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Thread: FX3 Wing Fuel Tank Recommendation?

  1. #1
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    Question FX3 Wing Fuel Tank Recommendation?

    According to the CubCrafters configuration web page at http://cubcrafters.com/carboncub/fx3/configure it outlines 2 x 22gal fuel tanks for a total of 44gal. I've heard through others that this is actually four tanks, 2 x 12 gal inboard tanks with a 10gal outboard of each with an interconnect that passes through a rib. As a result of the design there is about 5 gals unusable fuel. It is also suggested to delete the 10gal outboard tanks and use the default, I believe per the SS, of just the inboard 12gal tanks. This improves roll performance but the ideal setup is to instead us a belly tank for extended range operations.

    I have several concerns about modifying the default FX tank config.

    1) Resale: This is not something you're going to install later on your own and may make the sale a challenge unless a belly tank is included and desired
    2) The flight controls were designed for this so roll performance should not be an issue, I've heard nothing but great things about the new ailerons, etc.
    3) Nothing says you have to fill up the wing tanks beyond 25gal total keeping the outers dry if that was a concern
    4) If I'm traveling them I'm most likely wings level the majority of the time so the outer tanks are a non issue and may help make for a smoother ride

    What are your opinions on the wing tank configuration in a FX3? How would you order one?

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    Senior Member Andy's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by hawgdrvr View Post

    What are your opinions on the wing tank configuration in a FX3? How would you order one?

    I ordered with 44 gallon tanks and did not consider, even for a moment, having a reduced fuel capacity. If you will only do local flights, or will carry fuel in a belly pod, you may make a different choice.

    I find the roll rate and aileron response to be quite adequate with full fuel. Aren't the concerns about roll response for the earlier aileron configuration?

    "Nothing says you have to fill up the wing tanks beyond 25gal total keeping the outers dry if that was a concern"

    No, it doesn't work like that. The FX-3 filler neck is in the outboard tank. It is not possible to isolate the outers from the inners.


    Last edited by Andy; 09-03-2020 at 05:52 PM.

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    Senior Member turbopilot's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by Andy View Post
    I ordered with 44 gallon tanks and did not consider, even for a moment, having a reduced fuel capacity. If you will only do local flights, or will carry fuel in a belly pod, you may make a different choice.

    As you pointed out before the issue is weight. When CubCrafters added the two 10 gallon tanks outboard of the standard 12 gallon tanks, the result was not 20 gallons more fuel, it was 15 gallon of useable fuel. In addition because of the suboptimal design of using two tanks to achieve 22 gallons a side, the fuel tanks are heavier than a comparable single 22 gallon tank. I understand the tankage weight penalty was a few pounds. So all up the option to have 15 additional gallons of fuel cost 32+ lbs of cabin load. Given the 2,000 gross weight max of the FX that 30 lbs is not too significant but for light operations (or operations with the SS) you are always carrying around an extra 32+ pounds you cannot shed and has no useful purpose.

    Several of us have decided that keeping only 24 gallons in the wings and adding a 19 lb belly tank with no real unusable weight penalty is a much better way to limit weight. The belly tank comes off when you don't need it. And there are some unbelievable rumors that Carbon Cubs cruise faster with the belly tank owing to some aerodynamic effect that I don't understand. My new SS will come with 24 gallons in the wings and a belly tank so I will find out for myself. Apparently the belly tank capacity depends on the size of tires and extended landing gear used. Estimates I have heard suggest the capacity is well over 20 gallons. The higher the nose during fueling the more the tank holds.So question to be asked is an extra 15 gallons of useable fuel worth a 32 lbs weight penalty that is always with you even when you don't need that 15 gallons of fuel?
    Last edited by turbopilot; 09-03-2020 at 06:59 PM.
    Bob Anderson, CC11-00435, N94RA

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    Senior Member Andy's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by turbopilot View Post
    When CubCrafters added the two 10 gallon tanks outboard of the standard 12 gallon tanks, the result was not 20 gallons more fuel, it was 15 gallon of useable fuel.
    I have often wondered how much of that fuel is actually unusable in normal operations.

    "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."

    I was told the test condition that CubCrafters used but I'm not sure I have remembered it correctly. I think it was a Vx climb with a full rudder slip.



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    Senior Member Andy's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    From the CC11-160 maintenance manual:

    "1. STANDARD FUEL TANKS
    The aircraft is equipped with two tanks located on the inboard end of the wings. Each
    tank has a total capacity of 12.5 gallons for a total of 25 gallons. The total usable fuel is
    24 gallons. The tanks are made of aluminum.


    2. EXTENDED RANGE FUEL TANKS - OPTIONAL
    The aircraft can be equipped with two interconnected tanks on the inboard end of each
    wing. Between the two tanks, they have a capacity of 22 gallons for a total of 44 gallons.
    The total usable fuel is 40 gallons. The tanks are made of aluminum."


    From the CCX/CCK-2000 maintenance manual:

    "2 STANDARD FUEL TANKS
    The aircraft can be equipped with two tanks located on the inboard end of the wings. Each tank
    has a total capacity of 12.5 gallons for a total of 25 gallons. The total usable fuel is 24 gallons.
    The tanks are made of aluminum.

    3 EXTENDED RANGE FUEL TANKS
    The aircraft can be equipped with two interconnected tanks on the inboard end of each wing.
    Each tank has a total capacity of 22 gallons for a total of 44 gallons. The total usable fuel is 39
    gallons. The tanks are made of aluminum."

    Question 1 - why was a gallon of usable fuel lost for CCX-2000 vs CC11-160
    Question 2 - if unusable fuel is only 1 gal for the "standard" tanks why does it go up to 4/5 gallons when the outboard tanks are added.

