Fuel tank venting for dummies...:)

[Burgerilla]

This is a fascinating topic! FAA Part 25 aircraft (airliners) have faced this same problem with Jet A. But this is a "hydraulic" issue (in physics) and the type of fuel is irrelevant.

One solution we designed into the deHaviland Dash 8 Q400 that I worked with might spark (no pun intended) some thought: the outboard "wall" of each outboard-most wing fuel tank (these were integral tanks meaning the tank walls were formed by wing ribs and spars) had a sealed-off area that the fuel tanks all vented to. This "tank", located near each wingtip, was isolated from the regular fuel tanks which are filled by single-point refueling and is known as the "Overflow Tank". The highest point (on the ground) inside this Overflow Tank is vented to ambient at the wingtip (NACA vent). At the inboard, lower edge of this Overflow Tank are some small gaps in the bottom of the rib that allow any fuel that enters to flow back out through these gaps into the fuel tank. But fuel in the fuel tank was blocked from moving outboard into the "Overflow Tank" by rubber strips, called flapper valves, covering the outlet of these gaps.

Of course, Part 25 aircraft have a refuel shutoff that prevents overfilling the fuel tanks into the Overflow Tank, but there must be at least several different methods for us to fill our BH's without overfilling them (like positioning the filler such that the limited dihedral brings the level up into the neck prior to overfilling). We do not have integral fuel tanks or single-point refueling or automatic shutoffs, etc., but the idea could still work with a small (say one-half gallon capacity) cf tank located out near the wingtip with proper venting to ambient and drain lines with check valves returning to the fuel tank(s).

[Nev]

I’m considering whether to run a vent line between tanks and wanted to get thoughts on this idea as it pertains specifically to a Bearhawk with an electric fuel pump. Carbureted (high wing) engine designs typically don’t require an auxiliary pump, but fuel injected engines do. This opens up the possibility of “sucking” fuel down the lines given the right set of circumstances, and could make an unporting situation more eventful, cause a fuel imbalance, or lead to vapour lock. Also, we tend to use auxiliary pumps primarily at low altitudes (takeoff and landing), so any of these occurrences would be likely to happen at the worst possible time.

This was discussed a while back, but I want to open the discussion up again and get more thoughts. Incidentally, with the one or two forced landing incidents that occurred, was there ever a definitive csuse established?

[Gerhard Rieger]

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[sjt]

Is skinning starting from the root to the tip with each subsequent sheet being on top working out for you?
stan sjt p284

[Gerhard Rieger]

The wing skins was just hanged over the wing, started from the wing tip end. It worked out great.

[Gerhard Rieger]

Hi Nev. I made up my mind from the beginning that my IO540 will need a header tank, I added a , 2,2 usg header tank with two 1/2 inch vents each to a main tank, my aux tanks vent to the main tanks, if one vent is blocked I have another, and I added vents to the four tanks above the wing, I know it does not look that smooth, but its practical, I will have to park at a very steep angle before fuel will leak from the vents. The reason for all the vents is to assure that my header tank always stay full and if any bubbles do enter into it that it can easily vent back to the mains. The electric aux fuel pump is between my header tank and the engine. I also added a fuel drain to the lowest point of the header tank. I have one vent on the forward facing side of the header tank and the other on the opposite rear of the tank. This will cause a high point in the header tank through which air can escape in all attitudes of flight all the time.

[Battson]

I've been operating for almost a decade now with an IO-540 and a standard "Bob fuel system" apart from the extra EFII pump and filter. No venting, no header, no extra fuel drains, no single tank selection (always running both tanks). We regularly fly with minimum legal reserves in the circuit, for STOL purposes. This means that in unbalanced turns and slips one tank is certainly unporting from time to time.

We never had an issue with continuous fuel supply to the engine.

I think the type of pump is an important consideration here. Some pumps will suck air or fuel some pumps will only suck air if primed with fuel first, some pumps will not suck air at all.

[JoHo]

Don Hall who had a career designing fuel systems for large aircraft and is now involved with small EABs has boiled down his advice on vents an such into an Oshkosh presentation. This was one of the more valuable hours I spent at the show this year, the first side of his hit list is shown below. additionally, I forgot the name but the owner of the Patrol on display said that he tested deliberately un-porting a tank to see if the pump would suck air and it did not.

Based on these inputs, I pretty much abandoned plans to add cross-venting the tanks and adding a header tank.

Another topic I discussed with Don is the issue of return fuel. His strong recommendation is to use a duplex fuel valve but since the BH fuel valve is specifically made to fit in a particular spot, I want to just return it to the right tank and operate the plane accordingly. In his opinion, this is limited to a pilot workload and fool-proofing issue and a reasonable concession to keeping the project moving.

