Torq and ITT
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sharpless281
- Posts: 123
- Joined: Tue Feb 16, 2021 1:21 am
Torq and ITT
Could someone link me to a document or explain how the torq can be set at 90 when the ITT is still in the green at lower altitudes and as I get up to like 25000 feet the torq needs to be lower like around 65 because the ITT will go into the red. Sorry if I am not being clear.
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X24
- Posts: 6
- Joined: Thu Jan 29, 2015 2:08 am
Re: Torq and ITT
Hi.
Turboprops work like any other air-breathing engine, and Milviz have done a good job of modelling a turboprop engine that behaves realistically, unlike the classic default turboprop modelling that exists in the FSX, Prepar3D and MSFS platforms. In the default turbine model the engine characteristics work back to front so if that is what you are used to, this one will seem "wrong".
The basic explanation is that as the aircraft climbs and the air becomes less dense, the engine produces less power, so the throttle must be advanced, which then causes the ITT to rise.
The gas generator in these PT6 engines produces most of it's real power over a relatively small speed band way up in the 90-something to 100% range, in other words it's generally going flat out or close to it. Top speed is something like 36,000 RPM depending on the model. When we move the power levers, we think about wanting more or less power, hence the name. As we climb, we see the torque reducing, so it makes sense then to increase the thrust lever angle a little, which results in more fuel going to the gas generator, which must also cause the exhaust gas temperature to rise. The increased energy in the exhaust is then absorbed by the power turbine which transmits this through the gearbox to the propellor which enables the propellor to do more work, i.e it produces an increase in power.
At sea level in denser air the engine is generally capable of producing 100% torque without the gas generator having to work at it's absolute limit, so the torque will be the limiting factor. They are designed that way. As the power available tends to reduce with increasing altitude, we are demanding more and more from the gas generator to try and compensate until it does reach its thermodynamic limit which in this case is 820° C.
The net result is that as you climb you will see the ITT gradually increase as you try to maintain 100% Tq, until an altitude is reached where ITT is 820°, and all you can do now is maintain the ITT at its maximum and just accept whatever torque you can get from the engine.
You may well ask, "But if the engine is producing less power, why doesn't the ITT go down?" Horsepower is also required to drive the gas generator compressor section, and as we've seen, there is less of this to go around as we climb. Also, the turbine section is cooled a little by unburnt fuel and air, also a design feature. There is less of this too as we climb, so somewhere in that extraordinary ballet of aerodynamics, thermodynamics, chemical engineering, mechanics and metallurgy a delicate balance is reached where the core of that little engine is screaming as loud as it is allowed to in order to get the job done. Bottom line is that it is not the temperature of the exhaust that is doing all the work, it is the energy of the gas going through the power turbine, and that is what the gas generator has to work harder and harder to achieve as the aircraft climbs.
Somewhere in the literature I read that the limiting ITT is 820°C, however at least one pilot has described an SOP where they set 785°and maintain that until reaching cruise altitude. This will be kinder to the engines and use a bit less fuel too. I believe certain models of Learjet are flown around at max ITT - that is how you set thrust in them.
I hope this helps.
Turboprops work like any other air-breathing engine, and Milviz have done a good job of modelling a turboprop engine that behaves realistically, unlike the classic default turboprop modelling that exists in the FSX, Prepar3D and MSFS platforms. In the default turbine model the engine characteristics work back to front so if that is what you are used to, this one will seem "wrong".
The basic explanation is that as the aircraft climbs and the air becomes less dense, the engine produces less power, so the throttle must be advanced, which then causes the ITT to rise.
The gas generator in these PT6 engines produces most of it's real power over a relatively small speed band way up in the 90-something to 100% range, in other words it's generally going flat out or close to it. Top speed is something like 36,000 RPM depending on the model. When we move the power levers, we think about wanting more or less power, hence the name. As we climb, we see the torque reducing, so it makes sense then to increase the thrust lever angle a little, which results in more fuel going to the gas generator, which must also cause the exhaust gas temperature to rise. The increased energy in the exhaust is then absorbed by the power turbine which transmits this through the gearbox to the propellor which enables the propellor to do more work, i.e it produces an increase in power.
At sea level in denser air the engine is generally capable of producing 100% torque without the gas generator having to work at it's absolute limit, so the torque will be the limiting factor. They are designed that way. As the power available tends to reduce with increasing altitude, we are demanding more and more from the gas generator to try and compensate until it does reach its thermodynamic limit which in this case is 820° C.
The net result is that as you climb you will see the ITT gradually increase as you try to maintain 100% Tq, until an altitude is reached where ITT is 820°, and all you can do now is maintain the ITT at its maximum and just accept whatever torque you can get from the engine.
You may well ask, "But if the engine is producing less power, why doesn't the ITT go down?" Horsepower is also required to drive the gas generator compressor section, and as we've seen, there is less of this to go around as we climb. Also, the turbine section is cooled a little by unburnt fuel and air, also a design feature. There is less of this too as we climb, so somewhere in that extraordinary ballet of aerodynamics, thermodynamics, chemical engineering, mechanics and metallurgy a delicate balance is reached where the core of that little engine is screaming as loud as it is allowed to in order to get the job done. Bottom line is that it is not the temperature of the exhaust that is doing all the work, it is the energy of the gas going through the power turbine, and that is what the gas generator has to work harder and harder to achieve as the aircraft climbs.
Somewhere in the literature I read that the limiting ITT is 820°C, however at least one pilot has described an SOP where they set 785°and maintain that until reaching cruise altitude. This will be kinder to the engines and use a bit less fuel too. I believe certain models of Learjet are flown around at max ITT - that is how you set thrust in them.
I hope this helps.