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Thread: PA-18 fuel caps

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    PA-18 fuel caps

    I find myself needing new fuel caps for my stock 18. Univair or other?

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    www.SkupTech.com mike mcs repair's Avatar
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    I discovered Spruce has genuine Aeronca style fuel caps in both vented and non-vented types. Under $20 when I bought a pair as demos.

    Some Cubs need that little thing that looks like a pitot. CC puts them on the 180 hp Cubs. We need them on the J3 wing tanks - easy to make with copper tube.

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    Crash, Jr.'s Avatar
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    Quote Originally Posted by bob turner View Post
    Some Cubs need that little thing that looks like a pitot. CC puts them on the 180 hp Cubs. We need them on the J3 wing tanks - easy to make with copper tube.
    Okay, I've been curious about this: why do J3 wing tanks need a pressure cap to feed? Mine has a pressure cap and still takes ages to fill the nose tank back up. Is there something about the wing tanks that makes it so they don't just gravity feed without some assistance?

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    While we are talking about supercub fuel caps, I discovered the "hard way" that there are at least two thicknesses of gasket used ("thick" and "thin"). I replaced my gaskets and started to get moisture in my tanks when I tied down outside- eventually solved the problem with buying "thick" gaskets made of silicon. This solved the problem of water showing up in the sumps.

    Also the original style supercub fuel system has header tanks and vented fuel caps. The CC fuel system uses the "pitot" pressure caps and do not have header tanks. I know of one instance where icing plugged up one of the pitot tanks (in a completely rebuilt cub using the CC type fuel system-i.e. no header tanks) while flying thru Merrill Pass causing fuel starvation and a deformed fuel tank. Luckily the other tank was OK and got them home.

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    Crash - remember Bernoulli? A vented cap sits in a low pressure area. The little “pffft” on the top gets ram air, helping flow.
    I bought my tank when the STC did not specify plumbing or mounting, so I get to use 1/4” tubing. Flows plenty fast enough all the way to empty.
    It did flow without the “pitot” cap, but required a 2 G pull-up to start. Cap doesn’t have to look like a giant snorkel - I use 1/4” copper, soldered in.

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    skywagon8a's Avatar
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    Quote Originally Posted by bob turner View Post
    I bought my tank when the STC did not specify plumbing or mounting, so I get to use 1/4” tubing. Flows plenty fast enough all the way to empty.
    It did flow without the “pitot” cap, but required a 2 G pull-up to start.
    That's because the last time it was emptied, that 1/4" line filled with air to the level of the fuel in the main tank. Then when you next filled the aux tank with gas you trapped an air bubble in the line. The static weight of the gas is not enough to purge out that air bubble. Sooooo! the gas doesn't flow. I used to raise the wing and slip my T-craft to get the fuel flowing. If the aux fuel is shut off before the line runs dry, it will start flowing on it's own next time.
    N1PA
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    But I always run the aux dry. X-C, that's the safest. Locally, I use it to mix the 100LL with auto, but still run it dry.

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    skywagon8a's Avatar
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    Bob, I'm not suggesting you don't run it dry, only why you have to persuade it to flow next time.
    N1PA
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    mvivion's Avatar
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    The Biplanes, Inc. right wing tank STC for J-3 and PA-11 specifies a “standard Super Cub type vented cap, NOT the snorkel type cap.

    MTV
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    BC12D-4-85's Avatar
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    If using snorkel caps (my T-Craft G/W/C-85 STC requires them like some Cubs) drill a small breather hole on the rear vertical side. If the forward vent plugs the small hole might help maintain the vent and some fuel flow.

    Like Pete notes slip the plane towards the wing tank outlet next to the fuselage to force flow rate.

