Rating a Turboprop’s Power

To understand what's going on with the performance improvements from the more powerful Pratt & Whitney PT6 engines involved, you need to know that there are two fundamental measures of power. The most basic measure of power—and the one listed in the airplane specifications—is the maximum shaft horsepower (shp) of the engine. The other element in the power equation is how much power the engine can potentially produce at sea level on a standard 15 degree Celsius day, which are the international standard atmosphere (ISA) conditions.

The TBM 850, for example, has a limit of 850 shp. That means the airplane is approved for 850 shp to be delivered to the propeller. A shaft horsepower is essentially the same as horsepower developed by a piston engine, or an electric motor, for that matter. Horsepower is a measure of power, or torque, over a unit of time. We could accurately call the power delivered by an aircraft piston engine shp because the power is being delivered to the shaft that drives the prop. But because there are other measures of power output for a turbine engine, we specify for turboprop engines that shp is power delivered to the prop.

The shp of a turboprop engine is restricted by the strength of the gearbox that drives the propeller, and by the ability of the airframe and other components to handle the thrust developed by the prop. So the maximum amount of power—thrust, actually—that a turboprop engine is approved to produce at any time on a specific airplane is stated in shp and is a certified limitation.

Okay, that's the same as in a piston-powered airplane where engine power is a certified limit. But in the case of turboprop engines the actual turbine engine can produce more power than the maximum certified under many atmospheric conditions. And thus the confusion.

The power output of a turbine engine, jet, or turboprop is limited by internal temperature, pressure, and the rpm of its rotating components. If the temperature is too hot the crucial engine parts will break, or melt. If the pressure is too great the parts can break, or the entire engine case can even fail. And if the rotating components spin too fast they will at some point fly apart with explosive force.

As pilots we monitor these parameters to operate a turbine engine. The temperatures inside a turboprop vary from one section of the engine to another, but in the PT6 we monitor, and limit, the interstage turbine temperature (ITT). The rpm is also monitored, but instead of a gross number of revolutions—which is typically more than 30,000—we see a percentage of allowable rpm. There is no direct measure of engine pressure on a PT6 as there is on many large jet engines that use engine pressure ratio (EPR) as a measure of power output, but if the rpm and ITT are within limits the internal pressure of the PT6 will be, too.

There is another turboprop value—torque—that is also measured and reported to the pilot, and that is really just another way of measuring shp. The engine actually twists against the resistance of the propeller and the twisting force is measured and shown as a torque value. Torque is the limit of power the airplane can actually use, while temperature and rpm are limits on how much more or less power is available from the engine.

The PT6 is a free turbine engine, meaning the components of the turbine engine that actually generate the power are not physically linked to the propeller. The part of the engine that burns the fuel and makes the energy is called the gas generator, and the section that transforms that energy into shp is called the power section.

Air in the PT6 flows from rear to front. A compressor section in the aft part of the engine draws in air and compresses it through several stages. The hot compressed air enters the burner section where fuel is injected and ignited. The rapid expansion of the burning fuel-air mixture generates a powerful gas that forces its way forward over a turbine wheel. The turbine is connected directly to the compressor wheels to spin them and thus sustain the process. This rotating section is called N1.

As the expanding gases continue their rush forward toward the exhaust they force their way past another turbine, and this one is connected to the gearbox that turns the propeller. The gearbox is both complex and sturdy because it must reduce the many thousands of revolutions of the power turbine down to the 1,500 to 2,500 rpm that a propeller can effectively use. The rpm of this section is called N2 or prop rpm.

The power potential of the gas-producing section of the engine is totally dependent on the density of the air it is operating in. When air is dense—on a cool day at sea level, for example—the turbine section loafs along. The compressor has plenty of air to work with, so it feeds the burner section its maximum charge of air using only low rpm and relatively low compression ratios. But when the air is less dense, at high altitude, or when air temperature is above ISA, the compressor struggles to ram the same air charge into the burner. The air is hotter exiting the compressor and burns hotter. The compressor must spin faster to do its work. And at some point the density of the air available to the compressor just isn't enough for it to deliver the full charge of air into the burner before reaching the rpm limits, or the temperature limits, or both.

When the engine reaches its limits of temperature or rpm it is at its thermodynamic limit. Thermo, obviously, being temperature, while dynamic refers to the rotating speed of the components. That's why you'll see that a PT6 will have a limit of, say, 850 shp in the TBM, but have the thermodynamic rating of about twice that. The difference between the low and high power ratings is called flat rating, or de-rating. I like the term flat rating best because it accurately describes what is happening. The airplane and engine gearbox can only take so much shp, so the engine is capped at that value. Its power is held flat.

But the magic of flat rating is that you can use the extra thermodynamic power to increase climb and cruise speed. As the airplane climbs into less dense air there is plenty of margin in the compressor section to keep packing a full charge of air into the burner before rpm and temperature limits are reached. Just as a turbocharged piston engine continues to make full power as it climbs, the flat-rated PT6 delivers full-rated power at altitude by having the margin to increase rpm and ITT. The result is higher climb rates and true airspeed.

It wasn't always this way with the PT6. An early version of the engine in the Beechcraft King Air 90, for example, couldn't make full-rated power on the runway if the air temperature was hot, or the airport elevation high. Gradually Pratt & Whitney improved the design and materials of the engine to make it ever more powerful, even though certified shp remained the same. And over the past several years, versions of the PT6 are almost twice as powerful even though the external size and shape is about the same.

This available increase in thermodynamic power is what makes the engine conversions of existing airplanes so attractive. The new engines fit right in the space of the originals, are limited to the same maximum power to the propeller, but produce that power to a much higher altitude or air temperature. The results are many, many knots of increased cruise speed, much higher climb rate, and often a fuel flow increase that essentially matches the speed increase so range remains about the same. It is not a free lunch because the new engines are more expensive, but it is as close to a free speed increase as there is in aviation.

The reason newer PT6 engines can produce more power is better materials to withstand higher temperatures and pressures, and much improved aerodynamics that make the compressor and turbine more efficient. The same improvements have taken place in all turbine engines, but it's so remarkable in the PT6 because the engine has been used on the same airframes for more than 40 years.

I hope this explains flat rating, shp, thermodynamic power, and why turboprop airplanes continue to gain in climb and cruise speed. Flat rating puts power in the bank that you can draw on when conditions are less favorable. I think that's something we can all appreciate these days.