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  • Self Sealing Fuel Tanks.

    Hi all.
    I've been thinking about self sealing fuel tanks and would like to know if they had to be changed or patched after a post operation inspection found they had been punctured or were they just ignored if the hole wasn't too big?
    I remember reading something about an unexploded 20mm cannon shell found in a B17s tank and the crew being very grateful to the Czech factory worker who bodged the job.
    It seems to me that changing a fuel tank would be an enormous undertaking and judging by the volume of wing occupied by the tank,shrapnel penetrations must have been common place.
    I was thinking specifically of B17s and B24s but it would apply to all aircraft I expect.
    I think some of the Russian birds had a panel beneath the wing through which the tank was removed but what did the West have?
    There must be a simple explanation because if I remember rightly a squadrons reputation relied primarily on having the maximum amount of aircraft available for ops.

  • #2
    Most WW 2 era "self-sealing" fuel tanks used uncured rubber of a very low durometer value and double wall aluminum.
    If the tank was hit and teh damage minor, and detected, it would be possible to just patch the damage rather than replace the tank. The mechanic could cut away the damage, apply a patch of alumimum with rivets and a sealant and put a lump of new rubber in place.

    The alternative would be to replace the tank since in WW 2 these were not integral to the aircraft's structure like they normally are today.

    Comment


    • #3
      Originally posted by T. A. Gardner View Post
      Most WW 2 era "self-sealing" fuel tanks used uncured rubber of a very low durometer value and double wall aluminum.
      If the tank was hit and teh damage minor, and detected, it would be possible to just patch the damage rather than replace the tank. The mechanic could cut away the damage, apply a patch of alumimum with rivets and a sealant and put a lump of new rubber in place.

      The alternative would be to replace the tank since in WW 2 these were not integral to the aircraft's structure like they normally are today.
      No idea myself about them but did have a mate that was in the RAF and was on groundcrew and remember him talking about changing the damaged ones. lcm1
      Last edited by lcm1; 09 Dec 12, 07:27.
      'By Horse by Tram'.


      I was in when they needed 'em,not feeded 'em.
      " Youuu 'Orrible Lot!"

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      • #4
        Originally posted by T. A. Gardner View Post
        Most WW 2 era "self-sealing" fuel tanks used uncured rubber of a very low durometer value and double wall aluminum.
        If the tank was hit and teh damage minor, and detected, it would be possible to just patch the damage rather than replace the tank. The mechanic could cut away the damage, apply a patch of alumimum with rivets and a sealant and put a lump of new rubber in place.

        The alternative would be to replace the tank since in WW 2 these were not integral to the aircraft's structure like they normally are today.
        So I assume that checking the tanks was part of the groundcrews post operation routines.I imagine they'd do this by looking on the ground for fuel puddles.
        I remember seeing a SSFT at Duxford and it was made of a reddish brown slightly flexible material,possibly very thick canvas or similar.
        Removing one from an aircrafts wings must have been quite an undertaking, one would need a fair size access hatch for a start. There were overload tanks specifically designed to fit in some aircraft bomb bays,I assume the necessarry plumbing was designed into the plane,removing and replacing these would have been relatively simple.
        Aircraft that received direct flak hits were often described as an explosion with four fireballs falling to Earth,these would be the fuel tanks.
        BTW does anybody know the origins of the SSFT?

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        • #5
          Originally posted by flash View Post
          BTW does anybody know the origins of the SSFT?
          It seems that the original thought and experimentation goes back to WW1.

          http://1914-1918.invisionzone.com/fo...0#entry1605796

          According to posts on the above linked to Forum, a number of nations were experimenting with them but were largely forgotten about in the inter war years.

          Scroll through the above link for more discussion about the origins of SSFT's....and also about the origins of modern....ahem....feminine hygiene products.
          "It's like shooting rats in a barrel."
          "You'll be in a barrel if you don't watch out for the fighters!"

          "Talking about airplanes is a very pleasant mental disease."
          — Sergei(son of Igor) Sikorsky, 'AOPA Pilot' magazine February 2003.

          Comment


          • #6
            Originally posted by At ease View Post
            It seems that the original thought and experimentation goes back to WW1.

            http://1914-1918.invisionzone.com/fo...0#entry1605796

            According to posts on the above linked to Forum, a number of nations were experimenting with them but were largely forgotten about in the inter war years.

            Scroll through the above link for more discussion about the origins of SSFT's....and also about the origins of modern....ahem....feminine hygiene products.
            Aah,earlier than I thought.
            Interesting forum by the way.
            Thanks,Tony.

