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Thread: Difference between CC & Dakota headerless designs? Exp PA18 Fuel system decisions

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    Difference between CC & Dakota headerless designs? Exp PA18 Fuel system decisions

    Does anyone have the install drawings for the CC and the Dakota headerless fuel system (PA1 they can share?

    Any commentary on differences between them is welcomed!

    Am currently planning fuel system for Legend PA18 with Atlee 30.5g tanks, and there is musing at the Legend factory that a crossover tube may not be needed. There is also a suggestion to just go with simple on-off valves for each tank instead of a single L-R-Both-Off.

    I am of the thinking that if a vented cap gets clogged, a tank could be deformed/collapse from the fuel/air draw. Always thought the crossover tube, even with its potential sidehill parking issues of draining from one tank to another, was a good idea to equalize the pressure.

    J

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    If you wanted the crossover vent just put outboard vent ports like Skywagon mentioned then whatever valve you want. Best of both worlds. DENNY
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    On another note I know the older Atlee tanks had a outboard vent that came out the bottom of the tank. Does yours still have that? DENNY

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    stewartb's Avatar
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    My Cub’s fuel vents are Cessna style.

    If running on Both is it possible to collapse a tank if one vent is blocked? I wouldn’t think so, and a guy should recognize one tank is emptying before he runs out on the one tank.
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    Marty57's Avatar
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    Quote Originally Posted by stewartb View Post
    My Cub’s fuel vents are Cessna style.
    Where does the vent tube exit your tank? I'm not familiar with the Cessna set up. How do you prevent overflow out the vent tube?
    Thanks,
    Marty57
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    stewartb's Avatar
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    The vent is high on the outboard side. Like Cessna, it uses a one-way check valve so fuel doesn’t find a way out.

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    Marty57's Avatar
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    Quote Originally Posted by stewartb View Post
    The vent is high on the outboard side. Like Cessna, it uses a one-way check valve so fuel doesn’t find a way out.
    Got it, that's easy to add into a fuel system. Thanks
    Marty57
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    Gordon Misch's Avatar
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    I wonder about the check valve when the fuel tank is heated and everything tries to expand and vapor pressure increases. Does it maybe have a tiny hole for that purpose?
    Gordon

    N4328M KTDO

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    stewartb's Avatar
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    Standard vented caps. They cover the expansion problem while the under-wing vents allow higher flow rates for bigger engines.
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    Gordon Misch's Avatar
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    Ah, I see. Also provides vent redundancy. Thanks.
    Gordon

    N4328M KTDO

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    Marty57's Avatar
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    Quote Originally Posted by stewartb View Post
    The vent is high on the outboard side. Like Cessna, it uses a one-way check valve so fuel doesn’t find a way out.
    Something like this? Opens at 1.5 psi according to the specs.
    Marty57

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    skywagon8a's Avatar
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    Quote Originally Posted by stewartb View Post
    The vent is high on the outboard side. Like Cessna, it uses a one-way check valve so fuel doesn’t find a way out.
    Those Cessna valves are notorious leakers when the tanks are full. It could just be because of their flapper design? Also the Cessna tanks have a cross vent tube which is open inside the tanks on the outboard ends. So when the tanks are full with one wing low, the fuel does flow across ship and overboard until the level goes down a bit.

    Did you ever wonder how the fuel tanks are vented in the long wings on an airliner? The vent is connected to the tank at the outboard end of the tank, the vent tube then goes all the way across ship to the other wing tip where the vent exits under the wing. No cross flow at any wing angle. There is also a small collector tank at the outlet in case there is any overflow.

    If stewart's vent was connected to the top forward inboard corner of the tank with the vent mounted where it is, he wouldn't need the check valve. Fuel being as viscous as it is can work it's way through some of the best valves.
    N1PA

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    Steve Pierce's Avatar
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    Quote Originally Posted by JohnnyR View Post
    Does anyone have the install drawings for the CC and the Dakota headerless fuel system (PA1 they can share?

    Any commentary on differences between them is welcomed!

    Am currently planning fuel system for Legend PA18 with Atlee 30.5g tanks, and there is musing at the Legend factory that a crossover tube may not be needed. There is also a suggestion to just go with simple on-off valves for each tank instead of a single L-R-Both-Off.

    I am of the thinking that if a vented cap gets clogged, a tank could be deformed/collapse from the fuel/air draw. Always thought the crossover tube, even with its potential sidehill parking issues of draining from one tank to another, was a good idea to equalize the pressure.

