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Thread: Cross over vent between independently vented tanks. Yes? No? Why?

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    Cross over vent between independently vented tanks. Yes? No? Why?

    Clean sheet fuel system design. Each tank has a Cessna style vent. L-R-Both valve, no header tanks. Do you add a cross over vent? Why or why not?

    What if each tank also has a vented cap, does that change the answer?

  2. #2
    Gordon Misch's Avatar
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    I've thought about that often, and the only reason I've come up with is venting redundancy.

    Seems like a good reason, except if one tank's vent becomes inop it won't be apparent to the operator, so then there is no redundancy. Checking fuel tank vents at annual would minimize the likelihood of that scenario.
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    irishfield's Avatar
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    Yes!

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    Yes with an exclamation point follows why with a question mark?

    Why?????
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    irishfield's Avatar
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    To equalize the pressure in both tanks, to avoid uneven tank usage/flow when on both is the main reason. It's easy with dual vented tanks to create a pressure difference between tanks when flying even slightly uncoordinated. I've been flying mine for 23+ years now. Single fuel tank outlet per wing, vented caps, one ramp air tube for a bit of "push" and cross vented between the upper top of each tank. Have never had the engine cough and I do some pretty weird **** on floats when diving over tree lines into short lakes. Never a hiccup caused by fuel flow.
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  6. #6

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    You apparently didn't read the whole question.

    If both tanks have Cessna style vents AND vented caps, does the answer change? We're talking VENTS, not pressure tubes.
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  7. #7
    irishfield's Avatar
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    Sorry Stewart, been building home builts for 25 years just trying to help you. Carry on..

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    PerryB's Avatar
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    You apparently didn't read the whole answer. Wayne's right.
    After Monday and Tuesday, even the calendar says WTF !

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    Not an answer. I'm looking for discussion and hoping for information.

  10. #10
    Rob's Avatar
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    I'd like to hear more as well, as change in my fuel system is still possible. Anyone who has flown a larger Airtractor will remember the first time they flew without squaring away the rudder trim... The fuel vents are off the outboard end of the wings, and it leaves a spectacular trail off the dragging wing... Proof to the imbalance of pressure. But is that really a problem? I seem to remember the earlier 802's could flame out flying half cocked with low fuel on the advancing wing, but I don't think that had anything to do with crossovers or headers.

  11. #11
    irishfield's Avatar
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    "Cessna style" vents are ram air tubes! Hate when someone asks for help, then gets defensive and condescending when they don't like the reply.


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    Cross over vent between independently vented tanks. Yes? No? Why?

    No, they aren't. The specs for how the vents trail in the lee of the struts are very specific. I've owned Cessnas for the last 25 years. Been there, done that.
    Last edited by stewartb; 04-15-2018 at 11:17 PM.

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    The bigger question is why do we use crossovers? Because there's good reason or because that's how we've always seen it done? Standards change. Maybe crossovers are a good idea, maybe they aren't. The real answer is probably in between and depends on other conditions. I'd like to hear the evidence and make up my own mind.

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    I believe earlier Cessna had one hang down vent behind the pilot side strut, and non vented caps. Venting was through the left tank to the cross over to the right tank. AD requiring vented caps changed the whole basic setup, added secondary venting.
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    180Marty's Avatar
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    Looks to me like you could get by with no cross over since you may have two vents-----Cessna style and fuel cap. If you use a Atlee cap like I had on the PA12 that had the snorkel ram vent, then I'd say definitely no need for cross over. Some planes just have one tank----you'd have two individual that are connected at the valve.

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    PerryB's Avatar
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    As stated in post #5, the reason for the cross-over vent (redundancy notwithstanding) is to balance the static pressure in the tanks. This in turn causes them to feed at the same rate when on "both". In theory they should anyway, but in practice they might not. The location of a vent tube being off 1/4" from it's counterpart might cause a head pressure imbalance that skews fuel draw. The crossover tube solves this. With dual/independent vents in each tank you don't NEED a crossover.
    After Monday and Tuesday, even the calendar says WTF !
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  17. #17
    PerryB's Avatar
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    Quote Originally Posted by irishfield View Post
    "Cessna style" vents are ram air tubes! Hate when someone asks for help, then gets defensive and condescending when they don't like the reply.

    I'll bet money that the Cessna tube produces some degree of positive pressure. The put it behind the strut to limit the pressure, but I have no doubt its positive to some degree. Champion did the same thing on the Decathlon/Citabria/Scout family but I think they had a perforated cover over the tube.
    After Monday and Tuesday, even the calendar says WTF !
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    Are all airplanes with the "Both" valve equipped with crossovers? Mine have been, but mine have had single vents in each tank so I'm not sure whether the crossover's role is redundant venting or pressure equalization. My suspicion is it's about secondary venting. Many Cessnas have uneven fuel flow so the pressure equalization idea has a flaw. I read an article that suggests the crossover was to provide fuel expansion protection, to give the fuel a place to go without puffing up the tank. I'm not one to fill my tanks after every flight so that's not a problem. My kit has Cessna style tank vents and came with flush fillers and chrome plated killer caps. I had Atlee's make risers for the filler necks and needed caps. Steve instinctively handed me snorkel caps but I asked for standard vented caps since these aren't my primary vents. This is new territory. I've never had dual vents in each tank before.

