Muscle Beech – the Mighty Turbo Barons

Despite being manufactured in small numbers, the Model 56TC and A56TC were the most powerful Barons built and helped pave the way for development and production of the distinctive Model 60 Duke.

The decade of the 1950s and 1960s had been good to Beech Aircraft Corporation, and the company’s executive vice president, Frank E. Hedrick, reflected on those years in an address to the North America Newcomen Society in September 1967. He recalled that total sales (commercial and military) for Beech Aircraft in 1950 were $16.6 million, and by 1966 sales had exploded to 10 times that amount to $164.6 million. In addition, he anticipated that sales for 1967 would hit $175 million, and they did.

Hedrick said it took the company 25 years – from 1932 to 1956 – to achieve cumulative sales worth $1 billion, but only another 10 years to reach the ethereal $2 billion level. He also remarked, “Let it be recorded here and now that we at Beech Aircraft are profoundly proud of the heritage of our pioneering past – from 1933 with sales of $17,552 and 10 employees, to sales of $175 million and 10,000 employees [in 1967] with accumulative sales of $2.2 billion.”

One month later in Wichita, Kansas, more than 600 company officials and Beechcraft salesmen from around the world were attending the International Sales Spectacular and were told that two new airplanes would be entering the general aviation/business flying segment of the industry in only a few months. These were the Model 60 Duke (undergoing final development and flight testing) and the Turbo Baron 56TC that was entering production for the 1967 model year.

The Turbo Baron 56TC was based on the Model 95-D55 Baron airframe and a pre-production prototype, designated constructor (serial) number TG-1, made its first flight on May 25, 1966, with company engineering test pilot Bob Hagan at the controls. What gave the new Baron its muscle were two of Lycoming’s most powerful piston powerplants – the turbocharged, fuel-injected, opposed six-cylinder TIO-541-E1B4, each rated at 380 horsepower at 2,900 RPM and 41.5 inches of Hg (mercury) manifold pressure.

Impetus for development of the Turbo Baron was two-fold. First, Cessna Aircraft Company and Piper Aircraft Company were developing the twin-engine Model 401 series and the PA-31 Navajo, respectively. The sleek Cessna was powered by 300-horsepower Continental TSIO-520E engines, while the Navajo featured either the Lycoming IO-540-M rated at 300 horsepower or the optional TIO-540-A1A that produced 310 horsepower. As it had in the past, Beech Aircraft needed to respond to the competition and offer Baron buyers an airplane possessing overall performance equal to or better than the Model 401 and Piper Navajo.

The second reason was, perhaps, of primary importance. In the mid-1960s Beechcraft engineers and marketers had begun working on design and development of the all-new Model 60 Duke – a cabin-class, piston-powered business airplane that was intended to set a new standard of style and performance unmatched by any other aircraft in its class. The Turbo Baron would act as a platform for development and FAA certification of the Duke’s unique engine package, while offering Baron pilots the opportunity of owning one of the fastest lightweight twin-engine airplanes in the world.

The Turbo Baron’s Lycoming powerplants were housed in large, streamlined cowlings featuring nacelles that swept across the top of the wing and extended all the way aft to the wing trailing edge. Development of the Lycoming O-540-series engines had begun in 1959, and by the mid-1960s had evolved into the 380-horsepower TIO-541 and 450-horsepower TIGO-541. The engines were equipped with three-blade, constant-speed, full-feathering propellers manufactured by Hartzell. To satisfy the thirst of the Lycoming engines, Beech engineers increased the D55’s total fuel capacity to 182 gallons in bladder-type cells. In addition, maximum gross weight grew significantly to 5,990 pounds.

General dimensions of the 56TC remained the same as for the Model D55, with a wingspan of 37 feet 10 inches and the larger horizontal stabilizer/elevator of the Model 95-C55 that spanned 15 feet 11.25 inches.

The 56TC’s cabin essentially was unchanged from that of the D55 Baron and featured six seats, but the turbocharged Barons were among the first (if not the first) to have an (optional) Freon-based air conditioning system in a lightweight, twin-engine airplane. The installation was rated at 16,500 BTU and proved to be more than adequate to cool the cabin on hot days during taxi and climb to altitude.

The Model 56TC’s chief claim to fame, however, was not keeping its occupants cool, but was all about performance. At an altitude of 20,000 feet, the new Baron was restricted to 262 mph but was easily capable of achieving 300 mph (TAS) at 24,000 feet (full throttle). By the standards of 1967, that made the 56TC one of the fastest piston-powered, lightweight multi-engine airplanes in the world. Service ceiling was an impressive 32,200 feet.

