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martinlest2

Why do fuel tanks empty consecutively?

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I've noticed in a number of aircraft that the fuel tanks, rather than emptying concurrently, do so one after the other.. so of course you end up with one heavy tank and the others empty. The a/c is then not balanced, needless to say... My default b737-400 does this, for instance.

 

Opening crossfeed valves doesn't help (is it just a dummy switch?).

 

No way to fix it I suppose???

 

Thanks.

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Fuel in the center tank makes the aircraft noseheavy. Fuel in the wings do not affect the center of gravity that much. When the center of gravity  is forward because of fuel in the centertank you will need to use trim to get the center of gravity in the desired position. This increases the fuelburn. So you really want to fill  the centertanks last ( and use first).  Contrary to what you say the aircraft is better balanced when the tanks are used one after another in the correct sequence

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Martin

 

As has been explained most aircraft will use the centre tank first. This is achieved usually by having all fuel pumps on, but because the centre tank pumps are more powerful than the wing tank pumps, fuel is used from the centre tank first. Any good addon aircraft will simulate this, but the default 737 in FS9 does not and will take fuel from all three tanks evenly.

 

If your "imbalance" refers to left/right fuel difference then your default 737 is taking fuel from one wing tank only. This is usually caused by loading the 737 after one of the cessna or mooney aircraft which have click spots to select left or right tank. Save the cessna as the default flight with the fuel tank selected to "Both".

 

Set up correctly, the crossfeed switch in the default 737 will work, although it is not accurately modelled. Turning the crossfeed to "open" should have no effect on fuel imbalance unless the pumps in one of the tanks (L or R) are turned off.

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'cause that's how real airplanes work
the aircraft is better balanced when the tanks are used one after another in the correct sequence

 

The 'correct sequence' is the key of course...  You are saying that what should happen is that all the right-hand fuel tanks empty before any of the left-hand tanks are touched? (If you were right, and that were the way real a/c work, then most of my FS aircraft have a serious problem, as the tanks empty in a correct controlled sequence - Concorde is an extreme example).

 

Yes, I know that the centre tank will empty first (I should have said that), but then the right auxiliary and after that the right fuel tank(s) empty completely, leaving all of the left tanks full (or however full they were at take-off). The left fuel tanks only start emptying when all the right ones are spent. If you look at the balance of the a/c in the FS Menu, it's very over-heavy on the left side (obviously!). The weight of fuel is massive of course: in the real world it's important the aircraft is balanced laterally - think of how carefully cargo is stored in this respect. Well most of you are well aware of that of course...

 

If your "imbalance" refers to left/right fuel difference

 

Yes, I didn't make that clear enough. Sorry.

 

this is usually caused by loading the 737 after one of the cessna or mooney aircraft which have click spots to select left or right tank. Save the cessna as the default flight with the fuel tank selected to "Both".

 

Yes, the way I load FS9 these days, it always starts with the default Cessna, then I load my chosen a/c (prevents all kinds of issues with some complex payware addons). I will try to adjust things as you suggest - though not sure quite what you mean about saving the flight until I go to look in the sim itself. Thanks.

 

Turning the crossfeed to "open" should have no effect on fuel imbalance unless the pumps in one of the tanks (L or R) are turned off.

 

Need to check whether I had fuel pumps off - I think I neglected to do that!

 

M.

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Need to check whether I had fuel pumps off - I think I neglected to do that!

 

M.

 

I don't think you can do that in the default 737.

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Ok, that's not a problem. I'll try setting up the default flight now as you suggest Peter and see how it goes.

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It really depends on what a/c you are flying and whether the developer has paid any attention to fuel flow. Fs9 has a default fuel flow and start up availability. The default value is to draw fuel from the auxilliary tanks first before the main tanks.

FUEL_SELECTOR_LEFT Turns selector 1 to LEFT position (burns from tip then aux then main).
FUEL_SELECTOR_RIGHT Turns selector 1 to RIGHT position (burns from tip then aux then main)

Fs9 always initialises with "ALL" which in itself can be a problem for a developer wanting to model fuel flow. A good developer will do away with the different tanks and have just one but sub-divided. That way the developer's coding has full control at all times. For example the Super VC10 model has only one tank in each wing plus two centre tanks. The wing tanks are subdivided by gauges to mimic the real aircraft and therefore the fuelflow is proportionate from each imaginary tank. This models the real a/c extremely closely as that has proportionate tanks. It also allows for correct fuel management by pumping fuel from the centre to the wings either simulaneously or one wing at a time. And a bonus is that fuel jettison can also be correctly modelled. Another bonus with this method is that refuelling can be easily modelled too. The Super VC10 model cannot take fuel by simply entering a number. It "has to be" and can only be uplifted from an underwing point.

 

When you are flying jet transports always once airborne pump fuel from the centre to the wings as the wings need the extra weight to relieve bending stress.

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I love the JustFlight Concorde - it models the complex fuel tank procedures perfectly in auto mode (as evidenced if you fly following the real-world Concorde flight cockpit DVD that (I think) came with the FS9 disc. You can also switch to manual control, but you'd better be prepared to concentrate as Engineer!

 

I guess I shouldn't expect too much of the default FS9 aircraft (I don't fly them that often, but with addon cockpits, they can make a change from say PMDG, when you don't want to spend quite so much time in flight preparation). That said, I am flying the FS9 747-400 at the moment (with a much nicer addon panel), and it is emptying the fuel tanks as I would expect, left and right together and the a/c is still nicely balanced after a long flight. Not sure why the 737-400 doesn't do that (which is really why I posted the question, as I thought I had something set up incorrectly).

