Fuel tanks filling up the main tank?

[Kartoffel]

I landed with about 20 percent fuel in the main tank and no other tanks but i didn't have time to get a refuel so I just got bags, but for some reason they did not fill up the main tank so I had to crash land :/

 

Does anyone know why this happened? Surely they should feed into the main tank before being sent to the engine? I wasn't in afterburner also...

[mvsgas]

I don't know how is it modeled in DCS and not sure how it works in RL. But on the F-16 in real life, if the wing tanks are empty, you will not be able to get fuel out of the external tanks and the engine can burn fuel faster that the external tanks can feed. Maybe something like this is the reason.

[Svend_Dellepude]

In game fuel should be transfered from external tanks into internal tanks as soon as you have gear up.

Did you remember to retract gear after takeoff?

[dallas48]

I saw in a video that ralfiedude made he was telling his buddy that external tanks cant keep up with the feed tank when youre in afterburner.

[Svend_Dellepude]

OP already stated that he wasn't in burner.

[Kartoffel]

I saw in a video that ralfiedude made he was telling his buddy that external tanks cant keep up with the feed tank when youre in afterburner.

 

I wasn't in afterburner for a long time (only like 20 seconds for take off)

[Kartoffel]

In game fuel should be transfered from external tanks into internal tanks as soon as you have gear up.

Did you remember to retract gear after takeoff?

 

I'm pretty sure I did

[Kartoffel]

[ATTACH]141392[/ATTACH]

 

As far as I can tell from that is it should be going from external tanks straight to the main tank and then to the engine for combustion, have I got it wrong?

[Svend_Dellepude]

Did both quit at the same time or one at a time?

[Kartoffel]

Did both quit at the same time or one at a time?

 

The left one first then after a few minutes the right one

[mvsgas]

As far as I can tell from that is it should be going from external tanks straight to the main tank and then to the engine for combustion, have I got it wrong?

The thing is I don't know how it is modeled. In the diagram, tank 2 and 3A feed the engines. Externals feed the wings, tank 1 and tank 2. If it is modeled like this, it could have been that tank 2 ran out before it could feed fuel to 3A or before it could get fuel from the wings, tank one and the externals.

Edited May 29, 2016 by mvsgas

[esb77]

FLOW RATE

 

Where the plumbing is routed doesn't mean much if the output rate of what is feeding the engine is greater than the input rate from the external tanks.

 

To know if there's a problem you first need to know the fuel consumption rates of the engines for the flight profile you flew, then you need to know the max rate that the external tanks can output, then you need to know all the internal flow rates once the fuel is in the plane's internal system.

 

Then you can look for problems.

 

To answer, "should I have had flameout due to lack of fuel," you'd also need to know the tank contents for your starting point.

 

 

 

Impossible to tell without the relevant documentation.

 

 

From an engineering standpoint though, there's no reason that you'd need the feed rate of the external tanks to be greater than the consumption rate of the engines at AB, Mil, or even cruise. As long as the externals get emptied before the plane reaches the point in a "normal" flight plan where the pilot might want to jettison them, then they have enough flow rate.

Flow rate has a cost in mass, power, volume, and money. So in a lot of ways aiming for the minimum practical flow rate is optimal. A military jet is probably more likely to have excess flow rate capacity than a civilian design, but you shouldn't automatically assume that the excess capacity exists.

 

 

That's why this sort of thing is included in real aircraft manuals. It's stuff the pilot really needs to know before flying.

There'd be a chart or table where you could look up tank capacities and transfer rates, and calculate what engine throttle settings you could handle pulling from different tanks.

Edited May 29, 2016 by esb77

[Kartoffel]

The thing is I don't know how it is modeled. In the diagram, tank 2 and 3A feed the engines. Externals feed the wings, tank 1 and tank 2. If it is modeled like this, it could have been that tank 2 ran out before it could feed fuel to 3A or before it could get fuel from the wings, tank one and the externals.