    Gary

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    Gordon Misch's Avatar
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    That's because the last time it was emptied, that 1/4" line filled with air to the level of the fuel in the main tank. Then when you next filled the aux tank with gas you trapped an air bubble in the line. The static weight of the gas is not enough to purge out that air bubble. Sooooo! the gas doesn't flow. I used to raise the wing and slip my T-craft to get the fuel flowing. If the aux fuel is shut off before the line runs dry, it will start flowing on it's own next time.
    Only if that 1/4" line has a high spot. So depends on how that line is plumbed into the main tank. Near the top would be best. My T-craft didn't have that problem, for whatever reason. 12 gal main and 6 gal wing. I don't recall how it was plumbed.
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    wireweinie's Avatar
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    Quote Originally Posted by supercub9675 View Post
    The CC fuel system uses the "pitot" pressure caps and do not have header tanks. I know of one instance where icing plugged up one of the pitot tanks (in a completely rebuilt cub using the CC type fuel system-i.e. no header tanks) while flying thru Merrill Pass causing fuel starvation and a deformed fuel tank. Luckily the other tank was OK and got them home.
    This system was not installed correctly. The CC installation uses a 1/4" vent line between the left and right tanks so that if one vent clogs the aluminum line provides a backup vent to keep fuel flowing and the tank from collapsing.

    Web
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    Crash, Jr.'s Avatar
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    Interesting, guess I didn't think about the low pressure on top of the wing. My brother's Tcrate doesn't use a pressure cap and both his wing tanks flow great.

    Might just need to put a larger pressure pitot on that gas cap to make it flow better. Takes about 20 minutes to transfer fuel fully from wing to nose tank and even then I don't think I get all 12 gallons.

  15. #15
    Grant's Avatar
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    My J3 Main tank had a bung welded in the top. I plumbed the wing tank line to that and never had any issues. I ran the wing tank dry a few times with no problems.

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    skywagon8a's Avatar
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    Quote Originally Posted by Crash, Jr. View Post
    Might just need to put a larger pressure pitot on that gas cap to make it flow better. Takes about 20 minutes to transfer fuel fully from wing to nose tank and even then I don't think I get all 12 gallons.
    20 minutes doesn't sound bad for gravity fed 12 gallons. What size fuel line do you have? How many gallons does your J-3 burn in an hour? 20 minutes for about three hours flying. Sounds good enough for me.
    N1PA
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    Mine both in my J3 and in my Tcraft drain in 10 minutes. Wondering if I need the pressure caps on my 11 with headerless fuel system and 2 18 gallon tanks. I was planning to use that type...I just figured that pressure wouldn’t hurt.

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    J3 installations are usually designed to replenish the main. Doesn't make any difference how fast it feeds, so long as it refills the main tank as you fly.

    The CC I was referring to was 872CC, and it was almost new. It came with these giant periscope caps, which, as I recall, could be purchased from CC for around $100 each. It was a 180 Cub, heavy as all get-out. It would climb pretty good.

    The older Super Cubs have a vent system that should be independent of the cap. I have heard of them not working well. My Decathlon has a similar system that seems bulletproof, and requires sealed caps.

  19. #19

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    Quote Originally Posted by bob turner View Post
    J3 installations are usually designed to replenish the main. Doesn't make any difference how fast it feeds, so long as it refills the main tank as you fly.

    The CC I was referring to was 872CC, and it was almost new. It came with these giant periscope caps, which, as I recall, could be purchased from CC for around $100 each. It was a 180 Cub, heavy as all get-out. It would climb pretty good.

    The older Super Cubs have a vent system that should be independent of the cap. I have heard of them not working well. My Decathlon has a similar system that seems bulletproof, and requires sealed caps.
    My Piper Pa-150/ve CC180 2300# gross, 1000 usable. It has internal vents at each 25 gal tank. with the cross feed tube you are always vented. I took one look at the snorkel caps and called CC. The attract bugs when parked and rain, snow and ice when flying. The engineer at CC said FAA made them do it as part of the fuel system. I have 3/8 lines. I needed to drain it for OAS empty weight. I put 5 gal in both tanks. Took off with right tank selected. Full power climb to 1000' and not even a cough. composite prop so it almost climbs vertically . Came back put 5 gal in right tank and repeated with left tank. Only time the stupid caps are on is during FAA inspection. OAS never says a word and have looked at the internal vents. FAA never has.

    sandy
    Sandy
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  20. #20
    skywagon8a's Avatar
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    Quote Originally Posted by arcticairalaska View Post
    The engineer at CC said FAA made them do it as part of the fuel system.
    This is just a blanket statement meant to keep you happy. Any time an engineer says the FAA made him do something, he is just covering for the real reason he did it. I don't know the particular circumstance in this case, but it very likely was a quick fix to pass a certain FAA test towards certification. A time saver if you will.

