Fuel tank

The design of the Robin Fuel tank was based on a similar concept I used on my Wren. In keeping with the spirit of die hard home building,  I have designed the tank so it can be built as a one off design, but for my project I chose to build a full master model and female tool because I plan on offering this part as a semi kit. As part of the goal of eliminating as much drag as possible, I decided to recess the fuel filler neck into the FWD fairing. There is a fuel door that will be located just fwd of the canopy hinge. The tank capacity is 4.9 gallons US, this is in keeping with the US part 103 requirement of no more than 5 gallons or less fuel. 

Fuel tank mold



A master model was constructed from birch plywood and MDH board. This was for the production fuel tank. If a one off tank is being built, the tank is mocked up out of Styrofoam and covered with aluminum duct tape. This temporary "Master" is then set in place using dabs of automotive "Bondo" or body filler onto a flat plate of Masonite backed by MDH board. A 1/2" radius fillet is wiped around the lower edge. The master and the exposed Masonite plate is waxed  with at least 5 coats of carnuba wax.

  A word about waxing and waxes is in order here. One of the most expensive mistakes I made on the Wren production was to use Silicon based mold release wax with a high temp tool. everything was scrapped after the resin cured. Silicon anything has no place around a composite shop. Always keep this material far away. When waxing for mold release the technique here is to create numerous release planes. A good commercial mold release wax is used, but as an option you can also use Johnson's floor paste wax, this is also a Carnuba palm product. The first coat of wax is usually a very heavy one. Open pores in the mold will draw in the liquid state wax. The wax is allowed to dry and then buffed with a wheel or a rag. After each buffing the surface is water hardened or "spit" shined. This creates a hardened layer or release plane. Repeat this 4 more times at a minimum, the last coat however is not water hardened, just wiped and buffed, the reason for this is that the wax will retain some "tooth" or surface roughness that will not cause the gel coat of resin to bead up. 

overall view of the tank assembly


Once the outer shell is molded, it was released from the tool and all of the internal fittings were installed. The final step is to lay up another 3 plys of 8 oz cloth on a flat waxed Masonite surface. The precured tank mating flange is scuff sanded to remove all gloss. A mixture of cotton Flox and epoxy is applied to the flange surface and the tank is then set onto the uncured lower layer. I built a wooden picture frame that slid over the the tank and rested on the flange. I then applied a small amount of weight to the frame to ensure no gaps and to have positive squeeze out of the Flox.

detail view showing internal fitting reinforcement

I used precured- plates of 1/4" thick NEMA grade C fiberglass sheet to reinforce the fitting attach points. This worked quite well. The fittings are Brass barbed commercial fittings with a NPT pipe thread. I drilled and tapped the plates for this these. I scuff sanded the bonding surface on the plates and the inside of the tank. I then bonded the plates in place using T-88 epoxy. The fittings them selves were screwed into the backing plates and provided all the needed clamp pressure. This is an area where it does not pay to go light on the adhesive. All excess squeeze out is used to fillet seal the edges of the reinforcement plates.
The filler neck was purchased from Aircraft Spruce and Speciality, it is a standard weldable aluminum filler neck. An exact diameter hole is bored into the top of the tank and a double flox fillet is used to secure it in place, The aluminum must be scuff sanded and wiped down with MEK. 

Bottom view showing pick up tube


A copper tube is brazed to the fuel outlet fitting. This ensures that fuel will draw from the tank at high angles of attack. This image shows the lower surface after bonding. This surface is supported by the load shelf in the fuselage. The final step in building the tank is to slosh it with a half cup of resin. This is primarily for a fillet bond on the inside of the tank between the side wall and the lower surface. This is designed to react peel forces that would occur due to over pressurization.

view looking FWD


The rear of the tank has two additional barbed fittings. These will attach to a fuel line that attaches to a sight gauge on the instrument panel