    I suspect the answer to Q1 is that the higher power of the CCX/CCK-2000 allows a more extreme flight attitude for the test point.

    I suspect the answer to Q2 is that all the remaining fuel has moved to the outboard tank of the low wing.

    Wouldn't most pilots concerned about fuel exhaustion avoid "both" and burn all the available fuel in each tank by keeping the selected tank slightly high rather than very low?

    I'm not suggesting that any less than 5 gallons unusable should be used for flight planning but I don't think I'd let the engine stop with 5 gallons still in the tanks.

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    Senior Member turbopilot's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by Andy View Post

    I was told the test condition that CubCrafters used but I'm not sure I have remembered it correctly. I think it was a Vx climb with a full rudder slip.

    During my year trying to run down the specific weight differences between the SS and FX I could never really get a straight answer to several weight questions. One "normally reliable" CubCrafter source said 30+ lb unuseable fuel was a good number to use for normal operations. Another source said it is "technically" unusable but there are ways to get most of it out in flight if you need it. Since unusable fuel is not accounted for in the aircraft basic empty weight some folks treat it as not real. But the first time the tanks are filled it is real. The number should be easy to determine and must be know with extreme accuracy. All you need to do is weigh a new airplane before fueling, put the airplane in a flight attitude and drain the fuel. I could not get that number.

    I considered ordering the 44 gallon fuel option for my 2014 SS as that option had just become available. I was told then that the extra two 10 gallon tanks and hardware weighed 17 lbs and the unusable fuel was another 30 lbs. I cancelled the option before the build started.

    I guess we know one thing for sure, if you run through 39 gallons of fuel with the 44 gallon tanks there are some situations where the motor will stop. I would guess most smart pilots would not test that limit, so that sort of defines 5 gallons as unusable.
    Bob Anderson, CC11-00435, N94RA

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    Senior Member turbopilot's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by Andy View Post
    Question 2 - if unusable fuel is only 1 gal for the "standard" tanks why does it go up to 4/5 gallons when the outboard tanks are added.
    Been a while since I looked at the tank to tank interconnect with the fabric off on the 44 gallon tank system. My recollection is that the 10 gallon to 12 gallon tank interconnect is not flush with the bottom of both tanks. Somebody building an EX-3 can answer by a quick look. So my suspicion is the fuel is trapped in that outer 10 gallon tank that does not have a low point drain.
    Bob Anderson, CC11-00435, N94RA

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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by turbopilot View Post
    Since unusable fuel is not accounted for in the aircraft basic empty weight some folks treat it as not real. But the first time the tanks are filled it is real. The number should be easy to determine and must be know with extreme accuracy. All you need to do is weigh a new airplane before fueling, put the airplane in a flight attitude and drain the fuel. I could not get that number.

    My FX-3 was factory weighed with fuel tanks empty. The mass and CG were then adjusted for the mass and moment of 5 gallons of unusable fuel. The result is the "Running Basic Empty Weight".

    If you drained the fuel in level flight attitude there would be a lot less than 5 gallons left in the long range tanks! "Unusable fuel" is not determined in level flight attitude.

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    Senior Member turbopilot's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by Andy View Post
    My FX-3 was factory weighed with fuel tanks empty. The mass and CG were then adjusted for the mass and moment of 5 gallons of unusable fuel. The result is the "Running Basic Empty Weight".

    If you drained the fuel in level flight attitude there would be a lot less than 5 gallons left in the long range tanks! "Unusable fuel" is not determined in level flight attitude.
    Good idea. That is not the way Part 23 airplanes are done but I think it is a good idea. Understand about level flight test but it would be a good number to have. I have built two EAB airplanes. For both I ran a level attitude fuel available test on the ground. I never tried to determine fuel flow in other corner of the envelope with minimum fuel. Too dangerous.

    In any event based on available evidence it sounds like it is prudent not to count on using the last 5 gallons of a 44 gallon tank system in a Carbon Cub. So that is de facto unusable fuel.
    Bob Anderson, CC11-00435, N94RA

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    Senior Member kiwibob's Avatar
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    Default Re: FX3 Wing Fuel Tank Recommendation?

    Quote Originally Posted by Andy View Post
    I ordered with 44 gallon tanks and did not consider, even for a moment, having a reduced fuel capacity. If you will only do local flights, or will carry fuel in a belly pod, you may make a different choice.
    This was me too. at 24/25" LoP it burns 9-10gph and way more at full rich high power settings. Basically without an aux tank you would be restricted to around an 1.5 hours flight time to leave a decent reserve. I've also noted that the fuel in the sight glasses disappears from view much earlier than the level flight scale suggests (both the SS and FX3). I find this unnerving and and plan fuel (and pilot relief) stops accordingly.

    Until I read this thread I didn't even know a belly pod was a thing and have simply carted extra fuel around in 10l containers.

    Personally I just don't worry about weight too much as I find that the plane has way more than adequate performance for any where I'm likely to go. Essentially if I can fit it in it'll still leap off the ground and fly. I'm much more concerned to have the right kit for the trip, I like my comforts and a cold beer at the end of the day is well worth (imo) the few extra lbs, or even a lot of extra lbs, that entails.

    The only time that weight is a concern to me is our annual Healthy Bastards Bush Pilot Champs (STOL comps, I didn't fly on this particular day) but this is just a bit of fun and getting a good result is not as important to me as keeping the plane intact. I really don't try too hard and I find the result is more about landng on the chosen spot at the right speed rather than the shortness of the t/o or landing rolls. I find that generally goes for any landing on any strip.

    Others have equally valid opinions and you have what may look like too much choice!
    Bob Gray, FX-3 #38, ZK-FXC

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