TURBOPROP HISTORY

TPs continue their key role in bizav

New models emerge while older designs still serve.


No business turboprop is as well known as Hawker Beechcraft's venerable King Air series, which traces its lineage to the original piston-powered Beech 50 Twin Bonanza. Latest version of the King Air 200, the 200GT (above) is powered by 2 PT6A-52 engines of 850 shp each.

Garrett, now a division of Honeywell, also began work in the early 1960s on a turboprop engine that competed directly with Pratt & Whitney Canada's PT6A.

The Garrett TPE331, a single-shaft fixed turbine turboprop engine was soon appearing on a host of business aircraft, including the Rockwell (formerly Aero) Turbo Commander series, Cessna 441 Conquest II and Mitsubishi MU2, as well as regional aircraft such as the BAe Jetstream 31/32 and Fairchild (Swearingen) Metro.

Even the King Air, perhaps the best-known airframe using the PT6A, found itself equipped with the TPE331 for the launch of the B100 series. While used in fewer applications than the PT6A series, more than 14,000 TPE331s have been delivered since its advent in 1963.

Generally speaking, fixed-turbine turboprop engines like the TPE331 are simpler designs with fewer moving parts than a comparable free-turbine engine like the PT6A. As a result, they tend to be slightly smaller and more compact.

Yet, despite their comparative simplicity, they have their operational idiosyncrasies that are most manifest in the post-flight behavior of the crews that fly TPE331-equipped aircraft. Notably, on shutdown, pilots of these aircraft will often walk over to the propellers and spin them vigorously by hand in order to draw in additional air to cool the turbine.

The propellers on all turboprop engines are linked to the inlet or power turbines through a separate gearbox, but when the propeller stops on a fixed-turbine design so does the turbine itself. Spinning the propellers by hand also serves to expel hot air from the engine, which helps prevent shaft bow (as the TPE331 has relatively few main bearings considering its length).

This practice is also said to enhance fuel nozzle life and reduce air seal drag on the engine, which can lead to a hung start.

The Advanced Turboprop Project and the UDF

If the story of the turboprop engine ended with today's popular business applications, it would still be considered a widely successful powerplant.

But the story of the turboprop took an interesting turn in the 1970s-one that linked it forever to its fuel-efficient reputation and that resulted in the comparatively wild propeller designs found on newer turboprops.

In 1973, during the Yom Kippur war between Israel and the forces of Egypt and Syria, an oil embargo was imposed on the US, Europe and Japan, causing fuel prices to skyrocket overnight.

Six years later, as Ayatollah Khomeini ousted the Shah of Iran during the Iranian Revolution, panic struck the market. Again, oil prices soared. Volatility was the new norm, and as technology took on the task of producing more fuel-efficient engines, new life was breathed into the turboprop and its related accessory, the propeller.

Throughout the late 1970s and early 1980s, NASA engineers were working on an effort dubbed "the Advanced Turboprop Project"-a US government-funded study to examine the feasibility of developing the turboprop engine as a viable competitor to the larger, less efficient turbojet and turbofan designs found on airliners of the day.

The result of the NASA prototype and wind tunnel tests was a tractor-style turboprop design with a single row of computer-modeled scimitar-shaped blades that allowed extremely high subsonic cruise numbers without the typical drag penalties associated with propeller tips speeds as they approach supersonic.

Concurrently, and without the knowledge of NASA, engineers at General Electric were working on a similar design, but one which used 2 rows of counter-rotating blades in a pusher configuration-what would ultimately come to be known as the unducted fan (UDF). Unveiled in 1983, the GE36 UDF "propfan" was a highlight of the 1985 Paris Air Show.

By the following year, the UDF was being flown on a test bed, and Boeing announced the 7J7-a 150-seat airliner that would be powered by the UDF. Pundits and newspapers alike began to announce that the propeller was back to stay.

General Electric’s efficient GE36 unducted fan (UDF) was once seen as the future of airliner engine design. In the mid-1980s, the declining price of oil and the rise of the CFM56 engine sealed the fate of the powerplant. Its technology lives on today, largely in composite propeller designs affixed to turboprops.

The NASA team which led the Advanced Turboprop Project was awarded the Collier Trophy in 1987 for its work. Things were looking up for propeller-driven aircraft. But the story of the turboprop is inextricably linked to its fuel economy, which is revered only during times of high oil prices.

By 1987, the popular and efficient CFM56 turbofan was selling in wide numbers, and the market for the UDF evaporated as oil fell below $20 a barrel. The same collective national conscience that had once worried about fuel prices snubbed history instead, and went back to its business.

By 1989, the turboprop was again out of fashion, even as a lowly feeder aircraft-at least in North America-as Bombardier announced its Canadair Regional Jet (CRJ) program. Despite the disinterest in turboprops after the oil markets fell, the technology that came out of the Advanced Turboprop Project and the UDF lives on in today's turboprop engines and propeller systems.

Among these offspring are the Lockheed Martin C130J Hercules, which uses new Rolls-Royce AE_2100 turboprop engines coupled to all-composite, 6-bladed scimitar propellers. Similarly, EADS's 4-engine turboprop competitor to the C130J-the Airbus A400M-is equipped with all-composite 8-bladed scimitar propellers.

And even the US Navy's 1960s-era E2 Hawkeye, which still uses the trusty Allison T56A, has received a propeller update in the form of a composite 8-bladed arrangement like that on the A400M. Predictably, as oil prices peaked last summer, the propeller was again in vogue.

An Oct 24, 2008 General Electric press release entitled "GE and NASA to initiate wind-tunnel testing for open rotor jet engine systems" disclosed that GE36 UDF technology was being dusted off, albeit now under the moniker "open rotor" as opposed to "propfan." It appears that, finally, the turboprop and the propeller are here to stay.

Douglas Wilson started as a lineman at JGG (Williamsburg VA). An active pilot, he now serves as director of line operations and customer service for Galvin Flying Services at BFI (Boeing Field, Seattle WA).

 

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