Challenging the Pratt & Whitney Canada PT6 engine in the light turboprop market is a tall order, and GE Aviation knows it.
"The PT6 has been a legendary achievement for Pratt. That engine was first introduced I think in 1964. They've delivered 50,000 of them. Virtually all the light turboprops have the PT6. So if you can take on Pratt and the PT6 you're really doing something," says Brad Mottier, GE Aviation's vice-president and general manager of business and general aviation and integrated systems.
But that was precisely the job GE assigned Mottier in 2008, the year his business unit was formed out of a small portfolio of engine derivatives and joint ventures that at the time produced less than $100 million annually. Eight years later, GE plans to unveil a mock-up of what it now calls the Advanced Turboprop (ATP) engine at the Airventure fly-in now under way in Oshkosh, Wisconsin – an event hosted by the Experimental Aircraft Association.
The PT6 has never had the entire market completely to itself. Since the TPE331 was introduced by Cliff Garrett in the early 1960s, more than 13,000 examples have been delivered, with sales continuing under the Honeywell brand. But GE's ATP design reveals arguably the most technologically ambitious development in the small gas turbine market since the arrival of the PT6 itself in the early 1960s.
Mottier's list of specifications include several technologies normally associated with much larger turbofan engines designed for much less rugged applications than commonly associated with many PT6 operators. These include variable stator vanes to redirect the airflow path through the compressor, air-cooled turbine blades to allow the combustor to ignite the fuel at higher temperatures and a full authority digital engine control (FADEC) system to govern both fuel flow and component rotation speeds within the engine, and the speed and pitch of the propellers.
"This engine is unique," Mottier says. "This has more technology for this space than has ever been done before for this type of engine."
It was two years ago that GE revealed its ambition to challenge the PT6 with a new centreline engine design based on high technology, but it was only nine months ago that the gamble appeared to bear fruit.
Since consolidating Beechcraft and Cessna under the same corporate unit, Textron Aviation has been studying how to offer a single-engined turboprop, one of the only gaps in its diverse portfolio of piston- and turboprop-powered products aimed at general and business aviation customers.
Textron Aviation is set to unveil the new single-engined turboprop in Oshkosh, having already selected GE's ATP engine as the powerplant. Against a field of competitors ranging from the Piper M600 to the Daher TBM950 to the Pilatus PC-12NG, Textron will offer the only aircraft not powered by a version of the PT6.
Negotiations between three bidders — GE, Honeywell and P&WC — and Textron Aviation narrowed last fall to marathon sessions in Wichita, Kansas, with GE's ATP emerging as the winner with its first application, Mottier says.
Textron's selection rewarded GE's seven-year pursuit of the general aviation market. It began when GE purchased Czech engine maker Walter in 2008. By relaunching the Walter M601 as the new and improved GE H80 series, GE was able to learn about the needs and dynamics of the general aviation market, Mottier says.
The ATP's advanced technologies figured large in discussions with Textron executives.
The new technologies allow GE to offer more power and fuel efficiency than found on the most advanced versions of the PT6, he says. The five-stage, axial-centrifugal compressor in the ATP yields a 16:1 pressure ratio, giving the engine the most efficient and powerful architecture in its class. The pressure ratio is enabled by each of the new technologies embedded in the ATP, including digital controls to adjust the airflow with variable stator vanes and cooled blades to survive hotter temperatures without melting.
At the same time, all of that new technology begs several questions about whether the general aviation market values such advances, or can even afford it.
The modern PT6 shares only a high-level configuration with the original version that was introduced on the Beechcraft Queen Air in 1964. To fend off rivals and new challenges from small turbofans, P&WC has released a series of technology updates in the PT6 over the years, including uncooled single-crystal turbine blades in the early 1990s. One version of the PT6, which powers the AgustaWestland AW609 tiltrotor, features a compressor with a 13:1 pressure ratio, the highest that can be achieved without switching to cooled blade technologies.
Since 2014, P&WC officials have also committed to introducing FADEC technology in the PT6 in the near future. In other technology areas, P&WC officials have said that some advances are not appropriate for the general aviation market, due to either their acquisition cost or reliability. Although GE believes that cooled turbine blades are ripe for application in general aviation engines, P&WC still has concerns about the manufacturing cost and durability of such components in rugged environments. Higher pressure ratios may yield more fuel-efficient engines, but P&WC has argued that PT6 operators prioritise durability and maintenance bills over fuel costs.
In response, GE officials have to remind questioners that they build more than just large turbofans for airlines. Since the early 1970s, the company has delivered thousands of T700 turboshaft engines for combat helicopters, such as the Boeing AH-64 Apache and Sikorsky UH-60 Black Hawk. Those aircraft operate in even more austere environments than a Beechcraft King Air, Mottier says, adding that GE is leveraging that experience to build general aviation-style ruggedness into the ATP engine.
Mottier is also careful to describe the advanced nature of the technology offered on the ATP engine. GE has been building variable stator vanes in engines since the J79, the turbojet that powered the McDonnell Douglas F-4 Phantom beginning in the 1960s, he says. The exotic and costly materials used in the most advanced commercial engines, including ceramic matrix composites (CMCs), will not be found in the ATP.
"While this is a major step of technology infusion into this small [aircraft] marketplace, it is not all the latest expensive technology that we have like in the Leap and the GE9X," Mottier says.
In his view, cooled turbine blades do not belong to the category of technologies deemed too expensive for the general aviation market. He concedes that a cooled blade is more expensive to manufacture than an uncooled component. Such a blade is manufactured with hollow passages that pipe in cooling air from the compressor, allowing the blade to stay just cool enough to avoid melting against the heat of the exhaust gas flowing out of the combustor.
"We make millions of cooled blades today, and it's quite automated. Does it cost more than a non-cooled blade? Yes. But these are little, tiny blades, so everything kind of scales. I look at the cost, and that's not a problem. Cooled blades are not the problem," Mottier says.
GE also declines to concede a maintenance cost and durability advantage to the simpler technologies offered on the PT6. The FADEC is the key to GE's strategy for keeping maintenance costs down on the ATP. By digitising the engine controls, GE will have access to far more data about the health of each of the engine's components and systems than it would on a manually controlled engine, Mottier says. That information can allow GE to develop a maintenance programme tailored to how each operator uses the ATP engine, rather than simply following the generic procedures found in the manual.
Mottier acknowledges that regulators must first agree to allow condition-based maintenance programmes in general aviation, but he believes that such policy changes are coming.
"We're all at the start of the whole digital-industrial transformation. GE is out ahead of everybody. That will feed right into this marketplace," Mottier says.
GE plans to deliver the first ATP engine to test in 2017 and achieve certification three years later. That qualifies as sprinting in the engine development business, where engines such as the Leap and GE9X require at least seven years to reach certification. The five-year certification campaign for the ATP is partly enabled by an even heavier reliance on additive manufacturing. GE has used so-called 3D printers to make the fuel nozzles in the Leap engine, but the ATP will be significantly based on such additive manufacturing technologies, he says. The details of GE's manufacturing strategy are still being withheld by the company, however.
Source: FlightGlobal.com
