P&W freezes design of geared turbofan engines

 - June 5, 2009, 6:55 AM

Pratt & Whitney has (Hall 5 B20-B30) frozen the design configuration of the PW1000G geared turbofans for both the Mitsubishi Regional Jet and Bombardier C Series after finishing the final phase of demonstrator testing this past spring on an Airbus A340-600 testbed in and around Toulouse, France. Designated as the PW1217G for the MRJ and PW1524 for the C Series, the pair of engines will enter detailed design soon after the start of this week’s Paris Air Show, 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, according to the company.

“Initially we wanted to have a three-month difference between the two programs,” Saia explained. “We had 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.”

According to Saia, 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 the C Series’ engine could actually begin ground testing before the MRJ’s, some time around the middle of next year.

The company is now virtually finished with the program’s preliminary design phase and it expects both versions of the engine to enter detailed design imminently, when the design team produces electronic blueprints and begins working with suppliers and partners to produce actual parts.

“We’ve got a design that’s basically done,” Saia told AIN. “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, life, fuel efficiency. And we’re 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 trials 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 2gs,” 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 expected to finish this month.

From a performance perspective, the A340 testing revealed no surprises whatsoever, said Saia, as all the performance parameters measured have equaled those from the earlier tests on the 747.

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.”

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 a more effective cooling without using more air, he said.

Just ahead of the 2009 Paris show, Pratt had almost reached a firm conclusion about what material it planned to use for the fan blades after conducting projectile testing on a proprietary hollow metallic design and a composite construction at its facility in Connecticut.

“It’s a pretty close horse race,” Saia said. “They both have features of merit…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 performed ballistic testing on each in the months leading up to the show.