August 2012

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XSTOL and the Pratt & Whitney Canada PT6 engine

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Ahead of the 2011 National Business Aviation Association conference in Las Vegas, which begins on Sunday, Pratt & Whitney Canada invited me to take a closer look at the Pacific Aerospace P-750 XSTOL and the engine that powers it, the PT6A.

The XSTOL, which stands for extreme short take off and landing, is a designation given by the New Zealand-based airframer when an aircraft can take-off with a payload in excess of its empty weight in under 800ft. At its core, the P-750's missions include a 4,000lb payload on short field takeoff capability on semi-prepared runways, aerial survey, crop dusting, fire fighting or carrying up to 17 skydivers to dump at altitude. We'll talk about that last mission a bit more later on.

That performance is due, in part, to the Cresco's thick wing and a span of 42ft (12.8m), 305sq ft (28.34 sq m) wing area and pronounced outboard dihedral. The design of the P-750 was derived by combining the engine and wings of the older Pacific Aerospace Cresco crop duster with a new large fuselage design and modified empennage. The type made its first flight in 2001 and was certified in 2004.

The other part is the 750hp (560kW) PT6A-34 engine selected by Pacific to power the P-750, which is significant for an aircraft whose empty weight is only 3,100lbs. 

Pratt & Whitney Canada started mass production of the PT6 in 1963, and is arguably aviation's most resilient turbine engine design, evolving incrementally over time with each new application. The P-750 is one of 130 different aircraft that have been powered by the PT6.

The engine's first application, the Beechcraft Model 18, first flew in May 1961 and was intended as a replacement of the P&W Wasp radial engine. To date, PWC has delivered 46,000 PT6 engines, 26,000 of which are still flying today having accumulated 350 million hours in service.

With its 1961 first flight, the PT6 architecture is only eight years shy of having as many years removed from its first flight, as its own first flight was from Orville & Wilbur at Kitty Hawk.

PT6 Modules.jpg
The engine, which can provide power from 500shp up to 2,000shp - depending on the application - and is principally made up of three major components, a gas generator comprising the compressor and combustor, the power section made up of the turbine which is spun by the combusted fuel and air, and a gearbox that connects directly to a propeller or rotor.

Those three elements have been scaled and customized over the years to create a modular system that can be tailored depending on the application.

"We can mix and match these to get better thermal ratings, better flat ratings, make the PT6 a sort of tailored engine to the aircraft these people are trying to make," said John Saabas, PWC President in a Friday interview. "When you're a small aircraft maker we can give you choice. We can give you a power range, thermal range, mechanical range.

"Socatas need lots of thermal power because they want to go fast at altitude, not so much mechanical power. King Airs want to take off with heavy loads and don't go quite as fast need more mechanical power. So a different gear box for those two even those it may be the same basic engine behind it. It just depends on how its rated."

"From a first-cost perspective, we also bring an advantage over some other technologies, some of the other choices that are out there," he says, citing higher SFC but a lower acquisition cost of the PT6 than the GE T700 that powers civilian and military rotorcraft. 

For Saabas the extended investment over decades and pressure to deliver another another major leap in fuel burn improvement is tempered by the cost competitiveness and market establishment of the existing engine. 

To look at the engine through Red-Blue, London School of Economics academic Dr. Theodore Piepenbrock's work on business evolution, the PT6 serves as a prime example of incremental, scalable development that has benefitted from continuous product evolution to drive out cost and improve efficiency and fuel performance over its extended lifetime.

Though even with its modular design, Saabas cautions that the PT6 is not focused as an off-the-shelf engine as it appears: "You don't want to get to a point where you're a commodity, we're always trying to decomoditize ourselves."

Though he acknowledges, the mix-and-match capability of the PT6 allows airframers small and large flexibility, but the scale of the complexity is the differentiator from project to project.

"There are some applications that some people want to take a [PT6A-42] and they want to adapt it, but we're developing for the Eurocopter, right now, the [PT6C-67E] which is a brand new version of the PT6 for the EC175, we're putting a FADEC on to that."

Overall, Saabas attributes the success of the platform to its overall reliability, achieving an in-flight shutdown rate of two per every million flight hours. Though, he also acknowledges market pressures by relative new-comers in the market, General Electric and its M601 and H-80 engine, which grew out of its 2008 Walter Aircraft Engines acquisition.

The engine, says PWC, has improved in line with the industry standard, delivering an extra percent of specific fuel consumption improvement every year, translating into a 20% improvement over the last two decades.

In a 2010 interview, Saabas said its plans for the introduction of new technologies into the PT6 remained a higher priority that its plans for a replacement, calling it an "advanced design study", though he says in Friday's interview that the PT6 replacement will evolve from the PT6 itself: "We're doing a couple of neat technologies to see if we can't improve the efficiency of the PT6 by double digits, and some of them use PT6 architectures, some don't. So let's call them a general aviation engine replacement if you will."

Yet Saabas, who has been PWC's president since 2009, says the company has stuck with the PT6 for as long as it has because there have been "a lot of generations of leaders at Pratt Canada who have been customer focused and out there trying to adapt the product to different marketplaces and aggressively looking for these applications for the engine and spending the money to develop it with the confidence that there was a marketplace."

Video Credit Pacific Aerospace
Cutaway Credit Pratt & Whitney Canada

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