The reason behind this is that the micro swirl in turbulence creates local point of lower vacuum where the vapor pressure of air dissolved in kerosene equals the local level of vacuum. As soon as you open the container, its pressure drops and thousand of micro bubbles of CO2 start coming to the surface). What happens in our models is that air bubbles are created by both a certain level of vacuum AND turbulence (this is very close to what happens in a can of beer. You can see it by using a plastic syringe filled with fuel and pulling the piston while closing the tip. This depression does not have to be very high to create a fuel cavitation condition. This air is dissolved into the fuel until the condition for releasing is met: namely depression. It can absorb over 10 % of its volume of air. This may lead to a phenomenon that is not very well known to the modelers: fuel cavitation. This means that the suction upstream the pump, ie vacuum levels generated, can be very high. What happens with high fuel flow is that the pump is required to deliver a lot of pressure to the engine through the 4 mm/ 2.4 mm PU tubing. We make a lot of these for military / industrial UAVs and have an impressive reliability and dependability record.
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However our latest generation of professional bladders are extremely strong and fuel resistant.
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However, for hard core aerobatics flyers and people who like to fly at full thrust for a large portion of the flight and keep high thrust level in the dive, the amount of air that is sucked by the clunk can be up to 15 seconds of full thrust fuel flow!įor this reason I strongly recommend choosing and air trap that offers 30 seconds of full thrust fuel flow buffer capacity.īaldders are not legal in the US over rupture concerns, back when people were using blood/ plasma bags for their tanks. So as good as the older system are, they only offer a few seconds of maximum thrust fuel flow buffer. Just imagine that the biggest engines available on the market nowadays can burn 1 liter of kerosene per minute, whereas the first JPX turbines were burning a quarter of this amount. On this matter, the BVM UAT was designed for fairly reasonable fuel flows by today’s standards. On older, more conventional tank systems, however, an air trap is strongly recommended, adding complexity, increasing risk of failure and adding dead weight of about 2 to 3 lbs ( fuelled air trap ). As a result, an air trap is not necessary, reducing the weight of the fueling system significantly. Additionally, atmospheric pressure pushes on the bladder and contributes to reduce the effect of suction drag ( we will see this later ). The "tank" is always full, without the possibility of introducing air in the system. The bladder, if setup and filled properly, is designed to run without a vent and collapses as the engine drains fuel. This is the reason why fuel bladder systems are so interested for AUVs and competition models. During this 30 second sequence, one can see that the fuel clunk is out of the liquid for exactly 50% of the time, pickup up air instead of fuel during this period! This video is taken on board an aerobatic plane. Here is a video example illustrating the fact. Although it is heavy and mounted on a flexible line to stay in the fuel as much as possible, there will be situations when it will get out of the fuel and suck air! This will happen towards the end of the flight, as the tanks get mostly empty and/ or while performing aerobatics. The main reason why air would get into your fuel line is not because of a leak but because the rigid fuel tank concept and the fact that a clunk is not a perfect system. The idea behind this device was to be able trap a small air bubble that would otherwise make its way to the engine and create a flameout condition. The venerable BVM UAT was designed many years ago to protect the best ducted fans and the first JPX jet engines from sucking air bubble that would almost certainly lead to a flameout. These units are much more suited for high flow engines and some of them will protect your fuel system very efficiently from the dangers of cavitation as I will explain below.įurthermore, the latest technology in fuel bladders completely eliminate the need of using an air trap and increases the reliability of the fuel system by an order of magnitude, as we will explain further down. Quite a few companies have come to the market with new generation air traps, using a completely different approach from the original UAT.
However with the coming of bigger and more powerful engines, this little device that was designed about 15 years ago is reaching its limits. The air trap market has long been dominated by the BVM UAT and its derivatives.