    The real question needing to be asked is, what are those vents doing to pass a certain test which would not pass if they were not installed? Perhaps, without the extra snorkel vents this test would not pass?

    Sec. 23.955 — Fuel flow.
    (b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.

    Now, IF without the snorkels the fuel will only flow at 110% of the takeoff fuel consumption rate, you as a pilot would never see a difference. Yet, when an IA does his inspection they must be installed because they are part of the certification.

    Sandy, When you are using your caps without the snorkel, how are the tanks being vented? Do you have vented caps or is there an outside vent.
    N1PA
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    Quote Originally Posted by skywagon8a View Post
    This is just a blanket statement meant to keep you happy. Any time an engineer says the FAA made him do something, he is just covering for the real reason he did it. I don't know the particular circumstance in this case, but it very likely was a quick fix to pass a certain FAA test towards certification. A time saver if you will.

    The real question needing to be asked is, what are those vents doing to pass a certain test which would not pass if they were not installed? Perhaps, without the extra snorkel vents this test would not pass?

    Sec. 23.955 — Fuel flow.
    (b) Gravity systems. The fuel flow rate for gravity systems (main and reserve supply) must be 150 percent of the takeoff fuel consumption of the engine.

    Now, IF without the snorkels the fuel will only flow at 110% of the takeoff fuel consumption rate, you as a pilot would never see a difference. Yet, when an IA does his inspection they must be installed because they are part of the certification.

    Sandy, When you are using your caps without the snorkel, how are the tanks being vented? Do you have vented caps or is there an outside vent.
    Internal vents inside wing next to tank . Regular supercub vented caps.

    Sandy
    Sandy

  22. #22
    cubdriver2's Avatar
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    My Vagabond, J4 and Champ wing tank had snorkel caps. Never any problems in any type of weather, even monsoon rain. But CC Joe and I were out ski flying one day and on one of Joe's caps the snorkel froze shut. Noticed it when we stopped so he could add fuel from his bag.
    As for bugs, just roll up some window screen and place it in the snorkel.

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  23. #23
    skywagon8a's Avatar
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    Quote Originally Posted by arcticairalaska View Post
    Internal vents inside wing next to tank .
    Sandy
    I hope these are vented outside the wing? You wouldn't want any overflow to be inside. Vents do sometimes overflow.
    N1PA
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    Crash, Jr.'s Avatar
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    Quote Originally Posted by skywagon8a View Post
    20 minutes doesn't sound bad for gravity fed 12 gallons. What size fuel line do you have? How many gallons does your J-3 burn in an hour? 20 minutes for about three hours flying. Sounds good enough for me.
    It's 3/8" line. Burns 5-5.5gph so about 2 hours with some reserve on 12 gallons.

    Yeah, it may not be that much of an issue, it's just annoying. It's slow enough that unless I'm deliberately checking on the fuel gauge every couple minutes it's hard to notice it moving until it's totally full and spewing gas out the nose tank. No opportunity to transfer some now/some later sort of thing. If there's only 4-5 gallons in the wing tank then forget about it, you won't see the fuel gauge move and the only way I can tell it's transferring some of that fuel is by checking the wing tank with a flashlight after landing.

    Guess that's just old planes for ya. Would much prefer to have a sight gauge. Better hurry up and rebuild the old girl with some improvements I guess.

    As a side note, anybody know how long the fuel gauge stick is supposed to be for the nose tank? Seems like mine only reads about 8 gallons til empty and you just assume there's 4 left when it hits the cap.

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    Quote Originally Posted by skywagon8a View Post
    I hope these are vented outside the wing? You wouldn't want any overflow to be inside. Vents do sometimes overflow.
    Not if your ball is centered and the valve in the line doesn't let fuel out. Have you ever seen your standard cub caps vent unless parked on sidehill. Select downhill wing fuel selector to that side. If you don't do that it will take the fuel out of the uphill side and put it on the ground.
    Check out a 185 fuel system as it vents below the strut and it will [in short order] dump a lot of fuel out the low side.
    sandy
    Sandy

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    Yes on the nose tank stick. Mine are at the airport, but I can measure and report back. They are dead-nuts for both U and W tanks. Tie a string on the upper end - my partner has dropped it in the tank three times. I did it once.

