Alsim Engine Operation

This simulation shows how the PT6-42 engine in the Alsim MCC-200 works. As delivered in December 2007 there is an error in the ITT modeling such that ITT does not increase with altitude as much as it should. A real PT6 will reach redline ITT (800 °C) at 2230 torque climbing through 17,000 feet in ISA conditions. The simulation below allows you to switch between the parameters Alsim provided with those we believe it should use.

Instructions:

  1. Allow blocked content if required (click on bar at top of page.)
  2. Click spacebar to activate simulation - or click on simulation with mouse
  3. Choose Alsim Programming or Selkirk Programming (Alsim is the default)
  4. Advance throttles (black levers) to maximum torque, which is 2230 ft.lb (red line)
  5. click on "+" beside VSI to set desired rate of climb
  6. Reduce to climb power, which is 2230 x 1900 rpm (take note how much torque must be reduced to prevent over torque)
  7. maintain torque at 2230 while climbing
  8. Note that ITT rises - although not much in Alsim programming version
  9. If ITT reaches redline reduce torque as required (even better, keep ITT below 750° in climb.)

TIP: if the simulation does not fit on you computer screen click on F11 to enter full screen mode.

PT6 Temperature Limited Takeoff

Normal takeoffs are conducted with 2230 torque and max rpm (2000) however on a hot day, or if taking off at a high altitude the engine will be temperature limited. This means that ITT will reach redline making it impossible to use 2230 torque. To see this effect:

  1. Choose Selkirk Programming
  2. Set VSI to zero by clicking on the "0" key beside VSI
  3. Click on the "+" key beside the altimeter to set altitude to 3000 feet
  4. Click on the "+" key beside the thermometer to set temperature to ISA + 25
  5. Advance throttles as though starting a takeoff
  6. Note that ITT reaches redline when torque is approximately 2000 ft.lb

The above demonstrates that under these conditions it would not be possible to use the full rated power for takeoff.

You may want to experiment. For example at ISA + 25 is the aircraft temperature limited for a takeoff at sea level? (N0.) What amount of ISA deviation does result in a temperature limited takeoff at sea level.

Effect of Temperature on Fuel Flow

Turbine engines are more efficient in cold air. To see this try the following:

  1. Set temperature to ISA
  2. Set VSI to zero by clicking on the "0" key beside VSI
  3. Click on the "+" key beside the altimeter to set altitude to 24000 feet
  4. Set cruise power, which is 1700 rpm ITT 725
  5. Note the torque, which is just under 1600 ft.lb. Write down the fuel flow (it is about 340 lb/hr.)
  6. Reduce temperature to ISA - 20°. Notice that ITT drops and torque rises.
  7. Use throttles to reset torque to value observed in step 5.
  8. Note fuel flow - it has decreased

Power output is the same in each of the above cases but the efficiency of the engine (expressed as power/unit of fuel consumed) is better in the cold air. Engine efficiency is called specific fuel consumption and it has units of horsepower divided by fuel flow.

To learn more about horsepower and its relationship with torque and rpm see the textbook Aerodynamics for Professional Pilots.

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