Beech Aircraft Corporation tightened its grip on the twin-engine turboprop market by introducing the Model 200 Super King Air – an airplane that quickly established itself as the crown jewel of the company’s product line.
Previously published in part in the Sept./Oct. 2012 issue of this magazine.
In 1973 as Beech Aircraft Corporation approached the 10th anniversary of its highly successful King Air series of business aircraft, the time was ripe for the company to take the King Air platform to the next level of performance and utility. The existing flagships of the Beechcraft fleet were the Model A100 and B100 that introduced customers to a larger cabin and higher cruise speeds than the Model 90 series, but the company’s engineering and marketing departments knew they had to create a better King Air to expand the product line.
Research and development work was underway by 1969 and resulted in a concept known initially as the Model 101 that eventually was replaced by the designation “Model 200.” Early in the development process, it was decided that the new Beechcraft would be powered by Pratt & Whitney Canada PT6 engines that had powered the Model 90 and Model 100/A100 airplanes. The new “Super King Air,” however, would require engines of increased shaft horsepower to achieve performance goals that included a cruise speed in excess of 300 mph.
As part of the company’s research and development program for the new King Air, Beech engineers sought a more powerful version of the venerable PT6 to power the airplane and Pratt & Whitney Canada responded with the PT6A-41 engine. It was longer and heavier than the PT6A-20-series engines that had powered previous King Airs but incorporated an improved gas generator and power turbine design that, along with other systems and material upgrades, allowed the engine to produce
850 shp at outside air temperatures of up to 106° F.
The choice of the PT6A-41, however, drove a number of important design, performance and certification issues that would prove challenging to resolve. These included relocating the engines higher and farther outboard on the wing center section than those of the A100/B100 to accommodate the new propellers, which were designed with a larger diameter to absorb the -41 engine’s power.
In addition, relocating the engines farther outboard had the advantage of providing a wider center section that would increase total fuel capacity to 544 gallons, as well as positioning the propellers farther away from the cockpit and cabin to reduce noise and vibration. As a result of these modifications to the basic A100/B100, wingspan was increased to 54 feet, 6 inches and maximum takeoff weight rose to 12,500 pounds with a useful load of 5,275 pounds.
Although the changes were highly desirable from the standpoint of marketing and customer satisfaction, one undesirable result of the engine’s placement centered on meeting FAA regulations for minimum controllable airspeed with one engine inoperative. Engineering realized that the additional two-foot span (on each side) of the redesigned center section would increase the effect of yaw if one engine failed in flight. They also knew that the rudder played a crucial role in countering that yaw and providing the pilot with adequate directional control.
Unfortunately, it was determined through wind tunnel tests that the empennage configuration of the A100/B100 did not provide good airflow characteristics for the rudder under single-engine conditions, particularly at increased angles of attack normally associated with single-engine operations. As a result, Beechcraft engineers designed a T-tail configuration that not only proved acceptable under single-engine minimum controllable airspeeds but met FAA certification rules without imposing high rudder pedal forces on the pilot.
Compared to the Model A100/B100’s conventional empennage, the T-tail design required fewer pitch trim changes as flaps were extended, and with the horizontal stabilizer moved to the top and aft (because of the vertical stabilizer’s sweptback design), the elevators proved quite powerful even at aft CG loading. “Now, with a much smaller horizontal surface than the 100,” [Beech engineers] “could revert to conventional trim tabs on the elevators and yet achieve a 4-inch greater CG range for the same cabin dimensions,” King Air expert Tom Clements said.
With the empennage configuration established, the Model 200 program moved forward. In 1970 two prototype airplanes were built, BB-1 and BB-2 (the letters “BB” were chosen to identify the Model 200 series and remain in use for production aircraft). BB-1 first flew October 27, 1972, with Beech Aircraft engineering flight test pilot Bud Francis at the controls and he also commanded BB-2 during its inaugural flight December 15 of that year.
The T-tail and rudder combination, although it proved highly effective under single engine conditions, required a cautious approach to flight testing as pilots carefully probed its characteristics. Clements recalls that as a result, BB-1 was equipped with a stick shaker, stick pusher, a rudder boost system and one additional safety system – a drag chute housed in a special tail cone. If the airplane failed to recover from a stall under aft center of gravity (CG) and high angle of attack conditions by pitching nose-up instead of nose down, the chute could be deployed by an explosive charge and “break” the stall.
Following an exhaustive flight test program that spanned more than a year, the FAA certified the Model 200 in December 1973 and initial customer deliveries began in February 1974. In addition to commercial sales, the U.S. Army and Air Force were quick to order military versions of the Model 200 for use as VIP and utility transports.
In 1974, serial numbers BB-3, -4 and -5 were converted for the Army and carried the Beech Aircraft designation “A100-1.” That year the Air Force ordered a batch of 14 airplanes designated “C-12A” (Beech Model A200), and the factory produced another 20 airplanes in 1975 for the Army and Air Force. The last 20 C-12A aircraft were delivered to the Air Force in 1978.
A continuous program of airframe and engine systems and performance upgrades were incorporated into more than 830 Model 200s that rolled off the assembly line from 1974-1981. With the introduction of the Super King Air, Beech Aircraft Corporation had demonstrated once again the company’s determination to build the best business aircraft in the world. During the next 40 years, the Model 200 gradually came to dominate the upper end of the cabin-class turboprop business aircraft market. It proved not only a successful addition to the company’s list of crowning achievements but was the most worthy King Air yet to bear the Beechcraft name.
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 Piper Aircraft Corporation.1
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 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:
- Sullivan, Mark P.; “Dependable Engines – The Story of Pratt & Whitney”; American Institute of Aeronautics and Astronautics, Inc., 2008
- Harding, Stephen; “U.S. Army Aircraft Since 1947”; Airlife Publishing Ltd, 1990
* Numbers reflect what they were the year the article was first published.