Page 31 - Volume 15 Number 8

Piper Aircraft Corporation.
The Canadian manufacturer was well aware that its chief competitors, Allison and Rolls-Royce, already had excellent engines such as the T56 and the “Dart,” respectively, powering larger aircraft. In addition, Allison was hard at work developing a small, lightweight turboprop engine in the 250 shaft horsepower (shp) range, and the “Dart” was available in versions up to 2,000 shp. After considering market reaction and the competition, company officials decided to proceed with development of a new turboprop engine in the 450- to 500-shp class.
A key goal of the ambitious program was to build an engine that could operate for about the same costs as a piston engine of equal power. To help achieve that point, which would be a major factor in selling the engine to airframe manufacturers such as Beech Aircraft that was long accustomed to the attributes of the reciprocating engine, a team led by Kenneth Sullivan and Elvie Smith urged adoption of a free-turbine design instead of the more conventional fixed-shaft configuration.
There were a number of distinct advantages to the free- turbine approach. First, the engine would require fewer components because the gas generator and the power turbine were not connected to each other. Instead, the two sections were “coupled” only by the hot exhaust gases flowing from the gas generator across the power turbine, which drove the reduction gearbox and propeller. Second, starting would be simplified because the battery would be rotating only the gas generator, not the entire engine (particularly advantageous in extreme cold weather conditions). Third, the airframe structure required to support the engine would be simplified and weigh less than that required for a fixed-shaft configuration.
In February 1960, a prototype PT6 was undergoing testing but soon encountered a litany of technical problems and issues. Fortunately for the program, these eventually were resolved by a team of engineers led by Bruce Torrell. The next
1
The PT6 Engine’s Role
It is easy to overlook the highly significant role that the Pratt & Whitney Canada PT6 engine has played in development not only of the King Air, but of the business aircraft industry in both fixed-wing and rotary-wing designs. As early as 1958, Pratt & Whitney Canada had conducted widespread research aimed at determining market reaction to a new, proposed turbine engine. As part of that survey the company sent teams to various general aviation airframe manufacturers including Beech Aircraft Corporation, Cessna Aircraft Company and
year an engine was test-flown on the nose of a Royal Canadian Air Force Beechcraft “Expeditor” (Canadian military version of the Model 18 Twin Beech), and in July 1961 a Hiller “Ten99” helicopter became the first aircraft to be powered solely by a PT6 engine. Despite overall progress with the program, skeptical voices within New England-based parent company Pratt & Whitney called for an end to the PT6 initiative.
The future suddenly looked increasingly bleak for the PT6. Then, at just the right moment in time, an airframe manufacturer in Kansas played a key role in preserving the PT6 initiative and saving it from oblivion. Beech Aircraft Corporation engineers were casting about the industry in an effort to find a turbine engine to power the U.S. Army’s proposed NU-8F utility aircraft based on the piston-powered Model 80 Queen Air. Beech Aircraft combined the fuselage of the Model 80 with the wings of the Model 50 Twin Bonanza and installed two PT6A-20 engines each rated at 550 shp. Flight testing by the Army proved successful and led to production of 141 U-21A “Ute” transports in the mid-to-late 1960s.2
Beech Aircraft, long known worldwide as a premier builder of general aviation and business airplanes for both commercial and military customers, was quick to capitalize on the success of the NU-8F. In the wake of that program’s success and having won the approval of “mahogany row’s” chief official, Olive Ann Beech, company officers introduced the Model 90 King Air in July 1963.
An often overlooked and underappreciated fact is that by introducing both an unproven engine on an equally unproven airframe in a marketplace firmly dominated by the piston engine, the leadership team at Beech Aircraft Corporation took a tremendous gamble that business aircraft operators would buy the King Air. In the years ahead, that gamble would pay off handsomely and form the foundation of a mutually beneficial relationship between an airframe and engine manufacturer that continues unabated today. Having committed itself to the future of business aviation, Beech Aircraft’s prototype Model 90 made its first flight from Beech Field on January 24, 1964, and “the rest,” as the saying goes, “is history. ”
As for the PT6 turboprop series, as of 2012* Pratt & Whitney Canada had manufactured more than 43,000 of the “little engines that could” in more than 90 versions for aviation applications. According to the company, the engines currently are in service with more than 6,500 operators in 170 countries and have accumulated more than 335 million flying hours since the engine’s introduction in the early 1960s.
Footnotes:
1. Sullivan, Mark P.; “Dependable Engines – The Story of Pratt & Whitney”; American Institute of Aeronautics and Astronautics, Inc., 2008
2. Harding, Stephen; “U.S. Army Aircraft Since 1947”; Airlife Publishing Ltd, 1990
* Numbers reflect what they were the year the article was first published.
AUGUST 2021
KING AIR MAGAZINE • 29