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A good developer will do away with the different tanks and have just one but sub-divided. That way the developer's coding has full control at all times.

 

My DC-8 has 8 tanks and 4 fuel selectors. I can transfer fuel from any tank to any tank, crossfeed etc etc. A mammoth task but it can be done.

 

The 8 separate tanks are placed where they should be on the aircraft and of course affect the balance of the aircraft as they empty. If they were 1 tank subdivided ...I can't see how say a wing tank imbalance would adversely affect the FDE.

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Your trim should be able to cope with any inbalance but there's no point in deliberately leavign the a/c untrimmed for landing.

With the Super VC10 there is also a 5000kg on top of the fin. Fuel has to be transferred fro mthat at least an hour before landing as one is allowed only 50kg max in the fin tank for landings.
http://www.vc10.net/History/incidents_and_accidents.html#XR806%20Brize%20Norton%2018%20December%201997

Scroll down to XR806. The FE forgot to defule the fin tank before the wings.

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Your trim should be able to cope with any inbalance

 

I think you are confused. Trim won't help with lateral (left to right) imbalance  :wink:

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I think you are confused. Trim won't help with lateral (left to right) imbalance  :wink:

It can, to a certain degree IF you use aileron trim (& if the sim aircraft models it). I'm not sure how much l/r imbalance aileron trim is good for, but again that would vary from plane to plane.

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I think you are confused. Trim won't help with lateral (left to right) imbalance  :wink:

Aileron trim!

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Aileron trim can offset L/R fuel imbalance but most aircraft have a maximum differential that the manufacturer recommends, in the case of the 737 I think it is as low as 1,000 lbs.

 

Of course there is one aircraft that regularly crossed the atlantic at M 2.0 that would deliberately create a L/R imbalance so that excessive crosswind could be dealt with without the need for drag inducing elevon (it didn't have ailerons) trim.

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If cargo has shifted in the hold then I agree, but pilots don't use aileron trim to correct lateral fuel weight imbalance (except in emergency), they use the crossfeeds to balance the weight of the fuel more or less evenly between the right and left side tanks...

 

Of course there is one aircraft that regularly crossed the atlantic at M 2.0 that would deliberately create a L/R imbalance so that excessive crosswind could be dealt with without the need for drag inducing elevon (it didn't have ailerons) trim.

Indeed! I remember its first commercial UK flight - it flew across my house (we lived near enough to Heathrow that we could hear the afterburner roar at take-off). Still very sadly missed (even if I could never afford to travel on it!!).

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Work is progressing a pace on the final part of the Super VC10 manuel. With a fully functioning fuel panel, underwing uplift and the PO will also advise you of how many minutes left for jettisoning. Plus a fuel calculating spreadsheet with authentic loadsheet. What more could you ask for £0.00!

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I hardly ever fly 'legacy aircraft', ones that are no longer in the skies (Concorde being a notable exception) but I should probably take a look at this plane. Though I am not really up for any steep learning curves right now! Where's the download link please?

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I hardly ever fly 'legacy aircraft', ones that are no longer in the skies (Concorde being a notable exception) but I should probably take a look at this plane. Though I am not really up for any steep learning curves right now! Where's the download link please?

It's not as complex as Concorde. The main difference between it and modern a/c is that "you" are the FMC!

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Hi.

 

Anyone here fly Captain Sim's C-130? I've noticed it has a life-saving fuel flow bug where if the centre crossfeed valve is switched on, each engine takes fuel from the un-emptiable and supposedly unusable 'flow' tank (presumably the fuel in the virtual pipework). When I'm reaching the point of running on fumes, turn the centre crossfeed on, turn off each fuel pump and fly for ever...

 

(Handy for empirically determining V speeds as the plane's weight doesn't change).

 

Best regards,

D

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Fuel in my A2A Piper Comanche 250 appears to be drawn from both tanks at the same time.

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Pumps are not all the same, engines are not all the same. So there will be differences, slight of course (or intended to be slight at least).

 

I'm surprised to hear that happens in any default aircraft. I guess the engines of the default 737 are "exactly" the same, its pumps (i don't think it models any pumps, but rather fuel flow) will be exactly the same for each engine and so on.

 

Interesting thread, thanks for your comments vololiberista!

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Thanks for the comment Mitch!

 

 

 


No aviation equipment that I know of operates exactly 100% the same.

 

Yeah well, but that's what I implied to say :)

 

When I said that the default 737 should be "perfectly" balanced, I referred to the simulator FSX or FS9 737

 

I'm sorry if I still don't understand your point :)

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All "generic default" a/c either in FS9/FSX or P3D work in exactly the same manner. Particularly jet a/c. The airfiles are pretty much identical when it comes to fuel and engine management. You might get a couple of instances where the number of tanks might differ but the way the sims control the fuel flow is the same for each a/c.

A good developer will either write their own fuel flow system in C++ or use Doug Dawson's fuel dump gauge (as I do). His gauge allows for genuine fuel uplift i.e. not just typing in a number representing fuel needed but actually filling the tanks in real time. With it I can transfer fuel between tanks at a specific rate even gravity feed. And of course dump fuel!

Unfortunately they exist those that don't bother and in effect make you pay for a default aircraft!

Does your engine surge on start-up? If so you have default fuel settings across the board.

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