By Charlotte Adams, Contributing Editor
Despite the recession's toll on Pratt & Whitney (P&W) and its customers, the company remains bullish on its geared turbofan (GTF) technology and the design's maintenance advantages over conventional turbofan architectures. The company has invested more than $1 billion in engineering and development on the GTF engine over the past two decades. Initial launch customers for the engine family are the Mitsubishi Regional Jet (17,000-pound thrust) and Bombardier's CSeries aircraft, (24,000-pound thrust) both of which are expected to enter service in 2013.
Pratt & Whitney has high targets. The GTF architecture aims to reduce fuel burn at the engine level by 12 to 15 percent, carbon dioxide emissions by about half, NOx emissions by 55 percent, noise by half and overall operating costs to airlines over the life of the powerplant by $1.5 million per aircraft per year vs. comparable engines today. About half of that figure would be fuel savings and about one-third would be maintenance savings, says Bob Saia, vice president of next generation product family.
The GTF concept is simple - a gear system between the fan and the low compressor allows the fan and low turbine to run at optimal speeds. But the complexity of the gear system and the potential impact on maintenance have been a question for some airlines. The PW1000G GTF demonstrator engine looks short compared to the other engines on display at P&W's customer training center. That's because the GTF design eliminated about 1,500 airfoils, or about 30 percent of the airfoils in an engine of this class. Gone are three stages of low turbine and two stages of low compressor, along with the maintenance costs associated with them, explains Paul Adams, senior vice president of engineering. This "huge benefit" in maintenance costs, along with expected fuel, emissions and noise benefits, may be the "difference between an airline making and losing money," Adams says.
The absence of these 1,500 airfoils, most of which would have been in the turbines - "that's the real savings," Saia says. But there are other important points. "There are no life-limited parts in the fan or fan drive gear system," he says. Pratt & Whitney also plans to put borescope capability on all the compressor stages, "so we can do full blade blending of any part forward of the combustor without taking the engine off the wing." Life-limited parts, such as high compressor disks and turbine disks, will be good for 25,000 cycles, he adds.
Pratt & Whitney's geared turbofan engine has 30 percent fewer airfoils than current engines in this class.
Image courtesy of Pratt & Whitney
The GTF gear system, moreover, is simpler than those used by turboprops, the company points out. In a turboprop, the gear system tries to optimize propeller and turbine speeds. The speed ratio of a conventional turboprop gearbox is 10-to-1, which is accomplished in parallel stages. The PW1000G family, on the other hand, envisions a 3-to-1 gear ratio accomplished in a single stage. "The configuration and technology are pretty conventional," Adams notes. The GTF gear system is "a much simpler, much smaller, and much lower gear ratio version."
While the gear system allows the fan to run about one-third slower than it would in a comparable conventional architecture, the low turbine will be running about twice as fast as it would in a conventional engine of comparable thrust, Saia says.
But the turbine will not be running hotter relative to a conventional system, he continues. Temperatures are being driven by how you size the fan relative to the turbine. "We made a conscious decision, when we designed the PW1000G, that we would not elevate temperature. The value of a bigger fan is you actually run the engine cooler than what a conventional engine would." Pratt uses the gear system to "get efficiency with air vs. temperature."
The CSeries engine is designed to have a bypass ratio of 12 but will weigh less than a comparable 9.5 bypass ratio engine, Saia asserts. Based on computer studies, the current GTF engine family will be 500 to 1,000 pounds lighter than a conventional architecture of equivalent performance, Adams adds.
The GTF will also be less vulnerable to bird strike damage, the company says. Because the fan is running slower, less energy would be imparted from a bird strike. And because the amount of air going through the core is reduced, core exposure and the risk of thrust loss are lower.
P&W also has its eye on the airframers' timetables for next-generation narrowbody aircraft. It aims for a follow-on GTF engine in the 30,000-pound thrust class, Saia says. Pratt & Whitney is looking at higher bypass ratio, improvements to the fan, better materials, enhanced aerodynamics and a higher gear ratio - maybe a 4-to-1 ratio.
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