Auxiliary Fuel Tanks
Auxiliary fuel tanks or header tanks are commonly used on model helicopters for various reasons. Since they do not come standard with model helicopter kits one might wonder if they are really needed. We watch people completing 3-D manoeuvres and standard aerobatics with or without these devices added to the fuel system. To someone new to the hobby the question often asked is why. Even the most seasoned types might have different opinions of whether or not it is a necessity. It is not my intention to tell anyone what to do since many things need to be considered. It is my aim to give valid technical reasons which should help you to evaluate your specific situation. I will also serve as a guide for a competent installation.
For the person learning to fly, the header tank offers increased duration and possibly a better indication of fuel state. This is because some model helicopters have the fuel tank buried under the canopy hidden from view. If at the early stage you have no need for these two features, then why bother? It most certainly will not make the hovering machine operate better. Alternately for the beginner, even though the second tank adds slight complexity, it also teaches one how to adapt a basic non standard item. If something this simple becomes too difficult then surely helicopter setup and maintenance must be carefully approached. All I am saying is this alteration is not regarded as rocket science so don’t be afraid of trying it out. Care nonetheless is always needed with any part of the model.
For the advanced flyer the header tank acts as a bubble trap. It prevents air picked up by the clunk from reaching the carburettor. Just how much air gets trapped depends on the main and header fuel tank design and the type of flying performed.
Some main tanks are better than others for fuel delivery while others function better in specific attitudes. The person designing the system has to ask themselves what is the best compromise for fuel tank and clunk positioning inside the machine based on the available airframe space. The idea with the clunk is to have it always move with the fuel. Both fuel and clunk movement are subject to gravity and G force. Often the clunk has better access to the rear of the tank since most conventional aerobatic flying (debateable) causes fuel to be at this position. With 3-D and rearward flight becoming more popular this method of thinking might not be the best for these situations. In any event and more importantly, operating with a more fuel laden tank will reduce the amount of air picked up by the clunk. The lower the fuel level, the more chance for air to enter the clunk line. Knowing this fact has caused some people to adjust their flying schedule in so far as specific manoeuvres go. Others will just finish a flight with more fuel remaining. Understanding at which position in the main tank the clunk has poor fuel access can offer some insight.
Lets think about the main tank shape. I have seen rectangular (picture 1) and cylinder (picture2) shaped fuel tanks implemented for the most part.
Some of these allow better clunk positioning at differing flight attitudes. If the clunk line entering from the left side (diagram 1) of a long rectangular shaped tank extends to the far side wall then the left side will never be reached by the clunk.
During the knife edge part of a roll it is possible for the clunk to suck air. Luckily for most of us we spend little time here while a few guys enjoy falling out of the sky rotating around a vertical pitch axis. I wonder if these people have a favourite “this side up”! As you can imagine this tank configuration works best for horizontal pitching manoeuvres. It certainly would not be my first choice considering other possible alternatives.
If we installed the clunk centered on the short, long wall of a well shaped rectangular tank, less compromise will occur. The clunk can arc from side to side and top to bottom but will not reach fuel in the two far corners. The volume of one corner would suggest that a minimum safe fuel level be established for certain flying habits.
See diagram 1A for possible regions of unuseable fuel. It can be reasoned that with this tank the length is best designed at approximately twice the width. A spherical like half shape would seem an ideal model helicopter tank design from an arching clunk point of view. I have not seen such a tank shape nor do I expect to.
Another factor is main tank position in relationship to the carburettor. The higher the tank center line the more static fuel pressure. Ever wonder why water towers are used in large populated settlements. Ideally the tank center line would be set at the needle valve height with the clunk kept as close to the carb as possible. This affords the same pressure variance right side up as it does upside down. Unfortunately this is not a perfect world and so we do the best we can with what we have. Our model engines will tolerate a mixture variance and we can compensate with needle adjustments for the worst lean or rich condition. Peak engine performance will not be available under all fuel level states. The header tank addition will not alter this compromise and in some cases can actually aggravate the situation.
Let us consider how the header tank addition installed at an excessively high position can alter fuel delivery pressure. This variance occurs when the main tank is empty. The weight of the fuel no longer needs to be transferred from the main into the header so the static pressure at the carburettor is now higher. What can be relative here is the difference between the main and the header locations even if the header is closer to the needle valve height. The low main tank position along with the high header location compound matters. If there is a big difference in mounting height, the carb will see a change in mixture (empty main) due to a different static fuel pressure at the carb inlet nipple. Simplistically this has the same effect as moving the main fuel tank higher in the conventional tank configuration. The siphon effect of the main tank is no longer evident because it is empty. A classic example is the Raptor header tank mounting position shown in the picture 3. Some people like this feature as it gives a warning of low fuel by running rich. That’s fine if you are upright, but personally I’d not depend on it.
Due to the minor pressure changes possible, we as advanced model aviators should protect ourselves by not running or tuning our engines at the lean edge unless we do so under the specific or worst designed in low pressure situation. The opposite is true since sometimes engines can break into a four cycle during a flight if we go too rich. Some engines and helicopters are more tolerant so there is no exact rule of thumb for needle setting above maximum lean. Needle tapers and needle thread pitch vary from manufacturer to manufacturer. Fuel differences and nitro content will also play a role. Generally speaking a higher nitromethane content will allow a wider range of mixture settings.
