Turboprop power setting.

[Death made simple]

Forgive me for my ignorance, I have a job interview comming up and the sim check is in a Kingair. I have no turbine experience. How do I set engine power settings on a turbine engine?? Please help !

[adverse-bump]

it's easy. Set the power based on torque. All in %. And set the rpm based on, rpm!

That's it, easier than a piston

[dewet]

The Kingair's torque setting is in foot-pounds if I recall correctly. Use that as your 'manifold pressure' if you will. Allow for a short spool up time as well - not much, but there is an appreciable difference.

Also, compared to the pistons that you have been flying (assumption) you will not be used to the relatively huge amounts of power that a turboprop, particularly a 200 has to offer. So watch out for that...Other than that, enjoy!

[SNS3Guppy]

DMS,

With the turboprop, you're introduced to several new engine parameters that you didn't have before...but everything works essentially the same as what you've been flying, if you've been using a constant speed propeller.

The single most important parameter on the turbine engine is temperature. It's described as ITT or EGT in most cases, but may have other names...but your engine temperature is the single most critical instrument for your engine operation.

Your power (in most cases) in a turboprop will be set using torque, and the PT6 is no exception. As another poster noted, it's easy to think of torque as manifold pressure (though they've very different things), because you apply them in a similiar way. As you push the power up to takeoff, the propeller will spin up to whatever speed it's set to hold at takeoff...say for example, 2300 RPM. It will maintain this speed just like the constant speed propeller on a typical piston engine, by increasing the blade angle to absorb more energy as the power is increased. The blade angle takes a bigger bite of the air, meets the air at a greater angle of attack, and puts greater resistance against the propeller shaft. That twisting resistance is torque...the more power you put on that engine and the more power transferred to the propeller shaft against a load...the greater the torque.

The same thing occurs in a piston engine, but unless you've been flying large radial engines you wouldn't have had any cockpit instrumentation to show it...it's indicated as BMEP in bigger engines. Any time you push the engine harder, it's producing more torque, and it's torque that gets the job done. In the turboprop, you simply have a better, more direct indication of what's happening. How it happens, however, is VERY different from the piston engine.

In a typical piston engine, pistons apply a force to a crankshaft to turn the propeller...which is attached to the crank shaft directly, or through a gear arrangement. In the PT6A, the engine exists to do one thing only, and that's blow hot air. The engine does NOT turn the propeller. Hot air does that. The turbine engine produces bleed air for pressurization and anti-ice, and the exhaust gasses from the engine are used to turn a turbine wheel which is attached to the propeller gear box. The best way to visualize it is what might happen if you held onto the propeller during engine start...on a piston engine, you'll probably get hurt, because by engaging the starter, you're moving the propeller. Not so in the PT6A.

The Pratt PT6A is called a free turbine engine, because the engine isn't actually driving the propeller...just exhaust gasses are. If you hold the propeller during the engine start on a PT6, the engine can start and the propeller needn't move at all...because it's not attached to the engine. Of course, eventually enough of a turning, rotational force will be applied to that propeller (torque, remember), that it will probably toss you clear or hurt you...but the point is that unlike the piston engine, the turbine engine propeller action is actually independent of the turbine engine power shaft.

The PT6 has a propeller shaft which is relatively short, and connects the propeller to a gearbox which is turned by a turbine, and it's that turbine that's turned by the gas generator (or turbine engine) part of the entire engine assembly. Because the power section of the engine (gas generator) is separate from the propeller, two different instruments are used to monitor their speed. You'll have a propeller speed instrument for each propeller...it works exactly like any prop RPM gauge you've used before. You'll also have an Ng gauge, which is gas generator speed. This one is marked in percent of RPM...which sounds complicated, but it's not.

The engine itself, the gas generator, is turning at approximately 18,000 RPM, depending of course on your power setting. To simplify this, you'll have an instrument marked in % RPM, which begins with an arbitrary number at 100% (you don't need to know how many RPM that is), and allows you to see the speed of the engine in %Ng, or percent of gas turbine speed. This serves as a type of backup power instrument, but most importantly tells you a little about what the hot part of the engine is doing.

Also important in monitoring the hot parts is the temperature gauge, which works very much like an EGT gauge on a piston engine...except you're not monitoring multiple cylinders.

You may have two limits on your engine when pushing up the power...one will be a torque limit, the other a temperature limit. Sometimes torque will be limiting, sometimes temperature, depending largely on the density altitude where you're operating. You need to watch both closely.

To get the idea of the relationship between torque and propeller RPM, imagine setting your torque by pushing up the power, just as you'd set your manifold pressure. Let's say you set 2000 lbs of torque. Propeller RPM is 2200 with the controls all the way forward, and the propeller is governing on it's low pitch stops for takeoff just like in a piston airplane. If you use the propeller control (which may be combined with other controls, in some airplanes) to retard the propeller RPM, you'll see an increase in torque.

