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Name: Fred
Status: other
Grade: other
Location: NJ 
Country: N/A
Date: 9/16/2005

How does a fuel tank in an air plane work in all directions? i.e if the tank is upside down, right side up, or tilted any which way the fuel is supposed to come out to the last drop? How does this work?

Fred -

There are a number of techniques to accomplish this. Here are two -

1. The fuel uptake in the tank is a "flop tube" and it falls to the bottom of the tank in whatever position the plane finds itself.

2. The use of a "header tank" that is fed by the main tanks and designed to feed the engine any attitude. This tank holds enough to provide the fuel to the engine - for a period of time - even when the fuel from the tank is not flowing.

Many small aircraft use a gravity feed system that will not work upside down even with one of these accommodations. Either an engine driven or electrical fuel pump is required in some attitudes. Also, aircraft fuel tanks have an unusable quantity of fuel. In level flight, a typical light plane with 20 gallons in a tank will have a gallon or two unusable. The primary reason for this is to trap water and contaminants. A pilot drains a sample of fuel from the bottom of the tank to check for such before flight.

Larry Krengel

What you describe not true for all airplanes. Part of being "fully aerobatic" is having the right kind of gas-tank.

One way to do it is a flexible bladder, a bag full of gasoline, inside the rigid tank. as the bag slowly empties of fuel liquid, air enters into a vent-hole in the rigid tank to replace the volume. Another way is using surface tension and a well-sculpted mesh-filled tank. The liquid will stick hardest to the densest parts of the mesh or sponge. But this force is weak and the thick mesh slows down fuel flow, so it's more likely for low-G spacecraft or something else. Maybe one can afford to have multiple fuel-lines and actively switch to whichever one has liquid. If you are clever you might think of a passive mechanism open only the one tap with the right g-force or liquid coverage. Filling the tank with some ordinary mesh helps in this situation to minimize dynamic sloshing around, which could otherwise put lots of air-bubbles into the fuel lines.

Large tanks can be difficult to fix up right. Small reservoirs fed by pumps can give immunity for short periods of emergency maneuvering. Perhaps passenger planes have this. Sometimes these reservoirs are called "phase separators" because, given mixed liquid and gas-bubbles, they steer the liquid one way and the gas another.

I do not know which technologies are actually used. Seems to me there are a variety of options.

Jim Swenson

The most common method is a weighted pick-up. Imagine a flexible hose inside the tank, with a weight at one end. The end of the hose would be pulled to the 'bottom' of the tank, as determined by whichever direction the latest movements of the airplane are pushing the fuel. It is not a perfect system, but pilots and aircraft designers understand they will not be able to get every last bit of fuel out of a tank. In designing the aircraft, allowances are made for the unusable fuel, and in flying the aircraft, pilots keep enough extra fuel in the tanks to ensure they will not suddenly go dry when the pick-up cannot reach the last bit of fuel.

Another method involves pressurized fuel tanks, like a water balloon that has not been tied off.

Ryan Belscamper

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