D. Structures and Mechanisms · Flight

How Wings Make Lift

D2.1 D2.2 A1.2 A1.5About 45 minutes

Learning goals

I can explain that a wing makes lift by turning moving air downward.
I can explain that the air pressure under a wing is higher than the air pressure on top of it.
I can describe how wing shape, angle and speed affect lift.
I can explain why the "equal transit time" idea about wings is a myth.

Words to know

wing
A flat or curved surface that makes lift when air moves past it.

airfoil
The special cross-section shape of a wing: rounded at the front, curved on top and thin at the back.

angle of attack
How much a wing is tilted, front edge up, compared to the air flowing toward it.

leading edge
The front edge of a wing, which meets the air first.

trailing edge
The thin back edge of a wing, where the air leaves.

stall
A sudden loss of lift when a wing is tilted too steeply and the air can no longer follow its top surface.

myth
An idea many people believe that is not true.

Stick your hand out... safely!

If you have ever held your hand flat in a strong wind, or moved it through water in a pool, you may have felt something surprising. Tilt the front of your hand up a little and your hand gets pushed up. Tilt it down and your hand gets pushed down.

That push is the same force that holds a plane up. It's called lift. (Never put your hand out of a moving car window. Try it in a swimming pool or in front of a fan instead.)

Wings turn air downward

A wing is shaped and tilted so that the air flowing past it gets turned downward. Air flowing over the curved top follows the curve and leaves the back of the wing heading down. Air hitting the tilted bottom is pushed down too.

Here's the important part: when a wing pushes air down, the air pushes the wing up. Forces always come in pairs like this. When you push on a wall, the wall pushes back on you. When a wing pushes a huge amount of air downward every second, the air pushes the wing up. That upward push is lift.

Helicopter blades are spinning wings. If you stand near a helicopter (at a safe distance) you can feel the strong wind it blows downward. That downward rush of air is how it holds itself up.

A view out an airplane window of the wing over a sea of white clouds. — This wing is tilted and curved so that it turns air downward. When the wing pushes air down, the air pushes the wing up. AI-generated image

A wing seen from the side (the plane flies to the left, so air meets the wing from the left). The wing is curved and tilted. It turns the air that flows past it downward. Pushing air down pushes the wing up: that upward force is lift.

The same story, told with air pressure

Scientists can also describe lift using air pressure. Remember, air pushes on everything.

As the wing moves, the air flowing over the top of the wing speeds up and its pressure drops. The air pressure on top of the wing becomes lower than normal.
The air pressure under the wing is higher.
The air pushes harder on the bottom than on the top, so the wing is pushed up.

This is not a different reason. It is the same lift, described in a different way. The wing turns air downward and there is lower pressure on top and higher pressure underneath. Both happen together.

The air pressure on the bottom of the wing is higher than on the top, so the air pushes up harder than it pushes down. The difference is lift.

Busting a myth: "equal transit time"

You might read that air going over the top of a wing must travel farther, so it has to go faster to "meet up" with the air going underneath at the back edge. This is called the equal transit time idea, and it is a myth.

The air on top and the air below do not need to meet again. Nothing makes them.
Wind tunnel tests show the air over the top actually reaches the back of the wing sooner than the air underneath. It goes even faster than the myth says.
The myth can't explain why paper airplanes with flat wings fly, or why stunt planes can fly upside down. A tilted flat wing still turns air downward, so it still makes lift.

What makes more lift?

More speed: a faster wing turns more air down every second. Lift grows a lot as speed goes up.
More tilt (angle of attack): tilting the leading edge up a little more turns air down more strongly and makes more lift.
Bigger wings: more wing area moves more air.
Wing shape: a curved top (the airfoil shape) helps the air follow the wing smoothly and turn downward.
Thicker air: cold air and air near sea level are denser (more air in each litre), so wings make more lift there than in hot air or high up.

But there is a limit! If a wing is tilted too steeply, the air can't follow the curved top any more. It breaks away into swirls, and lift suddenly drops. This is called a stall. Pilots are trained to avoid stalls, and planes have warning alarms.

Tilting a wing more (a bigger angle of attack) turns more air down and makes more lift, but only up to a point. Too steep, and the air breaks away in swirls: the wing stalls and lift drops.

Worked example

Let's explain why a paper airplane flies

A paper airplane has flat wings with no curve at all. How can it make lift?

When you throw it, you give it speed, so air moves past the wings.
Its wings are tilted slightly, with the front edge up (an angle of attack).
The tilted wings push air downward as they move.
The air pushes the wings up. That is lift. Weight slowly wins because there is no engine, so the plane glides down.

Its flat wings are tilted and moving, so they turn air downward and the air pushes them up. A curved shape helps, but tilt plus speed is enough.

