Fuel Tanks

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Roger Mellema

Fuel cell leakage

The BD-4 has more than it’s share of fuel cell problems but don’t feel like the Lone Ranger – many airplanes have the same problem. The best way to insure good cells is to prepare the glass panels before wing assembly. There are many people who think. sloshing material is a lot better than it really is and hope it will fill all the cracks and pin holes. The panels should be inspected and the best ones chosen for fuel. These should be checked carefully for flaws and these should be fixed on the inside of the wing using fiberglass (some builders lay another layer of glass on the two end ribs of the tank). The inside of the cells should be sanded until the entire surface is roughed up (don’t sand into the cloth). The surfaces should be cleaned more than once with acetone or MEK – not just wiped but flow it on with one cloth and wipe it dry with another. I then painted four coats of thinned slosh material on all surfaces except the 1.5 inch edge to be glued. Make sure all the pin holes get filled. After the wing has been glued together, the tanks are sloshed with unthinned slosh. I like to keep turning the wing while a vacuum cleaner, hooked backwards, blows warm air into the fuel gauge hole.

This helps the slosh dry evenly on all surfaces. Be very careful to keep sparks from getting anywhere near the fumes as they are very explosive. I know of one person who blew up a wing.

Some people coat the entire outside of the tank area with a light layer of fiberglass. This works very well but the last time I just put 2 inch strips of glass over the cell joints. The 2216 used on the wings does not have very good peel strength. I had a couple joints come loose on my first BD-4. Av-gas eats asphalt beautifully so I quickly flew away before they decided to charge me for it. Make sure you fuel test your wings before you go to fly. Fill them up to the top and let them set for at least two days. It eaves a lot of embarrassment come test flight time.

Roger Mellema
Newsletter #13 (March 86)

Paul Wood from Lincoln, NE recently purchased a BD-4 and is having some trouble with leaky fuel tanks and other things. Sorry I haven’t written back sooner, Paul, but I will answer your questions here as maybe other readers have the same problem.

The fuel tank problem is a very common one and many people who buy a used Br) are told by the seller that “it never leaked when I had it”. This could be true but knowing BD-4’s, I rather doubt it. The leaks that occur in the outside skin are usually very easy to correct by putting some sealant over the leaky spot on the inside of the tank or by putting a small fiberglass patch on the outside.

Leaks that occur along the rear of the wing are usually caused by not sealing the flap/aileron attachment brackets properly. These again are not too difficult to fix if you don’t mind taking the flaps and ailerons off and mettind in and using some Pro-Seal to fix the oroblerii. By the way, I have seen builders pour unbelievable ariic-urits of slosh into the wine in the hopes of sealing a leak such as this and never oettina it done. Slosh is really only good for coating surfaces so oas can’t attack therii and sealing piri holes. The hardest leaks I’ve seen to seal are the ones that Occur in the ribs at each end of the fuel tank. The inboard rib seems to be the worst problem. This may be because of the slight flexing of the tubular spar which will be worse in the vicinity of the inboard rib than by the outboard rib. When you have a seep in one of these ribs, it usually requires cutting a hole in the last cell of the fuel tank, cleaning all the gunk (sloshing material and 2216 glue, etc.) off of the rib and then painting it with brushable Pro-Seal.

Before doing anything drastic, check the hose fittings and the fuel sender gasket to be sure they are not leaking. it sometimes helps to clean the area with solvent (or soap and water, or diesel fuel) and blow a thin layer of white flc-ur or powder oritc, the surfaces to make it easier, to see where the leak actually is. It is very difficult to tighten the fuel lines to the fittings that go into the tank so this is a frequent source of leaks. I like to coat the rotarig surfaces of the fittings with a little Pro-Seal to make sure they don’t leak.. Par-t of the problem here is that Bede supplied 2000 lb hydraulic lines to be used as fuel lines as they are extremely stiff. It would be a very good idea to get rid of these and put in something reasonable. The fuel pressures in this area are extremely small. When working with the hoses be careful to riot turn the fittings that go iritc- the tank as that will certainly cause leaks – it is very difficult to work on the aft fitting, sometimes requiring griridirig down wrenches etc..

When cleaning the ribs, be sure to pay special attention to the area j,ust around the spar. This is a very difficult area to get at but is by far the riiost iriiportant. The wing cell should be cut into so that you have good access to this area. If you think you are going to out a 6 inch hole in the top of the wing and work throumh it, you are very riiistakeri. What it really requires is to cut a hole in the BOTTOM of the cell that noes 12 or more inches forward of the spar and 12 or more inches aft of the spar. It should be cut to withir, 1.5 inches of each rib. The panel can be cut in an ‘HI shape with the center of the “H parallel with the spar. You will then have to find a way to hold the ‘HI open so that you car, work freely. It really is a lot easier to work if the whole panel is cut out but it is a little easier to close up the hole again if everything is not cut away. Next you should use acetone or MEK and/or a flat scraper (looks like a wood chisel) to scrape away the slosh around the hole and from the entire rib. Don’t get lazy and quit before the job is done – set everything up so you can sit down and take your time while doing this. It helps to use a trouble light held on the other side of the rib you are cleaning to see where you aren’t getting it right. This is a lot easier on the inboard rib where both sides are accessable.

The next job is to coat the rib with brushable Pro-Seal. A couple of good even coats should do it. The light will come in handy again here to see areas that have been missed. The fiberglass skin car, be closed up again by getting the cutout part to lay right and then covering the cut with about three layers of 8 oz fiberglass cloth (at least 2 inches wide) and polyester, r,esiri. The very early, very white opaque wing panels were made with epoxy and you will have to experiment to see what will work with these. If you think you need rilore strength than the fiberglass patch provides you can put manner- to the gas cap installation.

I guess the one point I want to make clear is that you CANNOT fix a leaking tank by dumping more slosh into it. I have seen a couple thousand dollars in slosh go down the drain with out doing much r3c-od. A couple of times I have seen the tanks stay sealed for a month or two and then take up right where they left off.

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