Fuel Flow Discussion, Moved from Float Mounting

[Archer39J]

Oh and to add, an additional vent on each tank would help mitigate this possibility, but I'm not going to be installing one. I'll be going with a check valve to prevent fuel stoppage if the vented caps get plugged, ice being my concern, McMaster has some I'm looking into. The check valve won't open at the pressures we've been discussing.

[Gerhard Rieger]

I have decided to use left and right and fit a two-gallon header tank, I have 3/8 fuel lines and 1/2 back from the header tank to the mains. IO540 C4B5. electrical and mechanical fuelpumps.

[JimParker256]

Rollie, I think if you put the cross-over vent at the highest point of the tank, and as far outboard as you can get it, you could avoid the "slope-transfer" effect. The fuel would naturally gravitate toward the inboard side of the uphill tank, thus away from the uphill vent. On the other hand, using a vent on the inboard side would have the opposite effect.
The Commander 114 that I used to have was a fuel-transferring demon... If you didn't place the fuel valve in the OFF position, it would transfer fuel until the tanks found a "level" point. Unfortunately, the nice, squishy trailing-link landing gear (which made for fantastic landings) allowed the now-heavier wing to sag even further as the fuel transferred. If left unattended on a slope with the fuel valve anywhere but OFF, it would transfer almost the entire 35 gallons in the uphill wing to the lower wing, which would then run out through the overflow vent. Many new Commander owners came back to their birds to find their newly-topped-off aircraft sitting in a puddle of 35 gallons of AvGas. An expensive mistake you only make once!

[Archer39J]

It's looking to me like in a property constructed gravity-fed Bob system a cross vent is unnecessary because the worst-case pressure differential isn't enough to be an issue. For engine stoppage at least, you may think it's a problem it's possible to dump fuel like this. Assuming a 100kt ram air pressure of about 0.23 psi, that means at most the pressure differential between the two tanks would be on the order of just under 0.5psi.

The fuel pressure head of a minimum fuel tank is on the order of 1.2psi (aft tank outlet to selector valve). So while fuel will be pushed from the tank with positive pressure to the one with the vent issue (and you'll dump fuel overboard if your tanks are full) the air won't reach the selector valve, and this is worst case when your tanks are effectively empty. Now in systems that don't have that pressure head this can absolutely cause a problem, but for us it doesn't look like this is too much of an issue. Even less so with more fuel and slower speeds.

This is of course for a gravity-fed system, if your moving massive quantities of fuel and your pump is sucking then I wish you well with your analysis.

[Rollie]

I think I might be starting to grasp this concept. I really want to understand it because I routinely fly down to about 10 gallons on board. And I always select both tanks.

After reading everything here, this is where I'm at: At first I thought that if there is enough pressure diff for one tank to suck the fuel out of the other and basically override the fuel pump and gravity, wouldn't it be enough to stop the fuel flow out of that tank even if the selector valve was on that tank instead of both? Maybe not. With the vent partially clogged on say the left tank, as the fuel level decreases, the air pressure in the left tank will decrease, maybe not to the point that it would stop the fuel from draining out of the tank but when you add in the right tank with the vent clear and oriented forward, the airflow might be partially pressurizing that tank. So now we have a differential. Fluids take the path of least resistance. (the one thing I understand about fluid dynamics) Usually that path is down from our tanks and into the engine, but in a rare case of the one tank being pressurized enough to be pushing fuel out and the other tank not having any significant pressure, the path of least resistance could become going down from the tank with the properly functioning tank, through the selector valve and up into the other tank. Once the fuel is flowing that direction, it would probably keep going that way and bypass the engine.

That is what I got from this discussion. I might be way off, I'm actually looking for clarification to see if I got it, I'm definitely not trying to say this is what happened.

Now I am also curious if check valves in the fuel lines just upstream from the selector valve would have the same result as a cross over vent line. It would prevent fuel from transferring so it seems like it would only be able to go to the engine where it belongs. I am not planning to add a cross over vent line because I park on slopes frequently requiring me to select L or R tank to prevent fuel from cross flowing and overflowing the downhill tank. I'm pretty sure if I had a cross vent the fuel would transfer through that in that situation.

[Nev]

My understanding at this stage is that the cross tank venting is added for two reasons:

1. Provide alternative venting in the case of a tank vent blockage.
2. Aid dispersion in the case of a fuel line vapor lock.

If the tank vents are operational, it could perhaps be a vapor lock issue that is only presenting itself very occasionally in the right conditions.

If certain geographical areas are providing the right conditions of high ground temp, low temp at altitude, low pressure (individual fuel system intricacies) etc then it may only manifest itself in specific countries/states, and during certain times of the year.

Just another 2c worth in what is a very interesting discussion.

[Ed.Meyer]

I was thinking the same thing. You said it very succinctly.

One additional thought that came to me is this: if there is significant difference in air volume in the tanks, like when one is near empty and the other is not, and there is altitude change then the tank with more air will have to vent more, either air coming in or going out, depending on increased or decreased altitude. This could create some imbalance until venting is complete. I have no idea how much this would or could be.

[JJohnston]

One thing that hasn't been adequately explained is how, without a malfunctioning tank vent, such a pressure differential arises. Technically, if both tank vents are working, the tanks ARE cross-vented - via the atmosphere - even if that doesn't satisfy the reg.

[Archer39J]

Of course not, and none of these "regs" apply to EAB, so there's no point in even discussing this. I just noted it exists, that'll teach me...

[zkelley2]

I mean, a POH for an experimental aircraft isn't what sets any rules. They're for your own info only, or more likely the next owner.
As far as on the certified planes, you can always land overweight. It usually triggers an inspection.
At work we generally talk about landing overweight after takeoff every flight. Not normal ops, but if you're in the mindset of dumping fuel, you're in an emergency and none of it matters.

[Archer39J]

In principle I agree. Though I recall some POHs prohibit this.

[zkelley2]

b(5) doesn't require you to jettison fuel unless you cannot make a safe landing over max landing weight. Which we know isn't the case.

[whee]

As noted above, the fuel system in the plane I flew was build according to Bob's specs. The Cub caps were the only difference. I do think Bob's caps make this failure less likely when properly installed because there orings seal well and aren't likely to leak. Replacement of the cork gasket on the Cub caps is almost a preventative maintenance item.

[whee]

🤦 good catch.

[Battson]

FWIW I flew 942vt for 500 hrs always on both. Lots of hard slips into tight places, steep banking turns down the valleys etc. Never a problem but it is carbureted so add that to the equation.

I hear that.

I have done 750hrs of the same in NJB, we are ALWAYS running on both tanks. We are often flying with little more than 30 minutes fuel and doing aggressive slips, steep turns, etc. We have bendix fuel injection.

We do always keep our tanks pretty well balanced though, I don't give it full power flying out of balance either.

Never had a problem with our tank vents either. I've flown for 45 minutes with a fuel cap off too, like STOL flying, steep turns, patterns, long cruises at high altitude - all sorts. We hardly lost any fuel at all. Flew back and recovered the misplaced cap....

I think this failure is a very remote possibility.