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Airplane Fuel System The airplane fuel tank system comprised a left wing tank, right wing tank, auxiliary fuel tank and tail fuel tank (see figure 1). The auxiliary fuel tank system beneath the center cabin had a forward, center, and aft tank that were interconnected by pipes and that were not isolated from each other by shutoff valves or check valves. The tail fuel tank system had two saddle tanks and a third tank at the rear of the tail cone. (see figure 1). Fuel tanks: You need fuel to power a plane—lots of it. An Airbus A380 holds over 310,000 liters (82,000 gallons) of fuel, which is about 25,000 times as much as a typical car! The fuel's safely packed inside the plane's huge wings. How do planes fly? Forces acting on a flying plane: thrust, weight, drag, and lift If you've ever watched a jet plane taking off or coming in to land, the first thing you'll have noticed is the noise of the engines. Jet engines, which are long metal tubes burning a continuous rush of fuel and air, are far noisier (and far more powerful) than traditional propeller engines. You might think engines are the key to making a plane fly, but you'd be wrong. Things can fly quite happily without engines, as gliders (planes with no engines), paper planes, and indeed gliding birds readily show us. Photo: Four forces act on a plane in flight. When the plane flies horizontally, lift from the wings exactly balances the plane's weight. But the other two forces do not balance: the thrust from the engines pushing forward always exceeds the drag (air resistance) pulling the plane back. That's why the plane moves through the air. If you're trying to understand how planes fly, you need to be clear about the difference between the engines and the wings and the different jobs they do. A plane's engines are designed to move it forward at high speed. That makes air flow rapidly over the wings, which throw the air down toward the ground, generating an upward force called lift that overcomes the plane's weight and holds it in the sky. So it's the engines that move a plane forward, while the wings move it upward. Photo showing jet engine afterburner at night. Diagram showing Newton's third law of motion applied to the wings and engines of a plane. Photo: Newton's third law of motion explains how the engines and wings work together to make a plane move through the sky. The force of the hot exhaust gas shooting backward from the jet engine pushes the plane forward. That creates a moving current of air over the wings. The wings force the air downward and that pushes the plane upward. Left photo by Julianne Showalter, right photo by Samuel Rogers (with annotations by explainthatstuff.com), both courtesy of US Air Force. Read more about how engines work in our detailed article on jet engines. How do wings make lift? Okay, so the wings are the key to making something fly—but how do they work? In most science books, you'll read that airplane wings have a curved upper surface and a flatter lower surface, making a cross-sectional shape called an airfoil (or aerofoil, if you're British): Photo: An airfoil wing has a curved upper surface and a flat lower surface. This is the wing on NASA's solar-powered Centurion plane. Photo by Tom Tschida courtesy of NASA Dryden Flight Research Center (NASA-DFRC). When air rushes over the curved upper wing surface, it has to travel further and go slightly faster than the air that passes underneath. According to a basic theory of physics called Bernoulli's law, fast-moving air is at lower pressure than slow-moving air, so the pressure above the wing is lower than the pressure below, creating the lift that holds the plane up. Although this explanation of how wings work is widely repeated, it's not the whole story. If it were the only factor involved, planes couldn't fly upside down. Flipping a plane over would produce "downlift" and send it crashing to the ground! Air flow around an airfoil wing in a wind tunnel Photo: An airfoil wing in a wind tunnel. You can see lines of smoke-filled air approaching from the right and deviating around the wing (tilted at a steep angle to the horizontal). Photo courtesy of NASA Langley Research Center. So other factors must be involved in producing lift as well. The best way to think about lift is also the most obvious, at least to a physicist: according to Isaac Newton's third law of motion, if air gives an upward force to a plane, the plane must give an (equal and opposite) downward force to the air. So a plane really generates lift by using its wings to push air downward behind it. That happens because the wings aren't completely flat, as you might suppose, but tilted back very slightly so they hit the air at an angle of attack: As a result, the wings direct the airflow downward, which pushes them upward and produces the lift. To produce extra lift at takeoff and landing (when the plane is moving slower), the planes have flaps on their wings they can extend to push more air down. Now a plane doesn't throw air down behind it in a completely clean way. (You could imagine, for example, someone pushing a big crate of air out of the back door of a military transporter so it falls straight down. But it doesn't work quite like that!) Each wing actually sends air down by making a spinning vortex (a kind of mini tornado) immediately behind it. It's a bit like when you're standing on a platform at a railroad station and a high-speed train rushes past without stopping, leaving what feels like a huge sucking vacuum in its wake. With a plane, the vortex is quite a complex shape and most of it is moving downward—but not all. There's a huge draft of air moving down in the center, but some air actually swirls upward either side of the wingtips. Wing vortex shown by colored smoke Wing vortex studied in a wind tunnel Photo: Newton's laws make airplanes fly: A plane generates an upward force (lift) by pushing air down toward the ground. As these photos show, the air moves down not in a neat and tidy stream but in a vortex. Among other things, the vortex affects how closely one plane can fly behind another. Left: Colored smoke shows the wing vortices produced by a real plane. The smoke in the center is moving downward, but it's moving upward beyond the wingtips. Right: How the vortex appears from below. White smoke shows the same effect on a smaller scale in a wind tunnel test. Both photos courtesy of NASA Langley Research Center. Not surprisingly, the bigger the wings, the more lift they create. That's why gigantic planes need gigantic wings. But small wings can also produce a great deal of lift if they move fast enough. Helicopters produce a huge amount of lift by spinning their rotor blades (essentially thin wings that spin in a circle) very quickly. How do planes steer? There's a steering control in the cockpit, but that's the only thing a plane has in common with a car. Planes are moved up and down, steered from side to side, and brought to a halt by a complex collection of moving flaps called control surfaces on the leading and trailing edges of the wings and tail. These are called ailerons, elevators, rudders, spoilers, and air brakes. Wikipedia's article on control surfaces is a pretty good explanation of what they all do with some very clear diagrams. |