PAST & PRESENT
50 years of success for P&WC's PT6 plus a peek into the future
An overwhelming number of turboprop and helicopter OEMs have built winning aircraft using this outstanding powerplant design.
On most smaller turboprop engines, the gearbox that drives the propeller is connected to the main shaft of the engine. This means that, when starting the engine, the battery must supply enough power to turn the compressor, the turbines, the gearbox and the propeller all at the same time.
This requires a lot of power, which in turn means a larger and heavier battery. The added inertia is more likely to cause a slow start, particularly in cold weather, which will raise the internal temperatures and cause increased thermal stresses.
The PT6, on the other hand, has a so-called "free" power turbine that drives only the gearbox and the propeller. This means that the starter only has to drive the relatively light "gas generator" consisting of the compressor and the compressor turbine. This results in faster starts that put less stress on the engine, particularly in cold weather.
There have been several medical evacuations from the South Pole in midwinter, when the temperature can be as low as –90°F. The first 2 were successfully completed by Twin Otters operated by Kenn Borek Air. More recent missions have used Basler DC3 conversions (again operated by Kenn Borek Air). The common factor of all these successful missions has been the PT6 engine, amply proving the cold weather capabilities of the design.
Another feature of the free turbine design is that it's possible to stop the propeller without shutting down the gas generator. This means that a commuter aircraft can stop the prop to board passengers without shutting down the engine, reducing the cycle count and stress on the engine.
On the lighter side, I once spoke to a pilot from northern Canada who had upgraded to a PT6-powered aircraft. His hobby was ridge soaring, and he enjoyed the free turbine design because he could feather the props for soaring and still have cabin heat from the gas generators.
In addition to the reduced weight and improved turbine efficiencies, the reverse flow arrangement of the PT6 has several practical advantages. The main one is that the power section can be removed from an engine without actually removing the engine from the aircraft.
The power section "unplugs," leaving the gas generator and all its accessories (starter/generator, hydraulic pump, fuel control, etc) on the wing. This greatly simplifies engine maintenance, hot section inspections, gearbox overhauls (in the event of a prop strike), etc.
The only part of the PT6 where the airflow direction is "normal," ie, front-to-back, is the combustor. Folding the combustor reduces the overall length of the engine. One unplanned benefit is that, as the hot gas leaves the combustor and is reversed in the turbine entry duct, the centripetal forces stratify the temperature distribution so that the temperature varies from cooler at the blade root to hotter at the blade tip—exactly what is wanted for both maximum efficiency and turbine disk longevity.
PT6 future evolution
Inertial particle separator (IPS) system. When activated by the pilot, a small door forward of the engine intake closes and another further aft opens (indicated by red circles). This forces the intake air around a sharp bend and debris carries on out the back of the intake duct.
Millar left Pratt & Whitney in 1963 to become a professor at Carleton University in Ottawa until his retirement. (I had the good fortune to take a number of Dr Millar's turbo-machinery courses as part of my engineering degree.)
To understand the evolution of the PT6 from that 1st version (in 1 or 2 aircraft) to the extensive family of engines that it is today, I spoke to Pratt & Whitney Canada VP General Aviation Denis Parisien.
The 1st PT6 was flat rated to 500 shp because of concerns about the ability of the gearbox to handle more power and the temperature limits imposed by the materials of the day.
Since then, tremendous advances have been made in blade materials, permitting much higher temperatures. New machining methods have allowed the development of blades with internal cooling (allowing still higher temperatures) and new gear technology has allowed higher power gearboxes.
All of this has combined to allow creation of a PT6 that can produce nearly 2000 shp, all within the same engine diameter—a remarkable feat.
These higher temperatures and improved manufacturing technologies have also improved the specific fuel consumption (SFC)—which is fuel flow divided by power—by up to 20% and increased the power-to-weight ratio by 40%.
Parisien highlighted the impressive reliability record of the PT6. In my first job, as a test engineer at Pratt & Whitney, I saw at first hand the extensive testing of proposed enhancements.
I recall one test that involved alternating periods of ground idle followed by slam accelerations to full power. This is very hard on an engine. We were doing this test in summer, so we were mystified when, immediately following one slam accel, the air of the test cell filled with what looked like snow.