            Comment


            • #7
              Originally posted by flash View Post
              So I assume that checking the tanks was part of the groundcrews post operation routines.I imagine they'd do this by looking on the ground for fuel puddles.
              I remember seeing a SSFT at Duxford and it was made of a reddish brown slightly flexible material,possibly very thick canvas or similar.
              Removing one from an aircrafts wings must have been quite an undertaking, one would need a fair size access hatch for a start. There were overload tanks specifically designed to fit in some aircraft bomb bays,I assume the necessarry plumbing was designed into the plane,removing and replacing these would have been relatively simple.
              Aircraft that received direct flak hits were often described as an explosion with four fireballs falling to Earth,these would be the fuel tanks.
              BTW does anybody know the origins of the SSFT?
              I would assume there are two ways to check:

              It's leaking. That would be pretty obvious.

              It won't hold pressure. Aircraft fuel tanks run at a slight overpressure to force the fuel into system. This is necessary since the plane could be doing something that might otherwise cause the fuel to move away from the fuel line.

              Comment


              • #8
                Originally posted by T. A. Gardner View Post
                It won't hold pressure. Aircraft fuel tanks run at a slight overpressure to force the fuel into system. This is necessary since the plane could be doing something that might otherwise cause the fuel to move away from the fuel line.
                Your statement implies that they are sealed.

                Aircraft fuel tanks are vented to the outside air, to compensate for changes in altitude, or more correctly, air density.

                Fuel pumps are what get the fuel into the fuel lines.

                Because aircraft fuel tanks are vented to allow for equalization of pressure at different altitudes, the nitrogen must be constantly fed into the tanks to displace the outside air that freely enters. Pressure valves regulate the flow so that the fuel tank

                isn’t over-filled, and FAA ground tests of the prototype showed that the nitrogen is quickly distributed throughout the tank, so no fans are needed to circulate the gas. The system operates in two modes: low-flow and high-flow. When the airplane has taken off, instruments signal OBIGGS, and the system enters the low-flow mode. OBIGGS remains in that mode through climb and cruise. Since air pressure is much lower at cruising altitudes, the system can displace all but two or three percent of the fuel tank’s oxygen.

                Conversely, during descent, air pressure rises, so more outside air flows into the tank, increasing the oxygen content. To compensate, OBIGGS goes into high-flow mode, pumping in additional nitrogen at a faster rate. Even so, the concentration of oxygen jumps from a few percent at cruising altitude to between nine and 12 percent at landing.
                http://www.airspacemag.com/how-thing...tml?c=y&page=2

                [The above source is referring to modern jet airliner tanks, but the venting principle is common to all.

                The pressurisation by nitrogen referred to above is a fire suppression measure, not a fuel supply measure]

                Aircraft designers rely on something other than pressure, of which there might be very little of at higher altitudes, to ensure a supply of fuel to the fuel pick up point(s) - (note the plural) if the aircraft has done something to move it out of "balanced flight" - a pilots term for "equilibrium", which might more commonly be understood as flying "straight and level"(but there is more to it which I won't bore anyone to death with an explanation of the aerodynamic formulae.

                INVERTED FUEL SYSTEMS

                Most aerobatic airplanes with inverted fuel and oil systems use fuel injection rather than a carburetor. When a carburetor is inverted, it can no longer meter fuel, and the float rises and cuts off the incoming supply. A fuel injector, which doesn’t care what attitude it is in, measures airflow and meters the proper ratio of fuel to each cylinder so that each receives a constant flow of the same fuel-air mixture.

                To ensure the flow from fuel tank to fuel injector, aerobatic aircraft with the fuel tank in the fuselage have a “flop tube,” (see pic below) a flexible hose with a weight in the free end, plugged into the fuel tank. In normal flight, the weighted end of the hose flops to the bottom of the tank and draws fuel from there. When the airplane rolls inverted, the weighted end flops to the top of the tank, with the fuel. Regardless of the aircraft’s attitude, fuel and flop tube end up in the same spot.

                Aerobatic airplanes that have fuel tanks in the wing use a small “header tank,” which is connected to the wing tanks. In normal flight, fuel gravity-feeds down to fill the header tank, which is connected to the suction side of the fuel pump. When the airplane rolls inverted, the header tank is now above the engine, and fuel gravity-feeds from the header tank to the fuel pump. A check valve in the line from the main tank to the header tank stops fuel from draining back into the main tank when the airplane is inverted.
                http://www.airspacemag.com/flight-to...side-Down.html





                I took the advice and design from a fellow RV builder and changed the anti-hang bar in the flop tube bay even after building the one as the plans show. This design allows for un-obstructed access to the tank bay after removing the cover. It's made in two pieces to allow this.
                Image #13 from here:

                http://www.wannafly.net/rv8/fuel_tank_construction.htm
                Last edited by At ease; 10 Dec 12, 08:32.
                "It's like shooting rats in a barrel."
                "You'll be in a barrel if you don't watch out for the fighters!"