Further flight tests revealed a two-engine rate of climb exceeding 2,000 feet per minute, although rate of climb with one engine inoperative plummeted to only 412 feet per minute (at maximum gross weight). To take full advantage of the airplane’s all-weather capabilities, Beech Aircraft offered the high-flying 56TC with a supplemental 66 cubic-foot oxygen system but also offered a 114 cubic-foot system as an option. The airplane was equipped with wing, empennage and propeller deicing and the majority left the factory so equipped. The FAA certified the Model 56TC on May 19, 1967. The powerful Baron was produced from 1967 through the 1969 model year and 82 were built. Yearly production numbers included 50 in 1967, 20 in 1968 and 12 in 1969.

Although sales of the 56TC were slow, the mighty Beechcraft did fill a gap between the Model E55 Baron and the much larger (but slower) Model A65 Queen Air. Beech engineers upgraded the airplane for the 1970-1971 model years as the Model A56TC. As with the 56TC, the A56TC could carry up to 300 pounds of baggage in the spacious nose compartment or a buyer could opt to install remote-mount avionics components, which often reduced baggage capacity (varied with the amount of avionics installed).

The only technical change was in response to customer feedback calling for more fuel, and Beech Aircraft responded by increasing capacity to 207 gallons of useable fuel. In addition, the 56TC’s wing-mounted pitot tube was relocated to the nose of the A56TC. The Lycoming engines were unchanged, and as with the 56TC, the turbocharger system was fully automatic and required little pilot workload to manage. Maximum gross weight remained at 5,990 pounds. Service ceiling with one engine inoperative was 18,600 feet at gross weight, increasing to 23,000 feet at a weight of 5,000 pounds. The factory rolled out only nine A56TCs during the 1970 model year followed by a mere two airplanes in 1971 when production was terminated in favor of the popular Model 60 Duke and the advent of the new Model 58 Baron.

The sleek Duke was introduced by Beech Aircraft Corporation for the 1968 model year. Combining speed, cabin comfort and an airframe that looked like no other business aircraft on the planet, design work on the Duke began early in 1965. Specifically, the airplane was intended to fill a gap in Beech Aircraft’s product line between the 56TC and the Model 65 Queen Air (production of the Model 50 Twin Bonanza was terminated after the 1961 model year).

The Duke also would answer a fresh challenge from competitor Cessna Aircraft Company with its Model 421 that featured a pressurized cabin that seated up to six occupants. A prototype first flew in October 1965. The pressurization system (4.2 psi maximum differential) allowed a cabin altitude of 8,000 feet at an airplane altitude of 20,000 feet. The Model 421 proved to be a great success for Cessna Aircraft, with 200 built in 1968, the first year of production. Powered by two Continental (T)urbosupercharged, (G)eared, (S)upercharged, (I)njected, (O)pposed six-cylinder piston engines (TGSIO-520-D) each rated at 375 horsepower, the new Cessna had a maximum speed of 286 mph at an altitude of 16,000 feet and a service ceiling of 26,000 feet. In terms of overall performance and cabin comfort, the Model 421 would prove to be a worthy competitor to Beech Aircraft’s powerful Duke.

By early 1966 construction of a pre-production prototype of the Model 60 (designated constructor number P-1) was well underway, and the airplane flew for the first time on December 29, 1966, with engineering test pilot Bob Hagan at the controls. As mentioned previously, the Duke would borrow heavily from the 56TC/A56TC’s fuel system, cowling and engine installations, all of which had be developed and proven on the Turbo Barons. The Duke’s engines were the same Lycoming turbocharged TIO-541-E1A4 units installed on those aircraft and turned Hartzell three-blade, constant-speed, full-feathering propellers that featured a diameter of 6.2 feet. Performance calculations indicated that the Model 60 would have a maximum cruising speed of 278 mph at an altitude of 25,000 feet, and a service ceiling of 30,800 feet (very similar to the Turbo Barons).

Although the Duke shared many of the basic airframe systems with the Model 56TC/A56TC, there were some differences. Whereas the 56TC used an NACA 23017-5 airfoil at the wing root, the Duke’s wings employed a NACA 23016-5 airfoil at the root, changing to the same 23010-5 airfoil used on the Model D55 Baron at the wing tip. The all-metal, cantilever wings spanned 39 feet 4 inches and featured a total area of 213 square feet. Wing chord was 9 feet 2 inches, dihedral 6 degrees, with incidence set at 4 degrees at the root and 1 degree at the tip. Wing loading was 31.6 pounds per square foot while power loading was 8.8 pounds per horsepower. Flaps were operated electrically, as was the tricycle landing gear.

One major advantage of the Duke over the 56TC/A56TC was its pressurized cabin that was only beginning to appear in an aircraft of the Duke’s class. Bleed air from the turbosuperchargers was routed into the cabin where valves automatically regulated cabin altitude in response to the pilot’s input to a controller in the cockpit. The system was capable of maintaining a maximum differential (difference between atmospheric pressure outside of the airplane compared to air pressure inside the cabin) of 4.6 pounds per square inch (psid). The differential provided sea level conditions in the cabin up to an airplane altitude of 10,000 feet, and a cabin altitude of 10,000 feet at an airplane altitude of 24,800 feet.

