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Incorrect engine power change when prop RPM changes


Calypte
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Power changes when prop RPM is changed from 96% to 80%. Incorrect, would not happen in the real-world.

 

Do a simple test:

1. Set some cruise power. For simplicity set 80% power (96% RPM and 40 PSI Torque). 

2. Note speed (after it stabilizes), Ng, Fuel Flow and T5 Temp.

3. Change RPM to 80%. 

4. Note TRQ and the same parameters you noted a moment ago (speed, Ng, FF, temp).

 

My values:

At 96% RPM:

TRQ = 40

Speed = 149 KIAS

Ng = 90,5%

FF = 299

T5 = 630

 

At 80% RPM:

TRQ = 45

Speed = 146 KIAS

Ng = 90,5%

FF = 299

T5 = 630

 

What exactly happens here? Somehow by changing prop speed we:

a) lost some power (as indicated on instruments)

b) lost some speed

c) did not change gas generator parameters a bit...

 

Engine Power. In a free turbine turboprop (such as PT6A) pilot sets power with the power lever (in alpha range). Of the events noted above only c) is correct - as we do not change the position of the power lever gas generator parameters stay the same. Prop lever governs Ng, FCU in the engine selects a proper fuel flow for the selected Ng (hence no change in the fuel flow) and the temp stays the same, because the whole process does not change. And the power does not change. Because we did not touch the power lever.

 

A bug:

a) The plane lost some power, which is indicated on the instruments. Power can be calculated from TRQ and Np indications and max rated power will be achieved at max allowed RPM and max TRQ. Any power changes will be in proportion to the equation, as power = trq * rpm. Keep in mind that power is generated in gas generator and is (in a free turbine turboprop) unrelated to the prop rpm). So the power does not change (as explained above - I did not move the power lever, the same amount of fuel goes to the engine). If you calculate the power with TRQ and Np indications... you will see a drop of several percent when prop RPM changes. Somehow some amount of power generated in the gas generator... evaporated before it reached the power section.

 

TRQ indication for 80% RPM should be (in this example and in my test conditions) 48 instead of 45.


A consequence of a but:
b) Speed change is related to the power change. This is correct. As the power dropped, the speed should drop too. But the power should not change (as all the indications for gas generator do not change).


This may seem as a limitation of MSFS, but it is not. Currently available planes show that correct power changes can be achieved and no change in speed can be achieved in normal prop speed range.

 

Speed change can not be explained with the change in prop efficiency. The prop efficiency does not change significantly in the usable Np range and the efficiency should not drop at lower RPM (it could actually increase).

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