Pratt readies GTF for MRJ, C Series
Pratt & Whitney has frozen the design configuration of the PW-1000G Geared Turbofans for both the Mitsubishi Regional Jet and Bombardier C Series after finishing the final phase of demonstrator testing on an Airbus A340-600 testbed in and around Toulouse, France. Designated as the PW-1217G for the MRJ and PW1524G for the C Series, the pair of engines will enter detailed design by late June or early July, in time for first engine runs by the middle of next year, according to Pratt & Whitney next generation product family vice president Bob Saia.
Originally expected to run the first MRJ engine by the end of this year, Pratt & Whitney has juggled the schedules to take advantage of the efficiencies inherent in running two programs in parallel, said Saia.
“Initially we wanted to have a three-month difference between the two programs,” he said. “We had the MRJ first, then the C Series following. What we’re finding now is that because the two designs are so common…we have the ability to cross-leverage. Right now, depending on what we do, we’re about one to two months [apart]. And instead of it always being the MRJ first, sometimes we do [parts of the] C Series first.”
Saia explained that when the larger version of a particular design element proves more “restrictive” than its smaller counterpart, the company will build the larger version first then scale it down for the smaller engine. The same principle applies to situations where a limitation resides with the smaller version; in that case, the larger version will follow the smaller one.
As a result, parts of the C Series engine have progressed faster than their corresponding items in the MRJ engine, even though schedules still call for first flight of the Mitsubishi jet first, at the end of 2011, while the C Series follows in the first quarter of 2012. In fact, Saia said he believes the C Series’ engine might actually begin ground testing before the MRJ’s, some time around the middle of next year.
Now in the midst of the program’s preliminary design phase, the company expects both versions of the engine to enter the detailed design phase by mid-year, when the design team produces electronic blueprints and begins working with suppliers and partners to produce parts.
“We’ve got a design that’s basically done,” said Saia. “The architecture is complete. We’ve taken the learning from the ground and flight-test programs and we’ve now optimized sizing… So we’ve got both configurations assigned, we’re in the final stages of optimizing for things like weight, lift, fuel efficiency and doing the tweaks in terms of things like how much cooling flow you use for a turbine airfoil based on [a given] temperature or pressure profile or how thick does the case get because you’ve got a certain speed at which you want to be able to hold containment.”
Testing of the GTF demonstrator, which used a PW6000 core, included 300 hours of ground testing and 12 flights on Pratt & Whitney’s Boeing 747SP flying testbed, followed by 75 hours of trials during 27 flights on the A340 out of Toulouse. All told, the engine ran for 406 hours, including 120 hours in flight–an extraordinary feat, according to Saia, given that such one-of-a-kind demonstrators typically run for no more than 100 hours.
Although the A340 carried only about half as much test instrumentation as Pratt’s 747, a separate test program under way at Airbus included nearly 60 high-angle-of-attack maneuvers onto which the GTF could “piggy back,” said Saia. “We were putting the engines under loads higher than 2 gs,” he noted. “We had simulated this in our laboratory, and didn’t expect to see any engine characteristic that would be limiting at all, but the engine operated flawlessly.”
A much newer airplane than Pratt’s 747, the A340 also offered the benefit of quieter engines, allowing for more precise noise measurements, assessment of which the company expects to finish in June. “That’s probably going to be three months away because there’s a lot of computational analysis that needs to be done by Airbus and our acoustics team,” explained Saia.
From a performance perspective, the A340 testing revealed no surprises whatsoever, said Saia, as all the performance parameters measured so far have equaled those from the earlier tests on the 747.
All told, the performance of the fan, of the low-pressure compressor and the fan-drive gear system met or slightly exceeded Pratt’s commitments for the MRJ and C Series, according to Saia. Cold-soak testing at altitude also confirmed that the GTF showed no less ability to perform a quick windmill start than a conventional engine. In fact, he said, it usually started a second or two faster than the average 60- to 65-second start times of a conventional engine.
Meanwhile, tests on the thermal management system–largely a measurement of the difference between the temperature of the oil going into the fan drive gear system and the oil coming out–showed that gearbox heat rejection proved 6 percent less than engineers predicted, allowing designers to reduce the size of the heat exchanger. “Gear efficiency stayed stable from first flight to the 39th flight,” said Saia. “We found no degradation at all.”
When Saia spoke with AIN in early March, he said he expected his engineers to have completely disassembled the gear system by the end of the month for a close inspection of any wear not implied in the temperature calculations.
Using an all new “centerline” core, the PW1000G’s new compressor has undergone both new-condition testing and end-of-service clearance measurements, said Saia. Tests conducted on the new combustor installed on a PW6000 found that it yielded
a 50-percent reduction in NOx, he added. The new turbine uses a completely new aerodynamic geometry to add efficiency and minimize airfoil count. Finally, a new internal cooling scheme has yielded more effective cooling without using more air, said Saia.
Still undetermined remains what material Pratt will ultimately use for the fan blades, as it continues projectile testing on a proprietary hollow metallic design and a composite construction in Connecticut. Saia said a decision should come by the middle of the year.
“It’s a pretty close horserace right now,” he said. “They both have features of merit, but we’ve got to get through these laboratory tests to really qualify the material…Whichever one we select we’ll probably still keep the other one working just because there might be an application on either a smaller engine or a bigger engine, where having a material substitution will help us for another model.”
The company also continues to evaluate the respective merits of two different composite chemistries for the fan containment case, and plans to perform ballistic testing on each during the second quarter.