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?

or allow us to modify/change them…

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] *** If “use_old_fuelflow_simvar” is set to 0.0,

[ThrustSpecificFuelConsumption]

then we use the modern FM formula to calculate the base_FF:

*THEN, in all cases, the*

**base_FF =**

inverse lookup of [corrected_n2_from_ff_table]inverse lookup of [corrected_n2_from_ff_table]

final_FF is scaled using the density_on_ff_table:

**If you enter a density_on_ff_table, you will be able**

*final_FF = base_FF **

[density_on_ff_table][density_on_ff_table]

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

It would be great if you could provide all the equations used by the sim to

calculate thrust from the configuration file parameters.

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?

Hello. This is being documented atm. Here’s a preview of the doc about this

table:

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

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”?)

Yes, we mean up to 10x2 entries, default values will be kept otherwise.

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:

**to the engines.cfg, but without any effect. This is**

*density_on_ff_table*probably due to the fact that this is a jet and not a turboprop. Using the

**, the FF gets scaled as the calculation is**

*old_fuel_flow_simvar = 1*based on thrust, which itself is scaled in the

**. I don’t think there is a need for a**

*n1_and_mach_on_thrust_table*specific table to scale the fuel flow and from the literature, fuel flow

should be scaled according to the following formula:

**with Delta 2 being the total pressure ratio and**

*final_FF = base_ff /*

(Delta2 * Theta2^0.5),(Delta2 * Theta2^0.5),

Theta2 being the total temperature ratio. Hopefully this can get corrected in

the upcoming SU6. ** __**

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

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:

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).

*for an additional density_over_ff_table as it*

**There is no need**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,

**and consists in**

*the fix is simple*multiplying the corrected fuel flow (determined by the corrected N2 from fuel

flow table) by a factor of

**. In both cases, those**

*Delta2 * sqrt(Theta2)*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.

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.

pretty close to FCOM published values.

FYI…Generalized fuel flow data provided by engine manufacturers to Boeing

are in the form: 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:

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.

Hello. Your feedback was read by engine devs and they agreed on it so it’s

been added to the development backlog. Can’t tell when it will be implemented

as they’re very busy people but it will. Thanks you for your feedback

Regards, Sylvain

Hello! That is great great news, with the density table, close results can be

approximated nut this will allow for correct FF at all Mach and altitudes.

Thanks again for taking this into consideration!