THE

SPORTPLANE BUILDER

By Antoni (Tony) Bingelis

EAA Designee Program Advisor

FUEL TANKS . . . MAKING THEM OF FIBERGLASS Part IV (All Photos by the Author)

1 HE FIBERGLASS LAYUP for a fuel tank is made in the same conventional manner as for overlaying any other kind of surface. But, before you begin, you should check your mold carefully to see that it is in good condition and that it has been adequately sealed, waxed or otherwise prepared for the application of the fiberglass laminations. Pre-cut fiberglass cloth into pieces large enough to overlay a complete side of the tank with a liberal allowance for overlapping the edges and corners. You will save time if you pre-cut enough to cover each side 4 times . . . and do it while everything is still clean, dry and not sticky. Fold each pre-cut piece taking extra care not to stretch or otherwise distort its dimensions. Place all like pieces in the same pile so that you w i l l know which piece is which, and that they don't get mixed up during the glassing operation. Strive for good working conditions; you know, good light, temperatures of between 75°F and 85°F and all the necessary tools and materials close at hand. Low temperatures and high humidity (damp conditions) affect the expeditious curing of the fiberglass layup. Try to avoid working when uncomfortably damp and/or cool conditions prevail. Along the same line, don't allow moisture, like sweat droplets or rain drops, to get onto your uncured glass surfaces. Allow yourself plenty of time in which to complete the layup without interference from other "essential" demands for your time and attention. Be sure to study all the instructions you have for the resin you will be using. Then follow the recom-

8509 Greenflmt Lane Austin Texas 78759

Layup Methods

Position the glass cloth on the mold and mix your resin, a small manageable quantity, precisely as recommended in the instructions. Pour some resin in the middle area of the glass cloth and spread it with a plastic squeegee cut from the lid of a coffee can or with a 2 inch brush. (See Figure 3) Don't be too stingy with the resin. Unlike making a structural layup, a gas tank layup needs to be a little on the resin-rich side. This is assured if you fill the cloth weave with resin so that no white areas show. Always work from the center toward the edges to minimize entrapment of air pockets. These bubbles must be removed from each layer of cloth applied because, should they remain, any of them could turn out to be a leak source. You will probably find it much easier to keep the air pockets and voids out of the last two plys than out of the first two. Lay on the second ply of glass cloth and smooth it down with the plastic lid squeegee, working always from the center of the area toward the edges. You may not need to add much resin to bed down the second layer but be sure it is wetted thoroughly. Remember, it is very important to work out all voids and to fill in all dry areas. Continue working until all the plys are laid to your satisfaction. If you prefer to make only one-half of the tank during a single work session, you should mark a parting line on the mold and mask off the lower half with masking tape or duct tape to protect it from resin runs. This will also provide you with a trim line and a uniform assembly edge later. Apply the full four layers of glass, one right after the other, and then allow the layup to cure. Flip the tank over and carefully remove the glass shell from the mold. Remask the parting line with tape and get on with the layup of the second half of the tank. This next method has little merit, but should you prefer it you could apply but one ply of glass cloth over

mended mixing ratios for the resin and catalyst very

the entire tank at one time and allow it to cure before

carefully.

adding another layer. Be prepared, however, for a lot of extra work. For example, you would have to smooth and sand off all of the gloss on that single cured layer before you can add another. Reportedly, builders have experienced delamination problems . . . not because

It might even be worth your time to first try some

resin on a small test sample, just to see how the mixture will set up and if there w i l l be any problems with its application. Tilt one resin-coated test piece so that its surface is almost vertical. You will see for

yourself if the resin has a tendency to run and sag, and if special care w i l l have to be taken to control

that tendency.

t h i s "one layer at a time" method won't work but

because some of them didn't always sand off the gloss or roughen the surface before adding additional laminations of glass cloth. SPORT AVIATION 61