As the propeller blade angle increases during your power increase, it maintains RPM, and a greater twisting force is imposed on the propeller shaft as a result...that's greater torque. You'll also see an increase in exhaust temperature as you push the power up...which of course is really internal engine temperature, too.

You don't have a mixture control in a turbine engine, because it's not needed. In a piston engine, when you add more fuel, the mixture gets richer, the exhaust temperature cools, etc. Not so in the turbine engine. As you add more fuel (by pushing the power up), the temperature just gets hotter, the engine just turns faster, and torque increases even more. The engine is perfectly happy, in fact, to just keep accelerating to destruction if more fuel keeps getting added; that's the nature of turbines. To prevent that, we have engine limits and some internal limiting features...including features in a reasonably sophisticated fuel control and fuel governor that prevent it from getting carried away. You'll learn about those later.

Also a little different is the way power is managed. In a piston engine, one has to be careful about cooling the engine too rapidly or pulling the power back too far during a descent. In the turbine engine, it won't hurt the engine (as a rule) to pull it back; the engine has a fire going on inside all the time, and stays reasonably normalized inside, temperature wise.

You'll find that the turboprop engine is easier to fly than a piston engine. It tends to be more reliable. You have more and better instrumentation to tell you about the health of the engine.

Where you don't have a mixture control, you have a condition lever,sometimes called a fuel lever. On some airplanes it's included with the propeller lever, and on some it's independent. Some airplanes have three levers per engine, some have two...even on the same type engine. Basically the condition lever functions are like mixture functions in that they provide an ability to cut off fuel, and introduce fuel. Other than that, there's usually three other primary positions for the condition lever aside from cutoff. One is the start position, which is where the lever is placed at a certain point in the starting process. Then there are ground and flight positions, usually referred to as the idle positions...ground idle and flight idle. These do nothing more than set the idle speed of the fuel controller, and don't affect the way the engine operates above idle.

When starting a turbine engine, the engine is turned using a starter motor to a particular pre-designated speed. Fuel is then introduced, along with the spark from ignitors. The engine lights off,and begins to increase in speed and temperature. This is closely monitored by the pilot, and cut off if the engine doesn't accelerate quickly enough, or if the temperature accelerates too high or too quickly. It's important that the engine be spinning as fast as possible when fuel is introduced, so it's usually run to "max motoring" speed...as fast as the starter can make it go, before applying fuel (moving the start lever or condition lever to the start position). This is necessary in a turbine engine because the fuel that's being introduced is kept from burning the internal parts of the engine by a wall of air...the engine has to be turning fast enough to produce enough cooling air to protect the engine and for a stable, continuous start to occur. (Most of the air passing through the turbine engine is for cooling, rather than burning).

If you're going for your first turbine job, you won't be expected to be an expert on turbine engines. What you should be concentrating on is basic flying procedure and technique; any particulars of operating the engine, simulator, or airplane will be given you by the operator. Relax, fly like you normally do, and look forward to easier flying than you've done...that's turbines for you.

[Death made simple]

Thanks heaps SNS3Guppy (http://www.pprune.org/members/121895-sns3guppy) and everyone else for the low down. That will help heaps!!

[Maude Charlee]

I think only the very last paragraph of Guppys post is all you need to know, especially if your check happens to be for Europe's largest regional airline. ;)

For your first airline position, it doesn't matter if they stick you in the sim flying Concorde; all they want to see is your basic competency at handling and simple instrument flying, and if you have a sim partner, your CRM skills.

[V1... Ooops]

I'll add a little post-script onto the end of SNS3Guppy's generally excellent post.

PT6A free turbine engines are equipped with two controls for the use of the pilot. The first control, commonly called the power lever, is hooked up to a governor that controls the rotational speed of the aft part of the engine (the compressor, or gas generator). The second control, commonly called the propeller lever, is hooked up to a governor that controls the rotational speed of the forward part of the engine (the power section, to which the propeller is directly connected).

The pilot sets these two governors to achieve the desired engine performance. Higher propeller speeds enable greater horsepower to be made, for this reason, the propeller is always set to the highest speed setting for takeoff and landing.

If you observe the performance of the engine closely as you climb, descend, accelerate, or decelerate, you might find that the torque varies even though you never touch the power lever, but the gas generator speed (the rotational speed of the back part of the engine) never varies. The power lever sets the speed of the gas generator, nothing more.

You cannot determine horsepower being produced by looking at any one instrument. Horsepower is a product of both torque and propeller speed. To calculate it, you multiply torque by propeller speed, then divide the result by a constant (K) which represents the efficiency of that particular engine model.

[SNS3Guppy]

PT6A free turbine engines are equipped with two controls for the use of the pilot. The first control, commonly called the power lever, is hooked up to a governor that controls the rotational speed of the aft part of the engine (the compressor, or gas generator). The second control, commonly called the propeller lever, is hooked up to a governor that controls the rotational speed of the forward part of the engine (the power section, to which the propeller is directly connected).


PT6 installations vary, some with three levers, some with two; on some the condition lever is combined with the propeller control, and on others, it's separate.