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The “bubbless fuel tank”
by Bruno Stuekerjuergen
The Bubbless fueltank is without doubt the finest fuel supply system currently available on the market, and is suitable for every methanol-fueled motor. The system offers advantages for every application, whether aerobatics, fun-flying, speed models or display work. Every one of the world’s pylon racing models is fitted with this fueltank system, so there must be something special in its design. As the name implies: “bubbless” simply means “without air bubbles”. Air in the feed line can be a nightmare for any pilot, as it makes the engine run unreliable. If the pilot cannot depend upon his power plant, then he is unable to concentrate fully on his essential activity - the flying. If the pilot cannot focus on flying and controlling his valuable model, sooner or later a crash is inevitable. The solution is simple: it is the “Bubbless fueltank”, which represents the total solution to the fuel supply problem. The system works just as efficiently in model helicopters. The only problem with helicopters is that some trainers are designed exclusively for a particular shape of tank. Even so, the Bubbless fueltank has one advantage to offer every modeller: It constitutes a fuel supply system which works unconditionally, making it much simpler and safer to handle any motor. This fueltank is for pilots who demand a constant motor run throughout every flight. A good motor deserves a good fuel supply, especially if it makes the motor even better and safer. When a modeller encounters motor problems, the fueltank is often overlooked or even not considered at all. This ultimate fueltank system eliminates all the disadvantages of conventional tanks: - no fuel foaming - no tendency for the motor to lean out as the fuel level falls - no clunk pick-up to get jammed - no tendency for the motor to run rich when flying inverted - no air bubbles in the feed line - no combustion residues and old oil in the fuel - constantly clean fuel - no foam padding required round the tank to counter vibration - rigid, space-saving installation, e.g. using cable ties - the full fuel capacity is available for each flight - rigid outer tank walls prevent pumping effects when you alter the throttle setting - only feed and pressure lines are required - no overflow
The “Bubbless” tank does not look substantially different to a “normal” fueltank; the only obvious distinction is the unusual location of the exhaust-fed pressure nipple. So what is the secret of this system? It is the “twin-wall” construction of the tank. The fuel is fed into a balloon which is located inside the main tank. In a conventional system, pressure from the exhaust is fed into the main tank, where the fuel is stored. In the Bubbless tank, pressure is fed into the space between the balloon and the outer wall of the fueltank. This principle means that the fuel cannot come into contact with air at all; it is simply impossible to suck air out of this tank.
Tips for handling the “Bubbless fueltank”: Before the fueltank is installed, it must be prepared by holding it under water (in a small bowl) and blowing into it through the fuel lines. The air pressure of the human lung is quite sufficient for this. Simply blow into the feed line and push the tank under the water’s surface. The fueltank works best when connected using only two fuel lines: the one tube acts as the fuel feed to the motor, and ends approximately in the centre of the tank balloon. This fuel line is continued through the stopper and into the balloon by means of the 3 mm Ø aluminium tube supplied. A length of silicone fuel tubing is fitted on the inside of the aluminium tube to prevent damage to the balloon. The end should project by around 1 cm on the inside; it should be cut in a wedge shape using a sharp knife blade, to prevent the possibility of fuel starvation if the balloon should be sucked against it. The outside end of the aluminium tube is connected to the carburettor. The second line is the exhaust pressure connection: locate the pressure nipple mounted on the side of the fueltank, and connect it to the exhaust system so that air is forced into the space between the fueltank and the balloon. This is all that is required for flying. Now all you have to do is fit some means of filling the balloon with fuel. Filling the tank: We recommend that you route the feed line between tank and carburettor out of the model, and install a separation point for refueling (see pictures). Before filling the tank, the balloon must be evacuated using the Tanker, i.e. any air in it must be sucked out. Before removing the Tanker, it is important to clamp off the feed line to the tank, otherwise air will be sucked into the balloon. A fuel stopper is used for this (see pictures); the stopper is left permanently attached to the fuel line. The Tanker can then be disconnected and filled with fuel (using a pump or the Tanker’s suction). The Tanker is then re-connected to the feed line. It is essential at this point to check that there is no air in the Tanker or in the feed line to the connecting point. When you are sure of this, remove the stopper on the feed line, and force fuel into the fueltank without introducing air. You will clearly feel when the balloon is full. When it is full, draw a little fuel back into the Tanker to ensure that the balloon itself is not under pressure. Any slight pressure in the balloon will cause the tank to overflow, but you can avoid this by clamping off the feed line again before disconnecting the Tanker. The stopper should then be re-opened just before you start the motor. When filling the Tanker it is essential to prevent fuel foaming. Some pumps mix a lot of air into the fuel, varying with the direction of rotation (manual pump). Simply check that fuel is fed into the Tanker without any air being introduced at the same time. The refueling operation sounds more complex than it actually is. This is an ingenious system whose operation soon becomes second nature; we recommend that you just practise the procedure a few times before installing the tank in the model. It is certainly possible to fill the tank with air-free fuel using a good manual pump once the tank has been evacuated. However, do not be tempted to use an electric pump! The tank has no overflow, and you will not notice when the balloon is full. The excess pressure could then ruin the balloon. Practice makes perfect - the motor will repay you with constant, totally reliable running. Rietberg, October 2003
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