Controlling Resin Sags and Runs Before you start making your layup is the best time to think about what you will need to do to minimize the inevitable resin sags and runs down the sides of your fuel tank. Furthermore, when you undertake to lay up the complete tank at one time, you will have to figure out a way to handle and turn that slippery thing without unduly disturbing the glass already in place. Using polyester resin you can, more or less, pace your work so that the glass cloth is at least partially gelled when you turn the tank to do another side. This is generally not feasible when working with epoxy. Wouldn't it be handy to have that tank mold suspended in your work shops airspace, at a nice working level, and have it so you could rotate it as you work? I've never tried it with such a large object but why couldn't you poke an "axle" (tube) through the tank mold, from end to end, and set the tank up between supports. The resin and fiberglass could then be applied to each surface, in turn, while occasionally rotating the entire unit, sort of like barbequing a chicken on a spit. This should keep the resin from misbehaving. After the tank layup has cured, it would be easy enough to remove the axle, scuff-sand around the holes and patch over them with several plys of glass cloth. A less challenging but an effective way to minimize the effects of gravity on the resin on a vertical surface is to immediately lay pieces of wax paper on the wet resin. This works best on fiat, or curved, vertical surfaces that have no compound curvatures. Wherever compound curves are present smaller pieces will have to be used. Although this treatment may result in the presence of small resin ridges between the overlapping pieces of waxed paper, the running tendency will be effectively curbed. Once you lay on the wax paper, do not disturb it until the resin has completely cured. The wax paper will peel off easily leaving a glassy smooth surface. Remove and level whatever bumps and ridges are present with a piece of broken glass, or a sharp plane blade, used as a scraper. Mold Removal Now, how are you going to get that mold out when there is no opening in the tank layup? Well, you have two ways. Cut the whole thing in half, or cut a large opening in one side of the fiberglass shell through which you can pull out the mold, in pieces. Don't fool yourself; it is just as much work to cut a large hole in the tank and later patch it as it is to cut the tank in half and later rejoin it. Having done it both ways my preference is now swayed toward cutting the tank in half for the removal of the mold residue. Where to cut the tank will be influenced by its shape. Instinct might rightly lead you to cut the tank top off, lid fashion, or would it be better to slice it vertically since the tank straps support each end? It really doesn't matter much how you choose to separate the tank. When you put it back together it will be just as strong as though it were made as a unit. However, do avoid cutting through areas where the filler neck, the sump, fuel gauge transmitter or vent outlets will be located. Any opening cut into the tank for the purpose of removing the mold must be large enough to work through, particularly, if you intend to install a baffle. The minimum size for such an opening should probably be on the order of 10" x 12". Mark your cutting lines with a black felt pen and do the cutting with a saber saw fitted with a fine tooth metal cutting blade. If you have to do it by hand, 62 NOVEMBER 1982

PRE-DRILL HOLES FOR POP RIVETS

USE RANDOM HOLES (I/4 ) TO IMPROVE EPOXY BOND

RIVETS IN LOWER PART OPTIONAL AS AREA IS ACCESSIBLE FOR CLAMPING

ASSEMBLY FLANGE (ALUMINUM/PLYWOOD/FIBERGLASS)

METHOD ONE -THE SEPARATED TANK FIGURE I.

METHOD TWO THE TANK WITH A LARGE CUT-OUT OPENING

ASSEMBLY FLANGE

FIBERGLASS TANK-FINAL ASSEMBLY

CUT AWAY ALL CORNERS

FOAM CORC

COMPOSITE BAFFLE

FIBERGLASS STRIP

B-B * - FLOX . . . A THICK MIXTURE OF EPOXY AND FLOCKED COTTON

••'0 \ FIBERGLASS STRIP

FIGURE 2. BAFFLE INSTALLATION - FIBERGLASS TANKS

CUT WITH SCISSORS

a hand held hacksaw blade with a handerchief wrapped handle will do. You can start your cut by scraping the broken point of a hacksaw blade along the marked line (about 2") back and forth until you scrape your way through the shell.Make the cut-out rectangular in shape and save it. Later you can reuse the cut-out piece to close the opening after you have finished all the inside work. To remove the mold from a tank cut in half, loosen the foam (or cardboard) from around the edges with a flat metal spatula-like tool, a trowel, a wood chisel or similar device. If an air hose is conveniently nearby, slip the nozzle into the loosened edge between the fiberglass tank shell and the mold and give it a blast of air. The whole thing will probably pop out if your mold sealing and preparation job was properly done. If not, proceed by braking out hunks of mold until the tank is free. Removing a mold through a hole, of course, means breaking up the foam into small hunks with a wood chisel or some other tool. A cardboard mold will be hard to rip out piece by piece. It is best removed by pouring in some water and allowing it to soak the cardboard mold until it becomes a soggy pulp. In that state, the cardboard form will easily pull out as a limp glob of stuff. Clean out the tank interior and inspect it closely for voids and imperfections. If wax has been used as an agent to facilitate mold removal, the inside of the tank shell will be "contaminated" with an invisible wax film. Do not apply a resin seal coat or attempt to bond anything to the tank interior unless you first remove that residual wax. The most reliable method for its removal is mechanical . . . by sanding. Also sand around the edges of the separated tank or, if you used the other method, around the edges of the large opening in the tank. Next, begin the installation of the various components. Drill the holes for the filler neck and sump fittings with a hole saw. Use an epoxy flox mixture to bond and imbed the fittings. Lay several additional patches of glass cloth around these bonded fittings to reinforce their installation and to further insure that they will be leak proof. Don't forget to bond in the vent fittings and, by all means, keep all openings taped over once they are installed. Baffle Installation

