http://www.sigmfg.com/cgi-bin/dpsmart.exe/IndexInformVideosF.html?E+SigFuel Tank Selection

Standard practice for almost all R/C airplanes is to use a "clunk-type" plastic fuel tank. This type of tank will continue to feed fuel regardless of the attitude of the model in flight. A weighted pickup on the end of a flexible fuel feed line inside the tank follows the movement of the fuel throughout the flight. There are several different brands, shapes, and sizes of plastic clunk tanks from which to choose.

How Big Should The Tank Be?

Most kits specify the exact size and shape of fuel tank that will work best in the model. Their recommendation is determined by the amount of space available inside the model. Round, rectangular, or slant shaped tanks perform equally well in flight, but the structure of the airplane may make one shape more desirable than the other for installation. The following list gives a general indication of the appropriate tank capacity to use with certain size engines. Note: Since four-stroke engines are more fuel efficient than two-stroke engines, the tank size can be reduced about 25% for equal running time.

Engine Capacity/Tank Size

.049 - .0901 to 02 oz.

0.10 - 0.1904 to 06 oz.

0.20 - 0.4006 to 08 oz.

0.40 - 0.4508 to 10 oz.

0.45 - 0.6012 to 16 oz.

0.60 - 0.9016 to 20 oz.

0.90 - 1.2020 to 24 oz.

1.20 - 1.8024 oz. or larger

Tank Assembly

When you buy a fuel tank of any brand, the final assembly of the tank is left to the modeler, who must first decide whether he wants to use a "single-vent" or "double-vent" set-up.

Single-Vent

This is the simplest and best fuel tank set-up for most R/C sport models. There are two brass tubes installed through the rubber stopper going into the tank - one for fuel feed to the carburetor and one for an overflow vent (see photo). Inside the tank, the vent tube should curve up to the top of the tank. The brass fuel feed tube stops just inside the rubber stopper and has a piece of flexible rubber tubing attached to it. The flexible tubing is just long enough to reach the back of the tank and has a heavy metal "clunk" on its back end. The clunk will stay submerged in the fuel no matter what the attitude of the aircraft. During assembly, be sure that the fuel feed line swings freely without hitting the back of the tank.

To fuel an airplane with this tank set-up, simply remove the fuel feed tubing from the carburetor and pump the fuel into that line. When fuel runs out of the vent line, the tank is full. Reconnect the feed line to the carburetor and you are ready to fly. This simple single-vent tank set-up is recommended for any R/C model that has the engine exposed enough for you to take the fuel feed line on/off of the carburetor.

Double Vent

Models which have the engine completely cowled in, or where the fuel feed line is hard to get at, will need to have three brass tubes installed through the rubber stopper - one for fuel feed to the carb, one for an overflow vent, and one for a filling vent (see photo). The brass tube that goes to the top of the tank is the overflow vent, the brass tube that goes to the bottom of the tank is the filling vent. Pump the fuel into the filling vent until it runs out of the overflow vent. Then plug the filling vent hose with a short bolt to keep the fuel from siphoning out of the tank, and you are ready to fly.

Check For Leaks

After assembly of the tank, check it for leaks before installing it in the model. Plug all but one of the tank lines, then pressurize the tank by forcing air into the remaining line (with a fuel bulb, fueling syringe, empty squirt bottle, or your lungs) while holding the tank under water and watching for air bubbles. If you see any bubbles, correct the problem before proceeding. A leaky fuel tank will not work!

Tank Installation

You need to plan for your fuel tank installation during construction of the model. The tank can be mounted with the stopper assembly poking completely through a 1" diameter hole in the firewall, or it can be installed with the stopper behind the firewall with only the fuel lines going through smaller holes in the firewall. Plan ahead so you can drill or cut the necessary holes in the firewall before you glue it in the fuselage.

Fuel Tank Location

A lot of engine problems can be traced back to improper fuel tank location. Model engines draw fuel by suction created as the intake air flows through the venturi in the carburetor. If the tank is mounted too low, too high, or too far back, the engine may not be able to draw fuel during the entire flight. The basic rule of thumb is to mount the tank so that when the plane is level, the centerline of the fuel tank is at the same height or slightly lower (no more than 1/2") than the high-speed needle valve on the engine. You should also keep the tank as close to the back of the engine as possible so the fuel lines can be kept short.

Fuel Tank Mounting

Modern plastic fuel tanks are virtually indestructible under normal use - bursting or cracking is very rare - so it is quite practical to mount the tank semi-permanently once the model is finished. Make scrap balsa supports for the bottom and back of the tank to hold it in place. Glue the supports to the fuselage sides. Seal around any holes in the firewall where the fuel lines come through with silicone rubber sealer to prevent exhaust oil from leaking inside the fuselage. Should the need ever arise to remove the tank for servicing, simply break away the balsa supports and silicone, and the tank can be taken out.

Some modelers prefer to wrap their fuel tank in foam rubber to help isolate it from engine vibration. While this is not always possible in some tight fitting installations, if you do have enough room inside the model it can provide additional insurance against fuel foaming. After getting the tank positioned correctly, more foam rubber can be stuffed around the tank to hold it in place.

Fuel Line Tips

There are several types of flexible rubber fuel line tubing available, but by far the most popular type is the kind made with silicone, like Sig's Heat-Proof Silicone Fuel Tubing. This fuel line tubing will not melt even if it is touching the hottest part of the engine. Also, it will not harden or deteriorate in raw fuel. Before using any fuel line, check it over carefully for pin holes or cracks. Any flaws in the line will be easy to see by stretching it while you look. When installing a fuel line, it is very important that it is not kinked and that it's not touching any sharp edges. Connect the feed line to the engine's carburetor, cutting it to its minimum

Hooking Up Muffler Pressure To The Tank

If your engine's muffler is equipped with a pressure tap, you can utilize it to pressurize the fuel tank to provide a more uniform fuel flow to the carburetor. The hookup for pressure is shown in the picture. Simply connect the pressure tap to the open vent line (the overflow vent in a double-vent set-up) of the tank with a piece of fuel line tubing. This makes the entire fuel system closed, with none of the lines open to the atmosphere.

Air pressure that is being bled off the muffler will now pressurize the tank and force feed fuel to the carburetor, instead of relying totally on the suction of the engine. During refueling, remove the line from the pressure tap so that you don't fill the muffler with fuel when the tank gets full. Do not try to fill the tank in reverse from the pressure line, the tank will not fill properly and fuel may be forced into the engine. One other important note - although muffler pressure helps improve the fuel flow, don't count on it to cure problems created by locating the fuel tank in the wrong place!

These glass-ruled engine mounts displayed above are provided in most Sig kits. The mounts are shown here bolted to a typical plywood firewall. The engine is fastened to the mounts using bolts and aircraft lock nuts.