    J
    CC and DC headerless fuel systems are identical in respect to plumbing. Only difference is the DC system calls out their fuel valve and gascolator.
    Steve Pierce

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    Is there much difference between DC’s gascolator and Steve’s gascolator?

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    Scott A's Avatar
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    I'm sure it has been mentioned somewhere already - for the simple inboard cross vent line a shutoff valve in the crossover line can be used to stop the side hill parking issue. And if you forget it in the closed position not really a big deal 99% of the time.

  16. #16
    stewartb's Avatar
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    I’ve never had a drip from my fuel vents in either airplane. The Cessna usually doesn’t get the tanks filled but the Cub does.

    Fuel cross-feed in my planes occurs from the fuel valve being on both. Easy to prevent but I’ve never bothered with it on the average slopes I park on. My valves stay on both unless shut off for maintenance.

  17. #17
    Steve Pierce's Avatar
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    Quote Originally Posted by KevinJ View Post
    Is there much difference between DC’s gascolator and Steve’s gascolator?
    The DC gascolator has the capacity for larger tanks than the standard Steve's.
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    Steve Pierce

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  18. #18
    stewartb's Avatar
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    Johnny, what's your target fuel flow? Mine was 30gph and I met that with a little extra. Flow is dependent on venting. Don't short yourself.

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    Quote Originally Posted by Gordon Misch View Post
    I wonder about the check valve when the fuel tank is heated and everything tries to expand and vapor pressure increases. Does it maybe have a tiny hole for that purpose?
    There is a pin hole in the flapper valve. The line is supposed to rise after exiting the tank to reduce dripping.

    Cessna vented caps only let air in.

    Sent from my SM-G965U1 using SuperCub.Org mobile app

  20. #20

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    Your point is well taken!
    I'll check with Aerosport. It's their IO-375 but with 7.8:1 pistons (for burning mogas/autofuel), so not likely to produce more than 190hp.
    Fuel Flow Calculations for Horsepower – performancedevelopments.com

    Quote Originally Posted by stewartb View Post
    Johnny, what's your target fuel flow? Mine was 30gph and I met that with a little extra. Flow is dependent on venting. Don't short yourself.

  21. #21
    Steve's Aircraft (Brian)'s Avatar
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    Difference between CC & Dakota headerless designs? Exp PA18 Fuel system decisions

    Quote Originally Posted by Steve Pierce View Post
    The DC gascolator has the capacity for larger tanks than the standard Steve's.
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    Both have the same 3oz bowl sediment capacity Steve. The only thing DC did different is the bowl retention with the screw..

    FAA regulations is 1 fluid oz per 20 gallon main tank size. 3 fluid oz is good for up to 60 gallon main take capacity.

    I have an optional 4 oz bowl for even larger fuel tanks up to 80 gallons.

    Brian.
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  22. #22
    Superbill's Avatar
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    I've got a header less system with no crossover or both valve. I had a vent failure cause fuel starvation. Fortunately it happened on landing. The tank was collapsed to the point that I couldn't get a grip on the cap. I pushed up the drain to let air in, got a new cap, and so far so good. It was a good lesson!
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    There is a reason for the balance vent tube between tanks and the forward facing Fuel caps that pressurize the tank.

    There is good reasoning for all these features. I installed it because these features make sense to me.

    Everyone always seems to have a better Idea but these ones are proven.
    Last edited by Dog; 11-08-2022 at 12:27 AM.
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    Steve Pierce's Avatar
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    Quote Originally Posted by Steve's Aircraft (Brian) View Post
    Both have the same 3oz bowl sediment capacity Steve. The only thing DC did different is the bowl retention with the screw..

    FAA regulations is 1 fluid oz per 20 gallon main tank size. 3 fluid oz is good for up to 60 gallon main take capacity.

    I have an optional 4 oz bowl for even larger fuel tanks up to 80 gallons.

    Brian.
    I was thinking DC told me they made their bowl with more capacity, maybe it was more capacity than the original. I remember working on a Super Cub with Atlee tanks and two gascolators and replacing it with your larger capacity bowl. It is easier to get the cotter pin retaining the bowl on your gascolator than the screw on the Dakota Cub gascolator.
    Steve Pierce

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    Steve's Aircraft (Brian)'s Avatar
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    Quote Originally Posted by Steve Pierce View Post
    I was thinking DC told me they made their bowl with more capacity, maybe it was more capacity than the original. I remember working on a Super Cub with Atlee tanks and two gascolators and replacing it with your larger capacity bowl. It is easier to get the cotter pin retaining the bowl on your gascolator than the screw on the Dakota Cub gascolator.
    Original Piper is 2oz. We went with 3oz because when we were developing the STC we wanted to match Husky bowl size and have it on the first AML.

