After years of intense development, PWC’s small gas turbine faced an uphill battle for the financial and market resources required to achieve certification and acceptance by prospective customers
Early in January 1959, Pratt & Whitney Canada (PWC) management recognized that with the PT6 program it had entered the highly competitive arena of engine manufacture, but it lacked much-needed experience to help move the project forward. The factory in Longueuil was geared toward the manufacture of spare parts for thousands of static, air-cooled radial engines such as the R-1340 and the R-1830 that were still in operation worldwide with commercial operators and military forces.
In January 1959, the infant PT6 program received a major boost when Thor E. Stephenson became president of PWC. He was well qualified for the job – in 1942 he earned a degree in aeronautical engineering at the University of Toronto, and during World War II served on the staff of Canada’s prestigious National Research Council. In 1946 he was awarded a master’s degree from the California Institute of Technology.1
One of Stephenson’s key initiatives upon taking the helm at PWC was supporting efforts to offer the PT6 as a viable powerplant for a military aviation application either in Canada or the United States. During the first six months of 1959 he oversaw work to sell the engine to one or both governments, in part because the $1.2 million in funding from the Canadian government seemed, at least in part, to depend on the company attracting interest in the small turbine from the United States military. It was a gamble Stephenson had to take, because the alternative was not to his liking: “If applications cannot be developed within a year it is doubtful whether the project has any merit,” he wrote.2
As a result, sales personnel covered the globe singing the praises of the PT6 in hopes of winning customer interest. Dozens of contacts were made, including 70 in the United States where the leading candidates were Beech Aircraft Corporation, Hiller helicopters and Republic Aviation. Although the campaign to sell the PT6 generated a lot of interest, nobody stepped forward with a commitment. One potential market, however, did emerge in 1960 when the United States Army moved forward with specifications for a Light Observation Helicopter (LOH) that would replace fixed-wing aircraft currently fulfilling that role.
The PT6 appeared to be the ideal engine for that application, and PWC quickly shifted its emphasis to the turboshaft version, placing the turboprop edition on the back burner for the time being. Unfortunately, none of the competitors (except Republic) showed serious interest in the PT6. The Army eventually chose the Hughes YHO-6A over Bell Helicopter’s YHO-4A and Hiller’s YHO-5A design. The Hughes aircraft was powered by Allison’s T63 turboshaft engine rated at 250 shaft horsepower (shp).3
Meanwhile, back in Longueuil PWC’s experimental department was hard at work designing the turboprop PT6. Among the many challenges faced by engineers was the configuration of the combustion section (combustor) and the axial compressor section. Plans called for these components to be operated for the first time in November 1958 and complete tests for a preliminary flight rating in December. Initial experiments with the combustion section were conducted using a clear plastic mockup immersed in the water tunnel operated by the National Research Council.
John Vrana, who oversaw the tests, recalled that water infused with tiny particles of metal was used to observe flow patterns within the combustor, and a camera recorded results. A series of “runs” were made that recorded temperatures at various locations inside the combustor under a range of pressures, altitudes and atmospheric temperatures. It took the team nearly a year to finalize a prototype design for the combustor, but the final configuration worked so well it became standard on production engines.
Another set of tests centered on evaluating the gas turbine’s structural integrity, which was deemed suspect after completion of a rigorous review of the powerplant’s aerodynamic and mechanical design. The concern focused on the single-stage centrifugal compressor that was originally fabricated from cast aluminum alloy. Although the compressor would be economical to manufacture and require only a small amount of final machining, it suffered a structural failure during a run in a test cell. The cast aluminum design was rejected in favor of one that was forged, and no further failures occurred.
In addition to structural issues, it was determined that the prototype engines, designated Mk. 1, weighed 20 percent more than the design weight of 250 pounds. That condition was corrected by performing a part-by-part weight reduction program that helped resolve the issue. Allan Newland remembered that during those early days of prototype engine testing, the PT6 did not achieve its design power and was plagued by high fuel consumption. He attributed these problems, at least in part, to anomalies in engineering drawings during the manufacturing process that were gradually eliminated through continuous design upgrades.
These and other changes were incorporated into what became the Mk. 2 PT6. Although the Mk. 2 was an improvement over the Mk. 1, it had its share of problems. These included the gas generator section that suffered from an uncontrolled vibration at about 20,000 rpm – a redesign of the thrust bearing solved that problem. Another issue centered on mounts for the turbine vanes. These failed, allowing the vanes to “collapse forward, severely damaging the rotor blades,” according to Newland. Redesign of the mounts solved that problem.
Despite these and other setbacks, in November 1959 Stephenson decided that time had come for PWC to show the world the PT6 in operation, or at least part of it. With a group of senior aerospace officials from Canada and the United States watching intently, the gas generator section was run in a test cell (the power turbine section and RGB were not installed). In the wake of that demonstration, coupled with the promise of funding from the Canadian government, further development of the PT6 looked promising.
