design that had been well thought out, the Canadians traveled down to Hartford where their proposal squared off against one from engineers at PWA. After considerable study, PWC’s DS-10 was selected for further development. Hartford also set aside $4.4 million to construct four prototypes and proceed with a 50-hour test program. One PWC team member recalled that in the wake of their victory, the engineering challenges that lie ahead would be significant. “Everyone understood that the engine program would evolve in response to the market,” and that PWC’s marketeers would have to work long and hard to sign customers if the program was to be successful.

In addition to the technical obstacles that would have to be overcome, financing was another concern. The majority of the cost burden in bringing the DS-10 (soon designated the PT6) to market would be borne by PWC. A large chunk of that money would come from the ongoing sale of spare parts for Pratt & Whitney’s R-1340 and R-1820 radial engines. In January 1959, the Canadian government agreed to provide $1.2 million to help carry the team through 30 months of tests leading to the 50-hour goal of flight qualification. The agreement called for PWC to provide four PT6A-2 engines for the tests and another four PT6A-2 or PT6A-B2 for further development.

Full of enthusiasm that was tempered by the reality of the risks associated with a major engine development program, the Canadian team went to work fabricating and building the first engine. “This was the first time we tried to put a gas turbine together,” said team member, Allan Newland. “It is not surprising that we showed a great deal of inexperience in what we did. We had no history, no experience as a team and only brought to the situation what background we had as individuals. This was a far cry from what would happen in a mature organization with a long history of design.”⁶

Newland further commented that, “Our inexperience did, however, have a positive aspect – we were uninhibited. We had no past failures, and we had all the expertise in Hartford to draw on and were smart enough to know which questions to ask when consulting with people there. They keenly shared their knowledge and experience. To say that this rubbed off on us would be an understatement.”⁷ As for PWC’s marketing department, they faced an uphill climb to attract customers to the untested, unproven PT6 engine. In 1959 they waged a worldwide campaign and managed to generate great interest, including 70 companies in the United States. Of these, six showed potential, including the Beech Aircraft Corporation.

The future looked bright for the PT6, but it would be another five years and many millions of dollars before the first production engine was shipped to a customer.

NOTES:

1. Sullivan, Kenneth H. and Milberry, Larry: “Power—The Pratt & Whitney Canada Story;” CANAV Books, 1989. Other companies already had development underway for small turbine engines, including programs in France, Great Britain and the United States.
2. Ibid
3. Ibid
4. The CL-41 was selected by the RCAF over the Cessna T-37, British Jet Provost and the French Fouga Magister, and 212 eventually were built. The JT12 (military J60) lost out to the General Electric J85 that was built under license by Orenda in Toronto. The JT12, however, was installed in the four-engine Lockheed Jetstar and North American Sabreliner business jets, versions of which also operated with the U.S. Air Force and Navy.
5. Sullivan, Kenneth H., and Milberry, Larry: “Power—The Pratt & Whitney Canada Story;” CANAV Books, 1989.
6. Ibid
7. Ibid