 

True, we may never know for certain thought :cry:

[DarkFire]

The -1 (non-classified flight manual) for the F-15 is publicly available, though the only one I've been able to find is behind a paywall :(

[codefox]

http://www.avialogs.com/index.php/en/aircraft/usa/mcdonnelldouglas/f-15eagle/to-1f-15a-1-flight-manual-f-15a-b-c-d-aircraft-block-7-and-up.html

 

http://www.avialogs.com/index.php/en/aircraft/usa/mcdonnelldouglas/f-15eagle/to-1f-15a-1-preliminary-flight-manual-f-tf-15a-aircraft-block-7-and-up.html

 

Free to read but DL costs.

[GGTharos]

What you experienced was (to the best of my knowledge, and possibly subject to certain conditions) incorrect. After the gear handle is UP, the external tanks should be pressurized and feeding to any internal tanks that will accept the fuel.

Stopping the tanks from feeding is a manual action IIRC, which isn't available in our virtual aircraft anyway.

 

True, we may never know for certain thought :cry:

[DarkFire]

http://www.avialogs.com/index.php/en/aircraft/usa/mcdonnelldouglas/f-15eagle/to-1f-15a-1-flight-manual-f-15a-b-c-d-aircraft-block-7-and-up.html

 

http://www.avialogs.com/index.php/en/aircraft/usa/mcdonnelldouglas/f-15eagle/to-1f-15a-1-preliminary-flight-manual-f-tf-15a-aircraft-block-7-and-up.html

 

Free to read but DL costs.

 

Great stuff, thanks for posting the links :thumbup:

[mvsgas]

Did the tanks stop feeding or did Kartoffel ran out of fuel before the system had a chance to transfer the fuel?

[esb77]

So I spent some time looking at the fuel system diagram and the fuel system sections of the manuals codefox linked.

 

Based on that, I strongly suspect that operating burners for takeoff drew down the engine feed tanks below the minimum combined engine feed tank level, which is 900 lbs of fuel.

 

The external tanks feed into the internal wing tanks and internal tank 1. They do this with bleed air pressure that is not available until after gear are up. From those tanks transfer pumps feed the engine feed tanks. Because of a check valve, as long as the engine feed tank pumps are operating it's improbable that the externals could feed directly to tanks 2 and 3.

 

 

From the engine tanks, some of the fuel is sent to the engines, but some is diverted to the fuel system radiators after which it cycles back to the internal wing tanks.

 

So there was heavy draw from tanks 2 and 3, but the feed flows would have been split up into 3 to 7 streams (depending on what you want to count as feed) that had some detours to go through before arriving at tanks 2 and 3.

 

So operator error is a plausible explanation, as is a modelling error. Without knowing what the normal flow rates (or pump outputs and system head loss) are it's not really possible to tell for sure.

 

I'm guessing the F-15C module doesn't let you operate the fuel gauge selector? I don't fly it enough to know without checking.

 

The feed tank fuel levels would be the ones you'd really need to know. In general if the low fuel warning light comes on then it means that you're down to a 600 lb margin above the combined engine feed tank minimum. If you kept the burners on after that lit up, you probably drew down the engine feed tanks too much.

[GGTharos]

Shift-D

 

Anyway, I tested this a bit.

 

It turns out that the tanks are feeding correctly in all the cases that I tested (I tested with two bags), however, with afterburner they may not be able to keep up with demand once you get really fast.

 

What I did:

 

 

• Started at Batumi with 4000lbs fuel.
  
• Re-armed with two bags.
  
• At this point, I started the engines and checked external wing tanks - not feeding. This is correct (gear handle is DOWN, so they are not pressurized)
  
• I taxi'ed and burned some fuel to verify that fuel was only being consumed from internal tanks - this was correct.
  
• I took off in AB, and the wing tanks begun to feed when I pulled the gear up - this was correct.
  
• I observed that in AB, the internal tanks were not being filled or drained. I did not go beyond 300kts in AB, but fuel consumption will only increase with speed at low altitude, and the wing tanks were already merely keeping up with feeding the engines. probably correct.
  
• I came out of AB and leveled out. Observed wing tanks feeding, and internal wing tanks and tank 1 filling. This is correct.
  
• Flew until wing tanks were exchausted and jetted them.
  
• Repeated with landing to ensure there wasn't a landing bug.
  
