GTF speeds toward date with the A340
Pratt & Whitney passes one of the most important milestones in its long history as the much-heralded Geared Turbofan engine takes to the skies this month. Installed and nearly ready to fly aboard the company’s Boeing 747 testbed in the days leading to the start of this week’s Farnborough airshow, the GTF demonstrator underwent 250 hours of ground testing since engineers first ran the engine in November 2007. 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 real 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. Certainly there’s a lot of fine tuning that we’ll do during what we call our design and development phase, but the overall characteristics look really good.”
During the flight test program, Pratt & Whitney (Hall 4 Stand F13) plans to run the GTF demonstrator another 150 hours or so, including 75 to 100 hours over a two-and-a-half-month period on an Airbus A340-600 starting in mid-September. By then the engine will have flown on the 747 for between 30 and 40 hours over a three-week period. During that time, the company plans to test starting, general operability, acceleration and deceleration, as well as evaluate the electronic control logic that manages engine response times and characteristics. Once finished with those tests, Pratt & Whitney plans to remove the engine and deliver it to Toulouse for installation on the A340, giving Airbus a first-hand look at a potential powerplant for the successor to its A320 narrowbodies, now planned for late next decade.
“It’s easy for Pratt to speak positively about a Pratt product,” said Saia. “But if you get a customer to speak positively about a Pratt product, they have a lot more leverage.” He explained that the A340 tests will also allow Pratt & Whitney to get an early start on evaluating installation and maintenance practices with Airbus. Then, 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.
MRJ Is First Target
Pratt & Whitney plans to start detailed design on the version of the GTF destined for the 70- and 90-seat Mitsubishi MRJ regional jets by the end of this year, followed by work on a slightly larger variant meant for the Bombardier C Series narrowbody airliner late in next year’s first quarter. Because schedules call for the first MRJ engine to enter testing in December of next year, Pratt & Whitney hasn’t decided yet what it plans to do with the GTF demonstrator once it finishes flying on the A340.
“Even though we may find value into running this demonstrator, we really want to put that energy– whether it’s engineers or financial resources–into the product,” said Saia. “But I’m sure we’ll find a way of using this motor downstream someplace.”
In March, Mitsubishi chose the GTF for its newly launched tandem of regional jets known as the MRJ70 and MRJ90. Every other week Pratt & Whitney and Mitsubishi engineers take turns visiting each other’s facilities for a week of workshops. At the sessions, they talk over subjects such as the integration of the engine with the aircraft’s electronic and environmental systems, or decide, for instance, the best position of certain engine components for installation on a particular pylon.
Next in the queue stands Bombardier’s C Series, the planned 110- to 130-seat pair of airliners 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. Of course, the company still has its eyes trained on bigger fish– namely, applications on the expected replacements for the Boeing 737 and Airbus A320 in the second half of the next decade. For now, however, a high-frequency regional application could serve as a proving ground for
a technology that skeptics have criticized as too complicated and potentially trouble-prone.
Pratt & Whitney counters that the engine actually contains fewer parts and weighs about 10 percent less than a conventional engine because the gear system allows for fewer compressor and turbine stages. “It has fewer parts because it has fewer stages,” said Saia. “It’s about five stages on average. From an airfoil count, we’re probably dropping in airfoils [by] around 1,500, which is a pretty significant [reduction].” The engine maker advertises a 40-percent reduction in airfoils compared with “today’s” engines.
Less Stress on the Wing
In simple terms, the gear 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 Pratt & Whitney to make the engine shorter, therefore moving its center of gravity forward and allowing it to position it closer to the wing centerline. That relieves stress from the wing, allowing airframe makers to use a lighter airfoil.
Meanwhile, designers consciously decided to include nothing particularly exotic in their choice of materials. “The GTF as it’s operating today is within the same design space as a conventional engine, so we haven’t caused it to put a load on a bearing material or something that’s beyond what we’ve done in commercial engines today,” said Saia.
“Lubrication is really important,” he added in response to questions about the heat and friction produced by a gear system that turns the low-pressure turbine and compressor 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, Connecticut, showed that engine operating temperatures would measure cooler than first predicted. “We actually found through our ground testing that we can reduce the size [of the heat exchanger] that we have in the demonstrator by 20 percent,” said Saia. “We’re finding that the gear system is more efficient than we initially predicted and, secondly, we’re finding that the overall characteristics of managing oil temperature were more favorable than what we had predicted.”