In February 1960, the first complete PT6 with a propeller installed was run in a test cell, and at least 8,000 more hours would be required before the engine finally entered production. In addition to testing complete engines, many components and parts were rigorously evaluated, often to the point of destruction, in small labs equipped with special equipment that made precision measurements. The equipment had to be calibrated and monitored for accuracy, and a significant amount of time was lost correcting problems with instrumentation. As work progressed, the engineering budget was increased to $1.7 million from $980,000, chiefly because of rapidly escalating costs. There was, however, good news: none of the prototype engines undergoing tests had suffered any catastrophic failures. As one engineer recalled, “Not only does this speak well for the general soundness of the design, but for care exercised in setting speed and other limitations, and hours of running per build and the degree of inspection between builds.”5
By July 1960 it was becoming increasingly apparent that the Canadian team would need help from engineering at Hartford to overcome rising costs associated with resolving technical issues. The Mk. 2 engine was running but was plagued by problems, and the decision was made to seek guidance from parent company Pratt & Whitney Aircraft. There was a lot riding on the PT6 program because it held promise for future applications in aircraft for the United States Army and Navy. After meetings between officials from both companies, it was agreed that a team from Hartford would transfer to Longueuil temporarily to help get the program on track. No one at PWA doubted the abilities of PWC’s engineers, but they lacked understanding of how to conduct engine development.
The American team was led by Bruce Torell, a native of Winnipeg and a graduate of the University of Minnesota. As did Thor Stephenson, Torell worked at the National Research Council during the war, but whereas Stephenson focused on aerodynamics, Torell specialized in engine design and development. He would prove to be just what the struggling PT6 project needed – a hard-driving, no-nonsense engineer who knew how to get things done right the first time. Early in 1961 the team from PWA took control of the technical aspects of the program.
As engineering manager Elvie Smith recalled, “We learned how to develop engines from Bruce Torell. None of us from PWC had ever run a major engine development program.” Torell, however, ruled the proceedings with an iron fist. He immediately put the engineers on a round-the-clock schedule – no more single-shift work as had been the case, and cost was no longer an impediment to progress. Torell obtained whatever he deemed necessary to secure the PT6’s detail configuration, and he spent whatever money was required to develop alternative designs. “If we had done things in sequence it would have taken forever,” Smith said.
Torell’s leadership soon began to pay off. In February 1961, both turboprop and turboshaft versions of the Mk. 2 powerplant were run successfully, followed in March by a 50-hour test of the turboprop engine at a rating of 450 shp. As a result of that test, in June the PT6 was cleared for installation in the nose section of a Beechcraft C-45 Expeditor borrowed from the Royal Canadian Air Force (see King Air magazine, May 2015, Page 20). During 1962 the future of the PT6 was still uncertain, but Torell kept it moving forward toward initial production.
Years later Stephenson reflected on the engine’s prospects: “The early days of the program were not encouraging, technically or sales-wise.” There was sufficient doubt about the PT6 program that James Young, the founder of PWC, and board member Hubert Welsford, traveled to Hartford in an attempt to scuttle the project entirely. The powers at Hartford, however, rejected their assault. The PT6 had dodged a very powerful and deadly bullet.6
After a pre-production engine had been installed in the C-45, the PT6 made its first flight (in a fixed-wing aircraft) on May 30, 1961. The Beechcraft, of course, had two radial engines to rely on if the gas turbine failed. By contrast, the Hiller Ten99 helicopter was the first aircraft to fly solely under the power of a PT6 engine. Company founder Stanley Hiller had been interested in PWC’s compact, lightweight powerhouse since 1960 when he was competing against Bell Helicopter for a light helicopter contract submitted by the Canadian Army. In the wake of a proposal from the United States Marine Corps for an assault support helicopter, and Hiller eventually built a prototype of its candidate, the Ten99, to be powered by a turboshaft PT6. PWC responded in April 1961 by shipping a PT6 Mk. 2 engine to Hiller for installation in the prototype, which flew for the first time in July. Hiller lost the competition but the Ten99 still exists as a resident of the Hiller museum in Redwood City, California.
Other rotorcraft manufacturers also experimented with PT6 power in their helicopters. These included the Piasecki 16H that flew with the turboshaft engine in March 1962, and Lockheed’s XH-51a Aerogyro powered by a single PT6B that flew in November of that year. Charles Kaman, whose signature design feature was twin main rotor blades that overlapped, had been the first manufacturer to fly a turbine-powered helicopter when the K-225 flew with a Boeing turboshaft powerplant in December 1951. His interest in the PT6 stemmed from the company’s K-1125 that became the first rotorcraft powered by twin PT6 engines, flying in April 1963. The first flight of any aircraft powered by two PT6 engines, however, fell to the deHavilland Otter on May 7, 1963. Seven months later the first deHavilland DHC-2 Beaver flew with a 550-shp PT6, but its high rate of fuel consumption compared to the R-985 radial engine-powered versions limited production of the Turbo Beaver to only 60 airplanes. The major goal of PWC’s small gas turbine engine program was attained in December 1963, when the Canadian government granted civil certification after the PT6 had undergone 11,000 hours of testing and 1,000 hours of flying.
