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Non-ISA Engine Corrections

Taz5150

Taz5150

Messages
70
Country
spain
Hi everyone,

I've been working on engine modeling for some time now, and as a basis for my work/ research, I've used Roy's "FS Thrust versus Altitude" paper.

The temperature ratio, represented as theta, is given by the following formula: (288.15-1.98*altitude/1000)/288.15. Quick question: does this take into account non-ISA temperature conditions?

Quick example:

ISA+0 @ 5,000 ft
theta = (288.15-1.98*altitude/1000)/288.15 = 0.965642
theta = Ta/To = (273.15 + 5.1) / 288.15 = 0.965642

ISA+10 @ 5,000 ft
theta = (288.15-1.98*altitude/1000)/288.15 = 0.965642
theta = Ta/To = (273.15 + 5.1 + 10) / 288.15 = 1.000347
 
The equations are used to model performance in a standard atmosphere then adjust to the actual conditions using the sensed or specified atmosphere in the sim. This will give a different result if the temperature is non-standard, but it will be the right solution.
Incidentally, the paper applies to FSX and P3D. There are some differences with MSFS which I'm studying at the moment. For example, it appears that fuel flow increases with altitude and the fuel flow equations use a PID controller which creates under and over swings of flow rate that can result in flame out with zero fuel flow.
Roy
 
Hi Roy ,
I noticed with the new documentation they have now the table:
JET_density_on_FF_table , I have it populated with the values
0.000460:0.1243, 0.000585:0.415, 0.000737:0.459, 0.000890:0.548, 0.001066:0.5375, 0.001267:0.5377, 0.001496:0.5342, 0.001756:0.6638, 0.002049:0.8177, 0.002378:1.0

which corresponds to (1000 feet) 60 ,40, 35 ,30 ,25, 20 ,15 ,10, 5 ,0
so got fuel flow to decrease without using the legacy tables.
 
The usual way is to multiply the thrust by the Thrust Specific Fuel Consumption value (TSFC) which is a constant. Since thrust varies with altitude/density, the method you refer to basically does the same thing but does allow for TSFC variation which some engines might have in their non afterburning power regime.
Roy
 
Not wanting to open up a new topic on this matter, and having read extensively the forum on this subject, this might be a very basic (even stupid) question. The reason I want to cross check with all of you is because of the discrepancies I'm seeing between theoretical calculations and real figures.

I have a FCOM maximum climb table as follows (excerpt), at ISA and IAS=250 knots. Please let me know if this sequence of calculations seem correct:

1640271084936.png


Problem: Flying at 2000 feet, and 250 KIAS (ISA conditions, thus OAT=+11ºC), I'd like to know what N1 would we have at 300 KIAS.

1) Find out what N1 corresponds to those conditions: TAT = OAT*(1 + 0.2M^2) = (273.15+11)*(1 + 0.2*(0.391^2)) = 292.84K = 19.69ºC. Interpolating between 18ºC and 22ºC TAT, gives me an N1 of 83.05%.

2) We correct N1 to obtain what rotational speed we would have at Sea Level & Static conditions: CN1 = N1 / SQRT(theta2), where:
* theta = Ta/288.15 = (273.15+11)/288.15 = 0.986118
* theta2 =theta * (1 + 0.2M^2) = 0.986118 * (1 + 0.2*(0.391^2)) = 1.01627
thus, CN1 = 82.38%. This is what my N1 would be at SLS.

3) Now we want to know what N1 we would have at same Altitude (2000 feet) and Temperature (11ºC), but at 300 KIAS (Mach 0.469): So N1 = CN1 * SQRT(theta2), where:
* CN1 = 82.38% (from step 2)
* theta = Ta/288.15 = (273.15+11)/288.15 = 0.986118
* theta2 =theta * (1 + 0.2M^2) = 0.986118 * (1 + 0.2*(0.469^2)) = 1.0295
thus, N1 = 83.59%.

Does this calculation sequence look right to you? I believe this is correct.
 
As a practical matter, the speed you achieve has to do with throttle setting. If you are at 2000ft and are steady at 250 KIAS, you clearly have to increase throttle and N1 to get to 300. How else would it happen?
If there are differences between theory and MSFS results it is because MSFS is not using the correct equations.
Roy
 
Thank you Roy. Yes, I see the physics and logic behind it. Even more, during climb, not just speed but gradient should be taken into account. I just doubted myself after a few days of tests and analysis.

What I’m trying to do is to “translate” the maximum climb performance tables (which have been developed for standard 250/300/0.78) to non standard speeds (say 220/300/0.78). I have to do some math but probably I will need to know what the thrust is for every altitude vs TAT pair.
 
I'm doing the same for the Salty 747. In the case of the climb, as this is the max climb rating, you cannot throttle up, but pitch down to increase speed. I have arrived to the same calculations than yours, but I cannot figure out how the N1 increases with altitude (it doesn't match the formula). If you feel like teaming up efforts, that would be nice!
 
For max power, N1 should stay at 100% or whatever it was at sea level as you climb up. CN1 is an intermediate term in the calculations and is not a gauged parameter. CN1 will vary as Tax5150 says but your N1 should stay put. There may be some minor variations depending on the used interval between altitude entries in the tables. These are the result of a straight line interpolation of a non-linear parameter.
Roy
 
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