If a wet fuel engine is used, some form of fuel tank becomes necessary. Commonly, these are separate items with some form of closure where supply, filler, and air lines are installed (with the filler readily accessible for refueling). They are available in a range of sizes and can be obtained for methanol- or gasoline-based systems (generally not interchangeable - note also that fuel pipes must be chosen to suit the fuel in use; gasoline rapidly degrades many forms of plastic pipe), see Figure 5.12. If SLS nylon structures are being used on the aircraft as we commonly do, the fuel tank can be formed as an integral part of the structure, as already noted in Chapter 4, allowing for very large capacities with minimal tank weight. In all cases, some form of fuel pick-up must be included that allows for the aircraft to roll and pitch without leading to engine fuel starvation. This is commonly in the form of a “clunk” - a heavy-weighted end that fixes to the pipe inside the tank so that it always lies at the bottom with the remaining fuel.
Figure 5.12 Large UAV fuel tanks. Note clunks and fuel level sensor fitting at rear left-hand corner of one tank.
The fuel tank should be sited either close to the engine (minimizing pipe runs) or near the center of lift (minimizing the impact of fuel usage on trim). The choice largely depends on the total fuel load: if only modest endurance is needed, then placing the tank close to the engine is good practice; for long-endurance aircraft where the weight of fuel may be many times that of the engine, a location near the main wing center becomes important. If a pusher propeller configuration is adopted, sometimes it is possible to achieve both aims at once. The aircraft of Figure 5.7 has a separate fuel tank placed just behind the main spar and is thus only a few centimeters forward of the engine. In the case of the SPOTTER aircraft of Figure 5.8, a large central monocoque tank is used. Since the OS GF40 engines used on this aircraft incorporate diaphragm pumps, the lengths of fuel pipe are not an issue. For engines without such systems, it may be necessary to install a dedicated fuel pump to pressurize the fuel lines, though this can lead to dangers of engine flooding at idle settings. It is important to test all such installations at a range of throttle settings and pitch and roll angles before flying, to ensure that fuel starvation or flooding is not a problem.
The final part of the fuel system is some means of checking on the amount of fuel remaining during flight. At the most basic level, the time in the air can be recorded and compared against known fuel consumption rates from earlier flights along with visual inspection of the amount of fuel on board before commencing flight. While this may suffice for short tests and smaller aircraft, it is not workable for long-endurance systems where some kind of in-flight fuel monitor is necessary. While float systems can be used for these purposes, we prefer to use optical sensors that detect low fuel levels and trigger alarms, typically at 25% and 10% remaining levels, see Figure 5.13. These sit on metal inserts glued into the nylon and are sealed with O-rings.