• Repeated again but jettisoned tanks without draining them.
  

 

In conclusion, whatever Kartoffel experienced was the result of his own doing. Sorry bud :)

 

 

I'm guessing the F-15C module doesn't let you operate the fuel gauge selector? I don't fly it enough to know without checking.

 

The feed tank fuel levels would be the ones you'd really need to know. In general if the low fuel warning light comes on then it means that you're down to a 600 lb margin above the combined engine feed tank minimum. If you kept the burners on after that lit up, you probably drew down the engine feed tanks too much.

[Kartoffel]

Did the tanks stop feeding or did Kartoffel ran out of fuel before the system had a chance to transfer the fuel?

 

So I spent some time looking at the fuel system diagram and the fuel system sections of the manuals codefox linked.

 

Based on that, I strongly suspect that operating burners for takeoff drew down the engine feed tanks below the minimum combined engine feed tank level, which is 900 lbs of fuel.

 

The external tanks feed into the internal wing tanks and internal tank 1. They do this with bleed air pressure that is not available until after gear are up. From those tanks transfer pumps feed the engine feed tanks. Because of a check valve, as long as the engine feed tank pumps are operating it's improbable that the externals could feed directly to tanks 2 and 3.

 

 

From the engine tanks, some of the fuel is sent to the engines, but some is diverted to the fuel system radiators after which it cycles back to the internal wing tanks.

 

So there was heavy draw from tanks 2 and 3, but the feed flows would have been split up into 3 to 7 streams (depending on what you want to count as feed) that had some detours to go through before arriving at tanks 2 and 3.

 

So operator error is a plausible explanation, as is a modelling error. Without knowing what the normal flow rates (or pump outputs and system head loss) are it's not really possible to tell for sure.

 

I'm guessing the F-15C module doesn't let you operate the fuel gauge selector? I don't fly it enough to know without checking.

 

The feed tank fuel levels would be the ones you'd really need to know. In general if the low fuel warning light comes on then it means that you're down to a 600 lb margin above the combined engine feed tank minimum. If you kept the burners on after that lit up, you probably drew down the engine feed tanks too much.

 

I had approximately 40% fuel before takeoff and taxi.

[Kartoffel]

Shift-D

 

Anyway, I tested this a bit.

 

It turns out that the tanks are feeding correctly in all the cases that I tested (I tested with two bags), however, with afterburner they may not be able to keep up with demand once you get really fast.

 

What I did:

 

 

• Started at Batumi with 4000lbs fuel.
  
• Re-armed with two bags.
  
• At this point, I started the engines and checked external wing tanks - not feeding. This is correct (gear handle is DOWN, so they are not pressurized)
  
• I taxi'ed and burned some fuel to verify that fuel was only being consumed from internal tanks - this was correct.
  
• I took off in AB, and the wing tanks begun to feed when I pulled the gear up - this was correct.
  
• I observed that in AB, the internal tanks were not being filled or drained. I did not go beyond 300kts in AB, but fuel consumption will only increase with speed at low altitude, and the wing tanks were already merely keeping up with feeding the engines. probably correct.
  
• I came out of AB and leveled out. Observed wing tanks feeding, and internal wing tanks and tank 1 filling. This is correct.
  
• Flew until wing tanks were exchausted and jetted them.
  
• Repeated with landing to ensure there wasn't a landing bug.
  
• Repeated again but jettisoned tanks without draining them.
  

 

In conclusion, whatever Kartoffel experienced was the result of his own doing. Sorry bud :)

 

Oh :/

Thanks, but I was going pretty damn fast as I stayed in afterburner for a bit too long, I think the fastest was around 480 knots

[GGTharos]

I re-tested, but I was unable to reproduce ... so maybe you really did run into a bug or ... you forgot the landing gear down? Maybe left the refueling door open accidentally? (I didn't check that one - tanks might not feed if it's open but should start to feed if min fuel is indicated).

 

I took off with 1200lbs internal fuel in full AB, but the feed tanks were filling, even if very very slowly. Even if they were draining it would have been quite slow - and I had to get up to 500kts+ to get the trend to go negative.