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Indiafoxtecho avatar image
Indiafoxtecho asked boufogre commented

Jet engine model equation, fuel flow and fuel consumption

On our aircrafts, fuel consumption and fuel flow on jet engines seem not to behave correctly.
In detail, the fuel flow does not seem to change with altitude and mach number, resulting in incorrect fuel consumption if compared to the real world flight manual and incorrect aircraft range.

Could you share the full jet engine model equations you are using for MSFS?

flightmodel
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Krazycolin avatar image
Krazycolin answered

or allow us to modify/change them....

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FlyingRaccoon avatar image
FlyingRaccoon answered boufogre converted comment to answer

Hello @Indiafoxtecho

Here's some information about the turbine fuel flow formulas

If "use_old_fuelflow_simvar" is set to 1.0, then we use the classic FSX formula to calculate the base_FF:
base_FF = [jet thrust] * [ThrustSpecificFuelConsumption]

If "use_old_fuelflow_simvar" is set to 0.0, then we use the modern FM formula to calculate the base_FF:
base_FF = inverse lookup of [corrected_n2_from_ff_table]

THEN, in all cases, the final_FF is scaled using the density_on_ff_table:
final_FF = base_FF * [density_on_ff_table]

If you enter a density_on_ff_table, you will be able to change the ff based on air density. (Checkout the default turboprop turbines to see an example of density_on_ff_table)

The density_on_ff_table parameter isn't shown in the doc at the moment. That's part of what @Nocturne plans to update.

Regards,
Sylvain

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It would be great if you could provide all the equations used by the sim to calculate thrust from the configuration file parameters.
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tml avatar image
tml answered FlyingRaccoon commented

Thanks. This explains the density_on_ff_table. It maps air density in slug/ft^3 to a fuel flow factor. (See for instance http://www.luizmonteiro.com/StdAtm.aspx for a nice page that calculates various atmospheric values according to the US Standard Atmosphere model. Input air density 0.000225 slug/ft^3 (which is what the density_on_ff_table starts with) and you will see that it corresponds to an altitude of 70,000 ft. The 0.002378 that the table ends with corresponds to sea level.)

But what about the density_on_torque_table table? What does that map air density to? A torque factor? Is that factor directly used for thrust, too?

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Hello.


This is being documented atm.
Here's a preview of the doc about this table:
1631523798564.png

It appears it's only used for turboprop aircraft so not sure if this is in the TURBINEENGINEDATA section because of a mistake or because we have plans for future support.

I'll clarify that. You can ignore that table for the moment.

Regards,
Sylvain

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1631523798564.png (123.0 KiB)
tml avatar image tml FlyingRaccoon ♦♦ ·
Thanks! Great to see documentation improving.


(By the way, when this documentation says that "the table expects 10x2 entries", I guess that should say that "it typically has 10x2 entries"?)
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Yes, we mean up to 10x2 entries, default values will be kept otherwise.
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boufogre avatar image
boufogre answered boufogre edited

Hello @FlyingRaccoon,


I have noticed a big problem with the fuel flow using the modern FM formula: by doing an inverse lookup of the corrected_n2_from_ff_table, the fuel flow calculated is not currently corrected for altitude. Cf. the screenshot down below:


1631603335992.png


I have tried adding the density_on_ff_table to the engines.cfg, but without any effect. This is probably due to the fact that this is a jet and not a turboprop.


Using the old_fuel_flow_simvar = 1, the FF gets scaled as the calculation is based on thrust, which itself is scaled in the n1_and_mach_on_thrust_table.


I don't think there is a need for a specific table to scale the fuel flow and from the literature, fuel flow should be scaled according to the following formula:

final_FF = base_ff / (Delta2 * Theta2^0.5), with Delta 2 being the total pressure ratio and Theta2 being the total temperature ratio.


Hopefully this can get corrected in the upcoming SU6.




1631603335992.png (433.6 KiB)
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Hello.


I see is your use_corrected_n2_from_ff_table parameter is set to 0.
If it doesn't exist yet, add it to your TURBINEENGINEDATA section and set it to one.

For some reason, the density_on_ff_table entry seems to be used only on turboprops.
I'll double check with devs if this is on purpose or not.

Regards,
Sylvain

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boufogre avatar image boufogre FlyingRaccoon ♦♦ ·

I have just tried, N1 is surging and not going beyond 56%. I have no thrust anymore.

The engine behaviour was spot on with this parameter set to 0, only that the fuel flow does not get corrected.

Here is a screenshot from a paper showing how to calculate corrected parameters and why there is no need for a specific table for density effect over fuel flow:


1631613555847.png


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1631613555847.png (30.1 KiB)
boufogre avatar image
boufogre answered

Hello,

Since there is no news on this topic, I feel the need to insist that at the moment, and as explained, the fuel flow calculated by the simulator is wrong in any of the two cases listed by @FlyingRaccoon.

In the case where use_old_fuelflow_simvar = 1, the fuel flow is wrong by a factor of 1 / sqrt(Theta2) and the density effect over the fuel flow is already applied through the effect of density over thrust (the two being connected by the TSFC). There is no need for an additional density_over_ff_table as it would apply the density effect twice.

In the other case, the fuel flow is simply not scaled at all and the sea level fuel flow is used at all altitudes and mach numbers. In that case, the fix is simple and consists in multiplying the corrected fuel flow (determined by the corrected N2 from fuel flow table) by a factor of Delta2 * sqrt(Theta2).

In both cases, those simple fixes would finally solve one of the most annoying bug of MSFS and its predecessors: the impossibility to have consistent fuel flow over the operating envelope.

Thanks for taking this into account.

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boufogre avatar image
boufogre answered

Hello again,


As sometimes an example is better than words, here is a screenshot showing what the simulator calculates as fuel flow in the engines debug UI and what it would look like on the FF gauges if applying the correction I mentioned in my comments above.

1632325142551.png

As you can see, the results are pretty close to FCOM published values.

1632325268632.png


1632325142551.png (2.1 MiB)
1632325268632.png (53.0 KiB)
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donstim avatar image
donstim answered boufogre commented

FYI...Generalized fuel flow data provided by engine manufacturers to Boeing are in the form:screenshot-1201.png

That is, they are generalized for the total pressure and total temperature ratios, with the exponent on the total temperature ratio different for different engine models. Another way to write it as a comparison to what you provided would be:

screenshot-1203.png


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Thanks for the input, you have obviously more detailed information and more knowledge on the subject than me. Obviously, I don't have access to the exponent, but I think 0.5 is a good enough approximation and it is certainly better than not correcting the fuel flow at all, or badly in the case of the old fuel flow simvar.
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