Post by Tom Goodrick on Dec 25, 2008 20:25:28 GMT -5
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.
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.