Worked example

Let's think like a pilot

A plane is taking off from Thunder Bay on a cold winter morning. The next day it takes off on a very hot summer afternoon with the same load. On which day will it probably need a longer run down the runway? Why?

Lift depends on how much air the wings can push down.
Hot air is less dense: there is less air in each litre.
On the hot day, the wings push down less air at the same speed, so they make less lift.
The plane must go faster before lift equals its weight, so it needs a longer run.

The hot summer afternoon. Thinner, hot air gives less lift at the same speed, so the plane must reach a higher speed.

Just for fun

A wing's motto: "If you push me up, I'll push you down." The air's motto: "Deal!" That deal, made millions of times a second, is what keeps every plane in the sky.

Hands-on activity

Build a wing that climbs a string

You need: a sheet of paper (about 21 cm by 28 cm), tape, a drinking straw, about 1 m of string or thread, a ruler and a hair dryer (with a cool setting) or a fan.

Safety: a grown-up should help with the hair dryer. Use the cool setting, keep it away from water, and unplug it when you are done.

Fold the paper in half so the short ends meet. Slide the top end back toward the fold about 2 cm (so the bottom layer sticks out a little at the back) and tape the top end down onto the bottom layer. The top layer bulges up into a curve, like an airfoil. The fold is the leading edge (front).
Push a pencil through both layers about a third of the way back from the fold, and make the hole just big enough for the straw. Push the straw through and tape it in place so it stands straight up and down.
Thread the string through the straw. Tie the bottom to a chair leg (or have a helper hold it) and hold the top so the string is tight and vertical.
Point the hair dryer or fan at the folded front edge of the wing, level with the wing. What happens?
Fair test: hold the hair dryer 60 cm from the wing, then 30 cm from the wing. Change only the distance (keep the same setting and the same height). Measure how high the wing climbs each time with the ruler. Then switch the hair dryer off: what happens?

What to look for: with air blowing at the leading edge, the wing rises up the string. It usually climbs higher when the dryer is closer (the air reaching it is faster), and it slides back down when the air stops. This shows that a wing makes lift only when air moves past it, and faster air makes more lift.

Your turn

In your own words, how does a wing make lift?

Compare the air pressure on top of a wing and under it.

What is angle of attack?

Name two things that give a wing more lift.

What is a stall?

Is it true that the air over the top of a wing must meet the air from underneath at the back edge? Explain.

How does a helicopter hold itself up?

More practice: warm-up

What do we call the front edge of a wing?

What do we call the special shape of a wing's cross-section?

True or false: a wing must be moving through air (or have air moving past it) to make lift.

When a wing pushes air down, which way does the air push the wing?

True or false: flat wings can never make lift.

More practice: on your own

Why do airplanes speed down a long runway before they lift off?

A pilot tilts the plane's nose up a little. What happens to lift? What happens if the pilot tilts it up far too much?

How can a stunt plane fly upside down?

Airports high in the mountains often have longer runways. Explain why.

Explain why the air pressure story and the "air turned downward" story are not two different reasons for lift.

Challenge

Maya's friend says, "The air on top of the wing goes farther, so it has to go faster to arrive at the back at the same time as the air underneath." Write two sentences Maya could use to show this is a myth.

In the string-wing activity, the wing rose higher when the hair dryer was set to high instead of low. Which factor changed, and which factor should have stayed the same for a fair test?

Some birds, like gulls, can hang almost still in a strong wind coming off a lake, without flapping. Use what you learned about lift to explain how.

Check your understanding

Quiz

How does a wing make lift?

Compared with the air under a wing, the air pressure on top of the wing is:

Which statement about the "equal transit time" idea is correct?

What is a stall?

Which change will usually give a wing more lift?

Why do planes need a longer run to take off on a very hot day?

For parents and teachers

Big idea (D2.1, D2.2): a wing makes lift by turning the air that flows past it downward; the air pushes back up on the wing. The same lift can be described with pressure: lower pressure on top of the wing, higher underneath. Lift grows with speed, angle of attack (up to a stall), wing area and air density.

About a common myth: many older books and websites say the air on top must go faster "to meet the air underneath at the back" (equal transit time). That is not true. The air over the top does move faster and the pressure there is lower, but not for that reason, and it actually reaches the trailing edge first. If your child finds the myth in a book, it's a great chance to talk about checking sources.

How to help: Let your child feel lift by moving a flat hand through water in a pool or bathtub, or in front of a fan, tilting it up and down. Never let a child put a hand out of a moving car's window.

Common difficulties:

Thinking wings need a curved top to work. Curves help, but tilt plus speed is enough (paper airplanes).
Thinking lift and air pressure are two competing explanations. They are two descriptions of the same thing.
Safety: the hair dryer is used on the cool setting with a grown-up, away from water.