How turboprop power lives up to its potential
"There's no substitute for power," goes an old aviation saying. The implication, of course, is that the more power, the better. High power-to- weight ratios mean shorter takeoff distances, heftier climb rates, and satisfying acceleration. At altitude, more power equals more airspeed, higher groundspeeds, and higher ceilings. Given all these advantages, it might be difficult to understand why manufacturers would want to limit an airplane engine's power. But with most turbine engines, that's the rule, not the exception. This limitation has come to be known as flat rating.
Why on earth would you ever want to limit an engine's power output? There are several very good reasons — especially in the case of turbine engines. Add up these reasons, and it becomes simple to realize why flat rating is commonplace.
The strength and design of airframes and engine mounts can dictate a cap on power. When manufacturers design an airframe, they typically begin by defining a target mission and performance. To meet these goals, a certain amount of horsepower (and the fuel capacity to make it last the duration of the mission) will be necessary. Any more power and the airframe and other components wouldn't be able to handle the extra stresses caused by higher airspeeds or higher twisting forces (torque, in the case of turboprops and turboshafts). Weight savings are another concern, particularly in the case of turboprops. A power limitation means that propeller gearboxes can be built lighter, because they won't have to endure the mammoth forces that highly torqued propeller shafts and reduction gears would generate. Also, more power means higher fuel consumption, which means more fuel capacity is needed — which means more weight.
Flat rating can also make it easier on the engine. Generally speaking, lower power ratings translate into lower temperatures in a turbine engine's hot section and potentially longer times between hot section inspection intervals. What's more, cooler hot section and turbine temperatures allow higher permissible torque values. That's because there's less of a chance of "overtemping" during a hot-day, high-altitude takeoff, when you might need plenty of power/torque.
The effect of very cold temperatures on turbine engines is another very important reason for flat rating. Turbine engines, much more so than piston engines, are extremely sensitive to ambient temperature. Without a means of limiting power, a turbine engine at takeoff power in sub-zero conditions could easily produce several times as much power as it would at high temperatures — high enough to completely destroy it. Pratt & Whitney says that its PT6s can produce more than twice as much power at -65 degrees Fahrenheit as at 130 degrees. A technician who performed cold- weather tests on a PT6A-28 engine — the 620-shaft horsepower, flat-rated version used in the Piper Cheyenne II — found that at -5 degrees F, the engine developed some 945 shp when pushed to its interturbine temperature (ITT) redline. Sound good? Think again. That kind of power just doesn't belong on a Cheyenne II.
Fine, you say, but how do manufacturers keep from going overboard with power limitations? In other words, how can designers guarantee that an engine will produce enough power over a wide enough range of ambient conditions and, at the same time, avoid the drawbacks we just talked about? The aircraft has to perform the way the marketing department says, but it can't destroy itself in the process, or the people in the company's service organization will have a fit — not to mention hapless pilots and passengers.
The answer: Install an engine with far more power than really needed, then limit its output. This is another aspect of flat rating, and the beauty here is that a powerful, but flat-rated, engine will produce all of its advertised horsepower when it's most needed (at takeoff) and throughout a very wide range elsewhere (at altitude) in the operating envelope. To put it another way, you could say that a flat-rated engine is loafing, or derated — never working harder than necessary at any time and certainly not living up to its total potential power rating. Flat rating is accomplished automatically, by the way, through governing devices in fuel control units and torque limiting devices — that, and published operating limitations the pilot must follow.
Take the Aerospatiale TBM 700, for example. Its Pratt & Whitney PT6A-64 engine is capable of putting out a whopping 1,570 shp, but TBM flat-rated it to a "mere" 700 shp. Even at 100 percent torque, the engine is shirking to the point of 870 shp. It's this kind of power margin that lets the airplane cruise at 300 KTAS and 30,000 feet with little chance of exceeding the engine's ITT limit. Little chance, that is, unless ambient temperatures are well above standard (more on this later).
The Garrett TPE331 series of turboprop engines are similarly flat rated. Those used in the Twin Commander 690 series are 840 shp, but they're flat rated to 717.5 shp. The same basic engine is used in the Cessna Conquest II, but in this application, the engines are flat rated even lower, to 700 shp. Again, the idea is to limit horsepower for takeoff so that you have more in reserve for performance at altitude.