    Brian.
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    I have not been able to get a hold of DC to see if steves gascolator will be acceptable under their STC

    I know CC stc will not accept the DC fuel valve.

    The Dilema was we have the DC fuel valve with the Steves larger Gascolator and the CC STC

    Unfortunately I did not have the STC to read before I bought it. At least the 24 gallon tanks are accepted under the CC stc
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    Quote Originally Posted by Dog View Post
    I have not been able to get a hold of DC to see if steves gascolator will be acceptable under their STC

    I know CC stc will not accept the DC fuel valve.

    The Dilema was we have the DC fuel valve with the Steves larger Gascolator and the CC STC

    Unfortunately I did not have the STC to read before I bought it. At least the 24 gallon tanks are accepted under the CC stc
    I have installed DC fuel valve in a Cub Crafters fuel system. The CC fuel system is installed under one STC and the DC valve is installed under another STC. It is up to the installer to verify that one STC does not negatively impact the other. Dakota Cub developed their own headerless fuel system STC to sell their fuel valve and gascolator. They give you the STC when you buy the parts. The STC was developed out of the certified Super 18. Mark (the owner) is a machinist and didn't want he type certified airplane to be dependant on a vendor gascolator when he could build his own.
    Steve Pierce

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    Some questions on fuel system design:

    I'm curious - How would a "Both" valve reliably ensure fuel flow from a tank with a blocked forward facing pressure cap if there was no crossover tube or other secondary venting provided for the tank?

    Couldn't the system be forced to only pull air off the "unblocked" empty tank on the other side because pulling from the "blocked" tank after sufficient vacuum was formed would require air to travel back through fuel in the feed lines of the blocked tank? See the areas marked with red arrows for sections that could completely cut off flow with an airlock.

    Seems like in order to safely capitalize on the benefits of the L/R/Both/Off valve, there needs to be a secondary vent system. Ram air fuel caps can suffer from icing, bug invasion, etc., as can any venting, but to rely on just one source of equalization?

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    skywagon8a's Avatar
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    You have just described several good reasons for installing a cross vent line. If there is any potential for vent blockage in your use of the plane, then absolutely install a cross vent. Make certain you connect it to the outboard ends of the tanks to ensure only air cross feeds.
    N1PA
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    The main reason for the cross vent tube is that if one tank runs empty and you do not have the cross vent tube the air pressure from the empty tank will go down the fuel line and push against the remaining fuel in the opposite tank - reducing or even stoping the flow. The cross vent will allow the top and bottom pressure to equal out to allow the remaining fuel to flow to the carb.

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    Gordon Misch's Avatar
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    Unless the full tank's vent is blocked, how could the air pressure from the empty tank exceed the air + fuel pressure (gravity head) from the full tank?
    Gordon

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    Quote Originally Posted by Gordon Misch View Post
    Unless the full tank's vent is blocked, how could the air pressure from the empty tank exceed the air + fuel pressure (gravity head) from the full tank?
    Thats right air pressure would equal out and all you have is gravity without air pressure to help fuel flow (that may or may not be enough with this system), now if the vent is clogged -- the most likely scenario when you have a fuel imbalance ( Because we all fly perfectly with the ball centred at all times ) you will have a fuel starvation problem.
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    stewartb's Avatar
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    Snorkel caps are a "fix" to increase fuel flow from a system that wasn't designed for that much flow. For a new build, why not design the venting to provide the required flow?

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    Quote Originally Posted by Dog View Post
    The main reason for the cross vent tube is that if one tank runs empty and you do not have the cross vent tube the air pressure from the empty tank will go down the fuel line and push against the remaining fuel in the opposite tank - reducing or even stoping the flow. The cross vent will allow the top and bottom pressure to equal out to allow the remaining fuel to flow to the carb.
    How would the pressure change? I.e., if there was 1/2 psi air, pushing fuel out of both tanks, down to the selector valve (set on 'both' for this scenario), if one tank runs dry, the 1/2 psi air pressure in the line would remain unchanged. And if 1/2 psi air pressure was still pushing fuel out of the other tank, how would the flow of fuel be disrupted?

    When no snorkels are installed, the pressure on each line is simply head pressure from the weight of the fuel. 'Left', Right', or 'Both', with or without cross venting, they seem to work fine under normal circumstances.

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    Steve Pierce's Avatar
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    Dakota Cub used 1/2" lines to provide required fuel flow at those crazy angles of attack with the slotted wings.
    Steve Pierce

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