Installing the Fuel Tank and Engine

In Part II of this article series, we covered the best way to build an ARF trainer Fuselage airframe. “How to Build an ARF Trainer Part II” detailed how to build the very best fuselage airframe possible. In truth, most ARF trainers would fly well if a few steps were left out, as long as the stabilizer and vertical fin were properly set. However, if all the steps were followed, your ARF Trainer would better than 99% of “First Timer” airplanes brought to the flying field.

This article, “Part III – Engine Choice, Installation and Final Assembly” will follow the same pattern. When selecting and installing the engine, and then completing construction, you will not need to perform every step detailed here for your airplane to fly OK. But if you want the best flying trainer at the field and one that will be problem free and extremely durable, try to follow these instructions as much as possible.

How much you are willing to do so will determine how well your trainer will perform, how easily and quickly you will learn to fly and how long your airplane will last. Remember that all Sport Aviator photos can be enlarged just by clicking on them,

Photo 41

The instructions included with today’s ARF trainers are almost always excellent. The Tower Hobbies Trainer is no exception as photo 41 shows. It is time to move towards the airplane’s nose area. Assembling the fuel tank is the first step. The front of the fuel tank’s “stopper”, which contains the fuel and vent hoses, protrudes from the firewall. Having the engine already installed means using lots of surgical clamps to pull the lines into their proper positions. Assemble and install the fuel tank first if your trainer resembles this setup.

Note: If your trainer has the fuel inlet line and vent (pressure) line easily accessible, use a two line system as shown here. If your trainer’s engine is completely cowled in, you will need a third line for filling. Almost all trainers use the two-line system so the third line installation is not detailed here. If you must have one, follow your airplane’s instructions about installing one.

Photo 42         Photo 43

Photo 42 shows all the fuel tank parts in the order in which they will be assembled. The first step is to slide two aluminum tubes into the “rubber” stopper from the smaller side. Two of the three holes in the stopper are drilled completely through. One is not. Do not puncture the latter if you are not using a three tube system. After inserting the tubes, slide the smaller metal nut plate over the two tubes. Position the larger metal plate over the tubing and holes from the top. Insert and tighten the screw about 50%. Your assembly should look like photo 43.

Photo 44

TIP: Pick one of the tubes as the fuel inlet tube (usually the shortest one). My preference is to use the tube on the side of the engines fuel inlet (the left in this case). Use a 1/32 in. nail set to slightly “flair” the fuel tank end of the tube. Do not separate the tubing, just make a slight flair. This flair keeps the tank’s pickup line from slipping off the fuel inlet tube. Do not bend the interior vent tube at this time. Make sure the stopper is not expanded yet.

Photo 45        Photo 46

Slide the transparent silicone fuel tubing over the pickup weight as shown. Make sure that the tubing is all the way on and slips past the slightly raised area as shown in photo 45. This keeps the weight, sometimes referred to as the “Clunk”, firmly attached to the pickup fuel line.

Note the clunk’s position inside the tank in photo 46. The idea is to get the clunk to be about 1/8 in. to 1/4 in. above the rear of the fuel tank when the stopper is fully inserted and the tank is held in the vertical position. Even though the clunk has a small slit cut into it (see photo 45), it is remains possible to starve the engine of fuel in a vertical climb if the clunk rests against the tank bottom. While not as critical in a trainer, this precaution is an absolute necessity in a sport aerobatic airplane.

Photo 47         Photo 48

Slide the tank fuel pickup line over the fuel pickup tubing about 3/4 in. (photo 47). Expand photo 47 and note the flair in the tubing that holds the line firmly onto it. Then place the stopper about even with the tank hole and note if the clunk would extend too far into the tank (photo48). It almost will always be too long.

Photo 49

The first remedy is to slide the pickup line all the way up the remaining tubing (photo 49). Try it again. Only this time, install the clunk and slide the entire stopper assembly into the tank. If the clunk touches the bottom, remove the stopper and cut the line using a sharp hobby razor knife (photo 49). Repeat this process until the clunk is in the correct position. (Now you know why you didn’t bend the vent tube since you will be inserting and removing that stopper too many times.)

Photo 50        Photo 51

Test bend the vent tubing as shown in photo 50. It sometimes helps to put the stopper over the tank’s exterior, even with the tank’s stopper hole but outside the tank. Bend the tubing until it looks like it will fit inside the tank pointing at the TOP of the fuel tank and nearly touching it. This tubing allows the muffler gases to pressurize the fuel tank, (usually to about 3-4 lb/sq. in.), to insure consistent engine runs. If not set high enough towards the tank top, it will not be possible to completely fill the fuel tank.

Once you have the correct height for the vent tube, keep the height but begin to bend the tubing until it points towards the top front of the fuel tank (photo 51). The correct position is for the tubing to point forward with the tubing top almost resting against the tank’s top. The “hole” must be clear.

TIP:  If this is the first time you have assembled the fuel tank, you might wonder how to insert the stopper assembly once the vent tube has been bent. Insert the fuel pickup, with the attached clunk, into the tank. Then tilt the stopper at about a 45-degree angle until the vent tube just clears the inlet. Then slide the stopper forward, while gradually decreasing the angle, until the vent tube is entirely inside the tank. It is possible that the pickup tube is too long to allow this. In that case, slide a bit more of the pickup tubing up into the stopper. It will now extend further out from the stopper on the exterior. This operation is harder to explain than it is to do.

Once all the assembly is complete, use a regular #2 Phillips screwdriver to tighten the stopper screw. Tighten it well so that the stopper can not rotate and you can see it swell in the tank neck, firmly sealing it. A small hobby screwdriver will not provide sufficient torque unless you happen to be an XFC contestant or an Olympic weight lifter. Make sure that the vent tubing is on the top side of the fuel tank.

Sliding the tank in place in the fuselage can be difficult. The short fuel lines provided in the kit do not help. It is possible to slide two 6-32 ball drivers into each line from the front firewall hole and then guide the lines out through the hole. But few new pilots have these tools. (Hint – go buy one 6-32 plus the 4-40 and 2-56 sizes as soon as you can.)

Photo 51A         Photo 51B

Here’s a better idea. Go to the hobby shop and buy a 5 ft. length of the Aerotrend® blue fuel line, standard size, and 5 ft. of the Prather® standard size fuel line. If these brands are not available use the equivalents in your area. Hook the blue line to the fuel inlet and the pink line to the vent tube (photo 51A). Note the flared ends on the two metal tubes to make sure that the lines do not leave their respective tubes during use.

Run the other ends of the 5 ft. fuel lines out through the firewall hole. Gently pull on the lines while pushing the tank into the fuselage from the wing saddle opening. Use the lines to guide the tank stopper into position in the firewall hole. The tank is installed and you didn’t need to learn any nasty new words to do it.

Photo 52

One great thing about the Tower Trainer is that all the holes for mounting the engine mount and nose wheel bearing are not only marked, but are drilled and the blind nuts installed (photo 53). This is not bad for such an inexpensive, but well designed and built, basic trainer. Other ARF trainers may not have the holes drilled; only marked.

In that case, drill the holes as marked and of the instructed size (directions can help sometimes so read them). Insert the blind nuts from the other side and imbed them into the rear of the firewall using the proper size bolt. Insert several larger washers under the bolt head to insure that the firewall front is not damaged during this process. Fortunately, Tower did this work for us.