Ground Testing Under Way
The Geared Turbofan demonstrator ran for 130 hours during the first phase of ground testing in Palm Beach, Florida, which ended in March. Running from idle to full power, Phase 1 testing validated the design of the fan, low-pressure compressor, fan drive gear system and the Hamilton Sundstrand-supplied thermal management system. Phase 2, which ended in late May, involved some 120 hours of testing to validate engine performance and ground acoustics with the Geared Turbofan engine’s flight-capable Goodrich nacelle system.
Phase 2 testing used an array of 32 individual microphones around the engine test stand and 16 sound pressure transducers inside the engine to get accurate data for noise modeling. “We achieved our target, running at Stage 4 minus 20 decibels,” said Saia. “We did absolute measurements at the engine level and then we also did tone measurements, and we were able to demonstrate much quieter, ear-pleasing tones that go with this lower noise signature.”
Pratt & Whitney’s ability to lower noise by an expected 50 percent would come by virtue of the GTF’s larger, slower-turning fan. Using a low-pressure compressor that turns much faster than the same part in a conventional turbofan engine, however, the GTF might have produced higher tonal frequencies if the company’s acoustic calculations proved inaccurate. As Saia expected, that didn’t happen, he said. “We weren’t counting on having a higher tone associated with a higher speed, but we needed to validate that, and the engine met our pre-test forecast.”
In fact, according to Saia, 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 Pratt & Whitney fully expects to improve on its fuel burn benefits by 1 percent a year–a benchmark that it bases on experience with other programs over the years. And because the GTF involves a completely new architecture, the company expects the improvements to come faster than usual.
“So by the time we get to 2020, we want the 12 percent to basically be 20 percent,” said Saia.
Some of those improvements might 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 avoided doing at this point.
“All of our development history told us that 3:1 was the right place to start,” said Saia. “It appeared to be the right balance between the turbomachinery–like a low compressor, a low turbine– and how you want to operate the fan. Once we get that knowledge under us, I’m sure our engineers will be able to decide, ‘OK, what do we want to do for our next generation?’
“For [the low-pressure compressor] to go higher in speed, we may have to use different materials in, for example, a bearing compartment or even in some of the attachments associated with the speed. The GTF, as it’s operating today, is within the same design space as a conventional engine…But as we go through these technology programs here, we look at better materials, better lubrication systems, it’ll give us that opportunity to make improvements.”
The engineering team continues to run individual systems under a variety of conditions on a series of test rigs; some six of the 15 total were running by mid-June. The ability to validate technology on individual test rigs helps identify multiple characteristics of each part during different phases of their service life, Saia explained. In the case of Pratt & Whitney’s low- and high-pressure compressor rigs, engineers found that the level of performance loss from normal wear measured less than projected. “So what it does is allow us to optimize the design to try to sneak out a little bit more efficiency, which is good for fuel and emissions, and secondly, as we look at how we offer the product, we can offer it with better characteristics,” he said.
Meanwhile, Pratt & Whitney uses PW4000s and V2500s undergoing endurance testing to perform sample selections of, for example, new turbine material. Such “piggybacking,” as Saia called it, saves the time and expense of building and running another dedicated demonstrator. “Our approach is to use rigs and demonstrators like we’re doing now, continuing to use those assets or those vehicles to validate new technologies…it could be a tweak or it could be a new material,” he said.
What new material and for what components would depend on the application because, in the world of turbine engines, size often determines feasibility. For example, while composite fan blades might save quite a bit of weight in a big application such as the GE90, for something as small as the 14,000- to 17,000-pound-thrust MRJ, the benefits might not prove so apparent. Furthermore, a fast-turning, small-diameter fan needs more strength and, therefore, weight to pass airworthiness tests than a slower-turning fan, making composites until now infeasible for regional jet applications, for example.
Now performing demonstrator testing with titanium fan blades, Pratt & Whitney plans to switch to either 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 kind of 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.”
Changes Planned for Next Version
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 the Generation 2 GTF, scheduled for service entry between 2015 and 2017–in time for the advent of Boeing’s and Airbus’ single-aisle replacements.
“On the next generation, we’re hoping we can take a little weight out of the fan drive gear system. How much 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. “But because we’ve had such a successful test, we won’t place that risk on the initial products currently under design.”
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, if the segment that now includes the 737 and A320 continues to account for 85 to 90 percent of all deliveries, one can’t blame Pratt for looking ahead. “Though we’re really pleased at being selected by MRJ and C Series, we really want to make
sure we’re ready for the big fish,” he said. “And we’re working hard to do that.”