Les Parson

Carenado PA31T & FSX Turboprop ITT Behavior

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With this pending release, I feel compelled to once again raise the topic of ITT behavior within FSX operating parameters. I know some folks believe this is a nit-picking item and should be forgotten with a "nothing to see here; move along". However, for those of us committed to realism to the extend possible, I must convey the following. A couple of weeks ago, a beautiful King Air 250 with the Blackhawk upgrades including the -XP52 engine upgrade flew into the small Huntsville, Texas Airport. I had the opportunity to chat with the pilot for over one hour about the upgrades, avionics and operating limitations. He confirmed managing ITT, Torque and a variety of other parameters are indeed a big deal, especially above FL280. The Flight 1 B200 does model the ITT increase with altitude. I think I heard the DA Cheyenne does also. Can someone confirm?

Please note Carenado comment below:

ITT do not behave as it should with altitude

Carenado Support

June 13, 2016 00:41

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Some customers have commented that the ITT behavior is not as it supposed to, that the ITT should increase as the altitude is increased, this behavior is true in all flat rated turboprops, controlled by an engine management computer, but in the turboprop engines in FS are not controlled by an engine management computer, and in turboprops engines not flat rated this behavior is correct, the ITT decreases as altitude is increased. The flat rated behavior may be simulated by forcing some values to show what it supposed to, but this will not affect the engine performance. This is why we don't simulate the ITT behavior in flat rated turboprop engines.

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The DA cheyenne have a correct and realistic behavior of the PT6 28 and 21. The ITT dont decrease with the altitude like all carenado turboprops. 

For this and other things i assume that the DA cheyenne still rocks and even his age will be better that carenado one.

Redgars

RB

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I have been watching actual ITT indications with stable Torque settings on climb out in the real King Air. This is a complicated subject due to a variety of factors.

The aircraft engine on take-off is going to be limited either by reduction gear box stress (i.e. torque limited) or thermodynamically due to higher density altitude (ITT limited.) Allot of King Air pilots spend most of their time in the first category in that they take-off at mostly sea level airports on relatively cool days and are torque limited. As they climb into less dense air the propeller is encountering less resistance (since torque is a measure of the engine working against the resistance of the propeller) and as torque drops the pilots add power (increase N1.) This has the effect of increasing ITT.

When the aircraft does take-off when the engine is limited by ITT, generally the ITT limit for take-off is less than the typical cruise climb ITT limit. Thus the pilot pulls back on power (decreasing N1) and lowering ITT.

So to truly see what the fuel control unit is doing you have to take the pilot out of the equation. In this regard, you find that the fuel control unit on PT6A engines is a pneumatically controlled and designed to maintain a set N1. So like the propeller as the aircraft climbs the compressor section is encountering cooler and less dense air. One would expect the engine would thus need to work less to spin at the same speed. Indeed you see that in relation to fuel flow.  However, my observations in a -42 engine has shown that while fuel flow is going down as the engine becomes more efficient at higher altitudes the PT6A when left to its own was slowly increasing in ITT while maintaining steady N1. This is a hardly noticeable effect but over the course of about 10,000' ITT increased about 4 degrees. I have observed this on numerous flights. Indeed ITT does slightly increase with altitude. Thus when you are climbing out right against an ITT limit you have to reduce power every so often. 

My understanding with talking to other developers a baseline turboprop behavior is built into FS and is hard coded. There is not an aircraft.cfg variable or .air file table you can modify to change the basic thermodynamic behavior of the engine. To make the gauges behave like the real aircraft takes some behind the scenes work with gauges and this can't be done using just xml. So I doubt you will see this adjustment in Carenado Turboprops and I doubt there will ever be a community fix. 

I should also note that YMMV. My observations were in a single aircraft with two low time to TBO engines. In an airplane with engines more worn out and in different subtype engines (-41, -52, -60, -65, -67, etc.) you may observe a different ITT values during the climb. 

  • Upvote 1

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Thanks for your detailed observation. As I mentioned from the King Air B250 guy above,  increasing ITTs are indeed factor and can be the limiting factor at the higher levels (e.g. FL270+). In addition, A Beech Royal Turbine Duke driver told me the same via forum. If I can find it, I could post it here if there's enough interest. The only FSX aircraft that I've used with realistic behavior is the Flight 1 B200. I understand there are others like the DA Cheyenne mentioned by RB but I have not tried that. I did put the DA Cheyenne .air file in the Carenado but that didn't work. I know others consider GTN 750 installation as the very highest priority but I focus on the realistic aircraft behavior/performance instead.

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I did type a statement backward.

Quote

When the aircraft does take-off when the engine is limited by ITT, generally the ITT limit for take-off is less than the typical cruise climb ITT limit.

 

I should have said it this way.

When the aircraft does take-off when the engine is limited by ITT, generally the ITT limit for cruise climb is less than the typical take-off  ITT limit.

Sorry, call it an elder moment.

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