As many of you know, I am just starting out in this world but for the sake of sharing, that's where I'm at!

[Nev]

Excellent, thanks.

[svyolo]

I am using EFI and was originally going to use a duplex valve and return fuel to the same tank. I added fittings to the top of the inboard side of both main wing tanks, which turned out to be the wrong place for a return line. For different reasons, I chose not to return fuel to the main tanks (probably requires 1/2 inch fuel lines), I am going to use those fittings to vent the two tanks together, and vent the small header or collector tank to a T in that line that I am using to return fuel to. I think the best is what AK did and vent both tanks together at the outside of the tanks, but with only 1 degree of dihedral I don't think it is a big deal. I couldn't figure out how to do it on the outboard edge without making maintenance and service a nightmare.

[PaulSA]

I was concerned about the "sucking air" issue with a pumped system should a feed become unported. After some discussion with my AP, I realised that it was not an issue. You don't empty an unported feed because the pipe will be gravity fed and topped up from the other 3 ports if you are on "BOTH" or the other port in the same tank if you are on "LEFT" or "RIGHT".

Just think about it. Took me a little while to see it ......

[Bcone1381]

Thanks Paul! I had never considered that and it alters how I envision things happening.

[Nev]

You don't empty an unported feed because the pipe will be gravity fed and topped up from the other 3 ports if you are on "BOTH" or the other port in the same tank if you are on "LEFT" or "RIGHT".

That’s the way it should work if everything is working normally. I think the issue a number of aircraft have had over the years is when Both is selected, and one tank feeds at a faster rate than the other (for some reason). The tank that is hesitant to feed becomes the fuller tank. If the opposite side becomes unported then the remaining side may not feed properly, and may prefer to suck air if available. This could also occur in a side slip when the “uphill” tank is empty. Shouldn’t normally happen, but there were enough unexplained engine failures on Cessnas in the 1960’s that they spent quite some time trying to find a solution.

Edit: My comment about sucking air isn't correct. The system is gravity fed, so as long as any fuel line is flooded, the pump inlet should be flooded.

[Mark Goldberg]

Or when flying pay attention to fuel levels in the tanks and try to keep them more or less level. Mark

[BravoGolf]

Not to be flippant why not fly without using the both position.

[jaredyates]

Flying on both, when the system works well on both, eliminates a pilot task and thus a possible pilot error that could lead to undeaireable outcomes.

[svyolo]

If you are running "both" and getting an imbalance, something is wrong. The solution might not be going L or R, but figuring out why it is happening. This is a painful discussion of simplicity versus complexity in the effort to get more redundancy. My local DAR wants more simple, which I like. But he wants a single feed line from each tank, instead of two, fore and aft. I like Bobs fore and aft feed lines..

But L or R on Bobs' fuel system doesn't solve the problem of suction feed on multiple fuel lines. The fuel valve is maybe 20 inches from a T fed by two fuel lines. If our fuel system can't feed that from gravity, it will be suction feed, which might end up sucking air instead of fuel. EFI or Continental FI might cause suction feed if you use 3/8 inch lines.

[Mark Goldberg]

It is common in the small planes to drain one tank more than another while running on a BOTH setting. Maybe you are flying slightly in a slip or skid. So little you do not even notice it but the tanks drain unevenly. Mark

[svyolo]

I can see that but I would think if you flew a long flight with the same slip them one tank would drain faster until they reached some equilibrium, then stay at that imbalance and drain down together. If you flew like that all the time I think it would seem normal.

[PaulSA]

Even if you empty one tank on BOTH, you still shouldn't suck air. The feed pipes on the empty side will still remain full due to gravity cross-feed through the selector. Unlike a low wing, which is why you switch LEFT/RIGHT and only feed one tank at a time.

[svyolo]

I agree if you are only gravity feeding. If you are flowing more fuel than gravity alone can feed then there is some suction going on. It doesn't seem to be an issue with a Carb or Bendix style. I don't know what the max fuel flow is for Conti engines, but my EFI is pumping 40-45 gph continuously. My solution was a small header tank and return fuel to that. Gravity only has to feed the fuel flow of the engine.

I don't know how much suction it would take to un-port a line. Too many variables to consider, at least for me. Bank angle, pitch angle, amount of suction...... too much for me to figure it out.

[PaulSA]

I guess that's an issue if you are returning to one main tank and the other one becomes unported. I have a standard injected style engine where the pump recirculates excess fuel. So if the engine is using 22gph at full power, it doesn't make any difference if it is a gravity fed carb or pumped injection - that's the rate it is coming out of the tanks.

So there are obvious advantages to more "modern" EFI systems but they aren't always 100% compatible with less "modern" fuel supply systems.....