An aircraft is a machine that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.
The human activity that surrounds aircraft is called aviation. Crewed aircraft are flown by an onboard pilot, but unmanned aerial vehicles may be remotely controlled or self-controlled by onboard computers. Aircraft may be classified by different criteria, such as lift type, aircraft propulsion, usage and others.
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.
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.
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. Photo by Samuel Rogers (with added annotations by explainthatstuff.com) courtesy of US Air Force. Read more about how engines work in our detailed article on jet engines.
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):
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!
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.
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.
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.
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.
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).
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!