Fixing A Turbopro Engine in FS9

[Tom Goodrick]

While it is true that modern turboprop engines that are set up to operate in a "flat-rated" mode are not modelled correctly in FS9, the old-fashioned turboprop engines of the 1980's are correctly modelled but may need a little help.

I have been looking for documentati9on that would indicate how best to do this. I found something that works for the Beechcraft King Air B200 - a very popular business plane for 6-8 passengers. I found a chart for setting cruise torque from the Pilot's Operating Handbook (POH) for this aircraft. The chart is a complex "carpet plot that requires some study. You select an altitude, climb to that altitude, read the outside air temperature, compare it to the standard or "ISA" temperature for that altitude, and then find the value of torque that you should set for those conditions of altitude and temperature. I made a table from this chart that is much easier to work with than the chart. below is a copy of this table. I have made the values in the table in percent of max torque which is how all the torque gauges in FS9 work. (I could make a real torque gauge like they use in real aircraft but it would have to be used only with one particular engine. It seems best to use the percent turque that works for all turboprop engines.)

RECOMENDED TORQUE SETTINGS FOR B200 CRUISE
SET 1700 RPM

TABLE GIVES PERCENT MAX TORQUE
MAX TORQUE IS 2230 FT LB
OAT VARIATION FROM ISA ALL DEG C
ISA____-20____-10____0____10____20____ALTITUDE
-16.7__100.0__100.0__100.0__95.5__87.9__16000
-20.7__100.0__98.2___94.2___89.2__83.4__18000
-24.6__95.5___91.0___87.0___83.9__78.5__20000
-28.6__88.8___84.8___81.2___77.6__72.6__22000
-32.5__82.1___78.5___74.9___71.7__68.2__24000
-36.5__74.9___72.2___69.1___65.5__62.8__26000

As an example, to cruise at 26,000 ft on a standard day this shows you set torque at 69.1%. Note that RPM must be set at 1700. I did this after some modifications to the engine and found it did indeed produce a nice cruise at 243 KTAS with an endurance of 4.74 hrs after a gross weight takeoff. The power was 63.38%.

There are two modifications to make, one to allow higher max torque settings at high altitude and the other to reduce the sensitivity of the engine temperature. First look at line 1508 in the .air file. It is a "graph with two points beyond zero-zero. The middle point should be raised from near zero to 0.25. The last point should be raised from1.9 to 2.10. These changes make higher torque possible. Then in line 1526 set the rate for ITT at 0.77. This will make it possible to set the throttle (actually the torque lever) properly for best torque and still keep ITT below 800 C.

The proceedures for operation of B200 were in my copy of Van Sickle's Modern Airmanship and I had not noticed. For takeoff, RPM is set at max, Torque is moved smoothly to 2000 ft lb (89.7%) and ITT is monitored. Adjust the torque lever to keep ITT below 800 C. As you transition to climb (gear and flaps up) set the RPM to 1900. This keeps the N2 speed from exceeding the maximum. The torque lever must be adjusted just a little as you climb to keep torque at 89.7%. I use the autopilot with HDG or NAV mode and ALT mode set. 2000 fpm was a good climb rate to 13,000 ft when I switched to 1600 fpm. At cruise altitude set 1700 RPM, read the temp and pick the torque value from the table. In the case of "Clear Weather" the temp will agree with the ISA value. But I flew in RW two days ago and had to use the -10 column.

One other strange thing about FS9 is that we must use N2% instead of N1% as the key parameter for engine speed. This is because they assummed the second stage drives the prop so max RPM times the gear ratio gives the max N2 value in the aircraft.cfg file.

This is not the technique you would use to fly a flat-rated turboprop. But it is the proper technique for flying the old-fashioned type of B200 from the 1980's. All parameters vary properly with altitude when this method is used.

The book provides some illumination of the use of the condition levers. Low idle is 51% N1 and high idle is 70% N1. (Our N2). Low is used most of the time. But high idle speed is used for battery charging after start, to handle high electrical loads, and when rapid prop response is needed as when landing and using prop reverse.

[camg]

This thread is related to the topics started in the "Turboprop Engine Parameters", so I'll reply here instead of the older one.

We have been discussing engine parameters including torque, turbine speed, ITT values and "flat rated" turboprop engines.

Tom: I'm not sure what you mean by "old-fashioned" turboprop engines. Since the start of the previous thread, I've talked with two turboprop pilots. They say that virtually all TP engines have some flat rating. That allows them to still develop takeoff power even at higher elevation airports and higher that ISA temps.

No TP engine is firewalled at takeoff--as you have said--since it would cause most to go over ITT limits. Take the PT6A-42 that powers the King Air B200 in your example. Takeoff torque is 2230 ft-lb. Full throttle produces over 2400 ft-lb.

The aviation program at Selkirk College has a PT6A-42 software program (on-line) that models all engine gauge parameters during takeoff and climb; I regard this as definitive for the B200.

Takeoff with a 2230 ft-lb setting results in in an ITT of 690 C. This is nowhere near the 800 C. limit. But if you hold 2230 in the climb, you will hit 800 C. at 17,000 ft., after which you reduce power to avoid going over temp. 17,000 is the limit of 100% power for this engine

Based on your table of cruise torque settings, I climbed the Selkirk B200 using your values. Those setting all resulted in an ITT of 750-740 C., which is the recommended cruise ITT from Pratt & Whitney for this engine. So both sets of data agree. So, instead of holding a constant power level and watching ITT increase, the data you found progressively reduces the power level to hold ITT constant. That makes sense.

