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FS2004 Fuel Flow Oddities

G

geoffco

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206
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unitedkingdom
Hi,

I have a couple of questions / observations:

  1. On engine startup, the fuel flow begins to rise immediately up to a value of about 50 PPH, even when the fuel valve is still commanded closed. Once ignition occurs (and the valve is open), it then rises very sharply to about 550 PPH. Why is there fuel flow before the valve is open??
  2. When taxiing, the fuel flow seems to be the same whether at idle or up to 40% N1. The thrust increases so surely there must be more fuel consumed?
  3. When reverse thrust is applied, the EGT, N1 and N2 all behave as one might expect. However, the fuel flow remains at the same state as in idle thrust! Surely that is not right? Is it a known little bug?

In all the above cases, I am referring to the 'default' FS2004 XML variables. I then have various modifiers in place for my own gauges. I just wondered whether there could be fixed with the .cfg or .air files rather than endless XML tweaking?

Geoff
 
If the fuel valve is closed then there should not be any fuel flowing and your engines will not start or they will shut down.
If you think you have the wrong fule flow for different throttle settings then check your airfile table 1505 which should look something like this:-
vololiberista
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Hmmm, mine looks a bit crappy then! All works fine but the started seems to 'pull' about 30 kg/hour without the valves open. I get more normal values - up to 300 in start - once the valves are open...

How to edit this though, assuming Y is N2 the first value is 64.93 and then the numbers go up all the way to nearly 200. Very odd! First value is 0, 0 which is good though! What kind of numbers should I be aiming for here?

Geoff
 

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It was also discussed at length here.


http://www.fsdeveloper.com/forum/showthread.php?t=389153

You need to set the fuel_flow_scalar to 1 in the a/c cfg.
My first vertical line is 23% n2 and my last is 105%n2.
You need to look carefully at your engine limitations schedule to establish how ling the engine takes to get to idle and also how long from idle to a high power setting when the throttle is slammed!
vololiberista
 
Last edited:
Ok, I have modified my 1505 table to look like the one in the example - hope it will improve my spool rates etc. However, what I now do not understand is how any of this relates to the actual amount of fuel used at any given throttle or N2..?
How do I modify the numbers to ensure that I get realistic values out of the (A:TURB ENG1 FUEL FLOW PPH, pounds per hour) variable...

I know, for example, that on the ground I should see:
330 kg/hr at N2=60
780 kg/hr at N2=77
2400 kg/hr at N2=90

In flight at cruise, it is much less - at N2=90 I should have around 1300kg/hr

This is rather important to get right because at the end of a flight, the fuel consumed needs to be realistic.

If I tried to put 'realistic' flue flow values in the 1505 table, my understanding is that the spooling times would be messed up! So, how to fix the fuel flow..? Or have I missed something obvious?

Geoff
 
Probably it will not be possible considering the way FS calculates fuel flow from net thrust unless you play with the relevant tables.

FS calculates instantaneous fuel consumption (PPH, per engine) as
PPH = TSFC * Fn (Fn: engine net thrust, lb)

However if PPH calculated this way is below a minimum value (often the case during taxi or idle descent), this minimum value is always used whatever the engine thrust is.
This minimum value (MinPPH) is
MinPPH = 0.065 * StaticThrust * TSFC (StaticThrust = static_thrust * thrust_scalar, both variables from TurbineEngineData section)

Hope it will clarify and help

Hervé
 
Right, now I get it (finally)! Actually, the fuel flow is now pretty good - it was already ok but I tweaked the thrust a little bit.
The startup and spooling is, however, now much better than it was. At least in a few quick ground tests, i now have much more realistic N1 and N2 values. I'll have to test now in some longer flights to check I didn't mess anything up...

Geoff
 
geoffco said:
Right, now I get it (finally)! Actually, the fuel flow is now pretty good - it was already ok but I tweaked the thrust a little bit.
The startup and spooling is, however, now much better than it was. At least in a few quick ground tests, i now have much more realistic N1 and N2 values. I'll have to test now in some longer flights to check I didn't mess anything up...

Geoff
Click to expand...

Make sure you do all your ground tests at an airfield close to sea level and ISA standard temperature. I did all my engine run tests at Shoreham.
vololiberista
 
Starting to look really good now. There are two more things I would like to tweak but I don't know how possible it is...

1) The N2 value at which the starter cuts out - currently this is about 60, but I want it to be more like 40 (the engine should continue to spool up to idle at 60).

) The N2 value at which ignition is commanded. FS seems to always have this at 22-24 but I think, from looking at a few videos, that I should have more like 30...

The idle values are pretty good so I don't really want to change them too much, although dropping the idle N2 value by about 2% would be good too.

Geoff
 
geoffco said:
Starting to look really good now. There are two more things I would like to tweak but I don't know how possible it is...

1) The N2 value at which the starter cuts out - currently this is about 60, but I want it to be more like 40 (the engine should continue to spool up to idle at 60).

) The N2 value at which ignition is commanded. FS seems to always have this at 22-24 but I think, from looking at a few videos, that I should have more like 30...

The idle values are pretty good so I don't really want to change them too much, although dropping the idle N2 value by about 2% would be good too.

Geoff
Click to expand...


1) if the starter is cut out, the start will hang. Fortunately, this can be overcome by reprogramming your starter gauge.

2) The autostart feature can't be changed. You would need to write your own starter code, and set up your tables accordingly.

3) change the CN2 vs Throttle table to reflect the correct CN2 at 0% throttle and 0 mach (for your given altitude and assumed temperature)
 
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