    I never fly a J3 without dipping. Same with the Decathlon if I am going somewhere.

    I mentioned a Super Cub feed problem - a 1950 18-90 converted to flaps/160 - not my airplane, not my responsibility. Needs CC caps to feed properly. Somebody probably screwed up the vent system.
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    BC12D-4-85's Avatar
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    Not sure about a Cub's nose tank but some Taylorcrafts with a C-85+ engine have minimum fuel limits for a climb. Their cap wire bottoms or Rochester dial gauge goes to "E" when there's 3 of 12 gallons left. Has to do with Pete's fuel flow requirement for head pressure under full power climb I've been told. The F-19's on placarded them with 9 gallons useable. Cubs may have had that with bigger engines?

    Gary

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    skywagon8a's Avatar
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    Quote Originally Posted by arcticairalaska View Post
    Not if your ball is centered and the valve in the line doesn't let fuel out. Have you ever seen your standard cub caps vent unless parked on sidehill. Select downhill wing fuel selector to that side. If you don't do that it will take the fuel out of the uphill side and put it on the ground.
    Check out a 185 fuel system as it vents below the strut and it will [in short order] dump a lot of fuel out the low side.
    sandy
    Do whatever you wish. It just is not safe to have the vent inside the wing.
    N1PA

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    skywagon8a's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    Not sure about a Cub's nose tank but some Taylorcrafts with a C-85+ engine have minimum fuel limits for a climb. Their cap wire bottoms or Rochester dial gauge goes to "E" when there's 3 of 12 gallons left. Has to do with Pete's fuel flow requirement for head pressure under full power climb I've been told. The F-19's on placarded them with 9 gallons usable. Cubs may have had that with bigger engines?

    Gary
    That would be this requirement. I know it's Part 23, all the different regs are basically the same as far as this requirement is concerned. Basically the higher the horsepower on any same type of airplane, the higher the nose will be under full power climbs. So that changes the relationship of the fuel tank outlet to the carb. There's a little more to it than this, but that is the basic idea.

    Sec. 23.959 — Unusable fuel supply.

    (a) The unusable fuel supply for each tank must be established as not less than that quantity at which the first evidence of malfunctioning occurs under the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank. Fuel system component failures need not be considered.(b) The effect on the usable fuel quantity as a result of a failure of any pump shall be determined.
    N1PA
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  30. #30

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    As a side note, anybody know how long the fuel gauge stick is supposed to be for the nose tank? Seems like mine only reads about 8 gallons til empty and you just assume there's 4 left when it hits the cap.

    I brought the stick home - will post measurements in a bit.

  31. #31
    BC12D-4-85's Avatar
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    https://www.supercub.org/forum/showt...-J3-float-cork

    Here's an earlier discussion about that J-3 float gauge and fuel reserve

    Gary

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    A blast from the past.

    Any old stick will do - I use a 5/8" x 1/2" x 19" piece of spruce. Mark it with ball point, leaning heavily enough to dent the wood:

    2 3/8" - 3 11/16 - 4 7/8" - 5 15/16" - 7" - 7 15/16" - 9 1/8" - 10 1/4" - 11 3/8" - 12 11/16"

    That gives you from one to ten gallons.

    Drill the stick near the top, and put a string on it.

    I have the J4 and the Dec measured the same way. I rarely fly without dipping.

    Every cork is different. Do not depend on corks. I personally do not depend on gauges. In the airlines we always compared gallons added to the gauges to see if they matched. Always.

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    Gordon Misch's Avatar
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    Longer stick, Bob. No string required. And - - - know YOUR cork. Then all is good.
    Gordon

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    BC12D-4-85's Avatar
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    My sticks have one mark for conventional gear and one for floats. Marked by adding fuel to an empty tank...can note fuel quantity or flight time at avg consumption.

    Gary
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