Freeflight Design Shop Forum


FS2K Specific Fuel Consumption Tutorial (by anonymous)

Posted by Sam Chin/FFDS on Tuesday, 14 August 2001, at 9:01 p.m.

Well, I thought it was near impossible to solve, but an "anonymous benefactor" emailed me a working solution to modify FDE fuel consumption. (I have no idea if this was published anywhere previously, but the author believes it is unprecedented...please let me know if there is contrary evidence to this). Note: it does not work for jets. In any case, I thank again immensely the person who emailed this to me, and I reprint the info here with permission:

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FS2K Specific Fuel Consumption Tutorial.

This tutorial explains how to set SFC for piston engined and turboprop powered aircraft in MSFS 2000.

The SFC of FS98 air files flown in FS2000 may be set using this methodology, but it is not the preferred method.

This methodology should also work in CFS 1 and CFS 2 since the piston engine code appears to be identical.

Setting SFC for engines without propellers is outside the scope of this tutorial.

Until the publication of CFS1 specific fuel consumption for aircraft with propellers was varied by setting data in section 500 of the air file. With the publication of CFS1 Microsoft began to implement changes which allowed FDE to be varied from within the aircraft.cfg instead of, or in place of, the filename.air. Within FS2000 it seems to have been Microsoft's intention that SFC for aircraft with propellers be varied from within aircraft.cfg. To some extent it remained possible to set SFC for piston engines, but probably not turboprops, by other more complicated means. These are not discussed here.

Unfortunately there is no single entry for SFC in aircraft.cfg. Setting SFC is however a simple process whether you are authoring a new Flight Dynamics Envelope (FDE) for uploading or just wish to alter the SFC of an aircraft which you have downloaded.

With the exception of rotary and diesel engined aircraft the SFC of piston engined aircraft differs only slightly at any specified percentage of rated power. The aircraft. cfg parameters;

[piston_engine] power_scalar = 1.0

[propeller] thrust_scalar = 1.0

are therefore frequently omitted by third party authors.

These are nevertheless the specific fuel consumption parameters.

In principle to halve the fuel consumption without altering the FDE you would instead include;

[piston_engine] power_scalar = 0.5

[propeller] thrust_scalar = 2

Half the power with double the thrust per horse power leaves the thrust opposing the drag unchanged but cuts the fuel flow in half. You will probably never need to make such gross changes in practice.

To increase SFC by 10% you increase the power by 10% and reduce the thrust to match.

[piston_engine] power_scalar = 1.1

[propeller] thrust_scalar = 0.909

Please note that you multiply and divide by 1.1. You must not add and subtract 10%.

power_scalar * thrust_scalar must always =1

If they do not the FDE will alter and you have changed more than SFC.

There are a couple of wrinkles however.

When you change mass fuel flow in unit time you will change the energy flow and this will alter the engine temperature gauge readings. This only matters if you are altering existing FDE. The new readings will be more accurate, but if you wrote the original FDE and you lovingly set the temperature gauges to read the cruise numbers with the wrong SFC you will now have to go back and fix those. Pressures will not alter.

The potentially larger wrinkle relates to the way that MSFS handles any airscrew defined as constant speed.

When you alter the thrust scalar to fix bad SFC it will force MSFS to change the prop pitch (slightly) to achieve the same demanded rpm. It will then read a slightly different efficiency from section 511. These changes are very small unless you make gross changes in SFC. They may however put the FDE slightly out of balance. Remember the new data are more nearly correct. Unless you are very certain of the profile drag in the air file you should change this very slightly as necessary to re- balance the FDE.

If you have a test panel which will read thrust and you are "fixing" a downloaded FDE and you are sure that it was "accurate" apart from the SFC you may wish to take the trouble to alter the thrust scalar in aircraft.cfg until the new thrust exactly matches the old thrust at the speed which you consider to be the most important for that aircraft. This is the logical alternative to altering the profile drag. The thrust before and after will not however match exactly at all speeds.

It follows that you should always research and write SFC data to the aircraft.cfg before commencing testing of new FDE.

Although the first aircraft.cfg parameter is;

[piston_engine]

power_scalar =

It really means any engine with a prop. The piston engine data in the aircraft.cfg is read if the engine is defined as turboprop in the air file. It follows that the procedure for setting the SFC of turboprops is identical. You may need to make larger variations and the need to re- balance FDE for c/s prop pitch changes becomes more likely.

The only wrinkle is that some FDE authors may set one or other of the aircraft.cfg parameters for a turboprop aircraft to a value other than 1.0 for reasons not related to SFC. Proceed as before but multiply and divide pre SFC factor parameters by the difference between default SFC and prototype SFC.

When altering existing turboprop FDE calculate power_scalar * thrust_scalar. The answer after the SFC change must be the same as before. For a turboprop it need not be exactly 1.

Three caveats;

1) If you make gross changes to the scalars it is possible that in certain regimes of flight you may force MSFS to try to read beyond the end of the prop efficiency tables. In these circumstances it may not be able to calculate an appropriate airscrew efficiency and the FDE will alter. If you understand sections 510-512 of the air file you may be able to rewrite the data to resolve this. Unless you are prepared to undertake this task it is necessary to recognise that there are undefined boundaries beyond which SFC cannot deviate from the default without imposing errors in the thrust equation. These boundaries vary from aircraft to aircraft depending to a large extent, but not solely, on the prop data in sections 510-512 of the air file. If you stick to real world SFCs you will probably never encounter the boundaries.

2) Using the scalars really does alter the horse power data output. Remember this when using a test panel prior to FDE upload. The required reading on the test panel is real world horse power * power scalar. The thrust gauge on the test panel should change only slightly whatever the SFC, (see above re c/s props in FS2000).

3) I believe there have been real world aircraft with (equivalent / shaft) horse power gauges on their panel but AFAIK these do not exist in MSFS except as test gauges. So long as MSFS (prop) panels have only gauges to enable power management by boost, rpm, or fuel flow the user will not be aware that the program is outputting scaled horse power which are rescaled to give the correct thrust. In the end it is always thrust not power which opposes drag.

Aircraft in which power management is by reference to fuel flow will of course benefit most from application of this tutorial.

Finally I recommend that you explain your aircraft.cfg entries to users so that the aircraft.cfg might read;

;.............................................

;Power and thrust scalars reduce default FS2000 fuel consumption by 5%

[piston_engine] power_scalar = 0.9524

[propeller] thrust_scalar = 1.05

;...............................................

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