TURBOPROP HISTORY

TPs continue their key role in bizav

New models emerge while older designs still serve.


Latest version of the original 1960s-era Grumman E2, the US Navy's Northrop Grumman E2D Advanced Hawkeye (upper) is an airborne early warning and battle management aircraft. Power comes from 2 Rolls-Royce T56A-427As coupled to 8-scimitar-bladed composite NP2000 propellers. Compare with the earlier E2C (lower aircraft).

Notably, it was the Mk 529 version of the Dart that would power one of the first purpose-built business aircraft-the Grumman G159 Gulfstream, which first flew in 1958. Soon, 4-engine turbojet aircraft filled the longhaul need, and short to medium-range turboprops filled the void left by retiring piston-engine aircraft.

Although the Dart was a workhorse, and remained in production into 1987, competition in the form of even bigger, more powerful turboprops from the US and USSR would eventually outmode it.

East meets west

In 1951, when the US Air Force announced a requirement for a medium-size logistic and transport aircraft to replace a fleet of aging piston-powered transport aircraft, Lockheed responded with the C130 Hercules, now one of the most recognizable turboprops of all time.

First flown in 1954, the YC130 was powered by the new Allison-designed T56A-9 turboprop engine driving wide-chord, 3-bladed Aero Product propellers. Later, with the C130B, the 3-bladed propellers would be replaced by the now highly familiar Hartzell 4-bladed versions.

By 1957, the T56A series would show up in civilian use, on the Lockheed L188 Electra-a medium-range 4-engine turboprop airliner similar in layout and configuration to the Dart-powered Vickers Viscount.

Eventually, some 18,000 T56 turboprop powerplants would be produced, powering not only the 4-engine C130 and Electra series, but the US Navy's twin-engine Grumman C2 Greyhound and E2 Hawkeye, as well as the Electra-derived Lockheed P3 Orion.

Interestingly, production of the T56A series continues today, albeit no longer under the Allison name. In 1994, Rolls-Royce, maker of the famed Dart, announced its intention to acquire Allison Engine Company, maker of the T56A. By 1995, that acquisition was official, and Allison became a subsidiary of Rolls-Royce, again bringing Rolls to the forefront of turboprop technology.

The current iteration of the T56A-the Series IV-is capable of producing 5250 shp at only 1940 lbs of weight. In comparison, the Pratt & Whitney R4360 Double Wasp, arguably the zenith of piston-engine development at the close of WWII, could muster only 4300 hp, yet its weight was twice that of the T56.

And, while powerful by US standards, the T56A is small by comparison to Russian standards of large turboprops.

Giants in the sky

At about the same time the C130 was entering the world stage and the cold war was beginning to heat up, the USSR was quietly developing a turboprop-powered intercontinental bomber of monstrous proportions-the Tupolev TU95 (NATO codename Bear).

Development of the Bear mimicked the concerns engineers in the west were facing in the 1950s. In effect, piston engines were of late WWII vintage and were simply not powerful enough, but pure turbojet engines lacked range due to high fuel consumption.

During this period, while the US flirted with the idea of supersonic intercontinental bombers, it produced the subsonic Boeing B52 Stratofortress in droves. It also began moving the 4-engine piston-powered KC97, which was based on the Boeing 377 airframe, to second-line duties, as an all-jet aerial refueling armada composed of Boeing 707-like KC135 Strato_tankers would supply the thirsty B52s.

Again it seemed that the propeller age was drawing to a close-or that, at best, propellers were to be relegated to only slow, lumbering transport aircraft. But Tupolev's TU95 was a radical turboprop design.

A long slender fuselage was attached to wings that swept back some 35 degrees. At first glance, it appeared the designers at Tupolev meant for turbojets to power the Bear. Instead, the TU95 is powered by 4 Kuznetsov NK12 turboprop engines each driving a pair of 4-bladed counter-rotating propellers.

Pilatus launched the PC12 NG next-generation version of its successful PC12 turboprop in 2006. It uses a 1200-shp P&WC PT6A-67P.

Producing some 15,000 shp, the current version of the Kuznetsov turboprop, the NK12MA remains the most powerful turboprop engine ever developed. Even in 1953, when the Bear first flew, early versions of the NK12 were producing around 12,000 shp-some 2.5 times the output of the most powerful Allison turboprop engine in production today.

While propeller-driven aircraft are typically regarded as slow by comparison to turbojet aircraft, the combination of ultra-large turboprop engines, counter-rotating propellers and a jetlike design means the TU95 is no slouch. With its maximum speed of 575 mph, or Mach 0.87, the Bear is faster than most of today's commercial jetliners.

Reported to have initially outaccelerated even afterburning western-built jet fighters, Tupolev's TU95 is easily the fastest propeller-driven aircraft in the world. And, as defense analysts have it, the Bear should remain in active service until at least 2040.

Business applications

While western and eastern militaries were experimenting with ever larger turboprops for a wide variety of aircraft, Pratt & Whitney Canada and Garrett AiResearch were working on civil and business applications for their small to midsize turboprop engines.

In 1961, the Pratt & Whitney Canada PT6A took to the skies in experimental form. By 1963, it was evident it was a winner, and the engine began a production run that continues to this day.

Eventually, the PT6A would become the most popular turboprop engine ever developed, with a notable 36,000 units delivered before its 40th anniversary in 2001. This remarkably adaptable engine, available today in models from 500-2000 shp, powers nearly the entire series of Hawker Beechcraft King Air, as well as the Cessna Caravan, Piaggio Avanti, Pilatus PC12 and Piper Cheyenne II/III.

In most applications the PT6A is mounted facing rearward-the intake side of the engine faces the rear of the aircraft and the power section and gearbox face forward, driving the propeller. Because of this design, the engine uses 2 separate shafts, making it a free (as opposed to fixed) turbine design.

Although slightly more complex than a single-shaft fixed turbine, the PT6A arrangement is a favorite among mechanics in the field as it allows ease of access to the engine's power or hot section, and it is much quieter during ground operations.

 

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