Mercedes Mullins
Aircraft fuel tanks are an essential component of the aircraft industry. There are three basic types of fuel tanks: integral fuel tanks, rigid removable tanks, and non-rigid removable or bladder tanks. The type of fuel tank construction varies, with metal tanks being lighter than fiberglass tanks, and aluminum being a popular choice for its clean, stable, and easy-to-work-with nature. Aluminum fuel tanks can be fabricated in a workshop with simple tools and homemade gadgets, or assembled with aircraft rivets to avoid welding. Baffles and rivets are important components that must be properly installed and sealed with fuel-proof compounds to ensure leak-proof seams. Fuel systems in aircraft can be complex, with considerations such as fuel lines, pumps, and pressure gauges, as well as the need for proper fuel management to prevent engine failure.
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What You'll Learn
- Aluminium tanks are lightweight, clean, stable, and easy to work with, but difficult to weld
- Flexible hoses are required for all fuel lines from the firewall forward
- A gravity flow fuel system won't work with many aircraft designs
- Baffles can be made of lighter weight material than the tank skins
- Integral fuel tanks provide the highest volume of space with the lowest weight but are the least crashworthy
Aluminium tanks are lightweight, clean, stable, and easy to work with, but difficult to weld
Aluminium tanks are lightweight, clean, stable, and easy to work with. They are a popular choice for aircraft fuel tanks. However, welding aluminium can be challenging for several reasons. Firstly, aluminium has a lower melting point than steel, but it dissipates heat much faster. This means that a higher heat level is required to create a weld puddle, and the process must be completed quickly to avoid burning through the material. Aluminium also requires specialised equipment, such as a machine with HF, which is more expensive than a regular MIG welder. Additionally, aluminium does not change colour as its temperature increases towards the melting point, making it difficult to determine when it is about to melt.
Aluminium's high thermal conductivity further adds to the challenge of welding. Its lower melting point can be suddenly reached due to this property, increasing the risk of accidentally melting the metal. To weld aluminium successfully, it is crucial to have expert instruction and ample practice. Builders who have not attempted to weld aluminium before may find the process daunting.
While welding aluminium requires skill and experience, it is not impossible to master. With the right tools and techniques, such as using a spool gun for MIG welding or employing TIG welding, builders can achieve strong and reliable welds. Additionally, some alternative methods, such as heliarc welding, can be used to join aluminium without the need for traditional welding techniques.
It is worth noting that aluminium tanks can also be assembled using aircraft rivets, eliminating the need for welding altogether. This method is commonly used for integral wing tanks in metal aircraft. The riveted seams and rivets must be sealed with a fuel-proof compound to ensure a tight and leak-proof seal. Standard aluminium weldable flanges can be purchased or machined from weldable aluminium stock for those who prefer welding.
In summary, aluminium tanks offer numerous advantages, such as lightweight construction, cleanliness, stability, and ease of use. However, welding aluminium can be challenging due to its unique physical properties and the need for specialised equipment and skills. Builders have the option to weld or rivet aluminium tanks, with each method requiring careful consideration and adherence to best practices.
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Flexible hoses are required for all fuel lines from the firewall forward
Flexible hoses made from synthetic-impregnated oil-resistant cotton braid with a seamless synthetic rubber inner tube reinforced with a single wire braid over a layered cotton braid are ideal for this purpose. They are tough and can withstand operating temperatures ranging from -65°F to 250°F, with a maximum operating pressure of 2,000-3,000 psi for the small hose sizes used in aircraft.
However, it is important to note that flexible hoses carrying flammables (fuel, oil, hydraulic fluid) should be fitted with firesleeves prior to installation. Firesleeves are protective sleeves that insulate the hoses against heat and flames, making them virtually "fireproof". They are usually made of braided asbestos covered with a flame, fuel, and oil-resistant neoprene substance. The FAA defines a fireproof installation as one that can withstand a direct flame for 15 minutes under operational conditions without failure.
When installing flexible hoses, it is important to avoid twisting the hose when tightening the fittings, as this can shorten the hose's lifespan. Finger-tight hose connections should also be avoided, as they can lead to cracked or deformed sealing seats. Instead, it is recommended to use stainless steel clamps to secure the hoses and trim any excess material protruding from the clamps. Additionally, the fuel line between the gascolator and the engine-driven fuel pump and/or the carburetor should be separated from hot exhaust stacks by a minimum distance of 1-1/4" to prevent heat damage and possible fuel vaporization.
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A gravity flow fuel system won't work with many aircraft designs
A gravity flow fuel system is the simplest, most basic type of aircraft fuel system. It is commonly found on single-engine high-wing aircraft. In this system, the fuel tanks are positioned higher than the engine, allowing the fuel to feed downwards from the tank to the engine using gravity. However, this type of system will not work in many aircraft designs, especially most low-wing types.
