How-To Article by Mike Buzzeo (MinnFlyer) Email me
Fuel tanks are one of those things that we veteran fliers usually take for granted, but that can be very frustrating to the beginner, so here is an article that will hopefully answer many of the questions the beginner might have about setting up a tank.
Let's start out with the type of tanks available. Frankly, there are far too many to list, but when you start out, there is only one type, although it may come in a variety of shapes: Square, round, straight, slant, etc. The type you use will depend on the specific plane you're building, or it may have come with your airplane.
The parts involved are:
Front plate for stopper (Or plastic cap)
Rear plate for stopper
Metal (or plastic) fuel tubes (usually two or three)
Other items that may be included with your tank are:
A stiff plastic tube
A basic rundown of the setup is as follows: Two metal tubes will go through the stopper. One is to vent the tank (And is usually connected to the muffler for pressure) and a piece of fuel line is attached to the other tube with a clunk at the far end, which acts as a weight to hold the fuel line below the fuel level.
So let's get started. The first thing you want to do is to inventory the parts and make sure the tank has no debris left inside it from the molding process. If the tank seems dusty inside, you could wash it out with water and a mild soap, but make sure it is fully dry before assembling. You can avoid this by washing it with denatured alcohol instead of water.
The next thing you'll want to do is assemble the stopper. Most stoppers are designed with three holes, but only two of them go all the way through. The third hole is used for a 3-line system, which we will go into later.
Push one of the metal tubes through one of the two through-holes so it protrudes about 3/4" past the front of the stopper. Lay a second tube along side the stopper and mark it so there will be 3/4" forward and aft of the stopper.
Cut the tube by placing a razor blade on the mark and roll it back and forth on your workbench until the tubing breaks (This is actually very easy to do). Once it cracks, you can finish the break with your hands. You may also need to touch up the end with sandpaper.
Insert the second tube into the stopper.
Now, place the front and rear plates over the tubes and put the screw through the front plate and thread it into the rear plate, but DO NOT tighten it yet. In fact, leave the plates slightly loose for now.
Next, you will want to bend the longer tube (Vent) so that once assembled, it will reach the top of the tank. It is not critical to get it perfect, but try to get it close (Without touching) - The closer it is, the more fuel will go into the tank before it reaches the vent. Make this bend carefully and try to avoid kinking the tube. Minor kinks are ok, but remember that air must easily flow through the tube. Some of the tricks used here are to fill the tube with salt or sand, or put a piece of heavy monofilament line (Like from a weed whacker) inside the tube before bending, but I find that these methods are only necessary in some severe cases.
The short tube will be for the fuel pick-up line. All that is really needed here is a piece of fuel line with the clunk at the far end, but adding a piece of stiff tubing (about 2" long) to the middle of this line is a good habit to start getting into now.
I find that best way to do it is to cut a piece of silicon fuel tubing about 1" long. Attach the clunk to one end and the stiff tube to the other. Now attach a piece of tubing to the other end of the stiff tube and lay this assembly and the stopper along side the tank and "eyeball" where to cut the fuel tube. Remember that it's better to cut the tubing a little long - you can always cut it shorter.
Attach the tubing to the feed line on the stopper and we're ready for final assembly.
Why the stiff tubing?
Ideally, the clunk should sit close to the rear of the tank without touching the back so it can swing freely to each of the rear corners. In other words, if you hold the tank with the stopper at the top, you should be able to swing the clunk all around the back of the tank without it ever touching bottom. However, if you hold the tank with the stopper at the bottom, the clunk should NOT swing down to the stopper side of the tank. This doesn't make sense to many beginners and we're often asked, "What keeps the clunk from sucking air when you're low on fuel and flying in a nose-down attitude?"
My highly technical answer to this question is, "I don't know", but it doesn't happen. And if it does, you're flying too long. Land sooner, and refuel, or put a larger tank in your plane (Of course, there are those who figure a larger tank just means they can fly even longer!) Most fliers use about one half, to two thirds of a tank per flight so there is always some "reserve fuel" if needed. But I digress.
