Airplanes

Last updated: May 2, 2010.

We take for it granted that we can fly from one side of the world to the other in a matter of hours, but a century ago this amazing ability to race through the air had only just been discovered. What would the Wright brothers—the pioneers of powered flight—make of an age in which something like 100,000 planes take to the sky each day in the United States alone? They'd be amazed, of course, and delighted too. Thanks to their successful experiments with powered flight, the airplane is rightfully recognized as one of the greatest inventions of all time. Let's take a closer look at how it works!

Photo: You need big wings to lift a big plane like this US Air Force C-17 Globemaster. The wings are 51.75m (169ft) wide—that's just slightly less than the plane's body length of 53m (174ft). Photo by Jeremy Lock courtesy of US Air Force.

How do planes fly?

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. Photo by Kemberly Dawn Groue courtesy of US Air Force.

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: 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!

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.

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.

Photo: The Wright brothers took a very scientific approach to flight, meticulously testing every feature of their planes. Here they are pictured during one of their first powered flights on December 17, 1903. Courtesy of Great Images in NASA.

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.

More parts of a plane

Here are some other key parts of planes:

• 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.
• Landing gear: Planes take off and land on sturdy wheels and tires, which are rapidly retracted into the undercarriage (the plane's underbody) by hydraulic rams to reduce drag (air resistance) when they're in the sky.
• Radio and radar: The Wright brothers had to fly their pioneering Kitty Hawk plane entirely by sight. That didn't matter because it flew near the ground, stayed in the air for only 12 seconds, and there were no other planes to worry about! These days, the skies are packed with planes that fly by day, by night, and in all kinds of weather. Radio, radar, and satellite systems are essential for navigation.
• Pressurized cabins: Air pressure falls with height above Earth's surface—that's why mountaineers need to use oxygen cylinders to reach extreme heights. The summit of Mount Everest is just under 9km (5.5 miles) above sea level, but jet planes routinely fly at greater altitudes than this and military planes have flown almost three times higher! That's why passenger planes have pressurized cabines: ones into which heated air is steadily pumped so people can breathe properly. Military pilots avoid the problem by wearing face masks and pressurized body suits.

Photo: You don't actually need landing gear to fly, but you can't take off or land very easily without it! Photo by Susan Cornell courtesy of US Navy.

Further reading

• The Beginner's Guide to Aeronautics: A great introduction to the science of flight (particularly aimed at students) from the NASA Glenn Research Center. Covers how planes and engines work, wind tunnels, hypersonics, aerodynamics, kites, and model rockets.