    I agree Cessna vents must have some element of positive pressure. Negative pressure at a vent would be a problem.

    Here's my Cessna's vent from the service manual. Look at the allowable tolerance. +/- .03" is pretty specific.
    Attached Images Attached Images
    Last edited by stewartb; 04-16-2018 at 09:43 AM.

  19. #19
    irishfield's Avatar
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    Quote Originally Posted by PerryB View Post
    I'll bet money that the Cessna tube produces some degree of positive pressure. The put it behind the strut to limit the pressure, but I have no doubt its positive to some degree. Champion did the same thing on the Decathlon/Citabria/Scout family but I think they had a perforated cover over the tube.
    As you note Perry, of course a tube into the slip stream produces a bit of positive pressure. If it created negative pressure, in the original design with non vented caps, there would have been zero fuel flow in the system on a gravity flow engine without pump.

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    So, I am no expert on this, but have some recent experience. My exp cub is headerless, off-left-right-both. However I do not have a crossover line. I do have ram air caps. In certain flight/cruise conditions when on both and full fuel I found unequal pressure and if crabbing in the wind some fuel(minimal) is pushed out the cap of the right tank.

    My run run mostly on “both” and have never had a fuel starvation issue in any configuration....a while back I reversed the ram air cap on the left tank, and left the right tank point forward...tanks did not burn evenly, but had no issues over a 3.5 hour flight.

    Not sure how to advance, or if the issue is a crossover. My first cub, a J5 had a crossover, ram air caps and also had uneven fuel burn.
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  21. #21
    Cub Builder's Avatar
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    It seems like no matter what you do the pressure between the tanks is never perfectly even, and they rarely feed at exactly the same rate when you have both tanks selected. FWIW, I use a single vent under the wing to each tank, with a single pick up at the back in each tank feeding a small header tank located behind the baggage bay. Both tanks feed all the time and there is no crossover vent. My tanks always feed a bit unevenly, but they feed as evenly (or more evenly) as compared to the average Cessna that does have a crossover. The pressure differential is so low as compared to the weight of the fuel, that it really doesn't make any difference. Both tanks will draw down to as low as I am willing to fly them. Since I can get 100% of the fuel without loosing fuel flow, and the differential in feed rate from the tanks is low enough that I can't feel it and it doesn't unbalance the plane, I guess it doesn't really matter for me.

    If you add a crossover vent, it will typically have to dip down below the top of the tank level to cross over to the other tank. So, the first time you top off a tank, that crossover vent will fill with fuel that will remain in the low spot of the line. With both tanks vented to the outside world, you will never see enough pressure to push the fuel up hill out of the crossover line to clear the crossover vent line.

    -Cub Builder
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  22. #22
    www.SkupTech.com mike mcs repair's Avatar
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    one thing to consider is the cessna caps are vented, but with a valve that only lets air in... i'm thinking those red ones... we used to check them at inspection to make sure they worked

  23. #23
    hotrod180's Avatar
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    Quote Originally Posted by eskimo77 View Post
    I believe earlier Cessna had one hang down vent behind the pilot side strut, and non vented caps. Venting was through the left tank to the cross over to the right tank. AD requiring vented caps changed the whole basic setup, added secondary venting.
    This was the case with the later model Cessnas.
    Earlier models, like my 1948 170 & my 1953 180,
    had a "snorkel" vent up on top of the cabin, tied into the crossover vent line.
    Plus my 180 has vented caps (the "red type") on both side.

    I understand the fuel vent was moved down to behind the strut on later models to eliminate icing over in icing conditions.
    Unfortunately there is an AD that requires this be done with the earlier models if you remove the LH fuel bladder.
    I lkike the vent where it is, IMHO this AD's a solution in search of a problem for a non-IFR airplane, but there you are.
    Cessna Skywagon-- accept no substitute!

  24. #24

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    IIRC, The later C-150 models had a single vent tube on the left side, and that vent was connected to the right tank. In addition, the left tank was vented into the right tank. That system equalized pressure between the two tanks. If the aircraft was not sitting level with full fuel, the venting design minimized fuel that would drain out of the tanks.

    If one built a system like the original poster asked about with both tanks vented, and with the addition of a cross over vent, I think he would discover a poor design. If the aircraft had full fuel and sat on an uneven surface, the higher wing tank would gravity feed a considerable amount of fuel over to the lower tank which would them pee it out onto the ground.

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