For another example, consider the Beech King Air F90 and the Cessna Caravan I. Both use the same basic PT6A engine, but the heavier King Air has its engines rated at 750 shp. The Caravan, a single, has its engine flat rated to 600 shp — to accommodate an airframe designed to fly slower and to limit torque effects.
There's another very good reason for flat-rating the Caravan I to 600 shp, and it has to do with takeoff performance on hot days. One often hears flat rating spoken of in terms of temperature, and in the Caravan's case, its engine is said to be flat rated to an ambient air temperature of 136 degrees F.
What does flat rating to a temperature mean? In this case, it means that the Caravan's PT6, at sea-level elevations and standard pressure (flat-rating temperatures are published for sea level, standard- pressure conditions), can develop its 600 shp even when temperatures go as high as 136 degrees. As we all know, power goes down when outside air temperatures and density altitudes go up. In turbine engines, this shows up in the form of high internal temperatures. When engine inlet air temperatures climb, cooling of internal components is impaired.
These internal temperatures can be high enough to dictate reductions in torque, in which case the engine is temperature limited. Now the pilot has to take off with something less than the airplane's full, published torque limits. To keep engines from going beyond temperature redlines, the pilot can't push the power lever(s) up to their usual setting(s) — at or slightly below the torquemeter redline — and has to accept a performance penalty for the sake of sparing the engine's hot section from heat-related damage. A longer runway will be needed, and the airplane won't be able to climb as advertised.
With a flat-rated engine, our Caravan pilot knows that he or she can get all 600 shp at sea level; i.e., go all the way up to torque redline with the power, as long as the temperature stays at or below 136 degrees. At higher density altitudes, more torque is available than would ordinarily be the case without flat rating. This gives the airplane great flexibility when operating in hot and high conditions, which is what you need in a utility hauler like the Caravan. It's a very rare day when the mercury hits 136, so for all practical purposes, the Caravan driver will never have to worry too much about runway requirements or reduced-power takeoffs when flying out of airports with relatively low elevations.
The King Air F90 has the same basic engine as the Caravan's, but in the F90, the engine is flat rated to 750 shp at 93 degrees — again, at sea-level, standard-pressure conditions. Producing those additional 150 hp over the Caravan will create additional internal engine heat, so the ambient flat-rated temperature limitation is 43 degrees lower.
Now come the compromises. Yes, the F90 driver has more potential power, but when the temperature climbs above 93 degrees at lower airport elevations — a fairly common occurrence in the summertime in the United States — takeoffs will have to be at reduced power, especially at airports with high elevations. This can present big problems when the pilot faces things like a large passenger load, the need to carry enough fuel for a long leg, a runway less than about 4,000 feet, obstructions in the departure path, and a high airport elevation. No one wants to be left behind, and no pilot likes to defuel, but these may be the only alternatives when high density altitudes prevail.
These temperature-limitation aspects show how the margin between an engine's potential power and its flat-rated power can translate not just into achieving design and performance objectives, but into operating limitations as well.
Is flat rating the same as derating? Not quite, although the two go hand in hand. All turbine engines have a flat-rating temperature. But when derating an engine's power value, the flat-rating temperature rises. That's why the 600-shp Caravan engine's flat rating is up at 136 degrees, while the 750-shp F90 engine is 93 degrees.
Are piston engines flat rated? Turbocharged variants can be thought of as flat rated, because they'll keep producing their rated power over a higher range of nonstandard temperatures, elevations, and altitudes than normally aspirated engines. But at its critical altitude, the turbocharged engine will have a fully closed wastegate. Climb any higher, and manifold pressure will drop off. Then the turbocharged engine begins to behave like a normally aspirated engine, losing power as it climbs. So you could say that turbocharged engines are flat rated to their critical altitudes.
Though some normally aspirated piston engines are flat rated by manifold pressure and other limitations, most normally aspirated piston engines don't fit in very well with a flat-rating analogy. They begin losing their rated power the moment conditions vary from standard and as soon as they climb. The only example that readily comes to mind is the Piper Comanche 400. You could say that this airplane's 400-hp, eight- cylinder Lycoming IO-720 is quasi-flat rated, in that it has gobs of power. It's got plenty for takeoff in all but the worst density altitudes. At altitude, so what if it's lost half its power? It still has 200 hp in reserve.
No, there's no substitute for power. Turbine-engine flat rating proves the saying in spades, even if it does involve power and temperature limits. Without a ton of excess horsepower to begin with, flat ratings would be so low that they'd offer little or no performance advantages. It's the power you can't use that makes all the difference.
