After two decades of study, Pratt’s PW1000G takes flight

 - July 30, 2008, 5:04 AM
Pratt & Whitney passed one of the most important milestones in its long history as the much-heralded PurePower PW1000G took to the skies last month. Formally known as the Geared Turbofan, or GTF, the PW1000G demonstrator flew on a Boeing 747SP testbed from the company’s Plattsburgh, N.Y. test facility, where engineers have validated various modes of operation and evaluated the electronic control logic that manages engine response times and characteristics. According to Pratt & Whitney, all indications point to a smooth transition from 20 years of study into a real-world model of efficiency from which the world’s airlines will reap benefits for decades.

“The engine is running really well,” said Pratt & Whitney next generation product family vice president Bob Saia. “The GTF modules that we have on this engine are meeting or exceeding the commitment levels we have made to [launch customer Mitsubishi Heavy Industries] and Bombardier.”

During the flight-test program Pratt plans to run the PW1000G demonstrator some 150 hours in total, said Saia, including between 30 and 40 hours over the course of about three weeks on the 747. Next, the company plans to remove the engine and deliver it to Toulouse, France, for installation on an Airbus A340-600, giving Airbus a first-hand look at a potential powerplant for the successor to its single-aisle A320 series. Schedules call for 75 and 100 hours of testing over the course of two-and-a-half months on the A340 starting in the middle of next month.

Saia explained that the A340 tests will also allow P&W to get an early start on evaluating installation and maintenance practices with Airbus. During the flight testing itself, the A340 will allow for more precise acoustic readings on the demonstrator because its engines produce less noise than the Pratt & Whitney JT9Ds on the older 747.

Pratt & Whitney plans to start detailed design on the version of the PW1000G destined for the 70- and 90-seat Mitsubishi MRJ regional jets, the MRJ70 and MRJ90, respectively, by year-end. Plans call for the first MRJ engine to enter testing in December of next year.

Next in the queue stands Bombardier’s C Series, the 110- to 130-seat pair of airliners launched last month and scheduled for market introduction in 2013. Saia estimated that the progress of Pratt & Whitney’s collaboration with Bombardier lags roughly six to eight months behind its work with Mitsubishi.

Faster Compressor, Fewer Stages
Although sized for different thrust ratings, the MRJ and C Series engines will use virtually identical architectures and the same gear system that distinguishes the PW1000G from other turbofan designs. In simple terms, the system allows the engine’s fan to turn at roughly one-third the speed of the low-pressure compressor and turbine. Because the low-pressure components turn much faster than those in a conventional engine, they can take in more air, allowing designers to remove stages and making the entire low-pressure side lighter and more efficient.

Removing stages also allows P&W to make the engine shorter, therefore moving its center of gravity forward and allowing it to be positioned closer to the wing centerline. That relieves stress from the wing, allowing airframe makers to use a lighter airfoil.

Meanwhile, “lubrication is really important,” Saia said in response to questions about the heat and friction produced by a gear system that sees the low-pressure turbine and compressor turning at three times the speed of the engine’s fan.
So Pratt & Whitney engineers were particularly encouraged when tests on the company’s fan drive gear rig in Middletown, Conn., showed the engine operating at cooler temperatures than first predicted.

Using a low-pressure compressor that turns much faster than the same part in a conventional turbofan engine, however, the PW1000G might have produced higher tonal frequencies if the company’s acoustic calculations proved inaccurate. As Saia expected, that didn’t happen, and in terms of overall noise the engine performed just as advertised, he said. “We achieved our target, running at Stage 4 minus 20 decibels,” he said. That equates to a 50-percent reduction in noise, thanks mainly to the PW1000G’s larger, slower-turning fan.

In fact, according to P&W, all of the GTF technologies have met or exceeded pre-test expectations, including a 12-percent fuel-burn improvement over any comparably sized design available today.

Saia said the company expects to improve on its fuel burn benefits by at least 1 percent a year–a benchmark that it bases on experience with other programs. “So by the time we get to 2020, we want the 12 percent to be 20 percent,” he added.
Some of those improvements could come from changes in the fan-to-low-pressure compressor ratio from 3:1 to 4:1 and perhaps even 5:1. Such extremes would probably require more exotic materials, however, something Pratt & Whitney has gone to lengths not to do at this point.

Although the company hasn’t ruled out materials changes to other parts of today’s engine, any changes to the gear system will have to wait for what Saia called Generation 2, scheduled for service entry between 2015 and 2017–in time for the advent of Boeing’s and Airbus’s single-aisle replacements.

“On the next generation, we’re hoping we can take a little weight out of the fan drive gear system and that will depend upon what we learn from destructive testing that we’re doing, to see how much margin we really have in the structure,” said Saia.

While P&W does not want to risk any modification to the fan drive gear system in this generation, other components, such as the fan, remain subject to change. Now performing demonstrator testing with titanium fan blades, P&W plans to switch to either a composite material or a lightweight proprietary metal. Structural testing on various composite chemistries has already begun, while the company plans to start bird ingestion testing on the metallic fan with a Pratt & Whitney Canada engine later this year.

“We’re leaning toward the lightweight metal, but there’s some value with the composite blade that we really want to evaluate,” said Saia. “As we go to larger engines, we will move to the composite early, let’s say on an MRJ engine, then we get revenue service experience on that composite before, say, a Boeing or Airbus application.”

For now, Saia and company feel secure in the knowledge–or at least the firm belief–that the MRJ can wrest a respectable piece of a regional airline market projected to produce a 20-year demand for some 5,000 to 6,000 airplanes. Of course, Pratt & Whitney is keenly aware that the segment that now includes the 737 and A320 could continue to account for 85 to 90 percent of all deliveries. “We want to make sure we’re ready for the big fish,” said Saia.