15 Gallon Fuel Tanks

The MKIII has very little room behind/below the seats for Stuff, and us Old Poops need to take Stuff with us when we go aviating. Also, the stock 10 gallons of gas was not much range with two people. 15 gallons is better.

The gap seal area is now fuel tank, and holds 13 gallons. It weighs about 15 pounds empty and is 5 to 9 plys thick, depending on the stress areas. It rests on all the upper fuselage tubes, each of which is covered with a rubber radiator or heater hose, split and glued to the tubes. It was not very difficult to make, but very tedious & messy.

The tank is held in place with seat belt webbing which also secures an aluminum angle that the front gapseal is attached to. Unpainted areas are marked for fuel level. The front part of the tank in these pictures had a built up section that elevated the fuel filler to a higher position. Around this elevated section was a balsa wood streamline that blended the airflow off the parachute launcher and then smoothly redirected the airflow up, over, and around the engine. That has since been cut off, and redesigned, but I didn't take any pictures while I did that part. You'll see the current version on another page.

The tank was made by forming it in place around the upper fuselage tubes. The tubes were wrapped with two thicknesses of heater hose to allow for the thickness of the tank to increase as extra plys were added to the outside as it was built up. The shape of the lower half of the tank was formed in place out of aluminum flashing from Lowe's, which was covered in duct tape, and braced with wood. Then the glass cloth and epoxy was laid up inside the aluminum onto the duct tape covered areas. Neither epoxy nor polyester resin will stick to duct tape. These pictures show the mold just before the layup began.

The glass plys were laid up inside and allowed to cure, then the mold was removed and stripped off. The top edge of the tank sides were cut to just below airfoil shape and a 1"x1"x1/16" aluminum angle was attached to the upper inside edge to provide an attach flange for the upper surface of the tank. The easiest way to securely bond a metal angle or similar item to a fiberglass surface is to drill and cleco it, and then when you get ready to glass it, mix up a batch of flox and epoxy to make a thick slurry, spread it on both surfaces and then secure everything with pop rivits. The tank top is flat and was made by covering a flat table with duct tape, laying the cloth on it, wetting it out with epoxy, and letting it cure. Then just pull it off and trim it to size. After the fuel pickups, vent lines and transfer lines were installed, (the lower surface of the tank divides into four sections to fit around the fuselage tubes, and you need to have a way for the front sections to drain into the back ones) the tank top was drilled, clecoed, slurried and rivited into place. Then the gas filler neck was glassed on and it was done. Then the whole tank got two more layers of glass, with extra layers in high stress areas. Due to the complexity of the plumbing, such as the fuel transfer lines between the front and rear sections of the tank, vent lines, and feeder lines, removing the tank is not something that is undertaken lightly...

The main tank feeds a 2.5 gallon hopper tank located behind the passenger seat. That area was modified with the addition of an extra brace tube welded in to give the tank something more to rest on, and all tubes were covered with split 3/8" fuel hose, glued in place. At left is the area modified and ready. Center, is the cargo bay floorboard with the Facet electric fuel pump installed. At right is the tank as seen from the rear side. The hopper tank is fed from the main tank and vented to it, making a closed system. The fuel pickup is 2.5" from the tank bottom, and the sump drain is at the bottom. It takes more than 1/2 pint of water in the tank before it gets to the fuel pickup.

The ELT is visible in the picture to the left, attached to the cargo bay floor behind the pilot's seat. The center picture shows the tank in place, the right picture shows it strapped in and plumbed. There is a clear sight tubing running along the left front edge of the tank for a fuel gauge. The cargo area is 13" high, as wide as the fuselage, and 15" front to back. The floor of the cargo area is 3/16" plywood. Between what you can fit in the cargo area, plus underneath it, there is lots of room for "Stuff", and it is all right on the center of gravity.

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