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Engine variables

JB3DG

JB3DG

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southafrica
Hi guys

Figured i would get a better answer here than what i got in the gauge forum.

In the SDK i have 3 variables for fuel flow and 2 for N2 RPM.

TURB ENG CORRECTED FF (the only one controllable via simconnect)
TURB ENG FUEL FLOW PPH
ENG FUEL FLOW PPH

TURB ENG CORRECTED N2
TURB ENG N2
(both controllable by simconnect)

My question is..which one is actually the correct one that indicates the actual amount of fuel being drawn from the tanks or the actual rate at which the engine is spinning? Also how do they differ from eachother?
 
Last edited:
Corrected means it has been realigned to account for power losses with temperature and pressure changes.

75% N1 at sea level and 75% N1 at 35,000 would give different thrust ratings. Corrected N1 would give thrust at all altitudes by Mach, and as a result is always higher than N1.


In flight sim, which is best to use depends on how much the aircraft you are modeling's thrust varies at the same %N1 with altitude. If you have a large variance, you may need to use N1 in the gauge. If not, Corrected would be easier because what you see in the table is what you see in the sim. Easier meaning less math and less time tweaking...not necessarily more accurate.


Fuel flow is the same concept, but I haven't actually tested it in FS to see the outputs.
 
So which fuel flow would be correct aside from the corrected version?
 
Corrected N1 would give thrust at all altitudes by Mach, and as a result is always higher than N1.
Click to expand...

That is true only for very low Mach numbers, because the relationship depends on Total Air Temperature Ratio. CN1 = N1/sqrt(Total air temperature ratio). Total Air Temperature ratio is Air Temperature ratio times(1+(M^2)/5)

Thus at M=0.8 at sea level 100% CN1 would be 106.2% N1, they are about the same around 18000 ft then CN1 does always exceed N1 at higher altitudes for 0.8 Mach. At Mach=1.0 the crossover occurs at around 25,000Ft.

My other post gives the relationship between corrected fuel flow and actual fuel flow,

Roy
 
I stand corrected: Corrected N1 would give thrust at all altitudes by Mach, and as a result is usually higher than N1.


Naruto,

Always use regular fuel flow in the gauge.. That will show the fuel burned from the tanks in weight/time that pilots need to see.
 
Jonathan,
jx_ is correct, you should use TURB ENG FUEL FLOW PPH. However, the output depends on what you have in the config file ThrustSpecificFuelConsumption entry. The default value is 0.5 and if that is what you have the result will be exactly half the TURB ENG JET THRUST value.

I have a bunch of flight test data on TURB ENG FUEL FLOW PPH versus TURB ENG CORRECTED FF both when static on the ground and in flight at around 500 MSL and around the tropopause, at various Mach numbers like 0.8 to 1.03, and various N1/CN1 settings. Even though I know the theoretical relationship between the 2 variables, none of the data makes any sense to me at the moment.

Roy
 
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