Certain manufactures have seen fit to install fuel pumps (and I hesitate to say fuel injection) on to engines for combatting fuel delivery pressure variances. This allows maximum power throughout a flight. The theory of diminishing returns is prominent when working at the edge, so don’t feel you need to go and buy one of these engines simply because your engine running is not perfectly consistent the whole flight. Accept the fact that you cannot have full power all the time, after all the drop in performance is minor between a safe and unsafe fuel mixture setting.
Items to look at are the header tank capacity as it relates to the size of the helicopter and the available mounting positions. A one or two minute reserve (1 or 2 oz) seems a nice round figure for me. For reasons mentioned the center of the tank should be as close as possible in height and horizontal distance to the carb needle valve. This is the best situation but other locations will work. Have another look at the Raptor picture. Again a caution, when the main tank is depleted the engine can shift to a richer fuel scheduling causing a change in engine note and power output. Manufacturers try to keep the main fuel tank vertical center close to the needle valve height for the most consistent fuel delivery pressure full to empty. Granted design compromises need to be made due to the helicopters compactness but they do strive to this end, as should we whenever possible.
With the header tank we have the choice of using an internal clunk or a standoff pickup pipe at the center of the tank. Both work but half the fuel is not accessible with a standoff. If you do not want this fuel stagnating until the next flying session in the small tank, another line (defueling) can be routed low into a better position. A defueling clunk could be used.
While the clunk pickup will extend endurance slightly it is also one more thing to go wrong. After all it is a moving part and subject to degradation. I choose to use a clunk because it is my personal choice, but I make sure that it is as far away from the back of the tank as functionally possible. I routinely monitor the complete fuel system when defueling from the carburettor supply line since the header should not reduce its fuel level until the main is empty. I also make sure the fuel being drawn from the helicopter contains no air bubbles. This type of thing should be done with all model helicopters. I have witnessed pranged machines due to fuel starvation after the header emptied because the main tank clunk line ruptured. Take your time with a proper preflight or one day you will get caught.
Three common methods can be used for the header tank mounting. All work well but you will have to decide which is best for your aircraft. My favourite involves using the Dave Brown “Hush Clamp Mount” and a cylinder shaped “Sullivan” tank. The long tank allows a clunk to be positioned more towards the center of the tank length. Therefore when a small amount of air gets into the header it will not be drawn into the system during horizontal pitching attitudes. It affords single bolt mounting, vibration isolation and the lowest removal and installation time. It is held securely to the mount by a fuel resistant “O” ring as can be seen in the
The cost is about five dollars for the tank and ten for the mount. The mount comes in two sizes and the tank in several capacities. Helicopters can include inferior clunk tubing and clunks. I have found the Aerotrend small silicone alternative tubing to work the best, being a very good compromise between flexibility and durability. Many tank/clunk assemblies are manufactured with sharp barbes using thin walled silicone tubing. Together this can cause the silicone tubing to be cut when flexing. Sanding the sharp edge off the barbs helps as does the slightly thicker Aerotrend tubing. Certain helicopter manufacturers/distributors claim that high nitro fuel will reduce the life of their specific thin walled clunk tubing and so recommend routine replacement. If any tank uses brass or metal rigid tubing then the sharp edges at the end should be sanded smooth.
The common rectangular tank can be purchased and mounted with tie raps. Sometimes it is hard to find a place to use the tie raps and small holes may need to be drilled or a tank position compromise made. I have even seen professional looking velcro rap mounting. Velcro can however become degraded by fuel and oil residue. A few after market companies supply a nice looking rigid header mounting system using single bolt attachment with a shiny metal bracket. Expect to pay more for the convenience and the privilege of someone else doing the thinking for you. Try Quick UK, HHI etc.
No matter which method you decide to use the plumbing will need to be done correctly. The main tank clunk will feed the header tank inlet. This will normally be located at the top of the header tank so that it may be filled to the top. The clunk or stand-off in the header will feed the carburettor. Again, it is not wise to have the header tank clunk near the back tank wall. We should still use a fuel filter location just before the carb inlet. You can fill the system by pulling the line off the back side of the fuel filter closest to the header tank or you can install a third line into the header tank which must be capped off after fuelling. Fuelling from the backside of the filter prevents dirt entering the clean side of the filter and later flowing into the carb. I do not recommend using a glow fueller valve since it can leak and cause problems. This is most likely due to dirt contamination or cut “O” rings. I have watched people painfully trouble shoot poor running engines due to this issue.
However, the fueller is often necessary on a scale ship (picture 6) or glass fuselage and so this becomes something you need to be aware of. The two (diagrams 2&3) show schematically both the clunk and stand off pipe pick up header tanks.
I hope you enjoyed this article and both the beginner and advanced types have come away with ideas to digest. Finally I’d like you to look at the main tank clunk in the Raptor picture 7/8 as it would be positioned in a vertical climb and dive. Moving the clunk aft by adding clunk tubing length would further reduce the useable nose down fuel supply, so the factory location appears the best compromise. I encourage you to think in this light with all flight situations.
Consider that the header tank has little effect on engine running when the main tank is still supplying fuel if ideal mounting arrangements become too difficult. Finally I hope you enjoy these articles, and I’ll do my best at the keyboard. We’d appreciate your input since this is after all a place meant to fill your needs.