Fuel Cross Feed

[Heretic]

I'm in dumb mode regarding fuel crossfeed. The theory is perfectly clear, but the physical effects are something of a mystery.

Assuming an aircraft with a left and right wing tank and a crossfeed line between them, I understand that said line connects the (normally isolated) fuel tank systems to each other to counter fuel imbalances.

In a basic setup with two liquid-filled containers, one of which contains more than the other. Once the containers are connected, the increased pressure of the fuller container will press the liquid into the one with less liquid until both contain equal pressure. So much, so easy. What I'm unsure about, however, is how exactly this happens in an aircraft.

Is the pressure difference between the left and right tank usually enough to permit near complete equalization in fuel pressures?
Imagining the two containers above, I consider them tall so that gravity does all the work and does it in a reasonable amount of time. On an aircraft, however, left and right wing tanks are usually more or less at the same level, especially when the wings have little dihedral. So I conclude that the process takes quite a while and will not provide perfectly equal pressures in the end(?).

Since aircraft fuel systems usually come with pumps and pipes, how much of a limiting factor is the flow rate of the fuel pump(?) in the tanks?
The fuel system diagram* of the aircraft in question (DC-9) shows bypass lines for each fuel pump. I assume that these bypass lines enable easier pressure equalization?

While writing this, I considered how to implement this. Something along the lines of Bernoulli's equation and mass flows and Bernoulli?
Since I'll be dealing with areas to calculate pressures, what's a good diameter for an aircraft crossfeed fuel line? 1 inch? 2 inch?


Sometimes it's the simplest things that you can't wrap your head around...and I'll hand my engineering degree in later.



* http://www.coolsky.no/images/storie...ght1-coolsky-mcphat-dc9-user-interface-06.jpg

 

[Vitus]

You have two tanks with a different fill level, connected by a tube of a given diameter. The pressure in the tube is the result of the different heights of the fill level and can be expressed with:

dP=p * g * dH

with dP: pressure differential
p: density of the medium
g: gravity
dH: height delta

Once you worked out the pressure in the tube, you can calculate the dynamic flow. hat can be done by using Hagen–Poiseuille:

Q=(pi * r^4 * dP) / (8 * eta * l)

with Q: flow rate
r: radius of the pipe
dP: the pressure differential
eta: dynamic viscosity
l: length of the tube

The dynamic viscosity is a bit tricky, because you need to estimate the Reynolds number and get the viscosity for Avgas. I use 6.04*10exp-4 Pa/s for eta.


I use the same equation in reverse to calculate the pressure at different points in the fuel system at a given flow rate.
Hope it helps!

 

[hairyspin]

You don't want too large a pipe if the system is gravity fed, unless your pilot always keeps the ball dead centre on every turn.

 

[EduHir]

you can't get any fuel from left tank into the right tank or vice versa, because the check values are one direction valves!!
So if you are unbalanced you have to play with the fuel pumps.
Left tank has more fuel, so open the crossfeed valve and switch off the right tank pumps, until you are balanced.
The system is similar to the B727 fuel system.
Also the Center pumps are overwriting the wing pumps.
In my B727 fuel panel it works this way. But of course i used in FS9 dsd_fueldump and in FSX xmltools.
But DC10-30 for example has it owns transferpumps and transfer lines.

 

[taguilo]

Assuming an aircraft with a left and right wing tank and a crossfeed line between them, I understand that said line connects the (normally isolated) fuel tank systems to each other to counter fuel imbalances.

Actually that is not the crossfeed concept.
Crossfeed lines do not connect fuel tanks but rather engine of one side with a tank on the opposite. In a crossfeed there is no transfer of fuel between tanks, that is another process available to a few group of aircraft.

Every crossfeed system needs a crossfeed valve and a standby fuel pump active on the opposite side, which provides the necesary pressure for the fuel to flow . Gravity is not involved.

Tom

 

[WarpD]

The Citation X has a gravity crossfeed... actually. It provides both pumped and gravity.

 

[taguilo]

The Citation X has a gravity crossfeed... actually. It provides both pumped and gravity.

That is not crossfeed but rather crossflow of fuel between left and right tanks. And yes, gravity is involved there.

As I stated before, crossflow, or fuel transfer between tanks is common in some aircraft models.

Tom

 

[n4gix]

In the simplest and most common situation, the crossfeed simply connects one engine to the opposite engine's fuel tank. One would leave crossfeed enabled until the fuel in the active tank is roughly equal to the fuel remaining in the inactive tank. At that point normal fuel flow is re-enabled.

 

[Roy Holmes]

Couple of points:
Fuel feed lines are usually pressurized to some extent to prevent vaporization creating air locks at high altitude and low ambient pressure. Gravity maybe used for fuel transfer but it basically just helps the process and provides an emergency supply.
Multi-engined airplanes usually are fed by dedicated tanks and in the event of engine failures the failed engines tanks can be used to transfer fuel to the remaining good engines tanks.
As in all things aviation there are unusual exceptions to these points.
Roy

 

[Heretic]

Thanks for the input.

As linked in the first post, this is the fuel system of a DC-9:

As you can see, there's no transfer pump. And assuming that the APU and start pump aren't running, that all pumps deliver the same output and that both engines consume the exact same amount of fuel, it leaves nothing but gravity.
But I'm noting only now that the tanks are connected with check valves, so the left won't fill at all if the greater pressure from the fuel in the right tank closes the valve. Will be definitely easier to implement.


Does anybody know what exactly "CROSS_FEED_TOGGLE" "FUEL_TANK_SELECTOR_ISOLATE" do? Couldn't I just switch the right fuel selector to the left tank instead?

 

[ddawson]

I'm fairly certain that FUEL_TANK_SELECTOR_ISOLATE will isolate the engine from the fuel system. Useful in the event of an engine fire or other unscheduled in-flight shut down.
Not exactly sure about CROSS_FEED_TOGGLE, I can only suggest a little experimentation.

 

[taguilo]

Bjoern,

In your example, crossfeed needs (Left to Right)

Crossfeed Valve Open
Left Boost Pumps (FWD/AFT) on
Right Boost Pums (FWD/AFT) off.

And that's it. Fuel pressure from left side feeds both fuel lines for left and right engine. No gravity.

Tom

 

[Heretic]

I'm fairly certain that FUEL_TANK_SELECTOR_ISOLATE will isolate the engine from the fuel system. Useful in the event of an engine fire or other unscheduled in-flight shut down.
Not exactly sure about CROSS_FEED_TOGGLE, I can only suggest a little experimentation.

Thanks. Honestly enough, I was hoping to get around the experimentation phase (acute laziness disease).

In your example, crossfeed needs (Left to Right)

Crossfeed Valve Open
Left Boost Pumps (FWD/AFT) on
Right Boost Pums (FWD/AFT) off.

And that's it. Fuel pressure from left side feeds both fuel lines for left and right engine. No gravity.

Well, since the MD-80's fuel system is basically the DC-9's, some quick research confirms this.

And it's easy enough to implement (I think).

 

[Heretic]

FYI:
(K:CROSS_FEED_TOGGLE)/(A:FUEL CROSS FEED, bool) and then selecting a suitable tank works like a charm. As does the fuel pump driven tank selection. One step closer to realism for the DC-9.