Turboprop engine parameters

[camg]

A basic flaw in the FS turboprop engine modeling is apparent: In FS, as you take a turboprop aircraft from take-off to altitude, both the ITT temperature and the N1 (or Ng) decrease as you climb (with either a constant torque setting or constant throttle setting).

But in "real life" the ITT and N1 slowly increase as you go higher. That is why you typically hit an ITT temp limit that prohibits further power increases above a certain altitude.

I'm assuming this engine and gauge behavior is coded somewhere other than in the .air file. Does anyone know if these relationships have been able to be modified, as we do other parameters in .air files and .cfg files?

Regards,
Cam

[Tom Goodrick]

This is a very interesting question. There are some parameters deep in the .air file that permit adjustment of the rate at which ITT increases, perhaps influencing N1 too. I am guilty of toning them down a little. I used to blow past ITT on every takeoff and so I reduced the rate just a little on many that I fly.

Please give a specific example. In FS I fly the Beech B200 and C90 in addition to the 350, also the Commander 1000 and 690, the Cheyenne II, the Merlin III, and the MU-2-60. It would be helpful to me if your example related to any of these. But whatever aircraft it relates too, we can get a model of it and work with these parameters. Once we get things working right for on aircraft, we can set it up for other aircraft. I should add I have a C-130, C-133 and several airliners.

If you can, give an approximate table of throttle, torque, ITT and N1 with altitude at several altitudes, indicating the altitude at which power becomes limited.

Is this feature affected by modern power management computers?

[Tom Goodrick]

The change I had made a couple years ago was in the digital ITT gauges I made for the turboprops. I put in a factor of 0.88 so we didn't have to get too careful about power on takeoff. Tonight I corrected that and then made a takeoff and climb in the Beech B200. I did have to back the throttle off to 70% on takeoff to keep ITT below 800. ITT did decrease with altitude slightly as I climbed at 80% throttle from 6000 ft to 30,000 ft. At 6000 ft ITT was 770 and power peaked at 90.11%. At 16,000 ft ITT was 707 and power was 88.91%. At 30,000 ft, ITT was 632 and power was 83.35%.

Note that N2 is related to prop rpm by the gear ratio. We don't see N2 in RPM but see it as a %max. Torque is the same. What I show as power is actually %shp calculated as we fly by a gauge I made. It is the product of N2% and TQ%. in this case I was climbing with a constant prop rpm of 2000 so N2 di not vary much. The changes related to torque were directly related to power.

I quickly reviewed several pilot reports in Aviation Week by their experienced author/pilot David M North. He does most of their jet and turboprop evaluations. He normally notes anything during climb that a pilot should be aware of. He noted that ITT was 790 for the B200 during takeoff which agrees with what I saw though that is subject to the adjustment in power I was making and, presumably, he was making. He never mentioned ITT again. The thing is that 790 is just 10 below the safe max of 800. He would not have left power alone in climb if there was a chance that ITT would increase beyond 800. He would have mentioned having to adjust power further. I read the report for the Beech 350 and the Piper Cheyenne III. He never mentions ITT beyond the takeoff where power is adjusted to keep ITT below 800. Certainly he never mentions a condition where power is limited by temperature.

Perhaps it only occurs in some aircraft.

I found ways of making some adjustments including making a special gauge that adjusts ITT sensitivity with altitude. But I won't be able to do any adjustments until you give me more direct information. it does not seem that the variation you describe is a general occurance.

[camg]

I have two specific examples: One is from a fantastic website: selair.selkirk.bc.ca/Training/systems/Alsim%20engine%20operation.html. In it they model the takeoff and climb to altitude of the P&W PT6A-42 turboprop engine. You set the takeoff torque, outside air temp, and a target altitude. It reads out the N1 and ITT as you hold constant torque by gradually increasing the throttle as you climb. At takeoff the ITT is 695 C. and increases as you climb until you hit 800 C. at 17,000 ft. Above that altitude you have to reduce the throttle as the climb continues to avoid going over 800 C.

The Piper Meridian uses the same PT6A-42, but flat rated from the 850 shp in the above program to 500 shp. So if you hold a value of 1330 ft. lb. of torque (which is the Meridian "max" power) you can maintain this to FL300--just like Piper says! At FL300 the ITT is only 775 C. due to the flat rating. But as you increase the throttle to maintain 1330 ft.lb. in the climb, both ITT and N1 increase.

Another website confirms this behavior of ITT and N1. It's www.pa46t.com. Go to "Flying Tips", then click on "Take a ride with me" at the lower left of the screen. You'll see a Shadin Engine Monitor Excel spreadsheet of many engine parameters from takeoff to cruise at FL260.
Again it confirms that for any given torque setting, ITT is hotter and N1 is faster the higher you go.

I suspect that whomever programmed the KA350 (which I think was the first turboprop in MSFS) followed the protocol of a fixed-pitch Cessna engine, where EGT drops off with altitude and eventually so does RPM. They made the ITT and N1 behave the same way. But in truth, it is just the opposite.

I would love to find out if the ITT and N1 gauges behavior could be reversed. I know the .air file has a "scale factor". Maybe a negative value could be input that would reverse the indicator trend as you climb. I'll try that this evening.

Regards,
Cam

[Tom Goodrick]

OK it is very clear to me what is happening here. The examples you mention are for flat rated engines. These are engines controlled by a computer to maintain constant maximum power capability with altitude. The variation in ITT you observe is a result that indicates how the engine operation differs from an engine whose power capability is based on ambient air conditions.