As for the cabin, a center aisle flanked on each side by two seats were standard, but six seats were optional (although when fitted with six seats the cabin was snug). Many customers opted for six seats based on mission requirements. Entry into and exit from the cabin was through a hinged door located on the left, aft side of the fuselage. A 32-cubic-foot baggage compartment was provided in the nose section and a second compartment offering 28 cubic feet of storage was located in the aft cabin.

To keep the cockpit and cabin at a comfortable temperature at high altitudes and on the ground in winter, a fuel-fed combustion heater rated at 45,000 BTU was standard along with an optional, electrically-driven vapor cycle air conditioning system rated at 14,000 BTU.  A list of optional equipment included (but was not limited to) fifth and sixth seats, aft-facing third and fourth seats, writing desks; cabinetry to house refreshments, a toilet, urethane paint and electric deicing for the windshield.

To feed avgas to the Lycoming powerplants, the standard fuel system featured two interconnected cells in each wing that held a total of 142 gallons, and an optional system with four interconnected cells in each wing was available that increased capacity to 204 gallons. Many Duke operators opted for the extra fuel that increased range to 973 statute miles at an altitude of 25,000 feet while cruising at 271 mph (at a 75 percent power setting). Maximum cruising speed was 278 mph at 25,000 feet, service ceiling 30,800 feet, and maximum gross weight 6,775 pounds.

Following FAA certification in February 1968, the Duke was built at Beech Aircraft’s Wichita facility, but in June 1968 final assembly was relocated to the company’s facilities in Salina, Kansas. During the 1968 model year, only 15 airplanes were built as production began to ramp up to full capability. The next year 91 aircraft were built, followed by 16 in 1970. During the latter half of the 1970 model year, engineering implemented a series of improvements that resulted in the Model A60 replacing the original Duke on the production line.

Externally, the two airplanes were indistinguishable from each other, but under the cowlings new, lightweight, improved turbosuperchargers were installed that provided extended component life and allowed the TSIO-541-E1C4 to develop maximum rated horsepower at a higher altitude. Although maximum speed was unchanged at 286 mph, service ceiling increased to 35,800 compared with 30,800 feet for the original Duke. In addition, fuel economy was improved and the cooler operating temperature of the turbosuperchargers were beneficial and served to increase engine longevity.

The cabin of the Model A60 also received a facelift, with new selections of interior fabric and leather, and the pressurization control system was revised to provide smoother control of the cabin altitude. The A60 remained in production until the 1974 model year, when it was replaced by the Model B60. The A60 had sold well, however, with 23 built in 1970, 27 in 1971, 28 in 1972 and 43 in 1973. In October of that year, the Duke experienced its best month of sales since 1968 when orders for 18 airplanes were received worth more than $3.3 million.

The Model B60 was the final version of Beech Aircraft’s handsome Duke. It featured a cabin that was slightly wider and longer that allowed the use of redesigned seats and improved overall passenger comfort. The engines used improved intake valves, and a new turbosupercharger overboost relief valve was installed. In addition, Beech engineers designed an electronic overheat detection system to monitor the nickel-cadmium battery’s cell temperatures, and added a new duct to provide cooling air to the battery compartment.

For the 1975 model year, the entire pressurization system was upgraded to a lightweight AiResearch design with new Lexan outflow and safety valves and smaller controller in the cockpit that saved space on the instrument panel. By 1976, customers could order wet-cell wingtip fuel tanks each holding 30 gallons and increased the Duke’s range to 1,287 statute miles at a power setting of 65 percent. Time between overhaul of the Lycoming engines increased to 1,600 hours in 1977 and maximum cruise speed was raised to an impressive 283 mph.

Twelve years after the Duke was introduced, in March 1979, Beechcrafters at the Salina facility rolled out the 500th Duke – a Model B60 “Special Edition” bearing a silver and black exterior paint design with a custom cabin interior to match. The high-performance, owner-flown Duke soldiered on until 1982 when lackluster sales forced the company to terminate production after more than 500 airplanes had been built. The last Duke delivered in 1982 was constructor number P-596. Only 16 airplanes were built that year. 

When introduced for the 1968 model year, the Duke sold for more than $600,000, but that figure could increase significantly depending on optional equipment and avionics. As of early 2017, used aircraft prices for the Model A60 and B60 Duke ranged from $99,000 up to $180,000 depending on total time airframe/engines and avionic equipment. Major drawbacks to the Duke’s resale value are the high operating, maintenance and replacement costs of the Lycoming engines, as well as airframe corrosion issues with the empennage structure.