PLASTIC COFFEE CAN LID

FIGURE 3. PLASTIC SQUEEGEES FOR SPREADING EPOXY CUT ON A BIAS

REGULAR

CUT

If you are going to install a baffle, now is the time. A good material for baffles is a V4 inch or '^ inch sheet of Clark foam or urethane foam overlaid with a layer of glass on both sides. Also sometimes used is a slab of medium density PVC foam. It, too, may be prepared jn the same manner. However, the PVC foam, a pale red material, is quite rigid and does not really require a glass layer on either side. A brush coat of resin on both sides should suffice. These foam baffles are bonded in with epoxy and the edges filleted with flox. Before the flox sets up. strips of fiberglass cloth are laid and wetted thoroughly with epoxy to complete the installation. Don't plug the corner openings . . . fuel must pass through.

If a baffle is rigid, as most composite baffles are, it need not extend all the way to the top of the tank.

FIGURE 4.

BEST FOR COMPOUND

CURVES AND CORNERS

Although a baffle's function is merely to prevent the

SELVAGE EDGE

GLASS CLOTH - BASIC CUTS

fuel from surging during maneuvers, some tank reinforcement benefits often accompany its installation. Some builders use aluminum baffles made of .032" or .040" 2024-T3 sheet with bent flanges for pop rivet attachment. These baffles are. ordinarily, used in metal

tanks but are also to be found in some fiberglass tanks. SPORT AVIATION 63

Unique step-design of the fuel tank permits increased capacity as it can be located closer to the instrument panel. Note the cut-out areas. One is for the radio stack and the lower is a passage tunnel for engine controls.

A 20 gallon fiberglass fuselage tank to be located just ahead of the instrument panel. Large cut-out area is for the radio stack, the smaller one for a long directional gyro. (A) is the filler drain scupper. (B) is the fuel float/transmitter unit. (C) is the vent fitting and auxiliary tank fitting.

The sliced tank has not yet been reassembled. The baffle on the left will be bonded and riveted in place. Note the rivet line on the right marking the location of the other baffle.

This view shows the 3/8" overboard drain line for the fiHer

overflow compartment. A flexible hose will connect to the end of the aluminum tube and wiH be routed overboard.

When installed in fiberglass tanks they are bonded in place and also pop riveted. (See Figure 2) NOTE: While it might be all right to use aluminum baffles in a fiberglass tank, aluminum should not be mixed with fiberglass as a major element of the tank's external construction. If the idea has occurred to you to use glass end pieces and a wrap-around aluminum sheet for the remainder of the tank (or vice versa), think again. Even if bonded and pop riveted together initially, you may be inviting future leak problems due to the dissimilar expansion rates for aluminum and fiberglass. Closing The Tank

Take another look inside the tank to be sure it is clean and that everything is in place (fuel transmitter, vent fittings, float, flop tube for aerobatic tanks, etc.). NOTE: The use of a sloshing compound inside a fiberglass tank, in my estimation, is questionable. There are no joints or seams for the sealant to penetrate, so why use it? Besides, the slick glossy interior, unless wax free and scuffed, may allow the sloshing compound to someday peel off and cause fuel flow problems. Before you can rejoin the two halves of a fiberglass tank (or seal over a large opening in one), you have to prepare and bond in a flange support around the inside perimeter (edge). This flange may be cut from a pre-formed fiberglass sheet, wood strips, plywood or aluminum. If clamping pressure is necessary to hold the flange frame in place while the epoxy cures, you can resort to the use of pop rivets, small spring clamps or clothes pins. 64 NOVEMBER 1982

The tank is extremely heavy and over-built but it will probably never leak. Note the passageway built thru the tank for the engine controls and the tachometer cable. The glass layup was at least 3/16" thick . . . enough material for two tanks.

The tank assembly is most frequently accomplished with an epoxy bond supplemented with pop rivets to provide the clamping pressure. If you have used the cut-out opening rather than the split tank method, the assembly is the same. However, you could reuse the original cut-out piece as a cover. The final chore is to sand around all joint areas. These are then overlaid with a couple of layers of glass cloth strips wide enough to overlap each seam by 2 inches or so. This final step completely seals and hides any pop rivets that you might have used. Still more on fuel tanks next month.