Fuel Selector Aerobatics

[mariopilot]

Hi All,
I will try to be as concise as possible. My aircraft has 2 engines and 3 fuel tanks:
-Left
-Center
-Right

I have devised a monstrous XML orchestra that puts the fuel selectors of each engine according to tank boost pumps and X-Feed status (all custom L: variables). It works as intended.
In some configurations, an engine will feed from more than one tank simultaneously, e.g. pumps for L and C tanks on will feed the left engine from L and C tanks at the same time. I sorted this out by flipping the fuel selector from L to C every second automatically. On the other hand, if you activate all pumps and the X-Feed, engine feeds from every tank, and this is accomplished with ALL state of fuel selector.
Is there a way I can feed from LEFT and CENTER (or any other two) without having to constantly flip the selector?
(L+R is BOTH, no problem there. Complicated XML orchestra takes care of empty tanks to avoid unwanted starvation as well.... it calculates a "tank status" as 0 empty, 1 fueled, 2 pump on, and proceeds to compare tank status to determine what feeds what...)

 

[SierraB]

Is there a way I can feed from LEFT and CENTER (or any other two) without having to constantly flip the selector?

Yes, and the answer to life, universe and everything is XMLTools.

It allows you to write directly to the fuel tanks' quantities.

My first idea would be to...

• set the fuel selector to any one of the desired tanks
• read out the engines fuel flow
• devide this value by the number of fuel tanks used (the ones that are meant to feed the engine)
• use XMLTools to reduce the tanks' quantities by the calculated rate
• FSX would still draw the complete fuel flow from the single tank selected by the fuel selector, therefore you will need to add some negative fuel flow back to that tank (which is exactly the sum of the fuel flows, that you allocated to the other tanks)

Hope this is understandable.

 

[mariopilot]

Yes, and the answer to life, universe and everything is XMLTools.

The answer to that is 42.

Yep, cannot do it in FS-9. Gotta be compatible with both FS-9 and FS-X... sorry! I could use D.Dawson Fuel dump gauge, but wanted to make sure that non-fsuipc users had still access to the center tank... also adding and/or subtracting fuel from tanks is subject to rounding errors...

 

[SierraB]

The answer to that is 42.

No, XMLTools replaced that answer for me.

Is there any good reason not to use fsuipc in 2016? But yes, the rounding errors are said to be a problem in FS9 (not in FSX though).

At least I don't see a method to overcome FS systems without fsuipc or simconnect. Maybe someone else can help.

 

[Roy Holmes]

I have used your method with both single engined (only one fuel selector) and twin engined (one selector to each engine) jets.

With the single engine I fed some of the center tank first (required for CG reasons) then fed outboard drop tanks (left and right), inboard drop tanks (left and right) internal wing tanks (left and right) and finally the center. I emulated the Hunter fuel balance system where the system feeds equally from left and right to avoid assymetric loads. The method I used was to calculate the total weight of left and right wing fuel and feed from left until there was a significant (though small) imbalance then feed from right and so on. I fed each tank down to where it had a small amount of fuel remaining, then changed the selector to feed the next tank in sequence. Once the internal wing tanks were almost empty I selected ALL to finish off remaining wing fuel and feed the center last. That way I only changed the fuel selector when there was an imbalance or the tanks were almost empty. This was an automatic process with no pilot control.

With the twin jet and two selectors, I just fed some center fuselage fuel (CG reasons) then fed the centerline drop until almost empty with both selectors, then fed the left wing drop to the left engine and right wing drop to the right engine, internal wings then ALL on both to drain tanks. This was automatic but with manual intervention if desired. Actually, in the F-4, the fuel feed from drop tanks was manually controlled, but I found it simpler to do it automatically in the sim after a couple of flame outs.

You did not mention whether you were using two selectors, normal procedure is to do that and only crossfeed if one engine has died.
Roy

 

[mariopilot]

Thanks for answers! I see no reason at this point to change the current logic... I will leave the 1-second fuel selector oscillator in place so the fuel level odometers will seem to smoothly move together...
Normally the aircraft takes off with engines feeding from their respective wing tank. After take-off the center tank pumps can be engaged (manually). Each engine will draw from its side tank plus the center tank. Each engine has its own fuel selector. Each tank has 2 pumps. side pumps only feed their same side engine normally. center tank pumps feed one to left and one to right. Cross-feed is located after the pumps and a Y connection between the side pumps and their matching center pump. it can be used for fuel balancing or to feed left engine from right for example. fuel will flow through X-Feed duct if valve is open and pumps are asymmetrically configured (turn off left pumps for instance, and all engines will feed from right+center. One-way valves imply that you cannot use the system to pump fuel from one tank to the other... you have to relay on engine fuel flow should a fuel imbalance occur...
In the end the system was complicated to program in detail, but it is fairly straightforward to operate... this seems to be the case with all systems on this Dassault Mercure aircraft. All internal complication makes the pilot's life fairly simple..!