The story of how the PT6 became the right engine at the right time for the Beech Aircraft Corporation was detailed in the May 2015 issue of King Air magazine, and will be reviewed only briefly here. In October 1958 Beech officials met with PWC to discuss size and configuration requirements for a small turbine powerplant, and six months later the first official presentation of the PT6 was made to company management and engineering. PWC, however, was not alone in pursuing the Wichita, Kansas-based airframe manufacturer. Allison was offering the military T63 and Boeing was touting the merits of its T60 engine.
It was not until 1961 that Beech Aircraft and Pratt & Whitney Canada agreed to cooperate on a new program that would install two PT6A-20 engines in a United States Army L-23F aircraft. The L-23F was a workhorse airplane that provided VIP and troop transport, light cargo and liaison capabilities for the Army. These airplanes were powered by two, eight-cylinder Lycoming IGSO-480-A1A6 engines, each rated at 340 horsepower. It was essentially a modified version of the commercial Model 50 Twin Bonanza that was introduced in 1951.
Behind the scenes, however, Beech engineering and marketing teams were secretly working on design of the next generation Beechcraft tentatively designated the Model 120. It was unveiled in mockup form at the 1961 convention of the National Business Aircraft Association, and PWC officials were stunned to learn that the airplane would be powered by French Turbomeca Bastan turboprop engines. The Canadians realized that the Model 120 targeted the exact market they foresaw as an application for the PT6. Reacting quickly, Thor Stephenson joined forces with long-time Beech engineer Jim Lew, who wielded a lot of influence with company CEO Olive Ann Beech. They offered to install two engines in an L-23F and fly the airplane for 100 hours to evaluate the combination. The Army agreed, but in exchange convinced both Beech Aircraft and PWC to sell it the modified airplane for the tidy sum of one dollar. The deal was struck.
A year later market interest in the Model 120 had weakened, and company management realized that the best potential for developing a turbine-powered commercial Beechcraft rested with the Army L-23F. PWC shipped two PT6A-6 engines to Beech Aircraft for installation in the modified Model 65 airframe. Company vice president Frank E. Hedrick monitored progress, and as Beech engineer John Calhoun recalled, “Hedrick came in to see the prototype one day. He was very impressed by the neat appearance of the PT6 installation, and that’s when he decided to back a commercial turboprop.” By May 1963, the PT6-powered Model 65 had been re-designated by the Army as the NU-8F, and made its first flight that month with test pilots Steve Tuttle and Jim Weber at the controls.
The Army Aviation Test Board at Fort Rucker, Alabama, took delivery of the airplane in March 1964, and put the NU-8F through a tough six-month experimental flight evaluation program. After years of faithful service, the airplane was retired and served as a training aid for mechanics before being placed on static display in the Army Aviation Museum. In the wake of the Army’s success with the NU-8F, management at Beech Aircraft Corporation were ready to invest in the PT6 program and ordered 29 engines from PWC. According to PWC, Beech paid $25,000 per engine. The PT6 would power the company’s next-generation business airplane, the Model 90 King Air that was introduced in July 1963, first flew in January 1964 and received FAA certification in May of that year.
The success of the King Air program probably has done more to establish the reputation of the PT6 and assure its future than any other airframe application. As of 2015, more than 41,000 engines had been manufactured and serve nearly 7,000 operators in 170 countries. In addition, the engines have accumulated more than 335 million flight hours. Except for the Beechcraft Model B100 that was powered by AirResearch TPE-331 turboprop engines, the PT6 has powered all King Air commercial models and military derivitives.7
The following basic list covers PT6 applications for the commercial Beechcraft King Air and military series.
Commercial:
- PT6A-20: Model 90/A90/B90/99
- PT6A-20A: Model C90
- PT6A-21: Model C90A/B90/C90-1
- PT6A-27: Model 99A
- PT6A-28: Model E90/B99/100/A100
- PT6A-36: Model C99
- PT6A-41: Model 200
- PT6A-42: Model B200
- PT6A-52: Model B200GT/250
- PT6A-60A: Model 300/350
- PT6A-65B: Model 1900/1900C-1
- PT6A-67: Model 1900D
- PT6A-135: Model F90
- PT6A-135A: Model F90-1/C90GT/C90GTi/C90GTx
Military:
- PT6A-20: A90-1/U-21A/RU-21E/U-21G
- PT6A-28: A90-4/RU-21E/RU-21H/U-21F
- PT6A-41: A100-1/A200/C-12A/A200C/UC-12B
- PT6A-65B: UC-12J
NOTES:
1. Sullivan, Kenneth H., and Milberry, Larry: “Power – The Pratt & Whitney Canada Story;” CANAV Books, 1989.
2. Ibid
3. Harding, Stephen: “U.S. Army Aircraft Since 1947;” Airlife Publishing Ltd., 1990.
4. Sullivan, Kenneth H., and Milberry, Larry: “Power – The Pratt & Whitney Canada Story;” CANAV Books, 1989.
5. Ibid
6. Ibid
7. Phillips, Edward H.: “Beechcraft—Pursuit of Perfection;” Flying Books, Eagan, Minnesota, 1992. A total of 137 B100s were delivered before production ended in 1983.
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