The turboprop engines in FS are not controlled by an engine management computer. Their power is allowed to vary based on the properties of the air coming in the inlet. I have no idea what program the engine management computers use to control the engine to maintain constant power when the density, pressure and temperature of the inlet air are changing. I cannot assist you in achieving this. I would like to because I know that many turboprop engines today are "flat rated." Obviously, the process involves use of internal controls in the engine to which we do not have access.

I would advise you not tinker around with those calibration properties in the .air file. You have no idea what you are doing. You will not simulate real performance just by forcing the parameters to behave that way.

I am glad you pointed this out. I have noted in the past that we cannot simulate exact performance when dealing with flat rated engines. I did not know how the parameters behave for such engines.

[camg]

I have another engine chart from the Flight 1 Cessna Conquest. It uses a Garrett TPE 331 turboprop. That series engine is also used in the MU-2.

On the Maximum Recommeded Cruise Power chart (section 5-3) it shows the power setting is torque limited (1738 ft. lb.) from sea level to about 16,000 ft. Then the max. power setting is limited by hitting the max. EGT of 450 C.

Everything I read implies ITT (or EGT for Garrett engines) rises with altitude and becomes the limit for setting power--if torque or Vmo isn't a factor. So I don't think it is just a charactaristic of flat-rated engines.

I'll see a corporate jet pilot friend of mine this Sat. at a holiday party. I think he used to fly a corporate King Air. I'll ask him about PT6A turboprops.

P.S.--You can download the Flight One 441 Conquest II users guide (flight manual) from their website.

[Tom Goodrick]

The variation in ITT and other engine parameters during climb is a combination of the environment and of what control changes the pilot, or the engine management computer, is setting on the engine. Obviously during climb some atmospheric parameters change so that power capability is reduced according to some control schedules but that other controls can be set to increase power output subject to the interrelationships of temperature within the engine as a measure of engine health.

I do not know what parameters are directly affected by the throttle. The details of variations in various temperatures within the turboprop engine are features I have not studied and do not have in my library. There is a suitable book on the subject I may ask Santa to bring me.

I don't think it is possible to compute parameter values and input them to the sim at the point where they would be used in the next calulation in the series. Thus even if we knew what to do, I don't think we could do it. Microsoft is open to suggestions on this sort of thing for the next version of FS. We could send them a note. But we would first have to get all our facts straight. I still have a problem figuring what is a natural variation and what is a variation forced by certain control settings to match charts in a POM.

I do know two facts:

1. Max available power decreases with altitude for any turboprop.

2. Turboprop engines are selected for a particular aircraft that have excess power at most lower altitudes.

These two facts relate to how the engines must be managed during climbing flight. The excess power can always damage the engine if the engine is not controlled properly. At 30,000 ft the thrust output of a jet engine is only 30% of what it would be at sea level. How this relates to power output for a turboprop, I don't know. Compression of air in the inlet turbine heats the air considerably. BUT the inlet air is a lot colder at 30,000 ft (-44C). One factor would lower the engine temperatures but other factors would raise the temperatures as more power is demanded from the engine. "Flat rating" means an engine rated at 1200 hp is limited to 800 hp during the operation of that particular aircraft. Usually the flat rating holds to a level such as 20,000 ft.

One thing I do know about flying turboprops in FS9 is that we can match the speed and fuel consumption figures published for real aircraft at various altitudes fairly closely. Thus Flight Simulator correctly predicts the qualities of turboprop flight that would be of use to a company considering how best to use this type of aircraft for their travel needs. With proper flight dynamics files, the FS aircraft show the proper range limits, flight times and costs.

I do want the data presented to the pilot to be as accurate and realistic as possible so I will see what can be done. I have known that Garret engines are controlled with an eye on the EGT gauge. This was stated in several pilot reports on the Commander series.

[camg]

Tom:

Let me know if you find out where the engine operating parameters are modeled. I assume it is in some fundamental FS software--not a specific aircraft's .air file or gauge file. Whether they turn out to be editable is another issue.

Cam

[Tom Goodrick]

The FS model for turboprop engines is perfectly fine except for the special case of "flat-rated engines." We can use the sim aircraft to accurately represent the way turboprop engines worked in the 1970's or earlier. It was only in the 1980's that the use on compueters built into the engiens permitted the flat rating.

The process of flat rating an engine involves significant variation from the normal process that relates torque, power and all the engine parameters used to observe the process. I have sent for a text that gets into the turboprop engine processes. That may at least tell me something about what can and cannot be done. The process is very much like that of a turbopcharged piston engine because it sets a range of altitude up to a "critical altitude" (normally 20,000 ft) at which the pilot can demand full rated power. Just as in the turbocharged process for piston aircraft, abnormal thermodynamic demands are placed on the engine. The pilot must watch certain temperatures to avoid harming the engine though I would expect the controlling computer would be better at watching these parameters and avoiding problems. We may never know the details of the processes because they are propriatary. Wecan observe published data but it would be difficult to deduce the internal processes that relate to the observations. I'll know more in a couple weeks.

Of course the FS code that runs the turboprop engines is internal to the sim and we have no access to it. I very much doubt there is anything we can do. I intend to fly the turboprops using old performance specs from the 1970's and earlier without flat ratings.