The bottom line is, the clunk should never swing to the front of the tank, and if it does, (Which can happen on an abrupt stop, like a nose-over on landing) it will get stuck there - and you probably won't be aware of it. Then you'll go crazy trying to figure out why your engine quits every time you do a loop!
By adding a piece of stiff tubing to the fuel pick-up line, the clunk CAN'T swing to the front of the tank, so it is a very good habit to get into!
One final note before putting the stopper in - They are TIGHT! I will often chamfer the outer edge of the tank neck with a #11 blade in a hobby knife - just be careful to remove any scraps that may have fallen into the tank!
Now that everything is ready, slip the clunk line into the tank, and hold the stopper at an angle so you can slip the vent line through the neck. Rotate the stopper as you insert it and firmly push the stopper into the tank - in many cases, this is easier said than done! But keep trying; it WILL go in there!
With the stopper in place, you'll want to check the location of the vent and make sure it is close to the top. Then, hold the tank upright (Stopper at top) and make sure that the clunk is near the bottom, but swings freely. If either is incorrect, you'll have to take the stopper out again and adjust the lines as necessary. Then you get to do it all over again!
At this point, if you choose to do so, you can use some soft wire to secure the fuel lines at each of the connections. These tie-wires are often supplied with a tank, but personally, I don't bother with them except on high-pressure systems.
Once everything is correct, tighten the screw. This compresses the rubber stopper so it tightens itself in the neck of the tank, but this is another place you need to be careful. Over-tightening can cause the neck to split! It might happen now, or a month from now, so don't go mid-evil on it. Just tighten it enough so you can't turn the stopper in the neck - this is something you may want to double check after a season of flying.
Your tank assembly is now complete!
There are many concerns when it comes to fuel tank installation. The first is tank placement. Ideally, the tank should sit so that its centerline is even with the carburetor's spray bar. If it is too high, your engine can suffer from flooding, and if it's too low, you can have a problem drawing fuel. Of course, unless you designed the plane yourself, in most cases you're stuck with where the designer placed it, but it usually at least close to being in the right place, and if your engine has a pump, placement is not critical; for instance, many people who fly gassers choose to place the tank over the CG.
Just as important as placement is padding. You want to pad the tank with foam rubber or something similar to isolate it from vibrations (which can cause the fuel to foam up like a shaken can of beer). This will cause the fuel mixture to run lean and could possibly do damage to the engine. On some ARF's, there is little if any room for padding, but do what you can.
Another concern is the front of the tank. Make sure there is room for the fuel and vent lines so they don't kink or otherwise get jammed into the firewall.
Speaking of the firewall, are the engine mount screws protruding too far into the tank compartment? If so, the tank could spring a leak if it should vibrate against them or if it is shoved into them on a bad landing.
The next concern is the fuel lines. Some planes are designed in such a way that the neck of the tank goes straight through the firewall, while others have the fuel lines connected to the tank and routed to the side of the firewall. Plan your plumbing so that the silicon fuel lines leave the metal tank tubes as straight as possible to avoid having the metal tubes cut through the silicon.
An advanced way to prevent any such problem is to replace the brass or aluminum tubes that came with the tank with 1/8" copper tubing (Available at Auto Parts stores). The copper tubing bends much easier than the stock tubes, and you can make them any length you like, so they can run from the tank, completely through the firewall and avoid any possibility of a leak in the tank compartment.
FILLING THE TANK
Once the tank is assembled and properly installed, the feed line will connect to the carburetor and the vent will attach to the pressure fitting on the muffler. It is a good idea to use two different colored lines to avoid mixing them up. I use red tubing for the vent/muffler pressure because it's "HOT" and clear or clear blue for the carburetor because you want to be able to detect any bubbles in that line.