The main limitation of a gravity flow fuel system is that it relies on the force of gravity to move the fuel downhill and does not generate enough pressure for a fuel-injected engine. Therefore, aircraft with gravity-feed fuel systems typically have carbureted engines, as gravity cannot create sufficient pressure to run an injected engine.
In contrast, aircraft with low-wing configurations or engines positioned above the fuel tanks require a more complex fuel pump system to draw the fuel upwards from the tanks to the engine. This system utilizes one or more mechanical pumps to provide the necessary pressure to deliver the fuel to the engine. While a pump feed system offers greater flexibility in aircraft design, it introduces potential failure points and complexities that are not present in a gravity flow system.
To ensure the reliable operation of a fuel pump system, it is mandatory to install a backup auxiliary fuel pump in addition to the primary engine-driven pump. This redundancy protects against fuel starvation in the event of a primary pump failure. Additionally, a fuel pressure gauge or a fuel flow meter is required to monitor the fuel pressure and ensure the system's proper functioning.
In summary, while a gravity flow fuel system offers simplicity and reliability, it is limited to aircraft designs where the fuel tanks are positioned higher than the carbureted engine. Aircraft with low-wing configurations or fuel-injected engines must rely on a fuel pump system, which adds complexity but provides the necessary pressure and flexibility to deliver fuel to the engine.
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Baffles can be made of lighter weight material than the tank skins
When it comes to constructing an aircraft fuel tank, one of the key considerations is weight. It is important to use lightweight materials wherever possible to reduce the overall weight of the aircraft, which can improve fuel efficiency and handling.
Baffles are an important component of fuel tanks, helping to distribute and control the shifting weight of the fuel. In aircraft fuel tanks, baffles are angled dividers with holes that slow down the front-to-back sloshing of fuel during acceleration, deceleration, and cornering. This helps to maintain stability and prevent weight imbalances that can affect the aircraft's handling and even lead to rollovers.
While the tank skins, or external walls of the fuel tank, need to be made from strong and durable materials such as aluminum or fiberglass, the baffles can be made from lighter-weight materials. This is because baffles are typically riveted in place and do not require welding, which means they are not subject to the same structural demands as the tank skins.
By using lighter-weight materials for the baffles, such as high-strength polymers or polyethylene, the overall weight of the fuel tank can be reduced. This can have a significant impact on the aircraft's performance and efficiency. Additionally, modern baffled tanks may incorporate flexible or movable baffle systems that can adjust according to the fuel level, further optimizing stability and weight distribution.
When designing and constructing aircraft fuel tanks, it is crucial to carefully consider the materials used for each component, such as the tank skins and baffles, to ensure optimal weight distribution, safety, and performance.
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Integral fuel tanks provide the highest volume of space with the lowest weight but are the least crashworthy
Integral fuel tanks are formed by sealing part of an aircraft's structure with a fuel-resistant two-part sealant to create a tank as a unit within the airframe structure. This type of tank provides the highest volume of space available with the lowest weight. Integral fuel tanks are most commonly found in the otherwise unused space inside the wings, with the wings themselves becoming "wet wings".
Integral fuel tanks are not considered to be crashworthy. In the event of a crash, the tank is a factor, and the pilot has already been killed by the engine. In addition, integral fuel tanks must be able to withstand the pressure developed during the maximum limit acceleration of the airplane when fully filled with fuel with the critical limit structural loads simultaneously applied.
To prepare an integral fuel tank for repairs, it must be emptied, and fuel vapours purged. Respiratory equipment must be used by the technician, and a full-time spotter must be positioned just outside of the tank to assist if needed.
Integral fuel tanks can be fabricated in a similar manner to integral wing tanks, which are routinely assembled with aircraft rivets so that no welding is necessary. The riveted seams and rivets must be sealed with a fuel-proof compound. Baffles can be made of a lighter weight than the material used for the tank skins, and riveted in place.
Rotationally moulded plastic tanks are considered good for crashworthiness, as the plastic is both puncture-resistant and able to deform without spilling fuel.
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Frequently asked questions
Fuel tanks fall into three basic categories: integral fuel tanks, rigid removable tanks, and non-rigid removable or “bladder” tanks.
Metal tanks are almost always lighter than a fibreglass tank of similar capacity. Aluminium is a clean, stable, and easy-to-work-with material that yields predictable results. However, aluminium is difficult to weld and most people do not have the equipment to do so.
Flexible fuel lines are easier to install than solid fuel lines, but they are more susceptible to being pinched. Solid aluminium lines are more resilient in a crash but can increase the force applied to the tank in the event of a wing being sheared off.
Fuel mismanagement is a common issue, which can lead to engine failure. This can be caused by a lack of standardisation in fuel systems, requiring pilots to switch tanks, read fuel gauges, and operate pumps and switches.