                "Talking about airplanes is a very pleasant mental disease."
                — Sergei(son of Igor) Sikorsky, 'AOPA Pilot' magazine February 2003.

                Comment


                • #9
                  Originally posted by At ease View Post
                  Your statement implies that they are sealed.

                  Aircraft fuel tanks are vented to the outside air, to compensate for changes in altitude, or more correctly, air density.

                  Fuel pumps are what get the fuel into the fuel lines.



                  http://www.airspacemag.com/how-thing...tml?c=y&page=2

                  [The above source is referring to modern jet airliner tanks, but the venting principle is common to all.

                  The pressurisation by nitrogen referred to above is a fire suppression measure, not a fuel supply measure]

                  Aircraft designers rely on something other than pressure, of which there might be very little of at higher altitudes, to ensure a supply of fuel to the fuel pick up point(s) - (note the plural) if the aircraft has done something to move it out of "balanced flight" - a pilots term for "equilibrium", which might more commonly be understood as flying "straight and level"(but there is more to it which I won't bore anyone to death with an explanation of the aerodynamic formulae.



                  http://www.airspacemag.com/flight-to...side-Down.html







                  Image #13 from here:

                  http://www.wannafly.net/rv8/fuel_tank_construction.htm
                  Ha,it's the relatively simple solutions that endure,fascinating.

                  Comment


                  • #10
                    Modern aircraft use Nitrogen but the Russians piped cooled engine exhaust gas to the tanks in their MIG 3 design,very clever I thought.

                    Comment


                    • #11
                      Originally posted by flash View Post
                      Modern aircraft use Nitrogen but the Russians piped cooled engine exhaust gas to the tanks in their MIG 3 design,very clever I thought.
                      Yes, it was innovative and I believe they were the first to do it.
                      "It's like shooting rats in a barrel."
                      "You'll be in a barrel if you don't watch out for the fighters!"

                      "Talking about airplanes is a very pleasant mental disease."
                      — Sergei(son of Igor) Sikorsky, 'AOPA Pilot' magazine February 2003.

                      Comment


                      • #12
                        Originally posted by At ease View Post
                        Yes, it was innovative and I believe they were the first to do it.
                        Do you know of any other type that used a similar system,I don't.
                        Perhaps it wasn't quite as successful as we thought.Maybe there was a bit of fuel contamination from the impurities in the exhaust?

                        Comment


                        • #13
                          Originally posted by At ease View Post
                          Your statement implies that they are sealed.
                          Integral tanks are sealed, although they do have a vent system; a "wrap-around" tube system in the Herc's case.
                          Bladder (SSFT) fuel cells are also sealed. And yes, bladders are a distinct pain in the a*s to replace. They require lots of talcum powder, patience and some knowledge of knots for the lacing.


                          Sealing of fuel cells are usually by use of a PRC (fuel tank sealant). That's the dark brown rubber compound in the corners in the pictures above.
                          example: http://www.lasaero.com/site/products...e?id=D02VK9NTH

                          Years ago a polyurethane top coat had to be applied over the sealant because the sealants couldn't stand up to the fuel, but modern PRCs can now be applied as a stand alone product along all rivet lines, joints, on all faying and fillet surfaces, rivets are wet installed, etc.

                          Aircraft fuel tanks are vented to the outside air, to compensate for changes in altitude, or more correctly, air density.
                          Fuel pumps are what get the fuel into the fuel lines.
                          Yep and yep



                          Aircraft designers rely on something other than pressure, of which there might be very little of at higher altitudes, to ensure a supply of fuel to the fuel pick up point(s) - (note the plural) if the aircraft has done something to move it out of "balanced flight" - a pilots term for "equilibrium", which might more commonly be understood as flying "straight and level"(but there is more to it which I won't bore anyone to death with an explanation of the aerodynamic formulae.

                          Yep.

                          In the Herc's case, a series of pick-up tubes (9 IIRC) at the bottom of the tank provided the fuel supply to the pump, which in turn was located inside a "surge tank" to allow for unusual flight attitudes for a short period of time.

                          I'm not a very big guy, so I sometimes feel that I spent the first 6 years of my career inside fuel cells.
                          Last edited by tigersqn; 10 Dec 12, 21:39.
                          Scientists have announced they've discovered a cure for apathy. However no one has shown the slightest bit of interest !!

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