But I also climbed the aircraft at a constant 2100 ft-lb (which was your 94.2% power setting). Using your altitude marks, ITT was 755 C. at 18000, 770 C. at 20000, 785 C. at 22000, and 800 C. at 24000. At this point I needed to reduce power 'cause ITT was redlined; so I held 800 C. to 26000 ft. It yielded 1920 ft-lb.

Both pilots I talked to agree with my original statement--that ITT increases with altitude if torque is held constant. In FS02 and FS04 ITT drops with higher altitudes--this is wrong.

Another relationship in the PT6A engine became apparent when I was "flying" the Selkirk College engine simulator. When you reduce the Prop RPM to transition from takeoff to cruise climb, the torque value rises as the prop RPM is lowered. FS04 does not recognise this-so this is another engine gauge error.

I'll start another thread in General Fying Operations regarding the flat rating of turboprops. I've learned more in the last couple of weeks.

[Tom Goodrick]

OK. I accept the validity of your model. Now you must understand something about FS. This is not simply a gauge problem. This is a fundamental flaw in the FS turboprop engine model. Just as the sim uses aerodynamic forces to determine accelerations and then velocities and then displacements by numerical integration, it determines engine thrust output from the control settings plus inlet flow as they change during flight. This calculation may involve closed-form equations or it may involve integration if the equations are not of closed form.

Present your data to Microsoft. They are the only people who can make changes in the model for the next version. They won't make them if nobody complains about the lack of realism. Your information indicates we cannot fly turboprop aircraft in FS with any degree of realism.

To me, it makes perfect sense for the ITT to decrease with altitude because the temperature and pressure of the inlet air are each decreasing significantly with altitude. This generallly results in a loss of maximum power or thrust with altitude. Thus, to me if the ITT is seen to increase with altitude, the controls are changing the way the engine operates to get more power. It certainly is not caused by the environment.

I gave up trying to find a book that deals with this problem. I tried one $60 book but it only discussed fanjet engines. There's an $80 book published by AIAA, a very respected organization that publishes books by true experts in the field. But I am not sure I want to pay $80 to find out I still have to through away all my nice turboprops.

[Tom Goodrick]

What we need to fly turboprops properly is a max usable power as a percent of max rated power. We could build a gauge for that, both in real life and in the sim, so that the pilot can see the actual power at all times and keep it set according to the spec. It may be a function of altitude.

Since percent power is merely the product of percent fan speed and percent torque which are two parameters readily available for use by a gauge, an accurate power gauge can be made with the same response as a torque gauge or an rpm gauge.

Use of percent power has worked for piston engines for decades although it only recently appeared in the cockpit as displayed on engine management computers even in small single-engine aircraft.

It has always worked because power is a two-way street. Power output makes the airplane fly. The business of producing power is what stresses most aspects of engine operation. Limit the power and you limit the stress on the engine.

Pratt and Whitney has a lot of technical literature about their engines and are happy to send copies to anyone who buys an airplane that uses their engines.

[Allen Peterson]

Would your TPower gauge work, or be a start to build this gauge?

[camg]

I am currently flying the Quantas Piper Meridian (Mike Stone's)with my custom analog panel. In it I use the Cessna Caravan torque meter, which reads 0 to 2500 ft-lb. I redrew the green arc and redline to show 1360 ft-lb. as max. power for the 500 SHP PT6A-42A engine.

But I set the max. torque in the .cfg file to 2230 ft-lb, since the rated power of the -42A is 850 SHP; it's flat rated to 500 for use in the Meridian.

So on takeoff I am careful to power up only to the 1360 ft-lb. As I climb, I increase the power lever (throttle) to maintain this torque value up to final cruise altitude.

Once at cruise I adjust the drag and fuel scalar to achieve the real-world maximum TAS and Fuel Flow as published in pilot reports. I ignore the ITT readings.

This probably is an appoximation of how the real A/C behaves, but I am satisfied to fly it this way.

Also--The Caravan gauge would indicate the 2230 ft-lb of King Air B200, so one could use it as is on a modified panel in this A/C, too.

[Tom Goodrick]

Yes, Allen, my TPower gauge is exactly what could be used.

Camg, there is one problem with the C208B torque gauge. It is probably only good for use with an engine that can produce only 1875 ft-lbs max torque such as the engine in the Caravan. The reason I say this is that I cannot find any parameter Microsoft makes available for gauge makers to use other than percent max torque. There is no direct torque value given. So it seems the way that Caravan guage must have been made is that they multiply the percent torque by the max torque and divide by 100. That is what I would have to do if I were to make a torque meter. Then I'd have to make different gauges for use on each different engine.

The TPower gauge I have made works with any engine and shows percent power which is best for those of us who fly several different aircraft.

Here is a table I made for the cruise condition of the King Air B200:

B200 Cruise Power Constraints

For ISO temp and constant 1700 rpm.

ALT__TQ%__Pwr%__KIAS__KTAS__FF/e
16k___100___88.2___199___250____309
18k___94.2___84.6___193___250____295
20k___87.0___78.9___185___249____272
22k___81.2___73.7___178___248____254
24k___74.9___68.7___171___246____235
26k___69.1___63.8___163___244____217

This is a bit of a strange table. It looks as though the main thing they try to keep constant is the true airspeed. They certainly don't keep the power anywhere near constant. In this case, with rpm held constant, the torque percent is proportional to the power percent.

This suggests the generalized power table:

For ISO temp.

ALT__Pwr%
16k___88.2
18k___84.6
20k___78.9
22k___73.7
24k___68.7
26k___63.8

I am going to check this out It may be reasonable to use for setting the cruise power on other aircraft. I would use 90% power for all climbs and for takeoff. It is notable that anytime the rpm is held constant as in the table discussed here by Pratt&Whitney for the B200 or as with the Piper Meridian you are flying where the rpm is locked at 2000 by Piper, the torque is the direct control of power.