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Model Airplane Engine Fuel Systems

January 21, 2009



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Airfield Models (http://www.airfieldmodels.com/)Fuel Systems for Model Aircraft

Fuel tanks come in a wide variety of shapes and sizes.  There is a commercially made fuel tank that is perfect for 99.9% of all R/C aircraft applications.

I will be using the terminology for Sullivan fuel tanks to describe the different tank shapes.  In addition to Sullivan tanks, I also recommend those made by Hayes and Dubro.  I have had no problems with any of these brands.

I do not recommend the tanks made by Hanger 9 due to the screw on caps.  These caps can come loose easily and then your model becomes fuel soaked.  Things become really unpleasant when this happens.

Not only is the fuel-oil extremely difficult to remove from the wood but it can also get into your radio gear or cause glue joints to fail.

Also see

 
 

Choosing a Tank Size

With some experience you will know what tank size to use for any given engine.  Until then compare the engine manufacturer's recommendation against the kit manufacturer's.

A mistake I made for years was picking the largest tank that would fit in the tank compartment.  The center of gravity changed significantly during each flight as the fuel level goes down.  Now I select a tank that will provide approximately ten minutes of flight. As a side note I think it's a good idea to time the flight of a new modeluntil it runs out of fuel.  But don't do it until you're comfortable flying the model and you know what it's low speed habits are.

Surround fuel tanks with latex foam rubber inside the tank compartment.  The foam should be snug but not compressed when the tank is put in the aircraft.  If the tank is a loose fit even with the foam, stuff more scraps of foam in the tank compartment until the tank stays in place.

If necessary I will use a smaller tank so I can wrap it with foam rather than a larger tank that does not leave room for foam padding.  The foam helps prevent vibration from foaming the fuel.

The foam will absorb some of the fuel if the tank leaks or breaks although that is really a minor point because your plane will become fuel soaked if the tank breaks whether you use foam or not.

It is always a good idea to use a coat of epoxy or some type of fuel-proof paint in the fuel tank compartment.

 
 

Tank Shapes

Tip: Most fuel tanks including every tank shown on this page can be turned sideways if necessary to fit the installation.

A variety of fuel tank shapes.

Typical fuel tanks.  At the lower right is hardware for one fuel tank.

Rectangular Square (RST) tanks (left and bottom right) have fuel lines that come straight through the front of the tank.  These tanks are used for most applications where the lines pass through middle of the firewall and are then fished between the engine mount and the engine.

Slant tanks (top right) are rectangular with a 45 bevel on the top front of the tank.  These tanks are used when the back of the engine mount is solid or there is some other reason why the lines can not pass through the center of firewall.  The bevel on the tank allows the fuel lines to exit at the top of the firewall without kinking.

Round tanks (not shown) have lines that exit straight through the front of the tank similar to the RST tanks.  They are used in applications where a rectangular tank will not fit, such as in engine nacelles or similar.

Oval tanks (not shown) are used in applications where the fuel tank compartment is too narrow for another type of tank.

The tank in the center of the image is a hybrid.  It is rounded on the top and bottom and flat on the sides.  But it is not narrow enough to be an oval tank.

There are other shapes of tanks as well but the above tanks are the most common.  In fact, almost all R/C aircraft use either an RST tank or a Slant tank.

Some models, such as those powered by turbines, have tanks molded in Kevlar and are shaped to fit a specific location in a specific aircraft.

There are also special shape fuel tanks made for boats that have a "V" shaped hull.

Tanks intended for R/C aircraft (all of the above) are called stunt tanks.  That means the engine can draw fuel regardless of the attitude of the aircraft.  This is accomplished by having a weighted pick-up line in the tank that can move to remain submerged in the fuel.  The weight is called a Clunk.  It is basically a metal fitting that has a hole in it that connects to the end of the fuel line.

 
 

Two and Three Line Tanks

A two-line fuel tank system attached to a model airplane engine.For most purposes a two-line tank is used as shown here.  The feed line connects to the engine's carburetor (with a fuel filter in the line) and the vent line is hooked up to a fitting on the engine's muffler.  The muffler provides pressure to the fuel tank which gives more consistent engine runs.

I recommend a two-line setup like this unless you have a specific reason to use a different setup.  It is simple and trouble-free in most cases.  The tank is filled by disconnecting the line from the fuel tank side of the fuel filter and pumping fuel into the tank.

A three line tank can still be pressurized as long as the third line is plugged.  Three lines are sometimes used in applications where the feed line cannot easily be disconnected.  In these cases, the third line comes through the cowl at some point for filling and is then capped.

The same thing can be accomplished by putting a T-fitting in the feed line, but you have to ensure the system works well so as not to pump the engine full of fuel.

Note the fuel filter in the feed line and the air filter on the carburetor.  This air filter was purchased from a hobby shop when I was in Germany.  I have not seen a similar filter here in the U.S. but there are other air filters available.

