On Time and Nearly On Spec, Pratt & Whitney’s PurePower Program Soldiers On
Delays associated with the Pratt & Whitney PW1217G-powered Mitsubishi Regional Jet (MRJ) haven’t deterred the U.S. engine maker from proceeding with its test program as planned. Meanwhile, the company is also preparing the PW1524G–the engine destined to power the Bombardier CSeries–for an imminent final round of flights aboard one of the company’s Boeing 747SP test beds based in Mirabel, Quebec.
Next, right around the time it expects the PW1524G to gain certification during this year’s fourth quarter, P&W plans to start ground testing the first PW1100G for the A320neo airliner–the third and, judging by early sales performances, most commercially promising of the four aircraft programs on which the company’s “geared turbofan” technology has already won a place. In the process of making parts for the first PW1100G, the company had completed more than 85 percent of that engine’s design by the time it held its annual “Media Day” at its Hartford, Connecticut headquarters in early May.
Having finished its 60-hour flight test program on the fourth PW1217G and in the process of preparing the sixth PW1524G for an imminent final round of flight trials, Pratt & Whitney hasn’t allowed delays associated with the MRJ or skepticism surrounding the commercial viability of the Bombardier CSeries to temper its resolve.
Current schedules show the CSeries engine program running some six months ahead of that for the MRJ, Pratt & Whitney head of development programs Bob Saia told AIN in May. At the time, P&W’s C Series program had produced five production, or test, engines. “I use the word production because if I call them development [engines] people sometimes think they’re just experimental engines, but they’re really product engines,” explained Saia.
The MRJ engine–the fourth of the series of eight test engines for that program–had just begun its flight test regimen on a 747SP specially modified with a “stub wing” attached to the upper part of the test bed’s fuselage. Featuring a 56-inch diameter, the PW1217G generates between 15,000 and 17,000 pounds of thrust–not enough, explained Saia, to safely operate in place of one of the four Pratt & Whitney JT9Ds on the company’s traditionally configured 747SP test bed.
“We’d probably be able to get the airplane home, but it wouldn’t be optimal,” said Saia. “For engines rated below 20,000 pounds of thrust, we needed to put on this stub wing to keep full safety and functionality of the aircraft. Any program from the MRJ and lower in thrust will go on the stub wing.”
Also planning to build eight PW1524G test engines before delivering the first customer example to Bombardier for installation on a C Series prototype, Pratt & Whitney expected to fly the sixth engine of that series “some time in mid-summer.” It plans to use that engine, designated number 806, to validate the final software load for first flight of the C Series, expected late this year or early 2013.
The Weather Factor
Originally planning to fly the PW1524G on its primary 747SP for some 60 hours, Pratt ultimately flew two test engines–numbers 804 and 805–for a total of 245 hours, in a desire to adjust software logic to improve high-altitude, low-speed start times, for example, and some uncooperative weather at the test center in Mirabel, Quebec, outside Montreal.
“In Montreal we’ve suffered quite a bit with a lot of icing and…a lot of unstable air [between 5,000 and 10,000 feet] during the winter months,” said Saia, “so part of the reason we got extra hours is it took us time to find the right quality of condition that we wanted to test in.”
Having accumulated more than 3,000 hours and 9,000 flight cycles combined on the PW1524G and PW1217G, Pratt & Whitney has already applied lessons learned to the design of the Neo’s PW1100G. For example, said Saia, designers have managed to minimize the parasitic fuel burn that can be produced by the application of air pressure to bearing compartments or oil cavities. “We actually tested that on an engine [at Pratt & Whitney’s test facility in West Palm Beach, Florida] recently,” said Saia. “So we did the test, saw the back-to-back improvement in fuel efficiency and we can actually predict what it’s going to be at altitude. When the next engine goes to test it will have that feature and then we’ll measure it.”
Typically, engineers expect an engine program to start with a 2- to 3-percent margin of fuel burn deficiency. As the program progresses, that margin gradually diminishes until, hopefully, the manufacturer approaches its guarantees by the time the product enters service. In the case of the PW1500G, Pratt & Whitney started with between a 1- and 2-percent margin due to all the ground- and flight-testing its GTF demonstrator performed before the first C Series engine went to test.In total, the GTF demonstrator completed more than 400 hours of testing, including 50 hours aboard a Pratt 747SP demonstrator during 12 missions and some 75 hours aboard Airbus’s A340 demonstrator over the course of 27 flights.
“We did a lot of early learning that typically we would do on the first product design,” stressed Saia. “The first engines we’re testing are within about a percent of our specification. We know why they’re there and we’ve got optimization throughout the engine. We’re confident that when the first customer engine is delivered we will be on our specification…Right now, I’m pleased.”
Saia also expressed satisfaction with the progress the company has made toward future fuel burn and other performance improvements. For example, he mentioned tests on a new coating for turbine airfoils that would lessen the cooling flow required while maintaining the part’s lifespan.
The Next Generation
In fact, according to Pratt & Whitney senior vice president for operations and engineering Paul Adams, the company already has begun studying not only the next generation of geared turbofans, but the one following that. “We think the geared turbofan is a step that, with continued improvement, could be the fundamental architecture of the future for the next twenty-plus years,” said Adams.
In terms of future applications beyond the size needed for the A320neo, Pratt & Whitney has studied increasing the fan diameter from 81 inches to as wide as 146 inches–enough to power, most notably, a new version of a Boeing 777. And, it has encountered no technical barriers, said Adams. He did acknowledge certain manufacturing challenges associated with the much higher thrust and bypass ratio that a 777 engine would require, however.
“We’d be breaking scale of items like fan blades larger by far than any we’ve produced before,” he said. “Right now our 777 engine has a 112-inch fan. We’d be looking at diameter ranges between 138 and 146 inches. So the physical size of the parts has some unique manufacturing challenges associated with it.”
Few machining facilities in the U.S. carry the capability to machine an inlet case that would need to span more than 12 feet in diameter while holding the kind of precision tolerances such an application would require. One fundamental challenge associated with the large engine business versus the small engine business, explained Adams, centers on the fact that as the sizes of the parts increase the sizes of the equipment to manufacture them also must increase. “In all honesty, I’m actually more concerned about the manufacturing elements of a large GTF than I am doing a [large] GTF from a technical standpoint,” said Adams.
In terms of materials, increasing the size of the GTF to power a 777-size airplane wouldn’t “fundamentally” require any changes apart from what ordinarily happens during the transition from one generation of engine to the next, he added. “We’re always being pushed to develop new materials in every product,” said Adams. “I don’t expect that to be size specific, but I do think as we go into this next generation of GTF, the expectations are higher…and that would force us to look at the technology, the engine architecture and also the material systems to try to push up the limits.”