To fill the tank, remove the lines from the carb and muffler and attach your fuel pump to the carb line, then fill until the fuel comes out the vent line. Some people like to attach a small bottle to the vent line to catch any overflow. If you cannot access the muffler line, just fill until fuel starts to drip out of the muffler (It's not the neatest way of doing it, but - that's what we do!).
Now re-attach the lines to the carb and muffler and you're ready.
One more note here is that it's a good idea to add a piece of metal or plastic tubing to one of these lines so that at the end of the day, you can attach them to each other to seal the tank and avoid dripping any residual fuel.
WHAT DO I USE TO GET THE FUEL INTO THE TANK?
You will need a method of getting fuel from your jug into the tank. This can be something as simple as a squeeze bulb to an electric fuel pump. Most modelers agree that a simple hand-cranked pump is the best method. They work well, are inexpensive and uncomplicated.
2-LINE SYSTEMS vs. 3-LINE SYSTEMS
The three-line system is identical to the two-line system but it adds a third line for filling and draining (de-fueling). The reason for this is that on many airplanes where the engine is enclosed in a cowl, it may be difficult or impossible to access the line to the carburetor. To set this system up, you just add a third line to the tank by pushing another fuel tube through the third hole in the tank stopper. This is the hole that does not go all the way through. Just push the tube in from the rear until it breaks through the front. You can help it along with a razor blade if you like. Some people like to add a clunk to the third line, but I find that by pointing the tube toward one corner of the tank, a clunk isn't necessary.
The third line is usually capped by a fitting known as a "Fuel Dot" that attaches somewhere on the outside of the plane. And in case you're wondering, I use GREEN silicon tubing for the third line.
WHAT ABOUT FUEL-FILLING VALVES?
You will find very few veteran fliers that still use fuel-filler valves. Most of us have tried them and decided that they just aren't worth the problems they create. The main problem is leaking (Both fuel and air leaks). Some people use them with no problems, but they are the rare exception to the rule. Generally, I recommend that people avoid them.
At any time you may experience problems with your fuel system. Here are some of the more common problems and things to look for.
One of the first things to look for in any situation is any sign of bubbles. Not all bubbles are bad, but you should know the bad ones from the acceptable ones. The first type of bubble is just a little foam that may accumulate in the fuel line (Pictured on top of illustration at left). It is usually just sits in a high spot in the fuel line and it may or may not have much movement. Pictured at the bottom of the illustration is a line of bubbles that are moving from the tank to the carb. This is a definite sign that you have an air leak somewhere, and it needs to be corrected.
An air leak is usually caused by a bad connection, a pinhole in a line, or a malfunction in some apparatus (Like a fuel filler or remote needle valve). Now it's time to do a little detective work!
Start by filling the tank. If the bubbles appear while the engine is running with a full tank, you can pretty much figure that the leak is outside of the tank because all of the tank's internal components are surrounded by fuel (That's a good thing - you don't have to take the tank apart!).
The next thing to check is the line from tank to carb. Is there anything between the two, like a fuel filler or remote needle valve? If so, do the bubbles start before or after it? These are the types of questions you'll need to find the answers to. For instance, if there are no bubbles from the tank to the filler, but the bubbles are seen between the filler and the carb, replace that piece of fuel tubing. If the bubbles persist, your fuel filler valve is leaking air.
If the bubbles only appear at a time when the tank is less than half full, then the leak is most likely inside the tank. The easiest remedy is to replace all of the internal tank lines.
Another common problem we hear about is someone complaining that as soon as they start to fill the tank, fuel starts to come out the vent. This is usually caused by one of two things: either you're filling through the vent instead of the feed line, or your tank is upside-down.
Here's another often-heard problem, "Every time I do a loop, the engine quits". Check your tank, chances are the clunk is stuck in the front, or the feed line fell off. This will allow the engine to draw fuel when the tank is full and the plane is sitting relatively level, but as soon as the nose comes up, the feed line will be sucking air.
As you can see, there's more to a fuel system than you might think. From assembly, to installation, to operation there is a lot that can go wrong. We hope you will find this article helpful the next time you need to install or troubleshoot a tank.