A common problem with model aircraft engines is that the needle valve ratchet becomes loose after a while and the needle will not hold its setting.  This condition will drive you nuts if you don't notice it.  A piece of fuel tube can replace the ratchet in most cases and solve the problem.  Additionally, the fuel tube will seal the needle against air leaks.

 
 

Fuel Line

Fuel line for glow engines is made of silicone and is very flexible.  It comes in four sizes that I know of:

  • Small

    Used for 1/2A to .10 engines.

  • Medium

    .15 to .60 engines.

  • Large

    .90 and up.

  • Larger

    This tubing is also called Large, but there is an obvious difference in size.  It is used for fueling pumps.

If you acquire new planes on a regular basis, then it is more economical to buy a 25 yard roll of fuel line rather than three foot pieces.

Before you purchase a full roll, make sure you buy a shorter length to find out if you like it.  Fuel line varies a lot between manufacturers and some is better than others.  I have never had "bad" fuel line, but I have had some that was very difficult to remove from fittings or tore when removing it.

Fuel line does not need to be extremely tight on fittings.  It just needs to be tight enough to seal and to stay in place under normal operating circumstances.

Before installing the fuel line, be sure to deburr all fitting on the fuel tank because silicone does cut easily.  Also, be sure to sand fuel line exits in the firewall so that vibration from the engine does not cause the line to abrade.

I also radius the fuel line exit holes on the front and back of the firewall using a counter sink so there are no sharp edges.  Seal the exits with epoxy or polyurethane before fuel proofing the entire firewall.  These two coats will make the exit nice and smooth assuming you sanded the holes smooth in the first place.

I normally cut each piece several inches longer than needed.  There are two reasons for this.  First, it makes it easier to fish the lines through the firewall.  Secondly, I would rather have the line too long than too short and have to pull the tank back out to put on new lines.

Ensure that the lines are not kinked and have large radius bends.  I have never had an engine that couldn't draw fuel because the line was too long.  Larger bends will help prevent kinking as well as give you extra length to make refueling easier.

 
 

Filters

Always use a fuel filter and an air filter.  A fuel filter will keep dirt from clogging the carburetor and an air filter will keep sand and dirt from getting into the engine.  Fuel filters are made by several manufacturers, are inexpensive and should not be considered optional.

Air filters are difficult to find though.  Bru-Line makes a good air filter that is adequate for flying.  This filter can extend into the propeller arc on some engines and can not be used.  In this case I will use a piece of lady's nylons rubber banded to the carb.  At the very least it will keep larger chunks of dirt out of the engine.

The most important thing you can do is not run your engine on the ground any more than possible especially if the engine is low to the ground (small aircraft or inverted engine).  Do not taxi model aircraft any longer than necessary.  If I can, I carry the model to the runway.  Taxiing around in the dirt does not do anything good for your engine.

Clean Fuel

To keep the fuel system as clean as possible, I wash out the fuel container in my flight box with bleach water a at least at the beginning of each season.  If I notice crud in the bottle, then I will strain the fuel and wash out the bottle as needed.  Coffee filters make good strainers.

I also filter the fuel going from the jug into the fuel tank.  The most important filter of all is between the tank and the carburetor.  If you are using muffler pressure then there is a lot of gunk going from the exhaust of your engine into your fuel tank.

 
 

Fuel Pumps

There are two types of fuel pumps.  One type is designed to get fuel from the fuel can into the fuel tank and the other is to get fuel from the tank to the engine.

Fueling Pumps

A reliable manual fuel pump.To get fuel from the can to the tank you can use a squeeze bulb or manual or electric pump.  Squeeze bulbs are good for smaller aircraft models, but become tedious with tanks larger than four ounces.

I prefer a manual pump because every electric pump I have owned has died after a few months of service.  The instructions state that all fuel must be removed from the pump at the end of the flying day.  I do that and they still break quickly.

Manual pumps seem to last forever.  I have never had one go bad so now I do not bother with electric pumps any more.  Pumps are made that either mount to the fuel container or the side of your field box.  Either type works.

Engine Fuel Pumps

Engine fuel pumps are made to give the most consistent flow of fuel to the engine possible.  These prevent the engine from going lean as the fuel level goes down, inconsistent performance during aerobatic maneuvers or to pull fuel from tanks that are located farther than normal from the engine.

I have owned only one engine with a fuel pump (K & B .40 with built-in pump).  Other than that I have not had a lot of experience with them and have not had a need for one either.

Nevertheless, I can understand how a pump could be useful especially for scale models where engine failure can be the demise of the model or aerobatic contest ships where an inconsistent engine run can cost you the victory.

For sport flyers they are just something else that can go wrong and to tinker with.  I suggest you stay away from them until you definitely have a need for a pump.

 
 

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How to Assemble a Model Airplane Fuel Tank

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Copyright 2003 Paul K. Johnson