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Front view of huge USAF C-17 globemaster plane waiting on a runway

Airplanes

by Chris Woodford. Last updated: June 28, 2013.

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?

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. 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 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?

Airfoils

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 showing airfoil wing on the NASA Centurion solar-powered plane.
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!

Angle of attack

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.

Generally, the air flowing over the top and bottom of a wing follows the curve of the wing surfaces very closely—just as you might follow it if you were tracing its outline with a pen. But as the angle of attack increases, the smooth airflow behind the wing starts to break down and become more turbulent and that reduces the lift. At a certain angle (generally round about 15°, though it varies), the air no longer flows smoothly around the wing. There's a big increase in drag, a big reduction in lift, and the plane is said to have stalled. That's a slightly confusing term because the engines keep running and the plane keeps flying; stall simply means a loss of lift.

Air flow around an airfoil wing in a wind tunnel

Photo: How a plane stalls: Here's an airfoil wing in a wind tunnel facing the oncoming air at a steep angle of attack. You can see lines of smoke-filled air approaching from the right and deviating around the wing as they move to the left. Normally, the airflow lines would follow the shape (profile) of the wing very closely. Here, because of the steep angle of attack, the air flow has separated out behind the wing and turbulence and drag have increased significantly. A plane flying like this would experience a sudden loss of lift, which we call "stall." Photo courtesy of NASA Langley Research Center.

Planes can fly without airfoil-shaped wings; you'll know that if you've ever made a paper airplane—and it was proved on December 17, 1903 by the Wright brothers. In their original "Flying Machine" patent (US patent #821393), it's clear that slightly tilted wings (which they referred to as "aeroplanes") are the key parts of their invention. Their "aeroplanes" were simply pieces of cloth stretched over a wooden framework; they didn't have an airfoil (aerofoil) profile. Nevertheless, the Wrights realized that the angle of attack is crucial: "In flying machines of the character to which this invention relates the apparatus is supported in the air by reason of the contact between the air and the under surface of one or more aeroplanes, the contact-surface being presented at a small angle of incidence to the air." [Emphasis added]

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.

Wing vortices

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 and it's particularly important near airports where there are lots of planes moving all the time, making complex patterns of turbulence in the air. 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.

How do planes steer?

What is steering?

Steering anything—from a skateboard or a bicycle to a car or a jumbo jet—means you change the direction in which it's traveling. In scientific terms, changing something's direction of travel means you change its velocity, which is the speed it has in a particular direction. Even if it goes at the same speed, if you change the direction of travel, you change the velocity. Changing something's velocity (including its direction of travel) means you accelerate it. Again, it doesn't matter if the speed stays the same: a change in direction always means a change in velocity and an acceleration. Newton's laws of motion tell us that you can only accelerate something (change its speed or direction of travel) by using a force—in other words, by pushing or pulling it somehow. To cut a long story short, if you want to steer something you need to apply a force to it.

An airplane banks at a steep angle

Another way of looking at steering is to think of it as making something stop going in a straight line and start going in a circle. That means you have to give it what's called a centripetal force. Things that are moving in a circle (or steering in a curve, which is part of a circle) always have something acting on them to give them centripetal force. If you're driving a car round a bend, the centripetal force comes from friction between the four tires and the road. If you're cycling around a curve at speed, some of your centripetal force comes from the tires and some comes from leaning into the bend. If you're on a skateboard, you can tilt the deck and lean over so your weight helps to provide centripetal force. In each case, you steer in a circle because something provides the centripetal force that pulls your path away from a straight line and round into a curve.

Photo: Steering a plane by banking at a steep angle. Photo by Ben Bloker courtesy of US Air Force.

Steering in theory

If you're in a plane, you're obviously not in contact with the ground, so where does the centripetal force come from to help you steer around a circle? Just like a cyclist leaning into a bend, a plane "leans" into a curve. Steering involves banking, where the plane tilts to one side and one wing dips lower than the other. The plane's overall lift is tilted at an angle and, although most of the lift still acts upward, some now acts sideways. This sideways part of the lift provides the centripetal force that makes the plane go round in a circle. Since there's less lift acting upward, there's less to balance the plane's weight. That's why turning a plane in a circle will make it lose lift and altitude (height) unless the pilot does something else to compensate (such as increasing the plane's speed).

Diagram showing the forces (lift, weight, and centripetal) on a plane as it banks at various angles.

Artwork: When a plane banks, the lift generated by its wings tilts at an angle. Most of the lift still acts upward, but some tilts to one side, providing centripetal force that makes the plane steer round in a circle. The steeper the angle of the bank, the more the lift is tilted to the side, the less upward force there is to balance the weight, and the greater the loss of altitude (unless the pilot compensates).

Steering in practice

There's a steering control in the cockpit, but that's the only thing a plane has in common with a car. How do you steer something that's flying through the air at high speed? Simple! You make the air flow in a different way past the wings on each side. 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, so I won't bother to go into more details here. NASA's basic introduction to flight has a good drawing of airplane cockpit controls and how you use them to steer a plane.

One way to understand control surfaces is to build yourself a paper plane and experiment. First, build yourself a basic paper plane and make sure it flies in a straight line. Then cut or rip the back of the wings to make some ailerons. Tilt them up and down and see what effect they have in different positions. Tilt one up and one down and see what difference that makes. Then try making a new plane with one wing bigger than the other (or heavier, by adding paperclips). The way to make a paper plane steer is to get one wing to generate more lift than the other—and you can do this in all kinds of different ways!

More parts of a plane

Here are some other key parts of planes:

Wright brothers flight at Kitty Hawk

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.

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Text copyright © Chris Woodford 2009, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2009) Airplanes. Retrieved from http://www.explainthatstuff.com/howplaneswork.html. [Accessed (Insert date here)]

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